Switching between cortical and subcortical sensorimotor pathways.

It is well known that the reaction times of visually guided saccades exhibit a bimodal distribution. Those with extremely short reaction times are termed 'express saccades'. In their case, visual input appears to be transformed into motor output via a 'short-loop', brainstem-mediated pathway. In contrast, those with longer reaction times are called 'regular saccades'. The latter are presumably executed via a cortically mediated, 'long-loop' sensorimotor pathway. The 'gate' that switches signal flow between the short and long loop is thought to be located in between the superficial and deeper layers of the superior colliculus (SC). Nonlinear signal amplification mechanisms, which operate in local circuits of the deeper SC layers may underlie this gating function, with switching of the gate regulated in a context-dependent manner by inputs from the cerebral cortex and basal ganglia.