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Abstract:
Beta (β) oscillations (~15-25 Hz) in the field potential are associated with immobility and stopping actions, but it is debated whether stopping involves a sustained increase in beta power or in the number of randomly timed ‘bursts’ (transient beta power increases). The role of β in movement control has been previously studied using the stop signal task, in which a cue instructs the subject to stop ongoing movement preparation. The time at which movement preparation stops is inferred from a ‘race’ model and provides an across-trial estimate of the movement preparation stop time. Thus, discrepancies about the role of β may be due to an inability to align brain activity with stopping on a trial-by-trial basis. Moreover, the causal role of β in stopping movements cannot be addressed without an observable stopping behavior. We used a new paradigm for head-fixed rats on a treadmill. The rats were trained to not run to a NoGo stimulus. On some trials, rats initiated a pre-potent running response but stopped before crossing a response threshold (distance) and returned to immobility. The peak velocity of these treadmill movements provides an unambiguous time when rats self-initiate stopping. We recorded 32-electrode EEG bilaterally over the entire cortex (39,366 trials, 306 sessions, 14 rats). Trial-averaged spectrograms centered on peak velocity revealed two distinct oscillations prior to stopping (9-12 Hz, α, and 15-25 Hz, β). Larger movements (and thus a greater need for stopping) were associated with greater α and β power prior to peak velocity. Moreover, α and β were apparent on more electrodes indicating greater spatial spread with greater need for stopping. These pre-stopping oscillations were observed globally and preceded initiation of stopping by 48 ms for β and 52 ms for α. Ongoing analyses are assessing Granger causality between EEG electrodes in the α and β bands and will report whether area 24a single unit spiking is phase locked to LFP oscillations prior to stopping. Our data show that, when field potentials can be aligned to overt acts of stopping, β power as well as α power are highly reliable predictors of the time of volitional stopping. JFDF and RAO are equal contributors and listed in alphabetical order.
