Guidance of eye movements during visual conjunction search: local and global contextual effects on target discriminability.

The composition of a visual scene influences the ability of humans to select specific details within that scene for discrimination or foveation with saccadic eye movements. With the goal of establishing an animal model to investigate the neural mechanisms underlying the deployment of visual attention and the guidance of saccades during visual search, we studied the visual behavior of three monkeys while they performed a conjunction (color + form) search task similar to those used in human studies. We found that search performance declined when distractors adjacent to the target shared its color, thereby revealing that color was more discriminable than form in these displays and suggesting that monkeys perceptually grouped stimuli by proximity and similarity. Search performance also varied with the overall composition of the display. Most importantly, saccades were biased toward distractors sharing the target color when there were few of them within the display and away from those distractors when they were numerous. Last, the monkeys initiated saccades with a fixed latency, suggesting that their responses to the display were automatic and that search strategies did not involve attentional resources beyond those recruited for regulating saccades. We conclude that monkeys adapt their visual strategies, largely via bottom-up processes, to both the local and the global context of the search. These findings suggest that the visual behavior of monkeys is guided by strategies similar to those observed in humans.