Electromyographic Record of Classical Conditioning of Eye Withdrawal in the Crab

Classical (Pavlovian) conditioning of the eye withdrawal reflex of the green crab, Carcinus maenas, was studied by recording electromyograms (EMGs) from the main abductor muscle of the eye (19a). The EMG record was a reliable indicator of the response, and it was always correlated with physical movement of the eye, whether evoked by the unconditioned stimulus (a puff of air to the eye), or by the conditioned stimulus (a mild vibration of the carapace). The EMG was used to study the acquisition of conditioned responses in animals with an immobilized eye. Six of eight experimental animals developed responses to the conditioned stimulus in a manner similar to that for animals with freely moving eyes; unpaired controls showed few responses. The results indicate that eye movement is not required for learning. Behavioral tests after conditioning and after the eyes had been freed supported this conclusion. The results exclude theories of classical conditioning of eye withdrawal that invoke a role for stimuli due to eye movement (such as a change in visual field).