Partial reinforcement and resistance to extinction in the earthworm.

Abstract

With light as the unconditioned stimulus, groups of earthworms, Lumbricus terrestris, were conditioned to perform a withdrawal response to a vibratory conditioned stimulus. The continuously reinforced group received 3 daily 50-trial acquisition sessions with the conditioned stimulus and unconditioned stimulus presented on each trial. The partially reinforced group received the same number of trials with the unconditioned stimulus omitted on a random half of the trials following an initial series of reinforced trials on the first 2 days. Fifty extinction trials were administered on day 4. The 2 groups learned readily and did not differ significantly during conditioning. The partially reinforced group demonstrated greater resistance to extinction than the continuously reinforced group. Pseudoconditioning and sensitization control groups yielded no increase in frequency of withdrawal responses during 3 daily sessions.