Changes in cerebral lateralization in aging

Decoding episodic memory in ageing: a Bayesian analysis of activity patterns predicting memory

Normal ageing is associated with a decline in episodic memory, and neuroimaging studies in older adults have shown reduced activity in prefrontal cortex and other regions critical for memory function in the young. However, older adults also activate additional regions, suggesting a degree of functional reorganisation that has been attributed variously to detrimental and adaptive changes. Evaluation of these competing hypotheses depends critically upon inferences about the relative location and distribution of activity that are not well supported by current univariate or multivariate analyses. Here, we employed a recently developed model-based multivariate 'decoding' approach (Friston et al., 2008) to re-analyse a rich episodic encoding dataset and examine directly how the patterns of activity change in ageing. We assessed which spatial activity patterns, within lateral prefrontal cortex, best predict successful memory formation. Bayesian model comparison showed that the older adults had more distributed and bilateral (fragmented) predictive patterns of activity in anterior inferior frontal gyrus and middle frontal gyrus. With this direct multivariate test for changes in patterns of activity, we replicate and extend earlier findings of reduced prefrontal lateralisation in ageing. These findings extend conclusions based on conventional analyses, and support the notion that ageing alters the spatial deployment of neuronal activity, to render it less spatially coherent and regionally specific. This greater distribution of activity in older adults was also linked to poorer individual memory performance, suggesting that it reflects neural ageing, rather than adaptive compensatory responses.

Age-related changes in hemispheric asymmetry depend on sex

A total of 92 participants, 50 younger (mean age 26.3 years) and 42 older (mean age 63.8 years), were tested for visual-field asymmetries. On a word-matching task, a right-visual-field (RVF) advantage increased with age, consistent with the theory that right-hemispheric function shows relatively greater decline with age than left-hemispheric function. On a figural-comparison task, a left-visual-field (LVF) advantage was marginally decreased with age in the men, but significantly increased in the women, probably because age-related changes in hormonal levels are more pronounced in women. This increase in LVF advantage is contrary to both the HAROLD theory that hemispheric asymmetry declines with age, and the theory of relative right-hemispheric decline.

Hemispheric asymmetry and aging: right hemisphere decline or asymmetry reduction

We review evidence for two models of hemispheric asymmetry and aging: the right hemi-aging model, which proposes that the right hemisphere shows greater age-related decline than the left hemisphere, and the hemispheric asymmetry reduction in old adults (HAROLD) model, which proposes that frontal activity during cognitive performance tends to be less lateralized in older than in younger adults. The right hemi-aging model is supported by behavioral studies in the domains of cognitive, affective, and sensorimotor processing, but the evidence has been mixed. In contrast, available evidence is generally consistent with the HAROLD model, which is supported primarily by functional neuroimaging evidence in the domains of episodic memory encoding and retrieval, semantic memory retrieval, working memory, perception, and inhibitory control. Age-related asymmetry reductions may reflect functional compensation or dedifferentiation, and the evidence, although scarce, tends to support the compensation hypothesis. The right hemi-aging and the HAROLD models are not incompatible. For example, the latter may apply to prefrontal regions and the former to other brain regions.

Hemispheric asymmetry reduction in older adults: the HAROLD model

A model of the effects of aging on brain activity during cognitive performance is introduced. The model is called HAROLD (hemispheric asymmetry reduction in older adults), and it states that, under similar circumstances, prefrontal activity during cognitive performances tends to be less lateralized in older adults than in younger adults. The model is supported by functional neuroimaging and other evidence in the domains of episodic memory, semantic memory, working memory, perception, and inhibitory control. Age-related hemispheric asymmetry reductions may have a compensatory function or they may reflect a dedifferentiation process. They may have a cognitive or neural origin, and they may reflect regional or network mechanisms. The HAROLD model is a cognitive neuroscience model that integrates ideas and findings from psychology and neuroscience of aging.

Age-related evolution of the contribution of the right hemisphere to language: absence of evidence

The age difference observed between Wernicke's and Broca's aphasics has been understood by some authors as an indication of a progressive diminution of the contribution of the right hemisphere to language throughout the life span. To test this hypothesis, 24 right-hemisphere-damaged (RHD) and 24 normal control adults were submitted to six tasks looking at different aspects of language abilities. Results showed that RHDs performed less well than normal subjects on 3 of these tasks, but that this difference was not linked with age (younger than 55 versus older than 65 years). Consequently, these results do not support models of functional brain organization suggesting a decreasing contribution of the right hemisphere to language abilities with age.

Performance on motor tasks as an indication of increased behavioral asymmetry with advancing age

Age-related asymmetrical functional decline was tested on a sample of 64 right-handed volunteers between 60 and 64 years of age who were free from neurological illnesses and physical handicaps. Increase in functional asymmetry was explored by examining performance indexes for each hand and superiority of the dominant hand on motor tasks of different complexities: the Finger Tapping Test, the Grooved Pegboard Test, and the Pin Test. Our study revealed an increase in superiority of the right hand with age on a highly demanding task (Pin Test). This finding is discussed in light of the hypothesis of a decline in callosal functioning with age and the alternative hypothesis of a greater vulnerability of the right hemisphere in the elderly.

Symmetrical and asymmetrical changes in brain tissue with age as measured on CT scans

Age-related changes in mean CT attenuation values of brain tissue were analyzed by an objective, automated technique. Study participants included 64 healthy men ranging in age from 31 to 87 years. Five CT slices from each individual were evaluated: the last ventricular slice and the next four supraventricular slices. Regression analyses indicated that specific regions in the brain change with age. A schematic reconstruction of these regions suggests that several brain regions are selectively altered with age. On the medial aspect of the cerebral hemisphere there was bilaterally symmetrical atrophy of the cingulate gyrus and sulcus, the adjacent interhemispheric frontal gyri, the parietooccipital sulcus, and the marginal branch of the cingulate sulcus. On the convexity of the cerebral hemispheres there was asymmetrical widening of the central and postcentral sulcus on the left, and the intraparietal sulcus on the right.

Lateralization for the expression and perception of facial emotion as a function of age

In order to test the hypothesis of right hemisphere changes with age, this study examined lateralization for facial emotion in young, middle-aged, and older women (N = 90). For expression, subjects were photographed while posing positive and negative emotions. Composite photographs were created and rated for intensity. For perception, subjects were required to make intensity judgements about emotional chimeric faces. Overall, subjects demonstrated significant left-sided facial asymmetry for expression and significant left hemispace biases for perception. The findings for facial expression were not influenced by emotional valence or resting face asymmetries. There were no changes in lateralization as a function of age for either expression or perception. Taken together, these findings lend support to the notion that the right hemisphere mediates emotional processing across the adult life span.

Changes in cerebral functioning associated with normal aging

It has been suggested that the normal aging process is characterized by a pattern of neuropsychological performance decline that implies relatively greater vulnerability of right-hemisphere functions. This hypothesis was tested in a sample of 68 volunteers aged 20-75 who were free of systemic and neurologic illness. Neuropsychologic measures of lateralized and focal function were specifically selected to eliminate systematic procedural differences among tests (e.g., timed vs. untimed, overlearned vs. unfamiliar). Inferences about the localizing significance of each measure were based on previously demonstrated double dissociation of function in lesion studies. Results suggested that declines in cerebral efficiency are not differentially lateralized. Age correlated performance changes implied bilateral reduction that was significantly more pronounced on operations associated with frontal-lobe function. Anatomic and theoretical explanations for this pattern were discussed.

Hemisphere asymmetries, spelling ability, and classroom seating in fourth graders

Fourth grade children (N = 159) from eight classes who were free to select their seating location in the classroom were tested for spelling ability. On 100 word spelling lists, the children (males and females) seated on the right side of the classroom (facing the teacher) were superior to children on the left. Analysis of spelling errors for reliance on phonological processing in a subsample (N = 28) revealed that right sitters made more phonetically inaccurate misspellings, whereas, on the left, females, but not males, committed more phonetically accurate misspellings. The results support the view that hemispheric information processing is correlated with classroom seating location or preference, but casual inferences regarding the neuropsychological mechanisms underlying these findings are unwarranted pending clarification of several major theoretical issues.

Sex-linked left lateralized central processor for hierarchically-structured material? Evidence from Broca's aphasia

Based on a study with seven nonfluent aphasics, Grossman (Neuropsychologia 18, 299-308, 1980) advanced the 'central processor' claim that the mental representation of hierarchical linguistic structure is functionally equivalent to processing hierarchically-structured, nonlinguistic forms. Grossman reported that both abilities are compromised in Broca's aphasia. We attempted to replicate this effect with eight nonfluent aphasics using essentially the same task requiring the reconstruction from memory of stick designs. With the addition of controls for post-stroke interval, responding hand and gender, only the female aphasics had difficulty executing the hierarchical arrangement of visual-spatial materials. This sex difference disputes the strong form of the central processor hypothesis and supports the view that females may be more likely to possess partial noncomplementarity of specialization (dependence on the same hemisphere for selective language and spatial functions).