The Task

This tutorial develops an implementation of some aspects of Atkinson & Shiffrin's (1968, 1971) so-called "Modal Model" of human memory (called "modal", apparently because most psychologists subscribed to it at one point). The Modal Model is concerned to explain the Recency and Primacy effects in free-recall serial position curves (Glanzer & Cunitz, 1966). In a free-recall paradigm, participants are presented with a list of words, one at a time, and instructed to memorise the words as well as possible, in order to recall as many as possible in a subsequent free-recall phase. Because the words are presented serially, it is possible to plot the average accuracy of recall at each point in the series. The typical finding is that the curve is roughly U-shaped: words at the beginning of the list are recalled relatively well, as are words at the end of the list, but those in the middle are poorly recalled. The peak at the beginning is known as the Primacy Effect and the peak at the end is known as the Recency Effect. To anticipate slightly (ultimately you will reproduce this), a Serial Position curve produced by a COGENT model is shown below.

Now we can explain the Recency Effect (the rising portion at the right of the diagram) if we postulate two distinct memory stores: a limited-capacity but relatively reliable Short-Term Store (STS), and an unlimited capacity but relatively unreliable Long-Term Store (LTS). By this account, the items in the Recency portion of the free-recall curve are recalled well, because they are still held in the STS, whereas those earlier in the curve are recalled relatively poorly, since they are only held in the decay-prone LTS. If Ss can recall from either store, we expect a peak at the end of the curve, spanning as many items as the postulated capacity limitation of STS will allow (eg ~7 items (Miller, 1956)).

The Primacy Effect (at the far left of the diagram) requires a different explanation; in the Modal Model, it is assumed to arise from the limited-capacity process of Rehearsal which is used to transfer information from STS to LTS. The explanation depends on the fact that at the beginning of the list, there are relatively few items to rehearse, so these items receive disproportionately more rehearsal than items later in the list, and so if more rehearsal implies better recall, they should be remembered better.

Students should note that this theory is obsolete, and nobody believes the human memory system is so simple nowadays. Still, it is a good example to illustrate how COGENT can be used to implement concepts and models from the psychological literature.