Neural mechanism for tactile stimuli selection

The ability to selectively respond to a desirable stimulus while ignoring the surroundings is essential for efficient decision making, such as sensing food amongst rocks. This requires accurate discrimination of the input features and their associated values. Current understanding of value-modulated sensory processing is highly biased towards the visual modality, and little is known about how tactile signals evolve across cortical and subcortical regions in the same goal directed task. It is also unclear whether task engagement and action outcome modulates neural representation of the sensory and value space.

To answer these questions, we investigated the mouse primary somatosensory cortex (S1) and the superior colliculus (SC) while the animal was performing an active spatial discrimination task. The mice were trained to actively touch and associate a whisker-dependent stimulus with reward (GO whisker) while ignoring tactile input to the adjacent whisker (NoGo whisker). We hypothesize that sensory information in the S1 neocortex is encoded in the form of physical features such as location, which is then converted into a map of stimulus value in the SC. We found that both S1 and SC neurons accurately discriminated between adjacent whisker stimuli with the SC displaying a much stronger preference than S1 for the higher valued stimulus. This bias was unique to SC wherein spiking activities were facilitated for the rewarded stimulus and suppressed for the negative stimulus. Importantly, removing the opportunity for positive stimulus selection reduced but did not abolish positive bias in the SC, reinforcing its role in value-based sensory modulation. Moreover, the spontaneous activity in SC but not S1 predicted response latency and performance accuracy. Taken together, this study demonstrated a transformation of stimulus priority from a somatotopic map in S1 to a value map in SC.