Operant discrimination of emotional calls in rats: are there biological constraints?

Rats emit 50kHz ultrasonic vocalizations (USVs) in positive contexts, while they produce 22kHz USVs in negative contexts. 50kHz USV is possibly associated with the mental state of pleasure, while 22 kHz USV with that of distress, and such associations may influence feasibility of discrimination learning using USVs. Here, we investigated whether operant conditioning which utilizes USV stimuli is affected by presumed innate connections, which are 50kHz USV with pleasure, and 22kHz USV with distress. We trained rats to discriminate 50kHz and 22kHz USV in two conditions. In the congruent condition, 50kHz USV was paired with a positive event, while 22kHz with a negative one. In the incongruent condition, 22 kHz USV was paired with a positive and 50kHz USV with a negative outcome. Rats were trained to serially reverse the learning conditions four times in total. Results show that the number of trials to the learning criteria did not significantly differ between the congruent and the incongruent conditions, both in the discrimination and reversal learning phases. These results suggest that rats did not learn the congruent condition faster than the incongruent condition, nor did the congruent condition more strongly hamper the acquisition of reversal learning than the incongruent condition. Namely, rats did not respond differently to the congruent and the incongruent conditions. It seems likely that the presumed innate connections (50kHz USV with pleasure, 22kHz USV with distress) did not influence the feasibility of discrimination and serial reversal learning which utilizes USV stimuli.