Second-order appetitive conditioning in goldfish.

Second-order appetitive conditioning was studied in a series of experiments on goldfish. The first-order conditioned stimuli were white lines of different orientations projected on a target, the second-order conditioned stimuli were diffuse colors projected on the target, the unconditioned stimulus was liquid food delivered through a nipple at the center of the target, and the measured response was target striking. In the first experiment, sustained second-order conditioning was demonstrated. Evidence of colon-line association then was sought without success in two further experiments--a sensory preconditioning experiment in which second-order pairings preceded the first-order pairings, and a first-order revaluation experiment in which responses to second-order stimuli were measured after reversal training with positive and negative first-order stimuli. The stimulus-response reinforcement principle affords the best interpretation of the second-order conditioning demonstrated here: Responding to a second-order stimulus is strengthened by a contiguous first-order stimulus that acquires reinforcing properties on first-order trials.