The responses of five pigeons were reinforced on concurrent variable-interval variableinterval reinforcement schedules in which changeover key responses changed the stimulus and reinforcement schedules associated with the food key. While the reinforcement availability in one component remained unchanged throughout the experiment, the reinforcement availability in the other component was, during several conditions, signalled by the onset of an additional discriminative stimulus. During unsignalled conditions, both the relative frequency of responding and the relative time spent in each component approximated the obtained relative reinforcement frequency in each component. The effect of signalling reinforcer availability in one component was to (1) reduce responding in the signalled component to near-zero levels, and (2) increase the relative time in the unsignalled component, without a corresponding increase in the obtained relative reinforcement frequency. The magnitude of the increase in relative time in the unsignalled component decreased as the overall frequency of reinforcement increased. This deviation in the matching relation between relative time and the obtained relative reinforcement frequency was eliminated if the overall reinforcement frequency was increased before the signal was introduced and then, without removing the signal, gradually reduced.

Pigeons' responses to a uniformly illuminated response key were either reinforced on a variable-interval one-minute schedule of reinforcement or extinguished for one-minute periods. When 1.5 second signals were presented at the beginning of each compornent, so as to differentially predict reinforcement, the pigeons pecked at the signals, at rates higher than rates during the remainder of the component. When the brief signals were not differentially predictive of reinforcement, pecking in their presence decreased to near zero levels. Similar results were obtained with signals based upon colors and upon line orientations. Changes in rates of (unreinforced) pecking occurred during the signal whether pigeons responded differentially during the remainder of the component or not. Experiment II demonstrated that the presence of the signal correlated with extinction was not necessary for pecking to develop at the signal which preceded the component in which responding was intermittently reinforced. The experiments demonstrated a clear dissociation of respondent control from operant control of a response. In addition, operant behavior was shown to be relatively insensitive to differing rates of reinforcement, as compared to the sensitivity of respondent behavior to differing rates of reinforcement produced by the very same operant behavior.Key words: elicited and operant responding, stimulus control, trace stimuli, mixed schedules, key peck, pigeons Considerable evidence has accumulated which suggests that the stimuli associated with the various components of a multiple schedule have an eliciting, as well as a discriminative function (Gamzu & Schwartz, 1973;Gamzu & Williams, 1973;Ricci, 1973;Schwartz, 1973). The alternation of two stimuli which are differentially predictive of food appears to be a sufficient condition for the development of elicited pecking directed at the stimulus correlated with the higher frequency of reinforcement.The recognition of stimulus-reinforcer dependencies in multiple schedules of reinforcement led to the development of the additivity theory of behavioral contrast (Gamzu & Schwartz, 1973). The theory proposes that behavioral contrast-response rate increases in one component (S+) of the schedule resulting from a reduction in the frequency of reinforcement in another component (S-)-occurs beThis report is based on data presented by the author at the meetings of the Psychonomic Society, San Antonio, Texas, November 1978. A related procedure and similar data were reported independently by Coleman, Brown, and Hemmes, at the meetings of the American Psychological Association, Montreal, Canada, September, 1980. Reprints may be obtained from Henry Marcucella, Department of Psychology, Boston, Massachusetts 02215. 51 cause a differential stimulus-reinforcer dependency is imposed upon an already existing response-reinforcer dependency. The two sources of control are said to interact additively to generate a higher response rate than either dependency alone.Support for this view of contrast is provid...

The responses of four pigeons were first reinforced in the presence of two different wavelengths (green and red) on a two-ply multiple schedule with identical variable-interval 3-min schedules of reinforcement associated with each component. While the constantcomponent reinforcement schedule remained unchanged during the experiment, the schedule associated with the variable component was changed to (1) signalled variable time, (2) unsignalled variable time, or (3) signalled variable interval. The probability with which the availability of the reinforcer was signalled in the variable-interval schedules was either 0.5 or 1.0. Positive contrast occurred in both signalled variable-interval and variable-time schedules, but only when the availability of all the variable-component reinforcers was signalled. Signalling the availability of only 50% of the reinforcers in signalled variable-interval schedules resulted in negative induction. The present data suggest that positive behavioral contrast resulting from signalled reinforcer availability is due to the presence of an extinction-correlated stimulus. sponse rate of the two components covary in the same direction, the interaction is labelled "positive induction" (rate increases in both components) or "negative induction" (rate decreases in both components). Reynolds (1961a) proposed that the occurrence of positive and negative contrast is a function of the relative reinforcement frequency of the two components. That is, the response rate in the constant component varies inversely with the reinforcement frequency in the variable component. Decreasing the reinforcement rate in the variable component results in positive contrast, while increasing the variable-component reinforcement rate results in negative contrast. Indeed, subsequent research has shown that the relative frequency of reinforcement in the multiple components is a powerful predictor of the magnitude of contrast effect (Bloomfield, 1967;Nevin, 1968; Reynolds, 1961b;Reynolds and Limpo, 1968).An alternative position has been offered by Terrace (1972). He proposed that the necessary and sufficient condition for occurrence of positive contrast is a suppression in the response rate of the variable component, which results as an emotional reaction to the discrimination situation. In this formulation, decreasing reinforcement density is understood to be only 199 NUMBER 2 (SEPTEMBER) 1976, 26,[199][200][201][202][203][204][205][206]

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