Behavioural and Electrophysiological Correlates of Source Memory in Normal Ageing
Scanlon, Páraic (2009) Behavioural and Electrophysiological Correlates of Source Memory in Normal Ageing. PhD thesis, National University of Ireland Maynooth.
Source memory recall involves remembering the context in which an event took place. Studies have shown that a decline in source memory is evident in older adults who have not shown deficits in other areas of memory or cognition. Using the interfering effect of intervening stimuli it is possible to create a task that tests source memory ability in a laboratory setting using sounds, pictures, words or a combination of these stimuli. Studies have shown that healthy young adults can be affected by such designs, exhibiting deficits in source memory when asked to recall whether a stimulus was previously studied as a target or is a novel stimulus; a distractor, or foil. By repeating novel stimuli throughout a test phase, participants may respond to the repetitions as targets, despite them not being present in the study phase. Since source memory seems to affect older adults before any other obvious signs of cognitive decline, any task that can measure the presence of these deficits could be useful in determining if a participant might suffer from future, more general, memory impairments. The current thesis uses such a task to measure source memory deficits, both behaviourally and electrophysiologically, in groups of young, middle-aged and old participants. The behavioural results show a similar pattern of decline in recall among healthy young and middle-aged participants for repeated words compared to their immediate presentation, while older participants show a lower level of recall, especially for repeated stimuli, compared to both the other age-groups. This suggests that the onset of decline in source memory beginning after the age of 60 in a sudden manner, rather than as a gradual decrease in function through the lifespan. The electrophysiological results show a pattern of greater frontal area activations for repeated stimuli over first-presentation, as well as later parietal scalp area differences between these stimuli in young participants. These differences were correlated with the prefrontal cortex and the medial temporal lobe, both of which show more activation for repetitions. Older participants do not show these activations to the same extent, with little early frontal positivity, and instead exhibit a greater degree of later parietal activity and a pattern of temporal and parietal area activations, suggesting that their decrease in source memory recall ability may be linked to inefficient use of prefrontal and medial-temporal resources. Finally, an examination on the effect of a deeper level of encoding during processes, with participants asked to concentrate on the semantic features of stimuli, showed that this ameliorates source memory deficits caused by the task in both young and old groups, with recognition for repeated words increasing to a level equal to that of immediate presentations. This is in line with examinations of other memory types, but had not been found to occur during such a repetition-based procedure previously in the literature. The complete results suggest a network of medial temporal and frontal areas combine to retrieve the source of a memory in younger participants, compensated for in part by older adults through the use of areas in the temporal and parietal areas, such as the precuneus.
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