Evidence of a collicular input to the dorsal lateral geniculate nucleus in rabbits -- electrophysiology

In anesthetized and paralyzed rabbits, unit responses of lateral geniculate nucleus (LGN) cells to focal electrical stimulation of the superior colliculus were studied. Geniculate responses to collicular stimulation (SCS) were compared with responses to optic nerve shock (ONS). A weak correlation coefficient suggested that collicular stimulation did not fire geniculate cells through collateral activation. Further differentiation between collicular and retinofugal inputs to LGN was made possible by repetitive stimulation. Geniculate cells which responded to collicular stimulation were relay cells as they were antidromically invaded from the visual cortex. This ruled out recordings from the ventral geniculate, since this area does not project to the visual cortex. A direct colliculo-geniculate pathway was revealed by antidromic activation of collicular cells by stimulation of the dorsal LGN. Finally, triggering flashes by collicular firing resulted in a marked modification of the geniculate test response. The results suggest that the superior colliculus sends fibers to the LGN and is capable of modulating the retino-cortical neuronal message at the level of the LGN.