Aging and the interaction of sensory cortical function and structure

[Interpretation of a concept for functional movement disorders from the perspective of older patients]

A recently published concept considers a significant proportion of the occurrence and persistence of functional movement disorders (FMD) to be explained by increased/incorrect weighting of the expected movement (feedforward signal) in the presence of decreased/altered actual feedback of the movement. In the context of aging and age-associated diseases, there is an increased likelihood that these prerequisites will occur, also in combination. For example, the feedforward signal can be enhanced by accumulation of a wealth of experience but can for example become prone to error due to changes in attention and (fear of) falling. Conversely, the actual feedback is subject to age-related changes, such as reduction of sensory functions. This could explain why FMDs also occur in old age and offer treatment approaches for this so far poorly studied disorder. It follows that a specific focus on (the correction of) feedforward signals and strengthening as well as training of the actual feedback are potentially promising therapeutic approaches for older people with FMD.

Perceptual Integration Compensates for Attention Deficit in Elderly during Repetitive Auditory-Based Sensorimotor Task

Sensorimotor integration (SI) brain functions that are vital for everyday life tend to decline in advanced age. At the same time, elderly people preserve a moderate level of neuroplasticity, which allows the brain's functionality to be maintained and slows down the process of neuronal degradation. Hence, it is important to understand which aspects of SI are modifiable in healthy old age. The current study focuses on an auditory-based SI task and explores: (i) if the repetition of such a task can modify neural activity associated with SI, and (ii) if this effect is different in young and healthy old age. A group of healthy older subjects and young controls underwent an assessment of the whole-brain electroencephalography (EEG) while repetitively executing a motor task cued by the auditory signal. Using EEG spectral power and functional connectivity analyses, we observed a differential age-related modulation of theta activity throughout the repetition of the SI task. Growth of the anterior stimulus-related theta oscillations accompanied by enhanced right-lateralized frontotemporal phase-locking was found in elderly adults. Their young counterparts demonstrated a progressive increase in prestimulus occipital theta power. Our results suggest that the short-term repetition of the auditory-based SI task modulates sensory processing in the elderly. Older participants most likely progressively improve perceptual integration rather than attention-driven processing compared to their younger counterparts.

Cross-sectional associations between cortical thickness and physical activity in older adults with spontaneous memory complaints: The MAPT Study

BACKGROUND

Age-related changes in brain structure may constitute the starting point for cerebral function alteration. Physical activity (PA) demonstrated favorable associations with total brain volume, but its relationship with cortical thickness (CT) remains unclear. We investigated the cross-sectional associations between PA level and CT in community-dwelling people aged 70 years and older.

METHODS

A total of 403 older adults aged 74.8 ± 4.0 years (mean ± SD) who underwent a baseline magnetic resonance imaging examination and who had data on PA and confounders were included. PA was assessed with a questionnaire. Participants were categorized according to PA levels. Multiple linear regressions were used to compare the brain CT (mm) of the inactive group (no PA at all) with 6 active groups (growing PA levels) in 34 regions of interest.

RESULTS

Compared with inactive persons, people who achieved PA at a level of 1500-1999 metabolic equivalent task-min/week (i.e., about 6-7 h of brisk walking for exercise and those who achieved it at 2000-2999 metabolic equivalent task-min/week (i.e., 8-11 h of brisk walking for exercise) had higher CT in the fusiform gyrus and the temporal pole. Additionally, dose-response associations between PA and CT were found in the fusiform gyrus (B = 0.011, SE = 0.004, adj. p = 0.035), the temporal pole (B = 0.026, SE = 0.009, adj. p = 0.048), and the caudal middle frontal gyrus, the entorhinal, medial orbitofrontal, lateral occipital, and insular cortices.

CONCLUSION

This study demonstrates a positive association between PA level and CT in temporal areas such as the fusiform gyrus, a brain region often associated to Alzheimer's disease in people aged 70 years and older. Future investigations focusing on PA type may help to fulfil remaining knowledge gaps in this field.

Hippocampal blood flow rapidly and preferentially increases after a bout of moderate-intensity exercise in older adults with poor cerebrovascular health

Over the course of aging, there is an early degradation of cerebrovascular health, which may be attenuated with aerobic exercise training. Yet, the acute cerebrovascular response to a single bout of exercise remains elusive, particularly within key brain regions most affected by age-related disease processes. We investigated the acute global and region-specific cerebral blood flow (CBF) response to 15 minutes of moderate-intensity aerobic exercise in older adults (≥65 years; n = 60) using arterial spin labeling magnetic resonance imaging. Within 0-6 min post-exercise, CBF decreased across all regions, an effect that was attenuated in the hippocampus. The exercise-induced CBF drop was followed by a rebound effect over the 24-minute postexercise assessment period, an effect that was most robust in the hippocampus. Individuals with low baseline perfusion demonstrated the greatest hippocampal-specific CBF effect post-exercise, showing no immediate drop and a rapid increase in CBF that exceeded baseline levels within 6-12 minutes postexercise. Gains in domain-specific cognitive performance postexercise were not associated with changes in regional CBF, suggesting dissociable effects of exercise on acute neural and vascular plasticity. Together, the present findings support a precision-medicine framework for the use of exercise to target brain health that carefully considers age-related changes in the cerebrovascular system.

Disturbances in primary visual processing as a function of healthy aging

For decades, visual entrainment paradigms have been widely used to investigate basic visual processing in healthy individuals and those with neurological disorders. While healthy aging is known to be associated with alterations in visual processing, whether this extends to visual entrainment responses and the precise cortical regions involved is not fully understood. Such knowledge is imperative given the recent surge in interest surrounding the use of flicker stimulation and entrainment in the context of identifying and treating Alzheimer's disease (AD). In the current study, we examined visual entrainment in eighty healthy aging adults using magnetoencephalography (MEG) and a 15 Hz entrainment paradigm, while controlling for age-related cortical thinning. MEG data were imaged using a time-frequency resolved beamformer and peak voxel time series were extracted to quantify the oscillatory dynamics underlying the processing of the visual flicker stimuli. We found that, as age increased, the mean amplitude of entrainment responses decreased and the latency of these responses increased. However, there was no effect of age on the trial-to-trial consistency in phase (i.e., inter-trial phase locking) nor amplitude (i.e., coefficient of variation) of these visual responses. Importantly, we discovered that the relationship between age and response amplitude was fully mediated by the latency of visual processing. These results indicate that aging is associated with robust changes in the latency and amplitude of visual entrainment responses within regions surrounding the calcarine fissure, which should be considered in studies examining neurological disorders such as AD and other conditions associated with increased age.

Age-related changes in excitatory and inhibitory intra-cortical circuits in auditory cortex of C57Bl/6 mice

A common impairment in aging is age-related hearing loss (presbycusis), which manifests as impaired spectrotemporal processing. Aging is accompanied by alteration in normal inhibitory (GABA) neurotransmission, and changes in excitatory (NMDA and AMPA) synapses in the auditory cortex (ACtx). However, the circuits affected by these synaptic changes remain unknown. Mice of the C57Bl/6J strain show premature age-related hearing loss and changes in functional responses in ACtx. We thus investigated how auditory cortical microcircuits change with age by comparing young (∼ 6 weeks) and aged (>1 year old) C57Bl/6J mice. We performed laser scanning photostimulation (LSPS) combined with whole-cell patch clamp recordings from Layer (L) 2/3 cells in primary auditory cortex (A1) of young adult and aged C57Bl/6J mice. We found that L2/3 cells in aged C57Bl/6J mice display functional hypoconnectivity of both excitatory and inhibitory circuits. Compared to cells from young C57Bl/6 mice, cells from aged C57Bl/6J mice have fewer excitatory connections with weaker connection strength. Whereas young adult and aged C57Bl/6J mice have similar amounts of inhibitory connections, the strength of local inhibition is weaker in the aged group. We confirmed these results by recording miniature excitatory (mEPSCs) and inhibitory synaptic currents (mIPSCs). Our results suggest a specific reduction in excitatory and inhibitory intralaminar cortical circuits in aged C57Bl/6J mice compared with young adult animals. We speculate that these unbalanced changes in cortical circuits contribute to the functional manifestations of age-related hearing loss.

Highly scalable maximum likelihood and conjugate Bayesian inference for ERGMs on graph sets with equivalent vertices

The exponential family random graph modeling (ERGM) framework provides a highly flexible approach for the statistical analysis of networks (i.e., graphs). As ERGMs with dyadic dependence involve normalizing factors that are extremely costly to compute, practical strategies for ERGMs inference generally employ a variety of approximations or other workarounds. Markov Chain Monte Carlo maximum likelihood (MCMC MLE) provides a powerful tool to approximate the maximum likelihood estimator (MLE) of ERGM parameters, and is generally feasible for typical models on single networks with as many as a few thousand nodes. MCMC-based algorithms for Bayesian analysis are more expensive, and high-quality answers are challenging to obtain on large graphs. For both strategies, extension to the pooled case—in which we observe multiple networks from a common generative process—adds further computational cost, with both time and memory scaling linearly in the number of graphs. This becomes prohibitive for large networks, or cases in which large numbers of graph observations are available. Here, we exploit some basic properties of the discrete exponential families to develop an approach for ERGM inference in the pooled case that (where applicable) allows an arbitrarily large number of graph observations to be fit at no additional computational cost beyond preprocessing the data itself. Moreover, a variant of our approach can also be used to perform Bayesian inference under conjugate priors, again with no additional computational cost in the estimation phase. The latter can be employed either for single graph observations, or for observations from graph sets. As we show, the conjugate prior is easily specified, and is well-suited to applications such as regularization. Simulation studies show that the pooled method leads to estimates with good frequentist properties, and posterior estimates under the conjugate prior are well-behaved. We demonstrate the usefulness of our approach with applications to pooled analysis of brain functional connectivity networks and to replicated x-ray crystal structures of hen egg-white lysozyme.

Physical Performance, Cardiovascular Health and Psychosocial Wellbeing in Older Adults Compared to Oldest-Old Residential Seniors

Background: This study analyzed physical, cardiovascular, and psychosocial health in different age groups at the far end of the lifespan. Methods: Sixty-two residential seniors participated in this cross-sectional study and were assigned according to age to either the older adults (n = 27; age: 74.8 (3.6); f: 23) or the oldest-old group (n = 35; age: 87.2 (5.0); f: 28). Gait speed, functional mobility, handgrip strength, and pulse wave velocity (PWV) were measured. Additionally, questionnaires to assess quality of life were applied. Mean between-group differences (Δ) and Hedge’s g with 95 % confidence intervals were calculated. Results: Oldest-old had moderately lower handgrip strength (Δ = −31.3 N, 95% CI [−66.30; −1.65], Hedge’s g = 0.49 [−0.97; 0.03]) and relevant lower gait speed than the older adults (Δ = −0.11 m/s [−0.28; 0.05], g = 0.34 [−0.89; 0.20]). All other physical parameters showed trivial differences. Very large effects were found in PWV in favor of the older adults (Δ = −2.65 m/s [−3.26; −2.04], g = −2.14 [−2.81; −1.36]). The questionnaires showed trivial to small differences. Conclusion: We found small differences in physical as well as psychosocial health between age groups with large inter-individual variance. Large differences were found in arterial stiffness, which increases with age. Exercise programs in nursing homes should consider physical, psychosocial, and cardiovascular variables more than age.

Synergistic associations of visual and self-reported hearing acuity with low handgrip strength in older adults: a population-based cross-sectional study

BACKGROUND

It is unclear whether visual and hearing acuity are independently or synergistically associated with muscle strength. We aimed to examine the associations of visual and self-reported hearing acuity with low handgrip strength and the additive interaction between visual and hearing acuity on low handgrip strength in people over 60 years.

METHOD

Data of 3,075 individuals aged over 60 years from the 2017 and 2018 Korea National Health and Nutrition Examination Survey were used for this cross-sectional study. Low handgrip strength was defined based on the 20th percentile of the study population (< 30.4 kg for male and < 17.7 kg for female). Visual and self-reported hearing acuity were each divided into three categories: good, moderate, and impaired. Multiple logistic regression and relative excess risk due to interaction (RERI) were performed.

RESULTS

Of the 3,075 participants, 993 (32.3 %) demonstrated low handgrip strength. Low handgrip strength was more prevalent in participants with moderate (adjusted odds ratio [AOR] = 1.54, 95 % confidence interval [CI] = 1.12-2.12) and impaired visual acuity (AOR = 2.00, 95 % CI = 1.34-2.96). Both moderate and impaired self-reported hearing acuity were significantly associated with low handgrip strength (moderate: AOR = 1.25, 95 % CI = 1.01-1.55; impaired: AOR = 1.66, 95 % CI = 1.15-2.38). The more severe the sensory function decline, the higher the association with muscle weakness. Moreover, combined sensory impairments were associated with deteriorating low handgrip strength (AOR = 8.38), with significantly strong additive interactions (RERI = 2.61, 95 % CI = 2.52-2.70).

CONCLUSIONS

Awareness is needed regarding the risk of reduced muscle strength in individuals with moderate and impaired sensory function. Older people with sensory function decline in clinical settings may benefit from programs such as exercise prescription to prevent muscle weakness.

Cortical Engagement Metrics During Reactive Balance Are Associated With Distinct Aspects of Balance Behavior in Older Adults

Heightened reliance on the cerebral cortex for postural stability with aging is well-known, yet the cortical mechanisms for balance control, particularly in relation to balance function, remain unclear. Here we aimed to investigate motor cortical activity in relation to the level of balance challenge presented during reactive balance recovery and identify circuit-specific interactions between motor cortex and prefrontal or somatosensory regions in relation to metrics of balance function that predict fall risk. Using electroencephalography, we assessed motor cortical beta power, and beta coherence during balance reactions to perturbations in older adults. We found that individuals with greater motor cortical beta power evoked following standing balance perturbations demonstrated lower general clinical balance function. Individual older adults demonstrated a wide range of cortical responses during balance reactions at the same perturbation magnitude, showing no group-level change in prefrontal- or somatosensory-motor coherence in response to perturbations. However, older adults with the highest prefrontal-motor coherence during the post-perturbation, but not pre-perturbation, period showed greater cognitive dual-task interference (DTI) and elicited stepping reactions at lower perturbation magnitudes. Our results support motor cortical beta activity as a potential biomarker for individual level of balance challenge and implicate prefrontal-motor cortical networks in distinct aspects of balance control involving response inhibition of reactive stepping in older adults. Cortical network activity during balance may provide a neural target for precision-medicine efforts aimed at fall prevention with aging.

Effects of age and left hemisphere lesions on audiovisual integration of speech

Neuroimaging studies have implicated left temporal lobe regions in audiovisual integration of speech and inferior parietal regions in temporal binding of incoming signals. However, it remains unclear which regions are necessary for audiovisual integration, especially when the auditory and visual signals are offset in time. Aging also influences integration, but the nature of this influence is unresolved. We used a McGurk task to test audiovisual integration and sensitivity to the timing of audiovisual signals in two older adult groups: left hemisphere stroke survivors and controls. We observed a positive relationship between age and audiovisual speech integration in both groups, and an interaction indicating that lesions reduce sensitivity to timing offsets between signals. Lesion-symptom mapping demonstrated that damage to the left supramarginal gyrus and planum temporale reduces temporal acuity in audiovisual speech perception. This suggests that a process mediated by these structures identifies asynchronous audiovisual signals that should not be integrated.

Impaired Sensorimotor Processing During Complex Gait Precedes Behavioral Changes in Middle-aged Adults

Gait impairment during complex walking in older adults is thought to result from a progressive failure to compensate for deteriorating peripheral inputs by central neural processes. It is the primary hypothesis of this article that failure of higher cerebral adaptations may already be present in middle-aged adults who do not present observable gait impairments. We, therefore, compared metabolic brain activity during steering of gait (ie, complex locomotion) and straight walking (ie, simple locomotion) in young and middle-aged individuals. Cerebral distribution of [18F]-fluorodeoxyglucose, a marker of brain synaptic activity, was assessed during over ground straight walking and steering of gait using positron emission tomography in seven young adults (aged 24 ± 3) and seven middle-aged adults (aged 59 ± 3). Brain regions involved in steering of gait (posterior parietal cortex, superior frontal gyrus, and cerebellum) are retained in middle age. However, despite similar walking performance, there are age-related differences in the distribution of [18F]-fluorodeoxyglucose during steering: middle-aged adults have (i) increased activation of precentral and fusiform gyri, (ii) reduced deactivation of multisensory cortices (inferior frontal, postcentral, and fusiform gyri), and (iii) reduced activation of the middle frontal gyrus and cuneus. Our results suggest that preclinical decline in central sensorimotor processing in middle age is observable during complex walking.

Cross-modal interference-control is reduced in childhood but maintained in aging: A cohort study of stimulus- and response-interference in cross-modal and unimodal Stroop tasks

Interference-control is the ability to exclude distractions and focus on a specific task or stimulus. However, it is currently unclear whether the same interference-control mechanisms underlie the ability to ignore unimodal and cross-modal distractions. In 2 experiments we assessed whether unimodal and cross-modal interference follow similar trajectories in development and aging and occur at similar processing levels. In Experiment 1, 42 children (6-11 years), 31 younger adults (18-25 years) and 32 older adults (60-84 years) identified color rectangles with either written (unimodal) or spoken (cross-modal) distractor-words. Stimuli could be congruent, incongruent but mapped to the same response (stimulus-incongruent), or incongruent and mapped to different responses (response-incongruent); thus, separating interference occurring at early (sensory) and late (response) processing levels. Unimodal interference was worst in childhood and old age; however, older adults maintained the ability to ignore cross-modal distraction. Unimodal but not cross-modal response-interference also reduced accuracy. In Experiment 2 we compared the effect of audition on vision and vice versa in 52 children (6-11 years), 30 young adults (22-33 years) and 30 older adults (60-84 years). As in Experiment 1, older adults maintained the ability to ignore cross-modal distraction arising from either modality, and neither type of cross-modal distraction limited accuracy in adults. However, cross-modal distraction still reduced accuracy in children and children were more slowed by stimulus-interference compared with adults. We conclude that; unimodal and cross-modal interference follow different life span trajectories and differences in stimulus- and response-interference may increase cross-modal distractibility in childhood. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Sensory dominance and multisensory integration as screening tools in aging

Multisensory information typically confers neural and behavioural advantages over unisensory information. We used a simple audio-visual detection task to compare healthy young (HY), healthy older (HO) and mild-cognitive impairment (MCI) individuals. Neuropsychological tests assessed individuals' learning and memory impairments. First, we provide much-needed clarification regarding the presence of enhanced multisensory benefits in both healthily and abnormally aging individuals. The pattern of sensory dominance shifted with healthy and abnormal aging to favour a propensity of auditory-dominant behaviour (i.e., detecting sounds faster than flashes). Notably, multisensory benefits were larger only in healthy older than younger individuals who were also visually-dominant. Second, we demonstrate that the multisensory detection task offers benefits as a time- and resource-economic MCI screening tool. Receiver operating characteristic (ROC) analysis demonstrated that MCI diagnosis could be reliably achieved based on the combination of indices of multisensory integration together with indices of sensory dominance. Our findings showcase the importance of sensory profiles in determining multisensory benefits in healthy and abnormal aging. Crucially, our findings open an exciting possibility for multisensory detection tasks to be used as a cost-effective screening tool. These findings clarify relationships between multisensory and memory functions in aging, while offering new avenues for improved dementia diagnostics.

Ageing-related changes in the cortical processing of otolith information in humans

Acoustic short tone bursts (STB) trigger ocular and cervical vestibular-evoked myogenic potentials (oVEMPs/cVEMPs) by activating irregular otolith afferents. Simultaneously, STBs introduce an artificial net acceleration signal of otolith origin into the vestibular network. VEMP parameters as diagnostic otolith processing markers have been shown to decline after the age of thirty. To delineate the differential effects of healthy ageing on the cortical vestibular subnetwork processing otolith information, we measured cVEMPs and the differential effects of unilateral STB in three age groups (20-40, 40-60 and 60+; n = 42) using functional neuroimaging. STB evoked responses in the main vestibular hubs in the parieto-opercular cortex. Whereas cVEMP amplitudes declined linearly with age, analysis of the BOLD response size depicted a u-shaped curve. Vestibular perception of the otolith stimulus on the other hand remained unchanged with age. Therefore, we propose that the comparably larger BOLD responses past the age of sixty could reflect a mechanism of central sensitisation for otolith perception to counterbalance the concurrent peripheral vestibular and somatosensory function decline.

Effects of Aging in Multisensory Integration: A Systematic Review

Multisensory integration (MSI) is the integration by the brain of environmental information acquired through more than one sense. Accurate MSI has been shown to be a key component of successful aging and to be crucial for processes underlying activities of daily living (ADLs). Problems in MSI could prevent older adults (OA) to age in place and live independently. However, there is a need to know how to assess changes in MSI in individuals. This systematic review provides an overview of tests assessing the effect of age on MSI in the healthy elderly population (aged 60 years and older). A literature search was done in Scopus. Articles from the earliest records available to January 20, 2016, were eligible for inclusion if assessing effects of aging on MSI in the healthy elderly population compared to younger adults (YA). These articles were rated for risk of bias with the Newcastle-Ottawa quality assessment. Out of 307 identified research articles, 49 articles were included for final review, describing 69 tests. The review indicated that OA maximize the use of multiple sources of information in comparison to YA (20 studies). In tasks that require more cognitive function, or when participants need to adapt rapidly to a situation, or when a dual task is added to the experiment, OA have problems selecting and integrating information properly as compared to YA (19 studies). Additionally, irrelevant or wrong information (i.e., distractors) has a greater impact on OA than on YA (21 studies). OA failing to weigh sensory information properly, has not been described in previous reviews. Anatomical changes (i.e., reduction of brain volume and differences of brain areas' recruitment) and information processing changes (i.e., general cognitive slowing, inverse effectiveness, larger time window of integration, deficits in attentional control and increased noise at baseline) can only partly explain the differences between OA and YA regarding MSI. Since we have an interest in successful aging and early detection of MSI issues in the elderly population, the identified tests form a good starting point to develop a clinically useful toolkit to assess MSI in healthy OA.

Representational Account of Memory: Insights from Aging and Synesthesia

The representational account of memory envisages perception and memory to be on a continuum rather than in discretely divided brain systems [Bussey, T. J., & Saksida, L. M. Memory, perception, and the ventral visual-perirhinal-hippocampal stream: Thinking outside of the boxes. Hippocampus, 17, 898-908, 2007]. We tested this account using a novel between-group design with young grapheme-color synesthetes, older adults, and young controls. We investigated how the disparate sensory-perceptual abilities between these groups translated into associative memory performance for visual stimuli that do not induce synesthesia. ROI analyses of the entire ventral visual stream showed that associative retrieval (a pair-associate retrieved in the absence of a visual stimulus) yielded enhanced activity in young and older adults' visual regions relative to synesthetes, whereas associative recognition (deciding whether a visual stimulus was the correct pair-associate) was characterized by enhanced activity in synesthetes' visual regions relative to older adults. Whole-brain analyses at associative retrieval revealed an effect of age in early visual cortex, with older adults showing enhanced activity relative to synesthetes and young adults. At associative recognition, the group effect was reversed: Synesthetes showed significantly enhanced activity relative to young and older adults in early visual regions. The inverted group effects observed between retrieval and recognition indicate that reduced sensitivity in visual cortex (as in aging) comes with increased activity during top-down retrieval and decreased activity during bottom-up recognition, whereas enhanced sensitivity (as in synesthesia) shows the opposite pattern. Our results provide novel evidence for the direct contribution of perceptual mechanisms to visual associative memory based on the examples of synesthesia and aging.

Selective Attention and Sensory Modality in Aging: Curses and Blessings

The notion that selective attention is compromised in older adults as a result of impaired inhibitory control is well established. Yet it is primarily based on empirical findings covering the visual modality. Auditory and especially, cross-modal selective attention are remarkably underexposed in the literature on aging. In the past 5 years, we have attempted to fill these voids by investigating performance of younger and older adults on equivalent tasks covering all four combinations of visual or auditory target, and visual or auditory distractor information. In doing so, we have demonstrated that older adults are especially impaired in auditory selective attention with visual distraction. This pattern of results was not mirrored by the results from our psychophysiological studies, however, in which both enhancement of target processing and suppression of distractor processing appeared to be age equivalent. We currently conclude that: (1) age-related differences of selective attention are modality dependent; (2) age-related differences of selective attention are limited; and (3) it remains an open question whether modality-specific age differences in selective attention are due to impaired distractor inhibition, impaired target enhancement, or both. These conclusions put the longstanding inhibitory deficit hypothesis of aging in a new perspective.

Cortical activity predicts which older adults recognize speech in noise and when

Speech recognition in noise can be challenging for older adults and elicits elevated activity throughout a cingulo-opercular network that is hypothesized to monitor and modify behaviors to optimize performance. A word recognition in noise experiment was used to test the hypothesis that cingulo-opercular engagement provides performance benefit for older adults. Healthy older adults (N = 31; 50-81 years of age; mean pure tone thresholds <32 dB HL from 0.25 to 8 kHz, best ear; species: human) performed word recognition in multitalker babble at 2 signal-to-noise ratios (SNR = +3 or +10 dB) during a sparse sampling fMRI experiment. Elevated cingulo-opercular activity was associated with an increased likelihood of correct recognition on the following trial independently of SNR and performance on the preceding trial. The cingulo-opercular effect increased for participants with the best overall performance. These effects were lower for older adults compared with a younger, normal-hearing adult sample (N = 18). Visual cortex activity also predicted trial-level recognition for the older adults, which resulted from discrete decreases in activity before errors and occurred for the oldest adults with the poorest recognition. Participants demonstrating larger visual cortex effects also had reduced fractional anisotropy in an anterior portion of the left inferior frontal-occipital fasciculus, which projects between frontal and occipital regions where activity predicted word recognition. Together, the results indicate that older adults experience performance benefit from elevated cingulo-opercular activity, but not to the same extent as younger adults, and that declines in attentional control can limit word recognition.

Use of Nerve Conduction Velocity to Assess Peripheral Nerve Health in Aging Mice

Nerve conduction velocity (NCV), the speed at which electrical signals propagate along peripheral nerves, is used in the clinic to evaluate nerve function in humans. A decline in peripheral nerve function is associated with a number of age-related pathologies. While several studies have shown that NCV declines with age in humans, there is little information on the effect of age on NCV in peripheral nerves in mice. In this study, we evaluated NCV in male and female C57Bl/6 mice ranging from 4 to 32 months of age. We observed a decline in NCV in both male and female mice after 20 months of age. Sex differences were detected in sensory NCV as well as the rate of decline during aging in motor nerves; female mice had slower sensory NCV and a slower age-related decline in motor nerves compared with male mice. We also tested the effect of dietary restriction on NCV in 30-month-old female mice. Dietary restriction prevented the age-related decline in sciatic NCV but not other nerves. Because NCV is clinically relevant to the assessment of nerve function, we recommend that NCV be used to evaluate healthspan in assessing genetic and pharmacological interventions that increase the life span of mice.

The sound-induced flash illusion reveals dissociable age-related effects in multisensory integration

While aging can lead to significant declines in perceptual and cognitive function, the effects of age on multisensory integration, the process in which the brain combines information across the senses, are less clear. Recent reports suggest that older adults are susceptible to the sound-induced flash illusion (Shams et al., 2000) across a much wider range of temporal asynchronies than younger adults (Setti et al., 2011). To assess whether this cost for multisensory integration is a general phenomenon of combining asynchronous audiovisual input, we compared the time courses of two variants of the sound-induced flash illusion in young and older adults: the fission illusion, where one flash accompanied by two beeps appears as two flashes, and the fusion illusion, where two flashes accompanied by one beep appear as one flash. Twenty-five younger (18–30 years) and older (65+ years) adults were required to report whether they perceived one or two flashes, whilst ignoring irrelevant auditory beeps, in bimodal trials where auditory and visual stimuli were separated by one of six stimulus onset asynchronies (SOAs). There was a marked difference in the pattern of results for the two variants of the illusion. In conditions known to produce the fission illusion, older adults were significantly more susceptible to the illusion at longer SOAs compared to younger participants. In contrast, the performance of the younger and older groups was almost identical in conditions known to produce the fusion illusion. This surprising difference between sound-induced fission and fusion in older adults suggests dissociable age-related effects in multisensory integration, consistent with the idea that these illusions are mediated by distinct neural mechanisms.

Functional brain networks formed using cross-sample entropy are scale free

Over the previous decade, there has been an explosion of interest in network science, in general, and its application to the human brain, in particular. Most brain network investigations to date have used linear correlations (LinCorr) between brain areas to construct and then interpret brain networks. In this study, we applied an entropy-based method to establish functional connectivity between brain areas. This method is sensitive to both nonlinear and linear associations. The LinCorr-based and entropy-based techniques were applied to resting-state functional magnetic resonance imaging data from 10 subjects, and the resulting networks were compared. The networks derived from the entropy-based method exhibited power-law degree distributions. Moreover, the entropy-based networks had a higher clustering coefficient and a shorter path length compared with that of the LinCorr-based networks. While the LinCorr-based networks were assortative, with nodes with similar degrees preferentially connected, the entropy-based networks were disassortative, with high-degree hubs directly connected to low-degree nodes. It is likely that the differences in clustering and assortativity are due to "mega-hubs" in the entropy-based networks. These mega-hubs connect to a large majority of the nodes in the network. This is the first work clearly demonstrating differences between functional brain networks using linear and nonlinear techniques. The key finding is that the nonlinear technique produced networks with scale-free degree distributions. There remains debate among the neuroscience community as to whether human brains are scale free. These data support the argument that at least some aspects of the human brain are perhaps scale free.

Aging causes a reorganization of cortical and spinal control of posture

Classical studies in animal preparations suggest a strong role for spinal control of posture. In humans it is now established that the cerebral cortex contributes to postural control of unperturbed and perturbed standing. The age-related degeneration and accompanying functional changes in the brain, reported so far mainly in conjunction with simple manual motor tasks, may also affect the mechanisms that control complex motor tasks involving posture. This review outlines the age-related structural and functional changes at spinal and cortical levels and provides a mechanistic analysis of how such changes may be linked to the behaviorally manifest postural deficits in old adults. The emerging picture is that the age-related reorganization in motor control during voluntary tasks, characterized by differential modulation of spinal reflexes, greater cortical activation and cortical disinhibition, is also present during postural tasks. We discuss the possibility that this reorganization underlies the increased coactivation and dual task interference reported in elderly. Finally, we propose a model for future studies to unravel the structure-function-behavior relations in postural control and aging.

Age-related neural correlates of cognitive task performance under increased postural load

Behavioral studies suggest that postural control requires increased cognitive control and visuospatial processing with aging. Consequently, performance can decline when concurrently performing a postural and a demanding cognitive task. We aimed to identify the neural substrate underlying this effect. A demanding cognitive task, requiring visuospatial transformations, was performed with varying postural loads. More specifically, old and young subjects performed mental rotations of abstract figures in a seated position and when standing on a force platform. Additionally, functional magnetic resonance imaging (fMRI) was used to identify brain regions associated with mental rotation performance. Old as compared to young subjects showed increased blood oxygenation level-dependent (BOLD) responses in a frontoparietal network as well as activations in additional areas. Despite this overall increased activation, they could still modulate BOLD responses with increasing task complexity. Importantly, activity in left lingual gyrus was highly predictive (r = -0.83, adjusted R(2) = 0.65) of the older subjects' degree of success in mental rotation performance when shifting from a sitting to a standing position. More specifically, increased activation in this area was associated with better performance, once postural load increased.

Defining nodes in complex brain networks

Network science holds great promise for expanding our understanding of the human brain in health, disease, development, and aging. Network analyses are quickly becoming the method of choice for analyzing functional MRI data. However, many technical issues have yet to be confronted in order to optimize results. One particular issue that remains controversial in functional brain network analyses is the definition of a network node. In functional brain networks a node represents some predefined collection of brain tissue, and an edge measures the functional connectivity between pairs of nodes. The characteristics of a node, chosen by the researcher, vary considerably in the literature. This manuscript reviews the current state of the art based on published manuscripts and highlights the strengths and weaknesses of three main methods for defining nodes. Voxel-wise networks are constructed by assigning a node to each, equally sized brain area (voxel). The fMRI time-series recorded from each voxel is then used to create the functional network. Anatomical methods utilize atlases to define the nodes based on brain structure. The fMRI time-series from all voxels within the anatomical area are averaged and subsequently used to generate the network. Functional activation methods rely on data from traditional fMRI activation studies, often from databases, to identify network nodes. Such methods identify the peaks or centers of mass from activation maps to determine the location of the nodes. Small (~10-20 millimeter diameter) spheres located at the coordinates of the activation foci are then applied to the data being used in the network analysis. The fMRI time-series from all voxels in the sphere are then averaged, and the resultant time series is used to generate the network. We attempt to clarify the discussion and move the study of complex brain networks forward. While the "correct" method to be used remains an open, possibly unsolvable question that deserves extensive debate and research, we argue that the best method available at the current time is the voxel-wise method.

Preserved discrimination performance and neural processing during crossmodal attention in aging

In a recent study in younger adults (19-29 year olds) we showed evidence that distributed audiovisual attention resulted in improved discrimination performance for audiovisual stimuli compared to focused visual attention. Here, we extend our findings to healthy older adults (60-90 year olds), showing that performance benefits of distributed audiovisual attention in this population match those of younger adults. Specifically, improved performance was revealed in faster response times for semantically congruent audiovisual stimuli during distributed relative to focused visual attention, without any differences in accuracy. For semantically incongruent stimuli, discrimination accuracy was significantly improved during distributed relative to focused attention. Furthermore, event-related neural processing showed intact crossmodal integration in higher performing older adults similar to younger adults. Thus, there was insufficient evidence to support an age-related deficit in crossmodal attention.

Association between prefrontal activity and volume change in prefrontal and medial temporal lobes in aging and dementia: a review

Functional neuroimaging studies have consistently reported age-related changes in prefrontal cortex (PFC) activity during a variety of cognitive tasks, including episodic memory. These changes are often interpreted within the context of one of the following three neural models of age-related changes in brain function: dedifferentiation, neural inefficiency, and neural plasticity and compensation models. Distinguishing between these competing models has proven difficult when interpreting results using functional imaging data alone. In this paper we suggest that a more accurate interpretation of age-related changes in PFC activity requires consideration of age-related differences in gray matter volume (GMv) in PFC and the medial temporal lobes (MTL). We review fMRI studies of cognitive aging that have directly examined the relationship between PFC activity and both local (PFC) and distal (MTL) GMv in older versus younger adults. We also considered how structure-function relationships may be further modified in pathological aging (i.e. mild cognitive impairment (MCI) and Alzheimer's disease (AD)). We found that when task performance was matched between age-groups there was a negative association between regional PFC volume and activity in older adults. However, when older adults performed worse than young adults we observed a positive association between volume and activity in right lateral PFC. Additionally during memory tasks, several studies revealed that PFC activity is positively related to GM volume in MTL in healthy older adults, but negatively related in MCI and AD patients. We conclude that PFC activity is related to age-related changes in local and distal GM volume reductions and that consideration of these structural measures aids the interpretation of fMRI results. Furthermore, the study of structure-function relationships may provide important insights into the biological mechanisms underlying healthy versus pathological aging.

Effects of age on negative BOLD signal changes in the primary somatosensory cortex

In addition to a contralateral activation of the primary and secondary somatosensory cortices, peripheral sensory stimulation has been shown to elicit responses in the ipsilateral primary somatosensory cortex (SI). In particular, evidence is accumulating that processes of interhemispheric inhibition as depicted by negative blood oxygenation level dependent (BOLD) signal changes are part of somatosensory processes. The aim of the study was to analyze age-related differences in patterns of cerebral activation in the somatosensory system in general and processes of interhemispheric inhibition in particular. For this, a functional magnetic resonance imaging (fMRI) study was performed including 14 younger (mean age 23.3±0.9years) and 13 healthy older participants (mean age 73.2±8.3years). All subjects were scanned during peripheral electrical median nerve stimulation (40Hz) to obtain BOLD responses in the somatosensory system. Moreover, the individual current perception threshold (CPT) as a quantitative measure of sensory function was determined in a separate psychophysical testing. Significant increases in BOLD signal across the entire group could be measured within the contralateral SI, in the bilateral secondary somatosensory cortex (SII), the contralateral supplementary motor area and the insula. Negative BOLD signal changes were delineated in ipsilateral SI/MI as well as in the ipsilateral thalamus and basal ganglia. After comparing the two groups, only the cortical deactivation in ipsilateral SI in the early stimulation phase as well as the activation in contralateral SI and SII in the late stimulation block remained as statistically significant differences between the two groups. The psychophysical experiments yielded a significant age-dependent effect of CPT change with less difference in the older group which is in line with the significantly smaller alterations in maximal BOLD signal change in the contra- and ipsilateral SI found between the two groups. Healthy aging seems to be associated with a decrease in intracerebral inhibition as reflected by smaller negative BOLD signal changes during fMRI tasks. This finding could constitute an important link between age-related neurophysiological changes and behavioral alterations in humans.

Age-related changes in the somatosensory processing of tactile stimulation--an fMRI study

Age-related changes in brain function are complex. Although ageing is associated with a reduction in cerebral blood flow and neuronal activity, task-related processing is often correlated with an enlargement of the corresponding and additionally recruited brain areas. This supplemental employment is considered an attempt to compensate for deficits in the ageing brain. Although there are contradictory reports regarding the role of the primary somatosensory cortex (SI), currently, there is little knowledge about age-related functional changes in other brain areas in the somatosensory network (secondary somatosensory cortex (SII), and insular, anterior (ACC) and posterior cingulate cortices (PCC)). We investigated 16 elderly (age range, 62-71 years) and 18 young subjects (age range, 21-28 years) by determining the current perception threshold (CPT) and applying functional magnetic resonance imaging (fMRI) using a 3.0 Tesla scanner under tactile stimulation of the right hand. CPT was positively correlated with age. fMRI analysis revealed significantly increased activation in the contralateral SI and ipsilateral motor cortex in elderly subjects. Furthermore, we demonstrated age-related reductions in the activity in the SII, ACC, PCC, and dorsal parts of the corpus callosum. Our study revealed dramatic age-related differences in the processing of a simple tactile stimulus in the somatosensory network. Specifically, we detected enhanced activation in the contralateral SI and ipsilateral motor cortex assumingly caused by deficient inhibition and decreased activation in later stages of somatosensory processing (SII, cingulate cortex) in elderly subjects. These results indicate that, in addition to over-activation to compensate for impaired brain functions, there are complex mechanisms of modified inhibition and excitability involved in somatosensory processing in the ageing brain.

Associations among imaging measures (2): the association between gray matter concentration and task-induced activation changes

The association between functional activation and gray matter (GM) structure has been revealed in clinical studies and studies of aging involving a small number of subjects. The purpose of this study was to investigate the association between functional activation maps and GM structures in young adults who do not show apparent GM atrophy and to investigate in detail the nature of this association using a large number of subjects. We used voxel-by-voxel regression analyses to investigate voxel-by-voxel associations between GM concentration (GMC) and contrast estimate images of brain activity during n-back working memory tasks. Associations were assessed for each voxel after regressing out the effects of age, sex, and mean signal intensity during functional magnetic resonance imaging scanning at each voxel using data from 248 normal, right-handed, young adult subjects. In our study, the concept of "the greater the GMC, the greater the task-related activation increase/task-related activation decrease (or the greater the task-related activation change from baseline)" was true for a wide range of activated and deactivated areas. However, in some minor regions, the other pattern of "the greater the GMC, the smaller the task-related activation increase" was observed. The first pattern is often observed at the borders of GM structures. These findings may have to be taken into consideration when group/individual differences in functional activation are investigated.

Lifespan age differences in working memory: a two-component framework

We suggest that working memory (WM) performance can be conceptualized as the interplay of low-level feature binding processes and top-down control, relating to posterior and frontal brain regions and their interaction in a distributed neural network. We propose that due to age-differential trajectories of posterior and frontal brain regions top-down control processes are not fully mature until young adulthood and show marked decline with advancing age, whereas binding processes are relatively mature in children, but show senescent decline in older adults. A review of the literature spanning from middle childhood to old age shows that binding and top-down control processes undergo profound changes across the lifespan. We illustrate commonalities and dissimilarities between children, younger adults, and older adults reflecting the change in the two components' relative contribution to visual WM performance across the lifespan using results from our own lab. We conclude that an integrated account of visual WM lifespan changes combining research from behavioral neuroscience and cognitive psychology of child development as well as aging research opens avenues to advance our understanding of cognition in general.

Effects of age on prefrontal subregions and hippocampal volumes in young and middle-aged healthy humans

There are limited data available regarding the effects of age and sex on discrete prefrontal gray and white matter volumes or posterior and anterior hippocampal volumes in healthy humans. Volumes of the superior frontal gyrus, anterior cingulate gyrus, and orbital frontal lobe were computed manually from contiguous magnetic resonance (MR) images in 83 (39M/44F) healthy humans (age range = 16-40) and segmented into gray and white matter. Volumes of the posterior and anterior hippocampal formation were also computed with reliable separation of the anterior hippocampal formation from the amygdala. There were significant age-by-tissue type interactions for the superior frontal gyrus and orbital frontal lobe such that gray matter within these regions correlated significantly and inversely with age. In contrast, no significant age effects were evident within regional white matter volumes. Analysis of hippocampal volumes indicated that men had larger volumes of the anterior, but not posterior hippocampal formation compared to women even following correction for total brain size. These data highlight age effects within discrete prefrontal cortical gray matter regions in young and middle aged healthy humans and suggest that the white matter comprising these regions may be more resistant to age effects. Furthermore, understanding the potential role of sex and age in mediating prefrontal cortical and hippocampal volumes may have strong relevance for psychiatric disorders such as schizophrenia that have implicated neurodevelopmental abnormalities within frontotemporal circuits in their pathogenesis.

Age and distraction are determinants of performance on a novel visual search task in aged Beagle dogs

Aging has been shown to disrupt performance on tasks that require intact visual search and discrimination abilities in human studies. The goal of the present study was to determine if canines show age-related decline in their ability to perform a novel simultaneous visual search task. Three groups of canines were included: a young group (N = 10; 3 to 4.5 years), an old group (N = 10; 8 to 9.5 years), and a senior group (N = 8; 11 to 15.3 years). Subjects were first tested for their ability to learn a simple two-choice discrimination task, followed by the visual search task. Attentional demands in the task were manipulated by varying the number of distracter items; dogs received an equal number of trials with either zero, one, two, or three distracters. Performance on the two-choice discrimination task varied with age, with senior canines making significantly more errors than the young. Performance accuracy on the visual search task also varied with age; senior animals were significantly impaired compared to both the young and old, and old canines were intermediate in performance between young and senior. Accuracy decreased significantly with added distracters in all age groups. These results suggest that aging impairs the ability of canines to discriminate between task-relevant and -irrelevant stimuli. This is likely to be derived from impairments in cognitive domains such as visual memory and learning and selective attention.

An exponential random graph modeling approach to creating group-based representative whole-brain connectivity networks

Group-based brain connectivity networks have great appeal for researchers interested in gaining further insight into complex brain function and how it changes across different mental states and disease conditions. Accurately constructing these networks presents a daunting challenge given the difficulties associated with accounting for inter-subject topological variability. Viable approaches to this task must engender networks that capture the constitutive topological properties of the group of subjects' networks that it is aiming to represent. The conventional approach has been to use a mean or median correlation network (Achard et al., 2006; Song et al., 2009; Zuo et al., 2011) to embody a group of networks. However, the degree to which their topological properties conform with those of the groups that they are purported to represent has yet to be explored. Here we investigate the performance of these mean and median correlation networks. We also propose an alternative approach based on an exponential random graph modeling framework and compare its performance to that of the aforementioned conventional approach. Simpson et al. (2011) illustrated the utility of exponential random graph models (ERGMs) for creating brain networks that capture the topological characteristics of a single subject's brain network. However, their advantageousness in the context of producing a brain network that "represents" a group of brain networks has yet to be examined. Here we show that our proposed ERGM approach outperforms the conventional mean and median correlation based approaches and provides an accurate and flexible method for constructing group-based representative brain networks.

Cerebral blood flow during rest associates with general intelligence and creativity

Recently, much scientific attention has been focused on resting brain activity and its investigation through such methods as the analysis of functional connectivity during rest (the temporal correlation of brain activities in different regions). However, investigation of the magnitude of brain activity during rest has focused on the relative decrease of brain activity during a task, rather than on the absolute resting brain activity. It is thus necessary to investigate the association between cognitive factors and measures of absolute resting brain activity, such as cerebral blood flow (CBF), during rest (rest-CBF). In this study, we examined this association using multiple regression analyses. Rest-CBF was the dependent variable and the independent variables included two essential components of cognitive functions, psychometric general intelligence and creativity. CBF was measured using arterial spin labeling and there were three analyses for rest-CBF; namely mean gray matter rest-CBF, mean white matter rest-CBF, and regional rest-CBF. The results showed that mean gray and white matter rest-CBF were significantly and positively correlated with individual psychometric intelligence. Furthermore, mean white matter rest-CBF was significantly and positively correlated with creativity. After correcting the effect of mean gray matter rest-CBF the significant and positive correlation between regional rest-CBF in the perisylvian anatomical cluster that includes the left superior temporal gyrus and insula and individual psychometric intelligence was found. Also, regional rest-CBF in the precuneus was significantly and negatively correlated with individual creativity. Significance of these results of regional rest-CBF did not change when the effect of regional gray matter density was corrected. The findings showed mean and regional rest-CBF in healthy young subjects to be correlated with cognitive functions. The findings also suggest that, even in young cognitively intact subjects, resting brain activity (possibly underlain by default cognitive activity or metabolic demand from developed brain structures) is associated with cognitive functions.

Word intelligibility and age predict visual cortex activity during word listening

The distractibility that older adults experience when listening to speech in challenging conditions has been attributed in part to reduced inhibition of irrelevant information within and across sensory systems. Whereas neuroimaging studies have shown that younger adults readily suppress visual cortex activation when listening to auditory stimuli, it is unclear the extent to which declining inhibition in older adults results in reduced suppression or compensatory engagement of other sensory cortices. The current functional magnetic resonance imaging study examined the effects of age and stimulus intelligibility in a word listening task. Across all participants, auditory cortex was engaged when listening to words. However, increasing age and declining word intelligibility had independent and spatially similar effects: both were associated with increasing engagement of visual cortex. Visual cortex activation was not explained by age-related differences in vascular reactivity but rather auditory and visual cortices were functionally connected across word listening conditions. The nature of this correlation changed with age: younger adults deactivated visual cortex when activating auditory cortex, middle-aged adults showed no relation, and older adults synchronously activated both cortices. These results suggest that age and stimulus integrity are additive modulators of crossmodal suppression and activation.

Exponential random graph modeling for complex brain networks

Exponential random graph models (ERGMs), also known as p* models, have been utilized extensively in the social science literature to study complex networks and how their global structure depends on underlying structural components. However, the literature on their use in biological networks (especially brain networks) has remained sparse. Descriptive models based on a specific feature of the graph (clustering coefficient, degree distribution, etc.) have dominated connectivity research in neuroscience. Corresponding generative models have been developed to reproduce one of these features. However, the complexity inherent in whole-brain network data necessitates the development and use of tools that allow the systematic exploration of several features simultaneously and how they interact to form the global network architecture. ERGMs provide a statistically principled approach to the assessment of how a set of interacting local brain network features gives rise to the global structure. We illustrate the utility of ERGMs for modeling, analyzing, and simulating complex whole-brain networks with network data from normal subjects. We also provide a foundation for the selection of important local features through the implementation and assessment of three selection approaches: a traditional p-value based backward selection approach, an information criterion approach (AIC), and a graphical goodness of fit (GOF) approach. The graphical GOF approach serves as the best method given the scientific interest in being able to capture and reproduce the structure of fitted brain networks.

A new measure of centrality for brain networks

Recent developments in network theory have allowed for the study of the structure and function of the human brain in terms of a network of interconnected components. Among the many nodes that form a network, some play a crucial role and are said to be central within the network structure. Central nodes may be identified via centrality metrics, with degree, betweenness, and eigenvector centrality being three of the most popular measures. Degree identifies the most connected nodes, whereas betweenness centrality identifies those located on the most traveled paths. Eigenvector centrality considers nodes connected to other high degree nodes as highly central. In the work presented here, we propose a new centrality metric called leverage centrality that considers the extent of connectivity of a node relative to the connectivity of its neighbors. The leverage centrality of a node in a network is determined by the extent to which its immediate neighbors rely on that node for information. Although similar in concept, there are essential differences between eigenvector and leverage centrality that are discussed in this manuscript. Degree, betweenness, eigenvector, and leverage centrality were compared using functional brain networks generated from healthy volunteers. Functional cartography was also used to identify neighborhood hubs (nodes with high degree within a network neighborhood). Provincial hubs provide structure within the local community, and connector hubs mediate connections between multiple communities. Leverage proved to yield information that was not captured by degree, betweenness, or eigenvector centrality and was more accurate at identifying neighborhood hubs. We propose that this metric may be able to identify critical nodes that are highly influential within the network.

The multifaceted interplay between attention and multisensory integration

Multisensory integration has often been characterized as an automatic process. Recent findings indicate that multisensory integration can occur across various stages of stimulus processing that are linked to, and can be modulated by, attention. Stimulus-driven, bottom-up mechanisms induced by crossmodal interactions can automatically capture attention towards multisensory events, particularly when competition to focus elsewhere is relatively low. Conversely, top-down attention can facilitate the integration of multisensory inputs and lead to a spread of attention across sensory modalities. These findings point to a more intimate and multifaceted interplay between attention and multisensory integration than was previously thought. We review developments in the current understanding of the interactions between attention and multisensory processing, and propose a framework that unifies previous, apparently discordant, findings.

Comparison of characteristics between region-and voxel-based network analyses in resting-state fMRI data

Small-world networks are a class of networks that exhibit efficient long-distance communication and tightly interconnected local neighborhoods. In recent years, functional and structural brain networks have been examined using network theory-based methods, and consistently shown to have small-world properties. Moreover, some voxel-based brain networks exhibited properties of scale-free networks, a class of networks with mega-hubs. However, there are considerable inconsistencies across studies in the methods used and the results observed, particularly between region-based and voxel-based brain networks. We constructed functional brain networks at multiple resolutions using the same resting-state fMRI data, and compared various network metrics, degree distribution, and localization of nodes of interest. It was found that the networks with higher resolutions exhibited the properties of small-world networks more prominently. It was also found that voxel-based networks were more robust against network fragmentation compared to region-based networks. Although the degree distributions of all networks followed an exponentially truncated power law rather than true power law, the higher the resolution, the closer the distribution was to a power law. The voxel-based analyses also enhanced visualization of the results in the 3D brain space. It was found that nodes with high connectivity tended have high efficiency, a co-localization of properties that was not as consistently observed in the region-based networks. Our results demonstrate benefits of constructing the brain network at the finest scale the experiment will permit.

Age-related increase in cross-sensory noise in resting and steady-state cerebral perfusion

Behavioral research indicates that healthy aging is accompanied by maintenance of voluntary attentional function in many situations, suggesting older adults are able to use attention to enhance and suppress neural activity. However, other experiments show increased distractibility with age, suggesting a failure of attention. One hypothesis for these apparently conflicting findings is that older adults experience a greater sensory processing load at baseline compared to younger adults. In this situation, older adults might successfully modulate sensory cortical activity relative to a baseline referent condition, but the increased baseline load results in more activity than younger adults after attentional modulation. This hypothesis was tested by comparing average functional brain activity in auditory cortex using quantitative perfusion imaging during resting state and steady-state visual conditions. It was observed that older adults demonstrated greater processing of task-irrelevant auditory background noise than younger adults in both conditions. As expected, auditory activity was attenuated relative to rest during a visually engaging task for both older and younger participants. However, older adults continued to show greater auditory processing than their younger counterparts even after this task modulation. Furthermore, auditory activity during the visual task was predictive of cross-sensory distraction on a behavioral task in older adults. Together, these findings suggest that older adults are more distractible than younger, and the cause of this increased distractibility may lie in baseline brain functioning.

Negative BOLD in sensory cortices during verbal memory: a component in generating internal representations?

People tend to close their eyes when trying to retrieve an event or a visual image from memory. However the brain mechanisms behind this phenomenon remain poorly understood. Recently, we showed that during visual mental imagery, auditory areas show a much more robust deactivation than during visual perception. Here we ask whether this is a special case of a more general phenomenon involving retrieval of intrinsic, internally stored information, which would result in crossmodal deactivations in other sensory cortices which are irrelevant to the task at hand. To test this hypothesis, a group of 9 sighted individuals were scanned while performing a memory retrieval task for highly abstract words (i.e., with low imaginability scores). We also scanned a group of 10 congenitally blind, which by definition do not have any visual imagery per se. In sighted subjects, both auditory and visual areas were robustly deactivated during memory retrieval, whereas in the blind the auditory cortex was deactivated while visual areas, shown previously to be relevant for this task, presented a positive BOLD signal. These results suggest that deactivation may be most prominent in task-irrelevant sensory cortices whenever there is a need for retrieval or manipulation of internally stored representations. Thus, there is a task-dependent balance of activation and deactivation that might allow maximization of resources and filtering out of non relevant information to enable allocation of attention to the required task. Furthermore, these results suggest that the balance between positive and negative BOLD might be crucial to our understanding of a large variety of intrinsic and extrinsic tasks including high-level cognitive functions, sensory processing and multisensory integration.