Alfredo Mayoral

Extended exposure to a discriminated, limited-hold temporal schedule does not produce stimulus control

Four rats were exposed to two different stimuli (either lights or tones), each stimulus being correlated with independent probabilities of water delivery in a temporally defined schedule. The schedule consisted of a 60 s T cycle with 30 s t(D) and t(-) successive subcycles; t(D) was correlated with a probability of water delivery of 1.0 and t(-) was correlated with a probability of water delivery of 0.0. The schedule was maintained during 180 sessions and extended for 25 extra sessions omitting the stimulus in t(-). The four rats showed low frequencies of responding, response frequency being slightly higher in t(-) than in t(D). The percentage of lost reinforcers was independent of response frequency. The rats which lost less reinforcers were those which obtained more water deliveries during the first 15 cycles of each session. These results show that stimulus control does not develop in limited-hold temporal schedules, and that response-reinforcer effectiveness may depend on the initial contact with reinforcers in the first cycles of the session.

Effects of auditory stimuli correlated with different probabilities of water delivery in a limited-hold temporal schedule

Four rats were exposed to two different tone frequencies, each tone being correlated with independent probabilities of water delivery in a temporally defined schedule. The schedule consisted of a 60-s T cycle, with 30-s t(D) and t(Delta) succesive subcycles. t(D) and t(Delta) were assigned complementary probabilities of water delivery (1.0-0, 0.75-0.25, 0.5-0.5 and 1.0-0) in succesive phases. Each of the first three experimental phases was followed by five consecutive probe sessions using the same probability values, and in which water was delivered independently of responding at the end of each subcycle. Three additional rats were exposed to the same conditions, except that only one tone was used in t(D) and t(Delta). The highest frequencies of responding were observed in two of the rats exposed to differential tone frequencies, an effect that was correlated with lower percentages of lost water deliveries, shorter response latencies, and less variability in the intervals between successive water presentations. In the last phase in which probabilities of water delivery in t(D) and t(Delta) were 1.0 and 0, respectively, response frequency was always higher in t(Delta). These results suggest that the number of water deliveries obtained in the early cycles of every session were responsible for the total number of responses and its correlated effects.

Acquisition of bar-pressing under interval and ratio-schedules in the presence and absence of stimuli correlated with water delivery

After one-session shaping, eight rats were exposed to interval or ratio schedules in the absence and attenuation of the usual dispenser-based stimuli correlated with the delivery of water, and eight rats to normal delivery water conditions. Half of the rats were exposed to a (fixed-interval) FI 1-min schedule; the other half was exposed to a (fixed-ratio) FR 10 schedule of water reinforcement. Experimental phases consisted of two exposures to an FI or FR schedule interpolated with one (continuous reinforcement) CRF schedule, and a final (fixed-time) FT period. Rats did not show typical scallop-like or break-and-run patterns of responding. Rats responded more in the absence of reinforcement delivery correlated stimuli during the CRF interpolated phase than rats in the presence of such stimuli. Responding and obtained reinforcers were higher during the CRF schedule than under the FI and FR schedules. Responding during the FT period was similar to that in the prior FI or FR phase. The results are discussed in terms of the central role of transitions from CRF reinforcement in order to achieve high rates and characteristic patterns of FI and FR schedules.