What starts an internal clock?

Five experiments with rats investigated under what conditions a stimulus is timed by the internal clock used in time-discrimination procedures. In Experiments 1-4, we trained rats to time one stimulus (e.g., light) and then asked whether they timed a stimulus from another modality (e.g., sound). The second stimulus was treated in three ways: exposed (presented alone), paired with food, and extinguished. Experiments 1 and 2 used the peak procedure, similar to a discrete-trial fixed-interval schedule, and paired the treated stimulus with food using instrumental training; Experiments 3 and 4 used a psychophysical choice procedure and paired the treated stimulus with food using classical conditioning. All four experiments found that there was cross-modal transfer of the time discrimination after pairing, but not after exposure or extinction. This suggests that the rat's internal clock timed the treated stimulus after pairing, but not after exposure or extinction. Experiment 5 tested a theory of extinction based on the results of Experiments 1-4; the results suggested that the decline of responding observed in extinction is not due to changes in timing. The main conclusion is that the internal clock apparently times stimuli with signal value (associative strength) and does not time stimuli without signal value.