Comment on “The Shift From Recency To Primacy With Increasing Delay” by Knoedler, Hellwig and Neath (Journal of Experimental Psychology: Learning, Memory and Cognition, 1999, Vol 25, No. 2, 474-487)”

by

Eugen Tarnow, Ph.D.

Avalon Business Systems, Inc.

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Knoedler, Hellwig and Neath presented measurements of the recency-primacy shift in which memory for early list items improves and memory for later items becomes worse as the retention interval between study and test increases. The authors interpreted the data to support the Dimensional Distinctiveness model. I present an alternative interpretation of their data in terms of an interference model, “Early Interference with Recall”, at five seconds after the presentation of a particular list item and show that this interpretation can qualitatively better account for the experimental data. Two predictions further separate the two models: Dimensional Distinctiveness predicts no recency-primacy shift for two-item lists, in contrast to Early Interference With Recall. As the overall time scale is changed, the former model also predicts no difference in recency or primacy while the latter shows large changes.

Neath (p. 151, 1998) wrote that “any theory of forgetting must explain not only why memory is often worse as time passes, but also why memory is sometimes better.” He and other authors have focused their attention on an interesting anomaly in experimental memory research, the recency-primacy shift. This shift was the subject of a previous paper in this journal, Knoedler, Hellwig and Neath (1999). The authors there presented data which showed the recency-primacy shift under a variety of conditions. They explained the experimental data using the Dimensional Distinctiveness model. The present comment suggests that the Dimensional Distinctiveness model, as elaborated in their paper, does not fit the data and that a better explanation is one in which the information storage process interferes with the recall process about five seconds after the presentation of a simulus.

I graph the data somewhat differently than was done in the original article. In Figure 1 is shown the proportion of correctly tested list items as a function of the delay in seconds between the test and the study of each item for four item series and four retention intervals. The recency-primacy shift is the secondary difference in proportion correct answers between the first and 4th position at 0 and at 5 second retention intervals - at 0 seconds this difference is negative, at 5 seconds it is positive (in each series the 4th and latest position shows up first in my graphs). As remarked in the paper and in Neath (1998), any theory of memory has to account for this shift, in particular, for the rather amazing result that the memory of the 4th position actually improves with increased study-test time! The recency-primacy shift is relatively small - the secondary difference is only about 0.1.

Figure 1. Mean accuracy (proportion correct as measured by hits) for four-item study/test series as a function of the delay between study and test for four retention intervals (0,1,2 and 5 seconds).

Figure 2. Simulation of the mean accuracy for four-item study/test series as a function of the delay between study and test for four retention intervals (0,1,2 and 5 seconds) using the Dimensional Distinctiveness model.

An unusual feature of the Dimensional Distinctiveness model is not shown in the picture: while the distinctiveness score is time dependent, it does not scale with time. I.e. if the experiment took place over 9 seconds or 18 seconds (or, indeed 18 years), the model would predict the same result as long as the list items had the same relative time relationship.

I simulate this interference process using an exponential function

centered at N seconds with a width s where ts is the time of study and tt is the time of the test. Using the parameters N=4.8 and a=0.074 and s=2.4 I obtain the graph of Figure 3. Two features are qualitatively similar to the experimental data in Figure 1. First there is a dip in the data around five seconds after the study. This dip automatically leads to the “recency-primacy” shift. Second, the intermediate list items are reproduced correctly - they vary just as much as the first and last list items, are not always the same and change relationships after the five second dip.

Figure 3. Simulation of the mean accuracy for four-item study/test series as a function of the delay between study and test for four retention intervals (0,1,2 and 5 seconds) using the Early Interference with Recall model.

If we add a small amount of inter-item interference the model fits the data even better and makes the test results dependent not only on the delay time between study and test, but also on the position of the item in the list. The result for 0.63% more erroneous recall for each previous presentation and N=4.6 and a=0.068 and s=2.1 in the series is shown in Figure 4. The secondary interference causes the curves to split.

Figure 4. Simulation of the mean accuracy for four-item study/test series as a function of the delay between study and test for four retention intervals (0,1,2 and 5 seconds) using the Early Interference with Recall model including a 0.63% inter-item interference term.

There are several other ways these two models predict different experimental results. Most importantly, the Dimensional Distinctiveness model predicts that there is no recency-primacy shift for two-item series, in contradiction to the prediction of the Early Interference with Recall model in Figure 5. Secondly, the Early Interference with Recall model depends strongly on the time scale, while the Dimensional Distinctiveness model has no dependence on the time scale - multiplying the experimental time scales by five will yield the same prediction for the Dimensional Distinctiveness model but the prediction of Fig. 6 from the Early Interference with Recall model. Finally, the Early Interference with Recall model shows an interesting balance of inter-item and intra-item interference effects. If one increases the ratio of intra-item to inter-item interference, the model predicts that the recency primacy shift should disappear.

Figure 5. Prediction of the mean accuracy for two-item study/test series as a function of the delay between study and test for four retention intervals (0,1,2 and 5 seconds) using the Early Interference with Recall model including a 0.63% inter-item interference term. Notice the predicted recency-primacy shift.

Figure. 6 Prediction of the mean accuracy for four-item study/test series as a function of the delay between study and test for four retention intervals using the Early Interference with Recall model including a 0.63% inter-item interference term. All time scales are five times larger than in previous figures.

Knoedler, A.J., Hellwig and Neath, I. (1999), “The Shift From Recency To Primacy With Increasing Delay “, Journal of Experimental Psychology: Learning, Memory and Cognition, 1999, Vol 25, No. 2, 474-487.

Neath, I (1998). “Human Memory”. Brooks/Cole. Pacific Grove. 148-151.