A true neuronal consensual pupillary reflex in chicks

The existence of a true neuronal consensual pupillary reflex (CPR) in birds has long been debated. In this century Noll (Noll, A. (1915). Archiv fur P hysiologie (Leipzig), 350-372.) claimed to observe a neuronal CPR in a pige on, but this was contradicted by Levine (Levine, J. (1955). Science, 122, 6 99.), who observed a direct transillumination effect (Durckleuchtungs-effek t) due to the retinas of the two eyes of the pigeon being in close appositi on. To determine if a neuronal CPR exists, we transected the optic nerves o f 28 chicks and observed and videotaped the direct and indirect pupillary r esponses. Twenty-one of the chicks exhibited no direct response in the oper ated eye but did exhibit an indirect pupillary response. The non-operated e ye showed a direct but no indirect response. These results conclusively dem onstrate for the first time that a true neuronal CPR does exist in chickens . (C) 1998 Elsevier Science Ltd. All rights reserved.