Conclusions and thesis outline

While the evidence for sleep based consolidation suggests a rich and multifaceted role in the processing of declarative memories the sheer diversity of studies exploring the phenomenon brings with it a risk of fragmentation in the literature. As the sleep

psychologist Robert Stickgold notes: “[…] one has to address the question of how five stages of sleep interact with at least six types of memories and six stages of post-encoding memory processing, for a combined total of 144 distinct sub-questions” (Stickgold, 2009, p.

305). The present thesis therefore brings together several of complementary approaches to consolidation outlined in this review to explore how the compatibility of new information in relation to prior-knowledge will influence its subsequent integration.

1.5.1 Research questions

Compatibility

The role of prior knowledge in consolidation will be explored by training materials in a manner that renders them congruent or incongruent with what is already known. Such a manipulation permits the delineation of potentially distinct integratory processes. For example, information that is congruent with pre-existing knowledge, according to the CLS and other models, should become rapidly consolidated, and should measurably benefit from the assimilative and abstractive qualities outlined in the previous sections (McClelland et al., 1995; Stickgold & Walker, 2004; Walker & Stickgold, 2010).

However, it is not clear from present theoretical and experimental accounts how consolidation operates on information that is incongruent with what is already known. It may be, for example, that such information will become restructured to conform to pre-existing schematic concepts through systems-level consolidation. This possibility fits well with notions in the present literature that consolidation aids in the reorganisation of new information to enhance its predictive value (Stickgold & Walker, 2013) as well as the recuperative benefits such processes have on more weakly encoded information, as noted in section 1.3.5. Alternatively, incompatible information may be more susceptible to rapid decay (i.e. forgetting) or actively suppressed through synaptic downscaling, active

consolidation or both (Lewis & Durrant, 2011).

Mechanisms

The second issue the present thesis relates to the mechanisms underlying mnemonic compatibility and consolidation. A full exploration of the potentially divergent routes of consolidation outlined above necessitates a complementary exploration of sleep physiology.

47 Although the field of sleep and memory research has been highly productive, the delineation of sleep specific benefits in this regard has sometimes produced divergent results (Smith 2001). SWS has been associated with both the suppression (Payne et al., 2009) and enhancement (Plihal & Born, 1997) of semantically compatible information, while both REM sleep and SWS have been linked with the more integratory dimensions associated with memory consolidation (Lau et al., 2010; Stickgold, Scott, Rittenhouse, &

Hobson, 1999). Similarly, underlying sleep spindle activity has been associated with multiple stages of consolidation including declarative enhancement (Cox et al., 2012; van der Helm et al., 2011) and mnemonic integration (Tamminen et al., 2010). Thus, while there is overwhelming evidence that sleep facilitates memory consolidation there is a need for experiments that attempt to distinguish the relative contributions of the sleep state in relation to some of these diverse observations.

It follows that information that is compatible with prior-knowledge should display the greatest relationship with SWS and its affiliated spindle activity, as these aspects of sleep physiology most directly relate to systems-level integration. However, as with the issue of incompatibility itself, it is not clear what mechanisms associated with sleep may be attributed to the processing of incompatible information. If this information is demoted through synaptic downscaling we may expect a negative relationship between the retention of this information and SWS. Alternatively, if systems-level dynamics aid in the restructuring and assimilation of incongruous information the resulting changes in memory should be positively associated with markers of systems-level integration during sleep. A third possibility is that the incongruent nature of such information will preclude any involvement of sleep-associated consolidation as it does not usefully contribute to predictive semantic memory. Here we might observe the rapid forgetting of incongruous information and no relationship with sleep physiology. These possibilities are more clearly outlined in the experimental chapters that follow.

1.5.2 Experimental chapters

To address these questions the present thesis contains five exploratory investigations across four experimental chapters. Broadly speaking these chapters will alternate between the two goals of this thesis: an exploration of (i) memory compatibility and consolidation, and (ii) the sleep-associated mechanisms involved with these processes.

While Experiments 1 and 2 (chapters 2 and 3) focus on the role of abstraction, assimilation and consolidation, Experiments 3, 4 and 5 (chapters 4 and 5) probe the role of memory compatibility in the recovery of memories from interference. All of these experiments include multiple measures of integration in an attempt to capture some of dynamic changes that may occur to memory.

48 Experiment 1 implemented a modified transitive inference paradigm to explore the role of time on consolidation and semantic compatibility (Bryant & Trabasso, 1971;

Ellenbogen et al., 2007). The experiment introduces a semantic dimension of size overlayed on premise pairs forming the underlying schematic hierarchy (e.g. ‘A>B>C>D>E>F’). Real world items were interleaved with novel information during training which ran counter, or in line with, expected size relationships. Such a manipulation permitted useful comparisons between novel and pre-existing items in transitive judgments (e.g. ‘B>D’, and ‘E>F’).

Furthermore, the introduction of a semantic component to this task allowed generalisation measures to be formed in relation to additional real world items presented at test. While Experiment 1 explored consolidation in participants after 20 minutes (immediate) or 24 hours (delayed) Experiment 2 took elements of this design on to a 12:12 overnight sleep study monitored by PSG.

Experiments 3, 4 and 5 follow a parallel line of investigation by exploring the role of semantic compatibility in the recovery of memories from retroactive interference. Here, in a similar approach to Experiments 1 and 2, a ‘classic’ A-B, A-C retroactive interference paradigm was modified by introducing an expected relationship of size across lists

(Koppenaal, 1963). Novel items (‘A’) were interleaved between two real word items (‘B’ or

‘C’) to create word triplets across lists that formed congruent or incongruent size relationships. This manipulation permitted an exploration of how the differential congruency of learned information may enhance or block the consolidation-associated recovery from retroactive interference (Drosopoulos, Schulze, et al., 2007). Experiments 3 and 4 explore this factor across 24 hours using immediate and delayed groups, with the night of sleep for delayed participants recorded using portable EEG devices. Experiment 5, as with Experiment 2, took this design on to an overnight study featuring PSG.

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