GENERALIZATION OF A CONDITIONED INHIBITOR OF RESPONSE SUPPRESSION IN RATS

Transcription

1 ACTA NEUROBOL. EXP. 1988, 46: GENERALZATON OF A CONDTONED NHBTOR OF RESPONSE SUPPRESSON N RATS Malgorzata WKSERSKA, James F. BRENNAN and Kazimierz ZELNSK Department of Neurophysiology, Nencki nstitute of Experimental Biology Pasteura 3, Warsaw, Poland Key words: stimulus control, conditioned inhibitor, conditioned suppression, *generalization, rat Abstract. Following acquisition of conditioned suppression of barpresses to a visual conditiondd stimulus (CS), rats were subjected to a particular variety of differentiation training procedures. An auditory conditioned inhibitor (C), which preceded and accompanied the CS on some trials, signaled that inescapable shock will not be given on that trial. When the C acquired the capacity to attenuate markedly the suppressive effects of the CS presented within the C + CS compound, the frequency dimension of the C was varied during generalization tests. Reliable within-subjects gradients of barpress enhancement were obtained when tone test frequencies were presented alone. Frequency gradients obtained from C variation in compound with the CS were rather flat anld reflected some residual suppression of the on-going barpressing. NTRODUCTON Stimulus control of behavior may be achieved and demonstrated in different ways. One of the most persuasive is the procedure by which one discriminative stimulus, termed the conditioned inhibitor (C), acquires the capacity to inhibit the excitatory properties of another discriminative stimulus, termed the conditioned stimulus (CS), otherwise paired with a biologically important event (the unconditioned stimulus, US). The acquisition of conditioned inhibitory properties is due to differentiation

2 training (28) in which two kinds of trials are presented. The CS is a common element of 'both types of trials. On some trials, the CS is paired with the US (e.g., inescapable shock) becoming a signal for the presentation of this biologically important event, and this acquired property is demonstrated by elicitation of a specific conditioned response (CR). On other trials, the same CS is preceded andlor accompanied by the C and never paired with the US. After some training, the CS partially or completely loses its ability to evoke the CR when presented with the backgmwd of the C, but retains this evocation property when presented without the C. Thus, a C exerts stimulus control of behavior by indirectly modifying the signaling value of the CS. Such a mechanism of conditioned inhibitory action, cmsi~sting of the acute cancellation of the signaling properties of the CS otherwise retained, was proposed by Pavlov (19, p. 87) and shared by his students (18, p ). This belief had consequences for designing experiments in which conditioned inhibitory properties were tested. No attention has been paid to direct effects from a C on behaviors not elicited by the CS. The method of conditioned inhibition of conditioned suppression (24-26) provides an opportunity to study the direct effects exerted by a C on behavior. First, the typical Estes-Skinner paradigm for training the conditioned emotional response (CER) was used (7). Rats initially learned to produce stable barpressing rate for food were then exposed to a CS signaling a brief inescapable shock. After several CS-US pairings, the CS acquired the property to suppress the on-going barpressing. Then another kind of trial was introduced in which the same CS was preceded and accompanied by a discriminative stimulus of another modality, the C, and never paired with shock. After brief training the CS lost most of its suppressive properties when presented in compound with the C and suppressed the barpressing when presented without C. The well trained C presented either alone or in compound with the CS accelerated the banpressing rate (24). This enhancing effect of the C on the on-going barpressing behavior was independent of the emotional response elicited by the CS, since neither extinction nor reinstatement of the suppressive properties of the CS changed the behavioral effects of the C (25, 26). Taking these results into account, the capacity of the C to accelerate the barpressing rate may be used as an index for studying the direct effects of a C on behavior. Moreover, the time parameters used in our studies allowed independent measures of the effects of the G presented alone or in mip pound with the CS. The aim of the present study ms to investigate generalization functions by changing one of the dimensions of the stimulus used as the C. Since the C is of another modality than the CS that has inhibited exci-

3 tatory properties, the shape of the generalization functions of the C presented alone should not be erased by the excitatory tendencies generalized from the CS. n this respect the shape of the generalization function of the C presented in compound with the CS is also of interest. MATERAL AND METHODS The experiment was conducted on 16 experimentally naive, male hooded rats, approximately 3 months old at the beginning of training. Since one subjects data were lost accidentally at the final stage of experiment, anly 15 rats completed the generalization tests. Training was conducted in four identical sound insulated operant chambers, each having an electrifiable grid floor, a single bar on one of the walls and a fornod-tray under it. A pilot light centered on the top of the back chamber wall provided illumination in the vicinity of the bar equal to 205 $. 5 lx. Tonal frequencies were generated to identical permanent magnet speakers located below the grid floor of each chamber positioned 'on the same wall as the bar and the food-tray. Equipment for automatic programming and recording of the experiment was located in an adjoining room. Experimental sessions were conducted daily at the same time for each subject. Prior to any training the rats were reduced to of their ad lib. body weight anld maintained at that weight on a 24 h feeding rhythm during all stages of the expeliiment. TW first day of barpress training consisted of initial presentations of 40 "free" 45 mg food pellets on a 1-min V schedule followed immediately by a period with a continuous reinforcement schedule until 120 food pellets were delivered in a single session. After this training, all consecutive sessions lasted 2 h, and the barpresses were reinforced according to 2.5-min V food reinforcement schedule both during and between trials. The sixth and seventh days of instrumental barpressing training constituted two Pretest days (P-days), during which an unconditioned effect of the to-be-cs, a 3 min period of darkness provided by offset of the pilot light, was tested. Four 3 min periods of darkness, starting at 14, 24, 49, and 72 min after the beginning of the session, were presented during each P-day. Training of the CER followed in six successive sessions, when each 3-min presentation of the CS terminated with 1 s of scrambled electric foot shock of 2-mA intensity. Onsets of the CS were at the same time as during P-days. During six subsequent C training sessions, constituting the next stage of experiment, the number of CS presentations was increased to eight with additional CS onsets at 38, 56, 79, and 101 min after

4 the beginning of the'2-h session. Four presentations of the CS-terminated with a 1, s, 2-mA shock as in the CER acquisition stage (CS+ trials), whereas four others were preceded by onset of a 3,000 Hz tone of 75 db (re: 20 FN/m2) intensity one min before the CS onset and coterminated without shock (C 4- CS trials), The order of CS+ anid C 4- CS trials was changed each day in predetermined arrangements (see 14) in which the same kind of trials was never given more than twice in a row. The last stage of the experiment, also lasting six sessions, involved testing of the generalization of conditioned inhibition. Essentially, a test sampling procedure was used wherein five frequency generalization values were varied on C + CS trials within the context of five regular CSf trials. That is, each session of this stage consisted of five CSf trials (3 min darkness, terminating with 1 s of foot shock) and five C + CS trials (4 min tone, together with the last 3 min of darkness). On each C + CS trial different tonal frequencies were used: 1,000, 2,000, 3,000 (the original frequency), 4,000, and 5,000 Hz. Each tone was of the same intensity as that used in original C training. Additional trial onsets were at 93 and 115 min after the beginning of the sessions. The order of CS+ and C + CS trials was changed each day and presented according to Gellermans series (9) in which the same kind of trial was never given more than twice in a row. The order of presentation of tonal frequencies in C + CS trials was also changed each day and partially contrabalanced over consecutive days and subgroups of 4 subjects each. During all experimental sessions the numbers of barpresses emitted in consecutive 30 s periods were collected and served as the main measure of behavior. The magnitude of changes in barpressing rate during the CS+ trials was measured by computing the "suppression ratio", BL(A + B), where B represents the number of barpresses emitted during the 3 min action of the CS and A is the number of responses during the 3min period immediately before the CS onset. For C + CS trials two ratios were computed, and in both cases A was the number of responses during the 3-min period immediately before the C onset. The effect of the tone alone, the C, was measured by the ratio 3B,/(A + 3B1), where B, represents the number of barpresses emitted during the one minute from tone onset to the darkness, the CS onset. The joint action ofl the C + CS was measured by the B/(A + B) ratio, similar to CS+ trial measure. Ratios were computed independently for each subject anld trial. Additionally, daily ratios were calculated by summing the numbers of responses emitted during corresponding periods of time for all CS+ or C + CS trials. The measures used in CER studies are not always normally distribu-

5 ted. Thus nonparametric tests were always used parallel to ANOVA tests.. For the same reason, medians were preferred over means for the illustration of the group data. RESULTS Pretest days. The initial presentations of the house light offset interfered with barpressing and decreased the rate of the on-going behavior. This effect was statistically significant during the first and the second presentations of darkness (P < 0.05, Wilcoxon matched-pairs, two-tailed test). Thus, two P-days were employed in this experiment to allow full habituation of the to-be-cs. Data presented in the Table indicate that during the 2nd P-day, changes in the barpress rate during light offset periods were unsystematic. Median differences in barpressing rate between the 3-min period before and 3-min period during the to-be-cs (darkness) action on the Pretest Days (A-B index) Consecutive vresentations CER acquisition. As shown in the CER training portion of Fig. 1, acquisition of conditioned suppression was rapid. Comparisons of the numbers of barpresses emitted in A and B periods independently showed that initially, during trials of the first day of CER training, the CS enhanced the barpressing rate, although this effect was statistically nonsignificant. The CS evoked a significant decrease of barpressing after four pairings with shock (i.e., the 1st trial of the 2nd CER training day, P < 0.05, Wilcoxon test) and this suppression was observed on all subsequent trials of the 2nd trough the 6th days of training (P's < 0.01 in all comparisons). Comparisons of daily scores for A and B periods showed that during the 1st CER training session, the CS enhanced (P < 0.05, Wilcoxon test) and on subsequent, 2nd-6th CER training sessions, decreased the barpress rate (P's < 0.01, the same tests). An analysis of variance confirmed these results showing a significant day effect on daily suppression ratios

6 C 0.- v , 1 l l 1 Sessions P 1 CER -training C - training Fig. 1. Median daily suppression ratios during P-days, CER acquisition and C acquisition. Filled circles denote the light offset CS presented with the shock US, while open circles denote the same stimulus presented without the shock US, and triangles indicate the 3,000 Hz tone. The effects of 1 min C action alone (triangles) were measured by the ratio, 3Bl/(A + 3B1), and the effects of 3 min CS+ (circles) and C+ CS (circles with triangle) presentations were measured by the ratio, B/(h + B). Crasses below the marks for daily ratios denote suppression and crosses abo:re the marks denfote enhancement significant at P's < 0.05 or better, as estimated by the A-B comparisons (Wilcoxon tests). (FEv5 = 25.11, P < 0.001). Further, Duncan tests showed differences between the first anld all others days of CER training (P < 0.001), between the second anld the subsequent days (P < for the 3rd and 4th days, P < 0.01 for the 5th day, and P < 0.05 for the 6th day) and no systematic differences between the third and subsequent days. Thus, the CER was acquired during the first two days of training. C acquisition. As seen from Fig. 2 the first presentation of the 3,000 Hz tone decreased the rate of the on-going barpressing behavior. This effect was significant as shown by the comparison of the number of barpresses emitted before and during the 1 min action of the C (P < 0.01, Wilcoxon test). n next trials the C rather increased the barpressing rate and such increase was significant in four trials (on the 2nd, 3rd and 6th days of the C training, P < 0.05, the same test). As seen from Fig. 2. at the beginning of training the suppressing effects of and of the C f CS compound were similar in magnitude. The A-B comparisons showed that during the first day.of C training each presentation of the C + CS compound resulted in significant suppression of barpressing similarly as observed during presentation of the CS alone (P < 0.01 in each comparison). Similar comparisons for the 2nd-6th days of C training indicate that the supressing effect of the CS was maintained (all but one A-B comparisons showed differences significant at

7 P < 0.05 or better), whereas the suppressive effect of the C +CS compound was less systematic. However, in this experiment the C + CS evoked significant suppression of barpressing on some trials even toward the end of training (on the 2nd day, three trials with significant suppression; 3rd day, one trial; '4th day, one trial; 5th day, two trials; and the 6th day, one trial). The slow extinction of the suppressive properties of the C + CS compound observed in this experiment was presumably related to thec modalities of stimuli used as CS and C, which will be discussed later. Fig. 2. Median numbers of barpress responses emitted in consecutive 30 s periods before, during, and after each presentation of the CS+ and the C+ CS during the first day of C acquisition. Numbers in the circles denote consecutive trials within the session. The introduction of the C + CS trials resulted in the attenuation of the suppressive effect of the CS+ trials clearly observed in Fig. 1. The differences between the daily suppression ratios estimated for the CS+ 2 - Aeta Neurobiologiae Exp. 1/86

8 and C + CS appeared on the 3rd day of the C training and were also observed on the 4th and 5th days of training (Table 11). The course of inhibitory training was additionally tested using a twoway analysis of variance of the daily suppression ratios. This analysis revealed significant main effects from stimuli used (F,,,, = 28.57, P < 0.001), and days of training (F5,75 = 5.95, P < 0.001) but showed Median daily suppression ratios during the CtCS training stage of the experiment calculated for the CS+, C, and CSCS periods independently CS+ C c+cs CS+ vs CtCS comparisons (P<) ( Days of C+CS training - 1st 2nd l d l? h l 5th 6th *** *** 0:394*** *** 0.402'"~ 0.470' NS NS 0.01 no interaction. Further, Duncan test showed that the greatest suppressive effect of the C was observed on the first day and it differed from all consecutive days (P's < 0.01 or better). Similarly, h e suppressive effect of the C +CS was the greatest during the first day and differed from all other days (P's < 0.001). Daily suppression ratios for the CSf trials were also lowest during the first C training day and differed from others days (P's < 0.05 or better) except for the fifth day of training. Generalization tests. This stage of the experiment was in some respect a continuation of the C training stage, since the CS+ trials (darkness with shock) were presented as before and only the frequency of the tone used as the C was changed each trial. This change in the procedure had no effect on the daily suppression ratios on the CS trials as shown by an ANOVA test in which CSf daily suppression ratios from all sessions of C acquisition and generalization test stages were compared. Similarly no changes between the daily A scores between these two stages of the experiment were observed. As seen from Fig. 3 the typical drop in the on-going barpressing rate at the beginning of the CER training followed by gradual recovery was observed in this experiment. An ANOVA of the mean rate of barpresses in A scores for all of the sessions showed a significant effect of successive experimental sessions (F11/238 = 8.35, P < 0.001). However, there were only small fluctuations of the "baseline" during the Cl training and the generalization test stages of the ex-

9 periment. Duncan tests showed significant differences between scores involving the CER training sessions, however, they are of no importance here. Nevertheless, the same tests indicated that only during the second day of generalization testing was the baseline responding lower than during the C training sessions and some of the other generalization testing sessions (P's < 0.01). The stability of performance during the generalization tests was additionally supported by an ANOVA of the suppression ratios for each CSf trial during the last stage of experiment. No effects from testing days, trials, or their interaction were obtained. Thus, it may be concluded that the on-going barpressing rate was rather stable over days in which generalization tests were conducted. Similarly, the suppression evoked by CS+ did not change significantly beween trials or days. 1 D P CER -training! C -training Generalization tests Fig. 3. Median numbers of barpress responses per min emitted in pre-cs periods during all consecutive stages of the experiment. The generalization functions may be seen in Fig. 4, which indicated that all C and C + CS values, independent of tonal frequency variation, were above the suppression ratio from the reference point, the third CS+ trial of the test sessions ). Similar to previous sessions, the C presented alone tended to enhance the barpressing rate, whereas the C presented in compound with CS only attenuated suppressin~g properties of The level of responding to the CS+ during the generalization tests was characterize'd by only one trial, the third trial within a given session. The seasonn for this strategy was an attempt to characterize performance during the generalization test sessions by figures based on the same number of raw data an4 having similar variability.

10 the CS. Of the two measures, generalization of the C value alone tended to be sharper than generalization of the C + CS compound. The overall analysis of the generalization of the C value based on "suppression" ratios indicated effects from tonal frequency variation (F4/,, = 7.36, P<0.001) and the interaction between frequency change and testing days (F = = 2.32, P < 0.001), but the main effect of days was not significant. Duncan comparisons revealed that response enhancement at the original C training value of 3,000 Hz was significantly different from all other testing frequencies except for the 5,000 Hz value (all P's < 0.01). The lowest suppression ratios were observed for the 4,000 Hz value resulting in distortion of the graldient. A similar overall analysis of the generalization of the C + CS value showed no significant effects of tonal frequencies, successive testing days, or their interaction x 1 0 ~ HZ Fig. 4. Generalization gradients based on suppression ratios estimated for the 1 min periods of the C presented alone (triangles) and for the 3 min periods of C given in compound with the CS (open circles with triangles). The mean suppression ratio for the thiad CSf trial of each testing session (filled circle) is given as a reference point. Each point in the Figure is a mean from six medians, thus representing data collected on one particular trial of a given session from 15 subjects. To explore further the change in generalization of the C across sessions, indicated by the interaction of sessions X test frequency in the overall analysis, separate analyses were conducted for each testing session and are summarized in Table 111. The a posteriori comparisons of the specific analyses supported the results of the overall analysis. n

11 Results of analyses of each generalization testing session for the variation of the C and C+CS values C generalization C+ CS generalization session PC P< three sessions significant effects of C frequency variation were obtained. The most symmetrical gradient was observed in Session 2 with the 3,000 Hz value similar to 2,000 Hz and 4,000 Hz values and different from 1,000 Hz and 5,000 Hz values (P < 0.05, Duncan test). On session 4 responding to the 4,000 Hz value was the lowest and differed from response levels of all other test stimuli (P < 0.005). On Session 6 responding to the 3,000 Hz value differed only from that to 4,000 Hz and 5,000 Hz (P < 0.05). Similar separate analyses were conjducted for the C + CS data and also presented in Table 111. As seen, in two sessions significant effects of C frequency variation on the C + CS data were obtained. On session 4 responding to 4,000 Hz C given in compound with CS was the highest and differed from responding to all other frequencies (P's < 0.05). On session 6 the highest level of responding was observed to thq 2,000 Hz frequency, but it differed.only from the 5,000 Hz frequency evoking the lowest level of barpressing (P < 0.01). DSCUSSON The major result of this study was the finding of reliable, withinsubjects gradients of barpressing enhancement as a result of variation in tonal frequency used as the conditioned inhibitor of conditioned barpress suppression. The effects of the C were tested in two situations: when presented alone during one min periods and -during the next three min intervals when presented in compound with the CS. The behavioral effects of the C in the two situations were different. The C presented alone was easily discriminated from the CS and after a few presentations ceased to suppress on-going banpressing. On the contrary, more than six days of training were necessary for the C to acquire the property of in-

12 hibiting the suppressive effects of the CS (24). Thus, at early stages of inhibitory training, the suppression ratios estimated for the C were close to the 0.5 value reflecting no change in on-going barpressing, whereas the suppression ratios for the C + CS compound were lower than 0.5 value denoting suppression of the on-going barpressing rate. With prolonged training, both the C and C $ CS acquired a tendency to enhance barpressmg rate (24-26). The present experiment showed that the enhancement effect was more readily acquired by the C presented alone than given in compound with the CS. The obtained generalization gradient for the C indicated that the strongest enhancing effect was evoked by the tonal frequency used as the C in the original training. Among all frequencies tested only the 4,000 Hz tone suppressed the on-going barpressing. The finding of suppression to the 4,000 Hz value, in conjunction with response enhancement to the 5,000 Hz value may be considered as an evidence of a contrast effect in the discrimination of the test values (22). This contrast effect in an aversively motivated context is similar to results reported by Hoffman (12) in a study of conditioned suppression of key pecking in pigeons and by Brennan and Riccio (3) in discriminative avoidance behavior in rats. The generalization gradient for the C + CS was rather flat and the overall analysis did not show a significant effect from variation of tonal frequency. Separate analyses did reveal an effect on two test sessions, although in neither case was the 3,000 Hz the preferred frequency. Broad generalization for the C + CS should be expected, since this stimulus situation had a mixed character, in that one element of the inhibitory compound was the excitatory CS. Earlier experiments on stimulus generalization of conditioned suppression, in which some parameters of the CS paired with inescapable shock were changed, yielded broad generalization gradients (4, 8, 10-13, 20, 29). These gradients sharpened with prolonged testing in extinction, when values for away from the CS+ eventually began to enhance the on-going barpressing. However, in the case of generalization around a C + CS oompound, an opposite process may be expected. A sharpening of the C + CS gradient would be related rather to prolonged training, when the properties of the C gradually became dominant within the C + CS compound. Significant effects of tonal frequencies on the C 4- CS generalization functions, which were \ observed only during the final testing sessions, support this supposition. The findings of the present experiment are related to the traditional discussion concerning the nature of conditioned inhibition. Pavlov distinguished two basic situations in which a CS ceased to evoke an acquired CR. One, occurring upon systematic presentations of the CS without

13 pairing with the US, produces the process of conditioned inhibition that overcomes the previously acquired conditioned excitation. The second, occuring when concurently with the CS another stimulus is presented evoking such strong excitation that the organism engages in another kind of activity. The first mechanism was termed internal inhibition, and the second - external inhibition. nternal inhibition, a synonymous with conditioned inhibition, reflected a notion that inhibition developed within the reflex arc of the CR under consideration. The term external inhibition denotes that lack of the CR or its diminution was not due to some changes in the reflex arc of this particular response, but rather related to influences from reflex arc of another response (19). This distincticon between the two mechanisms is not only of historical importance. n modern interpretations, this distinction was reflected in the two opposite definitions of conditioned inhibitory stimuli. Jenkins defined a conditioned inhibitory stimulus as "a signal for not responding" (15, p. 35), whereas Rescorla considered it as a stimulus controlling "a tendency opposite to that of the conditioned excitor" (21, p. 78). Properties of the conditioned inhibitor of conditioned suppression fit much better with Rescorla's than with Jenkin's definition. nhibition of conditioned suppression lealds to the enhancement of the on-going barpressing behavior. A previously published review (24) of experimental data showed that not only training of the conditioned inhibitor but also bther Pavlovian procedures, such as experimental extinction, differentiation learning and inhibition of delay training, produce stimuli that inhibit conditioned suppression and evoke enhancement of the on-going behavior. Thus, it may be concluded that in situations in which an organism is under the influence of different motivations to which specific response patterns are linked, inhibition of one response pattern elicits the opposite response. n consequence an important question may be asked, whether stimuli acquiring inhibitory properties for one response pattern at the same time acquire exeit'atory properties for the apposite response. There is no firm answer to this question (5). Stimuli acquire inhibitory properties in the course of leaning involving gradual elimination of other response tendencies. According to Terrace only learning with errors produce inhibitory stimuli; and "after discrimination is learned without errors, S- appears to function as a neutral stimulus" (23, p. 1678). The present experiment showed that the C presented alone acquired inhibitory proper'ties very easily, without, or with very few, "errors". Moreover, reliable generalization gradient of enhancement elicited by the C indicate that the conditioned inhibitor cannot be considered as a "neutral stimulus" but exerted specific control over conditioned enhancement - the behavior opposite to the suppres-

14 sion. Large amount of experimental data supported Terrace's state- ment. Thus, it has to be concluded that inhibitory properties of the C were acquired due to numerous "errors" commited when the C was given in compound with the CS. The proportion of trials in which the C 4- CS compound evokes signlficant suppression of the on-going behavior may be considered as an index of the amount of generalization from the CS. Comparison of the data from several experiments in which white -noise was used as the CS and darkness as the C (24, 25) showed that the more intense the CS the greater the proportion of C + CS presentations that produce significant suppression of barpressing during,the six days of inhibitory training: 42'O/o with an 50 db CS, 55O/o with an 70 db CS, and 75Q/d with an 80 db CS. n the present experiment the C + CS compound produced significant suppression in 50 /o of the trials. This showed that darkness may be considered as a weak CS for conditioned suppression, which is in agreement with other CER studies in which illumination of different intensities or flashing lights of differeht colours were used as CSi signaling inescapable shock (1, 2, 6, 16, 17, 27). This investigation was supported by Project of the Polish Academy of Science. Partkirpatim in this experimenit by the second author was made possible by a fellowship grant from the USA National Academy of Sciences. REFERENCES 1. BLACKMAN, D. E. and SCRUTON, P Conditioned suppression and discriminative control of behavior. Anim. Learn. Behav. 1: BRAHLEK, J. A Conditioned suppression as a function of the number of stimuli that precede shock. Psychoa. Sci. 12: BRENNAN, J. F. and RCCO, D. C Sitimulus control of avoidance behavior in rahs following differential or nondifferential Pavlovian training along dimensions of the conditioned stimulus. J. Comp. Physiol. Psychol. 85: BRENNAN, J. F. and RCCO, D. C Stimulus generalization of suppression in rats following aversively motivated instrumental or Pavlovian training. J. Comp. Physiol. Psychol. 88: DCKNSON, A Appetitive-aversion interactions: Facilitation of aversive conditioning by prior appetitive training in the rat. Anim. Learn. Behav. 4: DONEGAN, N. H., WHTLOW, J. W. Jr. and WAGNER, A. R Posttrial reinstatement of the CS in Pavlovian conditionilng: Facilitation or impairment of acquisition as a funection of individual differences in responsiveness to the CS. J. Exp. Psychol.: Anim. Behav. Proc. 3: ESTES, W. K. and SKNNER,'B. F Some quantitative properties of anxiety. J. Exp. Psychol. 29:

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