Movement planning is widespread across the animal kingdom, and is typically reflected in brain activity that gradually increases before the impending movement. A fundamental question in sensorimotor control is how planning activity is appropriately parsed into preparatory signals and movement commands. 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. In the gaze control system, inhibitory gating on the circuitry normally prevents the ramping preparatory activity from producing an eye movement. Reaching threshold is thought to release this inhibition, triggering a movement. This raises the question: would the preparatory activity be able to trigger movement before reaching threshold if the gating were no longer present?
In a recent paper published in e-Life, Removal of inhibition uncovers latent movement potential during preparation, Uday Jagadisan and Neeraj (Raj) Gandhi used a novel approach to answer this question. Monkeys were made to blink reflexively with an air-puff delivered to their eyes while they fixated a central spot on a screen, preparing to look at a peripheral visual target. Reflex blinks turn off inhibitory gating on the brainstem’s eye movement circuitry by the omnipause neurons, allowing the animal to make an early movement during the planning period. Jagadisan and Gandhi found that ramping activity in the superior colliculus (SC), a midbrain hub for gaze control, was correlated with kinematics of the early part of the eye movement, even before the eventual brisk component occurred (which was also correlated with the subsequent brisk increase in SC activity). They also showed that the activity required to initiate a movement is not fixed and can be flexibly modulated. Together, these results demonstrate that the neural activity involved in eye movement planning also has the potential to generate a movement when released from inhibition. The results have important implications for our understanding of how movements are generated, in addition to providing potentially useful information for decoding movement intention based on planning-related activity.
Figure caption: Instantaneous correlation between neural activity and velocity across trials (motor potential) is normally seen during a saccade after the eyes start moving (i.e.,after zero on the x-axis in top row, left panel). When inhibitory gating is released by a reflex blink, trial-by-trial variations in activity predict eye velocity even before onset of the saccade proper (highlighted area before zero on the x-axis in top row, right panel). This correlation occurs at the same putative projection delay as for normal saccades , ~12 ms (highlighted area in bottom panel)