Distinct neural population code and causal roles of primate caudate nucleus in multimodal decision-making.

Perceptual decision-making involves distributed networks spanning both association cortices and subcortical areas. A fundamental question is whether such a network is highly redundant, or each node is distinct with unique function. Using a visuo-vestibular decision-making task, here we show the subcortical caudate nucleus (CN) of male primates displays distinct population code compared to association cortices along the modality dimension. Specifically, in a low-dimensional state subspace, neural trajectory in the frontal and posterior-parietal association cortical activity during multimodal-stimulus condition evolves along the visual trajectory, whereas along the vestibular trajectory in the CN. We then show CN population activity is consistent with the animal's behavioral strategy employed within a generalized drift-diffusion framework. Importantly, causal-link experiments, including application of GABAa-receptor agonist, D1-receptor antagonist, and electrical microstimulation, further confirmed CN's critical contributions to perceptual behavior. Our results confirm CN's vital importance to decision making in complex environments with multimodal information.