Neonatal sucking behaviour: its development, modification, and individual variation

Abstract

In the first three weeks of life under a Fixed Ratio schedule of milk reinforcement, developmental increases were found in the number of responses emitted, in the intraburst sucking rate and in the amplitude of the suction component. In dry (non-nutritive) sucking, intraburst rate showed no change, while the number of responses and the time spent sucking increased. It is suggested that these divergent findings reflect increasing competancy in the role that each mode of sucking plays in early development. When the laboratory sucking performance in both modes of newborns whose milk intake during ward feeding was either high or low was compared, marked differences were found in both gross and burst pattern measures of sucking from the beginning of the two-minute trials. These differences could not be accounted for solely in terms of birth history and therefore an explanation in terms of congenital differences in intraoral responsivity is suggested. The performance of newborns under conditions of dry sucking and sucking on a Fixed Ratio and a continuous reinforcement schedule was then compared and it was found that the number of sucking bursts emitted in the two-minute laboratory trials was the sole measure to discriminate between all three conditions. Findings from a fine-grain analysis of sucking performance suggested that sucking on an FR10 schedule of milk reinforcement is more like sucking for continuous fluid than it is like dry sucking, and that on an FR10 schedule sucking shifts between the high rate lower amplitude sucking of the dry mode and the low rate higher amplitude sucking of the nutritive mode. No evidence was found to support Wolff's (1972) theory of two separate centrally-organised modes of sucking, and it is proposed that Burke's (1977) view of the two modes lying at either end of a continuum is the most parsimonious conception. An examination of sucking behaviour during simulated natural feeds in the first three weeks and on each of the first three days revealed that increased milk intake over the first three weeks of life results from a simple increase in the number of responses, while the burst and pause pattern of sucking remains essentially the same. Feeding performance shows generally high intercorrelations which increase across records, while improvement in feeding efficiency is shown by an increase in the amount of fluid ingested per unit time. Over a feed, sucking amplitude, intraburst rate and burst size declined, but the interaction between satiation and fatigue, which are assumed to control such changes, could not be clearly separated. In the first three days of life measures of sucking behaviour taken during the initial part of a feed show considerable variability with only suction amplitude in both the dry and nutritive modes revealing stable individual differences. Differences in the performance of breast and bottle-fed newborns were also found, and an explanation in terms of differential responsivity to the intraoral stimulation provided by the nipple or fluid is offered. The implications of the developmental findings, the usefulness of different parameters of the sucking response, the role of the expression or positive pressure component and other findings are discussed in relation to recent studies of sucking behaviour.