Eccentricity-Dependent Saccadic Reaction Time: The Roles of Foveal Magnification and Attentional Orienting

A hallmark of primate vision is the emphasis on foveal processing, accompanied by frequent saccades that bring the fovea to salient parts of the scene, or to newly appearing stimuli. A saccade to a new stimulus is one of the most fundamental sensory-motor transformations. In macaque monkeys, we show that foveal magnification is not only the reason for saccades, but it also governs the dynamics of saccade initiation. In a task where the monkeys made saccades to peripheral target onsets, saccadic reaction time (SRT) increased with target eccentricity. Notably, we effectively eliminated this increment by scaling the target size according to the foveal magnification factor in the superior colliculus (SC). We repeated the comparison between non- scaled and scaled targets, while changing the task to a delayed saccade task. In this task, the target was presented long before the saccade, and the saccade was triggered by fixation offset rather than target onset, such that target onset was essentially irrelevant for SRT. Crucially, in this task, we found that SRT increased with target eccentricity, for both non-scaled and scaled targets. Furthermore, this increment survived the addition of a salient foveal distracting flash. The results obtained with the delayed saccades task are consistent with an attentional scan from the fovea to the target, a recently hypothesized general mechanism of attention.