Correlates of head-fixed orienting movements in mouse superior colliculus and substantia nigra pars reticulata

Orienting movements are a critical component of the natural behavioral repertoire, but their underlying neural bases are not well understood. The deep superior colliculus (dSC) integrates input from several brain regions that influence the selection of targets for orienting movements and coordinates activity among brainstem motor nuclei to initiate and execute movement. Evidence suggests that one prominent dSC input, the substantia nigra pars reticulata (SNr), permits movement by disinhibiting its targets, but much is unknown about the relationship between SNr activity, dSC activity, and movement. Building on increasing application of the head-fixed mouse model to elucidate the neural basis of behavior, we examined neural activity recorded in dSC and SNr with high-density probes in mice performing several variants of a sensorimotor orienting task, from our labs and in data sets curated by the International Brain Laboratory. We found that dSC and SNr were active preceding and throughout movement, across task variants, suggesting that they were engaged by the required movements. Prior to movement, SNr activity reflected the outcome of the previous trial, consistent with a role in biasing movements towards the highest value target. However, the dependence of dSC activity on movement direction was weaker than in other directional orienting behaviors, and we found little evidence for strong suppression of dSC by SNr. These results complement and extend previous findings from other orienting tasks and suggest diverse roles for modulatory input from SNr to dSC in shaping motor behavior.