Electrophysiological correlates of visual search asymmetry in humans

Visual evoked potentials (VEPs) were recorded during a visual search task to evaluate parallel and serial models of visual processing. A target stimulus elicited a discrete posterior negative wave in the 150-300 ms latency range. Furthermore, there was a close correlation between the search performance and the activity of the negative wave. When the target had a unique feature, neither the search time nor the characteristics of the negative wave was affected by the number of items in display. However, when the target lacked a unique feature, both the search time and the latency of the negative wave increased with the number of items. These results were consistent with a claim that an object with a unique feature is detected preattentively.