Electrophysiologic correlates of visuo-spatial attention shift.

We investigated neurophysiologic correlates of shifting visual attention across the visual field. Event-related potentials (ERPs) were recorded in 12 normal subjects during a visual discrimination task in which target stimuli were presented at a predictable or unpredictable location in the eccentric visual field. Subjects were obliged to shift their attention from the expected site to the unpredictable site immediately after the presentation of shifted stimuli in order to detect the change of stimulus attributes. Shifted stimuli modulated the N1 component (130-200 msec), producing a larger amplitude at the posterior temporal site contralateral to the stimulus field and a smaller amplitude over the ipsilateral hemisphere. Furthermore, shifted stimuli uniquely evoked a positive ERP component with a latency of 200-300 msec, which distributed broadly over the skull maximally at the frontal and central electrode sites. Both the negative and positive components changed in amplitude as a function of shift distance and direction. These results suggest that modulations of the negative and positive deflections reflect the shift of covert visuo-spatial attention and that right hemispheric dominance does not exist at least in the early stage of shifting spatial attention.