Morphological and electrophysiological characteristics of the commissural system in the superior colliculi for control of eye movements.

Commissural connections between the superior colliculi (SCs) were well known anatomically, and assumed to be only inhibitory in relation to visual inputs. However, by recording intracellular potentials, we revealed that a strong monosynaptic excitatory commissural connection exists between the rostral SCs of the cat. Commissural excitation existed between the medial-medial or lateral-lateral parts of both SCs, while commissural inhibition existed between the medial SC on one side and the lateral SC on the opposite side. These commissural excitation and inhibition were also confirmed morphologically with the double-labeling method of HRP-conjugated gold particle and GABA. Similarity of the topography of commissural inhibition between the SC system and the vestibuloocular system supported the conclusion that the saccadic eye movement system uses the same semicircular canal coordinate as the vestibuloocular system. The commissural excitation may help to maintain Listing's law in saccadic eye movements.