Rapid development of a sensory attribute in young chickens.

Development of a consistent pattern in responses to frequency changes is revealed with individual differences scaling. Newborn chicks respond to changes in a background tone to which they have become habituated with momentary delays in their otherwise regular peeping. Durations of these delays are used as measures of perceived difference between all possible pairs of five frequencies between 500 and 608 Hz. Coordinates of five points are calculated to create a map of these frequencies such that distances between points are maximally correlated with delays. A steeply sloped s-shaped curve describes the rapid emergence, from the beginning to the middle of the first postnatal day, of maps that explain significantly more of the input data than expected by chance. This implies that over a few hours birds come to reliably rank different frequencies along a consistent scale. Controlled rearing conditions of decreased external stimulation or early exposure to frequency changes have no effect on this development. In conclusion, scaling is a replicable way to evaluate the development of consistent responses to frequency changes. Similar processes are likely important in other aspects of perceptual development, learning, and memory.