Parallel pathways can conduct visual CS information during classical conditioning of the NM response.

Single and combined lesions were made to the lateral geniculate nucleus (LGN), the superficial layers of the superior colliculus (SC), and the pretectal nuclei (Ptc) prior to conditioning of the nictitating membrane (NM) response in rabbit with a visual conditioned stimulus (CS). Due to technical considerations, lesions of the dorsal LGN were accompanied by lesions of the visual cortex, the only output of the dorsal LGN, in order to render the dorsal LGN nonfunctional. Single lesions to any one of the 3 target systems (LGN, SC, Ptc) did not alter the rate of conditioning. Furthermore, double lesions to any 2 of the systems did not prevent conditioning, although LGN + SC lesions significantly retarded acquisition. When all 3 systems were lesioned, however, animals never acquired to the visual CS, although they successfully conditioned to an auditory CS. The results indicate that in rabbit there are parallel visual pathways individually capable of supporting the acquisition of conditioned NM responses.