Abstract
The guidance of visual behavior involves a sequence of processes in which a target stimulus must first be discriminated from amongst several alternatives followed by the programming of an appropriate response. We have shown how visual search behavior is influenced by the composition of the scene and how search strategies are particularly affected by stimulus salience (Shen and Paré 2006). Here, we investigated the neural mechanisms underlying these contextual effects by recording 37 visuo-motor neurons from the intermediate layers of superior colliculus (SC) while three monkeys performed a visual conjunction (color+form) search task, in which color was the most discriminable feature. The target appeared pseudorandomly with 11 distractors in a concentric search array, in which the number of distractors sharing the color of the target was varied between three distractor ratios (2 same-color: 9 different-color; 6:5; 9:2). As previously observed, monkeys' search strategies changed flexibly depending on the display that was presented. Saccades were biased toward distractors sharing the target color when there were few of them within the display (i.e., when they were salient) and away from those distractors when they were numerous. The initial (first 25ms) neuronal response to the search target or a same-color distractor was invariant with distractor ratio and saccade bias. However, at the time neurons discriminated the target from distractors, neuronal activity associated with a same-color distractor was proportional to saccade bias: the greater the activity, the stronger the attraction to same-color distractors. That SC neurons represented stimulus salience was further evidenced by the target-related activity being similarly related to saccade bias. These preliminary results suggest that the sensitivity of SC neurons to visual salience reflects the allocation of attentional resources suitable for the current visual context.
This work was supported by the Canadian Institutes of Health Research, the J. P. Bickell Foundation, and the EJLB Foundation.