Infant sucking in response to variations in schedules of feeding reinforcement

JURNAL

OE’ ESI’EHIMESTAL

Infant

Sucking

CHILL)

13, 24&247

PSYCHOLOGY

in Response to Variations of Feeding Reinforcement’ DONALD Zirliversity

(19%)

in Schedules

HILLMAN 0)

Mebraskn

AND JEEOME Harvard

S. BRGNER University

Infants aged 1-2 months and 3-4 months were fed from a nipple through which milk delivery could be controlled to conform either to fixed interval or fixed ratios of reinforcement. A variety of fixed-interval schedules were used, with maximum nonreinforced delay of 2.0 sec. Various fixed ratios were also employed, with maximum of 4 sucks per drop of milk. The schedules were linked to negative or positive mouthing pressure in different babies. The principal response to the more sparsely reinforced schedules (FI and FR alike) was a decrease in length of sucking bursts and a concomitant increase in length of pauses between. Ko scalloping effects were found in the sucking patterns. The older infants were more sensitive to changes in sparseness of schedule. Differential reaction also exhibited itself in sucking arhythmia. The younger infants showed more marked differential reaction to schedules through arhythmia than did the older ones. The findings indicate an increase in alertness and information processing with increased difficulty in obtaining nutriment synchronously with reflex patterns of sucking, since pausing in sucking is associated with greater scanning activity.

The present study was designed to test the early capacity of infants to accommodate to different schedules of milk reinforcement for sucking. The mammalian infant at, bottle or breast usually receives a “drop” or (‘swig” of milk for each suck. Such sucking is usually divided int’o bursts consisting of 5-30 sucks, interspersed by pauses ranging from 2 set to 10 or 15 sec. Typically, a nutritive sucking record shows a quite high regularity in bursts and pauses, the pattern changing as satiety sets in (cf. H. Kaye, 1969). While the child is involved in committ,ed nutritive ‘This paper is based on a dissertation by the senior author for the Ph.D. in Psychology at Harvard University (Hillman, 1969). The research was supported in part by Grant MH-12623 from the Kational Institute of Mental Health to Harvard University, Center for Cognitive Studies. 240 @ 1972 by

Aca
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Inc.

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sucking, he is typically “intensively concentrated,” and seems to “Cut out” outside stimulatioll-as indicated, for example, by a decrease in scanning eye movements during sucking (Bruner, 1971). To what extent is this early, reflex pattern responsive to altered contingencies in the environment? It. is known, for example, that sucking alters with the nature of the object sucked on; for instance, half-inch rubber tubing evokes fewer sucks per unit time (averaging bursts and pauses) than a regular nipple (Lipsitt & Kaye, 1965). There is also evidence (Sameroff, 1968) that reinforcing sucking with milk also has the effect of increasing amount, of sucking during and immediately after reinforcement-again, averaging bursts and pauses. There is, however, nothing in the literature to indicate whether nutritive sucking is or is not responsive to schedules of feed reinforcement in the way in which “instrumental” sucking for arbitrary ends can bc shown to be-as when it. produces increased brightness in the visual field (e.g., Siqueland 8r. DeLucia, 1969) or clears blur from pictures (Kalnins, 1970; Kalnins & Bruner, 1971). The present study, then, is an exploration of the sensitivity of nutritive sucking to various schedules of feeding reinforcement. METHOD

Subjects Forty-eight full-term, bottle-fed babies were studied; 24 were l-2 months of age, and 24 were 3-4 months of age. Mothers and their infants were recruited by advertising in the college newspaper, thus assuring a fairly homogeneous middle-class group, and infants were assigned to groups at random. Roughly half the babies were of each sex. Apparatus A nipple was designed, based on the description given by Sameroff (1965). Three Teflon tubes (id = 0.042 in.) were threaded through a rubber plug inserted in the end of a standard pacifier. Two of the tubes were bonded through the mouth of the nipple, one of which measured negative sucking pressure, while milk was delivered through the ot)her tube when ordina,ry sucking pressure was applied. The third tube for measuring positive sucking pressure ended within the airtight, space enclosed inside the nipple by the tube at the nipple’s other end. The negative and positive pressure tubes were connected to two Grass Volumetric pressure transducers PT5A linked with a Beckman-Offner Dynograph. Tn turn, the outputs from the pressure transducers were connected via Schmidt t riggrrs to :t Grnson-Stndlw programmrr which operatetl :b

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milk-loaded liquid pump, Model LR-131, manufactured by Davis Scientific Instruments, according to various reinforcement schedules. Thus, when a child sucked at an amplitude of C.37 mm Hg or greater, a drop of milk, about 0.1 ml in volume could be delivered for either Positive Or negative sucking according to a predetermined schedule. Each reinforcement was recorded on the polygraph along with sucks and other relevant data.

Procedwe The infants came for three sessions, usually within a week and never more than 10 days. They were fed by their mothers in their normal feeding positions and at their normal feeding time in a separate room, relatively free from noise and visual distractions. The mothers were instructed to feed the child as if it were a normal feeding session. Though they were informed of the nature of the experiment,, they did not know at any given moment what schedule governed the baby’s feed. The feeding session was terminated when the mother so decided, usually when the baby had finished the amount normally given to it. The average session lasted about 30 min. The design of the experiment, was as follows. Over the three sessions of the experiment each infant’ was reinforced only for positive or for negative sucking, and received milk only on a fixed ratio (FR) or a fixed interval IFI) schedule. The reinforcement schedules were int,roduced after an initial 3-min period. During that initial period, a drop of milk was delivered for each suck, thus allowing the infant to become accustomed to his surroundings. For those babies on ratio sc,hedules, there was then introduced a set of ratio payoffs in which a drop of milk was given for every suck, every other suck, or every third or every fourth suck. Each infant, received 3 min of each of the four ratio schedules in random order. After this set was completed, another set of ratios was presented in random order. alost babies went, through two complete sets, and there was no difference in number of sets completed by younger and older babies. No intervals were used between schedules. In the fixed-interval schedules the first, suck after a certain nonreinforced interval was reinforced. The nonreinforced intervals employed in this study were 0.5, 1.0, 2.0 se? on one set, and 0.3, 0.75, and 1.5 set on the other set. One of the set,s was given to each baby in random order after the initial 100% reinforcement period of 3 min. Each contingency, as in the ratio schedules, was operative for 3 min and after the set was completed another pet, with a different random order was begun. Most babies completed between two and three PC+.‘. anrl tllcrc was no difference in thy number of rc,ts filli.shc(l by childrcll of the txvo ages.

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Pilot studies had indicated that longer intervals or sparser ratio schedules produced crying in the baby and disturbance in the mother. We, therefore, decided to limit the investigation to the schedules described.

The major effects may be summarized briefly at the outset. The infants do respond differentially to the different schedules of feed-forsucking, and t,he effects are in the same direction for the younger and older babies, whether one reinforces for positive sucking (i.e., mouthing) or negat,ive sucking, The effect itself is rather nonspecific. That. is to say, longer intervals in the FI schedule produce shorter bursts of sucking and longer pauses between bursts. In the FR schedule, the same effect is found as one goes from ratios of one drop of milk per two sucks to one drop per four sucks, the highest ratio used. For both schedules, ratio and interval, variance due to the schedule itself is highly significant (p < ,011 for both pause lengthening and burst ahortcning. There is no evidence in the record of anything approximating “scalloping” or bunching of sucks either toward the end of intervals in the FI schedule or toward the end of a number of sucks in the FR schedule. Whether reinforcement is associated with positive or with negat,ive sucking pressure seems to produce no discernible effect either on length or pauses or of bursts. Nor did reinforcing one form of sucking produce an increase in its amplitude and a decrease in its opl)osite-an effect reported by Sameroff (1967) .” What is of interest is that the effects described are more striking for the older children than for the young. They show a shortening of bursts and a lengthening of pauses sooner and more sharply than do the younger babies (Figure 1). Analysis of variance shows that age is a significant source of variance for burst length in both types of schedule (p < .05), and for pause length, though less so (p < .05 for FR, p < .lO for FI). The interactions of age and schedule all approach or exceed significance levels for both schedules and for bursts as well as pauses (;o < .05 for pauses, p < .lO for bursts). A fuller account of statistical analyses employed are in Hillman (1969). In the present paper the only measures reported are those derived from the analysis of variance. An effort to discover whether the rate of sucking within bursts altered with ec,hedule of reinforcement resulted in completely negative findings. ‘A study by Bruner and Gajdos (in progress) is concerned with and exclusive reinforcement of positive and negative components parameters that are controlling-such as latency of reinforcement-are complexity SO that any individual replication or failure of replication results cannot, IX very revealing.

the independent of sucking. The of sufficient of Samerops

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FIG. 1. Burst length and pause length for l- to 2-month-old and 3- to 4-month-old infants on the various ratio and interval schedules. Each point represents 12 subjects, half with schedules reinforced for the positive and half for the negative component of sucking.

It is the burst-pause pattern that is sensitive to reinforcement, not the rate of sucking which remains remarkably steady. Longer nonreinforced intervals in the FI schedule and higher ratios in the FR schedule were accompanied by greater arrhythmia in sucking (Figure 2). A given suck was considered arrhythmic if it deviated 4.5 cm in amplitude from the S’s average amplitude or (much rarer) deviated r/15of a second from S’s average peak interval time. Generally, arrhythmia expressed itself in amplitude changes rather than in rate changes. Figure 2 also indicates that for both the FI and FR schedules the incidence of arrhythmia was greater among younger infants at the longer intervals and the sparser ratios. The effect was a sharp one, with about 4 in 10 sucks on the part of the young babies being off the average rhythm at the most extended intervals and ratios. One qualitative difference between the two types of schedules expressed itself in the points at which arrhythmic sucking appeared. In the FI case, it was principally to be

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found just before and after reinforcement, whereas in the FR schedules it was scattered throughout, the record. Such a distribution in the two types of records suggests that the infants were sensitive to the nature of the schedules, though in neither schedule, as noted, was there any evidence of scalloping of the kind ordinarily found in response to such schedules in mature organisms. DISCUSSIOK

Reaction to schedule change in young human infants is less specific than in more mature organisms (e.g., Ferster & Skinner, 1957) or in the young of other species (Campbell, 1967). It takes the form of disengagement from committed sucking when feed becomes less readily available, though still predictable, as in a FR or FI schedule of reinforcement. The basic form of sucking remains-bursts of sucking at a rhythm of 1.5-1.7 sucks per second interspersed with pauses-but the length of pauses increases while the length of bursts decreases, arrhythmia entering besides. The older infan& (3-4 months) respond to reduced feed by a systematic reduction in length of sucking bursts and an increase in length of pauses. The younger infants (l-2 months) show this same tendency

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to a lesser extent, and respond as well to reduced feed-for-sucking by a form of disruption in sucking best described as arrhythmia. It may very well be the case that the more liquid the child gets in his mouth, the more likely his sucking is to continue, once started. This would account for one feature of our results: the longer bursts for richer schedules. But it would not so easily acccunt for the shorter pauses between bursts in such schedules. The lengthened pauses accompanying reduced feed-for-sucking probably indicate an increase in alerting, since other evidence (Bruner, 1971) goes on during pauses suggests that, more scannin, u of the environment than during bursts of sucking. Nonnut,ritive sucking reduces the number of eye movements per unit’ time in the presence of an activating visual stimulus. This suggests that when expectancy of steady food reinforcement is violated, the infant may respond by ceasing to suck or by pausing more often to inspect the situation. This may be a first form of nonspecific though useful response to a learning task. Similar initial patterns are also reported in studies by Ralnins an11 Bruner (1971) and by PapoGek and Bernstein (1969). An alternative explanation of the effect observed is that sparser schedules induce a pattern of nonuut,ritive sucking, marked more clearly by bursts of sucking intersperbed with pauses (1Volff, 1968). What the child may be doing, in effect, is making a discrimination between a “feed” and a “comfort” sucking situation and a,q feed reinforcement becomes sparser, the nonnutritive component becomes increasingly intrusive. In t,he present state of our knowledge, it is not yet possible to choose between the two alternative explanations or, indeed, to be sure that other more reasonable explanations have not been overlooked. REFERENCES

J. 8. Pacifier-induced visual buffering in human infants. In preparation, 1971. RRUNER, J. S., & GAJDOS, E. Analysis of negatire and positive components of sucking in infancy. In preparation. CAMPBELL, B. A. Developmental studies of learning and motivation in infra-primate mammals. In H. W. Stevenson, E. H. Hess, and Harriet 1,. Rheingold (Eds.), BRUNER,

Early

behavior:

comparatiele and developmental approaches. New York: John 1967. FERSTER, C. B., & SKINNER, B. F. Schedules of reinforcement. New York: AppletonCentury-Crofts, 1957. HILLMAN, D. F. Response to feeding contingencies in the development of sucking in human infants. ‘linpublished doctoral thesis. Harvard Universit,y, 1969. KALNINS. I. V. The use of sucking as an instrumental response to control focus. IJnplthlished doctoral thesis. University of Toronto, 1979.

Wiley and Sons,

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I. V., & BRUNER, J. S. Infant sucking used to change the clarity of a visual display. Submitted for publication, 1971. K.~YE. H. Infant sucking behavior and its modification. In L. P. Lipsitt and C. C. Spiker (Eds.), Advances in child development and behavior. Vol. 3. Sew York: Academic Press, 1967. Pp. 2-50. LIPSITT, L. P., SC KAYE, H. Change in neonatal response to optimizing and nonopt)imizing sucking stimulation. Psychonomic Science, 1965, 2, 221-222. PAIWU~H, H., & BERNSTEIN, P. The functions of conditioning stimulation in human neonates and infants. In A. Ambrose (Ed.), Stimulation in early infancy. New York: Academic Press, 1969. SAMERDFF. A. J. An experimental study of the response components of sucking in the human newborn. Unpublished doctoral dissertation, Yale University, 1965. SAMEROFF, A. J. The components of sucking in the human newborn. Joztrnal of ExKALNINS,

perimental

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Psychology,

1968. 6, 607-623.

E. R., & D~Luc1.4, C. A. Visual reinforcement of non-nutritive sucking in human infants. Science. 1969, 165, 1144-1146. WOLFF, P. H. Serial organization of sucking in the young infant. Pe&&r&, 1~8, 42, 943-956. SIQUELAND,