Task Engagement Gates Interareal Communication Geometry in the Mouse Thalamocortical-Midbrain Visual Circuit

Visual processing unfolds across hierarchically organized brain circuits. Existing theories largely explain changes in population geometry through local shifts in gain, firing-rate statistics, or recurrent dynamics, yet do not account for how interareal coordination interacts with local population geometry to constrain downstream population states. We used task engagement, compared to a passive condition, to probe this coordination in Neuropixels recordings spanning the mouse visual thalamocortical-midbrain circuit. Engagement reduced network activity, response participation, and dimensionality across the hierarchy. To account for this circuit-level organization, we developed a theoretical framework in which afferent population geometry interacts with local recurrent dynamics to constrain the accessible dynamics of downstream populations. Across the thalamocortical stages, population-wide afferent statistics predicted downstream activity and dimensionality. At the cortex--midbrain interface, engagement instead reorganized interareal communication geometry. Together, these results identify interareal input geometry as a key constraint on neural population dynamics and uncover a general principle by which behavioral engagement constrains neural state spaces across distributed visual circuits.