A fundamental issue in the analysis of working memory is the nature of the representation, or code, that mediates accurate performance across a retention interval. In this chapter we addressed the nature of working memory representations in DMTS tasks in which the samples differ in duration. In the standard choice version of the DMTS task, in which each duration sample is mapped in a one-to-one relation to a comparison stimulus, naive pigeons code samples retrospectively and analogically. We maintain that, in such instances, pigeons represent different event durations as codes which specify values along a dimension that changes in a continuous and cumulative manner as a function of the passage of time. Such a coding process is analogical because the representative dimension shares with duration the property of changing in a continuous and cumulative manner. As one example of an analogical-coding process, pigeons might represent different durations as different intensities of a visual image. On this view, the intensity of the image increases as time spent in the presence of the sample increases, and decreases as time since termination of the sample increases. The phenomena considered in the first major section of this chapter encourage the view that durations are coded analogically, and discourage the view that durations are coded asymmetrically or categorically (either retrospectively or prospectively).

Research considered in the second major section of this chapter suggests, however, that duration samples are not always coded analogically. In the successive DMTS task, in which accuracy is assessed by a go/no-go response rather than by choice behavior, duration samples are coded nonanalogically, perhaps in an instructional, prospective manner. Evidence of nonanalogical coding of duration samples has also been obtained in two instances in the choice DMTS task. First, pigeons initially trained in successive DMTS demonstrate nonanalogical coding when subsequently transferred to a choice DMTS task in which the contingencies are consistent with those of prior training. Substantial positive transfer from successive to choice DMTS suggests that pigeons continue to use the nonanalogical coding process adopted in the successive task when transferred to the choice task. Second, pigeons trained with two sets of samples, either both temporal or one temporal and one nontemporal, mapped onto a single set of comparisons (an MTO mapping) also demonstrate nonanalogical coding of the temporal samples. The results of mediated transfer testing suggest that samples associated with the same comparison stimulus activate a common code (e.g., "peck red", "event A"). One caveat to this conclusion is that an MTO mapping is less likely to reduce the tendency to code temporal samples analogically if accurate matching with temporal samples is acquired prior to acquiring accurate matching with nontemporal samples.

In summary, substantial progress has been made over the past decade in identifying the coding processes used by pigeons in DMTS tasks with duration samples, and in determining some of the factors that induce analogical or nonanalogical forms of coding. As yet, however, less is known about the generality of these results across species (e.g., Santi, Weise & Kuiper, 1995) or the coding processes used in other types of tasks that may require retention of temporal information.