Effects of limited-target availability on schedule-induced attack

Effects of limited-target availability on schedule-induced attack

Physiology & Behavior, Vol. 30, pp. 11-18. PergamonPress, 1983. Printed in the U.S.A. Effects of Limited-Target Availability on Schedule-Induced Atta...

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Physiology & Behavior, Vol. 30, pp. 11-18. PergamonPress, 1983. Printed in the U.S.A.

Effects of Limited-Target Availability on Schedule-Induced Attack RANDALL

K . F L O R Y A N D C A T E S B Y T. S M I T H

Psychology Department, Hollins College, Roanoke, VA 24020 R e c e i v e d 7 S e p t e m b e r 1982 FLORY, R. K. AND C. T. SMITH. Effects of limited-target availability on schedule-induced attack. PHYSIOL BEHAV 30(i) 11-18, 1983.--Pigeons exposed to a 180-sec fixed-time food schedule could attack a rear-projected conspecific target that was available either (a) continuously throughout the interfood interval, (b) randomly during one 30-sec portion of each interfood interval, or (c) during the final 90 sec of each interval. During continuous-target availability, attack was maximal shortly after food ingestion and progressively decreased thereafter. During random-target availability, five of seven pigeons attacked less per target-access period the later that period occurred within the interfood interval, whereas two subjects exhibited relatively high local attack rates even when access periods occurred within the final third of the interval. When the target was available only during the second half of the interfood interval, attack occurred as soon as the target was presented and progressively decreased throughout the remainder of the target-access period. In general, these results show that schedule-induced attack can be increased by limiting the availability of that target and also indicate that such attack can reliably occur at times other than shortly after food delivery. Schedule-induced attack Adjunctive behavior Pictorial target White King pigeons

Limited-target availability

A G G R E S S I V E behavior against live or inanimate targets has been induced by the transition from a period of frequent food delivery to one during which food is never delivered [3,34] and has also been induced by various responsedependent as well as response-independent schedules of food delivery [18,26]. In these investigations, extinction- and schedule-induced attack behavior typically occurred shortly after removal of the reinforcer and then decreased in probability with the subsequent passage of time. It is possible that attack predominantly occurs as a postreinforcement phenomenon because reinforcer delivery in most schedules, especially fixed or periodic ones, acquires discriminative properties that signal an ensuing period during which the probability of further reinforcement is either zero or relatively low [l, 12, 13, 33]. This account is supported by the observation that pigeons exposed to second-order schedules attacked following the presentation of a nonfood stimulus that signalled a period of food unavailability [4,17]. It is also possible that post-reinforcement attack is due not to the reinforcer's discriminative properties but rather to the aversive aftereffects of its withdrawal or interruption [3, 20, 23, 3 l] since constant probability schedules are sufficient to induce post-food attack [36]. Yet another account of the typical distribution of various schedule-induced behaviors has been offered by Killeen [24] who proposed that each intermittent reinforcer delivery generates a state of elevated arousal which increases the vigor of activities associated with available situational stimuli such as a conspecific target. As time progresses, however, behaviors appropriate to the forthcoming reinforcer increase in frequency and successfully compete with the induced activity. Although each of the above-mentioned accounts predicts

Fixed-time schedule

that attack against a continuously-available target should be maximal shortly after reinforcer delivery, the question arises as to whether comparable levels of such behavior can be induced against a target that is available only at other periods within the interreinforcer interval. In the only previous study conducted to answer this question, Muller and Cheney [29] investigated attack induced in pigeons by a fixed-time food schedule when access to a live but restrained conspecific target was restricted to various times following food delivery. When the target was available during a fixed final portion of the interfood interval, attack occurred as soon as the target was presented and then progressively decreased across the remainder of the target-access period. When the target was restricted to one randomly-determined quarter of each 120-sec fixed-time interval, less attack occurred in an access period the later that period occurred within the interfood interval. Muller and Cheney [29] found, however, that when the conspecific target was accessible only during a fixed 15-sec period of each fixed-time interval, the local level of attack against the restricted target was substantially higher than the level that occurred in the same 15-sec portion of the interfood interval when the target was continuously available. Through a systematic replication of the Muller and Cheney [29] study, the present experiment further assessed the effect on schedule-induced aggression of restricting target availability to varying periods of the interfood interval. In contrast to the Muller and Cheney [29] investigation, however, the present study employed naive vs. non-naive pigeons, a pictorial rather than animate target, a measure of attack based on physical contacts against the conspecific rather than one based on photobeam interruptions, and a

C o p y r i g h t © 1983 P e r g a m o n Press--0031-9384/83/010011-08503.00/0

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protective contingency between attacks and scheduled food delivery.

TABLE 1 SEQUENCE OF EXPERIMENTALCONDITIONSAND NUMBERSOF SESSIONS DEVOTEDTO EACH CONDITION

METHOD Sessions

Animals Seven experimentally-naive male White King pigeons, 1-2 years of age, were used. Each pigeon was maintained at 75% of its free-feeding weight and was housed in a wood and wire cage located in a temperature- and humidity-controlled room under a light cycle of 16 hrs on and 8 hrs off. Water and grit were always available in the home cages. Pigeons received supplementary feedings of mixed grain following experimental sessions to maintain stable 75% free-feeding weights.

Apparatus On one wall of the 36.3 by 40.0 by 34.0-cm black chamber, a grain hopper was located behind a 4.5 by 6.0-cm opening centered horizontally and 11.0 cm from the floor. The opposite wall of the chamber, 36.0 cm from the feeder opening, contained a horizontally-centered 12.0 by 16.5-cm opening the bottom edge of which was 9.8 cm above the floor. A 12.5 by 17.5-cm Polacoat plastic diffusing screen, covered with self-adhesive transparent vinyl to prevent scratching, was mounted directly behind this opening. The screen was hinged at the bottom edge, and a microswitch was set to operate when a force of 10 g or more was applied to any point on the upper half of the screen. Each operation of the microswitch defined an attack. A random-access projector [8] was used to display a color image of a conspecific (target B of [27]) on the screen. During initial sessions when no target was projected, the screen was dark, and chamber illumination was provided by two 1.6-watt houselights mounted on the ceiling of the chamber behind a white diffusing plastic panel. A ventilation fan and white noise source partially masked extraneous sounds during experimental sessions. Standard electromechanical programming equipment, located in an adjacent room, arranged and recorded experimental events.

Procedure Without the projected target image present and with the chamber dimly illuminated, each pigeon was trained to eat from the illuminated hopper during the initial two sessions and then exposed to a fixed-time (FT) food schedule that made grain available every 180 sec. A protective contingency, which arranged that food could not be delivered until at least 12 sec of no attack had elapsed, was employed to reduce the possibility of attack being reinforced adventitiously. During the initial sessions of exposure to the FT schedule, feeder durations were individually adjusted between 6 and 10 sec until each pigeon gained a minimum of 5 g per daily session. These individual feeder durations remained in effect throughout the study. Table 1 shows the sequence of experimental conditions, in order, for each bird as well as the number of sessions devoted to each condition. Following the first 15 sessions of exposure to the FT 180-sec schedule, during which the target image was not displayed and the chamber was dimly illuminated, the target was continuously displayed throughout each interfood interval. During the subsequent randomtarget condition, the rear-image target was presented according to a quasi-random sequence during one of the six 30-sec

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periods within each 180-sec interfood interval. In each session, the target randomly occurred three times in each 30-sec sixth of the interval. The sequence was changed every fifth session. Each bird was then returned to the continuoustarget condition followed by a second exposure to the random-target condition. During the second portion of the study, all birds except Pigeon 17 were given two separate exposures to a halfinterval target condition in which the target image was available only during the final 90 sec of the 180-sec interfood interval. Each half-interval condition was preceded and followed by the continuous-target condition. Pigeon 17 received only one exposure to the half-interval target condition. Finally, with the target continuously available, all birds were exposed to two baseline control conditions similar to those investigated previously [35]. To compare the amount of attack induced by the intermittent delivery of food with that induced when a comparable amount of food was delivered all at once in an equal-duration session, pigeons were exposed to a food-control condition in which 19 food deliveries, each separated by 1 sec, occurred at the beginning of each 54-min session. The subsequent no-food control condition allowed for comparison of session attack levels with and without intermittent food delivery. Throughout the study, experimental conditions remained in effect until attack rate showed little systematic variability over five consecutive sessions. Daily sessions each began with food delivery and terminated after the nineteenth hopper presentation. Across all conditions during which the target was presented, the houselights were turned off. RESULTS Figure 1 shows for each pigeon local rate of attack calculated only for the period of target availability. With one exception, local rates during each exposure to the randomtarget condition were higher than local rates during the preceding and subsequent continuous-target conditions. While particularly apparent for Birds 10, 11, and 17, this effect was not observed for Pigeon 15 during exposure to the initial random-target condition. With the exception of Pigeon 13, all

ATTACK AND TARGET AVAILABILITY

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FIG. 1. Attacks per min of target availability time for fixed-time 180-sec food schedule conditions in which a conspecific target image was accessible during the entire 180-sec interfood interval (continuous target), during one randomly-determined 30-sec period of each interval (random target), or during the second half of each interval (half-interval target) as well as for massed-food and no-food baseline control conditions. Data shown are medians and ranges based on the last five sessions at each condition.

birds showed sustained local rates when access to the target image was restricted to the final 90 sec of the 180-sec interfood interval. For Pigeons 10, 11, and 12, attack rates during each exposure to the half-interval condition were comparable to, or greater than, local rates during the preceding and following continuous-target conditions. For all seven birds, attack rates during the no-food control condition were lower than rates during the preceding

continuous-target condition and lower than rates in the food-control condition. Attack rates in the food-control condition, however, were not reliably lower than those in the final continuous-target condition across subjects. Figure 2 presents attack rates across successive 30-sec periods of the 180-sec interfood interval for both exposures to the random-target condition and for the first two continuous-target conditions. For six of the seven birds, at-

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FIG. 2. Attacks per rain of target availability time in successive 30-sec segments of each 180-sec interfood interval. Data are shown for conditions in which the target was available throughout the interfood interval (circles) or was available in one random 30-sec portion of the interval (triangles). Filled symbols indicate the first (I) exposure, and unfilled symbols indicate the second (2) exposure to a condition. Each data point represents the median attack rate based on the last five sessions at each condition.

tack rates during the two continuous-target conditions were highest in the first 30-sec period following food and then progressively d e c r e a s e d across the remaining interfood interval. F o r Pigeon 10, h o w e v e r , e x p o s u r e to the first random-target condition resulted in m a x i m u m attack rate during the second rather than the first 30-sec period following food delivery. Random-target rates show a similar decay function across the 180 sec b e t w e e n food deliveries for the five subjects (Pigeons 12, 13, 15, 16, and 17). The 30-sec

interval attack rates for Birds 10 and 11 in the first randomtarget phase, h o w e v e r , remained relatively high across twothirds (Pigeon 10) or all (Pigeon i 1) of the interfood interval. Figure 3 shows attack rate across successive 30-sec intervals of the 180-sec interfood interval for both exposures to the half-interval target condition and for the third and fourth exposures to the continuous-target condition. F o r five of the seven birds, attack rates in the fourth 30-sec segment of the interfood interval were higher for both half-interval condi-

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FIG. 3. Attacks per min of target availability time in successive 30-sec segments of each 180-sec interfood interval. Data are shown for conditions in which the target was available throughout the interfood interval (circles) or was available only during the final 90 sec of the interval (triangles). Filled symbols indicate the first (1) exposure, and unfilled symbols indicate the second (2) exposure to a condition. Each data point represent the median attack rate based on the last five sessions at each condition.

tions than for the two continuous-target conditions. The fourth period attack rates of Pigeon 13 during its second exposure to the half-interval target condition and of Pigeon 17 during its only exposure to this condition, however, were essentially equivalent to fourth period rates when the target was continuously available. When percent of 30-sec periods with at least one attack rather than attack rate was plotted as the behavioral measure, functions similar to those presented in Figs. 2 and 3 were obtained. Figure 4 shows sample event records of attack behavior for Pigeon 10 during each of the three target-availability conditions investigated. When the target was continuously available, attacks were initiated shortly after food delivery and occasionally continued throughout the first two thirds of the interfood interval. When the target image was displayed in random sixths of the period between food deliveries, attack typically occurred upon target availability. Similarly, during the half-interval condition, attacks occurred when the

target was first presented but seldom continued throughout the remainder of the interfood interval. Similar patterns of behavior were observed for the other experimental subjects. As the random and half-interval records show, food deliveries were postponed in several intervals due to the initiation of the 12-sec protective contingency by attacks that occurred late in the interfood interval. Similar delays were observed to occur occasionally for all seven birds during conditions of restricted target access. Visual observation revealed that the topography of attack against the rear-projected target image closely resembled that observed against live [3,20], taxidermically-prepared [11, 14, 15], and mirror-image targets [2,7]. Attacks were primarily directed at the eye and throat areas of the image and were often preceded by crouching, wing-flapping, and vocalizations. Negligible contacts occurred against the rear-projection screen in the absence of the ¢onspecific image.

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FIG. 4. Sampie event records of Pigeon 10 during continuous-target, random-target, and halfinterval target conditions. On each record, downward pen deflections indicate target availability in the upper channel, attacks in the middle channel, and 30-sec periods in the lower channel. Filled circles in the lower channel indicate food deliveries. Each record shows the third of the last five sessions of the first exposure to the experimental condition.

ATTACK AND TARGET AVAILABILITY DISCUSSION The present results, particularly those from the halfinterval target condition, demonstrate that attack induced by a 180-sec fixed-time food schedule is not limited to the immediate post-food period under conditions of restrictedtarget availability. Similar findings were reported by Muller and Cheney [29] who restricted access to a live rather than inanimate target, employed a 120-sec rather than 180-sec fixed-time food schedule, and defined attack as interruptions of a photo-beam in front of their target rather than as criterion-force contacts against it. These observations are also in agreement with previous reports of schedule-induced aggression at times other than shortly after reinforcer delivery. Whereas pigeons exposed to random-interval and random-time food schedules have been observed to exhibit bursts of attack late in long inteffood intervals [36], DeWeese [10] reported that monkeys exposed to fixed-interval food schedules exhibited little or no biting attack during a signalled post-food timeout period but did begin biting upon its termination (but see [25] which questions the generality of D e W e e s e ' s findings). In addition, studies employing water reinforcement schedules with rats [21] and pigeons [5] found that attack behavior was distributed throughout the interwater interval and was maximal in the middle rather than at the beginning of the interval. The observation that birds in the present study exhibit relatively strong attack when target availability was restricted to the latter portion of the interfood interval is in agreement with the results of studies reporting the occurrence of other schedule-induced behavior at times other than immediately following food ingestion. Several investigators [9, 16, 22, 32] have found schedule-induced drinking, or polydipsia, to occur whenever rats had access to water, although intake was attenuated somewhat when water availability was restricted to later portions of the interpellet interval. With few exceptions, attack rate calculated on the basis of target-availability time was higher when the target was restricted to a random sixth of the interfood interval than when it was continuously displayed (see Fig. 1). In addition, attack rate during the fourth 30-sec segment of the interfood interval was predominantly higher when the target was accessible only during the final half of the period between food deliveries than when it was always available (see Fig. 3). In agreement with these results, Muller and Cheney [29] found that restricting attack to one fixed portion of the interfood interval increased attack above the amount that occurred in the same portion of the interval when the target was continuously available, whereas other investigators [22,32] reported that restricting water accessibility to a specified portion of the interpellet interval increased schedule-induced drinking during that portion above the level observed when water was continuously available. As the present study found with attack, Gilbert [22] noted that when water availability was restricted to the final half of the interpellet interval, schedule-induced drinking increased until it approached the level observed when water was continuously available. Thus, limiting the opportunity to engage in either scheduleinduced attack or drinking appears to potentiate the level of either behavior. When the opportunity to engage in general activity was available to food-deprived pigeons throughout a 300-sec interfood interval, Killeen [24] found that activity increased to a maximum shortly after signal onset and then decreased

17 as food became emminent. Based on these and other findings, Killeen concluded that "general activity is not a simple reflexive response to food delivery, for it is modified by signals of impending food--inhibited by signals with close proximity to food, disinhibited and then inhibited more intensely by signals with moderate proximity to food" (p. 100). When target access was limited to the second half of the interfood interval in the present study, the temporal course of attack during target presentation closely paralleled that of general activity during the signalled portion of the interfood interval in Killeen's investigation. Thus, Killeen's [24] mathematical model of the temporal control of general activity as well as of various schedule-induced behaviors receives empirical support from the results of the half-interval target condition. In agreement with previous results [35], all birds in the present study attacked less during the no-food than during the massed-food control condition and, as in other studies [6, 7, 20, 35] attacked less during the no-food control than during the preceding FT 180-sec schedule condition. F o r two of seven pigeons (l 1 and 15), however, the massed-food control resulted in higher attack rate than did the food schedule condition. It has been suggested [28] that schedule-induced attack eventually becomes conditioned to those stimuli, such as food delivery and the experimental chamber, that are associated with the occurrence of this behavior. Accordingly, more attack would be expected during the baseline that included chamber stimuli plus food than during the no-food control condition that included chamber stimuli only. This expectation is also supported by the observation [3] that pigeons exhibited substantial levels of induced attack when an extinction period was preceded by a single extended reinforcement period. Restricting access to a target may enhance its salience or novelty by preventing the attacker's habituation to it. This possibility receives some support from the observation [19] that stimulus novelty increased the frequency of shockelicited aggression in rats and also by the finding [28] that the availability of a home-chamber target reduced the level of schedule-induced attack in a pigeon. Additional research is needed to more clearly assess the effect of target novelty and/or habituation on schedule-induced attack. It has also been suggested [36] that periods of limited-target availability generate relatively high levels of attack because the onset of these periods is reliably predictive of food unavailability for at least the duration of target access. According to this interpretation, target onset acquired discriminative SA properties. This account is supported by the observation [28,3 l] that withdrawal of a stimulus correlated with reinforcement is a primary determinant of induced aggression and by the occurrence of maximal attack during the initial 30 sec of target access in the half-interval condition of the present study. Further research is needed to determine whether enhancement of attack occurs during target-access periods when the onset of these periods is not exclusively correlated with food unavailability.

ACKNOWLEDGEMENTS This research was supported in part by a Faculty Research Grant from Hollins College to the first author. The authors thank T. A. Looney for his helpful comments and Betty Loving for her assistance in preparation of the manuscript. Reprints may be obtained from Randall Flory, Department of Psychology, Hollins College, Roanoke, VA 24020.

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FLORY AND SMITH REFERENCES

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