Learning in the green crab. Electromyograms reveal that movement of the eye is not required for classical conditioning of the eye withdrawal reflex

The eye withdrawal reflex of the green crab, Carcinus maenas , can be conditioned according to a classical (Pavlovian) procedure by pairing a mild vibration to the carapace as a conditioned stimulus (CS) with an air puff to the eye as an unconditioned stimulus (US). Stereotyped electrical activity accompanies the eye retraction and can be recorded as electromyograms (EMG). The EMG record was used as an indicator of behavior to determine if the pattern of acquisition of conditioned responses was the same in an immobilized eye as in one that was freely moving. The results indicate that acquisition is the same in most cases and suggest that, as in many cases of classical conditioning, integration of the two sensory stimuli (CS and US) is sufficient for learning.