Sumn2asy.-Rats and fish were trained to give a free operant response under partial versus consistent reinforcement and high versus low effort. Resistance to extinction was then measured in terms of number of responses to criterion ( R ) , time to criterion ( T ) , and rate to criterion (R/T). Effort failed to affect any of these measures in either species. In rats partial reinforcement produced greater resistance to extinction thzn consistent reinforcement on R and T, but not on R/T, whereas partially reirforced fish failed to differ from those consistently reinforced on R and T bur exceeded the consistently reinforced fish on R/T. The R and T measures were correlated in both groups of fish but only in the consistently reinforced rats. Partially reinforced fish were less variable than consistently reinforced fish on the T m t s u r e . It is suggested that the principal effect of partial reinforcement in rats is on mean time to extinction, but the effect in fish is on variabiliry of time to extinction.In their work on the partial reinforcement effect (i.e., increased resistance to extinction of a response resulting from intermittent omissions of reinforcement during training, PRE), Bitterman and his co-workers have studied several vertebrate and invertebrate species using both positive operant conditioning methods and negative classical conditioning methods. We are concerned here primarily with those studies in which positive reinforcement has been used with white rats and with the African mouthbreeding fish (Tilapia macrocephala). After training fish to suike at a target, training was continued, using a discrete trial technique and schedules of 50% and 100% reinforcement (Wodinsky & Bicterrnan, 1959). Response latency during subsequent extinction trials served as a measure of response strength. These procedures resulted in greater resistance to extinction in the 100% group than in the 50% group. Greater response strength for consistently reinforced Ss than for partially reinforced Ss also was demonstrated with widely spaced trials (Longo & Bitterman, 1960) and with extensive training (Wodinsky & Bitterman, 1360). The learning curves of the partial and consistent groups in the latter smdy also revealed a decrease in latency following nonreward after about 20 days of training at 20 trials per day. A later study demonstrated that this effect occurred whether the 50% schedule was random or alter-
A strong partial reinforcement effect was produced in 20 fish trained to a free operant striking response by injection of reserpine before extinction of the response. This was interpreted as a result of inhibition of responding during extinction of 20 Ss trained on a consistent schedule of reinforcement rather than facilitation of responding in Ss trained on a .4 variable ratio schedule of reinforcement.
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