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.]

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The ability of 5-8-month-old infants and of young adults to detect changes in the frequency of pure tones was investigated. A head turn for visual reinforcement technique was used to obtain difference thresholds for 14 infants and 5 adults at 1000, 2000, and 3000 Hz. With signals presented at 70 dB above adult detection thresholds, infants reliably detected frequency changes on the order of 2%, whereas adults could detect changes of about 1%. These data not only confirm the findings of previous, studies that infants can distinguish different frequencies but indicate that infants can make relatively fine discriminations.