Outstanding questions of PMN function

Our understanding of the PMN is still in its infancy. The hypothesis forwarded by Gilmore et al. (2015) reviewed a large area of the literature, but insufficient time has lapsed to begin to adequately test the novelty/familiarity hypothesis. I will now identify several

outstanding questions related to this network. These questions are meant to be answered by work described in later chapters of this dissertation, but by no means should this be considered an exhaustive list of “known unknowns” (for other outstanding questions related to PMN function, see Gilmore et al., 2015)

1.8.1 How does the PMN behave under incidental memory conditions?

All of the work that has been considered to formulate the novelty/familiarity hypothesis was conducted under explicit memory conditions. That is, participants were always either intentionally encoding materials in preparation for a later test, were always explicitly attempting to retrieve materials on a test, or both. However, we have hypothesized that neither explicit encoding nor retrieval is a necessary condition for deactivation to novelty or activation to familiarity. It seems that some amount of activation observed within the PMN can be driven by task, but in general it appears as though one can expect at least weak activation even in the absence of any explicit retrieval condition. Of course, it may well be the case that deactivations are not observed if participants are never directed to encode an item, and we may not observe activations within the PMN if participants were never instructed to consider their past experience with an item. To address this, one can easily invoke the logic used in incidental memory tasks, such as those used to study priming. In such cases, participants are asked to make decisions without being directed to their prior experiences, yet those very experiences can alter participant behaviors. For example, participants routinely show faster response times when repeatedly making judgments about the same stimulus (Schacter, 1987; Roediger, 1990; Roediger &

McDermott, 1993). This is a phenomenon known as behavioral priming, and can be

accomplished without explicit orientation to a previous time in which a given decision was made.

In this dissertation, I investigate how PMN regions respond during an implicit memory task. Participants were shown pictures of faces, pictures of scenes, or words, and were asked to make a simple semantic decision about each stimulus. Each stimulus was presented three times, and the semantic task was identical in each presentation. BOLD activity was measured for each presentation of each stimulus type. Results suggest that implicit memory conditions continue to exhibit initial deactivations and repetition enhancement within the PMN, but that the degree of activation appears to be very low or non-existent upon repeated exposures. The procedures employed and specific results of this experiment are discussed in Chapter 2.

1.8.2 Can training affect PMN responses?

An interesting approach to understanding the function of a system is to look at “outlier individuals.” For instance, a great deal was learned regarding the function of the medial temporal lobes by studying patients such as HM or KC, who had severe damage to these regions (Scoville

& Milner, 1957; Tulving, 1985). On the other end of the spectrum are individuals who have highly superior memory abilities. For some of these individuals, this appears to be a natural ability, as in the case of those with Highly Superior Autobiographical Memory (LePort et al., 2012). For others, superior memory abilities are learned. Such is the case with “memory athletes” who have extensively trained in the use of visual imagery to rapidly encode (and later retrieve) large amounts of information extremely quickly. These individuals participate in highly competitive national and international tournaments (e.g., The USA Memory Championships, http://www.usamemorychampionship.com; The World Memory Championships,

http://www.worldmemorychampionships.com) to fight for the title of “best memorizer.”

Surprisingly little research has been conducted on memory athletes in general, and even less has occurred in the context of cognitive neuroscience. A notable exception is work

conducted by Maguire et al. (2002), who found that athletes performed normally on standard cognitive batteries, did not have enlarged or shrunken brain structures, and were “special” only insofar as they used more effective strategies than normal individuals. In the context of PMN research, studying memory athletes may provide a valuable means of determining if the fundamental processing that occurs within the PMN can be altered in some way by

“overtraining” encoding or retrieval processes. One might hypothesize, for instance, that memory athletes should show particularly large levels of repetition enhancement, as they have trained themselves to be very effective encoders and are often able to retrieve information very vividly as a consequence of the techniques they use.

In this dissertation I present data from a single memory athlete who was asked to perform the same task described in Chapter 2. Behavioral and neural responses during the task are

compared to a control group consisting of the other participants who participated in this research.

In general, at least with respect to the implicit memory task, the athlete was generally within the range of normal responses both behaviorally and neurally. More complete results of this

comparison are presented in Chapter 3.