Fig. 1.

Simplified schematic of the brain-stem and cerebellar saccade circuitry for leftward (ipsiversive, Ipsi) and rightward (contraversive, Contra) saccades. A signal from the left caudal fastigial nucleus (cFN) reaches right inhibitory burst neurons (IBN), which project to the left abducens (ABD) nucleus, which, in turn, drives the left lateral rectus muscle that is the agonist for leftward (ipsiversive) saccades or the antagonist for rightward (contraversive) saccades. The left ABD nucleus contains motoneurons for the left lateral rectus muscle and internuclear neurons, which project to motoneurons for the right medial rectus muscle in the right oculomotor nucleus (III). Filled neurons are inhibitory. Open neurons are excitatory. Vertical arrows indicate the change in activity that we propose occurs after left cFN inactivation. After left cFN inactivation, leftward (ipsiversive) saccades become hypermetric, and rightward (contraversive) saccades become hypometric. We recorded ABD neurons that act as an agonist and antagonist during ipsi- and contraversive saccade, respectively.