With time on our side? Task-dependent compensatory processes in graceful aging

Semantic cognition in healthy ageing: Neural signatures of representation and control mechanisms in naming typical and atypical objects

Effective use of conceptual knowledge engages semantic representation and control processes to access information in a goal-driven manner. Neuropsychological findings of patients presenting either degraded knowledge (e.g., semantic dementia) or disrupted control (e.g., semantic aphasia) converge with neuroimaging evidence from young adults, and delineate the neural segregation of representation and control mechanisms. However, there is still scarce research on the neurofunctional underpinnings of such mechanisms in healthy ageing. To address this, we conducted an fMRI study, wherein young and older adults performed a covert naming task of typical and atypical objects. Three main age-related differences were found. As shown by age group and typicality interactions, older adults exhibited overactivation during naming of atypical (e.g., avocado) relative to typical concepts in brain regions associated to semantic representation, including anterior and medial portions of left temporal lobe (respectively, ATL and MTG). This provides evidence for the reorganization of neural activity in these brain regions contingent to the enrichment of semantic repositories in older ages. The medial orbitofrontal gyrus was also overactivated, indicating that the processing of atypical concepts (relative to typical items) taxes additional control resources in the elderly. Increased activation in the inferior frontal gyrus (IFG) was observed in naming typical items (relative to atypical ones), but only for young adults. This suggests that naming typical items (e.g., strawberry) taxes more on control processes in younger ages, presumably due to the semantic competition set by other items that share multiple features with the target (e.g., raspberry, blackberry, cherry). Together, these results reveal the dynamic nature of semantic control interplaying with conceptual representations as people grow older, by indicating that distinct neural bases uphold semantic performance from young to older ages. These findings may be explained by neural compensation mechanisms coming into play to support neurocognitive changes in healthy ageing.

When action prediction grows old: An fMRI study

Predicting the unfolding of others' actions (action prediction) is crucial for successfully navigating the social world and interacting efficiently. Age-related changes in this domain have remained largely unexplored, especially for predictions regarding simple gestures and independent of contextual information or motor expertise. Here, we evaluated whether healthy aging impacts the neurophysiological processes recruited to anticipate, from the observation of implied-motion postures, the correct conclusion of simple grasping and pointing actions. A color-discrimination task served as a control condition to assess the specificity of the age-related effects. Older adults showed reduced efficiency in performance that was yet not specific to the action prediction task. Nevertheless, fMRI results revealed task-specific age-related differences: while both groups showed stronger recruitment of the lateral occipito-temporal cortex bilaterally during the action prediction than the control task, the younger participants additionally showed a higher bilateral engagement of parietal regions. Importantly, in both groups, the recruitment of visuo-motor processes in the right posterior parietal cortex was a predictor of good performance. These results support the hypothesis of decreased involvement of sensorimotor processes in cognitive tasks when processing action- and body-related stimuli in healthy aging. These results have implications for social interaction, which requires the fast reading of others' gestures.

How the motor system copes with aging: a quantitative meta-analysis of the effect of aging on motor function control

Motor cognitive functions and their neurophysiology evolve and degrade along the lifespan in a dramatic fashion. Current models of how the brain adapts to aging remain inspired primarily by studies on memory or language processes. Yet, aging is strongly associated with reduced motor independence and the associated degraded interaction with the environment: accordingly, any neurocognitive model of aging not considering the motor system is, ipso facto, incomplete. Here we present a meta-analysis of forty functional brain-imaging studies to address aging effects on motor control. Our results indicate that motor control is associated with aging-related changes in brain activity, involving not only motoric brain regions but also posterior areas such as the occipito-temporal cortex. Notably, some of these differences depend on the specific nature of the motor task and the level of performance achieved by the participants. These findings support neurocognitive models of aging that make fewer anatomical assumptions while also considering tasks-dependent and performance-dependent manifestations. Besides the theoretical implications, the present data also provide additional information for the motor rehabilitation domain, indicating that motor control is a more complex phenomenon than previously understood, to which separate cognitive operations can contribute and decrease in different ways with aging.

Many aspects of neuronal control degrade with ageing, including motor control. Using a meta-analysis of functional MRI images, it is made apparent that the ageing brain relies more on visual strategies than sensory stimuli to maintain motor function.

Inter- and Intra-Hemispheric Age-Related Remodeling in Visuo-Spatial Working Memory

Electroencephalography (EEG) studies investigating visuo-spatial working memory (vWM) in aging typically adopt an event-related potential (ERP) analysis approach that has shed light on the age-related changes during item retention and retrieval. However, this approach does not fully enable a detailed description of the time course of the neural dynamics related to aging. The most frequent age-related changes in brain activity have been described by two influential models of neurocognitive aging, the Hemispheric Asymmetry Reduction in Older Adults (HAROLD) and the Posterior-Anterior Shift in Aging (PASA). These models posit that older adults tend to recruit additional brain areas (bilateral as predicted by HAROLD and anterior as predicted by PASA) when performing several cognitive tasks. We tested younger (N = 36) and older adults (N = 35) in a typical vWM task (delayed match-to-sample) where participants have to retain items and then compare them to a sample. Through a data-driven whole scalp EEG analysis we aimed at characterizing the temporal dynamics of the age-related activations predicted by the two models, both across and within different stages of stimulus processing. Behaviorally, younger outperformed older adults. The EEG analysis showed that older adults engaged supplementary bilateral posterior and frontal sites when processing different levels of memory load, in line with both HAROLD and PASA-like activations. Interestingly, these age-related supplementary activations dynamically developed over time. Indeed, they varied across different stages of stimulus processing, with HAROLD-like modulations being mainly present during item retention, and PASA-like activity during both retention and retrieval. Overall, the present results suggest that age-related neural changes are not a phenomenon indiscriminately present throughout all levels of cognitive processing.

Age-Related Changes in Hemispherical Specialization for Attentional Networks

Many cognitive functions face a decline in the healthy elderly. Within the cognitive domains, both attentional processes and executive functions are impaired with aging. Attention includes three attentional networks, i.e., alerting, orienting, and executive control, showing a hemispheric lateralized pattern in adults. This lateralized pattern could play a role in modulating the efficiency of attentional networks. For these reasons, it could be relevant to analyze the age-related change of the hemispheric specialization of attentional networks. This study aims to clarify this aspect with a lateralized version of the Attentional Network Test for Interaction (ANTI)-Fruit. One hundred seventy-one participants took part in this study. They were divided in three age groups: youth (N = 57; range: 20-30); adults (N = 57; range 31-64), and elderly/older people (N = 57; range: 65-87). The results confirmed the previous outcomes on the efficiency and interactions among attentional networks. Moreover, an age-related generalized slowness was evidenced. These findings also support the hypothesis of a hemispheric asymmetry reduction in elderly/older adults.

How aging affects the premotor control of lower limb movements in simulated gait

Gait control becomes more demanding in healthy older adults, yet what cognitive or motor process leads to this age-related change is unknown. The present study aimed to investigate whether it might depend on specific decay in the quality of gait motor representation and/or a more general reduction in the efficiency of lower limb motor control. Younger and older healthy participants performed in fMRI a virtual walking paradigm that combines motor imagery (MI) of walking and standing on the spot with the presence (Dynamic Motor Imagery condition, DMI) or absence (pure MI condition) of overtly executed ankle dorsiflexion. Gait imagery was aided by the concomitant observation of moving videos simulating a stroll in the park from a first-person perspective. Behaviorally, older participants showed no sign of evident depletion in the quality of gait motor representations, and absence of between-group differences in the neural correlates of MI. However, while younger participants showed increased frontoparietal activity during DMI, older participants displayed stronger activation of premotor areas when controlling the pure execution of ankle dorsiflexion, regardless of the imagery task. These data suggest that reduced automaticity of lower limb motor control in healthy older subjects leads to the recruitment of additional premotor resources even in the absence of basic gait functional disabilities.

Dynamic motor imagery mentally simulates uncommon real locomotion better than static motor imagery both in young adults and elderly

A new form of Motor Imagery (MI), called dynamic Motor Imagery (dMI) has recently been proposed. The dMI adds to conventional static Motor Imagery (sMI) the presence of simultaneous actual movements partially replicating those mentally represented. In a previous research conducted on young participants, dMI showed to be temporally closer than sMI in replicating the real performance for some specific locomotor conditions. In this study, we evaluated if there is any influence of the ageing on dMI. Thirty healthy participants were enrolled: 15 young adults (27.1±3.8 y.o.) and 15 older adults (65.9±9.6y.o.). The performance time and the number of steps needed to either walk to a target (placed at 10m from participants) or to imagine walking to it, were assessed. Parameters were measured for sMI, dMI and real locomotion (RL) in three different locomotor conditions: forward walking (FW), backward walking (BW), and lateral walking (LW). Temporal performances of sMI and dMI did not differ between RL in the FW, even if significantly different to each other (p = 0.0002). No significant differences were found for dMI with respect to RL for LW (p = 0.140) and BW (p = 0.438), while sMI was significantly lower than RL in LW (p<0.001). The p-value of main effect of age on participants’ temporal performances was p = 0.055. The interaction between age and other factors such as the type of locomotion (p = 0.358) or the motor condition (p = 0.614) or third level interaction (p = 0.349) were not statistically significant. Despite a slight slowdown in the performance of elderly compared to young participants, the temporal and spatial accuracy was better in dMI than sMI in both groups. Motor imagery processes may be strengthened by the feedback generated through dMI, and this effect appears to be unaffected by age.

Neurocompensatory Effects of the Default Network in Older Adults

The hemispheric asymmetry reduction in older adults (HAROLD) is a neurocompensatory process that has been observed across several cognitive functions but has not yet been examined in relation to task-induced relative deactivations of the default mode network. The present study investigated the presence of HAROLD effects specific to neural activations and deactivations using a functional magnetic resonance imaging (fMRI) n-back paradigm. It was hypothesized that HAROLD effects would be identified in relative activations and deactivations during the paradigm, and that they would be associated with better 2-back performance. Forty-five older adults (M age = 63.8; range = 53-83) were administered a verbal n-back paradigm during fMRI. For each participant, the volume of brain response was summarized by left and right frontal regions of interest, and laterality indices (LI; i.e., left/right) were calculated to assess HAROLD effects. Group level results indicated that age was significantly and negatively correlated with LI (i.e., reduced left lateralization) for deactivations, but positively correlated with LI (i.e., increased left lateralization) for activations. The relationship between age and LI for deactivation was significantly moderated by performance level, revealing a stronger relationship between age and LI at higher levels of 2-back performance. Findings suggest that older adults may employ neurocompensatory processes specific to deactivations, and task-independent processes may be particularly sensitive to age-related neurocompensation.

As Time Goes by: A rTMS Study on Age-Related Changes in Sentence Comprehension

It is well established that off-line sentence judgment tasks (oSJTs) typically rely on phonological working memory (WM), beyond specific linguistic processing. Nevertheless, empirical findings suggest that a juvenile level of performance in an oSJT could be associated with the recruitment of age-specific additional supportive neural network in healthy aging. In particular, in one of our previous study, healthy elderlies showed the additional activation of associative visual cortices when compared with young controls. We suggested that age-related hyperactivations, during an auditory sentence judgment task, might represent the neurofunctional correlate of the recruitment of compensatory strategies that are necessary to maintain a juvenile level of performance. To explicitly test this hypothesis we adopted repetitive transcranial magnetic stimulation (rTMS). Twelve healthy elderlies and 12 young participants were engaged in an off-line semantic plausibility judgment task while rTMS was delivered over: (1) the left inferior frontal gyrus (LIFG; i.e., a core region of the WM network); (2) the precuneus; and (3) a Control Site (vertex). Results showed a significant main effect of Stimulation Site and a significant Group-by-Stimulation Site interaction effect. In particular, the rTMS stimulation of the LIFG slowed down reaction times (RTs) both in young and healthy elderly participants, while only healthy elderlies showed an increment of RTs during the stimulation of the precuneus. Taken together our results further support the idea that the maintenance of a juvenile level of performance in graceful aging may be associated with task-specific compensatory processes that would manifest them-selves, from the neurofunctional point of view, by the recruitment of additional neural supportive regions.

How many deficits in the same dyslexic brains? A behavioural and fMRI assessment of comorbidity in adult dyslexics

Dyslexia can have different manifestations: this has motivated different theories on its nature, on its underlying brain bases and enduring controversies on how to best treat it. The relative weight of the different manifestations has never been evaluated using both behavioural and fMRI measures, a challenge taken here to assess the major systems called into play in dyslexia by different theories. We found that adult well-compensated dyslexics were systematically impaired only in reading and in visuo-phonological tasks, while deficits for other systems (e.g., motor/cerebellar, visual magnocellular/motion perception) were only very occasional. In line with these findings, fMRI showed a reliable hypoactivation only for the task of reading, in the left occipito-temporal cortex (l-OTC). The l-OTC, normally a crossroad between the reading system and other systems, did not show the same level of intersection in dyslexics; yet, it was not totally silent because it responded, in segregated parts, during auditory phonological and visual motion perception tasks. This minimal behavioural and functional anatomical comorbidity demonstrates that a specific deficit of reading is the best description for developmental dyslexia, at least for adult well-compensated cases, with clear implications for rehabilitation strategies. The reduced intersection of multiple systems in the l-OTC suggests that dyslexics suffer from a coarser connectivity, leading to disconnection between the multiple domains that normally interact during reading.

Age-related language lateralization assessed by fMRI: The effects of sex and handedness

Previous studies focusing on the relationship between lateralization of language function and age suffer from lack of a balanced distribution of age and handedness among participants, especially in the extremes of age. This limits our understanding of the influence of these factors on lateralization of language circuitry. The hemispheric asymmetry reduction in older adults (HAROLD) model suggests that under similar circumstances, involvement in cognitive processes of prefrontal (and potentially other) cortical areas tends to be less lateralized with age. In this study, we aimed to investigate the link between age, gender, and language lateralization in a large group of healthy participants with a relatively even distribution of age and handedness in order to further test the HAROLD model. 99 healthy men (33 left-handed; age range 18-74years) and 125 women (44 left-handed; age range 19-76) were recruited. All participants underwent fMRI at 3T with a semantic decision and a verb generation tasks and received a battery of linguistic tests. Lateralization indexes (LI) were calculated for each participant based on fMRI results for each task separately. LIs were found to be significantly decreasing with age only in right-handed men and only in temporo-parietal cortical area. LIs did not change with age in other brain regions or in left-handed subjects. Our results do not support the HAROLD model and suggest a potentially different relationship between aging and lateralization of language functions.

When I am (almost) 64: The effect of normal ageing on implicit motor imagery in young elderlies

Motor imagery (M.I.) is a cognitive process in which movements are mentally evoked without overt actions. Behavioral and fMRI studies show a decline of explicit M.I. ability (e.g., the mental rehearsal of finger oppositions) with normal ageing: this decline is accompanied by the recruitment of additional cortical networks. However, none of these studies investigated behavioral and the related fMRI ageing modifications in implicit M.I. tasks, like the hand laterality task (HLT). To address this issue, we performed a behavioral and fMRI study: 27 younger subjects (mean age: 31 years) and 29 older subjects (mean age: 61 years) underwent two event-related design fMRI experiments. In the HLT, participants were asked to decide whether a hand rotated at different angles was a left or right hand. To test the specificity of any age related difference in the HLT, we used a letter rotation task as a control experiment: here subjects had to decide whether rotated letters were presented in a standard or a mirror orientation. We did not find any group difference in either behavioral task; however, we found significant additional neural activation in the elderly group in occipito-temporal regions: these differences were stronger for the HLT rather than for the LRT with group by task interactions effects in right occipital cortices. We interpret these results as evidence of compensatory processes associated with ageing that permit a behavioral performance comparable to that of younger subjects. This process appears to be more marked when the task specifically involves motor representations, even when these are implicitly evoked.

Cognitive training can reduce the rate of cognitive aging: a neuroimaging cohort study

BACKGROUND

The neural mechanisms underlying the restorative effects of cognitive training on aging brains remain unclear. To address this issue, we examined the relationship between changes in spontaneous brain activity and cognitive performance that occur after cognitive training.

METHODS

Participants were older adults who were part of a randomized control trial within a larger longitudinal cognitive training study. We conducted single-domain and multi-domain cognitive training in two respective intervention groups. Participants were trained for 1 h, twice a week, for 12 weeks. Cognition was assessed in all participants and magnetic resonance images were obtained at baseline and 1 year after training. To assess spontaneous fluctuations in brain activity, we acquired resting-state fMRI data. Two indices-functional entropy and time-domain entropy-were used to measure the effects of training. Functional entropy increases with aging, and indicates disruptions in functional conectivity. Time-domain entropy decreases with aging, and indicates structural alterations in the brain and blood-flow reduction.

RESULTS

Seventy participants completed the study: 26 in the multi-domain cognitive training group (70.38 ± 3.30 yrs), 27 in single-domain group (70.48 ± 3.93 yrs), and 17 in a control group (68.59 ± 3.24 yrs). Functional entropy increased significantly less in the multi-domain (p = 0.047) and single-domain groups (p = 9.51 × 10(-4)) compared with the control group. In the multi-domain group, this was true in the paracentral lobule (p = 0.004, Bonferroni corrected p < 0.05). Time-domain entropy also improved with training. Compared with controls, time-domain entropy in the multi-domain group decreased less in the inferior frontal gyrus pars opercularis (p = 3.59 × 10(-4)), the medial part of superior frontal gyrus (p = 1.17 × 10(-5)), and the thalamus (p = 4.72 × 10(-5)), while that in the single-domain group decreased less in the cuneus (p = 2.58 × 10(-4), Bonferroni corrected p < 0.05). Additionally, changes in regional entropy for some regions such as hippocampus significantly correlated with improvements in cognitive performance.

CONCLUSIONS

Cognitive training can induce plastic changes in neural functional connectivity of healthy older people, and these changes may underlie the positive effect of cognitive training.

TRIAL REGISTRATION

ChiCTR-TRC-08000732 (Date of registration: 5th November, 2008).

Neural changes associated with semantic processing in healthy aging despite intact behavioral performance

Semantic memory recruits an extensive neural network including the left inferior prefrontal cortex (IPC) and the left temporoparietal region, which are involved in semantic control processes, as well as the anterior temporal lobe region (ATL) which is considered to be involved in processing semantic information at a central level. However, little is known about the underlying neuronal integrity of the semantic network in normal aging. Young and older healthy adults carried out a semantic judgment task while their cortical activity was recorded using magnetoencephalography (MEG). Despite equivalent behavioral performance, young adults activated the left IPC to a greater extent than older adults, while the latter group recruited the temporoparietal region bilaterally and the left ATL to a greater extent than younger adults. Results indicate that significant neuronal changes occur in normal aging, mainly in regions underlying semantic control processes, despite an apparent stability in performance at the behavioral level.

Better together: Left and right hemisphere engagement to reduce age-related memory loss

Episodic memory is a cognitive function that appears more susceptible than others to the effects of aging. The main aim of this study is to investigate if the magnitude of functional hemispheric lateralization during episodic memory test was positively correlated with memory performance, proving the presence of a beneficial pattern of neural processing in high-performing older adults but not in low-performing participants. We have applied anodal transcranial Direct Current Stimulation (tDCS) or sham stimulation over left and right hemisphere in a group of young subjects and in high-performing and low-performing older participants during an experimental verbal episodic memory task. Remarkably, young individuals and high-performing older adults exhibited similar performances on episodic memory tasks and both groups showed symmetrical recruitment of left and right areas during memory retrieval. In contrast, low-performing older adults, who obtained lower scores on the memory tasks, demonstrated a greater engagement of the left hemisphere during verbal memory task. Furthermore, structural equation model was performed for analyzing the interrelations between the index of interhemispheric asymmetry and several neuropsychological domains. We found that the bilateral engagement of dorsolateral prefrontal cortex and parietal cortex regions had a direct correlation with memory and executive functions evaluated as latent constructs. These findings drew attention to brain maintenance hypothesis. The potential of neurostimulation in cognitive enhancement is particularly promising to prevent memory loss during aging.

How does it STAC up? Revisiting the scaffolding theory of aging and cognition

"The Scaffolding Theory of Aging and Cognition (STAC)", proposed in 2009, is a conceptual model of cognitive aging that integrated evidence from structural and functional neuroimaging to explain how the combined effects of adverse and compensatory neural processes produce varying levels of cognitive function. The model made clear and testable predictions about how different brain variables, both structural and functional, were related to cognitive function, focusing on the core construct of compensatory scaffolding. The present paper provides a revised model that integrates new evidence about the aging brain that has emerged since STAC was published 5 years ago. Unlike the original STAC model, STAC-r incorporates life-course factors that serve to enhance or deplete neural resources, thereby influencing the developmental course of brain structure and function, as well as cognition, over time. Life-course factors also influence compensatory processes that are engaged to meet cognitive challenge, and to ameliorate the adverse effects of structural and functional decline. The revised model is discussed in relation to recent lifespan and longitudinal data as well as emerging evidence about the effects of training interventions. STAC-r goes beyond the previous model by combining a life-span approach with a life-course approach to understand and predict cognitive status and rate of cognitive change over time.

A systematic review of type 2 diabetes mellitus and hypertension in imaging studies of cognitive aging: time to establish new norms

The rising prevalence of type 2 diabetes (T2DM) and hypertension in older adults, and the deleterious effect of these conditions on cerebrovascular and brain health, is creating a growing discrepancy between the "typical" cognitive aging trajectory and a "healthy" cognitive aging trajectory. These changing health demographics make T2DM and hypertension important topics of study in their own right, and warrant attention from the perspective of cognitive aging neuroimaging research. Specifically, interpretation of individual or group differences in blood oxygenation level dependent magnetic resonance imaging (BOLD MRI) or positron emission tomography (PET H2O(15)) signals as reflective of differences in neural activation underlying a cognitive operation of interest requires assumptions of intact vascular health amongst the study participants. Without adequate screening, inclusion of individuals with T2DM or hypertension in "healthy" samples may introduce unwanted variability and bias to brain and/or cognitive measures, and increase potential for error. We conducted a systematic review of the cognitive aging neuroimaging literature to document the extent to which researchers account for these conditions. Of the 232 studies selected for review, few explicitly excluded individuals with T2DM (9%) or hypertension (13%). A large portion had exclusion criteria that made it difficult to determine whether T2DM or hypertension were excluded (44 and 37%), and many did not mention any selection criteria related to T2DM or hypertension (34 and 22%). Of all the surveyed studies, only 29% acknowledged or addressed the potential influence of intersubject vascular variability on the measured BOLD or PET signals. To reinforce the notion that individuals with T2DM and hypertension should not be overlooked as a potential source of bias, we also provide an overview of metabolic and vascular changes associated with T2DM and hypertension, as they relate to cerebrovascular and brain health.

Neuropsychological mechanisms of falls in older adults

Falls, a common cause of injury among older adults, have become increasingly prevalent. As the world's population ages, the increase in-and the prevalence of-falls among older people makes this a serious and compelling societal and healthcare issue. Physical weakness is a critical predictor in falling. While considerable research has examined this relationship, comprehensive reviews of neuropsychological predictors of falls have been lacking. In this paper, we examine and discuss current studies of the neuropsychological predictors of falls in older adults, as related to sporting and non-sporting contexts. By integrating the existing evidence, we propose that brain aging is an important precursor of the increased risk of falls in older adults. Brain aging disrupts the neural integrity of motor outputs and reduces neuropsychological abilities. Older adults may shift from unconscious movement control to more conscious or attentive motor control. Increased understanding of the causes of falls will afford opportunities to reduce their incidence, reduce consequent injuries, improve overall well-being and quality of life, and possibly to prolong life.

Older adults and psychoanalytic treatment: it's about time

It has become increasingly apparent that older adults may not only benefit from psychoanalysis and psychodynamic psychotherapy but may be particularly well suited to such treatment. Clinical evidence to support this is presented, including discussion of the successful psychoanalysis of a woman in her seventies. An overview of the psychoanalytic literature indicates that psychoanalytic beliefs about the feasibility of treating older patients have always been favorable, but have had difficulty gaining traction. The modern psychoanalytic literature is compatible with extra-analytic studies of aging that provide further rationale for the potential usefulness of psychoanalytically oriented interventions in the elderly population.

Correspondence of executive function related functional and anatomical alterations in aging brain

Neurocognitive aging studies have focused on age-related changes in neural activity or neural structure but few studies have focused on relationships between the two. The present study quantitatively reviewed 24 studies of age-related changes in fMRI activation across a broad spectrum of executive function tasks using activation likelihood estimation (ALE) and 22 separate studies of age-related changes in gray matter using voxel-based morphometry (VBM). Conjunction analyses between functional and structural alteration maps were constructed. Overlaps were only observed in the conjunction of dorsolateral prefrontal cortex (DLPFC) gray matter reduction and functional hyperactivation but not hypoactivation. It was not evident that the conjunctions between gray matter and activation were related to task performance. Theoretical implications of these results are discussed.

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.

Is a lone right hemisphere enough? Neurolinguistic architecture in a case with a very early left hemispherectomy

We studied the linguistic profile and neurolinguistic organization of a 14-year-old adolescent (EB) who underwent a left hemispherectomy at the age of 2.5 years. After initial aphasia, his language skills recovered within 2 years, with the exception of some word finding problems. Over the years, the neuropsychological assessments showed that EB's language was near-to-normal, with the exception of lexical competence, which lagged slightly behind for both auditory and written language. Moreover, EB's accuracy and speed in both reading and writing words and non-words were within the normal range, whereas difficulties emerged in reading loan words and in tasks with homophones. EB's functional magnetic resonance imaging (fMRI) patterns for several linguistic and metalinguistic tasks were similar to those observed in the dominant hemisphere of controls, suggesting that his language network conforms to a left-like linguistic neural blueprint. However, a stronger frontal recruitment suggests that linguistic tasks are more demanding for him. Finally, no specific reading activation was found in EB's occipitotemporal region, a finding consistent with the surface dyslexia-like behavioral pattern of the patient. While a lone right hemisphere may not be sufficient to guarantee full blown linguistic competences after early hemispherectomy, EB's behavioral and fMRI patterns suggest that his lone right hemisphere followed a left-like blueprint of the linguistic network.

Mental images across the adult lifespan: a behavioural and fMRI investigation of motor execution and motor imagery

Motor imagery (M.I.) is a mental state in which real movements are evoked without overt actions. There is some behavioural evidence that M.I. declines with ageing. The neurofunctional correlates of these changes have been investigated only in two studies, but none of the these studies has measured explicit correlations between behavioural variables and the brain response, nor the correlation of M.I. and motor execution (M.E.) of the same acts in ageing. In this paper, we report a behavioural and functional magnetic resonance imaging (fMRI) experiment that aimed to address this issue. Twenty-four young subjects (27 ± 5.6 years) and twenty-four elderly subjects (60 ± 4.6 years) performed two block-design fMRI tasks requiring actual movement (M.E.) or the mental rehearsal (M.I.) of finger movements. Participants also underwent a behavioural mental chronometry test in which the temporal correlations between M.I. and M.E. were measured. We found significant neurofunctional and behavioural differences between the elderly subjects and the young subjects during the M.E. and the M.I. tasks: for the M.E. task, the elderly subjects showed increased activation in frontal and prefrontal (pre-SMA) cortices as if M.E. had become more cognitively demanding; during the M.I. task, the elderly over-recruited occipito-temporo-parietal areas, suggesting that they may also use a visual imagery strategy. We also found between-group behavioural differences in the mental chronometry task: M.I. and M.E. were highly correlated in the young participants but not in the elderly participants. The temporal discrepancy between M.I. and M.E. in the elderly subjects correlated with the brain regions that showed increased activation in the occipital lobe in the fMRI. The same index was correlated with the premotor regions in the younger subjects. These observations show that healthy elderly individuals have decreased or qualitatively different M.I. compared to younger subjects.

Reassessing the HAROLD model: is the hemispheric asymmetry reduction in older adults a special case of compensatory-related utilisation of neural circuits?

The HAROLD (hemispheric asymmetry reduction in older adults) model, proposed by Cabeza in 2002, suggests that age-related neurofunctional changes are characterised by a significant reduction in the functional hemispheric lateralisation in the prefrontal cortex (PFC). The supporting evidence, however, has been derived from qualitative explorations of the data rather than from explicit statistical assessments of functional lateralisation. In contrast, the CRUNCH (compensation-related utilisation of neural circuits hypothesis) model posits that elderly subjects recruit additional brain regions that do not necessarily belong to the contralateral hemisphere as much as they rely on additional strategies to solve cognitive problems. To better assess the validity and generalisability of the HAROLD model, we analysed the fMRI patterns of twenty-four young subjects (age range: 18-30 years) and twenty-four healthy elderly subjects (age range: 50-80 years) collected during the performance of two linguistic/semantic tasks (a picture-naming task and a sentence judgment task) and two episodic long-term memory (eLTM) recognition tasks for the same materials. The functional hemispheric lateralisation in each group and the ensuing between-group differences were quantitatively assessed using statistical lateralisation maps (SLMs). The number of clusters showing a genuine HAROLD effect was proportional to the level of task demand. In addition, when quantitatively significant, these effects were not restricted to the PFC. We conclude that, in its original version, the HAROLD model captures only some of the age-related brain patterns observed in graceful ageing. The results observed in our study are compatible with the more general CRUNCH model, suggesting that the former patterns can be considered a special manifestation of age-related compensatory processes.

Dynamic functional reorganizations and relationship with working memory performance in healthy aging

In recent years, several theories have been proposed in attempts to identify the neural mechanisms underlying successful cognitive aging. Old subjects show increased neural activity during the performance of tasks, mainly in prefrontal areas, which is interpreted as a compensatory mechanism linked to functional brain efficiency. Moreover, resting-state studies have concluded that elders show disconnection or disruption of large-scale functional networks. We used functional MRI during resting-state and a verbal n-back task with different levels of memory load in a cohort of young and old healthy adults to identify patterns of networks associated with working memory and brain default mode. We found that the disruption of resting-state networks in the elderly coexists with task-related overactivations of certain brain areas and with reorganizations within these functional networks. Moreover, elders who were able to activate additional areas and to recruit a more bilateral frontal pattern within the task-related network achieved successful performance on the task. We concluded that the balanced and plastic reorganization of brain networks underlies successful cognitive aging. This observation allows the integration of several theories that have been proposed to date regarding the aging brain.

Neuroanatomical substrates of age-related cognitive decline

There are many reports of relations between age and cognitive variables and of relations between age and variables representing different aspects of brain structure and a few reports of relations between brain structure variables and cognitive variables. These findings have sometimes led to inferences that the age-related brain changes cause the age-related cognitive changes. Although this conclusion may well be true, it is widely recognized that simple correlations are not sufficient to warrant causal conclusions, and other types of correlational information, such as mediation and correlations between longitudinal brain changes and longitudinal cognitive changes, also have limitations with respect to causal inferences. These issues are discussed, and the existing results on relations of regional volume, white matter hyperintensities, and diffusion tensor imaging measures of white matter integrity to age and to measures of cognitive functioning are reviewed. It is concluded that at the current time the evidence that these aspects of brain structure are neuroanatomical substrates of age-related cognitive decline is weak. The final section contains several suggestions concerning measurement and methodology that may lead to stronger conclusions in the future.