Preliminary observations of infants’ detection of backward masked tones

Backward masked thresholds appear to have a prolonged developmental course compared to other measures of auditory capacity, but backward masking has not been studied in infant listeners. The present study examined 7–9-month-old infants’ detection of 20-ms, 1-kHz pure tones masked by a 50-ms, 2.5-kHz-lowpass noise. The interval between the offset of the tone and the onset of the masker was 0 ms. Both stimuli had 5-ms rise and fall times. The spectrum level of the masker was 30-dB SPL. Thresholds for the backward masked tone were determined adaptively for 6 young adults using a rule that converges on the 71% correct point on the psychometric function. The average threshold was 50-dB SPL (SD=12 dB). Thirteen infants were trained to respond to a backward masked 95-dB SPL tone, but not to the masker alone. These infants then completed 30 single-interval test trials, with 15 no-signal trials and 15 signal trials with the tone fixed at 85-dB SPL and the same backward masker. Infants achieved a p(C)max of about 0.7 (SD=0.05) in this task. These results suggest that at this age infants’ backward masked threshold is about 35 dB higher than the adults’.