Scalp topography of ERP source-densities during visually guided target practice.

"Whole-head" maps of ERP source densities were recorded at 42 locations, spaced radially at 6 cm in hexagonal arrays, from two human subjects (S) during a visually-guided target-practice task with manual button-pressing and visual feedback, in a series of six or more 9 h recording sessions. From one S a finer-grained map (3 cm spacing) was also obtained at 13 occipital placements. Corresponding results at a smaller number of locations were obtained from nine other Ss. The topographical effects of changing the hand used, reversing the direction and hemifield of target approach, and changing the difficulty of the task were investigated in detail. Only locations near and just anterior to Cz were significantly affected by exchanging hands. Contralateral preparatory negativities, and ipsilateral negativities in anticipation of feedback, were enhanced. Changing the stimulus hemifield mainly affected occipito-parietal locations, with enhancement of contralateral preparatory-phase negativities, and of ipsilateral pre-feedback negativities, both there and near Cz. At certain parietal locations, however, the profiles were unaffected. Increase of task difficulty enhanced the difference between evaluation potentials for success and failure at a few locations, but left the general map largely unaltered. When comparing topographic features of the source-density maps in the two main Ss, the standard 10/20 topographic landmarks proved surprisingly unreliable. For each phase of the task, there were also hemispheric asymmetries not linked to the hand used or the hemifield stimulated; but their topography varied from S to S. Source densities during the preparatory phase were strongest around the vertex, with accompanying activity in parietal as well as occipital areas. No endogenous predictors of success and failure could be seen in the preparatory source-density profile. The source density map of the evaluation potential elicited by feedback suggests a relatively deep location for its generator. The ERP correlates of success and failure differ mainly in their form; amplitude differences vary from S to S. When S participates only imaginatively as a spectator of target practice by others or by machine, the general form of the ERP map is similar over the whole cycle, but amplitudes diminish from rear to front of the head. The cognitive significance of the main ERP features is confirmed by their virtual disappearance, except near the occipital poles, when S's attention is distracted from the significance of the visual input.