Relationship between intensity and reaction time in normal-hearing infants and adults

Reaction time (RT) to sound is known to be related to loudness in adult listeners. The purpose of this study was to determine whether infants' RT to sound decreases systematically with intensity as it does in adults.

RT was measured for 24 6- to 9-mo-old infants and 11 19- to 26-yr-old adults. All participants were normal hearing, naïve listeners. The stimuli consisted of 4000 and 1000 Hz pure tones presented to the right ear through an insert earphone. Stimulus intensities ranged in 10 dB steps from 40 to 80 dB SPL for adults and 50 to 90 dB SPL for infants. Infant responses consisted of a head turn toward a reinforcer whereas adults responded by raising their hand. An additional three adults responded with a head turn. RT was defined as the time between the onset of the tone and an observer's button press indicating that a response had occurred. RT was corrected for the observer's reaction time and averaged over three to five repetitions at each level to obtain the mean reaction time (MRT) for each subject, frequency, and level.

MRT decreased with increasing intensity in both infants and adults. An examination of the MRT-intensity functions suggests that the infant functions may be steeper than those of adults, although considerable variability exists between listeners.

RT holds potential as a measure of loudness in infants. Whether differences in the MRT-intensity slopes exist between infants and adults is unclear. Future investigations using methods to reduce the variability of RT measurements are needed to examine potential slope differences further.