Resolving stages of processing in visual search: Frontal eye field neurophysiology with two degrees of difficulty

Behavior is the outcome of covert perceptual, cognitive, and motor operations that can be described by mathematical models and are produced by brain systems comprised of diverse neurons. Using the logic of selective influence, we are distinguishing stages of processing supporting visual search. Macaque monkeys searched for a color singleton among distractors. Two operations necessary for the task—search efficiency and stimulus-response mapping—were independently manipulated. Search efficiency was manipulated by varying the similarity of singleton and distractor colors. Stimulus-response mapping, or stimulus-response encoding, was manipulated by varying the elongation of stimuli that cued GO or NO-GO responses. The response times of both monkeys were modified selectively by the 2x2 (High vs Low efficiency) x (High vs Low encoding) manipulations. Single-unit spiking was sampled in frontal eye field of two monkeys. Neurons representing stimulus salience were distinguished from neurons mediating saccade preparation. The times of modulation of both categories of neurons were measured in the 2x2 (High vs Low efficiency) x (High vs Low encoding) manipulations. The manipulation of search efficiency influenced the time taken to resolve singleton location and the delay of saccade preparation of most neurons. The manipulation of stimulus-response encoding did not influence the time taken to resolve singleton location of most neurons but also delayed saccade preparation. The convergence of performance and neural results provide evidence that distinct operations during visual search can be resolved via the experimental logic of selective influence.