A Theory of Thalamocortical Loops in Evidence Accumulation and Decision-Making

A prominent property of the neocortex is that it forms reciprocal excitatory loops with the thalamus. Here, we propose a function for the loops between layer 5 pyramidal tract neurons in the cortex and matrix-type relay cells in the thalamus, which is based on known anatomy and biophysics. This pathway forms an amplifier circuit with a gain less than unity in the resting condition. However, descending cortical input to apical tufts of L5 neurons can raise the gain of these neurons, boosting the thalamocortical loop into either an integrating or an amplifying regime. Multiple thalamocortical loops compete via lateral inhibition in the thalamus, resulting in a winner-take-all global dynamic among the motor commands encoded by each loop. When L5 neurons are driven by sensory inputs, the integration regime closely resembles the drift-diffusion model of decision-making. We explore basic relationships among parameters of this model and compare broadly against neurophysiological data.