Superior colliculus saccade motor bursts do not dictate movement kinematics

Using two different comparisons of varying saccade types, a systematic analysis of saccade-related activity in superior colliculus (SC) neurons demonstrates that SC motor bursts do not dictate the kinematics of rapid eye movements. The primate superior colliculus (SC) contains a topographic map of space, such that the anatomical location of active neurons defines a desired eye movement vector. 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 constitute a rate code dictating movement kinematics. Here, using two complementary approaches, we demonstrate a dissociation between the SC rate code and saccade kinematics. First, we show that SC burst strength systematically varies depending on whether saccades of the same amplitude are directed towards the upper or lower visual fields, but the movements themselves have similar kinematics. Second, we show that for the same saccade vector, when saccades are significantly slowed down by the absence of a visible saccade target, SC saccade-related burst strengths can be elevated rather than diminished. Thus, SC saccade-related motor bursts do not necessarily dictate movement kinematics.

Automated summary

A systematic analysis reveals that the motor bursts in the superior colliculus (SC) neurons do not determine the kinematics of rapid eye movements. The spatial code and rate code of SC neurons exhibit dissociation, suggesting that saccade-related motor bursts in the SC do not necessarily dictate movement kinematics.