A Prospective for a Unified Model of Episodic Memory

Research in episodic memory has become somewhat fractured in recent years. Many investigations have focused on just one memory task, such as recognition memory, free recall, or serial recall. Despite the previous unification of episodic memory with the global memory models, these models were abandoned due to challenging data that arose in each of the aforementioned tasks. In this chapter, we present a review of each of the key constraints that arose in item recognition, free recall, and serial recall that led to the development of newer memory models. We then outline a prospective for how revisions to the global memory models such as sparse representations, likelihood ratio decisions, long distance associations, and simultaneous output of multiple items could potentially be used to overcome the aforementioned challenges. Unified Model of Episodic Memory 3 A Prospective for a Unified Model of Episodic Memory “A critical problem of long standing in psychological study of memory is concerned with the relation between recall and recognition. In what sense are they the same, and in what sense are they different? ” (Tulving & Watkins, 1973, p. 739). This quote began the well revered Gillund and Shiffrin (1984) article that answered Tulving and Watkins’s question in the form of the search of associative memory (SAM) model’s unification of free recall and recognition memory. The SAM model, along with the other global memory models such as the theory of distributed associative memory (TODAM: Murdock, 1982), MINERVA 2 (Hintzman, 1988), and the matrix model (Humphreys, Bain, & Pike, 1989), unified episodic memory as a whole. Collectively, the global memory models were extended to a wide variety of domains that were presumed to reflect retrieval from episodic memory, including schema abstraction (Hintzman, 1986), judgments of frequency (Hintzman, 1988), memory for serial order (Lewandowsky & Murdock, 1989), priming (Ratcliff & McKoon, 1988), probability estimation (Dougherty, Gettys, & Ogden, 1999), generation of semantic representations (Kwantes, 2005), lexical decision (Kwantes & Mewhort, 1999), and spoken word recognition (Goldinger, 1998). Despite the zeitgeist that occurred in the years following the publication of the Gillund and Shiffrin article, researchers have become increasingly focused on particular tasks instead of episodic memory as a whole. Hintzman (2011) criticized this tendency in a recent review of episodic memory research and compared researchers focusing on single tasks to the fable of the blind philosophers feeling different parts of an elephant. Just as observing only the trunk of an elephant will Unified Model of Episodic Memory 4 not allow one to understand the elephant, solely focusing on recognition or free recall may paint a misleading characterization of episodic memory. Hintzman’s criticism is somewhat unfair, as a number of current episodic memory models have been applied to multiple tasks. Rich Shiffrin’s own REM model (Shiffrin & Steyvers, 1997, 1998) which initially emphasized recognition memory has been applied to a variety of tasks such as cued recall (Diller, Nobel, & Shiffrin, 2001), free recall (Lehman & Malmberg, 2013), perceptual identification (Huber, Shiffrin, Lyle, & Ruys, 2001; Schooler, Shiffrin, & Raaijmakers, 2001), lexical decision (Wagenmakers et al., 2004), and the development of knowledge representations (Nelson & Shiffrin, 2013). Nonetheless, the level of task comprehensiveness of other current episodic memory models is still somewhat limited compared to the previous generation’s widespread applicability. With the exception of the grouping model of Farrell (2012), models of free recall and serial recall are applied to one task but not the other despite the similarities between the two tasks, and aside from the REM model of free recall, current recall models have not been applied to recognition memory. Even when one considers empirical research conducted currently, investigations that directly compare multiple tasks are relatively rare. In previous decades task comparisons were more common. Investigations of effects such as word frequency (Gillund & Shiffrin, 1984; Gregg, 1976), semantic similarity (Kintsch, 1968), presentation modality (Watkins, Watkins, & Crowder, 1974), depth of processing (Craik & Tulving, 1975), spacing of repetitions (Glenberg, 1976), and aging (Craik & McDowd, 1987) commonly involved comparisons across multiple tasks, such as recognition and recall or free and serial recall. The logic of the task Unified Model of Episodic Memory 5 comparison is that effects that persist across tasks may be more relevant to episodic memory as a whole. It has become somewhat rare to see such comparisons in recent years despite the continued relevance for theories of episodic memory. Why has the field become increasingly focused on single memory tasks? While we cannot answer this question with precision, our suspicion is that this change in focus came about due to several findings in recognition memory, free recall, and serial recall that challenged the global memory models and led to their abandonment. Researchers have built models that have overcome these challenges but have often omitted consideration of how these models could be extended to other memory tasks. In this chapter, we present an overview of the challenges that arose, brief discussions of the current models, along with a prospective for unifying the various memory tasks. Interested readers are encouraged to consult the original publications of these models. In this review, we placed somewhat more emphasis on the serial recall task than the others as memory for serial order has received little attention from long term memory researchers. We hypothesize that these challenges can be accommodated within the framework of the global memory models with only minimal revision to the core assumptions of these models.