Simultaneous masking in newborn chickens.

Simultaneous masking was used to investigate the development of frequency analysis in newborn animals. The ability of young chickens to detect a tone in the presence of a second tone was measured. In the first experiment data were collected with a 500-Hz masker presented at 15 or 25 dB above absolute threshold. Signals to be detected varied up to an octave from the masker (+/- 1/9, +/- 2/9, +/- 1/3, +/- 2/3, and -1 octave). Receiver operating characteristics (ROCs) were constructed from delays in ongoing peeps that occur when chicks detect the presence of the signal over the masker. Psychometric functions were fit by probit analysis using areas under these ROCs. Amounts of masking were estimated by the degree to which psychometric functions were shifted toward the right by the presence of the masker. There is no developmental change in these masking patterns, corroborating recent data from human infants. Masking was shown in a second experiment to be similar at a higher frequency (given intensities corrected for improving absolute thresholds). In conclusion, masking patterns derived from chicks' peeps are like those from adult humans. Frequency analysis, as measured by simultaneous masking, appears to be mature in chickens at hatching.