On-line brain potential correlates of right parietal patients' attentional deficit

EEG potentials evoked by cues and targets were recorded in Posner's visual cueing task from 10 patients with lesions of the right parietal cortex and from age-matched healthy subjects. The patients' N1 component evoked by left-side cues was reduced at the right parietal recording site, suggesting a general impairment in processing left-side visual input. As usual, patients' keypress responses were delayed when left targets were preceded by right cues. There were two correlates of this delay in the patients' EEG potentials evoked by the critical combination of right cue/left target: their mean amplitude 160-280 ms after target onset ('Nd') was less negative than with other combinations of cue and target, and the following frontal P300 was enhanced. The Nd reduction seems to be an on-line measure of patients' momentary decrease of attention for the left hemifield, while the frontal P300 might reflect the patients' attempts at reorienting. In conclusion, different components were sensitive to different aspects of the patients' disorder, suggesting the utility of this approach for developing detailed hypotheses on the mechanisms of attentional deficits involved in visual extinction and neglect.