Classical conditioning of the eye withdrawal reflex in the green crab.

Eye withdrawal in the green crab, Carcinus maenas was conditioned by pairing a mild vibration to the carapace as a conditioned stimulus (CS) with a puff of air to one of the eyes as an unconditioned stimulus (US). Animals subjected to repeated pairings showed an increased probability of eye retraction during CS presentation. Significantly less responding was found in several control groups subjected to backward conditioning, unpaired stimuli, stimuli alone, or simply time in the apparatus. Although conditioned animals showed few responses to CS alone after 24 hr, retention could be demonstrated by acquisition that was much more rapid on day 2 than on day 1. Conditioning could also be effected in the eye when it was restrained, a result consistent with reports in the literature that this reflex does not require proprioceptive feedback. Because the neuromuscular circuitry of eye withdrawal is already well defined in Carcinus, this is a promising candidate for studying the neuronal basis of classical conditioning.