A visually reinforced operant procedure was employed to obtain 2-point threshold-duration functions in 7-month-old infants and adults in two experimental paradigms. In Experiment 1, thresholds were determined for 10-and 100-msec,500-Hz tones and octave-band noise bursts presented in quiet and noise backgrounds. Threshold-duration functions were significantly steeper for infants than for adults under all experimental conditions, and did not differ in slope as a result of differences in either stimulus bandwidth or masking noise. In Experiment 2, thresholds for trains of brief 500-Hz tone pulses were examined in infant and adult subjects. Infant functions were adult-like for integration of multiple-pulse stimuli, suggesting that the traditional, long-term process of temporal summation is mature by 7 months of age. Differences between the present results and those previously obtained for 4-kHzstimuli appear to be consistent with the view that different mechanisms are involved in the detection of low-and high-frequency signals. 183Temporal summation refers to the well-documented finding that thresholds for detection of a signal decrease as the signal increases in duration. For auditory stimuli, thresholds decrease at a rate of 8-10 dB per decade increase in duration up to a maximum duration of approximately 200-300 msec. It is generally agreed that the slope of the function describing this relationship varies with signal frequency (Gerken, Bhat, & Hutchison-Clutter, 1990;Watson & Gengel, 1969), and perhaps also with bandwidth (Garner, 1947b). However, these effects are small and, in normal adult listeners, decreases in threshold closely approximate perfect integration of signal energy over a wide range of stimulus conditions (Gerken et al., 1990).Recent investigations oftemporal summation in young infants suggest that this generalization may not hold early in development. Thorpe and Schneider (1987) determined thresholds for 4-kHz octave-band noise bursts in a group of 6-month-old infants and found that the slope of their temporal summation function was exceptionally steep, exceeding 20 dB per decade of change in duration over the range of linear integration. Berg (1991) reported similar steep functions for 4-kHz octave-band noise bursts in 6-to 7-month-old infants, but found that their threshold-duration functions were adult-like for 4-kHz tones. When stimuli were presented in a background of continuous masking noise, infants' steep functions for octave-band noise bursts were significantly reduced in slope, while those of adults remained un- changed. More recently, Berg (1993) examined thresholds for 1/3-octave filtered clicks and 300-msec noise bursts, and found that differences between click and noise thresholds were significantly larger in infants than in adults at low and high ends of the frequency spectrum but not for stimuli in the midfrequency range. Considered together, results ofthese studies suggest that while factors such as stimulus bandwidth, stimulus frequency, and masking noise have only minimal effe...

Studies using burst comparison procedures to examine age-related changes in intensity discrimination have reported that the ability to discriminate differences in intensity does not reach maturity until late childhood. In the present study, developmental changes in intensity discrimination were examined in 1-to 3-year-old children, using an increment detection paradigm. Children and adults detected increments in a continuous standard presented at three levels ranging from 35 to 55 dB SPL.Adults were also tested at lower levels of the standard in order to permit age comparisons at equivalent sensation levels. Standard stimuli were two-octave bands of noise centered at either 400 or 4000Hz, and increments were 200 msec in duration. Discrimination performance improved significantly with both age and level of the standard. For all age groups, performance was significantly better for high-than for lowfrequency stimuli, but frequency-dependent differences in increment thresholds did not vary reliably with age. Age differences were largest at low levels ofthe standard. At the highest level (approximately 30 dB nHL), children's difference limens for both low-and high-frequency noise bands were adultlike by 3 years of age. These results suggest that the developmental time course of increment detection is more rapid than that previously reported in burst comparison studies.Although the ability to detect a change in the level of a sound is one of the most basic auditory capacities, little is known about the time course of its development. The few studies that have examined intensity discrimination in young listeners indicate that performance is relatively poor during infancy and continues to improve well into childhood, but, as yet, there is no consensus on the age at which adultlike performance is achieved. The results ofat least three studies suggest that the ability to discriminate differences in intensity matures relatively late. Both Faires and Lankford (1976) and Fior and Bolzonello (1987) determined children's intensity difference limens (DLs) for pure tones and found that performance continued to improve through at least 10 years of age. Similarly, Maxon and Hochberg (1982) reported a progressive improvement in the DL for tones through 12 years of age, the oldest group tested. In contrast to these findings, results reported by Jensen and Neff(1993) suggest a more rapid course of development. They used a video game format to obtain children's DLs for a 440-Hz tone and found that, for most children, performance was adultlike by 5 years of age.Werner and Marean (1996, p. 95) have recently questioned whether the ability to discriminate differences in intensity develops at all during childhood. Citing differences in procedure across studies and the great variability in performance among young listeners, they suggested that the sensory mechanisms underlying intensity discrimina- tion may mature early in life and that continued improvements in the discrimination performance of school-age children reflect the maturation of...

In adult listeners, the signal-to-noise ratio at masked threshold remains constant with increases in masker level over a wide range of stimulus conditions. This relationship was examined in 7-month-old infants by obtaining masked thresholds for .5- and 4-kHz tones presented in four levels of continuous masking noise. Adults were also tested for comparison. Masker spectrum levels ranged from 5 to 35 dB/Hz for .5-kHz tones, and from -5 to 25 dB/Hz for 4-kHz stimuli. Thresholds were determined for stimuli of both 10 and 100 msec in duration. The results indicated that infants' performance was more adult-like for 4-kHz stimuli. Although mean thresholds for both 10- and 100-msec, 4-kHz tones were approximately 7 dB higher in infants than in adults, E/N0 at threshold remained essentially constant over the 30-dB range of maskers employed. By contrast, infants' thresholds for .5-kHz tones were exceptionally high at lower levels of the masker. Threshold E/N0 decreased significantly as masker level increased from 5 to 35 dB/Hz, and this decrease was significantly greater for 10- than for 100-msec stimuli. Temporal summation of .5-kHz tones, measured as the difference between thresholds obtained at the two signal durations, was greater for infants than for adults at low levels of the masker. However, because infants' thresholds improved more rapidly with level for 10- than for 100-msec tones, age differences in temporal summation were no longer significant when masker spectrum level was 35 dB/Hz. These results suggest that the relationship between signal-to-noise ratio at masked threshold and level of the masker is dependent on both signal frequency and duration during infancy.