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Neuropsychologia 47 (2009) 1721–1727

Contents lists available at ScienceDirect

Neuropsychologia

journa l homepage: www.e lsev ier .com/ locate /neuropsychologia

vidence for long-range feedback in target detection: Detection of semanticargets modulates activity in early visual areas

icholas Hona,∗, Russell Thompsonb, Natasha Sigalab,c, John Duncanb

Department of Psychology, National University of Singapore, 9 Arts Link, Singapore 117570, SingaporeMRC Cognition and Brain Sciences Unit, 15 Chaucer Rd, Cambridge CB2 2EF, UKDepartment of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, UK

r t i c l e i n f o

rticle history:eceived 9 October 2008eceived in revised form 29 January 2009ccepted 6 February 2009vailable online 13 February 2009

eywords:

a b s t r a c t

In a variety of attention and search tasks, single-cell recordings of the primate brain have frequentlyshown an enhancement of responses in early visual areas to selected target stimuli. This enhancement isobserved only at longer latencies, suggesting the possibility that it reflects the action of feedback or returnsignals from upstream processing areas. However, in typical studies, targets are specified on the basis ofelementary visual features; as these are coded at multiple levels of the visual system, it is impossibleto determine where enhanced target processing begins. Using human functional magnetic resonanceimaging (fMRI), we demonstrate enhancement of activity in early visual areas even when low-level visual

arget detectionMRIeedback

information is insufficient for target detection to occur. We found enhanced activity in early visual areasto targets defined purely by semantic category, suggesting that feedback signals returning from at leastas far forward as temporal lobe semantic processing can influence visual responses. These findings alsosuggest feedback signaling as a mechanism by which early and late brain systems coding for differentproperties of a target object can integrate their activity, allowing for the target object to dominate overall

processing.

. Introduction

We are quite capable of seeking out a specific face in a crowdr looking out for a specific car in a crowded parking lot, whichemonstrates our ability to detect stimuli most relevant to our goals“targets”). Target objects can be defined on the basis of space, forxample, when a person or animal is cued to pay attention to apecific location in the visual field. Targets can also be defined onhe basis of non-spatial characteristics such as identity.

The neural mechanisms underlying certain aspects of targetetection are well described. Theoretical models of target detec-ion propose that, for accurate target detection to occur, sensorynput must be processed to the point at which it can be matchedgainst some advance description or template of the target cur-ently sought, a “target template” (Duncan & Humphreys, 1989).he specific nature of the template is determined by task or goalequirements. Findings from physiological studies are remarkably

onsistent with this theoretical proposal. In the brain, an instruc-ion dictating the goal context (e.g., an instruction to detect atimulus that appears at a particular location or of a particularolour) produces large sustained signals in many different regions

∗ Corresponding author.E-mail address: psyhonn@nus.edu.sg (N. Hon).

028-3932/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.neuropsychologia.2009.02.011

© 2009 Elsevier Ltd. All rights reserved.

(Desimone & Duncan, 1995; Kastner & Ungerleider, 2000; Kastner,Pinsk, De Weerd, Desimone, & Ungerleider, 1999), with the exactmodulated regions being determined by the nature of the task athand. For example, in both monkeys and humans, when a targetis defined by visual properties, sustained target-selective activityin various visual areas is observed during the interval between atarget-indicating cue and the presentation of the actual trial dis-play (Chelazzi, Miller, Duncan, & Desimone, 1993; Kastner et al.,1999). This preactivation of target-selective cells by biasing signalsis thought to give a competitive processing advantage to target,relative to non-target, stimuli (Duncan, 2006). Biasing signals andtheir action on target-selective cells can be conceived of as a neuralinstantiation of a target template. Accurate detection occurs whenincoming signals match the coding sensitivity of a preactivatedpopulation of cells (Bichot, Rossi, & Desimone, 2005).

Once a target stimulus is detected, enhanced activity isobserved in sites coding for many different properties of thattarget. Such enhancement is widely distributed across the multi-ple visual areas coding different visual features (Chelazzi, Miller,Duncan, & Desimone, 2001; O’Craven, Downing, & Kanwisher, 1999;

Schoenfeld et al., 2003). Of greatest interest to this study is targetenhancement of early visual areas. Single cell recordings of the pri-mate brain have frequently shown an enhancement of activity inearly visual areas when targets defined by their visual identity aredetected (Chelazzi et al., 2001; Motter, 1994). The modulation of

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ctivity in cells early in the visual processing pathway is particu-arly interesting because it is observed only at longer latencies, afterhe initial on-discharge to the presentation of the visual displayChelazzi, Duncan, Miller, & Desimone, 1998). Though several expla-ations are possible for such a result, one proposal is that targetffects on early visual activity result from feedback or return signalsrom later brain systems (Chelazzi et al., 2001; Roelfsema, Lamme,

Spekreijse, 1998), possibly those coding for target templates.In typical studies demonstrating target-enhancement of activ-

ty in early visual sites, though, targets are defined on the basisf elementary visual features. For example, in single-cell record-ng studies of the primate brain, detection of targets defined onhe basis of colour, luminance and orientation have been foundo enhance early visual activity at delayed latencies (Haenny,

aunsell, & Schiller, 1988; Motter, 1994). Similarly, in humans,nhanced activity in early visual areas is most evident whenbservers detect targets that differ from non-targets on the basis oflementary visual features (Bledowski et al., 2004; Clark, Fannon,ai, Benson, & Bauer, 2000). This raises two concerns for theeedback hypothesis. First, because elementary visual features areepresented at multiple levels of the visual system, it is impossibleo determine which region first distinguishes targets from non-argets. This necessarily implies a difficulty in establishing wherearget enhancement effects begin. A second, related, concern per-ains to the generality of the target effect in early visual areas. Asoted above, when enhancement is observed, targets are typicallyefined on the basis of low-level visual information. This raises theossibility that target effects on early visual areas might be obtainednly when targets are defined in a low-level way, or, in the lan-uage of detection models, only when a “target template” can benstantiated very early in the visual system. One possibility is that

odulation of activity in early visual areas may result from short-ange interactions between neighbouring early visual areas, whichave been shown to be functionally connected (Lamme, Super, &pekreijse, 1998). This raises the question of whether detection oftarget defined by higher-level information would have the same

ffect on activity in early visual areas.Taken together, these concerns highlight current uncertainties

ver the involvement of feedback signaling in target detection. Inhis study, to address both the concerns raised above, we examinedhether or not activity in early visual areas would be modulated

ven when low-level visual information is insufficient for targetetection to occur. To do this, we modified a simple detectionask in which observers were required to detect occurrences ofn infrequently occurring target. Tasks utilizing this experimentalaradigm with functional magnetic resonance imaging (fMRI) have

eported early visual activity when visually distinct targets wereontrasted with non-target stimuli (Bledowski, Prvulovic, Goebel,anella, & Linden, 2004; Bledowski et al., 2004; Clark et al., 2000),irroring the findings from single-cell physiology described earlier.

n the critical experiment of this study, we asked whether activity

ig. 1. Examples of stimulus sequences from both experiments. (a) Example of stimuluetected occurrences of the word “CLOCK”. (b) Example of stimulus sequence from Exper

ia 47 (2009) 1721–1727

in early visual areas would be modulated if a target was definedonly by meaning, a property necessarily extracted far forward inthe processing stream. The logic of this manipulation is as follows.When a target is defined by meaning, the “target template” thatmust be matched is likely to be instantiated in brain systems codingfor semantics. In large part, semantic systems are based in the tem-poral lobe (Davis & Johnsrude, 2003; Martin, 2007). Therefore, anytarget enhancement of activity in early visual areas when a seman-tic target is detected must originate from signals returning from asfar forward as temporal lobe semantic systems. We compared thissemantic condition (Experiment 2) with another in which the targetwas defined by exact word identity, potentially allowing detectionon the basis of elementary physical features (Experiment 1).

2. Methods

2.1. Participants

Sixteen healthy right-handed volunteers with normal vision (7 men and 9women, mean age = 24.8 years, S.D. = 5.9 years) gave written consent to participatein this study, which was approved by a local ethics committee.

2.2. Procedure

In Experiment 1, the sixteen participants were scanned while viewing a sequenceof serially presented five-letter words (Fig. 1a). Participants viewed the sequence inorder to detect occurrences of a target word (“CLOCK”). When participants detectedthe target, they made a button press response on a customized response box. Non-target words required no response. Each word was presented for 1000 ms and wasfollowed by a 500 ms blank frame. The words were printed in black 36 point Arialbold uppercase font and presented on a light grey background. The words subtendedapproximately 0.42o of visual angle vertically and 1.79◦ horizontally. Althoughappearing as a continuous sequence to the subjects, for the purposes of design andanalysis, the sequence was divided into 1500 ms long trials (i.e., each trial compriseda word presentation and the following blank frame). A total of 81 words were usedin this experiment. The target word was always the word “CLOCK”, which was pre-sented twenty times throughout the sequence. The remaining 80 words formed thenon-target set. Non-target words were never re-used. The exact sequence of targetand non-target words was randomized for each subject.

In Experiment 2, the same sixteen participants were scanned while viewing adifferent sequence of serially presented words (Fig. 1b). Experiment 2 was alwaysperformed after Experiment 1 in the same scanning session. Participants viewed thesequence in order to detect occurrences of target words. In this experiment, targetswere defined by their membership to a semantic category (“Any word denotingan animal”). Again, participants made a button-press response when they detectedthe target word. Presentation and timing parameters were the same as Experiment1. A total of 100 words were used: 20 target words and 80 non-target words. Thewords used in this experiment varied in length (between 3 and 6 characters long).The words had a vertical size of 0.42◦ of visual angle and a mean horizontal size1.44◦ of visual angle. The target words and non-target words did not differ on wordfrequency, concreteness or word length (all ts < 1). For both targets and non-targets,words were never re-used. The exact sequence of target and non-target words wasrandomized for each subject.

2.3. fMRI data acquisition and analysis

Scanning was performed on a 3T Siemens Trio scanner outfitted with a headcoil. Functional volumes were acquired with an echo-planar imaging sequence(TR = 2000 ms, TE = 30 ms, FOV = 192 mm x 192 mm, flip angle = 78◦). Each volume

s sequence from Experiment 1. Subjects made button-press responses when theyiment 2. Subjects made button-press responses to any word denoting an animal.

hologia 47 (2009) 1721–1727 1723

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Table 1Coordinates from the target–non-target contrasts of both experiments.

Experiment 1 Experiment 2

Side Coords (x, y, z) T value Coords (x, y, z) T value

MotorMotor L −30, −22, 56 12.82 −42, −22, 52 14.70

FrontalACC/Pre-SMA L −6, 30, 34 5.17 −4, 14, 42 9.12

R 6, 24, 46 6.71 0, 10, 50 9.93IFG L −56, 8, 24 5.85 −48, 8, 18 12.21

R 54, 12, 20 10.99 52, 12, 18 14.48MFG L −22, 40, 18 5.71 −40, 34, 20 7.77

R 40, 34, 20 7.29 34, 36, 28 7.64Premotor L −52, 2, 38 5.00 −54, 2, 38 8.35

R 46, 4, 42 6.96 42, 2, 40 6.99Insula L −34, 18, 2 10.39 −30, 24, 2 9.13

R 34, 28, 6 10.14 32, 26, −2 13.14

ParietalInferior Parietal L −34, −46, 38 5.59 −42, −34, 36 11.90

R 36, −44, 42 6.79 48, −32, 36 7.92

TemporalSTG L −46, −40, 14 7.78 −46, −48, 12 4.34

R 64, −40, 14 11.20 64, −42, 12 9.40MTG L −46, −66, 4 7.55 −48, −60, 2 6.23

R 50, −60, 6 7.43

VisualMOG L −36, −94, 6 5.68 −38, −80, 0 3.30

R 40, −86, 6 5.68 26, −98, 6 4.60Cuneus L −10, −92, −4 6.56 −4, −78, 8 3.46

R 20, −88, −2 6.67 16, −68, 6 4.03

SubcorticalThalamus L −10, −22, 0 9.43 −16, −24, 8 7.39

R 12, −24, 12 8.14 12, −26, 12 6.80Basal ganglia L −20, 4, −6 5.52 −24, 4, 0 5.51

R 16, 2, −6 7.37 20, 2, 10 6.01Cerebellum L −30, −54, −16 7.20 −32, −46, −28 5.23

R 20, −50, −16 10.60 32, −44, −30 12.22

All peaks passed a whole-brain false detection rate threshold of p < .01 (correctedfor multiple comparisons). Coordinates are given in MNI space. MFG: middle frontalgyrus; MTG: middle temporal gyrus; STG: superior temporal gyrus; MOG: mid-dle occipital gyrus; IFG: inferior frontal gyrus; ACC/Pre-SMA: anterior cingulatecortex/pre-supplementary motor area.

N. Hon et al. / Neuropsyc

omprised 32 slices, aligned with the anterior commissure-posterior commissureine. Slice thickness was 3 mm, in-plane resolution 3 mm × 3 mm. The fMRI data

ere processed and analysed using SPM 5 (Wellcome Department of Imaging Neu-oscience, London, UK). Prior to analysis, all images were subjected to slice-timeorrection, with the first slice in each volume taken as a reference. Images wereubsequently realigned into a standard spatial orientation using the first volume asreference, undistorted (Cusack, Brett, & Osswald, 2003) and normalised to MNI

Montreal Neurological Institute) space. The normalised images were then spatiallymoothed with an 8 mm full-width half-maximum Gaussian kernel.

There were two main levels of statistical analysis for both experiments. Inhe first level, event-related contrasts of interest for each subject were computedith the General Linear Model. Each voxel’s activity was fitted with a combi-ation of functions obtained by the convolution of the synthetic haemodynamicesponse function with the time series of events. For this purpose, each trial wasonsidered a single event of 1500 ms duration. In both experiments, there werewo trial types (targets, non-targets). These were explicitly modeled and enterednto the target minus non-target contrast. Because target detection was excellentn both experiments (Experiment 1: 100%; Experiment 2: 98%), all target trials

ere included in the analysis. Low frequency noise was removed with a high-ass filter. In the second level of analysis, individual subjects’ data from the targetinus non-target contrast were combined and subjected to a random effects anal-

sis.As a subsidiary analysis, we directly compared the activation from both experi-

ents. To do this, we entered the data from both experiments into the followingontrasts: [(targetExpt 1 − non-targetExpt 1) − (targetExpt 2 − non-targetExpt 2)] and itseverse contrast.

. Results

.1. Experiment 1: targets defined by physical identity

Targets were detected perfectly in this experiment. The meanorrect detection time was 466 ms. To study the effects of detect-ng an identity-defined target on neural activation, we contrastedctivation elicited by targets with that from non-target words.his contrast revealed an extended pattern of activation (Table 1nd Figs. 2 and 3). Consistent with other findings, the contrastetween targets and non-targets produced greater activation inrontal and parietal areas typically associated with cognitive controlnd goal-directed attention (Bledowski et al., 2004; Hon, Epstein,wen, & Duncan, 2006; Shulman, Ollinger, Linenweber, Petersen,Corbetta, 2001). Conspicuous bilateral activation was observed

long the intraparietal sulcus, extending in the inferior parietalobule towards the postcentral sulcus (Fig. 2). A large portion ofateral frontal cortex was also activated by this contrast, includinghe inferior and middle frontal gyrus extending to premotor cor-ex (Fig. 2), and the frontal operculum extending into the insulasee Fig. 3). On the medial surface of the frontal lobe, prominentctivation of the pre-supplementary motor area/anterior cingulateas observed (Fig. 2). Also evident was activation of left motor cor-

ex, likely related to the manual responses required by targets. Thisontrast also produced activation in the thalamus, basal ganglia anderebellum (Fig. 3).

Typical of experiments using word stimuli, we also observedctivation spreading from middle and superior temporal gyri toccipitotemporal areas. Posteriorly, this region of activity includedhe commonly-described lateral occipital complex (LOC, Fig. 3).

Of greatest interest to this report was the finding that targetetection activated early visual areas, including the middle occip-

tal and lingual gyri, as well as the cuneus (Fig. 3). This activity,hich includes much of the medial occipital surface immediately

urrounding the calcarine fissure, suggests enhanced target codingt the earliest cortical levels of visual representation. The reverseontrast revealed no activations.

.2. Experiment 2: targets defined by semantic category

In Experiment 1 and many earlier studies, target status could beetermined on the basis of specific physical features. As suggestedarlier, because visual features can be represented at multiple lev-

els of the visual system, it is impossible to determine from wheretarget enhancement effects originate. To assess the locus of tar-get enhancement of activity in early visual areas, we performeda second experiment. In this experiment, targets were defined bysemantic class. The design of this experiment allowed us to considermore directly the locus of the target-driven effects. In this experi-ment, every word presented was a different one. Target status couldonly have been established once membership of a particular seman-tic category had been determined. Therefore, any target-relatedmodulation of early visual activity must occur after semantic cate-gorization.

On average, subjects detected targets 98% of the time inthis experiment. Target detection took longer in this experiment(582 ms) than in Experiment 1 [t(15) = 9.17, p < .001]. The activationsobserved when targets were contrasted with non-targets in thisexperiment were very similar to those in Experiment 1 (Table 1).Frontal and parietal control areas were activated, along with leftmotor cortex (Fig. 2). Targets activated middle and superior tempo-

ral gyri and LOC more than non-targets (Fig. 3). Critically, as before,early visual areas, including the cuneus and the middle occipital andlingual gyri (Fig. 3), were engaged more by targets than non-targets.The reverse contrast revealed that non-targets did not activate anyareas more than targets.

1724 N. Hon et al. / Neuropsychologia 47 (2009) 1721–1727

Fig. 2. Dorsal activations observed with a whole-brain contrast between targets and non-targets. Data from both experiments are overlaid (Experiment 1: red; Experiment2: blue). All depicted activation passed a whole-brain false detection rate threshold of p < .01 (corrected for multiple comparisons). Both x- and z-coordinates are given in MNIspace. Parietal activation was conspicuous along the length of the intraparietal sulcus (IPS) and in the inferior parietal lobule (IPL). Lateral frontal activation was observedi he preo ent wm the m

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n inferior (IFG) and middle (MFG) frontal gyri, as well as in premotor cortex near tbserved in pre-supplementary motor area/anterior cingulate cortex (ACC). Consistotor activation. Also observable in the x = −50 section is temporal activation along

.3. Subsidiary analysis: direct comparison of target detection inxperiments 1 and 2

We also directly compared the neural responses elicited by tar-et detection from both experiments (see Section 2). This analysisevealed that two brain areas were activated to a greater extent byhe identity targets than the semantic ones (Fig. 4). Greater activa-ion by detection of identity targets was observed in the cerebellumnd right middle temporal gyrus.

. Discussion

Our results confirm that detecting a target on the basis of itsdentity has a pronounced effect on a number of occipitotemporalreas. Of greatest interest is the finding that target detection mod-lates activity in early visual areas in humans, and this is true evenhen target status can only be determined after semantic catego-

ization. Here, we found that target detection enhanced activity iniddle occipital and lingual gyri and the cuneus. In Experiment 1,

on-targets entailed the presentation of novel visual informationhereas targets were repeat presentations of the same stimulus.

n Experiment 2, both target and non-targets entailed the presen-

central sulcus (Premotor). On the medial surface, prominent frontal activation wasith the requirement for a right-handed motor response, targets elicited greater leftiddle temporal gyrus (MTG).

tation of new visual stimuli. Nevertheless, in both experiments,target detection evoked stronger visual activity than non-targets.As noted previously, many single-unit studies of target detectionhave reported enhancement of early visual activity at long laten-cies. One hypothesis is that this reflects feedback signals returningfrom upstream processing areas. The data from Experiment 2, inparticular, strongly support this hypothesis. In that experiment,target-related modulation of early visual activity could only haveoccurred after processing in upstream semantic systems, suggest-ing that source of the modulation must have been return signalsfrom those systems.

Additionally, these results speak to the magnitude of targeteffects. A typical finding in many studies is an attenuation of neuralactivity to repeated presentations of a stimulus, an effect commonlyreferred to, in fMRI studies, as adaptation (Grill-Spector, Henson,& Martin, 2006; Grill-Spector & Malach, 2001; Grill-Spector et al.,1999). The fact that the repeated targets of Experiment 1 produced

greater visual activity than the novel distractors suggests that targeteffects are capable of overwhelming adaptation effects.

In addition to early visual areas, target detection modulatedactivity in a number of areas normally associated with the pro-cessing of printed words. LOC was activated more by targets in

N. Hon et al. / Neuropsychologia 47 (2009) 1721–1727 1725

Fig. 3. Ventral activations observed with a whole-brain contrast between targets and non-targets. Data from both experiments are overlaid (Experiment 1: red; Experiment2 of p <c n of am STG:w n whi

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: blue). All depicted activation passed a whole-brain false detection rate thresholdritical finding is that target detection, in both experiments, resulted in modulatioiddle occipital gyri. LOC: lateral occipital complex; MTG: middle temporal gyrus;hite on sections z = −8, −4 and 0. The location of the calcarine sulcus is indicated i

oth experiments. Although often associated with the processingf object-level properties, the literature suggests that LOC may beunctionally heterogeneous, with different portions contributingo different linguistic processes: Posterior portions of LOC haveeen suggested to code for higher-level visual representations ofstimulus including those of printed words (Cohen et al., 2002;

rill-Spector et al., 1999; Kourtzi & Kanwisher, 2001), whereasore anterior aspects have been associated with semantic coding

Martin, 2007). In addition to LOC, we observed activation in morenterior areas also known to be involved in semantic processing:iddle and superior temporal gyri. The site of our left superior

ig. 4. Activations observed with interaction analysis. All depicted activation passedwhole-brain false detection rate threshold of p < .05 (corrected for multiple com-arisons). z-coordinate is given in MNI space. The interaction analysis revealed thathysical identity targets in Experiment 1 activated two areas, the cerebellum andiddle temporal gyrus, more than the semantic targets of Experiment 2. MTG:iddle temporal gyrus.

.01 (corrected for multiple comparisons). z-coordinates are given in MNI space. Thectivity in a number of early visual areas, including the cuneus and the lingual andsuperior temporal gyrus; MOG: middle occipital gyrus. The cerebellum is traced inte on section z = +8.

temporal activation closely resembles the traditional location ofWernicke’s Area, which has long been proposed to be involvedin comprehension (Geschwind, 1970). Middle temporal activationhas been observed to increase as intelligibility or comprehensionincreases (Davis & Johnsrude, 2003).

In models of detection, a “target template” defines the targetcategory for the current task (Duncan & Humphreys, 1989). Targetdetection takes place by matching visual input against this advancetarget definition. In simple feature tasks, the template may be a low-level description of the target colour, motion etc; in semantic tasks,however, matching necessarily requires extraction of word mean-ing. In our experiments, target-selective activity was observed in avariety of higher semantic areas. In Experiment 2, this activity insemantic systems provides a plausible site for the initial processof match between input word and target definition. In that exper-iment, enhanced target processing in early visual areas could onlyhave occurred after a target was first identified in semantic systems.

The setting of semantic target templates is somewhat reminis-cent of the phenomenon of conceptual priming. In both cases, priorinformation influences neural responses to subsequent stimuli,possibly via the preactivation of some body of cells. Nevertheless,the two are associated with qualitatively different effects: Whereaspriming is often associated with reduced neural activity (Henson &Rugg, 2003; Schacter & Buckner, 1998), template matching is asso-ciated with enhanced activation. A likely reason is that templatesact specifically to distinguish target stimuli of relevance to currentattention and behaviour from non-target stimuli that should beignored.

Various findings have demonstrated that early visual activity

can be influenced by contextual information (Kapadia, Ito, Gilbert,& Westheimer, 1995; Rossi, Rittenhouse, & Paradiso, 1996). Forexample, a V1 cell’s activity is enhanced at long latencies if itsreceptive field falls on a figure surface, relative to when it falls onthe background (Lamme, Rodriguez-Rodriguez, & Spekreijse, 1999;

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ipser, Lamme, & Schiller, 1996). One possible locus of contextualnfluence on early visual activity is short-range feedback signalsrom neighbouring visual areas: V1 cells failed to show figure-elated enhancement of activity when neighbouring extrastriateortex was lesioned (Lamme et al., 1998). Our results extend thesendings by demonstrating that feedback signals to early visualreas can have long-range origins, originating from outside theoundaries of visual cortex.

As noted earlier, targets can be defined on the basis of spa-ial location. Our findings are consistent with results from studiesxamining the neural effects of detecting spatially-defined targets.arious studies have found enhanced early visual activity to stim-li presented in a task-relevant location, relative to when the sametimuli are presented in an irrelevant location. In principle, a targetemplate specifying a specific location could be implemented veryarly in the visual system, as space is an early-coded visual feature.owever, like its object-based counterpart, this spatial target effect

s most strongly observed at long latencies, after the initial sensory-voked discharge of the cell (Di Russo, Martinez, & Hillyard, 2003;amme et al., 1998; Luck, Chelazzi, Hillyard, & Desimone, 1997;artinez et al., 2001). This suggests that spatial target effects on

ctivity in early visual areas also stem, at least in part, from delayedeedback signals from later brain systems. Possibly, feedback signalsre a general mechanism by which detection influences processingn early brain systems, regardless of whether targets are defined bypatial locations or identity.

In a variety of circumstances, top-down signals can influ-nce visual activity even in the absence of physical stimulation.or example, signals from higher-order brain regions can influ-nce visual activity following instructions to attend (Muller,artelt, Donner, Villringer, & Brandt, 2003) or imagine (O’CravenKanwisher, 2000), even without the presentation of a stimulus.

lthough likely related to those findings, here, our focus was oneedback or return signals from higher-order brain systems onlyfter a stimulus has been classified as a target.

Finally, our results speak to the neural effects of target detec-ion. A target object may be represented by many different brainystems, with different systems coding for different properties oreatures. One proposal is that, when an object is determined to betarget, brain systems supporting different representations of thebject co-operate by mutually enhancing each other, allowing forctivity related to this object to dominate the network (Duncan,umphreys, & Ward, 1997). Our results are consistent with this pro-osal. Here, when a word is determined to be a target after semanticategorization, representations of visual form are also enhanced.ikely, when detection occurs on the basis of semantic category,emantic representations send feedback signals to support the acti-ation of earlier representations, such that all representations of thearget are enhanced.

cknowledgements

This work was supported by the Medical Research Council (UK),ntramural program U.1055.01.001.00001.01. N.S. was supported byhe Royal Society. N.H. was supported by grant R581000057112/133rom the National University of Singapore.

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