Academic journal article Psychonomic Bulletin & Review

Source Accuracy Data Reveal the Thresholded Nature of Human Episodic Memory

Academic journal article Psychonomic Bulletin & Review

Source Accuracy Data Reveal the Thresholded Nature of Human Episodic Memory

Article excerpt

Published online: 29 November 2012

© Psychonomic Society, Inc. 2012

Abstract Episodic recollection supports conscious retrieval of past events. It is unknown why recollected memories are often vivid, but at other times we struggle to remember. Such experiences might reflect a recollection threshold: Either the threshold is exceeded and information is retrieved, or recollection fails completely. Alternatively, retrieval failure could reflect weak memory: Recollection could behave as a continuous signal, always yielding some variable degree of information. Here we reconcile these views, using a novel source memory task that measures retrieval accuracy directly. We show that recollection is thresholded, such that retrieval sometimes simply fails. Our technique clarifies a fundamental property of memory and allows responses to be accurately measured, without recourse to subjective introspection. These findings raise new questions about how successful retrieval is determined and why it declines with age and disease.

Keywords Human memory * Episodic memory * Recollection * Familiarity * Signal detection theory

Does recollection sometimes fail completely? This question has remained at the forefront of memory research for half a century. The common experience of forgetting where you leftyour keys fits well with threshold theories (Atkinson & Juola, 1974; Yonelinas, 1994) that state that retrieval can indeed fail. Alternatively, however, recollection may be a continuous signal (Green & Swets, 1966; Mickes, Wais, & Wixted, 2009), and retrieval "failure" may reflect weak or inaccurate, but not absent, recollection. Today, the fundamental nature of recollection remains as fiercely disputed as ever (Wixted, Mickes,& Squire, 2010; Yonelinas, Aly, Wang, & Koen, 2010).

Characterizing recollection correctly is necessary for it to be accurately dissociated from other memory processes. In particular, one highly influential dual-process theory distinguishes thresholded recollection from continuous familiarity (Yonelinas, 2002). This important functional dissociation underpins the widely used separation of recollection and familiarity using confidence ratings, yet it remains valid only if recollection really is thresholded (Wixted, 2007). If, instead, recollection were found to be continuous, many existing conclusions within memory research, from the specific decline of recollection in aging (Howard, Bessette-Symons, Zhang, & Hoyer, 2006) to the mapping between cognitive processes and neurobiological structures (Eichenbaum, Fortin, Sauvage, Robitsek, & Farovik, 2010; Peters, Thoma, Koch, Schwarz, & Daum, 2009), would need to be reinterpreted.

To date, the most widely cited evidence for a retrieval threshold comes from receiver-operating characteristic (ROC) curves (Yonelinas & Parks, 2007). Theoretically, the shape of the ROC provides information about the processes supporting memory. In practice, however, ROCs have been used to argue both for and against a recollection threshold (Wixted, 2007). One reason may be that ROCs, like old/new judgments, reflect subjective confidence rather than memory directly. The validity of this approach-especially the assumption that confidence and memory strength are directly and consistently related-is questionable (Bröder&Schütz, 2009;Malmberg, 2002; but see also Dube & Rotello, 2012). In short, unambiguous conclusions about the nature of recollection are difficult to draw from confidence ratings alone.

Here, we introduce an alternative measure of recollection: positional response accuracy. To assess accuracy, we used a source task that provides fine-grained assessments of retrieval errors (Fig. 1). At study, words were presented visually, each paired with a unique source location that participants would reproduce (Fig. 1a). At test, the studied words were represented, and participants recollected the associated source location as accurately as possible (Fig. …

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