Reference memory
A number of studies have manipulated the clock component of the model both in animals (Maricq, Roberts, & Church, 1981; Meck, 1983) and in humans (Droit-Volet & Wearden, 2002; Penton- Voak, Edwards, Percival, & Wearden, 1996) and have provided evidence for a pacemaker–accumulator clock of the type that SET proposes. Much less attention has been devoted to memory and decision components of the model. Some recent studies have explore timing in the probable absence of reference memory (Allan & Gerhardt, 2001; Rodriguez- Girones & Kacelnik, 1999; Wearden & Bray, 2001), but little research (apart from that of Meck, 1983, with rats) has attempted to manipulate the contents of the reference memory itself.
The reference memory component of the SET model serves two functions. The first is to provide a temporal reference for behavioural stability by storing “important” times. In animal studies the content of reference memory will typically be the remembered time of reinforcement; in human studies the reference memory is assumed to contain memories of standard durations important for the task in hand. The second role of reference memory is to generate the scalar property of timing: the requirement that the standard deviation of internal “estimates” of time is a constant fraction of the mean. This scalar property is a form of Weber’s law and assumes a timing process with constant sensitivity as the interval timed varies.
A key question in relation to reference memory is how the temporal representations develop..
Apriori, successive temporal memories could be stored in at least two distinct ways. In one of these, the successive individual examples could be aggregated together in some way (e.g., averaged into some mean time value), so increasing numbers of successive presentations of a standard would tend to make the aggregate value more representative or accurate than individual examples were. In this case, the contents of temporal memory would be transformed versions of the individual standard values provided. In contrast, another possibility is that the different examples of the standard are stored completely separately and then sampled. In this case, the individual identity of each previously presented standard would be maintained in memory, and the contents of reference memory might not statistically change with increasing numbers of standard presentations. Depending on how these separately stored examples of the temporal reference are used, increasing numbers of presentations of the standard may not necessarily change the temporal representation used.
Although we presented three experiments with consistent results above, perhaps the main contribution of the present article is that it argues for a reconceptualization of the nature of temporal reference memory, in humans and (most probably) also in animals. This reconceptualization considers the reference memory not as an extensive store of previous experiences, but as a much smaller store, either containing a single item or, as in the perturbation model, as a single item with upper and lower boundaries, which control whether or not change occurs as a result of some new experience. Perhaps the most surprising thing about this view of reference memory is that it is not only compatible with almost all previous results in both humans and animals, but also completely consistent with almost all the theoretical treatment of reference memory provided by Meck and colleagues (Meck, 1983, 1991; Meck & Angell, 1992; Meck & Church, 1987a, 1987b; Meck et al., 1984), in their development of the K* concept. The principal mechanism controlling variability of temporal references might be the K* transformation itself, rather than any aspect of the storage process of reference memory.