Instantaneous midbrain control of saccade kinematics

The ability to interact with our environment requires the brain to transform spatially-represented sensory signals into temporally-encoded motor commands for appropriate control of the relevant effectors. For visually-guided eye movements, or saccades, the superior colliculus (SC) is assumed to be the final stage of spatial representation, and instantaneous control of the movement is achieved through a rate code representation in the lower brain stem. We questioned this dogma and investigated whether SC activity also employs a dynamic rate code, in addition to the spatial representation. Noting that the kinematics of repeated movements exhibits trial-to-trial variability, we regressed instantaneous SC activity with instantaneous eye velocity and found a robust correlation throughout saccade duration. Peak correlation was tightly linked to time of peak velocity, and SC neurons with higher firing rates exhibited stronger correlations. Moreover, the strong correlative relationship was preserved when eye movement profiles were substantially altered by a blink-induced perturbation. These results indicate that the rate code of individual SC neurons can control instantaneous eye velocity, similar to how primary motor cortex controls hand movements, and argue against a serial process for transforming spatially encoded information into a rate code.