A deficit of spatial remapping in constructional apraxia after right-hemisphere stroke.
Russell C., Deidda C., Malhotra P., Crinion JT., Merola S., Husain M.
Constructional 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.