Prefrontal Manifold Geometry Explains Reaction Time Variability

ABSTRACT The stochastic drift-diffusion model proposes that the variability in reaction time is due to randomness during the accumulation of evidence until a decision threshold is reached. However, the neural mechanisms that explain both the randomness and implementation of the decision threshold in the model remain unclear. Here we address these questions using the dynamical systems approach to analyze primate frontal eye field activity and using microstimulation for causal manipulations. We built a mechanistic model in which signals associated with motor plans are bumped out of their attractor state by go-cue signals that emerge ∼60 ms after the go cue. The network then travels through a transition subspace towards a movement-initation subspace that emerges ∼35 ms before movement onset and implements the decision threshold. We postulate that the randomness in evidence accumulation, and hence in reaction times, is explained by the amplification of noise during movement preparation by the geometry of the frontal eye field manifold.