Real-world signals such as communication sequences unfold over time with a characteristic temporal structure. Recognizing temporally ordered patterns is key to survival. To explore how cortical neuronal activity underpins this capacity, we recently developed a task in which head-fixed mice distinguish between tactile ‘word’ sequences constructed from distinct vibrations delivered to the whiskers, assembled in different orders. Animals lick to report the presence of the target sequence (GO/NOGO design). Mice can respond to the earliest possible cues allowing discrimination of the GO stimulus, effectively solving the task as a ‘detection of change’ problem, but perform better when responding later, after more evidence could be collected.
We recorded and manipulated cortical activity with two-photon imaging and optogenetics while head-fixed mice performed the task. We expected that learning the task would induce cortical neurons to refine their sensory tuning by becoming more selective to the target GO sequence. Instead, two-photon imaging showed that, upon learning, neurons in both the primary somatosensory barrel cortex (S1bf) and posterior parietal cortex (PPC) became sensitive to multiple task variables, including sensory input but also the animal’s action decision (goal-directed licking) and the trial’s outcome (presence or absence of a predicted reward). Optogenetic inactivation showed that while S1bf was necessary for sequence discrimination, PPC was not. Moreover, classifiers trained on the activity of S1bf neurons robustly discriminated both the sensory type of a trial (GO/NOGO) and whether the animal licked on the trial, while those trained on PPC neurons could discriminate sensory information but not the animal’s actions.
Our results demonstrate that conditioning on a goal-directed sensory discrimination task results in neurons within S1bf whose activity reflects the learnt links between target stimulus and licking. They also show that PPC contains copies of task-relevant information even while playing no causal role in the animal’s performance.