Non-spatial extinction following lesions of the parietal lobe in humans

Changes in attentional breadth scale with the demands of Kanizsa-figure object completion-evidence from pupillometry

The present study investigated whether the integration of separate parts into a whole-object representation varies with the amount of available attentional resources. To this end, two experiments were performed, which required observers to maintain central fixation while searching in peripheral vision for a target among various distractor configurations. The target could either be a "grouped" whole-object Kanizsa figure, or an "ungrouped" configuration of identical figural parts, but which do not support object completion processes to the same extent. In the experiments, accuracies and changes in pupil size were assessed, with the latter reflecting a marker of the covert allocation of attention in the periphery. Experiment 1 revealed a performance benefit for grouped (relative to ungrouped) targets, which increased with decreasing distance from fixation. By contrast, search for ungrouped targets was comparably poor in accuracy without revealing any eccentricity-dependent variation. Moreover, measures of pupillary dilation mirrored this eccentricity-dependent advantage in localizing grouped targets. Next, in Experiment 2, an additional attention-demanding foveal task was introduced in order to further reduce the availability of attentional resources for the peripheral detection task. This additional task hampered performance overall, alongside with corresponding pupil size changes. However, there was still a substantial benefit for grouped over ungrouped targets in both the behavioral and the pupillometric data. This shows that perceptual grouping scales with the allocation of attention even when only residual attentional resources are available to trigger the representation of a complete (target) object, thus illustrating that object completion operates in the "near absence" of attention.

Perceptual Functioning

Perceptual disorders are not part of the diagnosis criteria for schizophrenia. Yet, a considerable amount of work has been conducted, especially on visual perception abnormalities, and there is little doubt that visual perception is altered in patients. There are several reasons why such perturbations are of interest in this pathology. They are observed during the prodromal phase of psychosis, they are related to the pathophysiology (clinical disorganization, disorders of the sense of self), and they are associated with neuronal connectivity disorders. Perturbations occur at different levels of processing and likely affect how patients interact and adapt to their surroundings. The literature has become very large, and here we try to summarize different models that have guided the exploration of perception in patients. We also illustrate several lines of research by showing how perception has been investigated and by discussing the interpretation of the results. In addition to discussing domains such as contrast sensitivity, masking, and visual grouping, we develop more recent fields like processing at the level of the retina, and the timing of perception.

Shared cognitive mechanisms involved in the processing of scene texture and scene shape

Recent research has demonstrated that the parahippocampal place area represents both the shape and texture features of scenes, with the importance of each feature varying according to perceived scene category. Namely, shape features are predominately more diagnostic to the processing of artificial human–made scenes, while shape and texture are equally diagnostic in natural scene processing. However, to date little is known regarding the degree of interactivity or independence observed in the processing of these scene features. Furthermore, manipulating the scope of visual attention (i.e., globally vs. locally) when processing ensembles of multiple objects—stimuli that share a functional neuroanatomical link with scenes—has been shown to affect their cognitive visual representation. It remains unknown whether manipulating the scope of attention impacts scene processing in a similar manner. Using the well-established Garner speeded-classification behavioral paradigm, we investigated the influence of both feature diagnosticity and the scope of visual attention on potential interactivity or independence in the shape and texture processing of artificial human–made scenes. The results revealed asymmetric interference between scene shape and texture processing, with the more diagnostic feature (i.e., shape) interfering with the less diagnostic feature (i.e., texture), but not vice versa. Furthermore, this interference was attenuated and enhanced with more local and global visual processing strategies, respectively. These findings suggest that the scene shape and texture processing are mediated by shared cognitive mechanisms and that, although these representations are governed primarily via feature diagnosticity, they can nevertheless be influenced by the scope of visual attention.

Bálint syndrome

This chapter starts by reviewing the various interpretations of Bálint syndrome over time. We then develop a novel integrative view in which we propose that the various symptoms, historically reported and labeled by various authors, result from a core mislocalization deficit. This idea is in accordance with our previous proposal that the core deficit of Bálint syndrome is attentional (Pisella et al., 2009, 2013, 2017) since covert attention improves spatial resolution in visual periphery (Yeshurun and Carrasco, 1998); a deficit of covert attention would thus increase spatial uncertainty and thereby impair both visual object identification and visuomotor accuracy. In peripheral vision, we perceive the intrinsic characteristics of the perceptual elements surrounding us, but not their precise localization (Rosenholtz et al., 2012a,b), such that without covert attention we cannot organize them to their respective and recognizable objects; this explains why perceptual symptoms (simultanagnosia, neglect) could result from visual mislocalization. The visuomotor symptoms (optic ataxia) can be accounted for by both visual and proprioceptive mislocalizations in an oculocentric reference frame, leading to field and hand effects, respectively. This new pathophysiological account is presented along with a model of posterior parietal cortex organization in which the superior part is devoted to covert attention, while the right inferior part is involved in visual remapping. When the right inferior parietal cortex is damaged, additional representational mislocalizations across saccades worsen the clinical picture of peripheral mislocalizations due to an impairment of covert attention.

Attention capture by salient object groupings in the neglected visual field

The integration of fragmentary parts into coherent whole objects has been proposed either to rely on the availability of attentional resources or to arise automatically, that is, from preattentive processing (prior to the engagement of selective attention). In the present study, these two alternative accounts were tested in a group of neglect patients with right-hemisphere parietal brain damage and associated deficits of selective attention in the left (visual) hemispace. The reported experiment employed a search task that required detection of targets in the left and/or right hemifields, which were embedded in configurations that consisted of variants of Kanizsa figures. The results showed that a salient, grouped Kanizsa triangle presented within the unattended, left hemifield can substantially improve contralesional target detection, though the very same triangle configuration does not facilitate target detection in the impaired hemifield when presented together with an ipsilesional, but non-salient (i.e., structurally non-integrated, isolated) target. That is, attention is captured by the grouped object in the impaired hemispace only when it is not engaged in the processing of an (isolated) object in the attended hemispace. This demonstrates that both part-to-whole-object integration and search guidance by salient, integrated objects crucially require attentional resources.

Topological difference between target and flankers alleviates crowding effect

In the crowding effect, object recognition in the periphery deteriorates when other items flank the target, especially if they share similarities. Here, we report that the similarity defined by topological property (differences in number of holes) influences the crowding effect. Orientation discrimination tasks suggested that the crowding effect was weaker with a topological different (TD) flanker than a topological equivalent (TE) flanker and an existing inward-outward anisotropy phenomenon. In another experiment, both an outer and an inner flanker were used to constitute four different conditions. The performance of an outer TD flanker and an inner TE flanker was superior to that of an outer TE flanker and an inner TD flanker, even though the items of the stimuli were the same. Different stimuli were used to control for local features. To eliminate the possible explanation of confusability, we selected pairs of letters with matched confusability, but one pair was TD and another was TE. The letter identification performance was better for the TD condition. Lastly, we investigated the digit identification under four conditions with varied spacing. Regardless of different spacing, the crowding effect was reduced by a topological different flanker. The results collectively suggest that topological property plays a role in the perceptual grouping, which modulates the crowding effect.

Feature-based attentional selection affects the perceived duration of a stimulus having two superposed patterns

The perceived duration of a visual event is highly related to stimulus attributes. It is well known that a moving stimulus appears to last longer than a static one does. Previous studies have demonstrated that the time dilation in a moving stimulus can be influenced by perceived motion, rather than by mere physical motion, and that a faster motion appears to last longer than a slower one does. However, whether a top-down attentional set for the feature value can modulate the time dilation in a moving stimulus when two different visual patterns coexist within the same region of the visual field is still unknown. To test this, in Experiment 1, we presented a moving and a static random-dot pattern simultaneously within the same region, and instructed the observer to attend to one of these two patterns. The results demonstrate that perceived duration was longer when attention was directed to the moving, rather than static pattern, although both patterns physically coexisted at the same time and place and for the same duration. In Experiment 2, slow and/or fast moving patterns were presented at the same time and place, and again, feature-based attentional selection affected the perceived duration of the identical physical display. These results suggest that attention to a moving stimulus is an essential factor that determines the time dilation in a moving stimulus. This study revealed that feature-based attention, as opposed to location-based attention, plays an important role in motion-induced time dilation.

A Tale of Two Visual Systems: Invariant and Adaptive Visual Information Representations in the Primate Brain

Visual information processing contains two opposite needs. There is both a need to comprehend the richness of the visual world and a need to extract only pertinent visual information to guide thoughts and behavior at a given moment. I argue that these two aspects of visual processing are mediated by two complementary visual systems in the primate brain-specifically, the occipitotemporal cortex (OTC) and the posterior parietal cortex (PPC). The role of OTC in visual processing has been documented extensively by decades of neuroscience research. I review here recent evidence from human imaging and monkey neurophysiology studies to highlight the role of PPC in adaptive visual processing. I first document the diverse array of visual representations found in PPC. I then describe the adaptive nature of visual representation in PPC by contrasting visual processing in OTC and PPC and by showing that visual representations in PPC largely originate from OTC.

Ventral lateral parietal cortex and episodic memory retrieval

With the advent of functional neuroimaging it quickly became apparent that successful episodic memory retrieval was consistently associated with enhanced activity in ventral lateral parietal cortex (VLPC), especially the left angular gyrus. Here, we selectively review recent neuropsychological and functional neuroimaging evidence relevant to the question of the functional significance of this activity. We argue that the balance of the evidence suggests that the angular gyrus supports the representation of retrieved episodic information, and that this likely reflects a more general role for the region in representing multi-modal and multi-domain information.

Transcranial direct current stimulation (tDCS) reveals a dissociation between SNARC and MARC effects: Implication for the polarity correspondence account

The concept of stimulus response compatibility (SRC) refers to the existence of a privileged association between a specific stimulus feature and a specific response feature. Two examples of SRC are the Spatial Numerical Association of Response Codes (SNARC) and the Markedness Association of Response Codes (MARC) effects. According to the polarity correspondence principle, these two SRC effects occur because of a match between the most salient dimensions of stimulus and response. Specifically, the SNARC effect would be caused by a match between right-sided responses and large numbers, while a match between right-sided responses and even numbers would give rise to the MARC effect. The aim of the present study was to test the validity of the polarity correspondence principle in explaining these two SRC effects. To this end, we applied transcranial direct current stimulation (tDCS) over left and right posterior parietal cortex (PPC), which is thought to be the neural basis of salience processing, during a parity judgement task. Results showed that cathodal tDCS over the PPC significantly reduced the MARC effect but did not affect the SNARC effect, suggesting a dissociation between the two effects. That is, the MARC would rely on a salience processing mechanism, whereas the SNARC would not. Despite this interpretation is in need of further experimental confirmations (i.e., testing different tasks or using different tDCS montages), our results suggest that the polarity correspondence principle can be a plausible explanation only for the MARC effect but not for the SNARC effect.

Feature Confirmation in Object Perception: Feature Integration Theory 26 Years on from the Treisman Bartlett Lecture

The Treisman Bartlett lecture, reported in the Quarterly Journal of Experimental Psychology in 1988, provided a major overview of the feature integration theory of attention. This has continued to be a dominant account of human visual attention to this day. The current paper provides a summary of the work reported in the lecture and an update on critical aspects of the theory as applied to visual object perception. The paper highlights the emergence of findings that pose significant challenges to the theory and which suggest that revisions are required that allow for (a) several rather than a single form of feature integration, (b) some forms of feature integration to operate preattentively, (c) stored knowledge about single objects and interactions between objects to modulate perceptual integration, (d) the application of feature-based inhibition to object files where visual features are specified, which generates feature-based spreading suppression and scene segmentation, and (e) a role for attention in feature confirmation rather than feature integration in visual selection. A feature confirmation account of attention in object perception is outlined.

Differences Between Objects and Nonobjects in Visual Extinction: A Competition for Attention

We used visual extinction as a method for investigating the influence of objects on selective visual attention Two subjects demonstrating extinction following damage to right parietal cortex were tested in a simple detection task The task required only that subjects detect the presence or absence of any item in the left and right visual fields Both subjects showed an advantage for good objects in the impaired field However, this advantage for good objects held only in the presence of a simultaneous stimulus in the undamaged field We suggest that objects are not simply easier to detect than nonobjects, instead we argue that objects are stronger competitors for attentional selection

Abnormal Center–Periphery Gradient in Spatial Attention in Simultanagnosia

Patients suffering from simultanagnosia cannot perceive more than one object at a time. The underlying mechanism is incompletely understood. One hypothesis is that simultanagnosia reflects “tunnel vision,” a constricted attention window around gaze, which precludes the grouping of individual objects. Although this idea has a long history in neuropsychology, the question whether the patients indeed have an abnormal attention gradient around the gaze has so far not been addressed. Here we tested this hypothesis in two simultanagnosia patients with bilateral parieto-occipital lesions and two control groups, with and without brain damage. We assessed the participants' ability to discriminate letters presented briefly at fixation with and without a peripheral distractor or in the visual periphery, with or without a foveal distractor. A constricted span of attention around gaze would predict an increased susceptibility to foveated versus peripheral distractors. Contrary to this prediction and unlike both control groups, the patients' ability to discriminate the target decreased more in the presence of peripheral compared with foveated distractors. Thus, the attentional spotlight in simultanagnosia does not fall on foveated objects as previously assumed, but rather abnormally highlights the periphery. Furthermore, we found the same center–periphery gradient in the patients' ability to recognize multiple objects. They detected multiple, but not single objects more accurately in the periphery than at fixation. These results suggest that an abnormal allocation of attention around the gaze can disrupt the grouping of individual objects into an integrated visual scene.

The neural coding of creative idea generation across adolescence and early adulthood

Creativity is considered key to human prosperity, yet the neurocognitive principles underlying creative performance, and their development, are still poorly understood. To fill this void, we examined the neural correlates of divergent thinking in adults (25–30 years) and adolescents (15–17 years). Participants generated alternative uses (AU) or ordinary characteristics (OC) for common objects while brain activity was assessed using fMRI. Adults outperformed adolescents on the number of solutions for AU and OC trials. Contrasting neural activity for AU with OC trials revealed increased recruitment of left angular gyrus, left supramarginal gyrus, and bilateral middle temporal gyrus in both adults and adolescents. When only trials with multiple AU were included in the analysis, participants showed additional left inferior frontal gyrus (IFG)/middle frontal gyrus (MFG) activation for AU compared to OC trials. Correspondingly, individual difference analyses showed a positive correlation between activations for AU relative to OC trials in left IFG/MFG and divergent thinking performance and activations were more pronounced in adults than in adolescents. Taken together, the results of this study demonstrated that creative idea generation involves recruitment of mainly left lateralized parietal and temporal brain regions. Generating multiple creative ideas, a hallmark of divergent thinking, shows additional lateral PFC activation that is not yet optimized in adolescence.

The Interaction of Spatial and Object Pathways: Evidence from Balint's Syndrome

An earlier report described a patient (RM) with bilateral parietal damage who showed severe binding problems between shape and color and shape and size (Friedman-Hill, Robertson, & Treisman, 1995). When shown two different-colored letters, RM reported a large number of illusory conjunctions (ICs) combining the shape of one letter with the color of the other, even when he was looking directly at one of them and had as long as 10 sec to respond. The lesions also produced severe deficits in locating and reaching for objects, and difficulty in seeing more than one object at a time, resulting in a neuropsychological diagnosis of Balint's syndrome or dorsal simultanagnosia. The pattern of deficits supported predictions of Treisman's Feature Integration Theory (FIT) that the loss of spatial information would lead to binding errors. They further suggested that the spatial information used in binding depends on intact parietal function. In the present paper we extend these findings and examine other deficits in RM that would be predicted by FIT. We show that: (1) Object individuation is impaired, making it impossible for him correctly to count more than one or two objects, even when he is aware that more are present. (2) Visual search for a target defined by a conjunction of features (requiring binding) is impaired, while the detection of a target defined by a unique feature is not. Search for the absence of a feature (0 among Qs) is also severely impaired, while search for the presence (Q among 0s) is not. Feature absence can only be detected when all the present features are bound to the nontarget items. (3) RM's deficits cannot be attributed to a general binding problem: binding errors were far more likely with simultaneous presentation where spatial information was required than with sequential presentation where time could be used as the medium for binding. (4) Selection for attention was severely impaired, whether it was based on the position of a marker or on some other feature (color). (5) Spatial information seems to exist that RM cannot access, suggesting that feature binding relies on a relatively late stage where implicit spatial information is made explicitly accessible. The data converge to support our conclusions that explicit spatial knowledge is necessary for the perception of accurately bound features, for accurate attentional selection, and for accurate and rapid search for a conjunction of features in a multiitem display. It is obviously necessary for directing attention to spatial locations, but the consequences of impairments in this ability seem also to affect object selection, object individuation, and feature integration. Thus, the functional effects of parietal damage are not limited to the spatial and attentional problems that have long been described in patients with Balint's syndrome. Damage to parietal areas also affects object perception through damage to spatial representations that are fundamental for spatial awareness.

Impaired texture segregation but spared contour integration following damage to right posterior parietal cortex

We examined the relations between texture segregation and contour integration in patients with deficits in spatial attention leading to left or right hemisphere extinction. Patients and control participants were presented with texture and contour stimuli consisting of oriented elements. We induced regularity in the stimuli by manipulating the element orientations resulting in an implicit texture border or explicit contour. Participants had to discriminate curved from straight shapes without making eye movements, while the stimulus presentation time was varied using a QUEST procedure. The results showed that only patients with right hemisphere extinction had a spatial bias, needing a longer presentation time to determine the shape of the border or contour on the contralesional side, especially for borders defined by texture. These results indicate that texture segregation is modulated by attention-related brain areas in the right posterior parietal cortex.

A world unglued: simultanagnosia as a spatial restriction of attention

Simultanagnosia is a disorder of visual attention that leaves a patient's world unglued: scenes and objects are perceived in a piecemeal manner. It is generally agreed that simultanagnosia is related to an impairment of attention, but it is unclear whether this impairment is object- or space-based in nature. We first consider the findings that support a concept of simultanagnosia as deficit of object-based attention. We then examine the evidence suggesting that simultanagnosia results from damage to a space-based attentional system, and in particular a model of simultanagnosia as a narrowed spatial window of attention. We ask whether seemingly object-based deficits can be explained by space-based mechanisms, and consider the evidence that object processing influences spatial deficits in this condition. Finally, we discuss limitations of a space-based attentional explanation.

Implicit location encoding via stored representations of familiar objects: neuropsychological evidence

We report data on the visual localisation ability of a patient with Balint's syndrome, GK. We show that, with relatively long exposures of displays, GK is better able to judge the spatial relations between separate objects (a "between-object judgement") than the spatial relations between a part and a whole object (a "within-object judgement") (Experiments 1-3). This dissociation occurred even when the same stimulus was used for both judgements, and the task instructions biased GK to parse the stimulus as either a single or as two separate objects (Experiments 2 and 6). However, when he could use a stored representation to make a within-object judgement, then performance was better than on a comparable spatial judgement of the relations between two separate objects (Experiments 4-7). The data demonstrate that stored representations of objects can support the spatial coding of parts to perceptual wholes. In the absence of stored representations, part-whole relations must be explicitly coded by attention, a process that is impaired in this patient.

Direct current stimulation (tDCS) reveals parietal asymmetry in local/global and salience-based selection

Data from neuropsychology and neuroimaging studies indicate hemispheric asymmetries in processing object's global form versus local parts. However the attentional mechanisms subtending visual selection of different levels of information are poorly understood. The classical left hemisphere/local-right hemisphere/global dichotomy has been recently challenged by studies linking the asymmetry of activation in the posterior parietal cortex (PPC) with the relative salience of the stimulus rather than with the local/global level. The present study aimed to assess hemispheric asymmetry in local-global and salience-based selection in hierarchical stimuli by using transcranial direct current stimulation (tDCS). To this end, tDCS has been applied to the PPC of both the hemispheres. Our data revealed that tDCS did affect the selection of the target on the basis of its relative salience in a manner that depended on the tDCS polarity applied to the two hemispheres. This result is in line with previous findings that the left PPC is critically involved in attention for low-salience stimuli in the presence of high-salience distractor information, while right PPC is involved in attending to more salient stimuli. Hemispheric asymmetries were also found in local/global selection. Overall the results suggest that neural activation in the PPC is related to both the salience and the level of stimulus representations mediating responses to hierarchical stimuli. The comparison of the results from Experiments 1 and 2 in local/global-based selection suggests that the effect of stimulation could be completely opposite depending on subtle differences in demands of attentional control (sustained attention vs task switching).

Dissecting the component deficits of perceptual imbalance in visual neglect: evidence from horizontal-vertical length comparisons

INTRODUCTION

Signs of left unilateral neglect often occur after damage to the right hemisphere and entail a left-right imbalance in stimulus processing. Typically, neglect patients deviate rightward when bisecting lines. An underestimation of the left portion of the line and/or a right overestimation could explain this effect.

METHODS

Here we dissected their respective contribution by asking participants to compare a vertical segment to a horizontal segment, either on the left or on the right. We also tested whether neglect patients exhibited the symmetry law, whereby normal participants underestimate symmetrically bisected lines as compared to asymmetrically bisected lines.

RESULTS

Controls and patients underestimated symmetric figures. Depending on the degree of left-right horizontal competition, neglect patients underestimated left-sided stimuli or overestimated right-sided stimuli.

CONCLUSIONS

We concluded that two independent deficits contribute to neglect signs: a deficit in attentional orienting to the left, which can be worsened by left hemianopia, perhaps depending on impaired functioning of right-hemisphere attentional networks, and a tendency for attention to be captured by right-sided stimuli, possibly resulting from the activity of an isolated left hemisphere. Finally, the symmetry law was preserved in neglect patients, and thus appears to be driven by pre-attentive mechanisms.

Neuropsychological evidence for a competitive bias against contracting stimuli

Two experiments examined extinction to stimuli presented either with contracting or expanding motion. Experiment 1 used solid shapes which either increased or decreased in size rapidly, consistent with looming motion. Experiment 2 employed random dots so that stimulus size was not confounded with type of motion. In both experiments extinction was modulated by the type of motion presented, with extinction most evident when a contracting object was in the weaker visual field. In addition, in Experiment 2 there was evidence for grouping modulating extinction, when there were looming stimuli in both fields. The results suggest that looming motion is a powerful determinant of stimulus salience in selective attention.

Assessing the weights of visual neglect: a new approach to dissociate defective symptoms from productive phenomena in length estimation

Right hemisphere damage often provokes signs of visual neglect, characterized by a prominent left-right imbalance in information processing. Neglect patients are biased towards right-sided objects and ignore left-sided events. Left-right imbalance may not only result from left neglect but also from right attraction, which have been considered as, respectively, defective and productive phenomena in neglect patients. However, the relative contributions of these two mechanisms to the final left-right imbalance remain uncertain. Using a novel experimental paradigm, we were able to separately test the contribution of left neglect and right attraction to neglect behavior. We used horizontal and vertical lines implemented in L shapes in a line extension task. The use of L shapes oriented either to the left or to the right made it possible to measure the left bias by comparing the length of a left-sided horizontal line to that of a centered vertical line, and to measure the right bias by comparing a centered vertical to a rightwards horizontal line. Our results showed that, in this experimental set, the left-right discrepancy is supported more by left neglect than by right attraction, with important implications about the role of left-right competition on the deployment of left neglect and right attraction.

The neural mechanisms of visual selection: the view from neuropsychology

In this review, we discuss how neuropsychological impairments in visual selection can inform us about how selection normally operates. Using neuroanatomical and behavioral evidence on the disorders of neglect, extinction, and simultanagnosia, we propose functional and anatomical links between different aspects of visual selection and distinct sites in the posterior parietal cortex (PPC). This includes linking: (i) bottom-up attentional capture and the right temporo-parietal junction (TPJ); (ii) top-down segmentation of displays and the medial PPC; (iii) grouping, individuation and identification, and the inferior intra-parietal sulcus (IPS) bilaterally; and (iv) the suppression of saliency and the left IPS. In addition, when neuropsychological studies are combined with fMRI, there is evidence that these regions of the PPC interact with striate and extra-striate cortical areas, which respond to specific properties of stimuli. Selection should be viewed as an emergent property of a network of areas involving both ventral and dorsal cortex.

The Left Intraparietal Sulcus Modulates the Selection of Low Salient Stimuli

Neuropsychological and functional imaging studies have suggested a general right hemisphere advantage for processing global visual information and a left hemisphere advantage for processing local information. In contrast, a recent transcranial magnetic stimulation study [Mevorach, C., Humphreys, G. W., & Shalev, L. Opposite biases in salience-based selection for the left and right posterior parietal cortex. Nature Neuroscience, 9, 740–742, 2006b] demonstrated that functional lateralization of selection in the parietal cortices on the basis of the relative salience of stimuli might provide an alternative explanation for previous results. In the present study, we applied a whole-brain analysis of the functional magnetic resonance signal when participants responded to either the local or the global levels of hierarchical figures. The task (respond to local or global) was crossed with the saliency of the target level (local salient, global salient) to provide, for the first time, a direct contrast between brain activation related to the stimulus level and that related to relative saliency. We found evidence for lateralization of salience-based selection but not for selection based on the level of processing. Activation along the left intraparietal sulcus (IPS) was found when a low saliency stimulus had to be selected irrespective of its level. A control task showed that this was not simply an effect of task difficulty. The data suggest a specific role for regions along the left IPS in salience-based selection, supporting the argument that previous reports of lateralized responses to local and global stimuli were contaminated by effects of saliency.

Disorders of visual attention and the posterior parietal cortex

Traditionally, both the monkey and human posterior parietal cortex (PPC) have been considered to have a privileged role in spatial perception or action. Lesions to this region of the human brain, particularly of the right hemisphere, undoubtedly lead to spatially lateralised deficits such as visual extinction or neglect. However, although studies in monkeys have revealed much about the spatial functions of the parietal lobe, the monkey PPC may not be a good model system with which to understand fully the disorders of attention that follow damage to the human parietal cortex. Several lines of evidence, from functional imaging as well as investigations of patients with parietal damage, demonstrate that parts of the human inferior parietal lobe (IPL) have non-spatial functions. Here, we argue that it is important to distinguish spatially lateralised from spatial deficits. Both spatial and non-spatial impairments might, in principle, contribute to a spatially lateralised behavioural syndrome such as neglect. In this review, we discuss the evidence for such a proposal and suggest that a better understanding of human parietal syndromes may emerge from considering both the spatial and non-spatial functions of this region.

Opposite biases in salience-based selection for the left and right posterior parietal cortex

Visual selection is determined in part by the saliency of stimuli. We assessed the brain mechanisms determining attentional responses to saliency. Repetitive transcranial magnetic stimulation (rTMS) was applied to the left and right posterior parietal cortices (PPC) immediately before participants were asked to identify a compound letter. rTMS to the right PPC disrupted the guidance of attention toward salient stimuli, whereas rTMS to the left PPC affected the ability to bias selection away from salient stimuli. We conclude that right and left PPC have opposite roles in biasing selection to and from salient stimuli in the environment.

Competition between simultaneous stimuli modulated by location probability in hemispatial neglect

Many aspects of spatial neglect can be explained as arising from competition for attentional selection, with salient ipsilesional stimuli emerging as the winner more often than contralesional stimuli. The outcome of the competition, however, can be affected both by bottom-up perceptual factors such as the gestalt properties of the display and by top-down factors such as expectancy or stimulus blocking. This study examines whether the competition for attentional selection can be modulated by manipulating the probability of the target's location in hemispatial neglect. Five patients with left-sided hemispatial neglect and a group of control participants performed a visual target discrimination task. In equal probability blocks, the target appeared randomly in any of six possible horizontal locations (three left, three right) whereas in biased blocks, the target appeared in the mid-location on the left on 50% of the trials and in each of the other locations on 10% of the trials. The target appeared either alone or was accompanied by a distractor on the opposite side. The results showed that the spatial bias facilitated detection of all left-sided targets in the neglect group, but was more spatially specific in the control group. Furthermore, while distractors on either side interfered with target processing in both groups, the patterns differed across the visual field. Finally, the magnitude of facilitation due to the bias was greatest in the condition with the most inhibition, i.e. a left-sided target accompanied by a right-sided distractor in the neglect group. These data underscore the competitive push-pull relationship between different bottom-up and top-down attentional factors, particularly within neglect patients, in whom a strong ipsilesional attentional bias already exists.

Attentional responses to unattended stimuli in human parietal cortex

Right-sided parietal lesions lead to lateralized attentional deficits which are most prominent with bilateral stimulation. We determined how an irrelevant stimulus in the unattended hemifield alters attentional responses in parietal cortex during unilateral orienting. A trial consisted of a central spatial cue, a delay and a test phase during which a grating was presented at 9 degrees eccentricity. Subjects had to discriminate the orientation of the grating. The unattended hemifield was either empty or contained a second, irrelevant grating. We carried out a series of functional MRI (fMRI) studies in 35 healthy volunteers (13 men and 22 women, aged between 19 and 30 years) as well as a behavioural and structural lesion mapping study in 17 right-hemispheric lesion patients, 11 of whom had neglect. In the patients with but not in those without neglect, the addition of a distractor in the unattended hemifield significantly impaired performance if attention was directed contralesionally but not if it was directed ipsilesionally. In the healthy volunteers, we discerned two functionally distinct areas along the posterior-anterior axis of the intraparietal sulcus (IPS). The posterior, descending IPS segment in both hemispheres showed attentional enhancement of responses during contralateral attentional orienting and was unaffected by the presence of an irrelevant stimulus in the ignored hemifield. In contrast, the right-sided horizontal IPS segment showed a strong attentional response when subjects oriented to a stimulus in the relevant hemifield and an irrelevant stimulus was simultaneously present in the ignored hemifield, compared with unilateral stimulation. This effect was independent of the direction of attention. The symmetrical left-sided horizontal IPS segment showed the highest responses under the same circumstances, in combination with a contralateral bias during unilateral stimulation conditions. None of the six patients without neglect had a lesion of the horizontal IPS segment. In four of the 11 neglect patients, the lesion overlapped with the horizontal IPS activity cluster and lay in close proximity to it in another four. The remaining three patients had a lesion at a distance from the parietal cortex. Our findings reconcile the role of the IPS in endogenous attentional control with the clinically significant interaction between direction of attention and bilateral stimulation in right parietal lesion patients. Functional imaging in neglect patients will be necessary to assess IPS function in those cases where the structural lesion spares the middle IPS segment.

A common parieto-frontal network is recruited under both low visibility and high perceptual interference conditions

A fundamental property of visual attention is to select targets from interfering distractors. However, attention can also facilitate the detectability of near-threshold items presented in isolation. The extent to which these two perceptually challenging conditions are resolved by the same neural mechanisms is not well known. In the present event-related fMRI experiment, subjects performed a letter identification task under two perceptually challenging conditions; when the luminance contrast of a target letter was reduced (perceptual visibility manipulation) and when the target letter was flanked by distractors (perceptual interference manipulation). Perceptual interference recruited the right parietal and mid-lateral frontal cortex, while perceptual visibility activated these regions bilaterally. The overlap of activated areas between the two perceptual manipulations suggests that a single parieto-frontal network is summoned under both perceptual visibility and interference conditions.

Attention modulates the visual field in healthy observers and parietal patients

Recent attention research suggests that factors other than low-level sensory processes modulate perception across the visual field, with right parieto-temporal cortex playing a critical role in directing visual attention to peripheral events. Here we examine how different degrees of attentional demand at fixation dynamically affect detection of abrupt visual onsets in the periphery. In young healthy subjects, peripheral detection was significantly disrupted bilaterally when there was high attention demand at fixation. Right parieto-temporal lesioned patients, tested with a simplified version of task, demonstrated bilateral shrinkage of their available visual field, worse to the contralesional side, under increased attentional demand at fixation. These findings demonstrate how the effective visual field is dynamically modulated by the deployment of attention in health and, more severely, following right parieto-temporal damage.

Bare hands and attention: evidence for a tactile representation of the human body

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or "wand", one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand. To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB's left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC's ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.