International Journal of Comparative Psychology

The accuracy of time judgments depends upon many factors, including the sensory properties of the to-be-timed stimulus. In auditory Pavlovian fear conditioning, an initially neutral tone (conditioned stimulus, CS) predicts the arrival of an aversive event (unconditioned stimulus, US) at a fixed time interval. The temporal relation between the CS and US events is encoded, leading to the development of a temporal pattern of responding. Little attention has been paid to the potential impact of the characteristics of the CS tone on the development of this temporal pattern. Here we compared the acquisition of the temporal pattern of conditioned responses of rats to different CS tone frequencies. Rats were first conditioned to lever press for food. Then, while lever pressing for food, they were presented with 60-s tones  of two very different frequencies 1kHz or 11kHz, each paired with a foot-shock given 30s after tone onset. This fear conditioning led to the appearance of conditioned suppression of the lever pressing. On probe trials the tone duration was 60 s, and the reinforcer was omitted. With training, a pattern of suppression evolved during the probe trials, showing a maximum of suppression near the programmed time of the shock US, however the 11kHz CS tone yielded better temporal control than did the 1kHz tone. A second experiment investigated rats’ abilities to discriminate between two times of shock arrival (10s or 30s) predicted by the different tone frequencies (1kHz or 11kHz), In this experiment, rats showed poorer discriminative timing performance when the lower frequency (1kHz) was associated with the longer duration (30s). Our results suggest a strong impact of the CS sensory properties on the expression of temporal learning within the context of auditory fear conditioning in rats.