STIMULUS COMPOUNDING IN FREE-OPERANT AND CLASSICAL CONDITIONING

Transcription

1 Psychological Bulletin 1972, Vol. 78, No. 3, STIMULUS COMPOUNDING IN FREE-OPERANT AND CLASSICAL CONDITIONING A REVIEW AND ANALYSIS 1 STANLEY J. WEISS» The American University Free-operant and classical conditioning studies reporting additive summation, suppressive summation, and response averaging to compounded stimuli are reviewed, and a stimulus control model applicable to additive and suppressive summation in both paradigms is presented. A symmetrical composite-stimulus continuum, denned by the on-off states of the discriminative or conditioned stimuli controlling behavior in training, is seen to be common to both types of summation, with the functions of the ail-on and ail-off continuum extremes interchangeable. Moreover, additive summation, suppressive summation, and response averaging appear to be dependent on discrimination training, and determined by the history of, as well as the immediate relations between, response outputs and reinforcement densities conditioned to particular stimulus values along the composite training dimension. This evidence, and the stimulus control model derived therefrom, are used to functionally relate summation to generalization peak shift and to suggest that these additive, suppressive, and averaging effects to compounded stimuli might further help in clarifying, and in placing in contextual perspective, what are currently thought of as "excitatory" and "inhibitory" mechanisms in learning. This review presents the current status of stimulus-compounding research in free-operant and classical conditioning, 3 and through this presentation it attempts to develop a comprehensive functional analysis of the variables involved in determining the various behavioral resultants reported within this experimental paradigm. The scope of the present paper does not extend to multiple-response or multiplechoice procedures, exemplified by the experiments of Schoeffler (1954) and Atkinson, Calfee, Sommer, Jeffrey, and Shoemaker (1964), but instead focuses on unitary response paradigms where variations in rates or magnitudes of response are of primary concern. Ultimately, the stimulus control analysis formulated is (a) discussed in terms of some 1 The preparation of this manuscript was supported in part by Grant MH from the National Institute of Mental Health, United States Public Health Service. 2 Requests for reprints should be sent to Stanley J. Weiss, Department of Psychology, American University, Washington, D. C Due to the limited availability of the Russian literature in English, stimulus compounding within classical conditioning is, unfortunately, probably not covered as extensively as the free-operant research in that area. Nevertheless, the classical conditioning studies presented are consistent enough in their findings, with exteroceptive conditioned stimuli, to adequately support the analyses presented. current conceptions of "excitation" and "inhibition" and (b) used to relate stimulus compounding research to other problems under investigation in the area of stimulus control. In stimulus-compounding research within free-operant and classical conditioning, subjects are customarily brought under the independent control of two or more stimuli, usually from different modalities; then, in testing, the response forthcoming to the simultaneous presentation of two or more of the independently conditioned stimuli is compared to the responses controlled by single-stimulus presentations. This procedure should be clearly distinguished from compound-stimulus conditioning, recently reviewed by Razran (1965) and Baker (1968). In compound-stimulus conditioning subjects are conditioned to a multielement stimulus, and the dependent measure is customarily the behavior controlled by the respective elements. This procedure is the mirror image of stimulus compounding. In the two-stimulus case there are four possible outcomes to stimulus compounding. These are additive summation, averaging, suppressive summation, and no difference in the response to individually presented and compounded stimuli. For additive summation to be evidenced, there is, at the minimum, at least a greater response forthcoming to the 189

2 190 STANLEY J. WEISS compound stimulus than to either individually presented component. When averaging occurs, the response to the compound is somewhere intermediate between those controlled by the two independently conditioned stimuli. Suppressive summation is observed when a lesser response is controlled_by the compound than by either of the stimuli presented singly. These several behavioral resultants to stimulus compounding are discussed in turn and compared. CLASSICAL AND FREE-OPERANT CONDITIONING EXPERIMENTS DEMONSTRATING ADDITIVE SUMMATION Classical Conditioning The first studies reporting additive summation were done in Pavlov's laboratory in the early twentieth century. Kimble (1961) cited a classical conditioning study performed by Pavlov in which one conditioned stimulus, the odor of oil of camphor, ordinarily evoked 60 drops of saliva; the other stimulus, a mild shock, usually produced 30 drops. When these stimuli were presented together they evoked 90 drops. Razran (1939) in his survey of studies in configural conditioning referred to two other Russian experiments that demonstrated additive summation. Leporsky 4 found that simultaneous presentations of several stimuli, each of which had been previously conditioned separately to the same response, produced conditioned responses of greater magnitude than any application of a single stimulus or of a combination of the same but fewer stimuli. Razran reported that Leporsky's results were corroborated by Yakovleva (1927) who also noted that the augmentation by summation is gradually diminished if the combination is continually reinforced. A similar decrease in response enhancement to compound-stimulus presentations was reported by Wolf (1963) in a free-operant paradigm where responding was maintained by food reinforcement. Hull (1940) detailed a classical conditioning experiment employing the galvanic skin response (GSR) in which eight human subjects were first conditioned with shock to respond to weak light, and next to a weak vibratory 4 N. I. Leporsky. Materials to the physiology of conditioned inhibition. Unpublished thesis, St. Petersburg, Russia, stimulus that was paired with the shock. Although the mean reaction, in millimeters, was approximately 3.55 to the light or vibrator separately, the compound stimulus elicited a GSR of 4.40 millimeters. Konorski (1948, pp ) also reported response enhancement upon the joint presentation of any two of three conditioned stimuli independently associated with food with the relative enhancement greater in those situations where, through reduced deprivation and/or decreased stimulus intensity, the separately presented conditioned stimuli controlled minimal salivation. Grings and O'Donnell (1956) differentially conditioned the GSR of 32 human subjects to three classes of colored-light stimuli. These stimuli were reinforced consistently, not paired with reinforcement, or not presented during training. During the test period, response magnitude was observed to pairs of stimuli. A pair of previously reinforced stimuli produced a significantly greater response than a reinforced stimulus paired with a nonreinforced or a nonpresented stimulus. The difference between the last two paired conditions was questionable. Grings, Uno, and Fiebiger (1965) directly replicated the Grings and O'Donnell (1956) experiment and confirmed that on the first test trial response magnitude tended to be ordered according to the principles of response summation. Unfortunately, since their conditioned stimuli (CS+) were continuously reinforced during training, the response elicitation capacity of those stimuli rapidly extinguished during testing. Timofeev (1959) mentioned results of Vediaev who reported increased conditioned responses (CRs) in both fishes and hens to the simultaneous presentation of independently conditioned exteroceptive stimuli. When Timofeev (1959) compounded independently, and sequentially, conditioned exteroceptive (40-watt light) and interoceptive (stretching of the wall of the anterior part of the alimentary tract via rubber tube insert) stimuli, additive summation was reported in fishes, but not in hens. Timofeev went on to cite that data similar to and even more pronounced than those forthcoming in his experiment with birds were reported in 1952 by Airapet'iants. When Airapet'iants presented an exteroceptive and an interoceptive agent simultaneously to dogs,

3 STIMULUS COMPOUNDING 191 conditioned as well as unconditioned reactions were inhibited. The difficulties in comparing methodologically and technically exteroceptive-exteroceptive and exteroceptive-interoceptive control, and the scarcity of detailed empirical information, forbids comparison and analysis by the author of these two types of control. For experimental purposes this review is primarily concerned with exteroceptive stimulus control in the training paradigms of both operant and classical conditioning studies exploring stimulus compounding. Free-Operant Conditioning Variable-interval schedules. Weiss (1964; 1971, Experiment 2) and Wolf (1963, Experiment 1) demonstrated within a multimodal training situation that additive summation could be produced in free-operant as well as classical conditioning. They trained rats to respond on a variable-interval (VI) food reinforcement schedule when a tone or a light was present. (On a VI schedule a reinforcement is produced by the first response occurring some average time, quantitatively denning the particular VI schedule, from the previous reinforcement.) During the simultaneous absence of these stimuli, light-out no-tone, extinction was in effect. In testing, all subjects summated to the simultaneous presentation of tone and light; that is, they emitted more responses to this test condition than to light or tone individually presented. 5 Weiss (1964) also re- 6 Cornell and Strub (1965), Miller (1971), and Wolf (1963, Experiment 2) also demonstrated summative effects to compounded stimuli when the independently conditioned discriminative stimuli were differentially located lights with Miller (1971) reporting that an intermodal compound maintained a higher percentage of combined component stimulus response frequencies than did an intramodal compound. Intensity dynamism effects per se have been conclusively shown not to be a factor in multimodal summation (Weiss, 1969a), and Miller (1971, Experiment 1) recently attempted a systematic investigation of this variable's effect during stimulus compounding when the compound stimulus turns out to be a greater intensity of physical energy to a single modality. He interpreted his results as minimizing the importance of compound intensity effects in intramodal compounding, although his between-intensity groups comparison was confounded by the training and testing experienced by the high-intensity group in a previous experiment. However, test performance of low- and moderate-intensity groups, who started the experiment naive, does support Miller's conclusion. ported summation when generalized stimuli were compounded. Fixed-interval schedules. Miller and Ackley (1970) and Miller (1971) produced summation when they compounded S D s individually associated with fixed-interval (FI) schedules. (On an FI schedule, a reinforcement is available for the first response emitted a specified time after the previous reinforcement.) Miller and Ackley trained four rats on a multiple FI FI extinction schedule where FI periods were signaled by tone or light, and S A by the simultaneous absence of tone and light. In spite of substantial rate overlap between single- and compound-stimulus presentations evidenced in test records, the overall test data reveal that (a) compound-stimulus presentations controlled a summative effect over the entire fixed-interval period for the three interval schedules these authors employed; and (b) for total test data, this summative effect could be adequately described as a simple additive function of the rates controlled by the single FI S D s. 6 Avoidance contingencies. Miller (1969b) separately conditioned a light and a buzzer to control a shock avoidance response in a shuttle box. Those rats that demonstrated a high percentage of avoidance responses, and fairly short latencies, to the single stimuli exhibited significantly shorter avoidance latencies to these stimuli compounded. Riess (1969) trained rats on a Sidman schedule in a shuttle box where hurdle crossing was the avoidance response. (On a Sidman schedule shocks are unsignaled, but a response postpones shock for a prescribed time in this instance, 20 seconds.) Subsequently, with the avoidance response restricted, subjects were classically fear conditioned. Tone and light independently signaled unavoidable shock. 6 Weiss, at the 1968 Convention of the American Psychological Association in San Francisco, reported a similar additive function when he compounded stimuli controlling responding on differential-reinforcement-oflow-rate (DEL) schedules. (A response is reinforced on a DRL schedule if a minimum specified time has elapsed between responses.) Five animals were trained on a multiple DRL 18-second DRL 25-second extinction schedule with tone and light counterbalanced over DRL schedules, and S 4 signaled by the simultaneous absence of tone and light. Also in the positive reinforcement situation, Miller and Price (1971) showed that when two stimuli each maintain a certain speed of running, their combination controls a significantly greater speed.

4 192 STANLEY J. WEISS Thereafter, when tone or light were superimposed on the Sidman base rate, conditioned acceleration, an increased avoidance rate, was observed (cf. Sidman, Herrnstein, & Conrad, 1957). However, conditioned acceleration to compounded tone and light was reliably higher in 22 of 24 daily subject performances than acceleration to the individually presented stimuli. Riess thus demonstrated summation of conditioned acceleration. Emurian and Weiss (1972) reported additive summation when they compounded S D s signaling a Sidman avoidance contingency during an extinction test as well as over a substantial majority of 28 maintenance test sessions in which the avoidance contingency was effective during single- and compound-stimulus presentations. Most striking was the fact that subjects received fewest shocks in the compound on 85% of the maintenance tests. In training, a tone and a light each independently signaled that a Sidman avoidance schedule was in effect and maintained responding, while light-out no-tone was a shock-free period that controlled response cessation. Discrimination and shock avoidance criteria were stringent. The relations between test response outputs to single (tone or light) and compound (tone plus light) test presentations appeared generally similar to those reported by Weiss (1969a) when VI S D s were compounded. This led Emurian and Weiss (1970) to conclude that free-operant additive summation is not incentive-specific. In the Riess (1969) and Emurian and Weiss (1972) studies, rate of avoidance responding was substantially higher during tone or light than during the simultaneous absence of these stimuli. Emurian and Weiss (1972) pointed out that the same general rate relations exist in training between these respective stimulus conditions when additive summation was reported to tone plus light when responding to tone and light was maintained by a food rather than a shock incentive. Emurian and Weiss (1972) therefore concluded that the discrimination training leading to those differential rates might be of prime importance in the production of additive summation to compounded stimuli. If this is the case, an interpretation of summative avoidance in terms of an interaction of "anxiety states" may not be warranted, or at best would be incomplete. CONSIDERATION OP STIMULUS CONTROL AND DISCRIMINATION TRAINING IN ADDITIVE SUMMATION PARADIGMS Weiss (1969a) has pointed out that although additive summation has been empirically established in both free-operant and classical conditioning, its dynamics remained obscure. To clarify them, he analyzed the stimulus relations between the various experimental conditions of training and testing in both classical and free-operant summation paradigms and discovered some startling consistencies. Composite-Stimulus Control Analysis Similar differential training is common to both classical and free-operant experiments demonstrating additive summation. In classical conditioning studies the presentation of an unconditioned stimulus (UCS) was signaled by the independent presentation of CSi or C 2 and the simultaneous absence of these stimuli (CSi + C^2) signaled extinction. That extinction was occurring to this latter condition was first reported by Pavlov who wrote: when conditioned reflexes are being established in dogs for the first time, it is found that the whole experimental environment... acquires at first conditioned properties.... But later on, when the special reflex to the single, definite and constant stimulus has appeared, all the other elements of the environment gradually lose their significance most probably on account of a gradual development of internal inhibition Q1927, p. 115]. In the instrumental studies described above, reinforcement periods in training were signaled by the independent presentation of S D i or S D 2. The simultaneous absence of these stimuli (S i + S D 2) customarily was systematically nonreinforced and therefore came to control response cessation just as (CSi + CS2) in the classical conditioning studies ceased to elicit conditioned responses. Furthermore, a composite comparison between the stimuli that control summation in these studies and those that are associated with reinforcement and extinction reveals additional consistencies. Weiss (1969a) noted: [A] composite continuum is defined by the on-off states of all relevant discriminative or conditioned stimuli, one

5 STIMULUS COMPOUNDING 193 TABLE 1 SCHEMATIC REPRESENTATION OF CLASSICAL AND FREE-OPERANT MULTIMODAL ADDITIVE SUMMATION EXPERIMENTS Composite stimulus continuum Light-out No-tone (f + I") Tone Light-out or Light No-tone (T+L) (L+T) Tone and light (T +L) 1. Extinction signaled by ail-off (T + L) extreme (a) Classical conditioning experiments 11 Extinction Training CSi CS 2 Summation obtained Test (ft) Free-operant experiments' 1 Extinction (S A ) Training S D 1 S D 2 Test Summation obtained 2. Extinction signaled by ail-on (T + L) extreme (a) Classical conditioning experiments No data available Training (ft) Free-operant experiments" Summation obtained S D i Extinction Test * Leporsky (1911), Yakovleva (1921), Pavlov (in Kimble, 1961), Hull (1940). Konorskl (1948), Timofeev (1959). >>Wolf (1963, Experiment 1), Weiss (1964; 1971, Experiment 2); Emurian & Weiss (1972), Miller & Ackley (1970), Miller (1971). «Weiss (1969a), Meltzer & Freeman (1970). continuum extreme anchored to the ail-off state, the other to ail-on. The points between these continuum extremes are scaled with reference to the number of elements common to each..., the number of points on any composite scale being equal to the number of independently conditioned S D s, or CSs, plus one [pp ]. Using tone (T) and light (L) as representative stimuli to make the example concrete, one notes that in both the reported free-operant and classical conditioning studies yielding additive summation the composite continuum starts at the ail-off extreme signaling extinction (T+L), progresses through the individual presentation of each S D or CS that maintains responding (T + L or L + T), to the ail-on extreme (T + L) that yields summation. A representative two-element, tone and light, three-step composite continuum is shown in the top row of Table 1 which schematically represents the training and test conditions of those multimodal classical and free-operant summation experiments that associated the absence of physical stimulus energy with extinction. 7 'Although multimodal composite dimensions are being primarily discussed because they (a) are most Comparable values along this dimension are associated with similar experimental conditions in both paradigms. Weiss (1969a) pointed out that a binary analysis of the on-off stimulus relations between reinforcement-associated stimuli and extinction in the top row of Table 1 reveals that prominent in the stimulus-compounding literature and (6) reduce the probability of interference or generalization between independently conditioned stimuli, this analysis can also be applied to experiments reported by Wolf (1963, Experiment 2) and Cornell and Strub (1965) who used different lights, Li and L 2, as their S D s. The composite continuum here would be: (Li + 2), (Li -f- L 2 ) or (Ei + Lz), (Li + L 2 ). This same reasoning might be applied conceptually to the unimodal classical conditioning studies of Grings and O'Donnell (1956) and Grings et al. (1965). Unfortunately, confirmatory data are not available. However, it should be emphasized that for an unambiguous application of the composite-stimulus control model in stimulus compounding (a) the characteristics of the discriminative or conditioned stimuli controlling the various responses in training must be clearly evident and not open to question or interpretation; and (V) the independently conditioned stimuli when presented simultaneously must not coalesce so as to camouflage the identity of the underlying composite elements. That is why multimodal designs are preferred in compounding research.

6 194 STANLEY J. WEISS each S D or CS contains an S A (extinction) element. But since additive summation was clearly demonstrated to T + L, or its equivalent, in testing, it appeared that the components of the T + L and L + T reinforcement-associated stimuli controlling responding were those S D or CS elements not also coincident with extinction, T and L, respectively. Thus, Weiss (1969a) observed: summation was forthcoming to the composite stimulus composed entirely of elements unique to an S A [or CS] function, which, lacking discriminated S A elements, was further removed from S A on the postulated continuum than either of the other test stimuli [p. 22]. Weiss' (1969a) position might be restated so as to make his underlying assumptions somewhat more explicit. When discrimination training occurs we have a situation that can be described as follows: (a) T + L, associated with extinction, controls response cessation (behaviori); (b) During T + L, responding, maintained by reinforcement, consists of response under control of T (behavior 2 ) and response cessation (behaviori) under control of L. Therefore, rate for this condition is lower than would be the case if L was not in effect. (c) Similarly, during L + T responding, also maintained by reinforcement, consists of response under control of L (behaviors) and response cessation (behaviori) under the control of T. Therefore, response rate in this condition is lower than would be the case if T were not in effect, (d) Finally, during the T + L test condition, response rate is higher (additive summation) because responding consists of a mixture of response under the control of T (behaviors) and response under the control of L (behaviors) which yields a higher rate than the mixture of either behavior 2 or behaviors with "response cessation" (behaviori). Of course according to the same reasoning if T + L controls behavior that results in higher rates of responding than T + L or L + T, then one would expect that when T -f- L was presented, alternation of responses to T and to L would result in lower rates than to either T + L or L + T. This latter prediction is confronted in subsequent sections of this paper (see "Suppressive Summation" and "Rate Relations during Additive and Suppressive Summation"). A prediction from this model more directly concerned with additive summation is what next concerned Weiss (1969a). From the foregoing analysis Weiss (1969a) then argued that if the stimulus control relations postulated for the two-element composite dimension did exist, the continuum's behavioral symmetry could be tested through a freeoperant experimental design that reversed the locus of S A to the T + L continuum j:xtreme while maintaining T + L and L + T as the independently reinforced S D s. Now the elements of each S D controlling responding should be L and T, respectively. This derives from a mirror-image analysis of those attentional processes postulated when S A was T + L. Now that composite condition composed entirely of elements unique to an S D function, thereby lacking S A elements and thus furthest removed from S A on the composite scale, is T + L, or "stimulus absence." Weiss (1969a) therefore predicted that additive summation should be forthcoming to the T + L test condition. The design of this experiment is shown in row 2b of Table 1. Weiss (1969a) did get summation to T + L to a degree that was directly comparable to that forthcoming to T + L in a mirror-image free-operant summation study that employed similar training and testing techniques. He therefore concluded: The postulation of a symmetrical composite stimulus dimension in summation designs has been supported. [Additive] summation will occur to the stimulus complex representing either extreme of a two-modality composite continuum when "single" stimulus presentations are reinforcement-correlated if the opposite extreme is conditioned as S a. Therefore the continuum is symmetrical, with the functions of the two extremes interchangeable [Weiss, 1969a, p. 25]. Extrapolating from Weiss' (1969a) compositestimulus control model and these results, it becomes evident that a "composite extreme" is denned in relation to the behavior it controls and not absolutely. It is not logically necessary for T + L and T + L to be dimension extremes. T + L and L + T could serve equally well if either were associated with extinction and controlled response cessation while T + L and T + L maintained responding through reinforcement. In the former case, where T + L was S A, "the composite stimulus composed

7 STIMULUS COMPOUNDING 195 entirely of elements unique to an S D function" is L + T, and it should control additive summation in testing. On the other hand, when S A is L + T, while T + L and T + L remain the S D s maintaining responding, it is T + L that should control additive summation since it contains only composite "elements unique to an S D function." Experiments substantiating these latter predictions have not yet been reported in the literature. 8 Influence of S A Training In addition to establishing a stimulus control model within which the additive summation studies of Table 1, as well as other suppressive summation designs to be discussed later, neatly fit, Weiss' (1969a) findings also implicitly focus upon the importance of the S A function in the discrimination training of these additive summation studies. Discrimination training, customarily consisting of periods of S A, has already been shown to be significant for the demonstration of several other phenomena of interest in the study of stimulus control. Jenkins and Harrison (1960) found that a flat generalization gradient was generated on a frequency dimension unless subjects were given discrimination training. Perkins (1953) showed that discrimination training was involved in the production of stimulus intensity dynamism effects. Terrace (1964) demonstrated that discrimination training, with errors, was necessary for generalization peak shift. Weiss (1971) endeavored to isolate the function of S A in the discrimination training of a free-operant stimulus-compounding study. Weiss (1971, Experiment 1) trained four rats on a two-ply multiple VI 30-second VI 120-second schedule where no S A contact was permitted. Subjects were even placed into, and removed from, the training chamber with one of the VI S D s, T + L or L + f, present. According to a "composite-stimulus attentional analysis" the "continuum extremes" (T + L and T + L) would each contain an equivalent representation of stimulus elements from each training condition that maintained responding. Therefore, if one assumes that the animal is sampling composite elements during com- 8 Donald Meltzer, in a personal communication to the author, presented data confirming these predictions. pounding, as is implicit in Weiss' (1969a) analysis, stimulus compounding should yield no difference in the number of responses to the two "continuum extremes." They should both control response averaging. This is precisely what Weiss (1971, Experiment 1) reported. He stated: In light of [these] results..., taken in the context of other comparable compounding studies cited, it seems safe to conclude that the discriminations afforded by S A training have been integral to previously reported instances of additive summation [p. 396]. These results of Weiss (1971, Experiment 1) are corroborated by findings of Lawson, Mattis, and Pear (1968, Experiment 2). Lawson et al. trained their four rats on a multiple VI 45- second VI 45-second schedule. Light and tone were S D s. During light, responding was maintained by food, and during tone by water. There was no 5 A. In testing, tone plus light controlled a response rate that was intermediate between that forthcoming to tone or light presented separately. Lawson et al. interpreted their failure to get additive summation to compounded S D s to the fact that different incentives maintained responding in the presence of each. However, in light of what is now known about the importance of discrimination training between extreme and intermediate composite stimuli for additive summation, it seems that the absence of S A training is the more likely reason for the averaging Lawson et al. found to tone plus light in testing (see also Van Houten & Rudolph, 1971). Qualitatively, the results of Lawson et al. (1968, Experiment 2) and Weiss (1971, Experiment 1) agree quite favorably with the stimulus-sampling theory analysis of stimulus compounding presented by Atkinson and Estes (1963). Atkinson and Estes cautioned that it may be unrealistic to assume that background stimulation from the apparatus and surroundings is negligible. In fact, the experimenter may have to count on an appreciable amount of background stimulation, predominantly conditioned to behaviors incompatible with the CR, to prevent "spontaneous" occurrences of the to-be-conditioned response during intervals between presentations of the experimentally controlled stimuli [p. 196]. When the "background stimulation," or the "composite training extreme" in the terminology of the current presentation, is not taken

8 196 STANLEY J. WEISS into account in the Atkinson-Estes analysis, the probability of a response to the compound presentation of two independently conditioned stimuli would be somewhere intermediate between the probabilities of each presented separately. (In response rate terms, if rate can be translated to probability of occurrence per unit time, this is precisely what occurred in the two experiments just described where subjects did not receive differential training to T + L.) On the other hand, when differential conditioning to the "background stimulation" is taken into account in the Atkinson and Estes model, and the strengths of the responses controlled by the separately conditioned stimuli are not overly disparate, additive summation is predicted to compound presentations. Later in the current paper it will become apparent that this is the only analysis of stimulus compounding to date that makes the necessity of discrimination training explicit for the occurrence of additive summation in conditioning. Consideration of Response Cessation and N onreinforcement Consequences o/5 A S A has been an intrinsic part of the training procedures of those free-operant experiments demonstrating additive summation to compounded S D s. Weiss (1971) has shown that it is probably necessary if summation is to be observed, and Weiss (1969a) analyzed its stimulus control function along the composite dimension he postulated to underlie additive summation. But it must be recognized that S A accomplishes several things behaviorally. In addition to its extinction signal function, S A comes to control nonresponding. Weiss (1971, Experiment 2) sought to isolate nonreinforcement from response reduction effects of S A in additive summation designs. Weiss trained two groups of animals on a three-ply multiple schedule generating similar rates to each respective component between groups. Tone and light were each S D s for one of two VI schedules for both groups. The absence of these stimuli, light-out no-tone (T + L) signaled extinction for one group (S A training) and a DRO (differential-reinforcement-of-behavior-other-than-bar-pressing) food reinforcement schedule for the second group (DRO training). In this experimental design S A and_dro both controlled response cessation to T + L, but, while S A was also correlated with nonreinforcement, DRO reinforcement density was maintained intermediate between that controlled by the two VI S D s. Therefore, response cessation was isolated from nonreinforcement during T + L, and the role of differential response rates to the composite-stimulus extreme and VI S D s in summation designs was investigated independently of differences in reinforcement density to the extent that response cessation controlled by DRO and extinction are dynamically similar. Total test responses were not changed by this manipulation, but rather the proportion of these responses emitted to the compound and the "single" S D s by S A and DRO subjects. Although both DRO and S A subjects summated to the T + L compound, DRO subjects emitted a significantly smaller proportion of their total test responses to this compound than did subjects of the S A group. Therefore, both response cessation and nonreinforcement consequences of S A appear to be factors determining distribution of total responses during stimulus compounding. If merely the DRO group had been run this multiple causation might have been overlooked, and response cessation or reduction to one of the composite extremes might have been considered the only variable contributing to additive summation. SUPPRESSIVE SUMMATION Suppressive summation is observed when a lesser response is controlled by a compound stimulus than by either of the independently conditioned components presented individually. It was reported by Miller (1969), Reberg and Black (1969, Experiment 1), and Van Houten, O'Leary, and Weiss (1970) in a conditioned suppression paradigm. Miller (1969) and Van Houten et al. (1970) stabilized rats on a VI base line correlated with the absence of tone and light (T + L). Subsequently they made tone (T + L) and light (L + f) a signal for unavoidable shock, producing conditioned suppression to each separately. In testing they observed summation of conditioned suppression; that is, a lower rate was forthcoming to the tone and light (T + L) presented simul-

9 STIMULUS COMPOUNDING 197 taneously than to the individual presentation of tone or light. Reberg and Black (1969, Experiment 1) demonstrated the same phenomena with shock signaled by darkness and by noise. Composite-Stimulus Control Analysis Weiss and Emurian (1970) demonstrated that suppressive summation could be produced to T + L as well as T + L. They reversed the composite extreme associated with the shockfree VI reinforcement contingency to T + L, while maintaining T + L and L + T as CSs, and got less responding to T -f- L in testing than to either of the CSs independently presented. Weiss and Emurian (1970) therefore successfully applied the composite-stimulus control model that Weiss (1969a) formulated for additive summation designs to predictions in suppressive summation. They stated: Summation of conditioned suppression will be observed to the stimulus complex representing either extreme of a two-modality composite continuum, when "single" stimulus presentations are shock correlated, if the opposite extreme is conditioned as the VI baseline. Therefore the [[composite] continuum [in suppressive summation paradigms] is symmetrical with the functions of the two extremes interchangeable [Weiss & Emurian, 1970, p. 208]. Kamin (1963) and Rescorla and Solomon (1967) have suggested that the conditioned suppression phenomenon offers a sensitive measure of Pavlovian conditioning. Therefore, by demonstrating that the "composite-stimulus control" model applies to suppressive summation controlled by conditioned suppression, Weiss and Emurian (1970) appear to have indirectly supported Weiss' (1969a) suggestion that the composite-stimulus analysis could be applied to classical as well as free-operant summation designs. The model has never been tested in a conventional classical conditioning experiment. ANALYSIS OF RATE RELATIONS IN ADDITIVE AND SUPPRESSIVE SUMMATION PARADIGMS Weiss and Emurian (1970) pointed out that in the midst of the many procedural differences between those free-operant experiments yielding additive and those yielding suppressive summation there is an underlying similarity. Response rates are systematically ranked, in opposite directions, over the symmetrical composite dimension in both instances. For example, in an additive summation study the training composite extreme (T + L) would control response cessation, the intermediate composite stimuli (T + L or L + T) would maintain responding, and summation would be forthcoming to the composite opposite to that employed in training (T + L). In suppressive summation, the training extreme (T + L) maintains responding, there is a reduction in rate to the intermediate composite stimuli, and T + L produces the lowest rate in testing. Therefore what might be the necessary condition for summation [on a multimodal composite dimension], whether additive or suppressive, is differential rates controlled by extreme and intermediate composite stimuli in training. The relation of these rates to each other, other variables held constant, could determine the direction of summation to the composite extreme opposite to that employed in training. [Weiss & Emurian, 1970, p. 209]. However, an incentive difference does exist between those studies reporting additive and those reporting suppressive summation. In those studies reporting additive summation responding to intermediate composites has been maintained by food as well as shock. When suppressive summation has been reported, suppression during training to the intermediate composites has only been controlled by a CS signaling an unavoidable shock in a conditioned suppression paradigm, punishment for responding (Van Houten & Rudolph, 1971), or shock avoidance (Weiss & Wiltz, 1972). The rate-relation approach to the analysis of summation would be strengthened if suppressive summation was forthcoming from a subject whose differentiated behaviors to the respective values along the composite continuum were solely controlled by positive reinforcement. A subject has recently yielded these data in the author's laboratory. A terminal training cumulative record of a rat trained on a multiple (VI 60-second) (conjunctive VI-60-second DRL 13-second) schedule is presented in Figure 1. During T + L, a VI 60-second food reinforcement schedule was in effect. When a tone (T + L) or a light (L + T) was present, a DRL 13-second sched-

10 198 STANLEY J. WEISS FIG. 1. Terminal cumulative record of rat trained on multiple VI 60-second (VI 60-second DRL 13- second) schedule where the VI 60-second component was signaled by light-out no-tone and the conjunctive VI 60-second DRL 13-second component was correlated with tone (T) or light (L). The latter periods are identified on the record by depression of the cumulative response pen. Slash marks by this pen, whether upward or downward, identify reinforcements. ule was superimposed on the underlying VI 60-second contingency. This meant that when a reinforcement was made available on the VI contingency, a response would produce it only if it occurred at least 13 seconds after the preceding response. In this animal the contingency described maintained a high response rate during T + L with responding appreciably reduced during T + L and L + T. In more general terms, responding to the training composite stimulus extreme was higher than that controlled by the intermediate composites. The results of the 12 block-randomized test replications, similar in format to that performed by Weiss and Emurian (1970) when they reported the summation of conditioned suppression, are presented in Table 2. Tone plus light (T + L), T + L, and L + T were each presented 1 minute per block, separated by 3-minute T + L periods. Only the VI 60- Test replications Total TABLE 2 TOTAL TEST RESPONSES T +L» Stimulus condition T +L L +T T+L » Represents the mean of the 1-minute periods preceding T + L, L + T, and T+L stimulus presentations of each test replication totaled of successive blocks of three replications. second contingency was effective during the test. Table 2 indicates that suppressive summation was clearly evidenced to T + L over the test. T + L and L -f- T controlled approximately one-third the response output of T + L, while T + L controlled roughly one-half as many responses as T + L or L + T. Although this evidence must be viewed as merely supportive of the general theme of the rate-relation analysis of suppressive and additive summation, rather than confirmatory, these preliminary data do lend substantive, and incentive, generality to that interpretation. However, Weiss (1971) cautioned that reinforcement density relations between the extreme and intermediate points along the composite continuum, the manner in which the differential rates between these stimuli are conditioned, as well as the terminal rate relations, could contribute to the control exerted by the compound stimulus represented by the composite extreme opposite to that employed in training. Moreover, recent evidence indicates that even considering all these factors might be insufficient when predicting the distribution of responses between single and compound stimuli in testing. Wiltz 9 just completed a dissertation under the author's supervision in which additive summation was clearly evidenced to combined stimuli (T + L) during extinction and maintenance tests, when responding produced food as well as mild 9 R. A. Wiltz, Jr. Combining discriminative stimuli signaling punishment. Unpublished doctoral dissertation, The American University, Washington, D. C., 1972.

11 STIMULUS COMPOUNDING 199 punishment on independent VI schedules during T + L and L + T, if f + L controlled response cessation from early training. However, if T + L first maintained a higher rate than T + L and L + T, when it was subsequently conditioned to control a lower rate than either of these stimuli, additive summation was not forthcoming to T + L. It therefore seems, at least in certain schedule situations where summation might be "costly," that the history of the rate relations between extreme and intermediate composite stimuli, as well as the immediate rate relations just prior to testing, must be taken into account in a comprehensive analysis of stimulus control. The stimulus-compounding assay appears uniquely sensitive and useful in this analysis. The rate-relation interpretation of summation in stimulus compounding could mean that compounding stimuli associated with the same schedule, VI 60-second for instance, could under one circumstance produce additive and under another suppressive summation. In fact Weiss and Wiltz (1972) have already demonstrated that compounding S D s controlling freeoperant avoidance on an RS 25-second schedule could produce suppressive as well as additive summation as a function of the rate and schedule relations between extreme and intermediate composite stimuli in training. The possible implications of such findings to a comprehensive understanding of "excitatory" and "inhibitory" mechanisms in conditioning as identified by compounding assays are discussed in a later section of this paper. AVERAGING Averaging might be said to occur during stimulus compounding if the response controlled by the compound is somewhere intermediate between the response outputs controlled by the independently conditioned stimuli composing the compound. Pavlov often used the degree of response reduction to CS+ on a summation test to measure the inhibitory strength of a stimulus (Rescorla, 1969a). This measurement technique was necessary because on a simple behavioral level the conditioned tendency not to make a response often cannot be distinguished from the absence of excitation. TABLE 3 LEPORSKY'S DATA SHOWING ALGEBRAIC SUMMATION OP CONDITIONED REFLEXES Time 1 :40 p.m. 2 :40 p.m. 2:10 p.m. 2 :27 p.m. 2:51 p.m. Stimulus during one minute Tone Rotating object + tone -+ flash Rotating object Simultaneous application of all three positive conditioned stimuli 4- tactile stimulus Rotating object + tactile object Drops of saliva Some Classical and Operant Conditioning Experiments Pavlov (1927) described an experiment by Leporsky that illustrates the effect of a conditioned inhibitor on positive conditioned reflexes of various strengths, as well as summation of conditioned reflexes paired with separate stimuli. Three alimentary reflexes were independently conditioned to a rotating object, to the flash of several lights, and to a musical tone. A tactile stimulus was firmly established as an inhibitor of the response to each of these stimuli. Table 3 not only shows clear summation of conditioned reflexes but also that the conditioned inhibitor, which could completely inhibit the response when presented simultaneously with any one of the positive stimuli, could only partially inhibit the reflex reaction evoked by all three positive conditioned stimuli acting simultaneously. This study clearly demonstrates the algebraic summation of response strengths. Szwejkowska and Konorski (1959) conditioned two positive CSs bell (Si) and bubbling water (82) while never pairing two other CSs with food metronome (83) and whistle (84). The coincidence of positive and negative CSs (Si + S s, S 2 + S 3, Si + S 4, and S 2 + S 4 ) usually produced a salivary effect in the range between the full effect to a positive CS and the minimal response to the negative CS. (Strangely, the simultaneous presentation of both positive CSs produced a CR indistinguishable from either presented separately.) Cappell, Herring, and Webster (1971), Hammond (1967), Reberg and Black (1969, Experiment 2), and Rescorla (1969b) demonstrated that a CS (non-shock-associated stimulus) can reduce suppression controlled by a CS+ 9 0

12 200 STANLEY J. WEISS (shock-associated stimulus) in a conditioned suppression paradigm when CS+ and CS are presented simultaneously. The CS is interpreted by these authors as inhibiting the conditioned emotional responses (CERs) elicited by the CS+, thereby reducing the response suppression produced by these CERs. 10 The results of several operant experiments employing compounding tests similar to Pavlov's are interpreted to indicate that a negative stimulus (S A ) exerts inhibitory control. Cornell and Strub (1965) employed differentially located light stimuli as S D s (signaling food availability on a VI schedule) or S A (signaling extinction) in free-operant training. During compounding tests, compound presentations of S D and S A lights led to response rates that were between those controlled by the separate presentations of S D and S A lights, indicating, according to Cornell and Strub, the "inhibitory" effect of S A. Brown and Jenkins (1967) recently performed an experiment that specifically controlled for Skinner's (1938) criticisms of the classical conditioning experiments on which Pavlov (1927) originally based the concept of inhibition. They demonstrated: A stimulus (tone) that has become a signal for no responding when paired with one excitatory stimulus (key color used in conjunction with a go/no-go auditory discrimination) also serves as a signal for not responding when it is combined with another excitatory stimulus (key color used for the transfer test) that is clearly dis- 10 Results recently reported by Coulter and Weiss (1971) suggested that a stimulus that previously served as a CS-)- in a conditioned suppression paradigm, but which currently signals safety and controls response acceleration, might not act as a "fear inhibitor." Their compound controlled suppressive summation, indicating that the previous conditioning history of, as well as the immediate rate control by, the single stimuli compounded must be appreciated when predicting behavior to n compound stimulus. Significantly, the compounding test behavioral assay appeared to permit the measurement of the residual "fear" still associated with the currently safe CS of Coulter and Weiss in spite of the fact that the response acceleration controlled in its presence made it indistinguishable from a CS that has always signaled safety. These results, as well as those presented earlier in the section "Rate Relations," support Konorski's (1967) conclusion "that the kind of primary training of a given stimulus, whether excitatory or inhibitory, determines its later properties: it is difficult to transform an excitatory CS into an inhibitory one..., as it is to transform a primarily inhibitory CS into an excitatory one... [p. 323]." criminated from the one employed in the original training.... [Since] extinction of responding to the combination of tone with one key color did not cause loss of response to the second key color,... generalized or secondary extinction cannot account for the result [Brown & Jenkins, 1967, p. 262]. The foregoing results fairly well establish through stimulus compounding techniques the functioning of what are customarily thought of as "inhibitory" mechanisms. Nevertheless, Weiss (1967) pointed out: When stimulus compounding produced additive summation, the separately conditioned stimuli had each been previously associated with reinforcement and maintained roughly similar, if not identical, response rates or magnitudes.... To yield response averaging though, one of the stimuli compounded was reinforcement-associated and maintained responding, while the other signalled extinction... and produced a negligible response.... Therefore, either the absence of reinforcement to one of the stimuli, or the concomitant low response rate, could be responsible for response averaging observed in these studies. Response and reinforcement variables are confounded in the comparison of additive summation and response averaging paradigms [italics added; Weiss, 1967, p. 535]. Weiss (1967) then went on to train animals on a multiple VI 30-second DRL 20-second extinction schedule. After the subjects stabilized, the widely different rates controlled by the VI and DRL S D s satisfied, or surpassed, the rate difference requirements that appeared necessary for free-operant averaging to compounded S D s. On the other hand, the reinforcement densities received during the VI and DRL S D s in training were similar to those associated with independently conditioned S D s that produced additive summation when compounded. Therefore, reinforcement and rate factors were separated. When the high-rate VI and low-rate DRL training S D s were compounded, they controlled an overall response output that was somewhere intermediate between that produced by these S D s individually presented. In general the same relationship held when generalized stimuli were compounded. Therefore Weiss (1967) concluded that "one of the stimuli compounded does not have to be associated with extinction for response averaging to occur [p. 540]." Widely discrepant rates appear to be sufficient.

13 Interresponse Time Analysis Although Weiss (1967) found response averaging when he compounded VI and DRL S D s he did not use these results to attribute "inhibitory" characteristics to the DRL schedule. He rather speculated that the averaging effect was probably due to a competition between the incompatible response repertoires controlled by the two S D s, just as S A or CS controls a response repertoire incompatible with that controlled by an S D or a CS+, respectively. He suggested that the behavioral form of this conflict might be more fully appreciated through an analysis of interresponse time (IRT) distributions. Bimodal IRTs during compound stimulus presentations could indicate alternating attention between [highrate] VI- and [low-rate] DRL-associated stimuli, or some intermediate distribution could indicate simultaneous attention to both stimuli [Weiss, 1967, p. 540]. Weiss (1969b, 1972) gathered IRT data during a stimulus-compounding test in which high- and low-rate stimuli were presented simultaneously and individually. A descriptive analysis of response patterning during the three test conditions strongly suggested that averaging total responses to each test condition presents a distorted description of the behavior emitted during the compound. An IRT analysis, as well as examination of test cumulative records, indicated that intermediate response totals to the compound of high- and low-rate S D s were not correlated with an intermediate distribution of responses. The compound distribution appeared to consist of IRTs characterizing the single S D s. The extent to which this finding applies to response averaging produced by the summation of "excitatory" and "inhibitory" response tendencies, when stimuli controlling less disparate response repertoires are compounded, is a ripe area for investigation. Blough (1963), Sewell and Kendall (1965), and Crites, Harris, Rosenquist, and Thomas (1967) have reported that IRT distributions to stimuli removed from the training stimulus on the continuum tested differed from training stimulus-response distributions during generalization only with respect to an increased frequency of long IRTs. Migler (1964) has also supplied convincing evidence in a twomanipulandum paradigm that generalization STIMULUS COMPOUNDING 201 gradients are a statistical artifact of combining mixtures of behaviors over time. It is obvious that the likelihood of finding averaging during compounding depends upon the differences between the response rates to each stimulus. However, more than these rate differences seem to be involved. It appears that the discrimination training received between extreme (T + L) and intermediate composite stimuli (T + L and L -f T) must be considered in analyzing response averaging as well as additive and suppressive summation. When the composite extreme controls a lower response rate than either of the intermediate composites, it seems reasonable to expect that extremely disparate rates between intermediate composites would be necessary for averaging. In this instance the rate relations between extreme and intermediate composite stimuli would lead to the prediction of additive summation to the composite extreme (T + L) opposite to that employed in training, while the incompatibility of rates between intermediate composites would make averaging likely to T + L. Averaging and additive summation processes were thus probably operating in opposition to each other when Weiss (1967, 1969b, 1972) reported averaging to compounded tone and light stimuli that customarily controlled rate differences of 25 or greater to one, while T + L controlled rates lower than either tone or light. (The same thing could probably be said for the Adams and Allen [1971] study where the average rate difference between tone and light in training was 130 responses per minute.) "Incompatible" rates between intermediate composites in training appeared necessary for averaging during compounding in these experiments. According to the composite stimulus control model discussed earlier, if, in training, different rates are forthcoming to the intermediate composites and no contact is permitted with the composite extreme, or if the rate controlled by the composite extreme is between those controlled by the intermediate composites, response averaging should be forthcoming during compounding. When Weiss (1971, Experiment 1) trained rats on a two-ply multiple VI schedule where tone signaled one VI and light another, and where subjects were allowed no contact with T + L, he found clear and con-

14 202 STANLEY J. WEISS sistent response averaging to T + L as well as T + L in testing in spite of the fact that the response rates to tone and light were not characteristically different from those controlled by these stimuli in studies reporting additive summation to T + L. (In these latter studies though [Weiss, 1964; 1969a; 1971 Experiment 2], subjects were given extensive training contact to T + L, in whose presence they were taught to cease responding.) On the other hand, response averaging reported in conditioned suppression studies is an instance of averaging when the composite stimulus extreme (VI base line) controls a rate between the suppression maintained by the CS+ and the facilitation customarily controlled by CS. Therefore, due to the different discriminations demanded between the several composite stimulus values in training, whether the freeoperant response averaging reported by Weiss (1967, 1969b, 1972) and Weiss (1971, Experiment 1) and the inhibition of suppression studies (Hammond, 1967; Reberg & Black, 1969; Rescorla, 1969b) are dynamically similar is open to question. To be more completely understood, response averaging, just as additive and suppressive summation, will more often have to be considered in the context of the total training schedule controlling the rates to the compounded stimuli. "EXCITATION," "INHIBITION," AND STIMULUS COMPOUNDING Stimulus compounding has been employed as an assay technique to measure the nature of control exerted by independently conditioned stimuli since the early twentieth century. Within the conceptual framework developed by Pavlov, the behavioral resultant to the simultaneous presentation of separately conditioned stimuli could be indicative of the mode of control "excitatory" or "inhibitory" conditioned to each. Although these terms are somewhat ambiguous they have, for example, been incorporated as explanatory mechanisms in the comprehensively formalized learning theories of Hull (1943,1952) and Spence (1956) and are rather loosely used by psychologists in a wide variety of situations. When compounding two conditioned stimuli results in a greater response than either stimulus presented singly, one observes what is empirically called additive summation. The learning mechanisms postulated to underlie these results would be considered "excitatory" in nature. Hull (1929), in an attempt at a functional interpretation of the conditioned reflex, spoke of it as an additive-subtractive type of reaction in which the number of elements from the original CS complex is the determiner of the intensity of the response. Hull (1929) predicted: if two distinct stimuli which have been independently conditioned to a given response are presented together, the intensity of the resulting response is likely to approach the arithmetical sum of the responses to the two stimuli presented separately [p. 502]. In 1943 Hull no longer emphasized the arithmetic sum of the separate responses, but rather the resulting habit strength if the number of reinforcements on all of the separate stimuli has been given consecutively in the presence of one stimulus. Skinner's (1938) law of spatial summation, which reads, "When two reflexes have the same form of response, the response to both stimuli in combination has a greater magnitude and a shorter latency [p. 31]," less formally says the same thing. Pavlov (1927, p. 76) also suggested that stimulus compounding might be the most direct method of measuring inhibition. When a known exciter of a response is compounded with another stimulus, and the response elicited by the compound is less than that controlled by the exciter alone, the second stimulus is an inhibitor. In addition to predicting the summation of compatible reaction potentials, Hull (1943) also noted that there can be a competition of incompatible reaction potentials that would lead to a less intense response, or no response at all if the antagonistic reaction potentials are momentarily equal. Functionally, Skinner (1957) accepted the same behavioral resultants when he stated: it is a well established principle in nonverbal behavior that separate sources of strength are additive. (Since some variables reduce the strength of verbal behavior, the addition must be algebraic) [p. 228]. Therefore, the results of stimulus compounding tests have identified the interaction of "exciters" and of "exciters" and "inhibitors." Following the same line of reasoning, if the simultaneous presentation of two independently conditioned stimuli produces a lesser

15 STIMULUS COMPOUNDING 203 response than either presented separately, the addition of "inhibition" should be indicated. Most likely this would be observed in classical conditioning when a lesser response was forthcoming when an "exciter" was compounded with two rather than with one "inhibitor." Suppressive summation in operant conditioning has been extensively discussed in a preceding section of this paper. Within the framework sketched "excitation" and "inhibition" are mutually exclusive processes that work in opposition to one another. However, on the basis of previous research it was predicted that the response rate and reinforcement density relations between the two stimuli compounded, and the composite extreme of a three-ply multiple training schedule, would be the factors determining direction of summation. Within this framework, where compounding precisely the same scheduleassociated stimuli can lead to additive or suppressive effects, the concepts of "excitation" and "inhibition" take on a definite relational character. This indicates that the stimulus control exerted by a component of a multiple schedule cannot be fully appreciated without due regard to the total schedule context of which it is a part. This is what Pavlov, Hull, and Skinner failed to make explicit when they analyzed stimulus compounding. In terms of Rescorla's (1969a) distinction, compounded stimuli could control tendencies "opposite" to one another, addition or suppression, in testing although they specifically were each associated with the same particular schedules of reinforcement controlling similar response repertoires. These results, not altogether speculative as data presented in prior sections of this paper indicate, would require a reformulation, at least as far as a stimulus-compounding index of these processes is concerned, of "excitatory" and "inhibitory" mechanisms in conditioning. However, these results of compounding would agree substantially with Hearst, Besley, and Farthing's (1970) recent redefinition of an inhibitory stimulus "as a stimulus that develops during conditioning the capacity to decrease response strength below the level occurring in its absence [p. 405]." According to this definition, if response strength may be translated to a rate measure, it could be the rate relations between components of a multiple schedule that determine inhibitory, and by implication excitatory, stimulus control, just as they can determine those circumstances that produce additive and suppressive summation during compounding. It appears, however, that once these mechanisms are defined in such a fashion that they lose their absolute properties, their conceptual usefulness, from an integrative point of view, is severely diminished. As Konorski (1967) suggested, it might be more useful and parsimonious to think in terms of the interaction of different excitatory mechanisms. GENERALIZATION PEAK SHIFT AND SUMMATION : SOME FUNCTIONAL SIMILARITIES Positive peak shift is observed during a generalization test when maximum responding is controlled by a stimulus removed from the S D controlling the highest response rate in training in a direction away from that stimulus controlling a lower training rate. If the on-off states of the stimuli employed in those stimulus compounding experiments that report additive summation might be conceptualized as forming a composite continuum extending from the ail-off extreme (T + L) through the onestimulus-on condition (T + L or L + T), to the ail-on extreme (T -f- L), a functional comparison of summation to peak shift would be facilitated (see the stimulus identification row of Table 1). Wolf (1963) and Weiss (1964) were the first to report that subjects summated additively to T + L in testing when responding in training was extinguished to T + L and maintained through reinforcement to T + L and L + T. On the composite continuum, T + L can be seen as further removed from T + L than either T -f L or L + T. This led Wolf to compare the response enhancement effect of combined S D s to generalization peak shift. He postulated that the composite stimulus dimension employed in summation experiments might be functionally equivalent to the unitary dimensions of the peak shift studies. Subsequent to Wolf's suggestion, evidence has been reported that is supportive of the peak shift-summation comparison. Response enhancement effects along the ratio scale continua employed in peak shift studies, as well as along the composite continuum postulated

16 204 STANLEY J. WEISS to underlie summation, appear to be controlled by similar variables. These include (a) intradimensional discrimination training that produces rate differences between stimulus values along the respective continua, with the possibility that contrast must be a by-product of that training if either enhancement phenomena is to occur; (V) reversibility of function between high- and low-rate training stimuli in positive and negative peak shift as well as additive and suppressive summation; and (c) the contribution of both rate and reinforcement differences between training stimuli in determining magnitude of response enhancement for peak shift and summation in testing. Positive peak shift will occur after discrimination training in which one value on a continuum is reinforced and another extinguished (cf. Hanson, 1959). Either value along the stimulus continuum can serve equally well in either S u or S A capacity, with the peak shifted one way or the other as a function of the position of S A relative to S D. Weiss (1969a) demonstrated this reversibility of function in an additive summation paradigm. Prior to this experiment, additive summation had been reported to T + L when T + L and L + T were each S D s for a VI food reinforcement contingency, while T + L signaled extinction (S A ). T + L and L + T were also the VI_S D s when summation was reported to T + L by Weiss (1969a), but in this instance S A was signaled by T + L- These results confirmed that the composite continuum was behaviorally symmetrical in additive summation, with the functions of the two extremes interchangeable in much the same way that 550 or 570 millimicrons (mju), for example, can be employed as S A to produce peak shift. Further, it is well documented that intradimensional discriminative training is necessary for peak shift just as Weiss (1971, Experiment 1) suggested that it was essential to additive summation. His subjects, who did not receive discrimination training between different values along the composite continuum, defined in terms of the number of composite elements "on," failed to summate additively to compound stimulus presentations. If additive summation might be conceptualized as having attributes in common with positive peak shift, suppressive summation might be compared to negative peak shift. Guttman (1965) and Hendry, Switalski, and Yarczower (1969) reported negative peak shift. In Guttman's experiment, responding in training was not completely eliminated to S A but was merely maintained at a lower level to S A than S D. In testing, the stimulus that controlled the lowest response output was not S A but a stimulus value removed from S A in a direction away from S D. During suppressive summation the composite extreme opposite to that which maintains responding controls a lower response output than either of the intermediate composites that control suppression in training, just as a stimulus further removed from S D than S A controls a lower response output than S 4 in negative peak shift. Guttman (1965) produced negative peak shift when 550 m/u was and 560 m/j, was S A, as well as when the opposite stimulus-schedule combination was employed. Similarly, Weiss and Emurian (1970) demonstrated that suppressive summation could be controlled by either extreme of their composite continuum after the appropriate training conditions. That rate relations between the components of a multiple schedule might be of fundamental importance in determining stimulus control has recently been demonstrated by Yarczower, Dickson, and Gollub (1966) and Terrace (1966, 1968). Yarczower et al. (1966) systematically varied response rate and reinforcement density relations during differential intradimensional training between components of a multiple schedule. Their manipulations led these authors to conclude that their "data imply that response rates at the end of training, rather than reinforcements per se, determine the characteristics of the generalization gradient [[Yarczower et al., 1966, p. 631]." When rates between multiple-schedule components were different, the peaks of their generalization gradients were shifted away from the lower rate stimulus. On the other hand, Yarczower et al. showed that as long as the rates in the two components were equal, the generalization peaks were not shifted even when reinforcement densities were not equal. More recently Yarczower, Gollub, and Dickson (1968) have suggested that reinforcement density and rate relations between the components of a multiple schedule might S D

17 STIMULUS COMPOUNDING 205 interact in determining gradient shape. Weiss (1971, Experiment 2) demonstrated that these same variables appear to be operating to determine magnitude of additive summation. Terrace (1968) affirmed the findings of Yarczower et al. (1966) while extending the rate relation analysis to behavioral contrast (cf. Reynolds, 1961) in addition to peak shift. Contrast is observed when, after discrimination training on a multiple schedule, the rate in the presence of one component that remains unchanged moves in a direction opposite to that produced by a change in the other schedule. Terrace (1968), from the results of a variety of experiments that produced differential responding to the components of a multiple schedule, concluded: that a reduction of the rate of responding to one of the two alternating discriminative stimuli is sufficient to produce contrast. The present results indicate also that a sufficient condition for the occurrence of the peak shift is a reduction in the rate of responding to one of two alternating discriminative stimuli... [with the peak shifted] away from the stimulus in the presence of which the rate of responding was reduced [1968, p. 739]. Rate reduction to a composite extreme has not yet explicitly been investigated in stimulus compounding yielding additive summation. Usually, though, when differential conditioning starts from the beginning of training, rate reduction to one of the components is implicit during acquisition of the discrimination. In fact, Weiss (1971, Experiment 2) documented this reduction. Therefore, although not explicitly investigated, rate reduction to the composite extreme employed in training has probably been implicit in those studies reporting additive summation. Thus, to the extent the evidence presented supports the contention that peak shift and summation possess some underlying functional similarities, the postulated operation of rate differences between training extreme and intermediate composite stimuli in determining stimulus control properties within compounding experiments is supported by the importance of that variable to peak shift. CONCLUSION Research interest in the area of stimulus compounding has increased substantially in the last several years. Additive summation is now a well-established phenomenon, with the freeoperant research in particular indicating that it is not schedule or incentive-specific. Rather, additive summation appears to be dependent upon discrimination training that leads to a higher rate, or response output, in the presence of each of the stimuli to be compounded than in their simultaneous absence. Research to date has customarily maintained a minimal response output to this latter condition by programming extinction in its presence. However, responding has been maintained to the individual stimuli subsequently compounded by VI, FI, DRL and signaled as well as Sidman avoidance contingencies in the various free-operant studies surveyed. The classical conditioning experiments yielding additive summation were more limited in their scheduling, although a variety of reinforcers were used as unconditioned stimuli. Unfortunately, attempts to predict the response output to a compound stimulus presentation as a function of the responses controlled by the independently conditioned stimuli composing it have not been overly successful in the literature of additive summation. The results have ranged from additive summation effects, just barely meeting the minimum requirements defined earlier in this paper, to several studies that fit a simple additive model, to others that report substantially greater responding to the compound than to the outputs controlled by the single stimuli combined. This variability is not surprising in view of the fact that it is just recently that some of the dynamics underlying additive summation have begun to be made explicit (Weiss, 1969, 1971; Weiss & Emurian, 1970), and a variety of training and testing procedures have been employed by different investigators. In particular, test length might be a significant factor in determining the magnitude of additive summation since several investigators employing extinction tests have specifically stated that the compound test condition continued to control responding when responding to each of the single stimuli composing it was completely extinguished. Suppressive summation has only been reported in the free-operant literature, and then only when response suppression to the individual stimuli comprising the compound was

18 206 STANLEY J. WEISS controlled by punishment or unavoidable signaled shock. Data presented in this paper suggested that the phenomenon is not, however, incentive-specific, but rather that it could also be observed when the rate reductions in training to the single stimuli compounded was controlled by food-maintained schedules. This finding lends further weight to the contention that suppressive summation is dependent upon, within limits, discrimination training that leads to a lower rate, or response output, in the presence of each of the stimuli to be compounded than in their simultaneous absence. Examining the influence of discrimination training on response distributions during stimulus compounding led to the development of an empirically substantiated compositestimulus control model applicable to additive and suppressive effects in both free-operant and classical conditioning. A symmetrical composite-stimulus continuum, defined by the on-off states of the stimuli controlling behavior in training, was seen to be common to both types of summation, with the functions of the ail-on and ail-off continuum extremes interchangeable. Moreover, additive and suppressive summation seem to be specifically determined by the relations between response rates and reinforcement densities conditioned to particular stimulus values along the composite training continuum with the most recent evidence indicating the importance of the history of these relations, as well as those conditioned immediately prior to testing, in determining the distribution of responses during a compounding test. Since, according to this formulation, compounding the same schedule-associated stimuli might lead to additive summation under one set of circumstances, suppressive summation under another, or even averaging in a third complex schedule context, absolute properties cannot be attributed to these schedule-associated behaviors without due regard to the total schedule context of which they are a part. This indicates that the continued exploration of those factors determining additive and suppressive effects in compounding might further help in clarifying, and placing in contextual perspective, what are currently thought of as "excitatory" and "inhibitory" mechanisms in learning thereby even stimulating the development of more advanced unifying principles of behavioral control. For the most part, stimulus compounding research to date has focused on twoelement composite control. As higher-element composites are explored, more complex interactions will inevitably be revealed by this powerful behavioral assay. REFERENCES ADAMS, D. L., & ALLEN, J. D. Compound stimulus control by discriminative stimuli associated with high and moderate response rates. Journal of the Experimental Analysis of Behavior, 1971, 16, ATKINSON, R. C., CALFEE, R. C., SOMMER, G. R., JEFFREY, W. E., & SHOEMAKER, R. A test of three models for stimulus compounding with children. Journal of Experimental Psychology, 1964, 67, ATKINSON, R. C., & ESTES, W. K. Stimulus sampling theory. In R. D. Luce, R. R. Bush, & E. Galanter (Eds.), Handbook of mathematical psychology. Vol. II. New York: Wiley, BAKER, T. W. Properties of compound conditioned stimuli and their components. Psychological Bulletin, 1968, 70, BLOTJGH, D. S. Interresponse time as a function of continuous variables: A new method and some data. Journal of the Experimental Analysis of Behavior, 1963, 6, BROWN, P. L., & JENKINS, H. M. Conditioned inhibition and excitation in operant discrimination learning. Journal of Experimental Psychology, 1967, 75, CAPPELL, H. D., HERRING, B., & WEBSTER, C. D. Discriminated conditioned suppression: Further effects of stimulus compounding. Psychonomic Science, 1970, 19, CORNELL, J. M., & STRUB, H. A technique for demonstrating the inhibitory function of S A. Psychonomic Science, 1965, 3, COULTER, W. R., & WEISS, S. J. Suppressive summation controlled by compounding a shock-associated and a shock-extinguished CS. Proceedings of the 79th Annual Convention of the American Psychological Association, 1971,6, CEITES, R. J., HARRIS, J. T., ROSENQUIST, H., & THOMAS, D. P. Response patterning during stimulus generalization in the rat. Journal of the Experimental Analysis of Behavior, 1967, 10, EMURIAN, H. H., & WEISS, S. J. Compounding discriminative stimuli controlling free-operant avoidance. Journal of the Experimental Analysis of Behavior, 1972, 17, ESTES, W., & SKINNER, B. Some quantitative properties of anxiety. Journal of Experimental Psychology, 1941, 29, GRINGS, W. W., & O'DONNELL, D. E. Magnitude of response to compounds of discriminated stimuli. Journal of Experimental Psychology, 1956, 52,

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