Extended Data Fig. 8: Connectivity between SNr and brainstem targets.

a, Simultaneous recording from an example SNr neuron and its postsynaptic Midbrain neuron during the forelimb task (top left). Z-scored cross-correlogram showing cross-correlation of spike times between pre and postsynaptic neuron with minimum at less than 2 ms, showing inhibition of the postsynaptic neuron by spikes of the presynaptic neuron (top right). Single trial spike raster for the presynaptic (SNr) neuron and the postsynaptic (Midbrain) neuron aligned to retraction start and sorted by reach start time (middle). Average spike rates with standard error of the mean from these trials for the presynaptic (SNr) neuron and the postsynaptic (Midbrain) neuron aligned to retraction start with overlay shown below. b, Example raster plot of one SNr neuron (middle) that was identified to have two putatively postsynaptic Midbrain neurons (top and bottom) highlighting the striking correspondence between spiking in Midbrain neurons and pauses in firing of the presynaptic SNr neuron. Interestingly, not all pauses in the SNr neuron corresponded to increases in the Midbrain neurons. c, Schematic indicating the detection of the change in firing rate during the forelimb reaching task of SNr, latRM and Midbrain neurons (top). Briefly, first, for every neuron a stringent search was employed to identify the earliest 20 ms time-bin in the 500 ms preceding the onset of reaching which showed a statistically significant modulation from baseline firing of the neuron (two-sided Mann-Whitney test with p-value ≤ 0.001). Subsequently, rolling back from this time-bin, a lenient search was used to identify when each neuron exhibited the first 20 ms time-bin with firing rate significantly different from its baseline (two-sided Mann-Whitney test with p-value ≤ 0.05) to identify the first statistically significant deflection in firing rate of the neuron (see Methods for details on the statistical tests used to detect modulation). Heatmaps of z-scored average firing rate of SNr neurons (middle) and Midbrain neurons (bottom) with positive modulation preceding the reach onset. d, Model for the control of forelimb movement mediated by the basal ganglia output structure, SNr, via target motor centres in the brainstem. The schematic summarizes the activity of SNr neurons and their downstream targets during the execution of movement A (top) and movement B (bottom). Single SNr neurons show forelimb movement specific pauses and increases in firing which exert a disinhibition or suppression of their downstream motor targets respectively, thereby licensing different motor programs to be executed contingent on excitatory drive to the motor centres (see Discussion).

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