MEMORY FOR EVENT DURATION IN RATS

Rats were trained to discriminate short or long durations of either houselight illumination (light group) or tone (tone group) using a choice procedure. During the test phase of each trial, the left and right levers were presented with the cuelight above one of them on (cued lever) while the other was off (uncued lever). For half of the rats, the correct response following the short sample was to press the cued lever, while following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. Delay testing conducted in the context of a 10-s intertrial interval (ITI) indicated that for the light group, accuracy declined equivalently on short and long sample trials as the delay interval increased. However, for the tone group, a choose-long bias was seen at the longer delays. When the ITI was increased from 10 to 25 s, both groups displayed a choose-long bias. These data suggest that rats in both groups had the clock running during the ITI prior to presentation of the nominal sample, and that both groups used a change in lighting condition within the chamber to stop the clock. For the light group, this event was offset of the houselight sample, while for the tone group it was onset of the cuelight itself. Subsequent delay testing in the context of a 25-s ITI indicated that, for the tone group, accuracy declined equivalently on short and long sample trials as the delay,interval was increased. However, for the light group, a choose-short bias was seen at the longer delay. Following the shift to a longer ITI, both tone and light groups appeared to evaluate the memory of the subjective duration of the sample as being ''shorter'' in the context of a long ITI than in the context of a short ITI. When trained to make choices based on the duration of a preceding event, both rats and pigeons make systematic errors when the delay between the event and the opportunity to choose is varied. The nature of the systematic errors displayed are complexly determined by the temporal context within which they are exhibited (in both rats and pigeons) and by the modality of the remembered event duration (in rats). (C) 1995 Academic Press, Inc.