Effects of stimulus frequency on vocal suppression in neonates

Three groups, each composed of 15 normal neonates, were exposed to different narrowband stimuli, all at 80 db SPL, representing low frequencies (peak level at 1 kc/s of 75 db, half-power bandwidth of 180 c/s), middle frequencies (peak level at 3 kc/s of 75 db, half-power bandwidth of 300 c/s), and high frequencies (peak level at 6 kc/s of 69 db, half-power bandwidth of 1 kc/s). Each S in each group was tested with the appropriate 4-min control condition for each S where a stimulus was not presented. For each group, Ss cried significantly less during the stimulus than for the control condition. There was a trend for suppression of crying to decrease as stimulus frequency increased (mean suppression of crying during the noise presentations, relative to the crying during the control conditions, were 148.3, 125.2, and 111.5 secs for the frequencies in order), but the differences among the groups were not significant. This suggests that neonates do not hear appreciably better at some frequencies than at others, and also that neonates do not necessarily interact differently with different stimuli because of frequency content. It seems, therefore, that neonates respond on a stimulus-response basis that is proportional in strength to stimulus level and spectral complexity. Loud complex noises are the strongest stimuli because they result in maximum neural excitation. However, narrowband noises can be used to indicate something of the neonate's frequency sensitivity.