Removal Of Inhibition Uncovers Latent Movement Preparation Dynamics

Abstract

The motor system prepares for movements well in advance of their execution. In the gaze control system, premotor neurons that produce a burst of activity for the movement are also active leading up to the saccade. The dynamics of preparatory neural activity have been well described by stochastic accumulator models, and variability in the accumulation dynamics has been shown to be correlated with reaction times of the eventual saccade, but it is unclear whether this activity is purely preparatory in nature or has features indicative of a hidden movement command. We explicitly tested whether preparatory neural activity in premotor neurons of the primate superior colliculus has \"motor potential\". We removed inhibition on the saccadic system using reflex blinks, which turn off downstream gating, and found that saccades can be initiated before underlying activity reaches levels seen under normal conditions. Accumulating low-frequency activity was predictive of eye movement dynamics tens of milliseconds in advance of the actual saccade, indicating the presence of a latent movement command. We also show that reaching threshold is not a necessary condition for movement initiation, contrary to the postulates of accumulation-to-threshold models. The results bring into question extant models of saccade generation and support the possibility of a concurrent representation for movement preparation and generation.\n\nSignificance StatementHow the brain plans for upcoming actions before deciding to initiate them is a central question in neuroscience. Popular theories suggest that movement planning and execution occur in serial stages, separated by a decision boundary in neural activity space (e.g., \"threshold\"), which needs to be crossed before the movement is executed. By removing inhibitory gating on the motor system, we show here that the activity required to initiate a saccade can be flexibly modulated. We also show that evolving activity during movement planning is a hidden motor command. The results have important implications for our understanding of how movements are generated, in addition to providing useful information for decoding movement intention based on planning-related activity.\n\nImpact StatementNon-invasive disinhibition of the oculomotor system shows that ongoing preparatory activity in the superior colliculus has movement-generating potential and need not rise to threshold in order to produce a saccade.