Cross-modal use of an internal clock.

Four experiments used time-discrimination procedures with rats to ask if light and sound are timed by the same internal clock and if measured durations of light and sound are stored in the same memory. Experiment 1 used a choice procedure and a cross-modal transfer-of-training design. Rats were trained to press one lever after a 1-sec signal, another lever after a 4-sec signal. At first, the signal was light (or sound); when performance was accurate, the signal was changed ot sound (or light). There was transfer of the time discrimination across modalities: To some extent, the rats treated the new signal (sound) as if it were the old signal (light). The rest of the experiments used the peak procedure, which is similar to a discrete-trials fixed-interval procedure. With the peak procedure, response rate reaches a maximum in the middle of the trial; the time of the maximum (peak time) is a measure of the clock and temporal memory. Experiment 2 found that changing the time of food during light (or sound) changed peak time during sound (or light). In Experiments 3 and 4, intervals of light (or sound) were followed by intervals of sound (or light). The interval of light decreased the peak time measured from the start to the interval of sound, whether the two intervals were adjacent (Experiment 3) or separated by 5 sec (Experiment 3). Experiments 1 and 2 suggest that measured durations of light and sound are stored by the same memory; Experiments 3 and 4 suggest that light and sound are timed by the same clock.