No abstract

Autobiographical memory comprises episodic and semantic components mediated by dissociable states of consciousness, one promoting the experience of the self at a specific moment in the past, and the other involving self-knowledge that does not require ''mental time travel.'' These components can be difficult to dissociate using retrospective autobiographical stimuli collection. In this study, we manipulated the episodic/semantic distinction within prospectively collected autobiographical stimuli. Over several months, participants made recordings documenting specific episodes, repeated episodes, and world knowledge. These recordings were later played back to participants during scanning with functional MRI. The results indicated overlapping but distinct patterns of brain activity corresponding to episodic and semantic autobiographical memory. Both episodic and semantic autobiographical memory engaged the left anteromedial prefrontal cortex associated with self-reference, but the episodic condition did so to a greater degree. The episodic condition uniquely engaged the medial temporal, posterior cingulate, and diencephalic regions associated with remote memory recovery. Whereas the episodic condition engaged the right temporo-parietal cortex involved in reconstruction of spatial context and attentional orienting, the semantic condition engaged the left temporo-parietal and parieto-frontal systems involved in egocentric spatial processing and top-down attentional control. Episodic recollection was also associated with suppression of emotional paralimbic regions. These findings support a functional neuroanatomical dissociation between episodic and semantic autobiographical memory, providing concordance to amnesic syndromes with disproportionate impairment in one of these two forms of autobiographical memory.

Voxel-based morphometry (VBM) was used to examine the relation between age and gray matter density cross-sectionally and to study the association between gray matter density and longitudinal decline in performance on cognitive tests in healthy, non-demented elderly individuals. Participants were neuropsychologically tested at baseline and again after 3 years. Thirty-seven subjects (mean age 72.5 years) who showed a decline in cognitive test performance at follow-up were compared with 38 individually matched control subjects (mean age 71.8 years) whose performance did not change over time. Magnetic resonance imaging scans were acquired at follow-up and individual differences in regional gray matter density were examined with VBM. The largest age effects were found in various regions in the prefrontal cortex, the (medial) temporal lobes and the striate cortex. Longitudinal cognitive decline was associated with decreased gray matter density in prefrontal areas, the (medial) temporal lobes and the posterior parietal cortex. These findings suggest that prefrontal and temporal cortical regions are of particular relevance both in aging and age-related cognitive decline in healthy elderly individuals.

Normal ageing is associated with a wide variety of disturbances in the structure and function of the human brain. Recent neuroimaging studies suggest that the prefrontal cortex (PFC) is particularly vulnerable to the effects of ageing. These findings are compatible with the so-called 'frontal ageing hypothesis' which has been formulated on the basis of neuropsychological research on non-pathological ageing. We will argue on the basis of recent structural and functional neuroimaging studies that this hypothesis needs to be refined, especially to acknowledge the possible relevance of a distinction between subregions within the PFC. In addition, findings with regard to a differential involvement of grey versus white matter suggest that both have to be considered in relation to age-related cognitive decline. Hence, neural networks and larger systems of interconnected brain regions and the functional activity in these circuits may be more important than specific cortical regions to explain age effects on cognitive functioning. Finally, it is important to consider individual variability due to sex differences and age-extrinsic biomedical factors in research which examines the relationship between brain structure or function and cognitive ageing.