Superior colliculus saccade motor bursts do not dictate movement kinematics

The primate superior colliculus (SC) contains a topographic map of visual field locations, such that the anatomical location of any given active neuron defines a desired eye movement amplitude and direction. Complementing such a spatial code, SC neurons also exhibit saccade-related bursts that are tightly synchronized with movement onset. Current models suggest that such bursts, and their properties, constitute a temporal rate code that may dictate moment-to-moment movement evolution. However, a recent result demonstrated altered movement properties with minimal changes in SC motor burst strengths (Buonocore, Tian, Khademi, & Hafed, 2021). Here, I support such a dissociation between the SC temporal rate code and instantaneous movement evolution: SC burst strength varies depending on whether saccades are directed towards the upper or lower visual fields, but the movements themselves have similar kinematics. Thus, SC saccade-related motor bursts do not necessarily dictate movement kinematics, motivating investigating other possible functional roles for these bursts.