Impaired spatial working memory across saccades contributes to abnormal search in parietal neglect

Visual neglect of left space following right parietal damage in humans invo lves a lateral bias in attention, apparent in many search tasks. We hypothe sized that parietal neglect may also involve a failure to remember which lo cations have already been examined during visual search: an impairment in r etaining searched locations across saccades. Using a new paradigm, we monit ored gaze during search, while simultaneously probing whether observers jud ged they had found a new target, or judged instead that they were re-fixati ng a previously examined target. A patient with left neglect following foca l right parietal infarction repeatedly re-fixated right locations. Critical ly, he often failed to remember that these locations had already been searc hed, treating old targets as new discoveries at an abnormal rate. In compar ison, healthy age-matched control subjects rarely re-fixated targets, and m istook old targets as new targets even more rarely. The frequency of such m istakes in the parietal patient, for different conditions, correlated with the severity of his neglect. Control experiments indicated no perceptual lo calization deficit in non-search tasks. These results suggest a deficit in retaining searched locations across saccades in parietal neglect, in additi on to the lateral spatial bias. Moreover, the former deficit exacerbates th e latter, such that patients do not realize that the rightward locations fa voured by their bias have already been examined during previous fixations a nd, for this reason, they saccade back to them repeatedly. The combination of the two deficits (a lateral bias plus a deficit in retaining locations a lready searched) may thus explain the pathological pattern of search that c haracterizes parietal neglect: why stimuli on the right are re-examined rec ursively, as if being searched for the first time, and hence why stimuli on the left continue to be ignored even with unlimited viewing time. These pr oposals accord with recent electrophysiological and functional imaging data , demonstrating posterior parietal involvement in the retention of target l ocations across saccades.