Topographic Organization of Excitatory and Inhibitory Commissural Connections in the Superior Colliculi and Their Functional Roles in Saccade Generation

Abstract

Our electrophysiological study showed that there are topographic connections between excitatory and inhibitory commissural neurons (CNs) in one superior colliculus (SC) and tectoreticular neurons (TRNs) in the opposite SC. To obtain morphological evidence for these topographic commissural connections between the SCs, tracers were injected into various parts of the SC, the inhibitory burst neuron (IBN) area and Forel's field H (FFH), in the cat. Retrogradely labeled CNs were classified into three types according to their somatic areas and identified as GABA-positive or -negative immunohistochemically. Caudal SC injections labeled small GABA-positive CNs (<200 μm2) in the deep layers of the opposite rostral SC. Rostral SC injections mainly labeled medium-sized GABA-negative CNs (200–700 μm2) in the upper intermediate layer of the opposite rostral SC and small GABA-positive CNs in its deeper layers. Lateral SC injections labeled small GABA-positive CNs in the opposite medial SC and mainly medium-sized GABA-negative CNs in its lateral part. Medial SC injections labeled small GABA-positive CNs in the lateral SC and medium-sized GABA-negative CNs in the medial SC. In comparison, TRNs projecting to the FFH or IBN region were large (>700 μm2) and medium-sized. Many of the medium-sized GABA-negative CNs were TRNs projecting to the FFH. These results indicate that mirror-symmetric excitatory pathways link medial to medial (upper field) and lateral to lateral (lower field) parts of the SCs, whereas upper and lower field representations are linked by reciprocal inhibitory pathways in the tectal commissure. These connections presumably play important roles in conjugate upward and downward vertical saccades.