Development of responsiveness to suprathreshold acoustic stimulation in chickens.

Developmental changes in an unconditioned response to acoustic stimulation were observed in young chickens. Specifically, durations of distress call (peep) suppression were measured after the onsets of tones that differed in intensity and frequency in 384 newly hatched and 4-day-old chicks. Resuppression was also measured after a 6% change in the frequency of these tones, once the animals had habituated to the original tone. The data showed that the suppression varied systematically as a function of age, intensity, and frequency: (a) the duration of suppression increased with increasing stimulus intensity, as expected; (b) responsiveness to high frequencies grew more rapidly over the first 4 days than responsiveness to low frequencies, an effect indicating a developmental gradient across frequencies with age; (c) resuppression to the 6% change in frequency increased in duration with age; and (d) young birds suppressed vocalizations longer to loud tones in the range of their species' maternal assembly call than to other frequency-intensity combinations. These developmental trends indicate rapid changes in "perceived loudness" and "perceptual sharpening" over the first few days of postnatal life.