Visual reinforcement: Operant acquisition in the presence of a free mirror

In Experiment I Siamese fighting fish performed operant responses that produced a mirror image to which the fish could display; consistent detectable display was not necessary for maintenance of operant performance. In Experiment II fish acquired and continued to perform an operant response that produced a mirror, although a “free” mirror was continually present; display rate to the contingent mirror increased while display rate to the “free” mirror was decreasing. An interpretation of these and previous data in terms of sensory reinforcement is suggested.