Bidirectional operant conditioning of heart rate in rats with sucrose reward

LEARNING

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488-501 (1979)

Operant Conditioning of Heart Rate in Rats with Sucrose Reward M. R. D’AMATO Rutgers

AND MARK

MEINRATH

University

Sucrose was used to reinforce heart rate (HR) increases in one group of unrestrained rats and to reinforce decreases in another. A third group received noncontingent sucrose presentations, and a fourth was presented with an empty dipper. After 10 conditioning sessions HR, as measured by changes in tonic levels, was significantly greater in the Fast group than in the Noncontingent group, which in turn maintained significantly higher levels than the Slow group. There was no difference between the Noncontingent and No-sucrose groups. Behavioral observations indicated that the Fast animals increased the percentage of time spent in such high-HR categories as rearing and walking; however, there were no corresponding systematic decreases in the Slow animals. Moreover, the Fast and Slow groups differed significantly with regard to the HRs associated with five behavioral categories. The overall pattern of results suggests that some degree of cardiospecific effect may have been exerted by the operant contingencies.

With few exceptions, investigations of operant conditioning of heart rate (HR) in noncurarized animals have employed shock avoidance as the reinforcer (e.g., Brener, Phillips, & Connally, 1977; Engel, Gottlieb, & Hayhurst, 1976; Regestein, Schwartz, & Lown, 1973, or shock avoidance in combination with food (e.g., Harris, Gilliam, & Brady, 1976). When appetitive reinforcers have been employed exclusively, they usually have taken the form of brain stimulation rather than more conventional reinforcers. A major aim of the present study was to evaluate the feasibility of using sucrose reward, which, unlike shock and brain stimulation, is not associated with strong unconditioned responses, to condition HR bidirectionally in unrestrained rats. An ECG monitoring system was employed that required no surgical intervention and placed a minimum of restraint on the animals’ activity. The degree to which conditioned changes in HR (and in other responses controlled by the autonomic nervous system) depend upon “mediation” This research was supported by NIMH Grant R03-MH 26545 and a grant from the Busch Memorial Fund. Reprints may be obtained from M. R. D’Amato, Department of Psychology, Busch Campus, New Brunswick, NJ 08903. 488 0023-%90/79/040488-14$02.00/O Copyright @ 1979 by Academic Press. Inc. All rights of reproduction in any form reserved

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by somatomotor responses is an important, and complex, issue (e.g., Cohen & Obrist, 1975; Harris & Brady, 1974; Kimmel, 1974). In the free-moving animal this issue becomes particularly acute because of the abundant opportunity for somatomotor mediation. Thus, an animal trained to increase its HR might simply learn to spend a good deal of time rearing or engaging in other high-HR behaviors. Although mediation of this sort is almost certain to occur in HR conditioning in the intact, unrestrained animal, particularly during the early stages of training, it is nevertheless possible that conditioned control of HR gradually shifts, at least in part, from overt somatomotor responses to more direct participation by the autonomic nervous system (ANS). For example, an animal conditioned to increase its HR might ultimately display in the experimental situation a heightened level of sympathetic tone. Presumably, such a conditioned increase in sympathetic tone would raise an animal’s heart rate generally, and as a consequence the HR contingency would often be met without the necessity of the animal engaging in high-HR somatomotor responses. To explore this possibility, during certain phases of the experiment the animals were observed and their behavior classified into a number of different response categories. These categories generated a HR profile, and the issue then was whether the conditioning procedures were effective in altering the baseline HR profiles in an appropriate and consistent manner. Discriminative stimuli are often used in HR conditioning studies to delineate periods when the contingency is in effect (SD) from those when it is not (SA). If these periods are relatively short, as is frequently the case, control of HR by conditioned somatomotor behavior is likely to be encouraged. One reason is that the animal can refrain, during SA periods, from performing the required somatomotor responses, which may reduce the pressure to seek alternative ways of meeting the reinforcement contingency. Another consideration is that if the ANS is to be used effectively to meet the HR contingency, it would have to adjust itself relatively rapidly to keep in phase with the cycling SD, SA periods. This may be a more demanding requirement than simply maintaining, say, an elevated sympathetic tone during the experimental session. Because of these and other considerations, we avoided the use of repeated SD, SA periods, and we used as our primary dependent variable tonic changes in HR as measured over a 30-min conditioning period (cf. Obrist, 1976). METHOD

Subjects. Sixteen male Sprague-Dawley rats, weighing 200-300 g, were divided into four groups (n = 4) matched according to weight. One week before testing, the animals were placed on a restricted food ration

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which reduced each animal to 85% of its free-feeding body weight. Water was available ad lib. Apparatus. The animals were tested in a large Plexiglas box (46 x 25 X 38 cm) equipped with a pair of stimulus lights and a liquid-dispensing dipper on the front wall and a second pair of stimulus lights on the rear wall. The experimental chamber was located in a larger sound-attenuating enclosure, equipped with a dim overhead light (7-W bulb) and a one-way mirror to permit behavioral observations. The behavioral observations were recorded on a seven-key event marker located adjacent to the one-way mirror. The stimulus lights, liquid dipper, and event marker were interfaced to a minicomputer, which controlled the experiment. HR was obtained by a biotelemetry system described in detail elsewhere (Meinrath, Collins, & D’Amato, 1977). Briefly this system consists of a small saddle-like transmitter harness, with two miniature surface electrodes attached to the cinch of the harness, obviating the need for implanted electrodes or skin pins. A small FM transmitter attached to the harness at a point above the animal’s shoulders amplifies and broadcasts the ECG signal, which is received by an FM tuner modified by adding a 3- to 300-Hz bandpass filter at the output. A voltage comparator and pulse-former circuit detects the R-wave of the ECG and transmits a digital pulse to the computer. Procedure: Adaptation. Several days before testing, the hair over the pectoral muscles was shaved, thereby insuring contact of the chest electrodes. The animals were tested for one 45-min session each day for a total of 20 days. To adapt the animals to the experimental apparatus and to the harness, during Days l-5 each animal was simply placed in the experimental apparatus for the 45-min period, no experimental manipulations being performed. However, HR was measured each day and behavioral observations (described below) were recorded on the last 2 days of adaptation (Days 4-5). Dipper training. On Days 6-10, three groups of animals were trained to drink 15% (w/v) sucrose solution from the dipper, which had a capacity of 0.03 ml. Activation of the dipper was signaled by a solenoid noise and a visual cue provided by powering the lights on the front wall. Dipper presentations occurred on a variable-time (VT) 30-set schedule. The fourth, No-sucrose, group received empty dipper presentations on the same schedule, to control for the possibility that an unconditioned HR response was elicited by the operation of the dipper mechanism. Dipper presentations did not occur during the first 15 min of the dipper training sessions. At the end of the 15 min, the pair of stimulus lights on the rear wall of the testing chamber was illuminated signaling the onset of the VT schedule, which remained in effect for the next 30 min. During the first 2 days of dipper training, the animals in the three groups that received sucrose were trained to approach and drink from the dipper. As in the

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adaptation phase, HR was measured each day during dipper training, and the behavioral observations described below were recorded on the last 2 days (Days 9-10). Conditioning. During Days 1 l-20, the response-independent dipper presentations were replaced in two groups of animals by dipper presentations contingent upon HR. The Fast group was reinforced for raising or maintaining elevated HR levels, and the Slow group was reinforced for lowering or maintaining low HR levels. A third, Noncontingent, group continued to receive sucrose on the VT 30-set schedule, and the Nosucrose group continued to receive empty dipper presentations on the same schedule. As before, the reinforcement schedules were in effect only during the last 30 min of each session, and the availability of reinforcement was signaled by the lights on the rear wall. HR was measured during each conditioning session and behavioral observations were taken during the last 2 days of conditioning. Behavioral observations. During the last 2 days of adaptation, dipper training, and conditioning, one experimenter (M.M.) observed each animal and classified its behavior by depressing an appropriate key on the event marker. The behavioral categories recorded were: (1) licking the dipper; (2) still, or motionless; (3) head movement only, which included bobbing the head, sniffing, or any behavior in which the body was stationary while the head moved; (4) walking, which included any ambulation involving the whole body; (5) rearing, or any orthostatic posture; (6) grooming; and (7) other behavior, i.e., any behavior not easily fitted into one of the preceding categories. These behavioral categories are similar to those employed by Elliott (1975) and Vanderwolf and Vanderwart (1970). When making these behavioral observations, the experimenter was unaware of the animal’s concurrent HR. HR measurement. The basic HR datum was the duration of time (measured to the nearest 20 msec) required to accumulate 10 “valid” heart beats (HB), i.e., HBs with interbeat intervals falling within the range of loo-240 msec. When a HB failed to meet this criterion, which was established to reject spurious signals due to noise and to detect “dropped” beats, the current HB sample was aborted and the computer waited for the first valid HB before beginning the next lo-beat sample. On the average, HR samples were obtained for approximately 80% of an experimental session. At the beginning of each conditioning session an animal’s initial criterion HR was established by subtracting (if in the Fast group, adding if in the Slow group) 120 msec from the duration of time required to accumulate 10 HBs for a representative sample taken during the first 15 min of the conditioning session. A titration schedule then decreased in the Fast group (increased in the Slow group), the criterion lo-HB interval by 40 msec whenever the animal achieved four or more reinforcements within 20 set (30 set in the Slow group). In the Fast group, the duration of the

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criterion lo-HB intervals was decreased by 40 msec if a reinforcement was not received for 30 set; in the Slow group the criterion was increased by 40 msec if a reinforcement failed to occur for 40 sec. Reward was delivered after each criterion IO-HB sample except that, to avoid activating the dipper mechanism while the animal was drinking, the minimum inter-dipper time was set at 3 sec. During the sessions that behavioral observations were taken, the computer sorted the animal’s HR in bins in accordance with the behavioral category identified at the beginning of the IO-beat sample. RESULTS Adaptation

and Dipper

Training

To assess whether there were any differences in HR among the groups during adaptation or dipper training, each animal’s average HR during the last 30 min of a session was calculated for Days 4-5 (last 2 days of adaptation) and Days 9-10 (last 2 dipper-training days). These data were subjected to a split-plot analysis of variance (ANOVA) in which Groups was the “between” factor and Phase (adaptation vs dipper training) was the “within” factor. In all statistical analyses, the rejection level was set at p < -05. Neither Groups nor the Groups by Phase interaction was significant (F values < 1). Phase was also insignificant (F < l), the overall mean HRs for Days 4-5 and 9-10 being 406.1 and 410.0 bpm, respectively. To provide a more powerful test of a possible unconditioned effect of sucrose presentations on HR, a t test based on the difference in HR between the two phases was performed which compared the No-sucrose group (x = -17.25) with the average of the other three groups (x = 11.25). It, too, was insignificant [t(12) = 1.401. One may conclude from these results that the preconditioning HRs were comparable in the four groups of subjects and that the schedule of noncontingent sucrose presentation employed during dipper-training did not result in a strong unconditioned HR response in the Fast, Slow, and Noncontingent groups. Because all animals were treated in an identical fashion during the adaptation sessions, the behavioral observations taken on Days 4-5 provided a means of evaluating whether the different behavioral categories were associated with reliably different HR levels and whether the HR profiles generated by the various categories were similar in the four groups of animals. To answer these questions each animal’s HR was calculated for each of the behavioral categories shown in Fig. 1, which presents the average HRs for all 16 animals. (Licking is not included because there were no dipper presentations during Days 4-5.) An ANOVA was performed with Groups as the between factor and Categories, the within factor. Because it occurred so infrequently (accounting for only 2% of the recorded observations), grooming was excluded from the analysis, although it is presented in Fig. 1 for purposes

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FIG. 1. Mean HR for all 16 rats as a function of behavioral category. The data are based on observations taken during the last 30 min of Days 4-5 (last two adaptation sessions). Because dipper presentations did not occur during these sessions, the licking category is not included.

of comparison. There was one instance in which an animal failed to engage in one of the target behaviors during the 60 min that the animal was observed; a value was generated for this missing score by the method suggested by Kirk (1968, p. 281). The ANOVA indicated that neither Groups [F(3,12) = .19] nor the Groups by Categories interaction [F(9,35) = .91] were significant factors. On the other hand, Categories was highly significant [F(3,35) = 83.371, accounting for approximately 40% of the total variance. We may conclude from these results that substantially different levels of HR were associated with the different behavioral categories and that the HR profiles generated by these categories were quite similar in the four groups of subjects. Conditioning In order to control for individual variations in HR level, difference scores are often employed. In the present experiment two possibilities existed. Because the reinforcement schedules were not in effect during the first 15 min of the conditioning sessions, a within-session difference score could be formed by subtracting the HR observed during the first 15 min from the HR of the last 30 min of a session, Alternatively, a between-

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session difference score was available derived from the HRs obtained during the last 30 min of a conditioning session and from corresponding periods of earlier “baseline” sessions. The following analysis led us to adopt the latter. Although a discriminative stimulus was available during the 30 min of each conditioning session that the reinforcement schedule was in effect, there is no guarantee that it gained complete control over the behavior of the animals in the Fast and Slow groups. Thus, if conditioning occurred in these groups, it might have been in evidence during the first 15 min of a conditioning session when neither the discriminative stimulus nor the reinforcement schedule was presented. To assess this possibility, HR difference scores were formed based on differences in mean HR obtained during the first 15 min of Days 19-20 (last 2 conditioning days) and the first 15 min of Days 9-10 (last 2 dipper-training days). A substantial decline in HR was shown by all groups but the Fast, which displayed a small increase. A t test based on the difference scores indicated that the Fast group differed significantly from the combined Slow, Noncontingent, and No-sucrose groups [t(12) = 2.791. To evaluate the conditioning data, therefore, the mean HR obtained during the last 30 min of Days 9-10 was subtracted from the mean HR of the last 30 min of each conditioning session. Figure 2 presents the mean difference scores in blocks of 2 days. The Fast group showed a rapid increase in HR which generally persisted over the 10 conditioning days. In contrast, a gradual decline in HR occurred in the Slow group. The two control groups revealed only a moderate decline in HR over the 10 conditioning sessions. A split-plot analysis of variance based on the difference scores produced a significant main effect for Groups [F(3,12) = 16.341. By Duncan’s multiple range test (a = .Ol), the Fast group was significantly different from the Slow group and both were significantly different from the Noncontingent group. The Groups by Blocks of Days interaction was not significant (F < I). The purpose of including the No-sucrose group was to evaluate whether the Noncontingent presentation of sucrose influenced HR during the conditioning sessions. It is evident from Fig. 2 that the presentation of sucrose without an explicit HR contingency had no discernible effect on HR. As might be expected, the number of reinforcements received by each group differed, but the differences were not directly related to HR level. The mean number of reinforcements received each conditioning session by the Fast, Noncontingent, and Slow groups were 121, 60, and 103, respectively. If the obtained HR changes were mediated by conditioned somatomotor responses, the Fast group should have spent more time in high-HR behavioral categories during conditioning than during the baseline sessions, and the reverse should be true for the Slow group. We

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FIG. 2. The results of the 10 conditioning sessions (Days 1 l-20) expressed as difference scores and averaged over blocks of two sessions. For each animal, the mean HR obtained on Days 9-10 served as the baseline HR score.

therefore calculated for each animal the relative percentage of time spent in each of the six behavioral categories on Days 19-20 (the last 2 conditioning days) and on Days 9-10 (baseline sessions). Relative percentages were derived by dividing the amount of time spent in a given category by the total amount of time spent in all six categories combined, which on the average accounted for approximately 68% of the 60-min total that each animal was observed. (Since the animals spent about 12% of this period in the “other” category, the six behavioral categories accounted for 85% of the recorded observations.) A difference score was formed for each category by subtracting the relative percentage time score of Days 9-10 from that of Days 19-20. It should be noted that because the groups differed little in the total amount of time consumed by the six categories, the difference scores based on relative percentages should accurately reflect changes in the absolute percentage time spent in the various categories. Figure 3 presents for the Fast, Noncontingent, and Slow groups the mean difference score for each category. (Grooming occurred very in-

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FIG. 3. Difference in percentage time spent in the various behavioral categories, derived from Days 9-10 (baseline sessions) and Days 19-20 (last 2 conditioning days) for the Fast, Noncontingent, and Slow group (see text).

frequently during Days 9- 10 and 19-20, and it therefore is not included in Fig. 3 or in the subsequent analyses.) The Fast group did indeed increase considerably the percent time spent in the rearing category, which is associated with high HRs. The slow group, however, did not show a consistent increase in time spent in low-HR categories; its pattern of changes was very similar to that generated by the Noncontingent group.

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To evaluate the increase shown by the Fast group in high-HR categories, the changes in percentage time displayed by this group for the walking and rearing categories were compared to the corresponding changes produced by the Noncontingent group. Licking was not considered in these analyses because of its dependence on the reinforcement schedule in effect. A split-plot analysis of variance produced significant results for Groups [F(1,6) = 26.371, Categories [F(1,6) = 55.101, and the Groups by Categories interaction [F( 1,6) = 11.331. Analysis of the simple effects of Categories revealed that the difference between the Fast and Noncontingent groups was significant for both the walking and the rearing categories. To evaluate whether the changes in percentage time spent in low-HR categories was different in the Slow and Noncontingent groups, a similar analysis was performed between these two groups based on the still and head-movement-only categories. In this case, neither the Groups nor the Groups by Categories interaction was significant (Fs < 1). Moreover, comparison of these two groups on the high-HR categories (walking and rearing) produced the same results. A somewhat more critical test of the somatomotor mediation interpretation is possible. If the conditioning procedures had no direct effect on HR but merely induced the animals in the Fast and Slow groups to engage in different behaviors, then the HR associated with a specific behavioral category should not have been influenced by the conditioning experience. On the other hand, if the conditioning procedures exerted a cardiospecific effect, raising HRs generally in the Fast group and lowering HRs in the Slow group, the level of HR observed for each of the behavioral categories should have been influenced accordingly. We therefore calculated for each subject in the Fast, Noncontingent, and Slow groups the HRs generated on Days 9-10 and 11-20 for each of the five behavioral categories (again dropping grooming). The mean difference between these HRs for each behavioral category is presented in Fig. 4. If an animal failed to engage in a behavioral category during either Days 9-10 or 19-20, no difference score could be formed. These instances are indicated in Fig. 4 by the number that appears in some of the bars, which gives the sample size for that particular category. The Noncontingent group evidenced a moderate decrease in HR for all five categories. The Slow group generally showed a greater decline in HR over the various categories than the Noncontingent group. In marked contrast, the Fast group produced an increase in HR in all five of the categories. To evaluate these results statistically, we calculated for each subject the average HR changes for those behavioral categories that were available, which numbered either 4 or 5. The mean differences for the Fast, Noncontingent, and Slow groups were 8.2, -6.2, and -27.5, respectively. A simple analysis of variance applied to the HR difference scores produced a significant result r F(2.9, = 4.92: for the Fast vs Slow comnarison

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FIG. 4. Mean HR differences for each behavioral category, obtained by subtracting HRs of the baseline sessions (Days 9-10) from the corresponding HRs of the last 2 conditioning days. Four animals contributed to each mean HR unless indicated otherwise by a number in a bar.

r(9) = 3.151. Thus, it appears that the profile of HR changes across the various behavioral categories differed significantly as a function of the reinforcement contingency applied to the Fast and Slow groups. DISCUSSION

.

The present results indicate that substantial increases and decreases in . TTr. 1 1 I~. 1 . A,-. .&.._I..- 2 __I Le. ------. ---A:*.‘--:--

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procedures which employ a conventional appetitive reinforcer, sucrose solution. The results also suggest that at least a portion of this conditioning may not be due simply to somatomotor mediation, that is, to the differential conditioning of high- or low-HR behavioral categories. The different HR profiles portrayed in Fig. 4 for the Fast and Slow groups provide support for this possibility. It should be pointed out that the No-sucrose group also showed uniform decreases in HR across the different behavioral categories, which suggests that the decline in overall HR that normally occurs as animals “adapt” to an experimental situation is not due solely to their spending more time in low-HR categories. Rather, there seems to be a general lowering of HR across all behavioral categories, as might be caused by a decrease in sympathetic tone. There are a number of studies in which HR changes have been obtained across behavioral categories that can be interpreted in a similar way. Vanderwolf and Vanderwart (1970), employing behavioral categories not unlike our own, found a significant within-session HR decline across all response categories. Although they were inclined to interpret this result somewhat differently, it is possible that the lowered HR profile resulted from a decline in the arousal which was initiated by the procedures associated with introducing the animals into the experimental space (e.g., handling). It is of some relevance in this connection that Bohus (1974) found that mean HR was consistently higher in rats whose ECG was obtained by means of attached wire leads rather than by biotelemetry. Moreover, in the former animals HR was uniformly higher in the four behavioral categories measured by Bohus. This result, as well as the finding by Elliott (1975) that water deprivation consistently and uniformly elevated HR across the eight behavioral categories measured by him, can be accounted for in terms of different levels of arousal (or sympathetic tone) associated with the different experimental conditions. However, deprivation does not uniformly have such a result, as Vanderwolf and Vanderwart (1970) found no differences in HR across their behavioral categories as a function of food-deprivation level. Although the HR-profile data obtained from the behavioral categories are suggestive, they are subject to alternative interpretations. It could be argued that the behavioral categories used by ourselves and others may consist of several separate subcategories, each with somewhat different characteristic HRs. If so, shifts from one subcategory to another as a function of reinforcement contingency would, without special observational techniques, go undetected. Although it is difficult to dispose of this possibility completely, it should be recalled that the four categories head movement only, still, walking, and rearing accounted for about 40% of the HR variance during Days 4-5 of adaptation. Correlations between HR and somatomotor activity (measured by stabilimeter-like devices) calculated over durations of approximately an hour typically range between .6

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and .7 (Meinrath & D’ Amato, 1979), thus accounting for approximately 36-49% of the HR variance. It appears, therefore, that the behavioral categories employed in this study, although few in number, account for about the same proportion of HR variance as continuous activity measures. Such comparability would not be expected if the behavioral categories encompassed a number of subcategories, each with significantly different HRs. A second potential limitation of the HR-profile data arises when the animals change markedly the distribution of time spent in the different behavioral categories. In the present experiment, for example, by the end of conditioning the Fast animals increased considerably the amount of time spent rearing, and it is conceivable that the relatively high HRs generated by rearing could have carried over to subsequent behaviors, thereby inflating their HRs. Unfortunately, the sequential data required to evaluate this possibility were not available to us. But even if such an effect played a role in the elevated HR-profile data of the Fast animals, it probably did not contribute significantly to the different HR profiles of the Slow and Noncontingent animals (Fig. 4), as the changes in time spent in the various behavioral categories were quite similar in these two groups (Fig. 3). Nonetheless, these are issues that require further investigation for their resolution. REFERENCES Bohus, B. Telemetered heart rate responses during free and learned behavior. Biotelemetry, 1974, 1, 193-201. Brener, J., Phillips, K., & Connally, S. R. Oxygen consumption and ambulation during operant conditioning of heart rate increases and decreases in rats. Psychophysiology. 1977, 14, 483-491. Cohen, D. H., & Obrist, P. A. Interactions between behavior and the cardiovascular system. Circulation Research. 1975, 37, 693-706. Elliott, R. Heart rate, activity, and activation in rats. Psychophysiology, 1975, 12, 298-304. Engel, B. T., Gottlieb, S. H., & Hayhurst, V. F. Tonic and phasic relationships between heart rate and somoto-motor activity in monkeys. Psychophysiology. 1976,13,288-295. Harris, A. H., & Brady, J. V. Animal learning-visceral and autonomic conditioning. Annutrl Review of Psychology, 1974, 25, 107-133. Harris, A. H., Gilliam, W. J., & Brady, J. V. Operant conditioning of large magnitude, 12-hour duration, heart rate elevations in the baboon. Puvlovinn Journcll ofthe Biologicd Sciences. 1976, 11, 86-92. Kimmel, H. D. Instrumental conditioning of automatically mediated responses in human beings. American Psychologist, 1974, 29, 325-335. Kirk, R. E. Experimentrtl design: Procedures for the hehaviorcd sciences. Belmont, Calif.: Brooks/Cole, 1968. Meinrath, M., Collins, P., & D’ Amato, M. R. A nonobtrusive heart rate telemetry system for rats. Behavior Research Methods ond Irtstrumentation. 1977. 9, 243-246. Meinrath, M., & D’Amato, M. R. Interrelationships among heart rate, activity, and body temperature in the rat. Physiology nnd Behavior, 1979, 22, 491-498. Obrist, P. A. The cardiovascular-behavioral interaction-as it appears today. Psychophysiology, 1976, 13, 95-107.

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Regestein, Q., Schwartz, G. E., & Lown, B. Operant conditioning of heart rate in freemoving dogs. Cardiology, 1975, 61, 138-145. Vanderwolf, C. H., & Vanderwart, M. L. Relations of heart rate to motor activity and arousal in the rat. Canadian Journal of Psychology, 1970, 24, 434-441. Received May 23. 1979