Saccade target selection in macaque during feature and conjunction visual search

To gain insight into how vision guides eye movements, monkeys were trained to make a single saccade to a specified target stimulus during feature and conjunction search with stimuli discriminated by color and shape. Monkeys p erformed both tasks at levels well above chance. The latencies of saccades to the target in conjunction search exhibited shallow positive slopes as a function of set size, comparable to slopes of reaction time of humans durin g target present/absent judgments, but significantly different than the slo pes in feature search. Properties of the selection process were revealed by the occasional saccades to distracters. During feature search, errant sacc ades were directed more often to a distracter near the target than to a dis tracter at any other location. In contrast, during conjunction search, sacc ades to distracters were guided more by similarity than proximity to the ta rget; monkeys were significantly more likely to shift gaze to a distracter that had one of the target features than to a distracter that had none. Ove rall, color and shape information were used to similar degrees in the searc h for the conjunction target. However, in single sessions we observed an in creased tendency of saccades to a distracter that had been the target in th e previous experimental session. The establishment of this tendency across sessions at least a day apart and its persistence throughout a session dist inguish this phenomenon from the short-term (<10 trials) perceptual priming observed in this and earlier studies using feature visual search. Our find ings support the hypothesis that the target in at least some conjunction vi sual searches can be detected efficiently based on visual similarity, most likely through parallel processing of the individual features that define t he stimuli. These observations guide the interpretation of neurophysiologic al data and constrain the development of computational models.