Eccentricity-dependent saccadic reaction time: The roles of foveal magnification and attentional orienting.

The primate visual brain is characterized by foveal magnification. Here, we show in macaque monkeys that foveal magnification affects the dynamics of saccade initiation. In a visually guided saccade task, saccadic reaction times (SRT) increased with target eccentricity. Notably, we effectively eliminated this increase by scaling the target size according to the foveal magnification factor in the superior colliculus. We then repeated the comparison between non-scaled and scaled targets while changing the task to a delayed, visually guided saccade task. In this task, the saccade was triggered by the foveal fixation offset rather than target onset, such that target onset long before the fixation offset was essentially irrelevant for SRT. In this task, we found that SRT increased with target eccentricity, with a similar rate for both non-scaled and scaled targets, consistent with an attentional scan from the fovea to the target, a recently hypothesized general mechanism of attention.