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P a r t C

SPATIAL AT TENTION

Chapter 7

Spatial Covert At tention: Perceptual

Modul ation

Marisa Carrasco

Attention is a Selective Process

Each time we open our eyes we are confronted with an overwhelming amount of infor-mation. Despite this, we experience a seemingly eff ortless understanding of our visual world. Attention allows us to selectively process the vast amount of information with which we are confronted, prioritizing some aspects of information while ignoring oth-ers by focusing on a certain location or aspect of the visual scene.

Attention is a selective process. Selection is necessary because there are severe limits on our capacity to process visual information. Th ese limits are likely imposed by the fi xed amount of overall energy available to the brain and by the high energy cost of the neu-ronal activity involved in cortical computation. Given that the amount of overall energy consumption available to the brain is essentially constant, the high bioenergetic cost of spikes requires the use of effi cient representational codes relying on a sparse collection of active neurons, as well as the fl exible allocation of metabolic resources according to task demands. Th ese energy limitations, which allow only a small fraction of the machinery to be engaged concurrently, provide a neurophysiological basis for the idea that selective attention arises from the brain’s limited capacity to process information (Lennie 2003).

Th e notion that stimuli compete for limited resources (Broadbent 1958; Kinchla 1980, 1992; Neisser 1967; Treisman 1960) is supported by electrophysiological, neuroimaging, and behavioural studies (for reviews see Beck and Kastner 2009; Desimone and Duncan 1995; Reynolds and Chelazzi 2004; Carrasco 2011). According to the biased-competition hypothesis, stimuli in the visual fi eld activate populations of neurons that engage in com-petitive interactions, most likely at the intracortical level. When observers attend to vis-ual stimulation at a given location, such competition is biased in favour of the neurons

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encoding information at the attended area. Th us, neurons with receptive fi elds at that location become more active while others are suppressed.

Changing an observer’s attentional state while keeping the retinal image constant can aff ect perceptual performance and the activity of ‘sensory’ neurons throughout visual cortex. In the 1980s and 1990s, there was much interest in categorizing mechanisms of vision as preattentive or attentive. Th e interest in that distinction has waned as many studies have shown that attention actually aff ects many tasks that were once considered preattentive, such as contrast discrimination, acuity, and texture segmentation.

Spatial Covert Attention

Knowledge and assumptions about the world, the behavioural state of the organism, and the (sudden) appearance of possibly relevant information in the environment, facilitate the processing of sensory input. Attention can be allocated by moving one’s eyes toward a location (overt attention) or by attending to an area in the periphery without actually directing one’s gaze toward it (covert attention). Th e deployment of covert attention aids us in monitoring the environment and can inform subsequent eye movements. Humans deploy covert attention routinely in many everyday situations, such as searching for objects, driving, crossing the street, playing sports, and dancing. Moreover, covert atten-tion plays an important role in social situations, when moving the eyes provides a cue to intentions that the individual wishes to conceal. Covert attention allows us to monitor the environment and guides our eye movements (overt attention) to locations of the visual fi eld where salient and/or relevant information is likely to be.

Th e focus of spatial attention has been likened to a spotlight (Posner 1980), a zoom lens (Eriksen, Webb, and Fournier 1990), or a Gaussian gradient (Downing and Pinker 1985), which enhances processing of visual stimuli within a confi ned region of space. Th e size of this attended region may be adjusted voluntarily. When attention is distrib-uted over a larger region of the visual fi eld, rather than being focused on one location, there is a loss in spatial resolution and processing effi ciency for any given subregion of the attended region (Eriksen, Webb, and Fournier 1990; Castiello and Umiltà 1990, 1992; Eriksen and Murphy 1987; Eriksen and Schultz 1979; Eriksen and St James 1986; Eriksen and Yeh 1985; Shulman and Wilson 1987).

Oft en we think of the need to selectively process information in cluttered displays with diff erent colours and shapes (i.e. in ‘Where’s Wally’-like displays). Even with very simple displays, however, attention is involved in distributing resources across the vis-ual fi eld. Th ere are processing trade-off s for simple, non-cluttered displays, in which only two stimuli are competing for processing; the benefi t at the attended location for contrast sensitivity and acuity has a concomitant cost at the unattended location (Herrmann, Montaser-Kouhsari, Carrasco, and Heeger 2010; Montagna, Pestilli, and Carrasco 2009; Pestilli and Carrasco 2005; Pestilli, Viera, and Carrasco 2007; Barbot, Landy, and Carrasco 2011). Th ese fi ndings are inconsistent with the idea that perceptual

spatial covert attention 185

processes have unlimited capacity (Eckstein, Th omas, Palmer, and Shimozaki 2000; Palmer, Verghese, and Pavel 2000; Solomon 2004), or that attentional selection is required only once the perceptual load exceeds the capacity limit of the system (Lavie 1995). Rather, they suggest that trade-off s are a characteristic of attentional allocation, which has a general eff ect across diff erent tasks.

Here I review studies that deal with the eff ects of covert attention—endogenous or exogenous—on perceptual performance of many detection, discrimination, and locali-zation tasks mediated by early visual processing, e.g. contrast sensitivity and spatial res-olution. Attention can aff ect perception by altering performance—how accurately and quickly we perform on a given task—and/or by altering the subjective appearance of a stimulus or object. Most of the studies described in this review deal with discrimi-nation tasks, because they depend on attention more than detection judgements do (Bashinski and Bacharach 1980; Bonnel and Miller 1994; Bonnel, Stein, and Bertucci 1992; Downing 1988; Muller and Findlay 1987; Shaw 1984). Th e perceptual distinction between larger or smaller sensory signals (detection) poses a much simpler problem for the visual system than the distinction between sensory signals that are equally large but diff er in qualitative ways (discrimination).

Types of covert spatial attention: Endogenous and exogenous

William James (1890) described two diff erent kinds of attention; one is passive, refl ex-ive, and involuntary, whereas the other is active and voluntary. We now refer to these as exogenous/transient attention and endogenous/sustained attention. A growing body of behavioural evidence has demonstrated that these two covert attention systems facilitate processing and select information. Th e endogenous system corresponds to our ability to voluntarily monitor information at a given location; the exogenous sys-tem corresponds to an involuntary, automatic orienting response to a location where sudden stimulation has occurred. Due to their temporal nature, endogenous attention is also known as ‘sustained’ attention and exogenous attention is also known as ‘tran-sient’ attention. It takes ~300 ms for observers to deploy endogenous attention, and they can sustain it voluntarily for as long as is needed to perform a task; the involun-tary deployment of attention is transient: it rises and decays quickly, peaking at about 100–120 ms (Cheal, Lyon, and Hubbard 1991; Hein, Rolke, and Ulrich 2006; Ling and Carrasco 2006a, 2006b; Liu, Stevens, and Carrasco 2007; Muller and Rabbitt 1989a; Nakayama and Mackeben 1989; Remington, Johnston, and Yantis 1992). In line with these studies, a single-unit recording study has demonstrated that in macaque area MT exogenous attention has a faster time course than endogenous attention (Busse, Katzner, and Treue 2008). 1

A seminal task in the study of attention is the Posner cueing task, in which observers have to respond as quickly as possible to a peripheral target, which is preceded by a central or peripheral cue (Posner 1980). Th is paradigm allows the comparison of

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performance in conditions where attention is deliberately directed to either a given location (attended condition), away from that location (unattended condition), or dis-tributed across the display (neutral or control condition). I explain this cueing proce-dure in detail below (see the third part of this section). Whereas the shift s of attention prompted by central/sustained cues appear to be under conscious control and observers can allocate resources according to cue validity (Kinchla 1980; Giordano, McElree, and Carrasco 2009; Mangun and Hillyard 1990; Sperling and Melchner 1978), it is extremely diffi cult for observers to ignore peripheral/transient cues (Cheal, Lyon, and Hubbard 1991; Nakayama and Mackeben 1989; Giordano, McElree, and Carrasco 2009; Jonides 1981; Yantis and Jonides 1996). Involuntary transient shift s of attention occur even when the cues are known to be uninformative and irrelevant (Montagna, Pestilli, and Carrasco 2009; Pestilli and Carrasco 2005; Pestilli, Viera, and Carrasco 2007; Barbot, Landy, and Carrasco 2011; Muller and Rabbitt 1989b; Prinzmetal, McCool, and Park 2005; Yeshurun and Rashal 2010), and when they impair performance (Eriksen, Webb, and Fournier 1990; Hein, Rolke, and Ulrich 2006; Carrasco, Loula, and Ho 2006; Talgar and Carrasco 2002; Yeshurun 2004; Yeshurun and Carrasco 1998, 2000; Yeshurun and Levy 2003; Yeshurun, Montagna, and Carrasco 2008).

Endogenous attention and exogenous attention show some common (Montagna, Pestilli, and Carrasco 2009; Hikosaka, Miyauchi, and Shimojo 1993; Suzuki and Cavanagh 1997) and some unique perceptual eff ects. For instance, with peripheral cues, but not with central cues, the eff ects of attention are larger for a conjunction search than for a feature search in a letter search task (Hikosaka, Miyauchi, and Shimojo 1993; Suzuki and Cavanagh 1997; Briand 1998; Briand and Klein 1987). Furthermore, transient attention improves performance in a texture segmentation task at peripheral locations but impairs it at central locations (Carrasco, Loula, and Ho 2006; Talgar and Carrasco 2002; Yeshurun and Carrasco 1998; Yeshurun and Carrasco 2000; Yeshurun and Carrasco 2008), whereas sustained attention improves performance at all eccen-tricities (Yeshurun, Montagna, and Carrasco 2008).

Endogenous and exogenous attention also show diff erential eff ects regarding con-trast sensitivity to texture patterns (Barbot, Landy, and Carrasco 2012), temporal order judgement (Hein, Rolke, and Ulrich 2006), vigilance (Maclean et al. 2009), and inhibi-tion of return (Chica and Lupiañez 2009). Furthermore, a study employing a speed–accuracy trade-off procedure, which enables conjoint measures of discriminability and temporal dynamics, showed that the attentional benefi ts of endogenous atten-tion increased with cue validity while costs remained relatively constant. However, the benefi ts and the costs of exogenous attention in discriminability and temporal dynamics were similar across a wide range of cue validities (Giordano, McElree, and Carrasco 2009).

Th e diff erent temporal characteristics and degrees of automaticity of these atten-tional systems suggest that they may have evolved at diff erent times and for diff er-ent purposes—the exogenous system may be phylogenetically older, allowing us to respond automatically and quickly to stimuli that may provide behaviourally relevant information.


Mechanisms of covert attention

Although it is well established that covert attention improves performance in various visual tasks, the nature of the attentional mechanisms and the levels of processing at which they modulate visual activity are not yet well understood. Explanations of how attention improves perception range from proposals maintaining that the deployment of attention enhances the signal, to those stating that attention improves sensitivity by reducing external noise, to those proposing that the eff ects are due to observers’ decision criteria and/or reduction in spatial uncertainty. Sensory enhancement, noise reduction, and effi cient selection are not mutually exclusive; rather, all three are likely to contribute to the computational processes by which attention improves performance (Pestilli and Carrasco 2005; Eckstein, Th omas, Palmer, and Shimozaki 2000; Palmer, Verghese, and Pavel 2000; Lu and Dosher 1998, 2000; Pestilli, Carrasco, Heeger, and Gardner 2011).

Th e signal enhancement hypothesis proposes that attention improves the qual-ity of the stimulus representation by increasing the gain on the signal within the locus of attentional selection (Bashinski and Bacharach 1980; Downing 1988; Ling and Carrasco 2006a; Lu and Dosher 1998, 2000; Cameron, Tai, and Carrasco 2002; Carrasco, Penpeci-Talgar, and Eckstein 2000; Carrasco, Williams, and Yeshurun 2002; Dosher and Lu 2000a, 2000b; Luck, Hillyard, Mouloua, and Hawkins 1996; Morrone, Denti, and Spinelli 2002; Muller et al. 1998). Th e external noise reduction hypothesis has two distinct, not mutually exclusive, formulations—noise exclusion and distractor suppression—that suggest diff erent underlying computations. With noise exclusion, attention changes the properties of perceptual fi lters, enhancing the signal portion of the stimulus and mitigating the noise (e.g. Lu and Dosher 1998; Dosher and Lu 2000a, 2000b; Lu, Lesmes, and Dosher 2002). Attention acts like a fi lter at a specifi c location, only letting specifi c information pass; diff erent fi lters process information outside the attention fi lter (i.e. distractors at other locations). With distractor suppression, attention enables the observer to utilize a specifi c fi lter and to disregard information outside the focus of attention. Noise-limited models of distractor suppression incorporate internal noise arising from sources such as spatial and temporal uncertainty of targets and dis-tractors, as well as external noise resulting from distractors and masks. Several studies have attributed attentional facilitation to reduction of external noise, either because a near-threshold target presented alone could be confused with empty locations (spatial uncertainty) or because a suprathreshold target could be confused with suprathresh-old distractors. According to these models, performance decreases as spatial uncer-tainty and the number of distractors increase, because the noise they introduce can be confused with the target signal (Kinchla 1992; Baldassi and Burr 2000; Cameron, Tai, Eckstein, and Carrasco 2004; Morgan, Ward, and Castet 1998; Shiu and Pashler 1994; Solomon, Lavie, and Morgan 1997). Presumably, precues allow observers to monitor only the relevant location(s) instead of all possible ones, reducing statistical noise with respect to the target location. Th is is known as reduction of spatial uncertainty (Kinchla 1992; Eckstein, Th omas, Palmer, and Shimozaki 2000; Shiu and Pashler 1994; Davis,

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Kramer, and Graham 1983; Nachmias 2002; Palmer 1994; Sperling and Dosher 1986; Verghese 2001). A comprehensive study describes physiological measures and behav-ioural predictions distinguishing reduction of spatial uncertainty from increased sensi-tivity models (Eckstein, Peterson, Pham, and Droll 2009).

Methodological considerations in behavioural studies of covert attention

As mentioned, the Posner cueing task (Posner 1980) is a seminal task in the study of attention. Th is paradigm allows us to compare performance when attention is directed to the target location (attended condition), away from that location (unattended condi-tion) or distributed across the display (neutral or control condition). For endogenous attention, a central cue indicates the most likely location of the subsequent target with a given cue probability (e.g. 70%). Central cues are typically small lines presented at fi xation that point to particular locations of the visual fi eld (e.g. upper left quadrant; Fig. 7.1 ); symbolic cues include diff erent numbers or coloured shapes that indicate

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Figure 7.1 Sequence of events in a trial. Observers perform a two-alternative forced-choice (2AFC) orientation discrimination task on a tilted target Gabor patch, which appears at one of four isoeccentric locations. Th e target is preceded by a central cue (instructing observ-ers to deploy their endogenous attention to the upcoming target location), a peripheral cue (refl exively capturing attention to the upcoming target location), or a neutral cue (baseline). Th e cue in this example is valid. Th e timing (precue and interstimulus interval) for endogen-ous and exogenous conditions diff ers (along with their respective neutral conditions), in order to maximize the eff ectiveness of the cues.


diff erent locations where the observer is to attend (e.g. a green triangle or number 1 indi-cates upper left quadrant, a red triangle or number 4 indicates lower right quadrant). For exogenous attention, a brief peripheral cue, typically a dot or a small bar, is presented adjacent to one of the target locations, and is oft en not predictive of the subsequent tar-get location. Peripheral cues attract spatial attention in an automatic, stimulus-driven manner within ~100 ms.

In the neutral trials, a small disk or bar appears in the centre of the display (central neutral cue) or several small bars appear at the centre pointing to or at all possible tar-get locations (distributed neutral cue for endogenous and exogenous, respectively), or two long lines encompass the whole display (distributed neutral cue), indicating display onset and that the target is equally likely to occur at any possible location. Performance is comparable with these diff erent types of neutral cues in acuity (Carrasco, Williams, and Yeshurun 2002), texture segmentation (Yeshurun and Carrasco 2008), letter iden-tifi cation (Talgar, Pelli, and Carrasco 2004), and temporal resolution (Yeshurun 2004). Many studies discussed in the third and fourth sections below use the Posner paradigm or a variant of it.

Th e following are some critical methodological issues to be considered when using spatial cues to test for sensory eff ects of attention.

To investigate covert attention, it is necessary to ensure that observers’ eyes remain fi xated at one location and to keep both the task and stimuli constant across conditions while manipulating attention. Given that ~200–250 ms are needed for goal-directed saccades to occur (Mayfrank, Kimmig, and Fischer 1987), the stimulus onset asyn-chrony (SOA) for the endogenous cue may allow observers to make an eye movement towards the cued location. Th us, to verify that the outcome of this manipulation is due to covert attention, it is necessary to monitor with an eye-tracker whether central fi xa-tion is maintained.

Spatial cues should convey only information that is orthogonal to the task, e.g. in a discrimination task they could indicate probable target location but not the correct response (e.g. the orientation of a stimulus; Fig. 7.2 ). Many experiments manipulate endogenous attention in detection tasks with cues indicating that a certain location has a given probability of containing the target. Although a high probability encour-ages observers to direct their attention to a particular location, it is unclear whether the enhanced detection is due to facilitation of information coding at that location, to probability matching, or to a decision mechanism, i.e. the higher probability encourages observers to assign more weight to information extracted from that probability location (Kinchla 1992). By using a two-alternative forced choice (2AFC) in which the observ-ers discriminate stimuli preceded by a cue (e.g. stimulus orientation: left vs right), even when the cue is 100% valid in terms of location, it conveys no information as to the cor-rect response. Th us, we can assess whether a cueing eff ect refl ects changes in sensory discriminability (e.g. with the help of the d΄ index), rather than decisional (criterion), processes.

Another critical factor is that of spatial uncertainty. According to noise-limited models, performance decreases as spatial uncertainty increases because the empty

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locations introduce noise that can be confused with the target signal. For instance, a spatial uncertainty eff ect is present for low-contrast pedestals but not for high-contrast pedestals (Foley 1998). Uncertainty about the target location produces a more notice-able degradation at low than at high performance levels (Eckstein and Whiting 1996; Pelli 1985), and uncertainty is larger for less discriminable stimuli (Pelli 1985; Cohn 1981; Nachmias and Kocher 1970). Th us, uncertainty models predict that the cueing eff ect would be greater for low-contrast stimuli and when localization performance is poor (Carrasco, Penpeci-Talgar, and Eckstein 2000; Carrasco, Williams, and Yeshurun 2002; Pelli 1985; Cohn 1981; Nachmias and Kocher 1970). Some studies have evalu-ated whether attention eff ects go beyond uncertainty reduction, considered to be a benchmark (e.g. Carrasco, Penpeci-Talgar, and Eckstein 2000; Carrasco, Williams, and Yeshurun 2002; Morgan, Ward, and Castet 1998; Eckstein, Shimozaki, and Abbey 2002). Physiological measures and behavioural predictions can distinguish uncertainty vs increased sensitivity models (Eckstein, Peterson, Pham, and Droll 2009).

Covert Attention and Contrast Sensitivity

Many studies described in this section involve an orientation discrimination task because this dimension has been well characterized both psychophysically and neu-rophysiologically, and a link between fi ndings obtained with these two approaches has been well established (De Valois and De Valois 1988; Graham 1989; Regan and Beverley 1985; Ringach, Hawken, and Shapley 1997). In addition, orientation discrimination is used to assess the eff ect of attention on stimulus contrast because performance on this task improves with increasing contrast (Nachmias 1967; Skottun et al. 1987), and because fMRI response increases monotonically with stimulus contrast (Boynton, Demb, Glover, and Heeger 1999). Moreover, the shared non-linearity between the contrast response function and the magnitude of the attentional modulation across diff erent areas of the dorsal and ventral visual pathways indicates a close link between attentional mechanisms and the mechanisms responsible for contrast encoding (Martinez Trujillo and Treue 2005; Reynolds 2005).

Th e eff ect of attention on contrast sensitivity has been well documented with studies employing an array of attention manipulations (e.g. Barbot, Landy, and Carrasco 2011; Solomon 2004; Barbot, Landy, and Carrasco 2012; Lu and Dosher 1998, 2000; Cameron, Tai, and Carrasco 2002; Carrasco, Penpeci-Talgar, and Eckstein 2000; Dosher and Lu 2000a, 2000b; Solomon, Lavie, and Morgan 1997; Foley 1998; Huang and Dobkins 2005; Lee, Itti, Koch, and Braun 1999; Lee, Koch, and Braun 1997; Morrone, Denti, and Spinelli 2004; Smith, Wolfgang, and Sinclair 2004). Psychophysical studies have shown that contrast sensitivity for the attended stimulus is enhanced in the presence of com-peting stimuli (e.g. Lu and Dosher 1998; Dosher and Lu 2000a, 2000b; Solomon, Lavie,


and Morgan 1997; Foley 1998; Huang and Dobkins 2005; Lee, Itti, Koch, and Braun 1999; Lee, Koch, and Braun 1997; Morrone, Denti, and Spinelli 2004; Carrasco and McElree 2001). In some studies, the target is displayed simultaneously with distractors and observers indicate whether the target has an increment or decrement in contrast. Performance deteriorates as number of stimuli increases. However, a cue improves per-formance and this eff ect increases with number of distractors (Cameron, Tai, Eckstein, and Carrasco 2004; Morgan, Ward, and Castet 1998; Foley 1998; Carrasco and McElree 2001; Carrasco, Giordano, and McElree 2006). Th ese fi ndings are attributed to result from spatial uncertainty reduction (Kinchla 1992; Shaw 1984; Solomon, Lavie, and Morgan 1997; Palmer 1994; Eckstein, Peterson, Pham, and Droll 2009; Foley 1998).

Other studies have shown that attentional benefi ts go beyond what is predicted by uncertainty reduction (Ling and Carrasco 2006a; Lu and Dosher 1998; Carrasco, Penpeci-Talgar, and Eckstein 2000; Carrasco, Williams, and Yeshurun 2002; Dosher and Lu 2000a, 2000b; Morgan, Ward, and Castet 1998; Lee, Koch, and Braun 1997). For instance, Lee et al. (1997) measured contrast and orientation thresholds, which are well characterized under normal viewing conditions (Rovamo and Virsu 1979; Virsu and Rovamo 1979), in the ‘near absence of attention’. Th ey asked observers to carry out two concurrent visual tasks, one of them highly demanding of attention (‘primary task’), in which optimal performance is reached only when attention is fully focused, and thus almost completely withdrawn from the other task (‘secondary task’). Th is paradigm ensures that less attention is devoted to the latter, thus the term ‘near absence of atten-tion’ (Braun and Julesz 1998; Braun 1994). Performing a concurrent task increased both orientation and contrast thresholds, and the eff ect was more pronounced for the former than the latter (Lee, Koch, and Braun 1997).

Signal enhancement and external noise reduction

Th ere are a number of mechanisms proposed to explain how attention may aff ect per-ception, many of which can be indexed within the perceptual template model (PTM). Th e PTM provides a theoretical and empirical framework to assess the mechanisms of attention by systematically manipulating the amount and/or characteristics of external noise added to stimuli and measuring modulation of perceptual discriminability.

According to the PTM, a signal is analysed by a perceptual template. Th ere are three characteristic mechanisms by which attention can interact with the perceptual tem-plate to improve performance: (1) Stimulus enhancement, by which attention turns up the gain on the perceptual template, improving performance through amplifi ca-tion of the signal stimulus. It is mathematically equivalent to internal additive noise reduction ( Fig. 7.2a ). Increased gain helps to overcome internal noise in low-noise displays, but has little eff ect in high external noise because the external noise and sig-nal stimulus are amplifi ed equally ( Fig. 7.2b ). (2) External noise exclusion reduces the eff ects of external noise through fi ltering (Fig. 7.2c), by focusing the perceptual template on the appropriate spatial region or content characteristics of the stimulus.

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Figure 7.2 (a) Stimulus enhancement aff ects both the signal and the external noise in the input stimulus in the same way; thus, there is no retuning of the perceptual template entering the decision. (b) Stimulus enhancement has an eff ect at low but not high levels of external noise. (c) External noise exclusion reduces the eff ects of external noise through fi ltering, or retuning of the percep-tual template that enters in the decision. (d) External noise exclusion improves performance in the region of high external noise, where there is external noise to exclude. (e) Internal noise reduc-tion. Multiplicative noise increases with increasing contrast in the stimulus display. (f) Performance improves across all levels of external noise. Reprinted from Vision Research , 49 (10), Zhong-Lin Lu, Hennis Chi-Hang Tse, Barbara Anne Dosher, Luis A Lesmes, Christian Posner, and Wilson Chu, Intra- and cross-modal cuing of spatial attention: Time courses and mechanisms, pp. 1081–96, Copyright (2009), with permission from Elsevier.


Excluding external noise improves performance only when there is signifi cant external noise in the stimulus to fi lter out ( Fig. 7.2d ). (3) Multiplicative internal noise reduc-tion ( Fig. 7.2e ) impacts performance across all levels of external noise with slightly larger eff ects in high external noise. Th is pattern is rarely observed. Th e magnitude of the attention eff ect is the same (on the log contrast axis) at diff erent performance levels (e.g. 75% and 85% performance threshold) for signal enhancement ( Fig. 7.2a ) or external noise exclusion ( Fig. 7.2b ) , but not for multiplicative noise reduction. Th us, were there an eff ect across all levels of external noise, testing at two performance levels would disambiguate a combination of both stimulus enhancement and external noise reduction from multiplicative noise reduction. In the fi rst case, the eff ect would scale, whereas in the second case it would not.

Th e PTM is an extension of models of the human observer (Burgess, Wagner, Jennings, and Barlow 1981; Pelli and Farell 1999) that incorporates known qualities of the visual system (for a review see Lu and Dosher 2008). Accuracy is constrained by internal limitations (receptor sampling errors, noise in neural responses, informa-tion loss during neural transmission) and external noise in the stimulus. Performance in d΄ (or % correct) for psychometric functions or contrast threshold from threshold versus contrast (TvC) functions is measured as a function of task, attention, exter-nal noise, and contrast. Th e parameters of the PTM model are estimated from psy-chometric functions (Dosher and Lu 2000a, 2000b; Lu and Dosher 2008) or from TvC functions from two or three measured threshold levels (Dosher and Lu 1998; Lu and Dosher 1999).

Using an external noise paradigm, cueing attention to a spatial location has revealed external noise exclusion and signal enhancement as the primary and secondary mecha-nisms underlying spatial attention. In the absence of uncertainty, one of the primary roles of spatial attention is to exclude external noise in the target region (Lu and Dosher 2000; Dosher and Lu 2000a, 2000b; Lu, Lesmes, and Dosher 2002). Transient attention increases contrast sensitivity in conditions of low noise, indicative of signal enhance-ment, and also improves performance in high-noise conditions, indicative of external noise reduction (Lu and Dosher 1998, 2000). However, these authors have attributed sustained attention eff ects exclusively to external noise reduction (Lu and Dosher 2000; Dosher and Lu 2000a, 2000b; Lu, Lesmes, and Dosher 2002); but see Ling and Carrasco (2006a). Figure 7.3 illustrates a trial sequence to manipulate sustained attention via a central precue. In this example the precue is invalid, because it cues a location away from the stimulus. Th e bottom panel shows that for this sample observer, the precue lowered the contrast threshold for high but not low levels of external noise, i.e. the signature of external noise reduction.

External noise paradigms have been widely used to probe diff erent levels of visual processing and examine properties of detection and discrimination mechanisms. Th ese paradigms assume that signal processing is noise-invariant: i.e. external noise adds to the variance of a single perceptual template, but that that fi lter remains unperturbed with diff erent levels of external noise. Some recent fi ndings, however, question this key assumption (Abbey and Eckstein 2009; Allard and Cavanagh 2011).

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Exogenous attention and contrast sensitivity

To assess the conditions for which the eff ect of attention can be attributed to signal enhancement, it is necessary to ensure that a performance benefi t occurs under con-ditions that exclude all variables that the external noise reduction models hold to be responsible for the attentional eff ect (Lu and Dosher 2000; Dosher and Lu 2000a, 2000b; Baldassi and Burr 2000; Morgan, Ward, and Castet 1998; Solomon, Lavie, and Morgan 1997; Davis, Kramer, and Graham 1983; Nachmias 2002). Th at is, the target should be suprathreshold (to reduce spatial uncertainty) and presented alone, with-out distractors or masks (Ling and Carrasco 2006a; Cameron, Tai, and Carrasco 2002; Carrasco, Penpeci-Talgar, and Eckstein 2000; Carrasco, Williams, and Yeshurun 2002; Golla, Ignashchenkova, Haarmeier, and Th ier 2004).

A study with these experimental conditions evaluated whether the eff ect of attention on contrast sensitivity could be mediated by signal enhancement. For a wide range of spatial frequencies, contrast threshold was lower when the target location was preceded by a peripheral cue than by a neutral cue. A signal detection theory (SDT) model of

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Figure 7.3 (a) Trial sequence illustrating an invalid cue: a central cue indicates a location; the stimulus and the response cue appear simultaneously indicating to the observer the tar-get location. (b) Contrast threshold versus external noise contrast data from a sample observer. Endogenous attention reduces contrast thresholds at high noise levels but not at low noise levels, suggesting tuning of the perceptual template to exclude external noise. Reprinted from Vision Research , 40 (10–12), Barbara Anne Dosher and Zhong-Lin Lu, Mechanisms of perceptual atten-tion in precuing of location, pp. 1269–92, Copyright (2000), with permission from Elsevier.


external noise reduction could account for the cueing benefi t in an easy discrimina-tion task (vertical vs horizontal Gabor patches), but not when location uncertainty was reduced (by increasing stimulus contrast to enable fi ne discriminations of slightly tilted suprathreshold stimuli, or presenting a local post-mask). An SDT model that incorpo-rates intrinsic uncertainty (the observers’ inability to perfectly use information about the elements’ spatial or temporal positions, sizes, or spatial frequencies) revealed that the cueing eff ect exceeded the one predicted by uncertainty reduction. Given that the attentional benefi ts occurred under conditions that exclude all variables predicted by the external noise reduction model, the results support the signal enhancement model of attention (Carrasco, Penpeci-Talgar, and Eckstein 2000). Th is fi nding is consistent with those of the low-noise conditions in the external noise plus attention paradigm (Lu and Dosher 1998, 2000).

Another study showed that transient attention decreased the threshold for contrast sensitivity in an orientation discrimination task across the contrast psychometric func-tion (Cameron, Tai, and Carrasco 2002). Two control experiments assessed the role of spatial uncertainty. First, attention increased performance to the same extent in condi-tions with diff erent uncertainty levels, even though observers required higher stimulus contrasts to perform the discrimination task with a small than with a large tilt. Second, for a large tilt discrimination and localization performance were tightly coupled, but performance on the localization task was much better than on the discrimination task with a small tilt, and yet the attentional eff ect was comparable for both orientation conditions. Th us, given that there was no added external noise and spatial uncertainty cannot explain the cue eff ect on contrast threshold, the observed attentional benefi t is consistent with a signal enhancement mechanism (Cameron, Tai, and Carrasco 2002).

Attention improves performance at the attended location, but how sensitive to con-trast are observers at the unattended locations? To assess contrast sensitivity at both cued and uncued locations, observers were asked to discriminate the orientation of one of two Gabor patches simultaneously presented left and right of fi xation. A response cue was presented aft er the Gabor stimuli, indicating which Gabor to respond to; in valid trials, the cue location and response cue match and in invalid trials, they do not match (Fig. 7.4a). Th e response cue equates location uncertainty across conditions (Montagna, Pestilli, and Carrasco 2009; Pestilli and Carrasco 2005; Pestilli, Viera, and Carrasco 2007; Barbot, Landy, and Carrasco 2011; Ling and Carrasco 2006b; Barbot, Landy, and Carrasco 2012; Lu and Dosher 2000; Eckstein, Shimozaki, and Abbey 2002). Contrast sensitivity was measured at the valid cue and invalid cue locations, and compared with the contrast sensitivity obtained at the same locations when the target was preceded by a neutral cue. Observers were told that the peripheral cue was uninformative: that it pre-ceded the target or the distractor with the same probability.

Despite the simplicity of the display, there is a performance trade-off : the cue increases sensitivity at the cued location (benefi t) and impairs it at the uncued location (cost), as compared to the neutral condition (Fig. 7.4b; Pestilli and Carrasco 2005); see also (Pestilli, Viera, and Carrasco 2007). Given that for an ideal observer the uninformative cue would not reduce uncertainty, this fi nding supports sensitivity-based explanations,

196 Marisa Carrasco

i.e. signal enhancement at the cued location and external noise reduction at the uncued location. Th is study provides evidence that exogenous attention aff ects visual process-ing in an automatic fashion (Cheal, Lyon, and Hubbard 1991; Muller and Rabbitt 1989a; Nakayama and Mackeben 1989; Yantis and Jonides 1996).

A performance trade-off has also been obtained for an intermediate stage of processing—second-order contrast (Barbot, Landy, and Carrasco 2011). Much of the early system, from the retina to the primary visual cortex, is dedicated to detecting changes in luminance (fi rst-order information). However, humans are also sensitive to patterns defi ned only by spatial variations of textural attributes (second-order informa-tion). Texture patterns can be homogeneous in terms of average luminance, but have diff erent regions segregated by means of their global grouping characteristics—regional similarity, spatial frequency, or orientation ( Fig. 7.5b ). Naturally occurring edges are usually defi ned by spatially coincident changes in both luminance and texture infor-mation (Johnson and Baker 2004). However, luminance-defi ned information generally originates from non-uniform surface illumination that does not correspond to object

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Figure 7.4 (a) A trial sequence. Following a fi xation point, a cue appears either above one of the two Gabor locations (peripheral cue) or at fi xation (neutral cue). Aft er an ISI, two Gabor stimuli are simultaneously presented (randomly oriented to the left or to the right) on the horizontal meridian. Th en a response cue appears at fi xation to indicate the target Gabor for which the observer had to report the orientation. On one third of the trials the response cue points to a precued Gabor (valid cue). On another third of the trials it points to the Gabor that was not precued (invalid cue). In the remaining trials the precue is presented in the centre of the screen and the response cue is equally likely to indicate the Gabor to the right or to the left of fi xation (neutral cue). (b) Th e histograms represent the distribution of the measured thresholds obtained for an individual observer in each cue condition at 4° and 9° of eccentricity. Top row—valid cue; middle row—neutral cue; bottom row—invalid cue. Dashed vertical lines indicate the median values. Reprinted from Vision Research , 45 (14), Franco Pestilli and Marisa Carrasco, Attention enhances contrast sensitivity at cued and impairs it at uncued locations, pp. 1867–75, Copyright (2005), with permission from Elsevier.


boundaries (Schofi eld, Hesse, Rock, and Georgeson 2006; Sun and Schofi eld 2011). Consequently, by increasing the sensitivity to second-order information, attention may considerably improve object detection.

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Figure 7.5 (a) Natural scene containing 1st- and 2nd-order boundaries. Th e boundary between the pedestrians and the walkway in the lower right is defi ned by a change in lumi-nance (1st-order information); the boundaries between the diff erent regions of the walkway are defi ned by changes in textural attributes (i.e. local orientation) without any changes in luminance (2nd-order information). (b) Texture patterns constructed by spatially modulating two orthogonal gratings (1st-order carriers ±45º) using a second horizontal or vertical grat-ing with lower spatial frequency (2nd-order modulator). (c) Eff ects of exogenous attention, and (d) Eff ects of change in 1st-order peak luminance contrast, on performance (d΄) across observers as a function of 2nd-order modulator contrast. Psychometric functions and param-eter estimates (c 50 : 2nd-order contrast yielding half-maximum performance; d΄ max : asymp-totic performance) for each cueing condition (C, peak luminance carrier contrast was 70%, and D, for diff erent 1st-order carrier contrast values). Adapted from Vision Research , 51 (9) Antoine Barbot, Michael S. Landy, and Marisa Carrasco, Exogenous attention enhances 2nd-order contrast sensitivity, pp. 1086–98, Copyright (2011), with permission from Elsevier.

198 Marisa Carrasco

Like fi rst-order luminance fi lters, second-order channels are tuned for orientation (Arsenault, Wilkinson, and Kingdom 1999; Dakin, Williams, and Hess 1999; Graham and Wolfson 2001) and spatial frequency (Landy and Oruc 2002; Scott-Samuel and Georgeson 1999), but have wider bandwidths (Landy and Oruc 2002). Exogenous attention also aff ects contrast sensitivity for second-order, texture-defi ned, information (Barbot, Landy, and Carrasco 2011). Specifi cally, attention increases second-order contrast sensitivity at the attended location, while decreasing it at unattended locations, relative to a neutral con-dition ( Fig. 7.5c ). A recent study shows that endogenous attention also alters second-order contrast sensitivity (Barbot, Landy, and Carrasco 2012). Th ese eff ects of attention on second-order contrast sensitivity could not be explained by changes in fi rst-order lumi-nance contrast because discriminability is the same at a wide range of contrasts ( Fig. 7.5d ). Together with the eff ects of covert attention on fi rst-order contrast sensitivity reviewed above, this study suggests that attention aids in the segmentation of the retinal image by increasing both fi rst- and second-order sensitivity at the attended location.

In a study using a diff erent attention manipulation, thresholds were compared when observers performed various tasks with full attention and transient attention focus-ing elsewhere on a concurrent, secondary task (Lee, Itti, Koch, and Braun 1999). Th e measurements included a dipper curve (contrast detection and contrast increment detection), contrast-masking thresholds (for masks of diff erent orientations and spatial frequencies), orientation discrimination thresholds, and spatial-frequency discrimination thresholds. Based on the psychophysical results and modelling (‘linear-fi lter-population-interacting-through-divisive-normalization’), the authors concluded that transient attention has a generic eff ect, that it boosts both excitatory and suppressive inputs prior to the divisive normalization, and that the fi nal eff ect on the population response depends on the relative strength of target and distractor stimuli.

Endogenous attention and contrast sensitivity

Using an external noise procedure, Lu and Dosher reported that exogenous attention is mediated by signal enhancement (at low levels of external noise) and external noise reduction (at high levels of external noise), whereas endogenous attention is only medi-ated by external noise reduction (Dosher and Lu 2000a, 2000b; Lu, Lesmes, and Dosher 2002; Lu, Liu, and Dosher 2000). Th is result is surprising given that electrophysiological studies have reported facilitation eff ects of attention in conditions of low noise. Ling and Carrasco (2006a) compared these two types of covert attention using the same task, stim-uli, and observers. Th ey evaluated whether a signal enhancement mechanism underlies both types of attention and investigated the neural model underlying signal enhancement by measuring the psychometric functions for both types of attention to assess whether they have similar or diff erent eff ects on the contrast response function. Observers per-formed a 2AFC orientation discrimination task on a slightly tilted Gabor patch.

Results indicated that both endogenous and exogenous attention have a similar eff ect on performance; both types of attention increase contrast sensitivity under zero-noise


conditions (the display contained nothing to be suppressed because there was no added external noise). Hence, both attentional systems can be mediated by a signal enhance-ment mechanism. However, they have diff erent eff ects on the contrast response func-tion: endogenous attention seemed to operate via contrast gain, i.e. a shift in threshold, whereas exogenous attention seemed to operate via response gain, i.e. a shift in asymp-tote (Pestilli, Viera, and Carrasco 2007; Pestilli, Ling, and Carrasco 2009), but see Herrmann, Montaser-Kouhsari, Carrasco, and Heeger (2010).

Th ese results cannot be explained by an uncertainty reduction model, which would predict that the attention eff ect should be most prominent with low-contrast stimuli (Pelli 1985) for both types of attention. Ling and Carrasco (2006a) proposed that the short 150 ms SOA may have precluded an emergence of the signal enhancement mech-anism in the Dosher and Lu studies (Dosher and Lu 2000a, 2000b). As mentioned, it takes ~300 ms for endogenous attention to be deployed (e.g. Liu, Stevens, and Carrasco 2007; Muller and Rabbitt 1989a; Nakayama and Mackeben 1989).

A population coding model based on few biological-plausible assumptions and only two free parameters—threshold and asymptote of the contrast response function—esti-mated attentional eff ects on population contrast response given psychophysical data (including those of Ling and Carrasco 2006a). According to this model, whereas endog-enous attention changes population contrast response via contrast gain, exogenous attention changes population contrast response via response gain (Pestilli, Ling, and Carrasco 2009; Fig. 7.6 ).

With a dual-task paradigm, a sustained attention task was used to investigate the inde-pendence of the attentional resources on the processing of luminance- and chromatic- modulated stimuli. Observers’ threshold vs contrast (TvC) functions were measured under conditions of full or poor attention. Th e eff ect of attention was constant at ped-estal contrasts above 0%, consistent with a response gain model (Morrone, Denti, and Spinelli 2004). Similar results have been obtained with visually evoked potentials (Di Russo, Spinelli, and Morrone 2001). Based on the fi ndings that there was interference only when both stimuli involved luminance or colour contrast, the authors concluded that the two attributes are processed separately and that they engage diff erent attentive resources. However, this conclusion is controversial; the attentional demand of the cen-tral task may have been higher for the same contrast than the diff erent contrast condi-tions (Huang and Dobkins 2005; Pastukhov, Fischer, and Braun 2009).

Using a similar dual-task paradigm, a subsequent study found evidence for both contrast gain and response gain, and proposed a hybrid model in which attention fi rst undergoes contrast gain, followed by a later-stage response gain modulation (Huang and Dobkins 2005). Th e authors attributed the diff ering fi ndings to experimental parameters; they state that the contrasts they tested captured the entire response functions better, and that the dual task used by Morrone et al. may have not been demanding enough.

Whereas the dual-task paradigm has the advantage of eliminating location uncertainty reduction as an alternative explanation, it also has drawbacks. Dual-task paradigms do not control the deployment of attention well, and make it hard to isolate the source of possible processing diff erences (Sperling and Dosher 1986; Pashler and Johnston 1998).

200 Marisa Carrasco

Ling and Carrasco (2006a) speculate that the diff erence in results with those obtained by Huang and Dobkins (2005) and Morrone et al. (2002, 2004) may be due to the way in which attention was manipulated. In dual-task paradigms, attention is not directed to a specifi c location, but the amount of resources being spread to all locations is manipu-lated. (For an alternative explanation for these diff erences, see the next section.)

A selective adaptation procedure, which assesses the spatiotemporal properties of the visual system, has also been used to further investigate the eff ects of endogenous atten-tion on contrast sensitivity. Prolonged exposure to a stimulus reduces sensitivity to it and to similar stimuli, thus allowing for the selective adaptation of a particular feature, such as spatial frequency or orientation (Graham 1989; Blakemore and Campbell 1969;

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Figure 7.6 (a) Model fi ts and averaged psychometric data for attended (red) and neutral attention (black) conditions with endogenous (top panel) and exogenous (bottom panel) atten-tion. (b) Normalized population contrast-response functions (pCRF) for contrast and response gain. Th e best model for endogenous attention is contrast gain; attention shift s the pCRF horizontally towards lower contrast. Th e best model for exogenous attention is response gain; attention increases pCRF incrementally from low to high contrast (bottom panel). (c) Average diff erence in responses (from b): neutral-minus-attended (blue) across all observers. Error bars in (a) are 95% confi dence intervals (C.I.), obtained by non-parametric bootstrap. Shaded areas in (B) show model parameters’ (C 50 and Rmax) variability obtained by fi tting the model to each one of the bootstrapped samples within the 95% C.I. in (a). Reprinted from Vision Research , 49 (10), Franco Pestilli, Sam Ling, and Marisa Carrasco, A population-coding model of attention’s infl uence on contrast response: Estimating neural eff ects from psychophysical data, pp. 1144–53, Copyright (2009), with permission from Elsevier.


Movshon and Lennie 1979). Capitalizing on this procedure, and on the fi nding that the magnitude of adaptation increases with the intensity of the adaptor stimulus (Langley 2002), the time course of attention’s eff ects on contrast sensitivity was assessed. Given that attention boosts signal strength, it initially enhances contrast sensitivity, but over time sustaining endogenous attention to an adapting pattern actually impairs sensitiv-ity because the adapting signal is stronger at the attended location. Conversely, contrast sensitivity at an unattended location is impaired at short adapting times, but this eff ect reverses with time, because the adapting signal is weaker at unattended locations (Ling and Carrasco 2006b).

A recent experiment has investigated the eff ect of attention (relevance) and expecta-tion (probability) on contrast sensitivity by using a signal detection task in conjunction with reverse correlation (Wyart, Nobre, and Summerfi eld 2012). Th e authors manipu-lated independently relevance and probability using two cues and showed that they pro-duce dissociable eff ects on contrast sensitivity. Both attention and expectation infl uence energy sensitivity, but they do so in a diff erent way ( Fig. 7.7 ): attention increases energy sensitivity mainly for signal-present stimuli (hits) whereas expectation increases the frequency and energy sensitivity mainly for signal-absent stimuli (false alarms). Based on a computational model, the authors propose that attention suppresses internal noise during signal processing via a multiplicative mechanism, consistent with electrophysi-ological fi ndings showing that attention reduces interneuronal correlations (Cohen and Maunsell 2009; Mitchell, Sundberg, and Reynolds 2009; see Maunsell’s chapter in Kastner and Nobre, forthcoming). Further, by comparing two models in which signal probability facilitates signal detection either at the input stage or at the response stage, the authors concluded that probability biases signal detection by increasing the baseline activity of signal-selective units, in agreement with ‘predictive-coding’ models of per-ception (Rao and Ballard 1999).

Model of enhanced contrast: Th e Normalization Model of Attention

A recent model, the Normalization Model of Attention, was proposed to reconcile pre-vious, seemingly contradictory fi ndings on the eff ects of visual attention, to unify alter-native models on attention, and to off er a computational framework to simulate new research questions (Reynolds and Heeger 2009). Based on the model ( Fig. 7.8 ), spatial attention can be characterized with an attention fi eld selective for a given spatial loca-tion, but not selective (constant) across features of a dimension (e.g. orientation). Th e stimulus drive is multiplied with the attention fi eld and then normalized, such that the extent of the stimulus and the relative extent of the attention fi eld can shift the balance between excitation and suppression. Th us, the model can exhibit diff erent eff ects of attentional modulation, described in the literature, such as response gain changes that increase fi ring rates by a multiplicative scale factor without changing the shape or width of neuronal tuning (McAdams and Maunsell 1999; Treue and Maunsell 1999); contrast

202 Marisa Carrasco

gain changes that increase responses, multiplying the stimulus contrast by a scale fac-tor (Martinez-Trujillo and Treue 2002; Reynolds, Pasternak, and Desimone 2000); a combination of both response gain and contrast gain changes (Williford and Maunsell 2006); or sharpening of neuronal tuning (Martinez-Trujillo and Treue 2004; Spitzer, Desimone, and Moran 1988).

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Figure 7.7 (a) Signal probability and (c) signal relevance increase the energy-sensitive profi le at the target orientation. However, they do so in a dissociable manner: Signal probability increases primarily sensitivity of false alarms (b), but signal relevance increases predominantly the sen-sitivity of hits (d), to parametric changes in signal energy. Reprinted from Proceedings of the National Academy of Sciences of the United States of America , 109 (9), Wyart, V., A. C. Nobre, and C. Summerfi eld, Dissociable prior infl uences of signal probability and relevance on visual contrast sensitivity, pp. 1594–600, fi gure 3, Copyright (2012), with permission from Th e National Academy of Sciences.


A key prediction of this model is that the eff ect of attention can systematically shift from a change in response gain to contrast gain with smaller stimuli and a broader attention fi eld. Th is prediction was recently confi rmed by using spatial uncertainty to manipulate attention fi eld size (Herrmann, Montaser-Kouhsari, Carrasco, and Heeger 2010). When the stimuli are large and the size of the window is small, both exogenous ( Fig. 7.9a ) and endogenous ( Fig. 7.9b ) attention yield response gain. However, when the stimuli are small and the size of the window is large, both exogenous ( Fig. 7.9c ) and endogenous ( Fig. 7.9d ) attention yield contrast gain.

An fMRI experiment showed that the attention fi eld was larger with spatial uncer-tainty than without it. As predicted by the Normalization Model of Attention, atten-tion modulates activity in visual cortex in a manner that can resemble either a change in response gain or contrast gain, depending on stimulus size and attention fi eld size.

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Figure 7.8 Th e Normalization Model of Attention. Stimuli, presented as input to the model, were two spatially noncontiguous locations identical in contrast, one of which was attended. (a) depicts the stimulus drive for a population of neurons with various receptive fi eld centres (horizontal) and orientation preferences (vertical). Brightness at each location in the image corresponds to the stimulus drive to a single neuron. (b) shows the attention fi eld when attending to the right location. Grey indicates a value of 1 and white indicates an attentional gain factor >1. Th e attention fi eld is multiplied point-by-point with the stimulus drive. (c) Th e normalization factors are computed by the product of the stimulus drive and the attention fi eld, and then pooled over space and orientation through convolution with the suppressive fi eld. (d) Neural image depicting the output fi ring rates of the population of simulated neu-rons, computed by dividing the stimulus drive by the normalization factors. Th e stimulus drive, attention fi eld, and normalization factor all had Gaussian profi les in space and orienta-tion. Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Neuroscience , 13 (1), Matteo Carandini and David J. Heeger, Normalization as a canonical neural computa-tion, pp. 51–62, © 2012, Nature Publishing Group.

204 Marisa Carrasco

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Figure 7.9 Eff ects of exogenous and endogenous attention on performance (d') as a function of contrast. (a, b) Large stimulus with small attention fi eld. (c, d) Small stimulus with large attention fi eld. Exogenous attention is shown in (a, c). Endogenous attention is shown in (b, d). Shown are plots of psychometric functions for each attentional condition (valid, neutral, and invalid precues) and parameter estimates (c50, contrast yielding half-maximum perfor-mance; d' max, asymptotic performance at high contrast). Each data point represents the mean across four observers. Error bars on data points are ±, i.e. error bars on parameter estimates are confi dence intervals obtained by bootstrapping. Reprinted by permission from Macmillan Publishers Ltd: Nature Neuroscience , 13 (12), Herrmann, K., L. Montaser-Kouhsari, M. Carrasco, and D. J. Heeger, When size matters: attention aff ects performance by contrast or response gain, pp. 1554–9, © 2010, Nature Publishing Group.

Th us, diff erences in the experimental protocols may also explain previous discrep-ancies among psychophysical studies (Pestilli and Carrasco 2005; Pestilli, Viera, and Carrasco 2007; Ling and Carrasco 2006a; Morrone, Denti, and Spinelli 2002, 2004; Di Russo, Spinelli, and Morrone 2001). With constant stimulus size, some studies have found that exogenous attention alters performance via a response gain change, whereas endogenous attention does so via a contrast gain change (Pestilli and Carrasco 2005; Pestilli, Viera, and Carrasco 2007; Ling and Carrasco 2006a; Huang and Dobkins 2005; Di Russo, Spinelli, and Morrone 2001). A response gain change could have been elicited by brief peripheral cues nearby the stimulus, whereas a con-trast gain change could have resulted from endogenous cues at fi xation, rather than


cues adjacent to the stimulus, which may have encouraged a narrower or larger atten-tion fi eld, respectively. Moreover, the diff erent results seem to be related to diff er-ences in stimulus size; for endogenous attention, response gain changes were reported with the largest stimuli (Morrone, Denti, and Spinelli 2002, 2004), a combination of contrast and response gain changes was observed with intermediate stimulus sizes (Huang and Dobkins 2005) and contrast gain changes were reported with smaller stimuli (Ling and Carrasco 2006a).

Attention , contrast, and appearance

From psychophysical and neurophysiological evidence indicating that covert attention increases contrast sensitivity, one might infer that attention changes contrast appear-ance. But does attention alter appearance? Whether attention can actually aff ect the per-ceived intensity of a stimulus has been a matter of debate dating back to the founding fathers of experimental psychology and psychophysics—Helmholtz (1866/1911), James, and Fechner. Surprisingly, until the last decade, very little direct empirical evidence has addressed the issue (Prinzmetal, Amiri, Allen, and Edwards 1998; Prinzmetal et al. 1997; Tsal, Shalev, Zakay, and Lubow 1994) and some methodological issues questioned the results of these studies (Carrasco, Ling, and Read 2004; Treue 2004). To investigate this issue further, Carrasco, Ling, and Read (2004) implemented a novel paradigm that assesses the eff ects of spatial cueing on appearance and tests subjective contrast, while circumventing methodological limitations of previous studies.

To test subjective perception, a paradigm has been developed in which observers perform a task contingent upon a comparative judgement between two stimuli, while an uninformative cue directs spatial attention to one of the stimuli (Carrasco, Ling, and Read 2004). For example, observers are asked to report the orientation of the higher-contrast stimulus. With one response, observers convey information regarding both properties, and implicitly report their subjective experience of contrast. Changes in apparent contrast are measured in terms of shift s of the point of subjective equal-ity (PSE), at which the two stimuli appear equal. An additional advantage of this 2X2 alternative forced choice (AFC) paradigm is that it provides concurrent assessment of appearance and performance: in addition to altering appearance, attention improves performance at the cued location, indicating that attention has been successfully manipulated. Th is AFC paradigm enables an objective and rigorous study of attention and subjective experience (Treue 2004; Luck 2004).

Exogenous attention signifi cantly increased perceived contrast. When a Gabor stimulus was peripherally cued, the point of subjective equality (PSE) was shift ed—the apparent contrast of the attended stimulus was higher than for the other Gabor. Th us, when observers attend to a stimulus, they perceive it to be of signifi cantly higher con-trast than when they perceive it without attention (Carrasco, Ling, and Read 2004). An attentional eff ect consistent with these fi ndings has been obtained using low-contrast sine-wave gratings (Hsieh, Caplovitz, and Tse 2005).

206 Marisa Carrasco

Multiple control experiments have ruled out alternative accounts of these fi ndings based on cue bias or response bias with regard to perceived contrast (Carrasco, Ling, and Read 2004; Carrasco, Fuller, and Ling 2008; Fuller, Rodriguez, and Carrasco 2008; Ling and Carrasco 2007), as well as with regard to the eff ects of exogenous attention on the subjective perception of other dimensions, such as spatial frequency (Gobell and Carrasco 2005), saturation (Fuller and Carrasco 2006), motion coherence (Liu, Fuller, and Carrasco 2006), stimulus size of a moving object (Anton-Erxleben, Henrich, and Treue 2007), fl icker rate (Montagna and Carrasco 2006), and speed (Fuller, Park, and Carrasco 2009; Turatto, Vescovi, and Valsecchi 2007). A bias should be of similar mag-nitude independent of:

(1) The temporal order of cue and stimulus; however, the effect disappears when the order of cue and stimulus is reversed (postcue) (Carrasco, Fuller, and Ling 2008; Gobell and Carrasco 2005; Anton-Erxleben, Henrich, and Treue 2007; Fuller, Park, and Carrasco 2009; Liu, Pestilli, and Carrasco 2005).

(2) The interval between cue and stimulus; however, when it is lengthened to 500 ms, the cue effect disappears, consistent with the transient timing of exogenous attention (Carrasco, Ling, and Read 2004; Ling and Carrasco 2007; Liu, Fuller, and Carrasco 2006; Turatto, Vescovi, and Valsecchi 2007).

(3) Stimulus properties; however, attention increases perceived saturation but not apparent hue, even though it improves performance in both cases (Fuller and Carrasco 2006).

(4) Visual field location; however, the attention effect on apparent contrast is greater at the lower than at the upper vertical meridian (Fuller, Rodriguez, and Carrasco 2008).

(5) The direction of the question; however, when observers are asked to report the orientation of the stimulus of lower, rather than higher, contrast, they choose the cued test stimulus less frequently (Carrasco, Ling, and Read 2004; Ling and Carrasco 2007; Gobell and Carrasco 2005; Fuller and Carrasco 2006; Montagna and Carrasco 2006; Fuller, Park, and Carrasco 2009; Turatto, Vescovi, and Valsecchi 2007; Liu, Abrams, and Carrasco 2009).

Notwithstanding all these controls, alternative explanations have been proposed for the eff ects of exogenous attention on perceived contrast (Schneider 2006) and reply by Ling and Carrasco (2007), Prinzmetal, Long, and Leonhardt (2008) and reply by Carrasco, Fuller, and Ling (2008), Schneider and Komlos (2008) and reply by Anton-Erxleben, Abrams, and Carrasco (2010).

It has been proposed that the observed increase in apparent contrast is largely due to sensory interactions occurring between the precue and stimulus, rather than to attention (Schneider 2006). Th is study reports that cueing eff ects only occur at contrasts near detec-tion threshold, and that there are confounding sensory interactions between the cue and stimulus at suprathreshold detection contrasts. In a reply to this study, Ling and Carrasco


fi rst outlined the key methodological diff erences between Carrasco et al.’s (2004) original study and Schneider’s study that are likely to account for the diff erent results, and explained how the sensory interaction explanation of the cue had been ruled out. Furthermore, they directly tested the prediction put forth by Schneider: if the eff ects were due to sen-sory interactions, reversing the luminance polarity of the precue should lead to diff eren-tial cueing eff ects. Ling and Carrasco’s results (Carrasco, Ling, and Read 2004) replicated their original fi ndings—an increase in apparent contrast of a high-contrast stimulus when it was precued. Moreover, they found that the black cue and the white cue had the same eff ect, thus ruling out the alternative explanation proposed by Schneider.

Prinzmetal et al. (2008) suggested that for stimuli of low visibility, observers may bias their response toward the cued location, and proposed a cue-bias explanation for previ-ous studies. In a reply to that study, Carrasco, Fuller, and Ling (2008) concluded that a cue-bias hypothesis is a plausible explanation for Prinzmetal et al.’s results given the characteristics of their stimuli, but that such an explanation had no bearing on the stud-ies by Carrasco and colleagues in which the stimuli were suprathreshold. Furthermore, an increase in apparent contrast for high-contrast stimuli when a stimulus was precued, but not when it was postcued (Carrasco, Fuller, and Ling 2008), further supported the claim that attention alters the contrast appearance of suprathreshold stimuli.

Lastly, Schneider and Komlos (2008) have argued that in a comparative paradigm, PSE and criterion are confounded, whereas in an equality judgement the PSE is inde-pendent of criterion shift s. Using the equality task, in which observers report whether two stimuli are the same or diff erent in contrast, they did not fi nd a signifi cant eff ect of attention on contrast appearance and thus concluded that the eff ects of attention in pre-vious studies were due to a bias. Th eir null results with the equality paradigm and their positive result with the comparative paradigm led them to argue that attention increases stimulus saliency, biasing decisions rather than altering perception. Th ese conclusions are unwarranted for three reasons: (1) the inconsistent theoretical relation among sali-ence, perception, and attention; (2) issues with criteria in the equality paradigm; and (3) the diffi culty in interpreting their null results (Abrams, Barbot, and Carrasco 2010).

In an experimental reply to Schneider and Komlos (2008), the sensitivity of equal-ity and comparative judgements of perceived contrast were compared with regard to physical contrast diff erences and attentional modulation (Anton-Erxleben, Abrams, and Carrasco 2010). Questioning the assumption of equal sensitivity of both types of judgements and an absence of bias from the equality judgement (Schneider and Komlos 2008), the reply study demonstrates several methodological limitations of the equality paradigm, which may contribute to decrease the reliability of PSE estimation and render the equality judgement less sensitive to shift s in perceived contrast. Notwithstanding these methodological limitations, in this study both paradigms revealed that attention enhances apparent contrast (Anton-Erxleben, Abrams, and Carrasco 2010).

A study investigating the eff ect of exogenous attention on contrast appearance with concurrent electrophysiological and behavioural measures (Stormer, McDonald, and Hillyard 2009) has lent further support to the view that attention enhances apparent contrast. Cross-modal spatial cueing of attention increased perceived contrast of the

208 Marisa Carrasco

stimulus at the attended location concurrent with an amplifi ed neural response in the contralateral visual cortex. Specifi cally, cueing attention to one of two identical stimuli boosted early processing (100–140 ms) of the attended stimulus in the ventral occipi-totemporal visual cortex. Moreover, the amplitude of the enhanced neural response correlated positively with the perceived contrast of the cued stimulus, which provides converging evidence that contrast appearance arises from early cortical processing of visual stimuli. Crucially, the cueing of attention enhanced neural processing in the same ventral regions of the visual cortex that are responsive to physical diff erences in contrast. Th ese results contradict the hypothesis that the eff ect of attention is due to a decisional bias (Schneider and Komlos 2008). By providing converging evidence from human electrophysiology and behaviour, this study demonstrates that the enhanced perceived contrast at the cued location is attributable to an eff ect of attention on early visual pro-cessing (Stormer, McDonald, and Hillyard 2009).

Th e appearance paradigm has been adapted to investigate the eff ect of endogenous attention, which also increases perceived contrast (Liu, Abrams, and Carrasco 2009). Th us, many studies provide evidence consistent with the ‘linking hypothesis’, which states that the attentional enhancement of neural fi ring is interpreted as if the stimulus had a higher contrast (Reynolds and Chelazzi 2004; Treue 2004). Th is proposal is sup-ported by converging evidence from neurophysiological, psychophysical, and neuroim-aging studies (for a review, see Carrasco 2011).

Another study has shown that endogenous attention can also modulate perceived brightness (Tse 2005). Th e author proposes that understanding this phenomenon may require new models that include higher-level mechanisms than gain control, such as surface, boundary formation, and inhibition among higher-level surfaces or objects.

Th e studies showing that attention alters appearance exemplify how the visual system operates on the retinal image to maximize its usefulness to the perceiver, oft en produc-ing non-veridical percepts. Th e visual system does not provide an internal one-to-one copy of the external visual world; rather, it optimizes processing resources. Attention is a pervasive example of this perceptual optimization: attention augments perception by optimizing our representation of sensory input and by emphasizing relevant details at the expense of a faithful representation (Treue 2004; Carrasco, Fuller, and Ling 2008; Fuller, Rodriguez, and Carrasco 2008; Carrasco 2009). Th e biophysical machinery of the brain engenders our phenomenological experience of the world: attention, both intra-modal and cross-modal, aff ects not only how we perform in a visual task but also what we see and experience (Carrasco 2011, 2009).

Covert Attention Increases Spatial Resolution

Spatial resolution, our ability to discriminate fi ne patterns, decreases with eccentric-ity. Signals from the central parts of the visual fi eld are processed with greater accuracy


and faster reaction times (Cannon 1985; Carrasco, Evert, Chang, and Katz 1995; Rijsdijk, Kroon, and van der Wildt 1980). In many tasks, these performance diff erences are eliminated when stimulus size is enlarged according to the cortical magnifi cation fac-tor which equates the size of the cortical representation for stimuli presented at diff er-ent eccentricities (Rovamo and Virsu 1979; Virsu and Rovamo 1979). However, there are also qualitative diff erences in neural processing between central and peripheral vision. Th us, compensating for cortical magnifi cation does not eliminate all diff erences (for a review see Kitterle 1986). Moreover, spatial resolution is not uniform across iso-eccentric locations. It is better along the horizontal meridian than the vertical one and better in the lower region than in the upper region of the vertical meridian (Talgar and Carrasco 2002; Fuller, Rodriguez, and Carrasco 2008; Abrams, Nizam, and Carrasco 2011; Montaser-Kouhsari and Carrasco 2009).

Several factors contribute to diff erences in spatial resolution across eccentricities. A greater proportion of cortex is devoted to processing input from the centre of the vis-ual fi eld than the periphery (cortical magnifi cation). In area V1, approximately 25% of cortex is devoted to processing the central 2.5 deg. of visual angle (De Valois and De Valois 1988). Neuronal RF sizes increase with eccentricity, as the RF density decreases. Th us, as eccentricity increases, information is pooled over a larger area, diminishing sensitivity to fi ne patterns. Moreover, the visual system’s peak sensitivity to spatial fre-quencies decreases with eccentricity (De Valois and De Valois 1988; Kitterle 1986).

A series of psychophysical studies provides evidence for the ‘resolution hypothesis’, which states that attention can enhance spatial resolution, and that the magnitude of such an eff ect increases with eccentricity (Carrasco, Williams, and Yeshurun 2002; Carrasco and Yeshurun 1998). When spatial attention is directed to a given location, performance improves in visual search, acuity, texture segmentation (unless resolution is already too high for the task at hand; see below), and crowding tasks, which are medi-ated by spatial resolution.

Attention and visual search

In a visual search task, observers are typically required to detect the presence of a pre-defi ned target appearing among other irrelevant items; for instance, a red vertical line appearing among red tilted lines in a feature search, or a red vertical line appearing among red tilted and blue vertical lines in a conjunction search (e.g. Treisman 1985). Performance typically decreases with the number of distractors (the set size eff ect) for conjunction search, whereas for feature search it does not. However, there are excep-tions to both: there are feature searches yielding a set size eff ect (Carrasco and Frieder 1997; Carrasco, McLean, Katz, and Frieder 1998; Dosher, Han, and Lu 2004) and con-junction searches that do not (Duncan and Humphreys 1989; Carrasco, Ponte, Rechea, and Sampedro 1998; McLeod, Driver, and Crisp 1988; Nakayama and Silverman 1986).

Performance in visual search tasks deteriorates as the target is presented at farther peripheral locations (Carrasco, Evert, Chang, and Katz 1995; Carrasco and Frieder 1997; Carrasco, McLean, Katz, and Frieder 1998; Carrasco and Chang 1995). Th is

210 Marisa Carrasco

reduction in performance has been attributed to the poorer spatial resolution at the periphery because performance is constant across eccentricity when stimulus size is enlarged according to the cortical magnifi cation factor (Carrasco and Frieder 1997; Carrasco, McLean, Katz, and Frieder 1998), but see Wolfe, O’Neill, and Bennett (1998). Similarly, a study in which attention was manipulated via a peripheral cue showed that attention eliminates the eccentricity eff ect for both features and conjunctions (Carrasco and Yeshurun 1998). Th ese results support the resolution hypothesis; by improving per-formance more at peripheral than at central locations, attention can minimize resolu-tion diff erences between the fovea and the periphery.

Th e idea that attention enhances spatial resolution was also supported by a study in which orientation thresholds were assessed in a visual search task (Morgan, Ward, and Castet 1998). Th e authors reported that when distractors were present, indicating the target location with a peripheral cue reduces orientation thresholds to the level found when the target was presented alone. Th e authors interpreted these eff ects to suggest that attention reduces the scale over which an image is analysed. Similarly, orientation discrimination thresholds increased with set size but cueing the target location elimi-nated this eff ect (Baldassi and Burr 2000). Th ese results are also consistent with a reduc-tion of spatial scale of processing and distractor exclusion.

Attention and acuity tasks

Acuity tasks are designed to measure the observer’s ability to resolve fi ne details. Performance in some of these tasks, e.g. detection of a small gap in a Landolt square, is limited by the retinal mosaic, whereas in other tasks, e.g. identifi cation of off set direc-tion with Vernier targets, performance is hyperacute and limited by cortical processes (Levi, Klein, and Aitsebaomo 1985; Th omas and Olzak 1986). Directing transient atten-tion to the target location improves performance in both acuity and hyperacuity tasks. A peripheral cue improved observers’ performance and the magnitude of this improve-ment increased with eccentricity. Similarly, directing attention to the location of a Vernier target allowed observers to identify smaller horizontal off sets (Yeshurun and Carrasco 1999). Th e same pattern of results emerged when all sources of added external noise were eliminated from the display; i.e. local masks, global masks, and distractors (Carrasco, Williams, and Yeshurun 2002). Consistent fi ndings emerged from a com-parative study that evaluated the eff ects of covert attention on Landolt acuity in humans and non-human primates (Golla, Ignashchenkova, Haarmeier, and Th ier 2004). Acuity was enhanced when the target location was precued and the attentional eff ect increased with eccentricity in both human and non-human primates ( Fig. 7.10 ). Moreover, cueing the location of a line enabled observers to better detect when the gap was present and to localize its location (Shalev and Tsal 2002). All these fi ndings further support the idea that attention enhances spatial resolution.

Increased spatial acuity at the attended location is coupled with decreased acuity at unattended locations (Montagna, Pestilli, and Carrasco 2009). For exogenous attention,


observers were explicitly told that the peripheral cue was uninformative, i.e. not predic-tive of target location or gap side. For endogenous attention, observers were informed that the cue would indicate the target location, but not the gap side, on 70% of the central-cue trials. For both exogenous and endogenous attention, gap-size thresholds were lower in the cued and higher in the uncued condition compared to the neutral baseline condition ( Fig. 7.11 ). Th e fact that acuity trade-off s emerge for very simple, non-cluttered displays, supports the idea that covert spatial attention helps regulate the expenditure of cortical computation.

Th e improved performance in acuity tasks could only be accounted for by the resolu-tion hypothesis—attention enhances spatial resolution at attended locations. Note that these attention eff ects could not be accounted for by any of the alternative hypotheses regarding attentional mechanisms—shift s in the decisional criterion, location uncer-tainty reduction, or reduction of external noise (see section ‘Mechanisms of covert attention’)—for the following reasons: (a) the peripheral cue only conveyed information regarding the target location, or conveyed no information regarding either the correct response or the target location; (b) the peripheral cue did not associate a higher prob-ability with one of the responses and observers could not rely on its presence to reach a discrimination decision; (c) a suprathreshold target could not be confused with the blank at the other locations and was presented alone, without other items to introduce external noise; (d) similar results were obtained when two suprathreshold targets were presented at fi xed locations, which could not be confused with the blank at other loca-tions; and (e) similar results were found with and without a local post-mask.

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Figure 7.10 Means and standard errors of acuity thresholds at 9° horizontal retinal eccentri-city for monkeys and humans under the valid (75% of trials) and invalid (25%) cueing condi-tions. Reprinted from Vision Research , 44 (13), Heidrun Golla, Alla Ignashchenkova, Th omas Haarmeier, and Peter Th ier, Improvement of visual acuity by spatial cueing: a comparative study in human and non-human primates, pp. 1589–1600, Copyright (2004), with permission from Elsevier.

212 Marisa Carrasco

Attention and temporal resolution Transient attention can alter temporal processing as it alters spatial processing. Given the trade-off between spatial and temporal resolution, enhancing the former necessarily results in degrading the latter (Yeshurun 2004; Yeshurun and Levy 2003; Yeshurun and Hein 2011; Yeshurun and Marom 2008). Several studies have found an impaired tempo-ral resolution at the attended location suggesting that transient attention degrades tem-poral segregation. For instance, a peripheral cue diminishes observers’ ability to indicate whether a target is continuous or fl ickering at the attended location (Yeshurun 2004; Yeshurun and Levy 2003; Rolke, Dinkelbach, Hein, and Ulrich 2008), but see Chica and Christie (2009). When observers are cued to a specifi c location, they improve at spatial-gap detection, but worsen at temporal-gap detection ( Fig. 7.12 ). Similarly, auto-matic orienting of attention impairs discrimination of the temporal order of two dots (Hein, Rolke, and Ulrich 2006). Furthermore, in apparent motion tasks, an attentional cue decreases observers’ accuracy, when indicating the motion direction of an appar-ently moving rectangle (Yeshurun and Hein 2011). Finally, transient attention leads to longer temporal integration (Megna, Rocchi, and Baldassi 2012) and prolongs perceived duration (Yeshurun and Marom 2008).

Yeshurun and colleagues have proposed that these results can be explained by an attentional mechanism that facilitates spatial segregation and temporal integration but impairs their counterparts—spatial integration and temporal segregation—due to per-ceptual trade-off s. Th ey further suggested that a possible physiological implementation of such mechanism is attentional favouring of parvocellular over magnocellular activity.

Exogenous attention Endogenous attention

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Figure 7.11 Average gap-size thresholds (75% localization accuracy) for both exogenous (top-left panel) and endogenous (top-right panel) attention for the cued, neutral, and uncued conditions. Error bars show +/− 1 SE. Reprinted from Vision Research , 49 (7), Barbara Montagna, Franco Pestilli, and Marisa Carrasco, Attention trades off spatial acuity, pp. 735–45, Copyright (2009), with permission from Elsevier.


Parvocellular neurons have higher spatial resolution and longer temporal integration than magnocellular neurons, but their temporal resolution is low. An attentional facili-tation of parvocellular activity on the expense of magnocellular activity could, there-fore, account for these various eff ects of attention (Yeshurun 2004; Yeshurun and Levy 2003; Yeshurun and Hein 2011; Yeshurun and Marom 2008; Yeshurun and Sabo 2012). Also consistent with this hypothesis is the fi nding that attention benefi ts discrimination of high- but not of low-spatial frequencies (Srinivasan and Brown 2006).

Attention and texture segmentation

Heightened resolution benefi ts many everyday tasks, such as reading or discriminat-ing objects. However, in certain situations resolution enhancement is not benefi cial; for example, when a global assessment of a scene is required (e.g. when seeing a whole tree rather than its individual leaves) or when navigating under poor atmospheric conditions (e.g. fog or haze). Th e fact that increasing spatial resolution can be detrimental in tasks in which performance is diminished by heightened resolution enabled a crucial test of the resolution hypothesis. If attention enhanced resolution, performance at the attended location should be impaired rather than improved in such a task (Yeshurun and Carrasco 1998). A basic texture segmentation task was used in which a to-be-detected texture tar-get was embedded in a background of an orthogonal orientation. Observers’ performance

1.00Temporal gap

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Figure 7.12 Performance (data averaged across observers) for detection tasks of a temporal gap (left panel) and a spatial gap (right panel). A peripheral cue improves performance in a spatial-gap task but impairs it in a temporal-gap task. Adapted from Yaff a Yeshurun and Liat Levy, Transient Spatial Attention Degrades Temporal Resolution, Psychological Science , 14 (3) pp. 225–31, copy-right © 2003 by SAGE Publications. Reprinted by Permission of SAGE Publications.

214 Marisa Carrasco

is highest at mid-peripheral locations, and drops as the target appears at more central or farther peripheral locations. Th is ‘central performance drop’ (CPD) is attributed to a mis-match between the average size of spatial fi lters at the fovea and the scale of the texture (Gurnsey, Pearson, and Day 1996; Kehrer 1997). Th e size of these fi lters at the fovea is too small for the scale of the texture and spatial resolution would be too high for the task. Th e fi lters’ average size increases gradually with eccentricity and is optimal around the perfor-mance peak. At farther locations, performance decreases because the fi lters are too big and the resolution is too low for the task. Th us, enlarging the scale of the texture shift s the per-formance peak to farther locations, whereas decreasing this scale shift s the peak towards the centre (Gurnsey, Pearson, and Day 1996; Kehrer 1997; Joff e and Scialfa 1995).

If attention enhances spatial resolution, attending to the target location should improve performance at the periphery where the resolution is too low, but should impair performance at the fovea where the resolution is already too high for the task (Yeshurun and Carrasco 1998). Accuracy was higher for the peripherally cued than the neutral tri-als at peripheral locations, but was lower at central locations ( Fig. 7.13a ). Th e size of the scale of the texture was manipulated by viewing distance. Attention impaired perfor-mance in a larger range of eccentricities with a larger scale of the texture. Conversely, with a smaller texture scale, performance was impaired at a smaller range of eccentrici-ties. Th us, transient attention improves performance when it is limited by resolution that is too low, but hinders performance when it is limited by resolution that is too high for the task. Th e scale of the texture and the average size of the fi lters at a given eccentric-ity determine whether attention helps or hinders performance. Th e fi nding that atten-tion aff ects performance even at the fovea is in accord with a study that demonstrated that targets that occur unexpectedly at fi xation capture attention in a stimulus-driven manner similar to attentional capture in the periphery (Coull, Frith, Buchel, and Nobre 2000). Th e impairment at central locations is predicted by the resolution hypothesis; no other model can predict these impairments (Yeshurun and Carrasco 1998). Shift s in the decisional criterion, location uncertainty reduction, or reduction of external noise would predict a benefi t on performance throughout all eccentricities.

To test the hypothesis that covert attention enhances spatial resolution by increas-ing sensitivity to high spatial frequencies, a peripheral cueing procedure was employed in conjunction with selective adaptation to spatial frequency (Carrasco, Loula, and Ho 2006). Th e selective adaptation procedure is used to assess the spatiotemporal proper-ties of the visual system (Graham 1989; Blakemore and Campbell 1969; Movshon and Lennie 1979). While keeping the stimulus content identical, the availability of spatial frequency information was manipulated by reducing observers’ sensitivity to a range of frequencies. Hence, by adapting to high spatial frequencies, the non-optimal fi lters would be removed from the normalization process and the magnitude of the CPD would be diminished. Furthermore, were the central attentional impairment due to an increased sensitivity to high frequencies and a reduced sensitivity to lower frequencies, adapting to high spatial frequencies should eliminate the attentional impairment at cen-tral locations and diminish the benefi t in the peripheral locations. Th e results confi rmed these predictions, indicating that the CPD is primarily due to the predominance of high


spatial frequency responses, and that covert attention enhances spatial resolution by increasing sensitivity to higher spatial frequencies (Carrasco, Loula, and Ho 2006).

When this texture is presented along the vertical meridian, performance peaks at far-ther eccentricities in the lower than in the upper vertical meridian, consistent with the resolution being higher in the lower meridian (Montaser-Kouhsari and Carrasco 2009),

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Figure 7.13 Observers’ performance as a function of target eccentricity and cueing condi-tion for (a) transient attention, the two viewing distances (indicated on each panel). Because viewing distance varied, the eccentricity values (abscissa) diff er in the two top panels. Adapted with permission from Macmillan Publishers Ltd: Nature . 396 (6706), Yaff a Yeshurun and Marisa Carrasco, Attention improves or impairs visual performance by enhancing spatial resolution, pp. 72–5 © 1998, Nature Publishing Group. (b) transient (left panel) and sus-tained (right panel) attention. Adapted from Vision Research , 48 (1), Yaff a Yeshurun, Barbara Montagna, and Marisa Carrasco, On the fl exibility of sustained attention and its eff ects on a texture segmentation task, pp. 80–95, Copyright (2008), with permission from Elsevier.

216 Marisa Carrasco

but the attention benefi t and cost are the same in both regions of the vertical meridian (Talgar and Carrasco 2002). Th ese fi ndings indicate that the vertical meridian asymme-try is limited by visual rather than attentional factors, and support the hypothesis that attention enhances spatial resolution at the attended location.

At the level of the visual cortex, texture segmentation theoretically involves pas-sage of visual input through two layers of spatial linear fi lters, separated by a pointwise non-linearity. Th e fi rst-order linear fi lters perform a more local analysis of spatial fre-quency and orientation, and are thought to refl ect the activity of simple cortical cells in area V1. Th e second-order linear fi lters which are of a larger scale perform a more global analysis on the output of the fi rst-order fi lters plus the intermediate non-linearity (Bergen and Landy 1991; Graham, Beck, and Sutter 1992; Sutter, Beck, and Graham 1989). To assess the level of processing at which transient attention aff ects spatial resolution, tex-tures composed of narrow-band stimuli were used to ensure that fi rst- or second-order fi lters of various specifi c scales would be diff erentially stimulated. For both fi rst-order low- and high-frequency conditions, accuracy was higher for cued than neutral trials at more peripheral eccentricities, but it was lower at central locations. In contrast, the atten-tional eff ect diff ered as a function of the second-order spatial frequency content: atten-tion impaired performance in a greater range of eccentricities for the low- than the high-frequency condition, and an attentional benefi t emerged only for the high-frequency condition. Th ese fi ndings suggest that attention operates at the second stage of fi ltering, possibly by reducing the size of the second-order fi lters (Yeshurun and Carrasco 2000).

Th e texture segmentation studies described thus far show that transient attention increases spatial resolution even when this is detrimental. Would the pattern of results be the same with sustained attention, which is considered to be more fl exible and capa-ble of adapting to task demands? A central cue was used to test whether the attentional eff ect would be similar to that found with peripheral cues (Yeshurun, Montagna, and Carrasco 2008). Unlike transient attention, sustained attention improves performance in texture segmentation tasks at all eccentricities ( Fig. 7.13b ). Th e fi nding that per-formance improves at fovea is in agreement with behavioural and neurophysiological enhancements at the fovea for focused versus distributed attention (Miniussi, Rao, and Nobre 2002). By comparing the eff ect of precues and postcues, the authors showed that the benefi t of the central precue went well beyond the mere eff ect of location uncertainty at the decisional stage (Yeshurun, Montagna, and Carrasco 2008).

Th e improvement at all eccentricities is consistent with the idea that sustained atten-tion is more fl exible than transient attention, and suggests that sustained attention may increase the resolution at peripheral locations but decrease the resolution at cen-tral locations where an increase would be detrimental. Alternatively, the fi ndings may be explained by an attentional mechanism that improves the signal to noise ratio at all eccentricities through reduction of external noise at early levels of processing (Dosher and Lu 2000a; Lu and Dosher 2004), possibly via distractor suppression (Shiu and Pashler 1994). A recent study has shown that sustained attention improves performance at central locations by decreasing the sensitivity of high-spatial frequency fi lters, thus decreasing resolution (Barbot, Montagna, and Carrasco 2012).


Attention, spatial resolution, and appearance

Th e studies discussed above show that spatial resolution is enhanced by exogenous/transient attention, regardless of whether this helps or hinders performance, and by endogenous/sustained attention when such an enhancement improves performance, but not when it would be detrimental. Does attention also aff ect appearance of the types of stimuli used to assess spatial resolution?

Using the paradigm developed to assess the eff ects of attention on contrast appear-ance (Carrasco, Ling, and Read 2004), it has been shown that exogenous attention increases both perceived spatial frequency and Landolt-square gap size (Gobell and Carrasco 2005). In line with this study, it has been found that exogenous attention also increases the perceived size of moving visual patterns ( Fig. 7.14a ) (Anton-Erxleben, Henrich, and Treue 2007). Th ese authors attribute their fi ndings to the fact that spatial attention shift s receptive fi elds in monkey extrastriate visual cortex toward the focus of attention (Womelsdorf, Anton-Erxleben, Pieper, and Treue 2006), so that they respond to a new location closer to the attentional focus, but without updating the position label

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Figure 7.14 (a) Average data across observers for test-cued, neutral, and standard-cued tri-als. Percentage of trials in which observers responded that the test stimulus was bigger than the standard stimulus, as a function of test size. Data are averaged across observers and then fi tted with a psychometric function using iterative likelihood maximization. Reprinted from Journal of Vision , 7 (11), Anton-Erxleben, K., C. Henrich, and S. Treue, Attention changes perceived size of moving visual patterns, p. 1–9, fi gure 2a © 2007, with permission from the Association for Research in Vision and Opthalmology. (b) Psychometric functions show-ing the proportion of trials on which the observers chose the test stimulus to be of higher spatial frequency than the standard stimulus, as a function of the physical spatial frequency of the test stimulus. Reprinted from Proceedings of the National Academy of Sciences of the United States of America , 106(52), Stormer, V. S., J. J. McDonald, and S. A. Hillyard, Cross-modal cueing of attention alters appearance and early cortical processing of visual stimuli, pp. 22456–61, Copyright (2009), with permission from the National Academy of Sciences.

218 Marisa Carrasco

(according to a labelled-line code). Th us, this distortion in the retinotopic distribution of receptive fi elds may increase the perceived size of attended stimuli.

Similarly, a rapid serial visual presentation (RSVP) paradigm developed to assess en dogenous attention and perceived contrast (Liu, Abrams, and Carrasco 2009) was adapted to investigate the eff ects of endogenous attention on spatial resolution, particu-larly on perceived spatial frequency. A demanding rapid serial visual presentation task was used to direct voluntary attention to a given location and perceived spatial frequency was measured at the attended and unattended locations (Abrams, Barbot, and Carrasco 2010). Attention increased the perceived spatial frequency of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task ( Fig. 7.14b ). Th ese studies included a number of control experiments that rule out possible alternative interpretations of the fi ndings of increased perceived spatial resolution, such as cue bias or response bias (see section above, ‘Attention, contrast, and appearance’).

A previous study had reported that sustained attention did not shift the mean appar-ent spatial frequency, but merely reduced the variance of the estimates (Prinzmetal, Amiri, Allen, and Edwards 1998). Th e discrepancy between this study and the studies reporting that attention increases perceived spatial frequency may result from meth-odological diff erences. In the study reporting no shift in appearance (Prinzmetal, Amiri, Allen, and Edwards 1998), the location of spatial attention was not manipulated; instead, a dual-task procedure was used, and the diffi culty of the primary letter identi-fi cation task (simultaneous vs sequential presentation) was varied to manipulate atten-tional deployment in the secondary appearance task. Furthermore, the results of this study are inconclusive because there was no independent measurement ensuring that attention had been deployed to the correct location.

Another line of studies supporting the view that attention aff ects appearance has shown that cueing the target location with a peripheral cue reduces perceived line length (Tsal and Shalev 1996). Th ese authors proposed that the visual fi eld consists of a grid of attentional receptive fi elds (ARFs), a hypothetical construct that operates as a functional receptive fi eld, whose operation follows an all-or-none principle. Th us, when a stimulus appears within its boundaries, this unit signals its entire length to the central processor (Tsal, Meiran, and Lamy 1995). Moreover, because the ARFs are smaller at the attended than the unattended fi eld, the attended line is systematically perceived as shorter than the unattended one (Tsal and Shalev 1996). Th e authors have proposed that smaller receptive fi elds increase spatial resolution, and have ruled out that spatial inter-actions between the cue and the target could aff ect line-length judgements (Tsal, Shalev, and Zakay 2005). Diff erences in the manipulation of attention and cueing parameters may explain the discrepancy with the results reported by others (Anton-Erxleben, Henrich, and Treue 2007).

Both endogenous attention and exogenous attention also aff ect perceived position by repelling briefl y presented Vernier stimuli away from its focus. Th is repulsion eff ect illus-trates that attention can distort the encoding of nearby positions and suggests an over-representation of space around the attended area (Suzuki and Cavanagh 1997); see also Wardak, Deneve, and Ben Hamed (2010). Spatial attention is also critical for observers’


ability to accurately report the relative position of two stimuli. When attention is pre-vented from shift ing to a target location by a concurrent task, performance on report-ing relative position of two features falls to chance (Lee, Itti, Koch, and Braun 1999; Pastukhov, Fischer, and Braun 2009; Li, VanRullen, Koch, and Perona 2002). Th e perfor-mance drops are large and fi t well with the hypothesis that attention enhances spatial res-olution (for a review, see Carrasco and Yeshurun 2009), as well as the enhanced resolution models (Lee, Itti, Koch, and Braun 1999; Deco and Heinke 2007; Deco and Schurmann 2000; Salinas and Abbott 1997; Womelsdorf, Anton-Erxleben, and Treue 2008) and single-unit studies on attention and resolution (Womelsdorf, Anton-Erxleben, Pieper, and Treue 2006; Anton-Erxleben, Stephan, and Treue 2009; Moran and Desimone 1985).

Adapting the attention and appearance paradigm developed by Carrasco and col-leagues (Carrasco, Ling, and Read 2004), a study has shown that attention also dis-torts perceived shape. Depending on cue placement inside or outside the contour of an oval, the aligned dimension (height or width) was perceived longer or shorter, respectively. Visual cues alter perceived shape so that the oval contours were repelled (Fortenbaugh, Prinzmetal, and Robertson 2011). Th ese results are consistent with those of Anton-Erxleben et al. (2007) and with the explanation based on RF shift s they pro-posed to account for eff ects of attention on the size of an object.

Conclusion

Th is review includes psychophysical studies of visual attention that allow us to probe the human visual system. I have described the eff ects of spatial attention on perceptual eff ects mediated by early vision—contrast sensitivity and spatial resolution—for which the best mechanistic understanding has been achieved by the confl uence of psycho-physical, electrophysiological, neuroimaging, and computational studies. I have shown that trade-off s in processing result in increased performance at the attended location and decreased performance at unattended locations, consistent with a selective repre-sentation of the world. Th ere is a fi xed amount of overall energy available to the brain, and the cost of cortical computation is high. Attention is crucial in optimizing the sys-tem’s limited resources. As a selective process, attention provides an organism with an optimized representation of the sensory input that emphasizes relevant details, at times even at the expense of a faithful representation of the sensory input.

Note

1. Th e terms endogenous attention and sustained attention have been used interchangeably or as synonyms. Th e term exogenous should be used only when the cue is uninformative regarding the target location, whereas the term transient does not necessarily imply that the cue is uninformative. Here I use the terms endogenous and exogenous, except in cases where the authors used a transient cue instead of an exogenous cue.

220 Marisa Carrasco

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