Insights into the Function and Mechanism of Saccadic Decision Making From Targets Scaled By an Estimate of the Cortical Magnification Factor

Here we address the shape of the saccadic latency-eccentricity function. Previously it has been proposed that the bowl-shaped nature of this function is a by-product of diminished stimulus representation in the periphery. A direct prediction of this theory is that saccadic latencies in the periphery should be speeded if stimuli are increased in size in proportion to the cortical magnification factor (M-scaling). Using a target-elicited saccade paradigm, ten subjects were shown M-scaled and unscaled Gaussian targets over a horizontal range of +/- 40 degrees. Saccadic latencies increased at an equal rate for peripheral targets regardless of whether targets were M-scaled or not. This suggests that the changes of latency with eccentricity are not a by-product of resources devoted to stimulus representation, but instead are a functional adaptation which takes account of the likelihood of saccades of each amplitude in the natural environment.