The zone of focal attention during active visual search.

We have examined target detection performance in two monkey subjects during visual search with eye movements as a function of stimulus density and the eccentricity of the target from fixation. As expected, search time and the number of fixations per trial were related to the number of array objects in conjunction style arrays. High probability detection of targets occurs only within a restricted area surrounding the fixation point. The size of this area is predictably controlled by stimulus density. When stimulus density is normalized using average nearest neighbor distances, detection probability as a function of eccentricity is equivalent across stimulus densities. Search in target unique feature arrays is a simple linear function of target eccentricity independent of stimulus density. Evidence suggestive of attentive scanning during the fixations of active search was not found. The effects of stimulus density were different in the initial 100-200 msec immediately after array presentation compared with the remainder of the trial for both conjunction and feature search. We suggest that the initial vs midtrial differences are related to surround integration time. Overall, our results suggest that during active search focal attention operates within a conspicuity area having an effective radius of about twice the average nearest neighbor distance.