Evidence has accumulated to suggest that normal ageing is associated with a decline in accuracy of the source of information retrieved from episodic memory (e.g. see
McIntyre and Craik, 1987; Schacter et al., 1981). The retrieval of source information requires recollection of contextual details surrounding the event, such as when and where the event occurred, along with executive functions, such as working memory, strategy use and planning, to monitor the contextual information retrieved. Due to the complexity of retrieving source information, strategic retrieval is required to maximise performance. Strategic retrieval is the retrieval of episodic information by
implementing a self-initiated process (Dzulkifli and Wilding, 2005).
While a decline in recollection with age has been widely linked to older adults source memory deficit (e.g. Dywan and Jacoby, 1990; Jennings and Jacoby, 1993), the role of executive functions, which also show an age-related reduction (e.g. see Moscovitch and Winocur, 1995), is less clear. Neuroimaging studies of young adults have provided considerable evidence that different regions of the prefrontal cortex, which sub-serves our executive functions, are active during episodic memory retrieval, possibly reflecting the engagement of various executive functions (e.g. Nolde et al., 1998; Wagner et al., 1998; Buckner and Wheeler, 2001; Wheeler and Buckner, 2003). One of the aims of the thesis was to establish if there is a relationship between the activation of different regions of the prefrontal cortex during source memory retrieval, and specific executive
functions, and whether this relationship is different for young and older adults. This approach will further our understanding of the role of executive functions in the
retrieval of source information from episodic memory in young adults. In addition, the approach will also help to understand if an age-related reduction in executive
functioning results in the engagement of less effective retrieval strategies, and contributes to the source memory deficit evident in older adults.
The thesis used neuropsychological profiling to assess four executive functions (working memory, strategy use, planning and flexibility of thinking and responding), along with non-executive short-term memory, and involved completing four tests from the Cambridge Automated Neuropsychological Test Battery (CANTAB): Spatial Span, Spatial Working Memory, Stockings of Cambridge, and the ID/ED Set-Shifting task.
The Spatial Span task is a non-executive task of spatial short-term memory capacity, while the Spatial Working Memory task is an executive task that assesses working memory for spatial stimuli and strategy use. The Stockings of Cambridge task and the ID/ED Set-Shifting task are executive tasks that measure spatial planning and flexibility of thinking and responding respectively. To a lesser extent, the Stockings of Cambridge task also taps into working memory and behavioural inhibition. The CANTAB is a computerised battery of tests that can be broken down in to their cognitive components in order to provide a detailed view of the functions that are impaired and those that are spared. This reason, along with the fact that the battery has an unrivalled degree of validation, is why its use was favoured over manual tests such as the Wisconsin Card Sorting Test or the Tower of Hanoi.
In addition to neuropsychological profiling, ERP data was recorded during the completion of a source memory exclusion task, to investigate the neural correlates of episodic memory retrieval in young and older adults. The exclusion task was chosen
because the majority of ERP studies investigating strategic retrieval have used this task.
Pressing one button during the test phase for target items and a second for non-target and new items allows participants to implement various strategies to maximise performance, and is therefore ideal for investigating strategic retrieval. Existing evidence shows that young adults either restrict recollection to target items only or recollect both target and non-target items. Furthermore, monitoring retrieved
information is restricted either to targets only or to both target and non-target items. In contrast to young adults, older adults appear unable to inhibit recollection of non-target information, recollecting both targets and non-targets.
A further aim of the thesis was to investigate the effect of the context of source
information on strategic retrieval. Previous ERP studies using the exclusion task have adopted intrinsic contexts (information inherent to the studied word e.g. a male or female voice) or extrinsic contexts (information not inherent to the studied word e.g.
rate the word for pleasantness) without any consideration of the effects of this on their findings. Two reasons highlight that the type of context may have an influence on strategic retrieval. Firstly, a meta-analysis on the effects of aging on memory for content and context (Spencer and Raz, 1995) noted that the elderly had reduced
accuracy for extrinsic context (spatial features of stimuli) compared to intrinsic context (auditory or visual aspects of stimuli), suggesting that the retrieval strategies might differ across contexts. Secondly, the previous ERP studies using the exclusion task in young adults have shown a different, and fairly consistent, pattern of results for the retrieval of intrinsic and extrinsic contexts: recollection of target and non-target items, and monitoring of targets only, has been observed for the retrieval of intrinsic contexts (Wilding and Rugg, 1997b; Cycowicz et al., 2001; Wilding and Sharpe, 2004). In contrast, recollection of targets only, and monitoring of targets and non-targets, has
been reported for the retrieval of extrinsic contexts (Herron and Rugg, 2003b; Dzulkifli and Wilding, 2005).