Activity of the crayfish caudal photoreceptor submitted to a conditioning paradigm.

The activity of the crayfish caudal photoreceptor (Ph) was studied during the application of a classical conditioning paradigm. This cell is a primary photoreceptor and, at the same time, a secondary neuron in the mechanoreceptive pathway. The electrical stimulation of afferent mechanoreceptor fibers, the analog of the conditioned stimulus (ACS), produced a slight increase in the Ph firing rate. The photic stimulation, the analog of the unconditioned stimulus (AUS), produced a marked increase in the Ph firing rate. After ACS-AUS pairing, a decrease in the Ph firing rate appeared during the ACS-AUS interval, and it was considered the analog of the conditioned response (ACR). This inhibitory response gradually vanished during extinction. The application of ACS or AUS series alone did not produce a response similar to the ACR, this excluding sensitization as a possible cuase. The ACR meets several conventional criteria accepted in learning, supporting the idea that the crayfish caudal photoreceptor is part of a small neuronal network able to learn in the isolated nerve cord preparation.