Memory search and visual spatial attention: An event-related brain potential analysis

Event-related brain potentials (ERPs) were recorded from subjects as they attended to one diagonal of a visual display. The task was to respond only to memory set items (targets) at the attended diagonal and to ignore stimuli at the other diagonal. The probability that the display contained either an attended or an unattended target was 0.30. The spatial separation between attended and unattended stimuli was 1.6°. The ERP elicited by stimuli at the unattended diagonal contained a sequence of phasic components. The early N₁₇₀ and P₂₅₀ components were elicited by the onset of the display and the later components N₄₈₀ and P₅₅₀ by the offset of the display. The presence of masks delayed N₁₇₀ and P₂₅₀. The ERPs elicited by attended non-targets, in addition, contained an increased N₃₅₀ (C_z, F_z) and P₄₁₀ (P_3a, P_z, C_z). The ERPs elicited by attended and unattended non-targets started to differ after 200 msec. This finding suggested that selection is relatively late if selection must be based on a conjunction of features (location and orientation) and if the spatial separation between attended and unattended stimuli is small. Memory-set size affected the ERPs after 250 msec. The ERPs elicited by attended stimuli contained a broadly distributed (F_z, C_z, P_z) negative endogeneous component. The amplitude of this component was related to memory-set size. Finally, the ERPs elicited by attended targets contained a large P_3b (P_z, O_z) with a peak latency around 600 msec. The ERP results suggested the existence of three processing stages: (1) orienting to the attended stimuli; (2) controlled search, and (3) target decision.