Additive summation by stimulus compounding irrespective of behavioral contrast during discrimination training: An investigation with positive reinforcement and avoidance schedules

Abstract

The similarity in the discrimination training leading to behavioral contrast and that preceding tests producing response enhancement to combined discriminative stimuli suggested that the two phenomena might be related. This was investigated by determining if contrast in discrimination training was necessary for this outcome of stimulus compounding. Responding to tone, light, and to the simultaneous absence of tone and light (1' + L) was maintained during baseline training by food reinforcement in Experiment I and by shock avoidance in Experiment II. During subsequent discrimination training, responding was reduced in T + L by programming nonreinforcement in Experiment I and safety or response-punishment in Experiment II. In the first e:g»e!i.ment, one rat exhibited positive behavioral contrast, i.e., tone and light rates increased while his T + L rate decreased. In Experiment Il, rats punished in 'if + L showed contrast in tone and light, this being the first demonstration of punishment contrast on an avoidance baseline with rats. The discrimination acquisition data are discussed in the light of current explanations of contrast by Gamzu and Schwartz (1973) and Terrace (1972). During stimulus compounding tests, all subjects in both experiments emitted more responses to tone-plus-light than to tone or light (additive summation). An analysis of the terminal training baselines suggests that the factors producing these test results seem unrelated to whether or not contrast occurred during discrimination training. It was concluded that the stimulus compounding test reveals the operation of the terminal baseline response associations and reinforcement associations conditioned on these multicomponent free-operant schedules of reinforcement. Additive summation refers to an increase in responding to the simultaneous presentation of two independently conditioned discriminative stimuli relative to that controlled by either stimulus presented alone. Weiss (1971, Experiment 1) has shown, however, that separate experience with the two stimuli ultimately compounded is, in itself, insufficient to produce additive summation when these stimuli are later presented simultaneously. Rats were trained on a two-component multiple schedule where responding during a tone produced food on the average of every 30 sec [variable interval (VI) 30 sec] while responding in light produced food every 120 sec, on the average. Responding to tone plus light in a later test was intermediate between the levels of responding to tone and light presented separately. The major procedural difference between the two-component multiple schedules of that experiment and the three-component schedules of experiments reporting additive summation was that in the latter experiments extinction was associated with the simultaneous absence of the positive discriminative stimuli.