Intensity discrimination abilities of infants and adults: implications for underlying processes

Abstract

It is known that infants' ability to recognize intensity variations, an important aspect of speech processing, is immature. However, processes contributing to this immaturity have not been identified. Measuring the intensity discrimination abilities of infants in two separate experiments provided an opportunity to examine these underlying mechanisms.In Experiment 1, individual psychometric functions for intensity discrimination were obtained for broadband noise from 6- to 9-month-old infants and untrained adults. Infant functions exhibited shallower slopes and poorer thresholds than adult functions. The average infant upper asymptote did not reach adultlike performance. These results provide support for a combination of reduced growth of neural excitation and immature attention as causes for immature intensity processing.In Experiment 2, intensity difference limens (DLs) were obtained from adults and infants in both gated- and continuous-pedestal paradigms for either pure tone or broadband stimuli. Adults showed the expected performance improvement in the continuous-pedestal paradigm compared to the gated-pedestal paradigm for both stimuli. While infants exhibited better continuous-pedestal DLs for broadband noise, there was no gated-continuous DL difference for the pure tone. The pattern of infant results is consistent with the predictions of models positing immature listening band strategies but mature across-channel mechanisms contributing to improved continuous pedestal DLs. However, this model cannot account for the overall size of the infant DLs. At this time, no model consistently accounts for all observations of infants' detection of sound.