Characteristics of visual seeking and evoked potentials in the extrastriate areas of the cortex in humans.

Studies in 11 young, healthy subjects addressed the characteristics of visual seeking (time taken, errors) on changes in the parameters of the target element to be sought (shape, color, and location) in an environment containing heterogeneous white distractors. Evoked potentials (EP) were recorded in six cortical leads (P3, P4, T3, T4, T5, T6) and the late endogenous components of EP were studied, i.e., the N2 and P3 components (standard terminology), as these components are known to change when the type of search changes, in the zone of so-called late selection. When the search difficulty increased (increased similarity between target and distractors), an increase in seeking time was accompanied by a delay in the P3 component and a decrease in its amplitude. Location of the target in a defined position resulted in a decrease in search time and a reduction in the latent period of the P3 component as compared with the situation in which the target position was indeterminate. Changes in the color of the target stimulus led to elimination of the inhibitory action of the distractors: EP parameters were no different from those recorded on presentation of single stimuli. A high level of correlation was found between search parameters and measures of the P3 component. Changes in EP in different types of search were essentially identical (no statistical differences) in the parietal and temporal leads. This suggests that on seeking the target in the environment, the parietal and temporal areas of the cortex function as a single system.