Pavlovian conditioning in Hermissenda consists of pairing light, the conditioned stimulus (CS) with activation of statocyst hair cells, the unconditioned stimulus (US). Conditioning produces CS-elicited foot shortening and inhibition of light-elicited locomotion, the two conditioned responses (CRs). Conditioning correlates have been identified in the primary sensory neurons (photoreceptors) of the CS pathway, interneurons that receive monosynaptic input from identified photoreceptors, and putative pedal motor neurons. While cellular mechanisms of acquisition produced by the synaptic interaction between the CS and US pathways are well-documented, little is known about the mechanisms responsible for the generation or expression of the CR. Here we show that in conditioned animals light reduced tonic firing of ciliary activating pedal neurons (VP1) below their pre-CS baseline levels. In contrast, pseudorandom controls expressed a significant increase in CS-elicited tonic firing of VP1 as compared to pre-CS baseline activity. Identified interneurons in the visual pathway that have established polysynaptic connections with VP1 were examined in conditioned animals and pseudorandom controls. Depolarization of identified type I e interneurons with extrinsic current elicited a significant increase in IPSPs recorded in VP1 pedal neurons of conditioned animals as compared with pseudorandom controls. Conditioning also enhanced intrinsic excitability of type I e interneurons of conditioned animals as compared to pseudorandom controls. Light evoked a modest increase in IPSP frequency in VP1 of conditioned preparations and a significant decrease in IPSP frequency in VP1 of pseudorandom controls. Our results show that a combination of synaptic facilitation and intrinsic enhanced excitability in identified components of the CS pathway may explain light-elicited inhibition of locomotion in conditioned animals.Pavlovian conditioning in Hermissenda results in conditioned stimulus (CS)-elicited inhibition of phototaxis (Crow and Alkon 1978;Crow and Offenbach 1983) and CS-elicited foot contraction (Lederhendler et al. 1986). While these two motor acts appear dissimilar, inhibition of CS-elicited ciliary locomotion and CS-elicited foot contraction may synergistically interact to contribute to the inhibition of phototactic behavior observed in conditioned animals. While some progress has been made in the identification of foot contraction motor neurons (Goh and Alkon 1984) and cellular neurophysiological correlates of conditioning in a contractile pedal motor neuron (Goh et al. 1985), little is known regarding cellular plasticity in the neural network supporting ciliary locomotion in conditioned Hermissenda. Recently, pedal neurons activating cilia on the anterior foot have been identified and synaptic connections between type I, II, and III interneurons and VP1 ciliary activating pedal neurons have been established (Crow and Tian 2003).In this report we show that Pavlovian conditioning results in light-elicited inhibition of the spike ...