Adaptive memory: Is survival processing special?

Do the operating characteristics of memory continue to bear the imprints of ancestral selection pressures? Previous work in our laboratory has shown that human memory may be specially tuned to retain information processed in terms of its survival relevance. A few seconds of survival processing in an incidental learning context can produce recall levels greater than most, if not all, known encoding procedures. The current experiments further establish the power of survival processing by demonstrating survival processing advantages against an encoding procedure requiring a combination of individual-item and relational processing. Participants were asked to make either survival relevance decisions or pleasantness ratings about words in the same categorized list. Survival processing produced the best recall, despite the fact that pleasantness ratings of words in a categorized list has long been considered a ‘‘gold standard” for enhancing free recall. The results also help to rule out conventional interpretations of the survival advantage that appeal to enhanced relational or categorical processing. 2008 Elsevier Inc. All rights reserved. Introduction ory evolved to be domain-general, or insensitive to conThe capacity to remember, to recover the past in anticipation of the future, almost certainly evolved (Darwin, 1859). Nature shaped the characteristics of our memory systems, primarily through natural selection, because fitness advantages accrued as a consequence of memory’s operation (see Klein, Cosmides, Tooby, & Chance, 2002; Tooby & Cosmides, 1992). Yet, to what extent do the operating characteristics of memory continue to bear the imprint of ancestral selection pressures? Are our memory systems ‘‘tuned” to achieve specific ends, particularly those related to survival and reproduction? Or, did memory evolve as an all-purpose machine, defined more by its flexibility than by its inherent constraints? Our laboratory has maintained that human memory likely does contain functional specialization (see Barrett & Kurzban, 2006). More specifically, we have suggested that memory is biased or tuned to remember fitness-relevant information (Nairne & Pandeirada, 2008; Nairne, Thompson, & Pandeirada, 2007). It is unlikely that mem. All rights reserved. airne). J. S., & Pandeirada, J.N.S 016/j.jml.2008.06.001 tent, because not all events are equally important to remember. For example, it is usually more important to remember the location of a food source, or a predator, than it is to remember random events. This is not to suggest that our brains come pre-equipped with contentspecific knowledge (e.g., edible versus inedible plants), but rather that fitness-relevant encodings are remembered particularly well. In support of this proposal, Nairne et al. (2007) found that memory was significantly enhanced relative to traditional deep processing controls (Craik & Lockhart, 1972) when random words were processed in terms of their relevance to a survival scenario. Participants were asked to imagine themselves stranded in the grasslands of a foreign land, without any basic survival materials, and to rate the relevance of words to finding steady supplies of food and water and protection from predators. Surprise free recall tests revealed an advantage for survival processing over a pleasantness rating task, typically considered to be one of the best deep encoding procedures (e.g., Packman & Battig, 1978), as well as over an alternative schematic control (moving to a foreign land) and to a condition requiring self-referential processing. More recently, we compared ., Adaptive memory: Is survival processing special?, Journal 2 J.S. Nairne, J.N.S. Pandeirada / Journal of Memory and Language xxx (2008) xxx–xxx ARTICLE IN PRESS survival processing to a host of deep processing controls— including forming a visual image, generation, and intentional learning—and survival processing produced the best recall (Nairne, Pandeirada, & Thompson, 2008). Mnemonic advantages for survival processing have now been demonstrated in other laboratories as well using alternative control scenarios (Kang, McDermott, & Cohen, in press; Weinstein, Bugg, & Roediger, 2008). For example, survival processing produced better memory than a control scenario involving the planning and execution of a bank heist (Kang et al., in press).The bank heist scenario was chosen to match the novelty and potential excitement of the survival scenario, something that may have been lacking in the moving control scenario used by Nairne et al. (2007). Our laboratory has also found survival advantages compared to scenarios in which (a) people were asked to imagine themselves vacationing at a fancy resort with all of their needs taken care of, (b) eating dinner at a restaurant, and (c) planning a charity event with animals at the local zoo (Nairne & Pandeirada, 2007; Nairne et al., 2007; Nairne et al., 2008). At this point, we believe, appealing simply to the schema-like properties of the survival scenario, or to its coherence or novelty, is unlikely to explain these advantages. Instead, these experiments support the hypothesis that it is the fitness-relevance of the processing that is important to memory. Information encoded as a consequence of fitness-based processing is especially accessible and memorable—more memorable, in fact, than that produced by most (if not all) known encoding procedures, at least when free recall is used as the retention measure. At the same time, these experiments have revealed very little about the proximate mechanisms that actually produce the survival benefit. Is survival processing special, arising from the action of some kind of special mnemonic adaptation, or can we explain the advantage using traditional explanatory tools? For example, one might claim that survival processing is simply another form of ‘‘deep processing”, albeit a particularly good one, leading to enhanced elaboration or distinctive encodings (see Hunt & Worthen, 2006). Another possibility is that rating words for survival mimics a categorization task. Perhaps participants essentially encode the rated words into an ‘‘ad hoc” category representing ‘‘things that are relevant in a survival situation.” Once primed by the rating task, the category structure could support an accessible and efficient retrieval plan (Tulving & Pearlstone, 1966). Put more generally, one can conceive of the survival rating task as inducing a form of relational processing. As people rate the items, they process ostensibly unrelated words along a common dimension of similarity—relevance to a survival context. It is well-established that relational processing of unrelated items leads to improved free recall, partly because the encoded dimension of similarity helps to restrict the set of possible recallable items at the point of test (Hunt & McDaniel, 1993; Nairne, 2006). Note this is a completely conventional account of the survival advantage: Survival ratings induce people to encode target items into a categorical structure that is particularly accessible during retrieval. Please cite this article in press as: Nairne, J. S., & Pandeirada, J.N.S of Memory and Language (2008), doi:10.1016/j.jml.2008.06.001 Such an account generates an obvious prediction: If the to-be-rated words are inherently related (e.g., if the list is categorized) then any relational processing induced by the survival rating task should be less useful to retention (see Burns, 2006; Mulligan, 2006). A number of studies have found that relational processing of items in a related list, such as sorting items from an obviously categorized list into categories, produces no particular mnemonic advantages, at least when compared to identical processing of words in an unrelated list (e.g., Burns, 1993; Einstein & Hunt, 1980; Hunt & Einstein, 1981). The category structure inherent in the list affords a sufficient retrieval ‘‘plan” for use in recall (e.g., the list contained pieces of furniture, weapons, and so on) so further relational processing is redundant (although see Engelkamp, Biegelmann, & McDaniel, 1998). In fact, encoding procedures that draw attention to the unique characteristics of the to-be-recalled items (such as rating items for pleasantness or familiarity) promote the best recall when lists are categorized. The list structure enables one to restrict the target search set effectively, and the individual-item processing helps one discriminate items within the search set that did or did not actually occur on the memory list (see Nairne, 2006). Researchers have been able to explain a variety of mnemonic phenomena by appealing to trade-offs between item-specific and relational processing (e.g., Hunt & Seta, 1984; Klein, Kihlstrom, Loftus, & Aseron, 1989; Klein & Loftus, 1988; Mulligan, 1999; Mulligan, 2001); it is certainly possible that similar logic can be used to explain the advantages seen after survival processing. The current experiments were designed to test these ideas, as well as to compare the mnemonic value of survival processing against yet another powerful encoding procedure: Individual item processing of words presented in a categorized list. In all three experiments, participants were asked to make rating decisions about words in a categorized list prior to a surprise free recall test. Experiment 1 used a between-subject design to compare the effects of survival processing to a prototypical individual-item processing task—rating items for pleasantness. Experiment 2 replicated the results of Experiment 1 using a within-subject design. Finally, in Experiment 3 a non-fitness-relevant scenario, vacationing at a fancy resort in a foreign land, was used instead of the survival scenario.