Stimulus generalization gradients for auditory intensity in rats and guinea pigs

Eight hooded rats and eight albino guinea pigs were given successive discrimination training involving variable-interval reinforcement (for barpressing in the presence of one intensity of a 1,000-Hz tone) randomly alternated with extinction (for pressing during another intensity). For half of the animals in each group, the S+ was 75 dB and the S− was 65 dB; for the other half, the S+ was 85 dB and the S− was 95 dB. All Ss were then tested in extinction for generalization along the intensity dimension with values of 65, 70, 75, 80, 85, 90, and 95 dB. The discrimination performance of the rats and guinea pigs was comparable, but the latter species showed less resistance to extinction during generalization testing. For both species, the discrimination employing the more intense training stimuli was mastered faster than that using the less intense ones. A peaked generalization gradient was obtained from most of the Ss, with maximal responding emitted to a stimulus displaced from S+ in the direction opposite S−, and reduced responding to stimuli still farther removed from S−. The group average function contained a significant quadratic component for each species, although occasional deviations from this pattern were observed.