Physiological correlates of a simple saccadic-decision task to extended objects in superior colliculus.

Saccadic eye movements direct gaze to objects of interest. Human studies show that saccade initiation latency depends on the size of the saccade target (the "size-latency effect"), perhaps reflecting a tradeoff between the cost of making a saccade and the expected information gain. Here, we investigated the neuronal correlates of the size-latency effect in macaque superior colliculus (SC). Analysis of saccade latencies within a stochastic accumulator framework predicted a steeper increase in activity for smaller targets compared to larger ones, and, surprisingly, an increase in saccade initiation threshold for smaller targets. We found that SC activity is in close agreement with these predictions. We also found evidence that these effects may be a consequence of the visual responses of SC neurons to targets of different sizes. The results shed new light on the sources of delay within the saccadic system, a system that we heavily depend upon for visuo-motor tasks.