Between-list manipulations of memory strength through repetition commonly generate a mirror effect, with more hits and fewer false alarms for strengthened items. However, this pattern is rarely seen with within-list manipulations of strength. In three experiments, we investigated the conditions under which a within-list mirror effect of strength (items presented once or thrice) is observed. In Experiments 1 and 2, we indirectly manipulated the overall subjective memorability of the studied lists by varying the proportion of nonwords. A within-list mirror effect was observed only in Experiment 2, in which a higher proportion of nonwords was presented in the study list. In Experiment 3, the presentation duration for each item (0.5 vs. 3 sec) was manipulated between groups with the purpose of affecting subjective memorability. A within-list mirror effect was observed only for the short presentation durations. Thus, across three experiments, we found the within-list mirror effect only under conditions of poor overall subjective memorability. We propose that when the overall subjective memorability is low, people switch their response strategy on an item-by-item basis and that this generates the observed mirror effect.
In recognition memory experiments, participants first study a list of items, and then, later, in a recognition memory test, they attempt to discriminate previously presented items (targets) from novel ones (distractors). A commonly used conceptual tool for understanding recognition memory performance is signal detection theory (SDT). According to SDT, targets and distractors on the recognition memory test are each distributed over a psychological strength-of-evidence dimension, with targets having higher mean strength than distractors (Figure 1). To make a recognition decision, participants are assumed to adopt a response criterion (C in Figure 1) somewhere along the strength-of-evidence dimension. If a test item has strength equal to or above the criterion, it is judged old; otherwise, it is judged new. The proportions of targets and distractors that are called old are dubbed the hit rate (HR) and false alarm rate (FAR), respectively.
The mirror effect is a phenomenon of recognition memory in which better old-new discrimination in one condition is manifested as both a higher HR and a lower FAR (e.g., Glanzer & Adams, 1985; Glanzer, Adams, Iverson, & Kim, 1993). The consistency with which the mirror effect has been observed with different recognition tasks and with different experimental manipulations led Glanzer et al. to describe it as a "regularity" (p. 546) of recognition memory.
Stretch and Wixted (1998) investigated the causes of the word frequency and the repetition-based mirror effects and concluded that the latter is a consequence of a shift in the placement of the SDT response criterion. An example of criterion shift is presented in Figure 2. In this case, strong targets (Ts) have been strengthened through study repetitions or increased presentation time so that they have higher strength-of-evidence than weak targets (Tw). In Figure 2, this graphically translates into the Ts distribution being shifted to the right on the strength-ofevidence axis relative to the Tw distribution. The distractor (D) distribution is low on the strength-of-evidence axis because distractors were not presented at study. Only one distractor distribution appears in Figure 2, because it is assumed that the strength manipulation exerts an effect only on targets, as it happens at encoding, and not on distractors. Placing response criteria at the intersection point of the target and distractor distributions for both the weak (Cw) and strong (Cs) conditions (i.e., the point corresponding to the optimal observer, C = 0; see Macmillan & Creelman, 2005) creates Cw and Cs, respectively. As can be observed in Figure 2, because of the positioning of the criteria and distributions, more old responses are given to Ts (with respect to Cs) than to Tw (with respect to Cw). …