[Neural Circuits of Saccadic Eye Movements: Insights into Active Vision]

This article deals with the neural circuits involved in the generation and suppression of saccadic eye movements. Voluntary eye movements are known to be organized in two-dimensional horizontal and vertical coordinate system, whereas the vestibulo-ocular reflex (VOR) uses a three-dimensional semicircular canal coordinate system. However, it is generally accepted that the neural circuits for saccades and the quick phase of vestibular nystagmus share a common pathway. Despite the discrepancy, this issue has not been noticed, because the output pathway for the vertical saccade system remains poorly understood. To resolve this discrepancy, we analyzed the pathway from the superior colliculus (SC) to vertical ocular motoneurons, using intracellular recording and staining techniques. We found that the saccade system and the vestibuloocular system share the common semicircular coordinate system. Based on this saccade generation circuit, we proceeded to analyze the saccade trigger system. Our results showed that the saccade-triggering signal originates from the saccade-driving pathway in the SC, passing through inhibitory burst neurons, to suppress the tonic firing of inhibitory omnipause neurons in the raphe nucleus. This triggering mechanism is then discussed in relation to the suppression pathway for saccades during fixation and active vision.