Etapas metodológicas

The hippocampus and parahippocampus are part of the larger medial temporal lobe (MTL) memory system that is involved in declarative memory formation (semantic and episodic memory). Given the critical role of the MTL area in the creation of new episodic memories, this brain region is a clear contender to be involved in certain age-related episodic memory deficits.

While the MTL region is anatomically less susceptible to ageing than the frontal lobes, age-related volume decreases have also been noted in this area (for a review, see Van Petten et al., 2004). Significantly, evidence has indicated that this volume decrease is associated with episodic memory impairment. Golomb et al. (1994) and Raz (2000) reported significant positive correlations between hippocampal volume loss and memory decline in the elderly, and longitudinal studies have shown hippocampal

reduction to be an indicator of memory impairment (De Leon et al., 1997; Golomb et al., 1996). However, other research has reported negative correlations between hippocampal volume loss and memory decline in ageing (Sullivan et al., 1995; for a meta analysis see Van Petten, 2004), showing that the evidence regarding the role of hippocampal reduction on episodic memory is not consistent.

Importantly, neuropsychological findings indicate that the contributions of the MTL and the frontal lobes to age-related memory impairment are distinct. Glisky et al., (1995) clustered older adults into high and low groups depending on their scores on tests of MTL and frontal functioning. The low MTL functioning group performed poorly on a simple item recognition test, whereas the low frontal group were unimpaired. In contrast, the low MTL group performed well on a source memory task, while the low frontal group showed a significant deficit. The dual process theory interpretation of these findings is that the MTL formation is principally involved in familiarity, whereas the frontal lobes are involved in recollection. The familiarity view of the MTL

contribution to memory decline is corroborated by an account that older adults with high MTL functioning were better than their low MTL counterparts at discriminating between lures showing differing levels of similarity to target stimuli (Rubin et al., 1999). However, reduced MTL functioning may also be related to impaired recollection: positive correlations have been noted between MTL test scores and performance on cued-recall tasks (Winocur et al., 1996) and memory for context (Henkel et al., 1998).

A main drawback of neuropsychological testing is that it can only provide indirect evidence of the association between the MTL formation and age-related memory

changes. In contrast, haemodynamic neuroimaging has permitted direct observations of alterations in MTL activity during episodic retrieval tasks, and the weight of evidence

suggests that the elderly rely more on familiarity. For example, in an fMRI study of memory and ageing, Springer et al. (2005) construed a link between an increased MTL BOLD response in the elderly and weak recognition performance as reflecting a greater reliance on familiarity (for comparable results from a PET study of cued-recall, see Backman et al., 1997). In addition, in a remember/know recognition test, the elderly produced a reduced hippocampal BOLD signal to correctly identified old words, along with an increase in parahippocampal activity (Cabeza et al., 2004). Because, in

comparison to the young group, the elderly also produced more know responses (even though overall accuracy was age equivalent), the authors concluded that the pattern of activation reflected the older adults’ increased reliance on familiarity.

2.4.1 Regional Account vs. Network Account of Age-Related Episodic Memory Decline

The aforementioned research, implicating age-related changes in the frontal lobes and the MTL system in episodic memory decline, assumes a regional account. According to this account, cognitive ageing is limited to separate brain areas. For example, a

reduction in right PFC activation during episodic memory retrieval in older adults only reflects an ageing change in the right PFC (Cabeza, 2002). The alternative network account proposes that a network of inter-connected brain areas mediates cognitive performance, with ageing influencing not only the function of separate brain regions, but also the myelinated connections between them (Greenwood, 2000). According to this account, therefore, a reduction in PFC activation during episodic memory retrieval in older adults could reflect an ageing change in the right PFC alone, or it could reflect changes in the connection between brain areas.

Evidence for the network account comes from reports of collaborative MTL and frontal activations during the encoding and retrieval of object identity and location only in younger adults (Schiavetto et al., 2002). The older group appeared unable to recruit these encoding and retrieval networks, and alternatively showed decreased MTL

activations, along with increased PFC activations. In addition, using structural equation modelling during episodic encoding and retrieval tasks, Cabeza et al. (1997) showed age-related changes in connectivity both within the PFC and between the PFC and other brain areas.

2.5 Conclusion

The detrimental effect of ageing on working memory and, in particular, episodic memory has been well reported. As working memory is important for monitoring the retrieval of information from episodic memory, ageing studies have suggested a close relationship between working memory and episodic memory. However, the effect of normal, healthy ageing on memory can be influenced by common health problems that occur with ageing, such as depression. These health problems must therefore be controlled for where possible before investigating the effects of normal ageing on memory.

The episodic memory literature shows that while the elderly are reasonably good at simple item recognition tasks, they show marked impairments in source or context memory tasks. The dual process theory proposes that the age-related decline in source memory occurs because recollection becomes impaired with ageing, resulting in an increased reliance on familiarity. The speed of processing theory, reduced inhibition theory, reduced processing resources theory and the frontal lobe theory provide

additional accounts, suggesting that age-related changes in episodic memory occur due to a reduction in the speed at which many cognitive processes operate, a reduced ability to suppress goal-irrelevant information, a reduction in the availability of attentional resources and changes in both the neuroanatomy and neurochemistry of the frontal lobes, respectively. Importantly, these theories are not in competition, and are likely to operate together to produce age-related memory impairments. The frontal lobe theory has proved particularly influential due to combined neuroanatomical and

neuropsychological evidence that relate cognitive changes to neural changes in the frontal lobes. Despite the importance of the frontal lobes to episodic memory (Wheeler et al., 1995; 1997), and in particular to the retrieval of source information, the MTL system is nonetheless widely considered to be fundamentally involved in episodic memory (Eichenbaum et al., 1994; Aggleton and Brown, 1999; Fortin et al., 2004).

While some neuroimaging studies have suggested that the age-related anatomical changes in the frontal lobes are associated with lower frontal activation in older adults during episodic retrieval, the more common finding is more widespread and often equivalent levels of activation. In particular, PFC activation may be more bilateral in elderly adults compared to young adults, and although the involvement of the

hippocampus in recognition memory may show an age-related decrease, parahippocampal activation seems to increase with age.