Functional neuroimaging studies in normal aging

A Review of the Measurement of the Neurology of Gait in Cognitive Dysfunction or Dementia, Focusing on the Application of fNIRS during Dual-Task Gait Assessment

Poor motor function or physical performance is a predictor of cognitive decline. Additionally, slow gait speed is associated with poor cognitive performance, with gait disturbances being a risk factor for dementia. Parallel declines in muscular and cognitive performance (resulting in cognitive frailty) might be driven primarily by muscle deterioration, but bidirectional pathways involving muscle–brain crosstalk through the central and peripheral nervous systems are likely to exist. Following screening, early-stage parallel declines may be manageable and modifiable through simple interventions. Gait–brain relationships in dementia and the underlying mechanisms are not fully understood; therefore, the current authors critically reviewed the literature on the gait–brain relationship and the underlying mechanisms and the feasibility/accuracy of assessment tools in order to identify research gaps. The authors suggest that dual-task gait is involved in concurrent cognitive and motor activities, reflecting how the brain allocates resources when gait is challenged by an additional task and that poor performance on dual-task gait is a predictor of dementia onset. Thus, tools or protocols that allow the identification of subtle disease- or disorder-related changes in gait are highly desirable to improve diagnosis. Functional near-infrared spectroscopy (fNIRS) is a non-invasive, cost-effective, safe, simple, portable, and non-motion-sensitive neuroimaging technique, widely used in studies of clinical populations such as people suffering from Alzheimer’s disease, depression, and other chronic neurological disorders. If fNIRS can help researchers to better understand gait disturbance, then fNIRS could form the basis of a cost-effective means of identifying people at risk of cognitive dysfunction and dementia. The major research gap identified in this review relates to the role of the central/peripheral nervous system when performing dual tasks.

Hand asymmetries of tactile attention in younger and older adults

Background/Objectives: The allocation of attention can alter the perception of magnitude. When performing line bisections, young adults deviate leftward (pseudoneglect), a bias thought to be induced by right hemisphere dominance for allocating spatial attention. However, when performing body bisections young adults deviate rightward, suggesting left hemisphere dominance for allocating body-centered attention. With aging, there is a reduction of pseudoneglect thought to result from either an age-related decrease in right hemispheric functions (right hemi-aging) or from hemispheric asymmetry reduction in older adults (HAROLD). The goal of this study was to learn if there are tactile body-centered perceptual-attentional right-left asymmetries in that change with aging. Methods: The participants were younger and older healthy adults. Semmes-Weinstein Monofilaments were used to test for differences in the perceived magnitude of pressure differences between a reference stimulus and test stimuli applied to the right and left palmar thenar eminence. Results: Young adults perceived the magnitude of difference in the lightest pressure stimuli applied to the right hand as being greater than the older adults. Young adults perceived the magnitude of difference between the lightest pressure and reference applied to the right hand to be greater than the left hand, but older adults perceived a lighter stimulus greater on the left compared to the right. Conclusions: Whereas the right hemisphere appears to be dominant in mediating spatial attention, the left hemisphere may play a dominant role in the allocation of body-centered attention. Like the aging-related reduction of the visual perception of the magnitude (length) of the left side of a line, this tactile reduction in magnitude (pressure) perception in older adults suggests that with aging, there is a reduction of left-hemispheric mediated allocation of tactile attention, and this result is not fully consistent with either the HAROLD or hemi-aging models.

Dissociating Normal Aging from Alzheimer's Disease: A View from Cognitive Neuroscience

Both normal aging and Alzheimer's disease (AD) are associated with changes in cognition, grey and white matter volume, white matter integrity, neural activation, functional connectivity, and neurotransmission. Obviously, all of these changes are more pronounced in AD and proceed faster providing the basis for an AD diagnosis. Since these differences are quantitative, however, it was hypothesized that AD might simply reflect an accelerated aging process. The present article highlights the different neurocognitive changes associated with normal aging and AD and shows that, next to quantitative differences, there are multiple qualitative differences as well. These differences comprise different neurocognitive dissociations as different cognitive deficit profiles, different weights of grey and white matter atrophy, and different gradients of structural decline. These qualitative differences clearly indicate that AD cannot be simply described as accelerated aging process but on the contrary represents a solid entity.

Compensatory networks to counteract the effects of ageing on language

BACKGROUND

Word-retrieval difficulties are a common consequence of healthy ageing and are associated with a reduction in asymmetrical recruitment of the dorsolateral prefrontal cortex (DLPFC), although the significance of this reduction has not yet been clarified. Using repetitive transcranial magnetic stimulation (rTMS) it has been demonstrated that an asymmetrical involvement of the DLPFC during action naming in young subjects, whereas bilateral involvement was shown in elderly participants. By using rTMS during a naming task in a group of elderly subjects, the aim of the present work was to investigate whether the magnitude of DLPFC asymmetry (left-right rTMS effect) during action naming correlates with task performance, proving the presence of a compensation strategy in some but not all elderly participants.

METHODS

We aimed to test if there was a correlation between DLPFC asymmetry (left-right rTMS effect) and naming performance in a group of elderly subjects.

RESULTS

The results show that rTMS affects action naming differently according to individual naming ability. In particular, the predominance of a left vs. right DLPFC effect was observed only in the low-performing older adults, while an asymmetric reduction was selectively shown in the high-performing group. Interestingly, high-performing older adults also displayed better performances on a phonemic fluency test.

CONCLUSION

The present data suggest that successful ageing is linked to less prefrontal asymmetry, an efficient strategy for counteracting age-related declines in cognitive function.

Cortical correlates of self-generation in verbal paired associate learning

Behavioral studies have shown that verbal information is better retained when it is self-generated rather than read (learned passively). We used fMRI and a paired associates task to examine brain networks underlying self-generated memory encoding. Subjects were 49 healthy English speakers ages 19-62 (30 female). In the fMRI task, related word pairs were presented in a "read" condition, where subjects viewed both words and read the second word aloud, or a "generate" condition, where the second word was presented with only the first letter and the subject was required to generate the word. Thirty word pairs were presented in each condition. After the fMRI scan, words that were read or generated were presented, each with two foils, in a forced-choice recognition task. On the recognition post-test, words from the "generate" condition were more correctly recognized than from the "read" condition (80.0% for generated words versus 72.0% for read words; t(48)=5.17, p<0.001). FMRI revealed increased activation for generate>read in inferior/middle frontal gyri bilaterally (L>R), anterior cingulate, and caudate nucleus and the temporo-parietal-occipital junction bilaterally. For the "read" condition, better subsequent memory performance across individual subjects was positively correlated with activation in the cuneus bilaterally. In the "generate" condition, better subsequent memory performance was positively correlated with activation in the left superior temporal gyrus. These results suggest that self-generation improves memory performance, that enhanced cortical activation accompanies self-generated encoding, and that recruitment of a specific brain network underlies self-generated encoding. The findings may have implications for the development of procedures to enhance memory performance.