Information accumulation on the item versus source test of source monitoring: Insights from diffusion modeling

Abstract

Source monitoring involves attributing previous experiences (e.g., studied words as items) to their origins (e.g., screen positions as sources). The present study aimed toward a better understanding of temporal aspects of item and source processing. Participants made source decisions for recognized items either in succession (i.e., the standard format) or in separate test blocks providing independent measures of item and source decision speed. Comparable speeds of item and source decision across the test formats would suggest a full separation between item and source processing, whereas different speeds would imply their (partial) temporal overlap. To test these alternatives, we used the drift rate parameter of the diffusion model (Ratcliff, Psychological Review, 85, 59-108, 1978). We examined whether the drift rates, together with the other parameters, assessed separately for the item and source decision varied as a function of the test format. Threshold separation and nondecision time differed between the test formats, but item and source decision speeds represented by drift rates did not change significantly. Thus, despite facilitation on the source decision when the item decision was immediately followed by a test for source memory than when item and source were tested in separate blocks, findings did not suggest that source information already begins accumulating in the item test in the standard format. We discuss the temporal sequence of item and source processing in light of different assumptions about the contribution of familiarity and recollection.

Keywords: Diffusion model; Item memory; Source memory; Source monitoring; Temporal sequence.

Fig. 1

Illustration of the decision process as proposed by the diffusion model (Ratcliff, 1978). Here, the upper and lower thresholds correspond to decisional outcomes (alternatively, correct and incorrect responses). The distance between the thresholds is represented by a. Information accumulation starts at z (here centered between the two thresholds) and continues over time with speed |v| (denoted by the upward pointing arrow) until it reaches either of the two response alternatives. Random influences lead to unsteady fluctuations in the sample path. The duration of processes outside the decision process (e.g., encoding or response execution) are accounted for t₀. The response time distribution for choosing response X (response Y) is shown above (below) the respective threshold

Fig. 2

Example visualizations of the test formats. (A) In the standard format, source decisions for each recognized test trial were collected in immediate succession to item decisions. (B) In the blocked format, source decisions for all recognized test trials were collected as a separate test block after the completion of the item tests

Fig. 3

Empirically observed mean accuracy rate and correct response times (RTs) across conditions with their estimated counterparts. Error bars represent standard errors. Mean RT = mean response time of correct responses

Fig. 4

Bar plots of the diffusion model parameters drift rate, threshold separation, and nondecision time. Error bars represent standard errors. For drift rates, absolute values are shown

Fig. 5

Graphical displays of model fit. Concordance of the empirical and predicted statistics for the accuracy of responses and the .1, .3, .5, .7, and .9 quantile of correct response time distributions for each person in each condition. RT = response time; r_item = correlation between empirical and predicted statistics for item test; r_source = correlation between empirical and predicted statistics for source test