Two groups of rats were trained on operant discriminations in which the discriminanda were two different sound pressure levels of a 4-kHz tone. The discriminanda were chosen so that the loudness difference between stimuli was equivalent for each group when calculated from a power function with an exponent of .35. Half of each group learned the discrimination in quiet, and the other half learned it in a background of white noise. Within the quiet and the noise conditions, the asymptotic discriminability of stimuli separated by equal loudness differences was equivalent, and discriminability was lower in noise. This is consistent with both the human literature on masked loudness and a model of psychophysical scaling (Pierrel-Sorrentino & Raslear, 1980) in which animals judge perceived differences between stimuli.
Rats and chinchillas were food rewarded for lever pressing in the presence of one of two alternating sound intensities of a 4000-Hz tone. Discrimination accuracy was indexed by percentage of presses made to the rewarded intensity, food-availability cues eliminated. Stimulus intensity pairs yielding equivalent discrimination performance were used to define equal-loudness differences. Six groups of rats were each trained on a different intensity pair. Equivalent performances at a high level of discrimination accuracy were obtained for three groups, whereas, behavioral matching occurred across the other three groups but showed a considerably lower discrimination index. Chinchillas, similarly trained, demonstrated behavioral matching at high, medium, and low discriminability levels. The various intensity pairs yielding the same discrimination indices differed when their sound-pressure-level separations were expressed in decibels but were similar (within species) when stated in loudness units calculated from a power function with SPL referenced at 20 μN/m2. For rats the exponent is 0.35, and for chinchillas, it is 0.25. This relates the growth of loudness to intensity in accord with Stevens' power law. [Work supported by NINDS.]
Two chinchillas were trained on a series of two-valued auditory intensity discriminations. Lever presses were reinforced when no tone was present and not reinforced in the presence of a four-kiloHertz tone. The intensity of the nonreinforced tone was successively decreased, increasing the difficulty of the discrimination, until differential responding resembled that on a mixed schedule (no-tone-no-tone). Response data were partitioned in such a way as to provide a continuing assessment of the relative amounts of control exerted by the reinforcement schedule and the sound intensity, respectively. Control by reinforcement density was a direct function of discrimination difficulty, whereas the control exerted by intensity was inversely related to difficulty. For these chinchillas, the absolute threshold value obtained at four kiloHertz was about two decibels referenced to 20 microNewtons per meter squared.
The present study examined the course of extinction of a learned taste aversion using prolonged exposure to the aversive food as the extinction test. A conditioned aversion to sucrose was established in deprived rats. They were then exposed for a 4-h period to sucrose alone where it was (1) response contingent, (2) delivered intermittently, or (3) freely available. The aversion extinguished in the 4-h period under all three test conditions. The results suggest that the extinction rate of a learned taste aversion is based on the duration of the exposure to the aversive food after the poisoning experience, not on the time since poisoning.
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