An auditory psychometric function from newborn chicks.

This study shows how the hearing of newborn chicks can be quantified with receiver operating characteristics (ROCs). A detailed auditory psychometric function was constructed from areas under ROCs, derived from delays in ongoing vocalizations elicited by pulsing pure tones. Over 20,000 delays were analyzed from 160 birds tested at four frequencies (0.25-2 kHz), eight intensities (-3 to 18 dB above estimated threshold), and two ages (0 and 4 days posthatch). Areas under these ROCs increase in an S-shaped function over intensity, as expected. Maximum responsiveness at approximately 75% correct shows that these neonates never achieve the near-perfect performance of mature listeners. Thresholds appear best estimated by the level required for 65% correct. Results replicate previous findings, showing that thresholds mature at low frequencies before high. Although there is no simple way to alleviate decreasing responsiveness caused by habituation over trials, improved performance results from presenting different frequencies, but only in the younger birds, and primarily with louder stimuli.