Loudness scaling in rats and chinchillas.

Chinchillas and rats were trained on operant discriminations in which the discriminative stimuli were two different sound pressure levels of a 4-kHz tone. Two or more of these two-intensity discriminations were used at each of three levels of discriminability: high, medium, and low. For any given level, each of the stimulus pairs used differed in decibel separation but were similar in loudness-unit differences calculated from a power function. Different groups of animals trained on stimuli separated by equal numbers of loudness units produced equivalent performances at each of the three levels of discriminability. It is concluded that loudness growth for both of these species, as for man, is well described by a power function (Stevens' law). For the chinchilla the exponent is .25, and for the rat it is .35.