The hippocampal system mediates logical reasoning about familiar spatial environments

It has recently been shown that syllogistic reasoning engages two dissociable neural systems. Reasoning about familiar situations engages a frontal-temporal lobe system, whereas formally identical reasoning tasks involving unfamiliar situations recruit a frontal-parietal visuospatial network. These two systems may correspond to the "heuristic" and "formal" methods, respectively, postulated by cognitive theory. To determine if this dissociation generalizes to reasoning about transitive spatial relations, we studied 14 volunteers using event-related fMRI, as they reasoned about landmarks in familiar and unfamiliar environments. Our main finding is a task (reasoning and baseline) by spatial content (familiar and unfamiliar) interaction. Modulation of reasoning toward unfamiliar landmarks resulted in bilateral activation of superior and inferior parietal lobules (BA 7, 40), dorsal superior frontal cortex (BA 6), and right superior and middle frontal gyri (BA 8), regions widely implicated in visuospatial processing. By contrast, modulation of the reasoning task toward familiar landmarks, engaged the right inferior/orbital frontal gyrus (BA 11/47), bilateral occipital (BA 18, 19), and temporal lobes. The temporal lobe activation included the right inferior temporal gyrus (BA 37), posterior hippocampus, and parahippocampal gyrus, regions implicated in spatial memory and navigation tasks. These results provide support for the generalization of dual mechanism theory to transitive reasoning and highlight the importance of the hippocampal system in reasoning about familiar spatial environments.     

The role of the angular gyrus in visual conjunction search investigated using signal detection analysis and transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) delivered over the posterior parietal cortex increases choice reaction times in visual search for a target defined by a conjunction of features. Some recent studies of visual search have taken an approach based on signal detection theory, the findings of which are not addressed by studying the disruptive effects of TMS on reaction time. Here we investigated the role of the posterior parietal cortex in visual search by applying TMS while subjects performed unspeeded feature and conjunction visual search tasks matched for level of difficulty. TMS over the right, but not the left angular gyrus (AG) in the parietal cortex, nor vertex decreased subjects' sensitivity on the conjunction but not the feature search task, as measured by the signal detection measure, d'. Changes in bias, specifically the tendency to make false positive responses, were less clear. We consider the findings in terms of four possible explanation: binding, attentional control, spatial localisation and visuomotor co-ordinate transformations.     

Timing of visuo-spatial information processing: electrical source imaging related to line bisection judgements

This study deconvolves the temporal dynamics of the neural processes underlying line bisection judgements (i.e., the landmark task). Event-related potentials (ERPs) were recorded from 96 scalp electrodes in 10 healthy right-handed male subjects while they were judging whether horizontal lines were correctly prebisected. In the control task, subjects judged whether or not the horizontal line was transected by a vertical line, irrespective of its position. Using a current density reconstruction approach, source maxima in the time range from 50 to 400 ms after stimulus onset were localized and the time courses of activation were elaborated. Five regions, corresponding to those revealed by our previous fMRI studies (e.g., [Fink, G. R., Marshall, J. C., Shah, N. J., Weiss, P. H., Halligan, P. W., Grosse-Ruyken, et al. (2000). Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI. Neurology, 54, 1324–1331]), were identified as contributing significant source activity related to line bisection judgements: right middle occipital gyrus (Brodmann area; BA18); bilateral inferior occipital gyrus (BA19); right superior posterior parietal cortex (BA7) and right inferior posterior parietal cortex (BA40). Temporal deconvolution indicated sequential activation of these regions starting at BA18 as early as 90 ms post-stimulus onset, followed by the successive activation of the right superior posterior parietal (BA7), bilateral inferior occipital (BA19) and right inferior posterior parietal cortex (BA40). Three of these areas (BA18, BA17 and BA19) became reactivated within 250 ms of stimulus onset.

The data provide evidence for an early involvement of the right hemispheric parietal network in visuo-spatial information processing. Furthermore, the temporal deconvolution of the electrophysiological data suggest that iterative processes between and within parietal (dorsal path) and occipital areas (ventral path) mediate bisection judgements.     

Specialization in the default mode: Task‐induced brain deactivations dissociate between visual working memory and attention

The idea of an organized mode of brain function that is present as default state and suspended during goal‐directed behaviors has recently gained much interest in the study of human brain function. The default mode hypothesis is based on the repeated observation that certain brain areas show task‐induced deactivations across a wide range of cognitive tasks. In this event‐related functional resonance imaging study we tested the default mode hypothesis by comparing common and selective patterns of BOLD deactivation in response to the demands on visual attention and working memory (WM) that were independently modulated within one task. The results revealed task‐induced deactivations within regions of the default mode network (DMN) with a segregation of areas that were additively deactivated by an increase in the demands on both attention and WM, and areas that were selectively deactivated by either high attentional demand or WM load. Attention‐selective deactivations appeared in the left ventrolateral and medial prefrontal cortex and the left lateral temporal cortex. Conversely, WM‐selective deactivations were found predominantly in the right hemisphere including the medial‐parietal, the lateral temporo‐parietal, and the medial prefrontal cortex. Moreover, during WM encoding deactivated regions showed task‐specific functional connectivity. These findings demonstrate that task‐induced deactivations within parts of the DMN depend on the specific characteristics of the attention and WM components of the task. The DMN can thus be subdivided into a set of brain regions that deactivate indiscriminately in response to cognitive demand (“the core DMN”) and a part whose deactivation depends on the specific task. Hum Brain Mapp, 2010. © 2009 Wiley‐Liss, Inc.     

Frontal and parietal theta burst TMS impairs working memory for visual-spatial conjunctions

In tasks that selectively probe visual or spatial working memory (WM) frontal and posterior cortical areas show a segregation, with dorsal areas preferentially involved in spatial (e.g. location) WM and ventral areas in visual (e.g. object identity) WM. In a previous fMRI study [1], we showed that right parietal cortex (PC) was more active during WM for orientation, whereas left inferior frontal gyrus (IFG) was more active during colour WM. During WM for colour-orientation conjunctions, activity in these areas was intermediate to the level of activity for the single task preferred and non-preferred information. To examine whether these specialised areas play a critical role in coordinating visual and spatial WM to perform a conjunction task, we used theta burst transcranial magnetic stimulation (TMS) to induce a functional deficit. Compared to sham stimulation, TMS to right PC or left IFG selectively impaired WM for conjunctions but not single features. This is consistent with findings from visual search paradigms, in which frontal and parietal TMS selectively affects search for conjunctions compared to single features, and with combined TMS and functional imaging work suggesting that parietal and frontal regions are functionally coupled in tasks requiring integration of visual and spatial information. Our results thus elucidate mechanisms by which the brain coordinates spatially segregated processing streams and have implications beyond the field of working memory.     

Selective and divided attention during visual discriminations of shape, color, and speed: functional anatomy by positron emission tomography

Positron emission tomography (PET) was used to identify the neural systems involved in discriminating the shape, color, and speed of a visual stimulus under conditions of selective and divided attention. Psychophysical evidence indicated that the sensitivity for discriminating subtle stimulus changes in a same-different matching task was higher when subjects selectively attended to one attribute than when they divided attention among the attributes. PET measurements of brain activity indicated that modulations of extrastriate visual activity were primarily produced by task conditions of selective attention. Attention to speed activated a region in the left inferior parietal lobule. Attention to color activated a region in the collateral sulcus and dorsolateral occipital cortex, while attention to shape activated collateral sulcus (similarly to color), fusiform and parahippocampal gyri, and temporal cortex along the superior temporal sulcus. Outside the visual system, selective and divided attention activated nonoverlapping sets of brain regions. Selective conditions activated globus pallidus, caudate nucleus, lateral orbitofrontal cortex, posterior thalamus/colliculus, and insular-premotor regions, while the divided condition activated the anterior cingulate and dorsolateral prefrontal cortex. The results in the visual system demonstrate that selective attention to different features modulates activity in distinct regions of extrastriate cortex that appear to be specialized for processing the selected feature. The disjoint pattern of activations in extravisual brain regions during selective- and divided-attention conditions also suggests that preceptual judgements involve different neural systems, depending on attentional strategies.     

Neural evidence for representation-specific response selection

Response selection is the mental process of choosing representations for appropriate motor behaviors given particular environmental stimuli and one's current task situation and goals. Many cognitive theories of response selection postulate a unitary process. That is, one central response-selection mechanism chooses appropriate responses in most, if not all, task situations. However, neuroscience research shows that neural processing is often localized based on the type of information processed. Our current experiments investigate whether response selection is unitary or stimulus specific by manipulating response-selection difficulty in two functional magnetic resonance imaging experiments using spatial and nonspatial stimuli. The same participants were used in both experiments. We found spatial response selection involves the right prefrontal cortex, the bilateral premotor cortex, and the dorsal parietal cortical regions (precuneus and superior parietal lobule). Nonspatial response selection, conversely, involves the left prefrontal cortex and the more ventral posterior cortical regions (left middle temporal gyrus, left inferior parietal lobule, and right extrastriate cortex). Our brain activation data suggest a cognitive model for response selection in which different brain networks mediate the choice of appropriate responses for different types of stimuli. This model is consistent with behavioral research suggesting that response-selection processing may be more flexible and adaptive than originally proposed.     

Monitoring for target objects: activation of right frontal and parietal cortices with increasing time on task

The right prefrontal and parietal cortices have been implicated in attentional processing in both neuropsychological and functional neuroimaging literature. However, attention is a heterogeneous collection of processes, each of which may be underpinned by different neural networks. These attentional networks may interact, such that engaging one type of attentional process could influence the efficiency of another via overlapping neural substrates. We investigated the hypothesis that right frontal and parietal cortices provide the neuroanatomical location of the functional interaction between sustained attention and the process of selectively monitoring for target objects. Six healthy volunteers performed one of two tasks which required either selective or non-selective responding. The task lasted continuously for 18 min, during which time 3 Positron Emission Tomography (PET) scans were acquired for each task. This was repeated to obtain 12 PET measurements of regional cerebral blood flow (rCBF) for each subject. The right inferior frontal and parietal cortices were differentially activated by increasing time on task during the selective (S) vs non-selective (NS) task. Specifically, rCBF decreased with increasing time spent performing the NS task but not the S task. This result suggests that the normal deactivation in these areas as time on task increases is counteracted by the extra cognitive demands of selectively responding to target objects. Therefore, we have confirmed our hypothesis that right frontal and parietal cortices provide the neuroanatomical location for the modulation of object selection by sustained attention. We also identified the neuroanatomical correlates of each process separately, and confirmed earlier reports of prefrontal cortex and anterior cingulate activation associated with selective responding, and a fronto-parietal-thalamic network associated with sustained attention.     

Dissociable roles of the human somatosensory and superior temporal cortices for processing social face signals

Faces are multi-dimensional stimuli bearing important social signals, such as gaze direction and emotion expression. To test whether perception of these two facial attributes recruits distinct cortical areas within the right hemisphere, we used single-pulse transcranial magnetic stimulation (TMS) in healthy volunteers while they performed two different tasks on the same face stimuli. In each task, two successive faces were presented with varying eye-gaze directions and emotional expressions, separated by a short interval of random duration. TMS was applied over either the right somatosensory cortex or the right superior lateral temporal cortex, 100 or 200 ms after presentation of the second face stimulus. Participants performed a speeded matching task on the second face during one of two possible conditions, requiring judgements about either gaze direction or emotion expression (same/different as the first face). Our results reveal a significant task-stimulation site interaction, indicating a selective TMS-related interference following stimulations of somatosensory cortex during the emotional expression task. Conversely, TMS of the superior lateral temporal cortex selectively interfered with the gaze direction task. We also found that the interference effect was specific to the stimulus content in each condition, affecting judgements of gaze shifts (not static eye positions) with TMS over the right superior temporal cortex, and judgements of fearful expressions (not happy expressions) with TMS over the right somatosensory cortex. These results provide for the first time a double dissociation in normal subjects during social face recognition, due to transient disruption of non-overlapping brain regions. The present study supports a critical role of the somatosensory and superior lateral temporal regions in the perception of fear expression and gaze shift in seen faces, respectively.     

An event-related functional magnetic resonance imaging study of an auditory oddball task in schizophrenia

Schizophrenia is a diffuse brain disease that affects many facets of cognitive function. One of the most replicated findings in the neurobiology of schizophrenia is that the event-related potentials to auditory oddball stimuli are abnormal, effects believed to be related to abnormalities in attentional and memory processes. Although event-related potentials provide excellent resolution regarding the time course of information processing, such studies are poor at characterizing the spatial location of these abnormalities. To address this issue, we used event-related functional magnetic resonance imaging techniques to elucidate the neural areas underlying target detection in schizophrenia. Consistent with recent event-related functional magnetic resonance imaging results, target processing by control participants was associated with bilateral activation in the anterior superior temporal gyri, inferior and superior parietal lobules, and activation in anterior and posterior cingulate, thalamus, and right lateral frontal cortex. For the schizophrenic patients, selective deficits were observed in both the extent and strength of activation associated with target processing in the right lateral frontal cortex, thalamus, bilateral anterior superior temporal gyrus, anterior and posterior cingulate, and right inferior and superior parietal lobules. These findings are consistent with the evidence for abnormal processing of oddball stimuli suggested by event-related potential studies in schizophrenic patients, but provide much more detailed evidence regarding the anatomical sites implicated. These data are consistent with the hypothesis that schizophrenia is characterized by a widespread pathological process affecting many cerebral areas, including association cortex and thalamus.     

What is "odd" in Posner's location-cueing paradigm? Neural responses to unexpected location and feature changes compared

Within the parietal cortex, the temporo-parietal junction (TPJ) and the intraparietal sulcus (IPS) seem to be involved in both spatial and nonspatial functions: Both areas are activated when misleading information is provided by invalid spatial cues in Posner's location-cueing paradigm, but also when infrequent deviant stimuli are presented within a series of standard events. In the present study, we used functional magnetic resonance imaging to investigate the distinct and shared brain responses to (i) invalidly cued targets requiring attentional reorienting, and (ii) to target stimuli deviating in color and orientation leading to an oddball-like distraction effect. Both unexpected location and feature changes were accompanied by a significant slowing of manual reaction times. Bilateral TPJ and right superior parietal lobe (SPL) activation was observed in response to invalidly as compared to validly cued targets. In contrast, the bilateral inferior occipito-temporal cortex, the left inferior parietal cortex, right frontal areas, and the cerebellum showed stronger activation in response to deviant than to standard targets. Common activations were observed in the right angular gyrus along the IPS and in the right inferior frontal gyrus. We conclude that the superior parietal and temporo-parietal activations observed here as well as previously in location-cueing paradigms do not merely reflect the detection and processing of unexpected stimuli. Furthermore, our data suggest that the right IPS and the inferior frontal gyrus are involved in attentional selection and distractor processing of both spatial and nonspatial features.     

Hemispheric lateralization of cerebral blood-flow changes during selective listening to dichotically presented continuous speech

Regional cerebral blood flow (rCBF) was measured with positron emission tomography (PET) while subjects were selectively listening to continuous speech delivered to one ear and ignoring concurrent speech delivered to the opposite ear, as well as concurrent text or letter strings running on a screen. rCBF patterns associated with selective listening either to the left-ear or right-ear speech message were compared with each other and with rCBF patterns in two visual-attention conditions in which the subjects ignored both speech messages and either read the text or discriminated the meaningless letter strings moving on the screen. Attention to either speech message was associated with enhanced activity in the superior temporal cortex of the language-dominant left hemisphere, as well as in the superior and middle temporal cortex of the right hemisphere suggesting enhanced processing of prosodic features in the attended speech. Moreover, enhanced activity during attention to either speech message was observed in the right parietal areas known to have an important role in directing spatial attention. Evidence was also found for attentional tuning of the left and right auditory cortices to select information from the contralateral auditory hemispace.     

Habituation of attentional networks during emotion processing

Dysfunctional emotion processing is a key aspect of many neuropsychiatric disorders. This dysfunction may be due to an abnormal magnitude of neural substrate activation during emotion processing or due to an altered time course of the neural substrate response. To better understand the temporal characteristics of the neural substrate activation underlying implicit emotion processing, nine healthy female controls were repeatedly exposed to pictures of affective faces while performing a gender identification task in an fMRI. As the salience of the stimuli decreased with repeated exposure, brain areas implicated in a right hemispheric spatial attention network (including the posterior parietal cortex (BA 40) and the frontal eye fields (BA 6)) habituated while brain areas lateralized to the left hemisphere (including the angular gyrus (BA 39), posterior superior temporal gyrus (BA 39) and insula (BA 13)) sensitized. These results provide strong evidence that the time course of activation is a critical component when assessing the function of neural substrates underlying emotion processing (specifically whether habituation is altered) in neuro-psychiatric patients.     

Brain regions involved in human movement perception: a quantitative voxel-based meta-analysis

Face, hands, and body movements are powerful signals essential for social interactions. In the last 2 decades, a large number of brain imaging studies have explored the neural correlates of the perception of these signals. Formal synthesis is crucially needed, however, to extract the key circuits involved in human motion perception across the variety of paradigms and stimuli that have been used. Here, we used the activation likelihood estimation (ALE) meta-analysis approach with random effect analysis. We performed meta-analyses on three classes of biological motion: movement of the whole body, hands, and face. Additional analyses of studies of static faces or body stimuli and sub-analyses grouping experiments as a function of their control stimuli or task employed allowed us to identify main effects of movements and forms perception, as well as effects of task demand. In addition to specific features, all conditions showed convergence in occipito-temporal and fronto-parietal regions, but with different peak location and extent. The conjunction of the three ALE maps revealed convergence in all categories in a region of the right posterior superior temporal sulcus as well as in a bilateral region at the junction between middle temporal and lateral occipital gyri. Activation in these regions was not a function of attentional demand and was significant also when controlling for non-specific motion perception. This quantitative synthesis points towards a special role for posterior superior temporal sulcus for integrating human movement percept, and supports a specific representation for body parts in middle temporal, fusiform, precentral, and parietal areas.     

Neural networks of response shifting: influence of task speed and stimulus material

Functional magnetic resonance imaging (fMRI) was used in 14 healthy subjects to measure brain activation, while response shifting was performed. In the activation phase, subjects were asked to shift their attention between two different types of visually presented stimuli. In the baseline phase, subjects were required to attend to one stimulus type only. Subjects responded by pressing a left or right key according to the side of presentation of the target stimuli. In a verbal task, subjects were required to switch between letters and numbers. In a figural task, subjects reacted to round and square shapes. Stimuli were presented for 750 or 1500 ms. Response shifting revealed significantly increased activation compared to non-switching in the bilateral superior parietal cortex, right occipital cortex, left inferior frontal cortex, left and right striatum, and bilateral dorsolateral prefrontal cortex (DLPFC). Superior parietal and occipital cortex activation may be due to spatial analysis during response shifting. Subvocal rehearsal of the task instructions may have led to activation in the left inferior frontal cortex. Activation in the striatum was related to prefrontal activation and may represent the association between basal ganglia and prefrontal activation during executive control. However, the most important brain region involved in the execution of response shifting was the bilateral DLPFC. Higher task speed increased executive top-down attentional control and, therefore, significantly increased activity in the bilateral DLPFC. Brain activation did not differ significantly between verbal and figural stimulus material. This result suggests that brain activation in the present study illustrates the brain regions involved in the basic cognitive mechanisms of response shifting.     

Gating by induced Α–Γ asynchrony in selective attention

Visual selective attention operates through top–down mechanisms of signal enhancement and suppression, mediated by α‐band oscillations. The effects of such top–down signals on local processing in primary visual cortex (V1) remain poorly understood. In this work, we characterize the interplay between large‐scale interactions and local activity changes in V1 that orchestrates selective attention, using Granger‐causality and phase‐amplitude coupling (PAC) analysis of EEG source signals. The task required participants to either attend to or ignore oriented gratings. Results from time‐varying, directed connectivity analysis revealed frequency‐specific effects of attentional selection: bottom–up γ‐band influences from visual areas increased rapidly in response to attended stimuli while distributed top–down α‐band influences originated from parietal cortex in response to ignored stimuli. Importantly, the results revealed a critical interplay between top–down parietal signals and α–γ PAC in visual areas. Parietal α‐band influences disrupted the α–γ coupling in visual cortex, which in turn reduced the amount of γ‐band outflow from visual areas. Our results are a first demonstration of how directed interactions affect cross‐frequency coupling in downstream areas depending on task demands. These findings suggest that parietal cortex realizes selective attention by disrupting cross‐frequency coupling at target regions, which prevents them from propagating task‐irrelevant information.     

Neural activation during response competition

The flanker task, introduced by Eriksen and Eriksen [Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143--149], provides a means to selectively manipulate the presence or absence of response competition while keeping other task demands constant. We measured brain activity using functional magnetic resonance imaging (fMRI) during performance of the flanker task. In accordance with previous behavioral studies, trials in which the flanking stimuli indicated a different response than the central stimulus were performed significantly more slowly than trials in which all the stimuli indicated the same response. This reaction time effect was accompanied by increases in activity in four regions: the right ventrolateral prefrontal cortex, the supplementary motor area, the left superior parietal lobe, and the left anterior parietal cortex. The increases were not due to changes in stimulus complexity or the need to overcome previously learned associations between stimuli and responses. Correspondences between this study and other experiments manipulating response interference suggest that the frontal foci may be related to response inhibition processes whereas the posterior foci may be related to the activation of representations of the inappropriate responses.     

The parietal cortex and attentional modulations of activities of the visual cortex

We recorded high density event-related brain potentials (ERPs) from a patient with focal left parietal damage in a covert visual orienting task requiring detection of targets in the attended or unattended hemifield. A positivity peaking at 120 ms (P1) to the left visual field stimuli was enlarged when attended than unattended and was localized to the right extrastirate cortex. However, spatial attention did not influence the ERPs to the right visual field stimuli. The leftward cue elicited an enlarged P1 relative to the rightward cue. The results suggest that human parietal cortex is critical for the attentional modulation of the neural activities in the extrastriate cortex associated with stimuli in the contralateral hemifield.     

Attention-dependent suppression of distracter visual input can be cross-modally cued as indexed by anticipatory parieto-occipital alpha-band oscillations

Recent studies show that in addition to enhancing neural processing for attentionally relevant stimuli, selective attention also operates by suppressing the processing of distracter stimuli. When subjects are pre-cued to selectively deploy attention during voluntary (endogenous) attentional tasks, these mechanisms can be set up in advance of actual stimulus processing. That is, the brain can be placed in a biased attentional state. Two recent cueing studies have provided evidence for the deployment of such biased attentional states [J.J. Foxe, G.V. Simpson, S.P. Ahlfors, Neuroreport 9 (1998) 3929-3933; M.S. Worden, J.J. Foxe, N. Wang, G.V. Simpson, J. Neurosci. 20:RC63 (2000) 1-6]. Specifically, these studies implicated oscillatory activity in the alpha frequency-band (8-14 Hz) as an anticipatory mechanism for suppressing distracter visual stimulation. The current study extends these findings by showing that this alpha-suppressive effect is also invoked by cross-modal cues. Auditory symbolic cues were used in an intermodal attention task, to direct subjects' attention to a subsequent task in either the visual or auditory modality. Cueing attention to the auditory features of the imminent task stimuli resulted in significantly higher parieto-occipital alpha amplitude in the period preceding onset of this stimulus than when attention was cued to the visual features. Topographic mapping suggests that this effect is generated in regions of the inferior parietal cortex, areas that have been repeatedly implicated in the engagement and maintenance of visual attention. Taken together, the results of this series of studies suggest that these parietal regions are capable of integrating sensory cues from multiple sensory modalities in order to program the subsequent deployment of visual attention.     

Unilateral attention deficits and hemispheric asymmetries in the control of visual attention

The present study sought to determine the roles of the two hemispheres in arousal and the selective components of attention. Ten patients with left and right parietal lesions and ten with left and right temporal lesions participated in the experiment. The hypothesis that posterior parietal lesions, whether left or right, cause two selective attentional deficits, namely, a reduced reactivity to stimuli in the visual field contralateral to the lesion, and a reduced reactivity to any stimulus which occupies a relative contralateral spatial position, was tested by asking the patient to tilt their head either to the left or to the right by 90 degrees and to respond to two stimuli displayed above and on either side of fixation mark. The arousal component of attention was studied by analysing the overall RT to visual stimuli independent of their spatial positions. The results showed that (1) patients with either left or right parietal damage are impaired in shifting attention from the ipsilateral to the contralateral visual field, and, within each visual field, in a direction contraversive to the lesion and (2) these two attentional deficits are more severe after right than after left parietal damage. Furthermore, the results show that the difficulty in maintaining a high level of alertness is a specific deficit of patients with right hemispheric lesion and not of patients with an extinction syndrome, insofar as there is no significant difference in overall RT between patients with parietal and temporal lesions.