INFANTS DETECTION OF INCREMENTS IN LOW-FREQUENCY AND HIGH-FREQUENCY NOISE

A visually reinforced operant paradigm was employed to examine the rel ationship between the difference limen (DL) for intensity and level of the standard during infancy. In Experiment 1, 7-month-old infants and adults detected increments in continuous noise presented via headphon es at each of four levels ranging from 28 to 58 dB SPL. Noise stimuli were 2-octave bands centered at either 400 or 4000 Hz, and increments were 10 and 100 msec in duration. Infants' DLs were significantly larg er than those of adult subjects and significantly larger for low- than for high-frequency stimuli. For the high-frequency noise band, infant s' DLs were generally consistent with Weber's law, remaining essential ly constant for standards higher than 28 dB SPL (3 dB SL) for 100-msec increments and 38 dB SPL (13 dB SL) for 10-msec increments. For low-f requency noise, infants' absolute thresholds were exceptionally high, and sensation levels of the standards were too low to adequately descr ibe the relationship. In Experiment 2, 7-month-old infants detected 10 - and 100-msec increments in 400-Hz noise stimuli presented in sound f ield. Infants' low-frequency DLs were large at low intensities and dec reased with increases in level of the standard up to at least 30 dB SL . For both low- and high-frequency noise, the difference between DLs f or 10- and 100-msec increments tended to be large at low levels of the standard and to decrease at higher levels. These results suggest that the relationship between the DL and level of the standard varies with both stimulus frequency and duration during infancy. However, stimulu s-dependent immaturities in increment detection may be most evident at levels within approximately 30 dB of absolute threshold.