An electrophysiological investigation of the spatial distribution of attention to colored stimuli in focused and divided attention conditions.

In the present experiment ERPs were recorded to colored stimulus bars (red or blue) which were randomly presented at one out of eight different spatial locations on a visual display. The locations were situated on a hemi-circle around the fixation point, with four locations lying in each visual half-field. The subjects were instructed to attend to one stimulus location (the most peripheral left or right location: the relevant location) in a focused attention condition and to all four locations within the same visual half-field in a divided attention condition, and were instructed to respond to stimulus bars in one color presented at the relevant location(s). In the focused attention condition, spatial attention resulted in early positivity in the P1 latency range (ca. 100-175 ms), followed by a prolonged negativity in the N1, P2, and N2 latency range (ca. 175-350 ms). These effects generalized to locations in the same visual half-field as the relevant location. The effect of attending and responding to the target color consisted of a number of different effects. An early anterior positivity, occipital negativity was observed for the relevant location and for locations in the same visual half-field as the relevant location, but not for the locations in the opposite visual field. A later central negativity (N2b) appeared to be confined to the relevant location and one location adjacent to it. Finally, a late parietal positivity (P3b) was exclusively evoked by target stimuli at the relevant location. In the divided attention condition, the ERPs evoked by stimuli presented at each of the four locations within the relevant visual half-field showed increased early positivity (enhancing P1 amplitude) as compared to the ERPs to stimuli in the opposite (irrelevant) visual half-field. The early color selection effect was also found to all stimuli within the relevant half-field, but the N2b component was evoked both by stimuli within the relevant half-field and by stimuli at the location in the irrelevant half-field closest to the midline. The P3b was present to target stimuli at the three most lateral positions within the relevant half-field but it was absent to the relevant location closest to the midline. The data suggested that the attentional spotlight encompassed at least approximately 2 degrees in the focused attention condition, and an efficient selection by visual half-field in the divided attention situation.(ABSTRACT TRUNCATED AT 400 WORDS)