Differential Effects of Value and Salience on Saccade Latencies

Express saccades, a mode of visually guided saccades, distinguished from regular saccades by extremely short reaction times, are triggered by inserting a temporal gap between the fixation dot and the saccade target. It is usually assumed that they are produced by a specific pathway in which the superior colliculus plays a key role. Whether and how this pathway deals with information on the subjective value of a saccade target is unknown. We, therefore, studied the influence of varying reward expectancies and compared it with the impact of the presence and absence of a temporal gap between the disappearance of the fixation dot and the appearance of the target on the visually guided saccades of two rhesus macaques (Macaca mulatta). We observed that the introduction of a gap shifted the entire saccadic reaction time distribution to shorter latencies while increasing the probability of express saccades. On the other hand, promoting the monkeys reward expectancy shortened reaction times and increased peak velocities of regular saccades, and increased the probability of express saccades. Importantly, we observed that the reaction time and peak velocity of express saccades were not sensitive to the value of the saccade target, suggesting that the express pathway does not have access to information on value. We propose a new model on express saccades that treats the salience of visual objects in the scene differently from the subjective value assigned to them.