dynamic communication and connectivity in ontal fr etworks n · 13.07.2011 · dynamic...

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How do we maintain a stable percept of the world in the face of the powerful drive of neuroplasticity in both health and disease? Th is dichotomy forms one of the most fundamental unanswered questions in neuroscience concerning the balance between the dynamic, plastic underpinnings of our neurobiology and the relative stability of our cognition. Th e brain undergoes massive changes in size, morphol-ogy, and connectivity during normal development (Fig. 6.1 ; Gogtay et al., 2004 ) and aging (Sowell et al., 2003 ) as well as in response to brain injury (Alsott et al., 2009 ; Carmichael, 2003 ), yet we can maintain a r elatively stable sense of c ogni-tion and self during the lifespan. Human brains, each with over 100 billion neu-rons, develop similarly despite the wide variations in environment and experience. However, within the bounds of this stability there exists a wide range of variabil-ity and capacity f or change. H ere we w ill discuss the r ole of neur oplasticity in frontal lobe-dependent cognition by examining the localization of attention and memory functions in the br ain and how these seeming ly fi xed locations ma y refl ect fl exible neural networks that change communication properties as required by behavior.

Localization of Cognitive Functions

Localization of cognitive functions in the human brain poses a major problem in modern neuroscience (Brett, Johnsrude, & O wen, 2002 ; Young, Hilgetag, & Scannell, 2000 ). First, there is the problem of comparing localization of function data across methodologies and across subjects and rectifying fi ndings from vari-ous neuroimaging and neuropsychological methodologies — each with their own

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Dynamic Communication and Connectivity in Frontal Networks

B R A D L E Y V O Y T E K A N D R O B E R T T . K N I G H T

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110 MIND AND THE FRONTAL LOBES

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1 limitations and underlying assumptions — with computational, lesion, and animal studies. Th is presents a daunting prospect for any investigator. Second, neurosci-entists face the inherent morphological variability across subjects; currently, any claims to cortical functional specifi city are probabilistic claims in that — barring direct cortical stimulation mapping — one cannot guarantee that a specifi c cortical region plays a specifi c functional r ole. For example, direct cortical stimulation mapping suggests frontal, temporal, and parietal sites are all involved in language functions, yet the specifi c neuroanatomy of these sit es diff ers widely acr oss subjects (Sanai, Mirzadeh, & Berger, 2008 ).

Th ese problems are not just theor etical or didactic issues: neurosurgeons performing surgical tissue resections must use intraoperative cortical stimulation mapping to ensure that the cortical tissue to be removed is not “eloquent” (lan-guage or motor) cortex. Such stimulations are performed while the patient is awake and performing cognitive and behavioral tasks. During this testing period the surgeon electr ically stimulates diff erent brain regions to monitor speech or motor arrest. Th is method — although decades old — is still widely employ ed because of the known variability in functional localization and cortical morphol-ogy across subjects.

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Figure 6.1. Changes in gray matter volume with normal development (adapted from Gogtay et al., 2004 ). Th is fi gure illustrates the structural plasticity of the neocortex in the developing human brain, especially in association cortex, during childhood. Note the relative stability of primary sensorimotor and visual areas by puberty, in contrast to the plasticity of the childhood frontal and temporal association cortices.

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Dynamic Communicat ion and Connect iv i ty in Frontal Networks 111

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1 Although the functional localiz ation story appears bleak at the level of a sing le individual, cerebral regions of functional localization are clearly observed when aver-aged across a group of subjects with neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission t omography (PET). Most studies rely upon the pr inciple of c ognitive subtraction, originally estab-lished in reaction time studies by Franciscus Donders (Donders, 1868 ). Th e under-lying assumption in these studies is that activity in br ain networks alters in a task-dependent manner that becomes evident after averaging many event-related responses and comparing those against a baseline condition. Deviations from this baseline refl ect a change in the neuronal processing demands required to perform the task of interest.

Although both the cognitive subtraction method (Friston et al., 1996 ) and assumptions regarding baseline activity (Gusnar d & R aichle, 2001 ) have their own problems, these methods provide details of functional localization that can then be t ested and c orroborated using other me thodologies, including lesion studies. Th e interpretation of these localiz ation results is confounded, however, by a lack of clarity in what is meant for a “function” to be localized. For example, Young and colleagues ( 2000 ) noted that for a given function to be localizable that function “must be capable of being considered both structurally and functionally discrete,(p.155)” a property that the br ain is incapable of assuming due t o the intricate, large-scale neuronal interconnectivity.

Th us, discussing behavioral functions outside the context of the larger cortical and subcortical networks involved with that function is a poorly posed pr oblem. Th erefore, the scientifi c study of cognition requires detailed neuroanatomical and connectivity information to complement functional activity fi ndings. Th e current eff ort to map a human connectome (Sporns, Tononi, & Kötter, 2005 ) will provide researchers with the neuroanatomical roadmap necessary to examine changes in large-scale cortical network activity during cognition.

The Lesion Method

While functional neuroimaging techniques such as fMRI and PET have advanced our understanding of regional specifi city, the lesion method provides the stron-gest case in the argument for causality in functional neuroanatomy; that is, brain region A can be assumed to play an important role in the network supporting function X if a lesion t o A impairs function X . Research on humans with f ocal brain lesions (Fig . 6.2 ) has pr ovided seminal inf ormation with r egard to our understanding of which brain regions contribute to specifi c behavioral, sensory, and cognitive functions (Ror den & Karnath, 2004 ). For example, because pre-frontal cortex (PFC) and basal ganglia lesions lead to working memory defi cits (Müller & Knight, 2006 ; Tsuchida & Fellows, 2009 ); Voytek & Knight, 2010 ), the

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1 PFC can be said t o play an important, if not necessary, role in working memory networks.

By combining lesion studies with neur oimaging techniques, researchers can identify other br ain regions associated with a c ertain behavior. For example, research using scalp electroencephalography (EEG) has shown that unilateral PFC lesions cause lateralized defi cits in top-down modulation of activity in visual extrastriate cortex during attention (Fig. 6.3 ; Barceló, Suwazano, & Knig ht, 2000 ; Yago et al., 2004 ) and working memory (Voytek & Knight, 2010 ), which makes EEG a power ful tool for investigating the network dy namics subserving cognition.

While the underlying notion of brain damage disrupting function is fairly obvi-ous — damaging parts of a machine prevents the machine from working optimal-ly — the specifi c eff ects of brain damage are neither obvious nor always predictable. Th ere are several factors that prohibit accurate prediction of which de fi cits will manifest after a given brain lesion. Th is is largely due to the fact that we are still uncertain about the ac curacy of regional localization of function and the poorly posed nature of the functional localization question in general. Because the prob-ability distribution of functional localiz ation across subjects is broad, especially across cortical association areas (Sanai, Mir zadeh, & B erger, 2008 ), the impor-tance of distributed cortical networks in behavior and subsequent recovery cannot be ignored.

Nevertheless, working with patients with cir cumscribed frontal brain lesions provides us with insig ht into how fr ontal cortex interacts with the r est of the brain to give rise to cognitive functions. W hen combined with c omputational and behavioral methodology and/or neur oimaging, the lesion me thod allows researchers to examine exactly which areas are critical for which cognitive func-tions. For example, recent work by B adre and colleagues took advantage of the inherent diff erences in lesion siz e and ex tent in their patient populations t o examine the rostral–caudal organization of cognitive and action c ontrol in the

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Figure 6.2. Patient lesion reconstructions. Th ese structural MRI slices illustrate the lesion overlap across six patients with unilateral PFC lesions. All lesions are normalized to the left hemisphere for comparison, although two patients had right hemisphere lesions (adapted from Voytek et al., 2010 ). Examining groups of patients with stereotyped lesions allows researchers to test the role of specifi c regions in behavior. Software reconstructions were performed using MRIcro (Rorden & Brett, 2000).

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1 frontal cortex (Badre et al., 2009 ). While this “messiness” of lesion siz e, extent, and location has tr aditionally been vie wed as a ma jor drawback of the lesion method, it is the c ornerstone of voxel-based lesion-symptom mapping ( VLSM) (Fig. 6.4 ; Bates et al., 2003 ). Th is method r equires a detailed neur oanatomical scan of every patient; t -tests are then performed at every voxel on a var iable of interest (e.g., a c ognitive task) wher e the statistical “ groups” are defi ned by whether the patient has a lesion in that specifi c voxel or not. Th is clever technique allows researchers to map voxel by voxel which regions are most important for a cognitive function.

Recent work has ex panded the lesion method int o computational modeling. Using a c ortically plausible network ar chitecture, researchers have shown the eff ects of lesions on functional connectivity (Alstott et al., 2009 ; Young, Hilgetag, & Scannell, 2000 ) and on oscillatory dynamics (Honey & Sporns, 2008 ) demon-strating activity changes in remote brain areas (Reggia, 2004 ) not directly con-nected to the lesioned br ain region (Young, Hilgetag, & Scannell, 2000 ). Th ese fi ndings suggest that lesions to highly connected critical hubs — including frontal and parietal regions — result in widespr ead changes in functional c onnectivity and oscillatory communication.

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1 Recovery and Compensation

Predicting the c ourse of r ecovery from brain damage is c onfounded by a lack of understanding about the ex tent and time c ourse of recovery possible across diff erent regions of the c entral nervous system. Neural plasticity is cr itical for functional r ecovery after brain damage, with impr ovement possible even 20 years after the initi al injury (Bach-y-Rita, 1990 ). Th ere are several theories of recovery of function (Gr afman, 2000 ), including c ortical compensation by perilesion and intact homolo gous brain regions (Wundt, 1902 ) or subc ortical (Van Vleet et al., 2003 ) structures; diaschisis reversal (von Monakow, 1969 ); unmasking (Lytton, Williams, & Sober , 1999 ); distributed cortical representa-tions (Jackson, 1958 ); and axonal sprouting and neurogenesis (Carmichael et al., 2001 ). Many of these theor ies predate neuroimaging and were based on clinical observations of patients with br ain damage. In 1902, W ilhelm Wundt noted that:

in both simple and complex disturbances, there is usually a gradual restoration of the functions in the course of time. Th is is probably eff ected by the vicarious functioning of some, generally a neighboring cortical region in place of that which is disturbed (in disturbances of speech, perhaps it is the opposite, before untrained, side that comes into play) . (p. 206)

Th is latter point was proved in a recent paper in which Blasi and coworkers demonstrated that patients who ha ve recovered from Broca’s aphasia due to left frontal stroke show fMRI activation in the right frontal Broca’s area homologue (Fig. 6.5A ; Blasi et al., 2002 ).

Th e fact that the brain is not a static machine, but rather a fl uctuating (plastic), self-repairing organ (Cramer, 2008 ), provides an important confound to lesion-based research. For example, most lesion studies that demonstr ate behavioral defi cits in humans ar e performed on pa tients who ha ve had sudden (acut e) brain damage (e.g., stroke or tr auma) precisely because these patients show the strongest behavioral defi cits. In contrast, patients who have undergone surgi-cal resections to remove cancerous cerebral tissue t end to show f ewer defi cits before and after their surgeries (Desmurget, Bonnetblanc, & Duff au, 2007 ) compared to a patient with a comparably sized lesion from a stroke. Th is phenom-enon is interpreted as recovery processes resulting from compensation by other brain regions in cases of slow -growing lesions. B ecause the lesions ar e slow-growing rather than r apidly occurring (such as fr om stroke), the h ypothesis is that the defi cits resulting from the lesion are minimized because the incremen-tally slow rate of growth permits compensatory processes to mask those defi cits. By defi nition, acute lesions, on the other hand, r esult in r apid tissue d amage that cannot be (immedi ately) compensated for. Th us, although patient work is

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1 invaluable, the temporality of the lesion (both onset time and time since damage) should not be discounted.

Given the number of brain regions needed to support cognitive functions, it is not unreasonable, given the variety of recovery theories, to hypothesize that cogni-tive recovery could be supported by any part of the c ognitive network. Th e PFC, however, plays an impor tant role in c ognitive networks by bi asing information fl ow in other regions to favor positive behavioral outcomes (Miller & Cohen, 2001 ). Th erefore, the PFC may play a privileged role in cognitive compensation. For exam-ple, although patients with lateral PFC lesions have lasting attention and working memory defi cits (e.g., Voytek & Knight, 2010 ; Barceló, Suwazano, & Knight, 2000 ), cognitive functions can r ecover somewhat over time ( Voytek et al., 2010 ). Numerous studies sug gest that the PF C plays a diverse r ole in a wide r ange of cognitive functions involved in the allocation and control of visual attention and working memory. One h ypothesis is that the PF C maintains an associ ation between endogenous elements in wo rking memory while an unknown neur onal mechanism compares these endo genous representations to exogenous visual information as it is pr ocessed in extrastriate visual areas (Barceló, Suwazano, & Knight, 2000 ; Kimberg & Farah, 1993 ).

It is important to note that neuropsychological testing alone can be misleading concerning the extent of recovery after PFC damage. For example, if, during an attention task, visual stimuli are presented full-fi eld (that is, presented in the center of the visual fi eld and with unr estrained eye movements), patients with unilateral PFC lesions do not show obvious visual att ention defi cits. However, if visual stimuli are lateralized to the left or right visual hemifi eld by a matter of a few degrees and c entral fi xation is maintained, then de fi cits in visual w orking memory (Voytek & Knight, 2010 ) and attention (Barceló, Suwazano, & Knight, 2000 ) are evident.

Visual stimulus lat eralization takes advanta ge of the neur oanatomy of the mammalian visual system such that stimuli pr esented to the r ight visual hemi-fi eld preferentially activate the left visual cortex (and vice versa) before that infor-mation is then transferred to the opposite visual cortex via the corpus callosum. Such lateralized designs increase statistical power in that patients can ser ve par-tially as their own c ontrols (i.e., “ good” hemifi eld vs. “ bad” hemifi eld; see Fig . 6.3A ), thus allowing for a within-subjects comparison of the eff ects of the brain lesion on a cognitive function for contralesionally versus ipsilesionally presented stimuli.

Nevertheless, even in lateralized visual attention and working memory paradigms, patients with unilat eral PFC damage — though worse than c ontrol subjects when stimuli ar e presented contralesionally — still perform well above chance levels. Th is fi nding is somewhat in contrast to what is obser ved in lesion and neuroimaging studies of pr imary cortical functions. Neuroimaging studies of movement or visual processing localize these processes to motor and visual cortex, respectively. Lesions to primary motor or pr imary visual cortex lead to

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1 striking and per manent defi cits (hemiparesis or c ortical blindness, in these specifi c cases). Con versely, while functional neur oimaging studies show task -dependent PFC activation during attentional control and working memory, lesions to the PFC lead to an incomplete loss of those functions. Th is discrepancy may have any number of underlying causes , including an y combination of the following: (1) Research paradigms used to assess cognitive defi cits may be less sensitive and less specifi c than those used t o examine motor or sensory defi cits; (2) Cognitive processes dependent on associ ation cortex may be mor e widely distributed across a broader network than those dependent on pr imary cortex, making cognitive processes more resilient to a sing le focal lesion; and (3) Compensatory mechanisms may be facilitating damaged cognitive functions more than primary functions.

For neuronal activity diff erences to be considered “compensatory,” Davis and associates ( 2008 ) have outlined at least two criteria that must be met. First, novel activity increases not seen in nor mal controls (but seen in, e.g ., lesion patients) must be associated with correct behavioral outcomes. Second, defi cits in process-ing by one r egion must be associ ated with increases in activity in the putative compensatory region. Th ese criteria are important because activity incr eases interpreted as “compensatory” may in fact more simply refl ect a global increase in cortical activity due to increases in diffi culty in per forming a task f or lesion patients compared to control subjects (Hillary et al., 2006 ). In other wor ds, because of the lesion, more cognitive resources are recruited to correctly perform the task compared to controls.

In the c ontext of unilat eral PFC damage and its e ff ects on att ention and working memory, Voytek and c oworkers ( 2010 ) hypothesized that the intact , undamaged PFC compensates for the damaged cortex in a load-dependent manner as required by task demands. What was observed (Fig. 6.5B ), consistent with the fi rst criterion for compensation, was that increases in activity over the intact PFC are enhanced on c orrect trials when the d amaged PFC is challenged with lateralized visual work ing memory or att ention demands. W ith regards to the second criterion, their ex perimental designs preferentially challenged the damaged hemisphere in patients with unilat eral PFC damage, and incr eases in activity over the intact PFC were seen in conjunction with top-down defi cits in the visual ex trastriate cortex of the d amaged hemisphere. It is impor tant to highlight that the decreased posterior extrastriate responses seen in cognitive experiments with patients with unilat eral PFC damage (Barceló, Suwazano, & Knight, 2000 ; Voytek & Knight, 2010 ) are seen only when stimuli ar e presented to the c ontralesional hemifi eld. If we ar e to assume that these post erior responses normally index beha vior and per formance — and PFC patients show attenuated extrastriate responses even when correctly performing the task — then logically there must be some other br ain regions compensating for the lesioned cortex.

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1 As previously stated, research indicates that the per ilesion cortex and the homologous intact contralateral cortex may both be involved in recovery and that there is long-r ange, intracortical reorganization of beha viorally and r ecovery-relevant pathways (Dancause, 2006 ; Nudo, 2007 ). Th us, Voytek and c oworkers proposed that the visual information delivered to the contralesional hemisphere is transferred trans-callosally to the intact hemisphere, where the intact PFC then assumes task control as needed on a tr ial-by-trial basis. Support for this conten-tion is provided by studies in non-human primates revealing that top-down PFC control over visual cortex during memory retrieval relies on callosal information transfer (Hasegawa et al., 1998 ; Tomita et al., 1999 ). Th us, if trans-callosal infor-mation transfer could be blocked, then behavioral defi cits should be enhanced.

As discussed previously, in contrast with cognitive defi cits, primary motor and sensory functions rarely recover in adults who suff er cortical damage, although other modalities may take over intact sensory cortex deprived of input due to peripheral damage (Sadato et al., 1996 ). Unlike adults with pr imary cortical damage, children who ha ve had a surg ical hemispherectomy, for example, can regain motor control of the aff ected limbs (Benecke et al., 1991 ); such recovery can be seen even in childr en with massive and severe cortical damage (e.g., Distelmaier et al., 2007 ). In contrast, others have observed a surprising normality among patients missing massive amounts of their c ortical tissue (Lewin, 1980 ). While defi cits caused by lesions t o PFC are more likely to recover if they oc cur later in life — and this recovery may be dependent upon ha ving some amount of intact PFC (Kolb & Gibb, 1990 ) — children with PFC damage may have lasting cog-nitive impairment (Kolb & Gibb, 1990 ). Th e interaction between age and location of lesion with behavioral recovery may refl ect a deeper relationship with the evo-lution of cognitive and sensory functions in primates (Anderson, 2007 ) wherein cognitive functions, having evolved more recently, are more distributed across cortex and thus more resistant to focal brain damage once those functions have developed in adulthood.

Integrating all of the prior points, it may be that the farther away from primary cortical areas a region is, the less predictable the function becomes. Th is phenom-enon may help explain why we have fairly robust sensory and motor homunculi in the primary (“lower”) cortical areas, but no reliable mapping in the “higher” sen-sory and motor association cortices. Th is is illustrated by example from clinical observations: a patient with d amage to the pr emotor cortex is mor e likely t o recover motor functions than a patient with a lesion of pr imary motor cortex, who in turn is more likely to naturally recover than a person with a lower mot or neuron lesion in the spinal c ord. A network theor y view of this phenomenon would suggest that diff erences between the focal networks of primary regions and distributed networks of the functions subserved by association cortex may account for these diff erences in recovery. Given the above caveats, to study human cortical recovery of function one must carefully balance recovery likelihood with

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4948474645444342414039383736353433323130292827262524232221201918171615141312111098765

4

3

2

1 probability of functional localization — that is, in theory, one is more likely to fi nd a reliable defi cit across subjects with damage to primary cortical regions, but less likely to observe recovery in these patients.

References

Alstott , J. , Breakspear , M. , Hagmann , P. , Cammoun , L. , Sporns , O. , & Friston , K . ( 2009 ). Modeling the impact of lesions in the human br ain . Public Library of Science Computational Biology , 5 , 1 – 12 .

Anderson , M . ( 2007 ) Evolution of cognitive function via redeployment of brain areas . Neuroscientist , 13 , 13 – 21 .

Bach-y-Rita , P . ( 1990 ). Brain plasticity as a basis for recovery of function in humans . Neuropsychologia , 28 , 547 – 554 .

Badre , D. , Hoff man , J. , Cooney , J. , & D’Esposito , M . ( 2009 ). Hierarchical cognitive control defi cits following damage to the human frontal lobe . Nature Neuroscience , 12 , 515 – 522 .

Barceló , F. , Suwazono , S. , & Knight , R. T . ( 2000 ). Prefrontal modulation of visual processing in humans . Nature Neuroscience , 3 , 399 – 403 .

Bates , E. , Wilson , S. , Saygin , A. , Dick , F. , Sereno , M. , Knight , R. T. , & Dronkers , N . ( 2003 ). Voxel-based lesion–symptom mapping . Nature Neuroscience , 6 , 448 – 450 .

Benecke , R. , Meyer , B. U. , & Freund , H. J . ( 1991 ). Reorganisation of descending motor pathways in patients after hemispherectomy and severe hemispheric lesions demonstrated by magnetic brain stimulation . Experimental Brain Research , 83 , 419 – 426 .

Blasi , V. , Young , A. C. , Tansy , A. P. , Petersen , S. E. , Snyder , A. , & Corbetta , M . ( 2002 ). Word retrieval learning modulates right frontal cortex in patients with left frontal damage . Neuron , 36 , 159 – 170 .

Brett , M. , Johnsrude , I. , & Owen , A . ( 2002 ). Th e problem of functional localization in the human brain . Nature Review Neuroscience , 3 , 243 – 249 .

Carmichael , S . ( 2003 ). Plasticity of cortical projections after stroke . Neuroscientist , 9 , 64 – 75 . Carmichael , S. T. , Wei , L. , Rovainen , C. M. , & Woolsey , T. A . ( 2001 ). New patterns of intracortical

projections after focal cortical stroke . Neurobiology Discussion , 8 , 910 – 922 . Cramer , S . ( 2008 ). Repairing the human brain after stroke: I. Mechanisms of spontaneous recovery .

Annals of Neurology , 63 , 272 – 287 . Dancause , N . ( 2006 ). Vicarious function of remote cortex following stroke: recent evidence from

human and animal studies . Neuroscientist , 12 , 489 – 499 . Davis , S. W. , Dennis , N. A. , Daselaar , S. M. , Fleck , M. S. , & Cabeza , R . ( 2008 ). Que PASA? Th e

posterior-anterior shift in aging . Cerebral Cortex , 18 , 1201 – 1209 . Desmurget , M. , Bonnetblanc , F. , & Duff au , H . ( 2007 ). Contrasting acute and slow-growing lesions:

a new door to brain plasticity . Brain , 130 , 898 – 914 . Distelmaier , F. , Richter-Werkle , R. , Schaper , J. , Messing-Juenger , M. , Mayatepek , E. , & Rosenbaum ,

T . ( 2007 ). “ How much brain is really necessary?” A case of complex cerebral malformation and its clinical course . Journal of Child Neurology , 22 , 756 – 760 .

Donders , F.C . ( 1868 ). Die Schnelligkeit psychischer Prozesse . Archiv für Anatomie und Physiologie und wissenschaftliche Medizin , 657 – 681 .

Friston , K. J. , Price , C. J. , Fletcher , P. , Moore , C. , Frackowiak , R. S. , & Dolan , R. J . ( 1996 ). Th e trouble with cognitive subtraction . NeuroImage , 4 , 97 – 104 .

Gogtay , N. , Giedd , J. N. , Lusk. , L. , Hayashi , K. M. , Greenstein , D. , Vaituzis , A. C. , Nugent , T. F. , Herman , D. H. , Clasen , L. S. , Toga , A. , Rapoport , J. L. , & Th ompson , P . ( 2004 ). Dynamic mapping of human c ortical development dur ing childhood thr ough early adulthood . Proceedings of the National Academy of Sciences , USA , 101 , 8174 – 8179 .

Grafman , J . ( 2000 ). Conceptualizing functional neuroplasticity . Journal of Communication Disorders , 33 , 345 – 355 .

06-Levine-Ch06.indd 120 5/9/2011 4:54:06 PM



545352515049484746454443424140393837363534333231302928272625242322212019181716151413121110987654321 Gusnard , D. A., & Raichle , M. E . ( 2001 ). Searching for a baseline: functional imaging and the resting

human brain . Nature Review Neuroscience , 2 , 685 – 694 . Hasegawa , I. , Fukushima , T. , Ihara , T. , & Miyashita , Y . ( 1998 ). Callosal window between prefrontal

cortices: cognitive interaction to retrieve long-term memory . Science , 281 , 814 – 818 . Hillary , F. , Genova , H. , Chiaravalloti , N. , Rypma , B. , & DeLuca , J . ( 2006 ). Prefrontal modulation

of working memory performance in br ain injury and disease . Human Brain Mapping , 27 , 837 – 847 .

Honey , C., & Sporns , O . ( 2008 ). Dynamical consequences of lesions in cortical networks . Human Brain Mapping , 29 , 802 – 809 .

Jackson , J. H . ( 1958 ). A study of convulsions . In: J. Taylor (Ed.), Selected writings of John Hughlings Jackson . London : Staples .

Kimberg , D. Y., & Farah , M. J . ( 1993 ). A unifi ed account of c ognitive impairments following frontal lobe damage: the role of working memory in complex, organized behavior . Journal of Experimental Psychology General , 122 , 411 – 428 .

Kolb , B., & Gibb , R . ( 1990 ). Anatomical correlates of behavioural change after neonatal prefrontal lesions in rats . Progress in Brain Research , 85 , 241 – 255 .

Lewin , R . ( 1980 ). Is your brain really necessary? Science , 210 , 1232 – 1234 . Lytton , W. W. , Williams , S. T. , & Sober , S. J . ( 1999 ). Unmasking unmasked: neural dynamics follow-

ing stroke . Progress in Brain Research , 121 , 203 – 218 . Miller , E. K., & Cohen , J. D . ( 2001 ). An integrative theory of prefrontal cortex function . Annual

Review of Neuroscience , 24 , 167 – 202 . Müller , N. G., & Knight R. T . ( 2006 ). Th e functional neuroanatomy of working memory: contribu-

tions of human brain lesion studies . Neuroscience , 139 , 51 – 58 . Nudo , R. J . ( 2007 ). Postinfarct cortical plasticity and behavioral recovery . Stroke , 38 , 840 – 845 . Reggia , J . ( 2004 ). Neurocomputational models of the remote eff ects of focal brain damage . Medical

Engineering and Physics , 26 , 711 – 722 . Rorden , C., & Karnath , H.O . ( 2004 ). Using human brain lesions to infer function: a relic from a past

era in the fMRI age? Nature Reviews Neuroscience , 5 , 813 – 819 . Sadato , N. , Pascual-Leone , A. , Grafman , J. , Ibañez , V. , Deiber , M. P. , Dold , G. , & Hallett , M . ( 1996 ).

Activation of the pr imary visual c ortex by Br aille reading in blind sub jects . Nature , 380 , 526 – 528 .

Sanai , N. , Mirzadeh , Z. , & Berger , M . ( 2008 ). Functional outcome after language mapping for glioma resection . New England Journal of Medicine , 358 , 18 – 27 .

Sowell , E. R. , Peterson , B. S. , Th ompson , P. , Welcome , S. E. , Henkenius , A. L. , & Toga , A . ( 2003 ). Mapping cortical change across the human life span . Nature Neuroscience , 6 , 209 – 215 .

Sporns , O. , Tononi , G. , & Kötter , R . ( 2005 ). Th e human connectome: a structural description of the human brain . Public Library of Science Computational Biology , 1 , e42 .

Tomita , H. , Ohbayashi , M. , Nakahara , K. , Hasegawa , I. , & Miyashita , Y . ( 1999 ). Top-down signal from prefrontal cortex in ex ecutive control of memor y retrieval . Nature , 401 , 699 – 703 .

Tsuchida , A., & Fellows , L.K . ( 2009 ). Lesion evidence that two distinct regions within prefrontal cortex are critical for n-back per formance in humans . Journal of Cognitive Neuroscience , 21 , 2263 – 2275 .

Van Vleet , T. M. , Heldt , S. A. , Pyter , B. , Corwin , J. V. , & Reep , R. L . ( 2003 ). Eff ects of lig ht deprivation on r ecovery from neglect and ex tinction induced by unilat eral lesions of the medial agranular cortex and dorsoc entral striatum . Behavioral Brain Research , 138 , 165 – 178 .

von Monakow , C . ( 1969 ). Die lokalisation im grosshirn und der abbau der funktion durch kortikale herde . In: K. H. Pribram (Ed.), Mood, states and mind . London : Penguin Books .

Voytek , B., & Knight , R. T . ( 2010 ). Prefrontal cortex and basal ganglia contributions to visual working memory . Proceedings of t he National Academy of S ciences, USA , 107 ( 42 ), 18167 – 18172 .

Voytek , B. , Davis , M. , Yago , E. , Barceló , F. , Vogel , E.K. , & Knight , R.T . ( 2010 ). Dynamic neuroplas-ticity after human prefrontal cortex damage . Neuron , 68 ( 3 ), 401 – 408 .

06-Levine-Ch06.indd 121 5/9/2011 4:54:06 PM



7

654321 Wundt , W . ( 1902 ). Outlines of psychology ( 2nd ed. ). Leipzig : Engelmann .

Yago , E. , Duarte , A. , Wong , T. , Barceló , F. , & Knight , R. T . ( 2004 ). Temporal kinetics of prefrontal modulation of the extrastriate cortex during visual attention . Cognitive, Aff ective, & Behavioral Neuroscience , 4 , 609 – 617 .

Young , M. , Hilgetag , C. , & Scannell , J . ( 2000 ). On imputing function to structure from the behav-ioural eff ects of brain lesions . Philosophical Transactions of t he Royal Society B : Biological Sciences , 355 , 147 – 161 .

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5

Age

20

1.00.90.80.70.60.50.40.30.20.10.0

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y m

atte

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Plate 3. Changes in gray matter volume with normal development (adapted from Gogtay et al., 2004). Th is fi gure illustrates the structural plasticity of the neocortex in the developing human brain, especially in association cortex, during childhood. Note the relative stability of primary sensorimotor and visual areas by puberty, in contrast to the plasticity of the childhood frontal and temporal association cortices.

Pre

fron

tal 6

54321

Plate 4. Patient lesion reconstructions. Th ese structural MRI slices illustrate the lesion overlap across six patients with unilateral PFC lesions. All lesions are normalized to the left hemisphere for comparison, although two patients had right hemisphere lesions (adapted from Voytek et al., 2010). Examining groups of patients with stereotyped lesions allows researchers to test the role of specifi c regions in behavior. Software reconstructions were performed using MRIcro (Rorden & Brett, 2000).

16-Levine-CPS.indd 3 5/9/2011 5:55:38 PM


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ntal

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e 5.

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nila

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Plate 6. Example of voxel-based lesion-symptom mapping (adapted from Bates et al., 2003). Th ese maps show speech fl uency (A–C) and language comprehension (D–F) in 101 aphasic stroke patients. Color represents the eff ect of lesion on behavior, with large t-values suggesting a signifi cant relationship between the presence of a lesion and a behavioral defi cit.



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nly

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and

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man

ds.



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