TOP-DOWN MODULATION OF PARIETAL CORTEX TO VISUAL DISCRIMINATION LEARNING

The brain has limited capacity to process sensory inputs from the environment. Top-down modulation is essential to selectively extract behaviourally relevant information; however, the neural mechanisms are not fully understood. Here, we investigated the role of the posterior parietal cortex (PPC), a hierarchically higher-level associative area involved in decision making, and how its top-down projections modulate neural responses in the lower-level primary visual cortex (V1) during a visual discrimination task. We used two-photon calcium imaging to measure activity in V1 and PPC, and inactivated PPC using optogenetics. Mice were trained to discriminate two orthogonal grating orientations using a go/no-go discrimination task, and we imaged neural activity before and after mice learned the task. We found that PPC neurons develop a strong preference for the rewarded go orientation while V1 neurons show increased selectivity to both rewarded and nonrewarded orientations after learning. When PPC was silenced after learning during the task, the learning-related improvement in V1 neuronal selectivity was abolished due to increased responses to non-preferred features. These findings support the idea that V1 selectivity is sharpened by PPC-mediated suppression after learning, and that PPC activity is required to maintain learning-related selectivity changes in V1.