A deficit of spatial remapping in constructional apraxia after right-hemisphere stroke

This Article is provided by the Brunel Open Access Publising Fund - Copyright @ 2010 Oxford University PressConstructional apraxia refers to the inability of patients to copy accurately drawings or three-dimensional constructions. It is a common disorder after right parietal stroke, often persisting after initial problems such as visuospatial neglect have resolved. However, there has been very little experimental investigation regarding mechanisms that might contribute to the syndrome. Here, we examined whether a key deficit might be failure to integrate visual information correctly from one fixation to the next. Specifically, we tested whether this deficit might concern remapping of spatial locations across saccades. Right-hemisphere stroke patients with constructional apraxia were compared to patients without constructional problems and neurologically healthy controls. Participants judged whether a pattern shifted position (spatial task) or changed in pattern (non-spatial task) across two saccades, compared to a control condition with an equivalent delay but without intervening eye movements. Patients with constructional apraxia were found to be significantly impaired in position judgements with intervening saccades, particularly when the first saccade of the sequence was to the right. The importance of these remapping deficits in constructional apraxia was confirmed through a highly significant correlation between saccade task performance and constructional impairment on standard neuropsychological tasks. A second study revealed that even single saccades to the right can impair constructional apraxia patients’ perception of location shifts. These data are consistent with the view that rightward eye movements result in loss of remembered spatial information from previous fixations, presumably due to constructional apraxia patients’ damage to the right-hemisphere regions involved in remapping locations across saccades. These findings provide the first evidence for a deficit in remapping visual information across saccades underlying right-hemisphere constructional apraxia.European Commission Marie Curie Intra-European Fellowship (011457 to C.R.) and a Wellcome Trust Senior Fellowship (to M.H.)

A computational theory of visuo-spatial mental imagery

The thesis develops a new theory of visuo-spatial mental imagery: the perceptual instantiation theory. The theory is concretized in a formal framework and implemented as a computational model. The theory and its model are evaluated against a set of empirical phenomena and compared to the contemporary theories of mental imagery. The new theory is shown to provide explanations for the considered phenomena that partly go beyond those of the contemporary theories

Dissociable components of spatial neglect associated with frontal and parietal lesions

Spatial neglect is a complex neuropsychological disorder, in which patients fail to detect and respond to contralesional stimuli. Recent studies suggest that these symptoms may reflect a combination of different component deficits, associated with different lesion substrates. Thus, damage to right lateral prefrontal and inferior parietal regions produce different degrees of left neglect on cancellation and line bisection tasks, respectively. Here we tested for dissociable behaviors across two tasks designed to assess distinct cognitive processes possibly mediating such components, in 14 patients with right focal lesion in either the frontal or parietal lobe. In the “distractor filtering” task, patients had to respond to a visual target presented centrally, with or without a lateralized distractor. Only frontal-lesioned patients showed a marked slowing of reaction times when a central target appeared with a simultaneous right distractor (compared to center and left distractor). In the “spatial coding” task, patients had to detect a target among successive visual stimuli presented horizontally with three sequence conditions (regular/predictive or irregular/non-predictive). Only parietal-lesioned patients were unable to benefit from the predictability of the target position, with similar reaction times across all sequence conditions. By contrast, frontal patients showed faster reaction times on trials with a regular succession of stimuli (compared to random order). Taken together, these results suggest that frontal damage may contribute to left inattention by disrupting top-down control and resistance to distractors on the ipsilesional side, whereas parietal damage may disrupt the maintenance of stable locations in space across gaze shifts or time. This further supports the notion that left neglect may arise as a combined breakdown or impaired connectivity between frontal and parietal mechanisms involved (respectively) in the selective control and memory storage components of spatial attention

Cross-modal extinction in a boy with severely autistic behaviour and high verbal intelligence

Anecdotal reports from individuals with autism suggest a loss of awareness to stimuli from one modality in the presence of stimuli from another. Here we document such a case in a detailed study of T.M., a 13-year-old boy with autism in whom significant autistic behaviors are combined with an uneven IQ profile of superior verbal and low performance abilities. Although T.M.'s speech is often unintelligible and his behavior is dominated by motor stereotypies and impulsivity, he can communicate by typing or pointing independently within a letter board. A series of experiments using simple and highly salient visual, auditory, and tactile stimuli demonstrated a hierarchy of cross-modal extinction, in which auditory information extinguished other modalities at various levels of processing. T.M. also showed deficits in shifting and sustaining attention. These results provide evidence for mono-channel perception in autism and suggest a general pattern of winner-takes-all processing in which a stronger stimulus-d riven representation dominates behavior, extinguishing weaker representations

DIRECTION SPECIFIC COSTS TO SPATIAL WORKING MEMORY FROM SACCADIC AND SPATIAL REMAPPING

Right parietal lesions often lead to neglect, in which patients fail to attend to leftward stimuli. Recent models of neglect suggest that, in addition to attentional impairments, patients demonstrate impairments of spatial remapping and/or spatial working memory (SWM). Although spatial remapping could be considered a kind of spatial memory process itself (i.e., updating remembered locations based on anticipated saccade outcomes), the two processes operate on very different time scales (milliseconds versus seconds). In the present study, the influence of saccadic and spatial remapping on SWM was examined in healthy individuals. An initial control condition, in which participants had to respond to a probe stimulus (i.e., “is the probe in the location previously occupied by the target?”) following a 1500 ms delay, was contrasted with conditions in which the fixation point moved (left, right, up, or down) at the onset of the delay. In a second version of the task, participants made covert shifts of attention at delay onset requiring covert spatial, rather than saccadic, remapping. In both tasks SWM performance was best when no remapping was required. Decrements in SWM were largest overall in the spatial remapping task, whereas for both saccadic and spatial remapping, a consistent cost was observed for remapping the target array into right visual space. Results are discussed in terms of hemispheric biases in attention and differences in performance for peripersonal versus extrapersonal space

Role of right posterior parietal cortex in maintaining attention to spatial locations over time

Recent models of human posterior parietal cortex (PPC) have variously emphasized its role in spatial perception, visuomotor control or directing attention. However, neuroimaging and lesion studies also suggest that the right PPC might play a special role in maintaining an alert state. Previously, assessments of right-hemisphere patients with hemispatial neglect have revealed significant overall deficits on vigilance tasks, but to date there has been no demonstration of a deterioration of performance over time--a vigilance decrement--considered by some to be a key index of a deficit in maintaining attention. Moreover, sustained attention deficits in neglect have not specifically been related to PPC lesions, and it remains unclear whether they interact with spatial impairments in this syndrome. Here we examined the ability of right-hemisphere patients with neglect to maintain attention, comparing them to stroke controls and healthy individuals. We found evidence of an overall deficit in sustaining attention associated with PPC lesions, even for a simple detection task with stimuli presented centrally. In a second experiment, we demonstrated a vigilance decrement in neglect patients specifically only when they were required to maintain attention to spatial locations, but not verbal material. Lesioned voxels in the right PPC spanning a region between the intraparietal sulcus and inferior parietal lobe were significantly associated with this deficit. Finally, we compared performance on a task that required attention to be maintained either to visual patterns or spatial locations, matched for task difficulty. Again, we found a vigilance decrement but only when attention had to be maintained on spatial information. We conclude that sustaining attention to spatial locations is a critical function of the human right PPC which needs to be incorporated into models of normal parietal function as well as those of the clinical syndrome of hemispatial neglect

Brain networks of visuospatial attention and their disruption in visual neglect

Visual neglect is a multi-component syndrome including prominent attentional disorders. Research on the functional mechanisms of neglect is now moving from the description of dissociations in patients' performance to the identification of the possible component deficits and of their interaction with compensatory strategies. In recent years, the dissection of attentional deficits in neglect has progressed in parallel with increasing comprehension of the anatomy and function of large-scale brain networks implicated in attentional processes. This review focuses on the anatomy and putative functions of attentional circuits in the brain, mainly subserved by fronto-parietal networks, with a peculiar although not yet completely elucidated role for the right hemisphere. Recent results are discussed concerning the influence of a non-spatial attentional function, phasic alertness, on conscious perception in normal participants and on conflict resolution in neglect patients. The rapid rate of expansion of our knowledge of these systems raises hopes for the development of effective strategies to improve the functioning of the attentional networks in brain-damaged patients