A longitudinal investigation of individual variability in brain activity during episodic encoding and retrieval

In general, the strategic nature of memory has been ignored in existent neuro-cognitive models of episodic memory, resulting in a static and possibly incomplete view of a dynamic declarative memory system. A complete discussion of the cognitive neuroscience of human memory necessitates consideration of the strategic processes that are involved and the processes that are variably recruited to support memory at the individual level. To address this issue, this investigation systematically assessed the structural and cognitive factors that give rise to the extensive variability that has been observed in brain activity using functional neuroimaging. Of particular interest was to what degree variability in brain activity is related to differences in cognitive strategy and further, how is this variability related to variability in anatomy, physiology and other possible sources of individual differences? Three experiments were conducted to address this question. All 3 experiments used a similar paradigm in which participants were scanned while they intentionally encoded lists words and completed subsequent recognition memory tests. After scanning, subjects completed a number of questionnaires to assess strategy, cognitive style and personality. In the first experiment, participants studied high imagery words, which allow the use a variety of encoding strategies. In Experiment 2, low imagery words were included as well, which restrict the number of strategies that can be used. In Experiment 3, twelve subjects from Experiment 2 completed 4 more sessions identical to their first. Together, these experiments showed that a number of factors give rise to differences in brain activity including connectivity, coherence of resting state activity, visual/verbal cognitive style, performance and personality. Furthermore, there was strong evidence that different strategies are related to neurally distinct activity patterns. First, it was shown that differences in strategy predict activity differences between individuals. Further, differences in strategy also predicted differences within an individual when strategy was implicitly manipulated. Finally, spontaneous changes in strategy over time also predicted differences within an individual. This lends support to the prediction that differences in strategy cause differences in brain activity.