This article describes an observer-based technique for assessing auditory capacities of infants from 3 to 12 months of age. This technique, referred to as the Observer-based Psychoacoustic Procedure (OPP), combines features of the Forced-choice Preferential Looking technique developed by Teller (1979) and of Visual Reinforcement Audiometry (Moore ; Thompson, & Thompson, 1975). The rationale behind the procedure and the specific techniques used in its application are detailed here. Psychometric functions and thresholds for pure-tone detection and frequency discrimination obtained with OPP are also presented. The results for 6-month-olds are compared with results from previous studies employing a visually reinforced head-turn procedure. 627

The ability of 3-, 6-, and 12-month-old infants to detect pure tones in quiet was tested at frequencies ranging from 250–8000 Hz. Stimuli were presented monaurally via headphone. Signal trials consisted of ten repetitions of a 500-ms tone burst, with 10-ms rise-fall time and 500 ms between bursts; no-signal trials were 10-s intervals of quiet. The infant's response to a tone was judged by an observer, who, blind to trial type, decided whether or not a tone had been presented on each trial, based on the infant's behavior. Comparison of infant thresholds, determined using an adaptive rule, to those of adults tested under similar conditions showed a progressive improvement in threshold from 3–12 months. Three-month-olds' threshold were relatively poorer at 250 and at 8000 Hz compared to adults. The 6- and 12-month-olds' thresholds were somewhat closer to those of adults at 4000 and at 8000 Hz than at lower frequencies. Maturation of absolute sensitivity, of frequency resolution, and of nonsensory processing may all contribute to these age-related changes. [Work supported by NIH.]

Frequency difference limens (FDLs) were estimated for 3-, 6-, and 12-month-old infants and for adults using pure tones at 500, 1000, and 4000 Hz. Each listener provided an FDL at 40 dB and at a higher (80 dB, in most cases) sensation level (SL). An observer-based behavioral testing technique was used. The FDLs of 3-month-olds were worse than those of adults at all three frequencies, and increased with increasing frequency. The FDLs of 6- and 12-month-olds were worse than those of adults at 500 and 1000 Hz, but not at 4000 Hz. Decreasing the SL led to an increase in the FDL of about the same magnitude at all ages, and the same age differences were found at both SLs. Thus infant-adult differences in FDL are not a simple consequence of differences in absolute sensitivity. Infant FDLs at one SL were also found to be significantly correlated with the FDL at the other SL. The FDLs at one age were, in general, predictive of the FDL at a later age in a longitudinal sample of infants. Models that might account for these age-related differences are discussed.

Pure-tone thresholds at frequencies ranging from 250 to 8000 Hz were estimated for 3-, 6-, and 12-month-old infants and for adults, using the Observer-based Psychoacoustic Procedure (OPP). Sounds were presented monaurally using an earphone. Psychometric functions of infants were similar to those of adults, although 3-month-olds had shallower functions at higher frequencies. The thresholds of 6- and 12-month-old infants were 10-15 dB higher than those of the adults, with the difference being greater at lower frequencies. This result is in general agreement with results from other laboratories. The thresholds of 3-month-olds were 15-30 dB higher than those of adults. The greatest difference between 3-month-olds and adults was at 8000 Hz. This threshold difference is smaller than that reported in earlier behavioral studies; higher thresholds at high frequencies have been previously reported for newborn and 3-month-old infants. The relative contributions of sensory and nonsensory variables to these age differences are discussed.

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