Behaviour of ‘impulsive’ and ‘non-impulsive’ humans in a temporal differentiation schedule of reinforcement

Person. indiuid. DI$ Vol. 8, No. 2, pp. 233-239, 1987 Printed in Great Britain. All rights reserved

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0191-8869/87 $3.00 + 0.00 1987 Pergamon Journals Ltd

BEHAVIOUR OF ‘IMPULSIVE’ AND ‘NON-IMPULSIVE’ HUMANS IN A TEMPORAL DIFFERENTIATION SCHEDULE OF REINFORCEMENT M. D. VAN DEN BROEK, C. M. BRADSHAW and E. SZABADI Department of Psychiatry, University of Manchester, Stopford Building, Oxford Road, Manchester M 13 9PT, England (Received 20 May 1986)

Summary-The operant behaviour of psychometrically defined ‘impulsive’ and ‘non-impulsive’ subjects, under a temporal differentiation schedule of reinforcement, was examined. Four impulsive and four non-impulsive females were selected on the basis of their scores on the Matching Familiar Figures Test. The subjects participated in fifteen 45min sessions in which they were exposed to an inter-response-timegreater-than-lo-set schedule ofmonetary reinforcement. During Phase I (sessions 1-5) no information was provided about the reinforcement contingency. During Phase II (sessions 610) a light on the response panel was illuminated whenever a reinforcer became available. At the start of Phase III (session 1I) the subjects were given explicit information about the reinforcement contingency. At the start of Phase IV (sessions 12-15) the subjects were told that the light would no longer be operative although the contingency would remain unaltered. During Phase I the impulsive subjects earned fewer reinforcers, and emitted a greater proportion of non-reinforced responses (inter-response times less than IO set) than the non-impulsive subjects. During Phase II both groups increased their earnings, although the performance of the non-impulsive group remained superior to that of the impulsive group. In Phase III both groups performed equally well. In Phase IV the performance of both groups deteriorated, the impulsive group performing more poorly than the non-impulsive group. The results demonstrate the sensitivity of operant performance maintained under temporal differentiation schedules to personality dimensions such as ‘impulsiveness/non-impulsiveness’.

A major difficulty encountered in studying the operant behaviour of humans is the wide variation between the performances of individual subjects, which is seen even under standard laboratory conditions using conventional schedules of reinforcement (see Harzem, 1984). This problem is especially acute in the case of performance maintained under interval and temporal differentiation schedules (Weiner, 1969; Lowe, Harzem and Bagshaw, 1978a, 1978b). In some instances the variability may be attributable to the use of weak, ‘non-consumable’ reinforcers and responses requiring little effort, which may allow uncontrolled variables to exert a greater influence upon behaviour than the reinforcement schedule itself (e.g. Weiner, 1970; Matthews, Shimhoff, Catania and Sagvolden, 1977; Bradshaw, Ruddle and Szabadi, 1981). In other cases, the subjects’ differing histories of carrying out tasks similar to that required by the schedule may contribute to their individual patterns of operant performance; in particular, it is well established that performance on interval and temporal differentiation schedules is highly sensitive to prior experience of different reinforcement contingencies (Weiner, 1970; Weisberg, 1970; Singh, 1971; Stein, 1977). Verbal instructions and self-generated verbal cues may also play a role in determining individual performance on fixed-interval and temporal differentiation schedules (Matthews et al., 1977; Lowe et al., 1978a, 1978b). Recently it has been proposed that individual differences in operant responding may be related to individual differences in other aspects of behaviour, which have traditionally been regarded as 1984). In the present experiment we have examined whether personality variables (Harzem, individuals differing on the personality dimension ‘impulsiveness/non-impulsiveness’ (Eysenck and Eysenck, 1978; Barratt, 1983) would show systemic differences in their performance on a temporal differentiation schedule of reinforcement. The schedule we used was an inter-response-time-greaterthan-t schedule (IRT > t: Zeiler, 1977). These schedules specify that a response is only reinforced if it occurs at least t set after the previous response. The possibility that performance in these schedules might be sensitive to individual differences in ‘impulsiveness’ was suggested by the finding that clinical groups in whom impulsive behaviour is regarded as a pathological feature (hyperactive children: Gordon, 1979; delinquent adolescents: Barratt, 1981) perform poorly on these schedules. 233

METHOD

SuhjecLs Eight female subjects were selected from a sample of 78 individuals who had undergone psychometric assessment in a standardization study of various neuropsychological tests (results to be presented elsewhere). The eight subjects were selected on the basis of their psychometric profile (see below), so as to provide an ‘impulsive’ group (n = 4) and a ‘non-impulsive’ group (n = 4) approximately matched for age, general intellectual level and trait anxiety. The subjects were recruited by advertisement and personal contact. All were experimentally naive, and none had had any previous training in psychology. Three were secretaries, three were hospital domestics. one was a nursing auxiliary and one was unemployed.

Each subject

underwent

the following

tests:

W~~chsler Adult Intelligence Scale (Wechsler, 1955). The following subtests were used: comprehension, similarities, vocabulary, block design and object assembly. The pro-rated full-scale IQ was determined for each subject. Strrtc-Trait Anxiety Scale (Spielberger, Gorsuch and Luschene. 1970). The subjects were asked to complete Form X-2 in order to obtain a measure of trait anxiety. Matching Familiar Figures Test (Kagan, 1966). The adolescent/adult version was used, and was scored in the usual way. Two measures were derived: the mean latency to the first response, averaged across the 12 items of the test, and the total number of CJJOJS made on all 12 items. (Previous studies have shown that ‘impulsive’ individuals tend to have low latency scores and high error scores (Kagan, 1966; Glow, Lange, Glow and Barnett. 19X3). The performance of the normative sample (n = 78, see above) indicated that latency scores were normally distributed (mean + SD: 45.7 + 23.9 set); however, the distribution of the error scores was skewed, and a square-root transformation was required in order to obtain a normally distributed measure (3.23 i 1.27). Using these normative data, standard scores (z) were obtained for each subject on the latency (~7’) and transformed error (zE) scores. An ‘impulsiveness index’ (1) was then derived for each subject using the formula I = :E - zT.

Apparatus. Experimental sessions took place in a sound-attenuated room. Subjects sat at a table on which was placed a rectangular box (21 cm long, 12 cm wide and 8 cm high). In the centre of the upper surface of the box was a five-digit counter. above which were two red light-emitting diodes (LEDs), and below which was a single green LED. Four cm to the right of the counter was mounted a response button which could be operated by a force of 0.6 N. Each depression of the button operated a relay in the box which provided the subject with auditory response feedback. A single red LED was located 2cm above the response button. Stimulus presentation and recording of the response were accomplished using an RR8 and a TIM module respectively (Biodata Instruments Limited) controlled by a CBM 8032 microcomputer situated in another room. Inter-response-times (IRTs) were recorded to an accuracy of 1 msec. Data were stored on diskettes for later analysis and graphical reconstruction using an X-Y plotter (model HP 7470A, Hewlett-Packard). Prowduie. The subjects participated in 15 sessions which took place on consecutive working days. each session lasting about 45 min. The sessions were divided into five 6 min working periods with interposed 3 min rest periods, when the box was inoperative. The experiment consisted of four phases. During Phase I (sessions l---5), reinforcement was available under a standard IRT > 10 set schedule. At the beginning of the session the subjects were given the following instructions: “This is a situation in which you can earn money. sounds and this green light comes on. When the The session will consist of periods when you can should rest. You can earn money by using this

You can earn money after a hooter green light is ofT you should rest. earn money and periods when you button. The score on this counter

Reinforcement

235

Table 1. Ages and psychometric test results from individual subjects Age

Subject ‘Impulsive’

Anxiety scoreb

Impulsiveness index (I)

group

I-A I-B I-C I-D Mean SD ‘Non-impulsive’ N-A N-B N-C N-D Mean SD

IQ

48 53 24 33 39.5 13.4

91 93 80 87 89.3 7.4

30 32 51 33 36.5 9.8

+ 2.89 f3.33 f2.92 + 3.33 +3.12* 0.25

91 99 100 95 96.3 4.1

27 53 38 41 39.8 10.7

-0.02 -0.37 -0.47 -1.10 -0.50’ 0.45

group 26 43 32 49 37.5 10.4

“Full-scale IQ (Wechsler Adult Intelligence Scale). bTrait Anxiety (Spielberger State-Trait Anxiety Inventory). ‘Derived from Matching Familiar Figures Test (see text). *P < 0.001.

shows how much you have earned, in pence, and every time you earn some money, these two lights will flash on and off, and three pence will be added to your score on the counter. At the end of the session we will take a reading from the counter and note down how much you have earned. You will be paid in a lump sum at the end of the experiment.” During Phase II (sessions &lo), the red LED above the response button was illuminated whenever a reinforcer became available, and remained illuminated until the reinforcer had been collected. In every other respect the schedule was the same as in Phase I, and no additional information was provided to the subjects. During Phase III (session 1 I), the contingencies were the same as in Phase II; at the beginning of the session the following instructions were given to the subjects: “The way to earn money is by delaying your button-press on. You should only press when the light is on.”

until this red light comes

During Phase IV (sessions 12-15), the schedule was identical to that operating (i.e. the standard IRT > 10 set schedule, without the cue light). The subjects following additional instructions: “Once again, the way to earn money is by delaying this light won’t be coming on to help you.”

your button

during Phase I were given the

press. However,

now

RESULTS Psychometric

data

The ages and psychometric data from the individual subjects are shown in Table 1. The impulsive and non-impulsive groups differed significantly with respect to their impulsiveness indices [t(6) = 4.09, P < O.OOl]. However, there were no significant differences between the groups with respect to age [t(6) = 0.24, P > 0.11, full-scale IQ [t(6) = 1.65, P > 0.11, or trait anxiety [t(6) = 0.45, P > 0.11. Operant

behaviour

Two measures were used to compare the performance of the two groups in the IRT > t schedules: rate of reinforcement in each session, and the proportion of IRTs that were less than 10 set in duration in each session. Reinforcement

rate

Figure 1 shows the mean reinforcement rates for the two groups of the experiment. In Phase I the non-impulsive subjects obtained P.A I 0. 8,2--F

in each session of each phase a significantly higher rate of

M. D. VAN DEN BROEK et al.

236

Fig, 1. Mean

reinforcement

ratio

for the impulsive of the four phases

(0) and non-Impulsive of the experiment.

(0)

groups

in each session

reinforcement than the impulsive subjects [F( I, 6) = 13.44, P < 0.021; the effects of sessions [F(4,24) = 2.201 and the interaction [F(4,24) = 0.821 were not statistically significant (P > 0.05). In Phase II the non-impulsive groups again obtained a significantly higher rate of reinforcement than the impulsive group [F( 1,6) = 11.30, P < 0.021; the effects of sessions [F(4,24) = 1.551 and the interaction [F(4,24) = 1.241 were not statistically significant. In Phase ITT there was no significant difference between the rates of reinforcement obtained by the two groups [r(6) = 1.341. In Phase

IMPULSIVE SESSION

5

SESSION

10

SESSION

NON-IMPULSIVE

15

Fig. 2. Mean inter-response time frequency distributions obtained for the impulsive and non-impulsive groups during the final session of Phase 1 (session 5), the final session of Phase II (session IO), Phase III (session I I) and the final session of Phase IV (session 15). Inter-response times are collected into I set bins, with the exception that all inter-response times greater than 20 set are accumulated in the twenty-first bin. Inter-response times greater than the criterion value of IO set (i.e. reinforced inter-response times) are shown as filled columns.

Reinforcement

Fig. 3. Mean proportions of inter-response and the non-impulsive group (0)

times less than 10 set in duration for the impulsive group in each session of the four phases of the experiment.

237

(0)

IV the non-impulsive group again achieved a higher rate of reinforcement than the impulsive group [F(l, 6) = 6.74, P < 0.051; the effect of the sessions [F(3, 18) = 3.151, and the interaction [F(3, 18) = 0.901 were not statistically significant. Inter-response times. Figure 2 shows the mean IRT frequency distributions obtained from the two groups during the final session of Phase I (session 5), the final session of Phase II (session lo), Phase III (session 11) and the final session of Phase IV (session 15). It is apparent that during Phase I the non-impulsive subjects showed a clear temporal differentiation of responding, with their modal IRT falling close to the criterion value of 10 set, whereas the impulsive group showed a preponderance of short IRTs and little evidence for temporal differentiation. In Phase II both groups showed an improvement in their temporal differentiation, but the proportion of reinforced IRTs was greater in the non-impulsive than in the impulsive subjects. In Phase III both groups showed unimodal IRT distributions with nearly all the IRTs falling between 10 and 11 sec. Finally, in Phase IV, both groups showed some broadening of their IRT distributions, although this was more marked in the case of the impulsive group. Figure 3 shows the mean proportions of IRTs less than 10 set (i.e. the proportion of IRTs that were not reinforced) for the two groups in each session of each phase of the experiment. During Phase I the proportion of IRTs less than 10 set in duration was significantly greater for the impulsive group than for the non-impulsive group [F( l(6) = 11 .OO, P < 0.021; the effects of sessions [F(4,24) = 0.421 and the interaction [F(4,24) = 0.131 were not significant. During Phase II the main effect of subject group was not significant [F(l, 6) = 1.661, and the effect of sessions [F(4,24) = 3.361 and the interaction also fell short of statistical significance. In Phase III the proportion of IRTs less than 10 set was virtually zero for both groups [t(6) = 01. In Phase IV the difference between groups fell short of statistical significance [F(l, 6) = 4.941, as did the effect of sessions [F(3, 18) = 1.291 and the interaction [F(3, 18) = 2.151. DISCUSSION The impulsive and non-impulsive subjects were selected on the basis of their performance on the Matching Familiar Figures Test (Kagan, 1966), a test which has been widely used in previous studies of impulsiveness (see Glow et al., 1983; Barratt, 1983). The ‘impulsiveness index’ which we used in the present experiment was obtained by algebraically summing the standard scores (z) for the latency and error measures yielded by the test. A similar approach to scoring the Matching Familiar Figures Test has been used by Glow et al. (1983) in a psychometric study of a student sample; the mean latency and error scores reported by these authors were similar to those found in our study. However, Glow et al. did not transform their error scores; we found such a transformation was essential in order to obtain a normally distributed measure in our sample (see Method). The impulsive and non-impulsive groups were approximately matched for age, IQ and trait anxiety. The relationship between general intellectual ability and performance on temporal differentiation schedules is unclear. McClure and Gordon (1984) found no significant correlation

M. D.

238

VAN

DES

BROEK

et ml.

between IQ and ‘efficiency’ of performance (proportion of reinforced responses) on an IRT > 6 set schedule in hyperactive and normal children. However, Harzem (1984) has presented data suggesting that more gifted university students may adjust to an IRT > t contingency more readily than their less gifted peers. In the present experiment the difference of 7 points between the mean IQs of the two groups was not statistically significant; it is therefore unlikely that differences in IQ contributed appreciably to the differences in operant behaviour between the two groups. Matching for trait anxiety was considered important, since it has been reported that performance on IRT > t schedules is affected by this variable (Singh, 1971). During Phase I when the standard IRT > t schedule was in operation, the impulsive subjects consistently earned fewer reinforcers than the non-impulsive subjects, and this was evidently due to the impulsive subjects emitting a higher proportion of short IRTs. A similar pattern of behaviour was found by Gordon (1979) and McClure and Gordon (1984) in hyperactive children. These authors suggested that the poor performance of hyperactive children under the IRT > t schedule was a reflection of their ‘impulsive style’. The present results obtained with normal adults support the notion that poor temporal control of responding is related to impulsive personality. The poor temporal control shown by the impulsive group was maintained during Phase II, when reinforcer availability was signalled by the cue light. This may reflect a lower sensitivity to the signalling procedure on the part of the impulsive group. However. it should be noted that the impulsive subjects had a lower rate of reinforcement than the non-impulsive subjects at the start of Phase II. It is therefore possible that the poor performance of the impulsive group was due to their having less experience of the cue light than the non-impulsive group, and thus less opportunity to learn about its significance for predicting reinforcer availability. Some support for this suggestion is provided by the near-optimal performance shown by both groups in Phase III when explicit information was provided about the significance of the cue light. In Phase IV, when the cue light was no longer available, both groups showed some decline in the accuracy of their performance. However. this was significantly greater in the impulsive group than in the non-impulsive group, as judged by the lower rate of reinforcement obtained by the former. Since the two groups both started Phase IV from a baseline of virtually optimal performance achieved during Phase III. and were told explicitly that reinforcement would continue to depend upon delayed responding, it is reasonable to interpret the poorer performance of the impulsive group as reflecting an inability to withhold responding for the IO set required by the schedule, rather than being due to inadequate knowledge of the contingency. This conclusion is consistent with the suggestion by Barratt (1981) that impulsive individuals have poor time perception. However, poor performance on IRT > t schedules could be due to other factors which are not excluded by the present results, for example. heightened sensitivity to the incentive aspect of the reinforcer, or an inability to delay responding despite an accurate perception of the passage of time. Finally. it should be mentioned that impulsiveness is a complex phenomenon, probably involving other behavioural features, in addition to poor temporal control of responding. For example, Mischel (1958) has pointed out that a characteristic feature of the behaviour of young children, which he regards as ‘impulsive’, is the inability to tolerate delay of gratification. When faced with a choice between a small immediate reinforcer and a larger delayed reinforcer, young children tend to choose the former, whereas older children opt for the latter. Quantitative models of impulsiveness based on delay-of-reinforcement paradigms have been developed and extensively tested using animal subjects (e.g. Ainslie, 1975; Herrnstein, 1981). but have not yet been applied to the study of individual differences in impulsiveness in humans. A[,linoM,ledgmlolrs-Thjs work was supported by the North technical assistance of Mr R. Langley and Mr T. Kershaw.

Western

Regional

Health

Authority.

We are grateful

for the

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