Local excitability in the superior colliculus influences evoked responses of lateral geniculate cells in rabbits.

In anesthetized, immobilized rabbits recordings were made simultaneously from cells in the Lateral Geniculate Nucleus (CGL) and Superior Colliculus (CS), in order to study how the CS influences the CGL. The experimental protocol consisted of three steps. In the initial step (first control) the light stimulus was triggered electronically. In the second step (Test), the same stimulus was triggered by a spontaneous spike arising from a collicular cell. Thus the stimulus presentation was time-locked to collicular endogenous activity. The third step was the same as the first and constituted a second control. The frequencies of stimulus application were gated to be approximately the same. The results indicated that the CS exerts two separate effects on CGL units. In 37 pairs (26%), conditioning the stimulus presentation to collicular firing produced a significant enhancement of geniculate responses. In 24 pairs (17%), the geniculate responses declined. In 82 pairs (57%), no significant influence was noted. The colliculo-geniculate influence is transient. The effects peaked between 100 to 200 msec after the collicular spike and returned to their control levels within 300 msec. Collicular cells producing a decline were encountered mostly in the ventral part of the stratum griseum superficiale, and the stratum opticum, whereas collicular cells that were related to an increased geniculate response were more frequently found dorsally. Increments were more pronounced if the distance (D) between receptive fields was short (0 degrees less than D less than 40 degrees) or if the collicular and geniculate fields were far apart (120 degrees less than D less than 180 degrees). The decrement effect was attenuated as the distance separating the two receptive fields. This study suggests that the superior colliculus is capable of generating an internal signal powerful enough to modulate at the geniculate nucleus the visual message conveyed toward the visual cortex. A possible role of the CS in the initiation of the corollary discharge is briefly discussed.