Neural Basis of the Set-Size Effect in Frontal Eye Field: Timing of Attention During Visual Search

Department of Psychology, Vanderbilt Vision Research Center, Center for Integrative and Cognitive Neuroscience, Vanderbilt Brain Institute, Vanderbilt University, Nashville, Tennessee Submitted 12 January 2009; accepted in final form 21 January 2009 Visual search for a target object among distractors often takes longer when more distractors are present. To understand the neural basis of this capacity limitation, we recorded activity from visually responsive neurons in the frontal eye field (FEF) of macaque monkeys searching for a target among distractors defined by form (randomly oriented T or L ). To test the hypothesis that the delay of response time with increasing number of distractors originates in the delay of attention allocation by FEF neurons, we manipulated the number of distractors presented with the search target. When monkeys were presented with more distractors, visual target selection was delayed and neuronal activity was reduced in proportion to longer response time. These findings indicate that the time taken by FEF neurons to select the target contributes to the variation in visual search efficiency. Address for reprint requests and other correspondence: J. D. Schall, Vanderbilt University, Wilson Hall, 111 21st Ave. S., Nashville, TN 37240 (E-mail: jeffrey.d.schall{at}vanderbilt.edu )