Impaired visual processing of contralesional stimuli in neglect patients: a visual-evoked potential study

Transient visual-evoked potentials (VEPs) were recorded in 11 patients with right brain damage and spatial neglect. High-resolution EEG was recorded using focal stimuli located in the four visual quadrants. VEPs to left stimuli, i.e. located in the neglected side, were compared to VEPs to right stimuli. Results showed that bottom-up processing of a visual stimulus located in the neglected hemifield was intact up to approximately 130 ms from stimulus onset. Hemispheric differences were not significant for either C1 or P1 components representing the activity of striate and extrastriate areas, respectively. In contrast, visual processing in more dorsal areas adjacent to the superior parietal lobe was changed from normal. We failed to record the N1a component for left visual field stimuli expected in the 130-160 ms time range. Furthermore, the N1p (140-180 ms) and P2 (180-220) components were delayed and/or reduced in amplitude for stimuli located on the neglected side. The source of the N1a was previously localized in the intraparietal sulcus in the dorsal occipital cortex; N1p may represent a reactivation of area V3A and P2 reactivation of occipital visual areas including V1 due to top-down feedbacks. Six patients with left brain damage (LBD) and no neglect and 21 healthy subjects were also tested in the same experimental conditions used for patients with neglect. In LBD patients, all components evoked by contralesional stimuli were comparable to ipsilesional components. Overall, data allow localizing in time and space the processing deficit specific for patients with neglect. The first takes place around 130 ms in the bottom-up processing at the level of the anatomically intact dorsal parietal areas; the second is located at the level of the reactivation of the striate and extrastriate areas via feedback connections from higher visual areas. The two functional impairments were limited to left-field stimuli.