Distinct roles of cortical layer 5 subtypes in associative learning

Adaptive behavior is critically dependent on associative learning, where environmental cues are linked with subsequent positive or negative outcomes. At the core of this process, primary neocortical sensory areas serve as pivotal nodes, processing stimuli and distributing information to cortical and subcortical networks. Layer 5 (L5) of the cortex comprises two types of pyramidal projection neurons---intratelencephalic (IT) and extratelencephalic (ET) neurons---each with distinct downstream targets. Sensory cortex exhibits plastic changes after forming a sensory-reward association and L5 neurons plays a critical role in this formation. However, the specific contributions of these L5 neuronal subtypes to associative learning remain poorly understood. By deciphering their specific functional roles during stimulus-reward associations, we can understand their interplay and computations throughout the learning process. In the present study, we distinguished IT and ET neurons in the primary somatosensory cortex (S1) and examined their roles in a whisker-based frequency-discrimination learning task. Longitudinal two-photon calcium imaging revealed distinct response characteristics between IT and ET neurons throughout learning. Interestingly, the activity of IT neurons showed stimuli encoding from the beginning that hardly changed over the five days of learning. In contrast, ET neurons became increasingly responsive to stimuli associated with reward. Chemogenetic silencing of either IT or ET neurons both impaired learning, but in strikingly different ways. By modeling the response characteristics of IT and ET neurons using a reinforcement learning framework, we show that IT neurons primarily encode sensory stimuli, and their representations are critical for forming stimulus-reward associations. ET neurons instead represent the value of the stimulus, used for refining behavior. Thus, our results delineate the distinct roles of L5 IT and ET neurons, underscoring their integral and complementary contributions to associative learning.