Dale Bull

Binaural unmasking in infants

Localization responses to a broadband noise signal presented against a broadband noise masker were obtained from 12-month-old infants and adults. Two loudspeakers, one to the left and one to the right of the listener, continuously presented identical broadband maskers. On a trial, a broadband signal was added to one of the loudspeakers. Subjects were required to identify the loudspeaker producing the signal. Noise signals were either coherent (from the same noise generator) or incoherent (from an independent noise generator). Both infants and adults found it easier to locate the incoherent signals even when the two types of signals were adjusted to produce equal increments in power. Since monaural performance, after this adjustment, should be equivalent for the two cases, superior performance for incoherent signals implies that binaural processing is involved. The same result was observed in control experiments in which coherent and incoherent signals were presented over earphones to adults. These results suggest that the mechanisms responsible for binaural unmasking are operative by 12 months of age.

The masking of octave-band noise by broad-spectrum noise: A comparison of infant and adult thresholds

Localization responses to a 4,000-Hz octave-band noise in a background of broad-spectrum noise were obtained from infants, 6, 12, 18, and 24 months of age, and adults. A two-alternative, forced-choice procedure was used to determine thresholds at each of two levels of masking noise, 42 and 60 dBC. Adults were also tested for their localization of pure tones in noise and their detection of octave-band noises with the more traditional two-interval, forced-choice task. Increasing the masking noise from 42 to 60 dBC resulted in comparable threshold shifts for all age groups. However, infant thresholds were 16–25 dB higher than those obtained for adults. The theoretical implications of these findings are discussed.

Effect of Reinforcement on Infants' Performance in an Auditory Detection Task

Centre for Research in Human DevelopmentUniversity of Toronto, Mississauga, Ontario, CanadaLocalization responses to octave-band noises with center frequencies at 400 and4000 Hz were obtained from 12-month-old infants, first without reinforcementand with a 5-sec response interval and then with reinforcement and an unlimitedresponse interval. The percentage of correct responses was substantially greaterin the reinforced than in the nonreinforced condition. In a second experiment,12-month-old infants were tested in nonreinforced and reinforced sessions, as inthe previous experiment, with the exception that both sessions incorporated the5-sec response interval. Again, performance was superior in the reinforced ses-sion. It is suggested that auditory detection techniques that omit reinforcementmay be yielding attentional thresholds rather than thresholds of audibility.

PESTI: A Procedure for Estimating Individual Thresholds in Infant Listeners*

Abstract An adaptive procedure for rapid estimation of adult thresholds (PEST: Parameter Estimation by Sequential Testing; Taylor & Creelman, 1967 ) was modified and applied to the estimation of auditory thresholds in 6-month-old infants (PESTI). This procedure yielded similar absolute thresholds to those obtained in previous research with the method of constant stimuli. Advantages of the present procedure are outlined.