Evaluating the contributions of top-down and bottom-up processing on eye movements during parallel visual search.

In the current study, we used an efficient visual search paradigm in a pseudo-realistic environment, with well-controlled search stimuli that allow a simultaneous evaluation of the impact of top-down and bottom-up factors on eye-movement patterns. Our stimuli varied along the color dimension to manipulate target-distractor similarity and our displays contained a salient stimulus of higher salience than target and other less-salient distractor stimuli. We manipulated task instructions, introducing a free-view instruction condition to serve as a baseline for how bottom-up contrast guided eye movements in one group of participants, and a top-down search instruction in a second group, where subjects were asked to find the red target in the scene. Experiment 1 assessed the impact of set size of less-salient distractors across both instructions. Experiment 2 examined target-distractor similarity effects for the less-salient distractors. We compared the likelihood that the first fixation in a trial would be selective towards the target (top-down) versus the high-salience singleton (bottom-up) and studied how this selectivity varied as a function of initial saccade latency. Interestingly, the results from the free-view conditions showed selectivity for the high-salience item during the first fixation was sustained across saccade latencies, yet the high-salience items capture very few saccades in the search task, suggesting attention might be in limbo early in the trial. Indeed, the results also showed that it takes time for saccades to be correctly directed at the target in a search task.