The influence of target position and response hand on efficient feature search

In event-related potential recordings (ERPs), the N2pc component, an enhanced negativity over parietal electrodes contralateral to the target hemifield about 270ms after stimulus onset has been earlier suggested as neurophysiological correlate for spatial shifts of visual attention (e.g. Woodman & Luck, 1999, Nature 400 :867–869). The presence of this component in efficient feature search (FS) thus indicated that at least one attentional shift is involved in this type of search. Here we asked whether possible attentional processes involved in efficient feature search are purely search-driven, or are modulated by motor response and target position. We measured in 17 subjects the influence of responding hand (left versus right) and target location (left versus right hemifield) on reaction times (RT) and on ERPs during FS. For RTs, a 2 (hands) × 2 (hemifields) ANOVA revealed a significant effect of the hemifield (left < right), and significant interactions between hand and hemifield, provoked by longer RTs for targets in the right hemifield during left hand responses. Taking the N2pc component and its ipsilateral counterpart (N2pi) into account, ERP peak latency analysis allowed us to isolate influences on cortical search dynamics of both responding hand and of target location, respectively. We show that ERP latencies for both N2pc and N2pi are shorter over electrode sites contralateral to the responding hand, thus independently of the target hemifield. Moreover, a target located in the left visual hemifield evokes faster N2pc and N2pi potentials than a target in the right hemifield, irrespective of the responding hand. We conclude that, in efficient search, both speed of target detection and cortical dynamics of the suggested neurophysiological correlate of attention depend on executive processes of the motor response, and the target hemifield.