Remembering the order of serially presented objects: A matter of time?

Remembering the sequence in which stimuli are encountered or events have occurred, is a key process in episodic memory and can also facilitate recognition memory. Rodents when presented with a sequence of objects, will explore the object encountered first, yet whether this behaviour is because the rodents spontaneously encode the order of stimuli presentation or because of relative familiarity or temporal decay is unknown. Here we tested sequence memory in rats using a series of spontaneous preference tasks. Experiment 1 demonstrated that when rats are presented with a sequence of four objects, with an inter-sample interval (ISI) of 5 min or 1h, they preferentially explored the object presented earlier in the list irrespective of the ISI. We then demonstrated that such memory for order was not affected by increasing or decreasing the ISI between the middle objects (Experiment 2). Finally, we showed that memory for order is not a function of absolute object familiarity, as animals showed clear discrimination between the objects presented in the sample phases and a novel object, independent of the sample objects position in the sequence (Experiment 3). These results show that animals are able to encode the order of objects presented in a sequence, and as such temporal order memory is not achieved using the process of relative, or absolute familiarity or temporal decay.