Patients with Balint' s syndrome are typically impaired at perceiving multiple objects simultaneously, and at evaluating the relationship between multiple objects in a scene (simultanagnosia). These deficits may not only be observed in complex scenes, but also when local elements of individual objects must be integrated into a perceptual global whole. Thus, unlike normal observers, patients with simultanagnosia typically show a bias towards the local forms, even to the extent that they cannot identify the global stimuli. However, we have previously shown that global processing is still attainable in Balint patients in certain scenarios (e.g., when local elements are unfamiliar). This suggests that in addition to a possible perceptual deficit that favors the local elements in these patients, impaired attentional control may be at the core of their unique performance. To test this hypothesis we manipulated the perceptual saliency of the local and global elements in a compound letter task so that it included global-more-salient or local-more-salient displays. We show that a Balint patient was able to accurately identify both global and local targets as long as they were the salient aspect of the compound letter. However, substantial impairment was evident when either the global or local elements were the less salient aspect of the compound letter. We conclude that in Balint' s syndrome there is a failure of flexible top-down attention both in biasing attention away from salient irrelevant aspects of the display (salience-based-selection) and in impaired disengagement from irrelevant but salient items once they have been selected.
We review neuropsychological evidence for visual selection operating in different reference frames. There is general agreement that there may be a separation of coding space near to and farther from the body, and that deficits in selecting stimuli within each form of spatial representation may be impaired in patients with unilateral neglect. However, there remains a lack of consensus about whether all forms of spatial representation relate to the body or whether there are spatial representations based on reference frames abstracted from the body (allocentric and object-centered spatial codes). Here we will review the evidence for spatial coding in these more abstracted reference frames (allocentric and object-centered but also environmental) and argue for the psychological reality of (at least) allocentric spatial coding. We discuss computational accounts of how such codes may be created as objects are selected.
Physiological evidence indicates that different visual features are computed quasi-independently. The subsequent step of binding features, to generate coherent perception, is typically considered a major rate-limiting process, confined to one location at a time and taking 25 ms per item or longer (A. Treisman & S. Gormican, 1988, Feature analysis in early vision: Evidence from search asymmetries, Psychological Review, Vol. 95, pp. 15-48). We examined whether these processing limitations remain once bindings are learned for familiar objects. Participants searched for objects that could appear either in familiar or unfamiliar colors. Objects in familiar colors were detected efficiently at rates consistent with simultaneous binding across multiple stimuli. Processing limitations were evident for objects in unfamiliar colors. The advantage for the learned color for known targets was eliminated when participants searched for geometric shapes carrying the object colors and when the colors fell in local background areas around the shapes. The effect occurred irrespective of whether the nontargets had familiar colors, but was largest when nontargets had incorrect colors. The efficient search for targets in familiar colors held, even when the search was biased to favor objects in unfamiliar colors. The data indicate that learned bindings can be computed with minimal attentional limitations, consistent with the direct activation of learned conjunctive representations in vision.
Motor extinction refers to a deficit of motor production on the side opposite a brain lesion that either only becomes apparent or disproportionately worsens during bilateral motor activity. It may arise due either to a contralesional deficit in setting the motor activation level (an intentional deficit) or a deficit in contralesional awareness of the sensory consequences of movement (an attentional deficit). In this study, we investigate the nature of motor extinction in a patient (LR) with a right fronto-temporal lesion through the kinematic analysis of unimanual and bimanual circle-drawing movements. While the ipsi- and contralesional limbs performed comparably for unimanual movements, the contralesional limb demonstrated marked bradykinesia and hypometria during bimanual movements. Furthermore, these deficits were not overcome when visual feedback of the contralesional limb was provided (Experiment 1). However, when performing bimanual movements in the presence of a visual template (Experiment 2), LR was able to overcome the contralesional hypometria but not the bradykinesia which proved intractable across both experiments. Both the bradykinesia and hypometria could result from an intentional deficit of motor production. However, in Experiment 2, LR also demonstrated an abnormal level of positional drift in the contralesional limb for bimanual movements indicative of an additional attentional deficit. We conclude that LRs presentation of motor extinction is the result of a primary intentional deficit and a secondary attentional deficit.
Actions taking place in the environment are critical for our survival. We review evidence on attention to action, drawing on sets of converging evidence from neuropsychological patients through to studies of the time course and neural locus of action-based cueing of attention in normal observers. We show that the presence of action relations between stimuli helps reduce visual extinction in patients with limited attention to the contralesional side of space, while the first saccades made by normal observers and early perceptual and attentional responses measured using electroencephalography/event-related potentials are modulated by preparation of action and by seeing objects being grasped correctly or incorrectly for action. With both normal observers and patients, there is evidence for two components to these effects based on both visual perceptual and motor-based responses. While the perceptual responses reflect factors such as the visual familiarity of the action-related information, the motor response component is determined by factors such as the alignment of the objects with the observers effectors and not by the visual familiarity of the stimuli. In addition to this, we suggest that action relations between stimuli can be coded pre-attentively, in the absence of attention to the stimulus, and action relations cue perceptual and motor responses rapidly and automatically. At present, formal theories of visual attention are not set up to account for these action-related effects; we suggest ways that theories could be expected to enable action effects to be incorporated.
Because of our limited processing capacity, different elements of the visual scene compete for the allocation of processing resources. One of the most striking deficits in visual selection is simultanagnosia, a rare neuropsychological condition characterized by impaired spatial awareness of more than one object at time. To decompose the neuroanatomical substrates of the syndrome and to gain insights into the structural and functional organization of visuospatial attention, we performed a systematic evaluation of lesion patterns in a group of simultanagnosic patients compared with patients with either (i) unilateral visuospatial deficits (neglect and/or extinction) or (ii) bilateral posterior lesions without visuospatial deficits, using overlap/subtraction analyses, estimation of lesion volume, and a lesion laterality index. We next used voxel-based morphometry to assess the link between different visuospatial deficits and gray matter and white matter (WM) damage. Lesion overlap/subtraction analyses, lesion laterality index, and voxel-based morphometry measures converged to indicate that bilateral parieto-occipital WM disconnections are both distinctive and necessary to create symptoms associated with simultanagnosia. We also found that bilateral gray matter damage within the middle frontal area (BA 46), cuneus, calacarine, and parieto-occipital fissure as well as right hemisphere parietal lesions within intraparietal and postcentral gyri were associated with simultanagnosia. Further analysis of the WM based on tractography revealed associations with bilateral damage to major pathways within the visuospatial attention network, including the superior longitudinal fasciculus, the inferior fronto-occipital fasciculus, and the inferior longitudinal fasciculus. We conclude that damage to the parieto-occipital regions and the intraparietal sulcus, together, with bilateral WM disconnections within the visuosptial attention network, contribute to poor visual processing of multiple objects and the loss of processing speed characteristic of simultanagnosia.
Patients with left neglect were tested with "chimeric" figures composed of the right and left halves of two different objects. The connectivity relation was modulated between the two half figures. For some displays, the two chimeric halves were separated by a small gap, while in others, the separate halves were connected by a line segment. In line with previous reports, performance on reporting the left half improved when the chimera were separated; but when a line connected the two separated halves the advantage was lost. If the connecting line was broken, the performance was again enhanced. The results suggest an important role for connectedness in the representation of perceptual objects and in the distribution of attention in neglect.
The present study examined the relations between the lesions linked to visual and tactile extinction (VE and TE), and those related to visual field defects and spatial neglect. Continuous variations in patients performance were used to assess the link between behavioural scores and integrity of both grey and white matter (GM and WM). We found both common and distinct neural substrates associated with extinction and neglect. Damage to angular and middle occipital gyri, superior temporal sulcus (STS) and insula were linked to VE. Lesions involving the supramarginal gyrus (SMG), intraparietal sulcus, middle frontal and superior temporal gyri (MFG and STG) were associated exclusively with spatial neglect. Lesions affecting the temporo-parietal junction (TPJ), the middle temporal region, middle frontal area (BA46) as well as the insula and putamen were linked to both spatial neglect and VE. Analysis of the relations between VE and TE highlighted the TPJ as the common site for both modalities. These findings suggest that the TPJ plays a general role in identifying salient events in the sensory environment across multiple modalities. Furthermore, WM analyses pointed to superior longitudinal fasciculus (SLF) as critical for interconnecting components of the visuospatial attention network. We demonstrated that functional disconnections resulting from SLF damage contribute to altered performance on attention tasks measuring not only neglect but also VE and TE. We propose that the SLF supports interactions between functionally specialized regions involved in attentional control across multiple sensory modalities.
Patients who have had a stroke resulting in the deficit of visuo-spatial neglect are normally not provided with a powered wheelchair, as they are either considered or found to be unsafe navigating about their environment. As these patients are relatively unlikely to regain functional mobility by walking, the denial of alternative forms of mobility is of particular concern. Modest progress has been made over the past two decades with regards to the rehabilitation of neglect but there have been calls for further research which addresses "real world" measures of independence such as wheelchair navigation. In this study, we investigated the ability of patients with neglect to improve their performance when navigating a powered wheelchair by using theoretically-driven strategies that have shown promise in previous studies (spatial cueing and limb activation). Strategies were applied and tested in the most realistic and practical manner for each individual, based on their abilities and concurrent deficits. Performance was improved by the experimental strategies. The data suggest it is possible to apply theoretically-driven strategies to improve wheelchair navigation in patients with neglect and are supportive of further studies that could lead to improved access to powered mobility by this population in the future.
We demonstrate that right-handed participants make speeded classification responses to pairs of objects that appear in standard co-locations for right-handed actions relative to when they appear in reflected locations. These effects are greater when participants "weight" information for action when deciding if 2 objects are typically used together, compared with deciding if objects typically occur in a given context. The effects are enhanced, and affect both types of decision, when an agent is shown holding the objects. However, the effects are eliminated when the objects are not viewed from the first-person perspective and when words are presented rather than objects. The data suggest that (a) participants are sensitive to whether objects are positioned correctly for their own actions, (b) the position information is coded within an egocentric reference frame, (c) the critical representation involved is visual and not semantic, and (d) the effects are enhanced by a sense of agency. The results can be interpreted within a dual-route framework for action retrieval in which a direct visual route is influenced by affordances for action.
Grouping between contra- and ipsilesional stimuli can alleviate the lateralised bias in spatial extinction (Gilchrist, Humphreys, & Riddoch, 1996; Ward, Goodrich, & Driver, 1994). In the current study we demonstrate for the first time that perceptual grouping can also modulate the spatio/temporal biases in temporal order judgements affecting the temporal as well as the spatial coding of stimuli. Perceived temporal order was assessed by presenting two coloured letter stimuli in either hemi-field temporally segregated by a range of onset-intervals. Items were either identical (grouping condition) or differed in both shape and colour (non-grouping condition). Observers were required to indicate which item appeared second. Patients with visual extinction had a bias against the contralesional item appearing first, but this was modulated by perceptual grouping. When both items were identical in shape and colour the temporal bias against reporting the contralesional item was reduced. The results suggest that grouping can alter the coding of temporal relations between stimuli.
Patients with extinction show a characteristic impairment in the identification of objects when two items are presented simultaneously, typically reporting the ipsilesional item only. The effect is thought to be due to a spatial bias advantaging the ipsilesional item under conditions of competing concurrent stimulation. Action relations between objects can result in recovery from extinction as the object pair may be perceived as a single group rather than competing perceptual units. However, objects interacting together can also have implied motion. Here we test whether implied motion is necessary to generate recovery from extinction. We varied orthogonally whether animate and inanimate objects were paired together in positions related or unrelated to action. Implied motion was greater when an animate object was present than when both stimuli were inanimate. Despite this, recovery from extinction was greater when actions were shown between inanimate objects. We suggest that actions between inanimate objects are perceived more easily due to the surfaces of these stimuli being designed for functional goals (e.g., the flat surface of a hammer head is designed to hit the flattened head of a nail). Attention is sensitive to the fit between potential action and the functional properties of objects, and not just to implied motion between stimuli.
Two experiments are reported that use patients with visual extinction to examine how visual attention is influenced by action information in images. In Experiment 1 patients saw images of objects that were either correctly or incorrectly colocated for action, with the objects held by hands that were congruent or incongruent with those used premorbidly by the patients. The images were also shown from a 1st- and 3rd-person perspective. There was an overall reduction in extinction for objects colocated for action. In addition, there was an extra benefit when the objects were held in hands congruent with those used by the patients and when the objects were seen from a 1st-person perspective. This last result fits with an effect of motor simulation, over and above a purely visual effect based on positioning objects correctly for action. Experiment 2 showed that effects of hand congruence could emerge with images depicted from a 3rd-person perspective when patients saw themselves holding the objects. The data indicate 2 effects of action information on extinction: (a) an effect of colocating objects for action, which does not depend on a self-reference frame (a visual effect), and (b) an effect sensitive to object-hand congruence, which does depend on a self-reference frame (a motor-based effect). The self-reference frame is induced when stimuli are viewed from a 1st-person perspective and when an image of the self is seen from a 3rd-person perspective. Both visual and motor-based effects of action information facilitate the spread of attention across objects.
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.
We discuss two commentaries that we have received on our target article (Humphreys et al., 2010). We elaborate on the evidence for action effects on extinction and discuss whether these effects occur pre or post the selection of a response. In addition, we discuss the neural basis of the effects of action relations on extinction and on the generalization of results on action relations to real-world examples.
Bilateral arm training with rhythmic auditory cueing (BATRAC) improves hemiparetic upper extremity (UE) function in stroke. It is unknown whether a similar exercise for the hemiparetic lower extremity (LE) is effective.
Extinction is an example of how stimulus selection may be affected by an imbalance in competition for attentional selection. Patients with extinction are able to process stimuli in either hemispace, but only when presented in isolation. Following brain injury, stimuli will not be processed as efficiently in the damaged hemisphere and so may fail to be detected when other stimuli are competing for selection. In this review we discuss some of the factors that contribute to the recovery from extinction, and consider their implications for functional and neural theories of selection. Work shows that extinction can be modulated by multiple bottom-up factors including: low-level visual grouping (e.g., reflecting Gestalt properties in an array) and grouping based on higher level factors (such as the lexical identity of a stimulus or action relations between objects). Top-down factors (such as holding items in working memory) can also facilitate recovery from extinction. Furthermore, the competition for selection may also be modulated by the programming of action to a given location, consistent with pre-motor feedback to perceptual processes. While often discussed in terms of spatial biases, non-spatial extinction can also be demonstrated (dictated by the coherence of stimuli). In contrast to extinction, a phenomenon of anti-extinction has also been documented where patients are better at report when two items rather than single items are presented. Although superficially distinct, evidence indicates that grouping may be important in both cases, with temporal grouping being important in generating the anti-extinction effect. Overall, the work indicates that the disorder of extinction plays an important role in the understanding of attentional selection.
We discuss evidence indicating that human visual attention is strongly modulated by the potential of objects for action. The possibility of action between multiple objects enables the objects to be attended as a single group, and the fit between individual objects in a group and the action that can be performed influences responses to group members. In addition, having a goal state to perform a particular action affects the stimuli that are selected along with the features and area of space that is attended. These effects of action may reflect statistical learning between environmental cues that are linked by action and/or the coupling between perception and action systems in the brain. The data support the argument that visual selection is a flexible process that emerges as a need to prioritize objects for action.
The authors present neuropsychological evidence distinguishing binding between form, color, and size (cross-domain binding) and binding between form elements. They contrasted conjunctive search with difficult feature search using control participants and patients with unilateral parietal or fronto/temporal lesions. To rule out effects of task difficulty or loss of top-down guidance of search, the authors made conjunction search easier than feature search. Despite this, parietal patients were selectively impaired at detecting conjunction targets in their contralateral field. In contrast, the parietal patients performed like the other participants with form conjunctions, with form conjunctions being easier to detect than difficult feature targets. These data indicate a qualitative difference between binding in the form domain and binding across form, color, and size, consistent with theories that propose distinct binding processes in vision.
We report the results of a new form of therapy for unilateral spatial neglect. Functional electrical stimulation (FES) applied to the left forearm extensor muscles reduced the symptoms of severe left unilateral visual neglect in three patients, with the benefits being measurable at 6 months post-treatment. We suggest that FES activates a proprioceptive map within the right parietal lobe whose level of activation is otherwise diminished by the lesion. This both increases awareness of the contralesional side and stimulates functional interactions with the environment.
Following lesions to (usually) the right parietal lobe, patients may fail to report stimuli on their contralesional side if a stimulus is also presented ipsilesionally. The problem can be ameliorated if the stimuli form part of a common action (e.g., a bottle pouring into a glass), when the contralesional item may be brought to awareness. We examined whether this improved awareness depended on implied motion from one object to another. This was tested using pairs of stimuli in which one had implied motion towards or away from the other stimulus. The results showed that patients were more aware of the presence of two objects on trials when one object had implied motion towards the other, compared with when motion was directed away from the second object. This held when the implied motion was in the contralesional as well as when it was in the ipsilesional field. In a single case, this effect held even when the direction of motion could not be explicitly discriminated. The data suggest that motion was coded implicitly and that it helped to link objects together as a perceptual unit. Coding objects as a single perceptual unit reduces the spatial bias in selection that produces extinction.
The left ventral occipito-temporal cortex (LvOT) is thought to be essential for the rapid parallel letter processing that is required for skilled reading. Here we investigate whether rapid written word identification in skilled readers can be supported by neural pathways that do not involve LvOT. Hypotheses were derived from a stroke patient who acquired dyslexia following extensive LvOT damage. The patient followed a reading trajectory typical of that associated with pure alexia, re-gaining the ability to read aloud many words with declining performance as the length of words increased. Using functional MRI and dynamic causal modelling (DCM), we found that, when short (three to five letter) familiar words were read successfully, visual inputs to the patients occipital cortex were connected to left motor and premotor regions via activity in a central part of the left superior temporal sulcus (STS). The patient analysis therefore implied a left hemisphere "reading-without-LvOT" pathway that involved STS. We then investigated whether the same reading-without-LvOT pathway could be identified in 29 skilled readers and whether there was inter-subject variability in the degree to which skilled reading engaged LvOT. We found that functional connectivity in the reading-without-LvOT pathway was strongest in individuals who had the weakest functional connectivity in the LvOT pathway. This observation validates the findings of our patients case study. Our findings highlight the contribution of a left hemisphere reading pathway that is activated during the rapid identification of short familiar written words, particularly when LvOT is not involved. Preservation and use of this pathway may explain how patients are still able to read short words accurately when LvOT has been damaged.
The validity and functional predictive values of the apraxia tests in the Birmingham Cognitive Screen (BCoS) were evaluated. BCoS was developed to identify patients with different forms of praxic deficit using procedures designed to be inclusive for patients with aphasia and/or spatial neglect.
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