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

BOLD cardiorespiratory pulsatility in the brain: from noise to signal of interest

Functional magnetic resonance imaging (fMRI) based on the Blood Oxygen Level Dependent (BOLD) contrast has been extensively used to map brain activity and connectivity in health and disease. Standard fMRI preprocessing includes different steps to remove confounds unrelated to neuronal activity. First, this narrative review explores how signal fluctuations due to cardiac and respiratory activity, usually considered as "physiological noise" and regressed out from fMRI time series. However, these signal components bear useful information about some mechanisms of brain functioning (e.g., glymphatic clearance) or cerebrovascular compliance in response to arterial pressure waves. Aging and chronic diseases can cause stiffening of the aorta and other main arteries, with a reduced dampening effect resulting in greater transmission of pressure impulses to the brain. Importantly, the continuous hammering of cardiac pulsations can produce local alterations of the mechanical properties of the small cerebral vessels, with a progressive deterioration that ultimately affects neuronal functionality. Second, the review emphasizes how fMRI can study the brain patterns most affected by cardiac pulsations in health and disease with high spatiotemporal resolution, offering the opportunity to identify much more specific risk markers than systemic factors based on measurements of the vascular compliance of large arteries or other global risk factors. In this regard, modern fast fMRI acquisition techniques allow a better characterization of these pulsatile signal components due to reduced aliasing effects, turning what has been traditionally considered as noise in a signal of interest that can be used to develop novel non-invasive biomarkers in different clinical contexts.

Across the adult lifespan the ipsilateral sensorimotor cortex negative BOLD response exhibits decreases in magnitude and spatial extent suggesting declining inhibitory control

Ipsilateral sensorimotor (iSM1) cortex negative BOLD responses (NBR) are observed to unilateral tasks and are thought to reflect a functionally relevant component of sensorimotor inhibition. Evidence suggests that sensorimotor inhibitory mechanisms degrade with age, along with aspects of motor ability and dexterity. However, understanding of age-related changes to NBR is restricted by limited comparisons between young vs old adults groups with relatively small samples sizes. Here we analysed a BOLD fMRI dataset (obtained from the CamCAN repository) of 581 healthy subjects, gender-balanced, sampled from the whole adult lifespan performing a motor response task to an audio-visual stimulus. We aimed to investigate how sensorimotor and default-mode NBR characteristics of magnitude, spatial extent and response shape alter at every decade of the aging process. A linear decrease in iSM1 NBR magnitude was observed across the whole lifespan whereas the contralateral sensorimotor (cSM1) PBR magnitude was unchanged. An age-related decrease in the spatial extent of NBR and an increase in the ipsilateral positive BOLD response (PBR) was observed. This occurred alongside an increasing negative correlation between subject's iSM1 NBR and cSM1 PBR magnitude, reflecting a change in the balance between cortical excitation and inhibition. Conventional GLM analysis, using a canonical haemodynamic response (HR) function, showed disappearance of iSM1 NBR in subjects over 50 years of age. However, a deconvolution analysis showed that the shape of the iSM1 HR altered throughout the lifespan, with delayed time-to-peak and decreased magnitude. The most significant decreases in iSM1 HR magnitude occurred in older age (>60 years) but the first changes in shape and timing occurred as early as 30 years, suggesting possibility of separate mechanisms underlying these alterations. Reanalysis using data-driven HRs for each decade detected significant sensorimotor NBR into late older age, showing the importance of taking changes in HR morphology into account in fMRI aging studies. These results may reflect fMRI measures of the age-related decreases in transcollosal inhibition exerted upon ipsilateral sensorimotor cortex and alterations to the excitatory-inhibitory balance in the sensorimotor network.

Importance of the Primary Motor Cortex in Development of Human Hand/Finger Dexterity

Hand/finger dexterity is well-developed in humans, and the primary motor cortex (M1) is believed to play a particularly important role in it. Here, we show that efficient recruitment of the contralateral M1 and neuronal inhibition of the ipsilateral M1 identified by simple hand motor and proprioceptive tasks are related to hand/finger dexterity and its ontogenetic development. We recruited healthy, right-handed children (n = 21, aged 8–11 years) and adults (n = 23, aged 20–26 years) and measured their brain activity using functional magnetic resonance imaging during active and passive right-hand extension–flexion tasks. We calculated individual active control-related activity (active–passive) to evaluate efficient brain activity recruitment and individual task-related deactivation (neuronal inhibition) during both tasks. Outside the scanner, participants performed 2 right-hand dexterous motor tasks, and we calculated the hand/finger dexterity index (HDI) based on their individual performance. Participants with a higher HDI exhibited less active control-related activity in the contralateral M1 defined by the active and passive tasks, independent of age. Only children with a higher HDI exhibited greater ipsilateral M1 deactivation identified by these tasks. The results imply that hand/finger dexterity can be predicted by recruitment and inhibition styles of the M1 during simple hand sensory–motor tasks.

Reduced frontal white matter microstructure in healthy older adults with low tactile recognition performance

The aging of the nervous system is a heterogeneous process. It remains a significant challenge to identify relevant markers of pathological and healthy brain aging. A central aspect of aging are decreased sensory acuities, especially because they correlate with the decline in higher cognitive functioning. Sensory and higher cognitive processing relies on information flow between distant brain areas. Aging leads to disintegration of the underlying white matter tracts. While this disintegration is assumed to contribute to higher cognitive decline, data linking structural integrity and sensory function are sparse. The investigation of their interrelation may provide valuable insight into the mechanisms of brain aging. We used a combined behavioral and neuroimaging approach and investigated to what extent changes in microstructural white matter integrity reflect performance declines in tactile pattern recognition with aging. Poor performance in older participants was related to decreased integrity in the anterior corpus callosum. Probabilistic tractography showed that this structure is connected to the prefrontal cortices. Our data point to decreased integrity in the anterior corpus callosum as a marker for advanced brain aging. The correlation between impaired tactile recognition and disintegration in frontal brain networks could provide an explanation why the decrease of sensory function predicts cognitive decline.

The impact of movement sonification on haptic perception changes with aging

Combining multisensory sources is crucial to interact with our environment, especially for older people who are facing sensory declines. Here, we examined the influence of textured sounds on haptic exploration of artificial textures in healthy younger and older adults by combining a tactile device (ultrasonic display) with synthetized textured sounds. Participants had to discriminate simulated textures with their right index while they were distracted by three disturbing, more or less textured sounds. These sounds were presented as a real-time auditory feedback based on finger movement sonification and thus gave the sensation that the sounds were produced by the haptic exploration. Finger movement velocity increased across both groups in presence of textured sounds (Rubbing or Squeaking) compared to a non-textured (Neutral) sound. While young adults had the same discrimination threshold, regardless of the sound added, the older adults were more disturbed by the presence of the textured sounds with respect to the Neutral sound. Overall, these findings suggest that irrelevant auditory information was taken into account by all participants, but was appropriately segregated from tactile information by young adults. Older adults failed to segregate auditory information, supporting the hypothesis of general facilitation of multisensory integration with aging.

Retinotopic variations of the negative blood-oxygen-level dependent hemodynamic response function in human primary visual cortex

Functional magnetic resonance imaging (fMRI) measures blood-oxygen-level-dependent (BOLD) contrast that is generally assumed to be linearly related to excitatory neural activity. The positive hemodynamic response function (pHRF) is the positive BOLD response (PBR) evoked by a brief neural stimulation; the pHRF is often used as the impulse response for linear analysis of neural excitation. Many fMRI studies have observed a negative BOLD response (NBR) that is often associated with neural suppression. However, the temporal dynamics of the NBR evoked by a brief stimulus, the negative HRF (nHRF), remains unclear. Here, a unilateral visual stimulus was presented in a slow event-related design to elicit both pHRFs in the stimulus representation (SR), and nHRFs elsewhere. The observed nHRFs were not inverted versions of the pHRF previously reported. They were characterized by a stronger initial negative response followed by a significantly later positive peak. In contralateral primary visual cortex (V1), these differences varied with eccentricity from the SR. Similar nHRFs were observed in ipsilateral V1 with less eccentricity variation. Experiments with the blocked version of the same stimulus confirmed that brain regions presenting the unexpected nHRF dynamics correspond to those presenting a strong NBR. These data demonstrated that shift-invariant temporal linearity did not hold for the NBR while confirming that the PBR maintained rough linearity. Modeling indicated that the observed nHRFs can be created by suppression of both blood flow and oxygen metabolism. Critically, the nHRF can be misinterpreted as a pHRF due to their similarity, which could confound linear analysis for event-related fMRI experiments.NEW & NOTEWORTHY We investigate dynamics of the negative hemodynamic response function (nHRF), the negative blood-oxygen-level-dependent (BOLD) response (NBR) evoked by a brief stimulus, in human early visual cortex. Here, we show that the nHRFs are not inverted versions of the corresponding pHRFs. The nHRF has complex dynamics that varied significantly with eccentricity. The results also show shift-invariant temporal linearity does not hold for the NBR.

Functional brain changes in the elderly for the perception of hand movements: A greater impairment occurs in proprioception than touch

Unlike age-related brain changes linked to motor activity, neural alterations related to self-motion perception remain unknown. Using fMRI data, we investigated age-related changes in the central processing of somatosensory information by inducing illusions of right-hand rotations with specific proprioceptive and tactile stimulation. Functional connectivity during resting-state (rs-FC) was also compared between younger and older participants. Results showed common sensorimotor activations in younger and older adults during proprioceptive and tactile illusions, but less deactivation in various right frontal regions and the precuneus were found in the elderly. Older participants exhibited a less-lateralized pattern of activity across the primary sensorimotor cortices (SM1) in the proprioceptive condition only. This alteration of the interhemispheric balance correlated with declining individual performance in illusion velocity perception from a proprioceptive, but not a tactile, origin. By combining task-related data, rs-FC and behavioral performance, this study provided consistent results showing that hand movement perception was altered in the elderly, with a more pronounced deterioration of the proprioceptive system, likely due to the breakdown of inhibitory processes with aging. Nevertheless, older people could benefit from an increase in internetwork connectivity to overcome this kinesthetic decline.

Altered Cingulate Cortex Functional Connectivity in Normal Aging and Mild Cognitive Impairment

Purpose

Resting-state functional Magnetic Resonance Imaging studies revealed that the brain is organized into specialized networks constituted by regions that show a coherent fluctuation of spontaneous activity. Among these networks, the cingulate cortex appears to play a crucial role, particularly in the default mode network, the dorsal attention network and the salience network. In the present study, we mapped the functional connectivity (FC) pattern of different regions of the cingulate cortex: the anterior cingulate cortex, midcingulate cortex and posterior cingulate cortex/retro splenial cortex, which have been in turn divided into a total of 9 subregions. We compared FC patterns of the cingulate subregions in a sample of mild cognitive impairment patients and healthy elderly subjects.

Methods

We enrolled 19 healthy elders (age range: 61-72 y.o.) and 16 Mild cognitive impairment patients (age range 64-87 y.o.). All participants had comparable levels of education (8-10 years) and were neurologically examined to exclude visual and motor impairments, major medical conditions, psychiatric or neurological disorders and consumption of psychotropic drugs. The diagnosis of mild cognitive impairment was performed according to Petersen criteria. Subjects were evaluated with Mini-Mental State Examination, Frontal Assessment Battery, and prose memory (Babcock story) tests. In addition, with functional Magnetic Resonance Imaging, we investigated resting-state network activities.

Results

Healthy elderly, compared to mild cognitive impairment, showed significant increased level of FC for the ventral part of the anterior cingulate cortex in correspondence to the bilateral caudate and ventromedial prefrontal cortex. Moreover, for the midcingulate cortex the healthy elderly group showed increased levels of FC in the somatomotor region, prefrontal cortex, and superior parietal lobule. Meanwhile, the mild cognitive impairment group showed an increased level of FC for the superior frontal gyrus, frontal eye field and orbitofrontal cortex compared to the healthy elderly group.

Conclusion

Our findings indicate that cognitive decline observed in mild cognitive impairment patients damages the global FC of the cingulate cortex, supporting the idea that abnormalities in resting-state activities of the cingulate cortex could be a useful additional tool in order to better understand the brain mechanisms of MCI.

Developmental Changes in Task-Induced Brain Deactivation in Humans Revealed by a Motor Task

Performing tasks activates relevant brain regions in adults while deactivating task-irrelevant regions. Here, using a well-controlled motor task, we explored how deactivation is shaped during typical human development and whether deactivation is related to task performance. Healthy right-handed children (8-11 years), adolescents (12-15 years), and young adults (20-24 years; 20 per group) underwent functional magnetic resonance imaging with their eyes closed while performing a repetitive button-press task with their right index finger in synchronization with a 1-Hz sound. Deactivation in the ipsilateral sensorimotor cortex (SM1), bilateral visual and auditory (cross-modal) areas, and bilateral default mode network (DMN) progressed with development. Specifically, ipsilateral SM1 and lateral occipital deactivation progressed prominently between childhood and adolescence, while medial occipital (including primary visual) and DMN deactivation progressed from adolescence to adulthood. In adults, greater cross-modal deactivation in the bilateral primary visual cortices was associated with higher button-press timing accuracy relative to the sound. The region-specific deactivation progression in a developmental period may underlie the gradual promotion of sensorimotor function segregation required in the task. Task-induced deactivation might have physiological significance regarding suppressed activity in task-irrelevant regions. Furthermore, cross-modal deactivation develops to benefit some aspects of task performance in adults.

Ageing of the somatosensory system at the periphery: age-related changes in cutaneous mechanoreceptors

Decline of tactile sensation associated with ageing depends on modifications in skin and both central and peripheral nervous systems. At present, age-related changes in the periphery of the somatosensory system, particularly concerning the effects on mechanoreceptors, remain unknown. Here we used immunohistochemistry to analyse the age-dependent changes in Meissner's and Pacinian corpuscles as well as in Merkel cell-neurite complexes. Moreover, variations in the neurotrophic TrkB-BDNF system and the mechanoprotein Piezo2 (involved in maintenance of cutaneous mechanoreceptors and light touch, respectively) were evaluated. The number of Meissner's corpuscles and Merkel cells decreased progressively with ageing. Meissner's corpuscles were smaller, rounded in morphology and located deeper in the dermis, and signs of corpuscular denervation were found in the oldest subjects. Pacinian corpuscles generally showed no relevant age-related alterations. Reduced expression of Piezo2 in the axon of Meissner's corpuscles and in Merkel cells was observed in old subjects, as well was a decline in the BDNF-TrkB neurotrophic system. This study demonstrates that cutaneous Meissner's corpuscles and Merkel cell-neurite complexes (and less evidently Pacinian corpuscles) undergo morphological and size changes during the ageing process, as well as a reduction in terms of density. Furthermore, the mechanoprotein Piezo2 and the neurotrophic TrkB-BDNF system are reduced in aged corpuscles. Taken together, these alterations might explain part of the impairment of the somatosensory system associated with ageing.

Effects of age and individual experiences on tactile perception over the life span in women

Tactile perception results from the interplay of peripheral and central mechanisms for detection and sensation of objects and the discrimination and evaluation of their size, shapes, and surface characteristics. For different tasks, we investigated this interaction between more bottom-up stimulus-driven and rather top-down attention-related and cognitive processes in tactile perception. Moreover, we were interested in effects of age and tactile experiences on this interaction. 299 right-handed women participated in our study and were divided into five age groups: 18-25 years (N = 77), 30-45 years (N = 76), 50-65 years (N = 62), 66-75 years (N = 63) and older than 75 years (N = 21). They filled a questionnaire on tactile experiences and rated their skin as either very dry, dry, normal, or oily. Further they performed three tactile tests with the left and right index fingers. Sensitivity for touch stimuli was assessed with von Frey filaments. A sand paper test was used to examine texture discrimination performance. Spatial discrimination was investigated with a tactile Landolt ring test. Multivariate ANOVA confirmed a linear decline in tactile perceptual skills with age (F(3, 279) = 76.740; p < .000; pEta² = 0.452), starting in early adulthood. Largest age effects were found for the Landolt ring test and smallest age effects for the Sand paper test, indicating different aging slopes. Tactile experiences had a positive effect on tactile performance (F (3,279) = 4.450; p = .005; pEta² = 0.046) and univariate ANOVA confirmed this effect for the sand paper and the Landolt ring test, but not for the von Frey test. Using structural equation modelling, we confirmed two dimensions of tactile performance; one related to more peripheral or early sensory cortical (bottom-up) processes (i.e., sensitivity) and one more associated with cognitive or evaluative (top-down) processes (i.e., perception). Interestingly, the top-down processes were stronger influenced by age than bottom-up ones, suggesting that age-related deficits in tactile performance are mainly caused by a decline of central perceptive-evaluative capacities rather than by reduced sensitivity.

Marked bias towards spontaneous synaptic inhibition distinguishes non-adapting from adapting layer 5 pyramidal neurons in the barrel cortex

Pyramidal neuron subtypes differ in intrinsic electrophysiology properties and dendritic morphology. However, do different pyramidal neuron subtypes also receive synaptic inputs that are dissimilar in frequency and in excitation/inhibition balance? Unsupervised clustering of three intrinsic parameters that vary by cell subtype – the slow afterhyperpolarization, the sag, and the spike frequency adaptation – split layer 5 barrel cortex pyramidal neurons into two clusters: one of adapting cells and one of non-adapting cells, corresponding to previously described thin- and thick-tufted pyramidal neurons, respectively. Non-adapting neurons presented frequencies of spontaneous inhibitory postsynaptic currents (sIPSCs) and spontaneous excitatory postsynaptic currents (sEPSCs) three- and two-fold higher, respectively, than those of adapting neurons. The IPSC difference between pyramidal subtypes was activity independent. A subset of neurons were thy1-GFP positive, presented characteristics of non-adapting pyramidal neurons, and also had higher IPSC and EPSC frequencies than adapting neurons. The sEPSC/sIPSC frequency ratio was higher in adapting than in non-adapting cells, suggesting a higher excitatory drive in adapting neurons. Therefore, our study on spontaneous synaptic inputs suggests a different extent of synaptic information processing in adapting and non-adapting barrel cortex neurons, and that eventual deficits in inhibition may have differential effects on the excitation/inhibition balance in adapting and non-adapting neurons.

Positive and Negative Somatotopic BOLD Responses in Contralateral Versus Ipsilateral Penfield Homunculus

One of the basic properties of sensory cortices is their topographical organization. Most imaging studies explored this organization using the positive blood oxygenation level-dependent (BOLD) signal. Here, we studied the topographical organization of both positive and negative BOLD in contralateral and ipsilateral primary somatosensory cortex (S1). Using phase-locking mapping methods, we verified the topographical organization of contralateral S1, and further showed that different body segments elicit pronounced negative BOLD responses in both hemispheres. In the contralateral hemisphere, we found a sharpening mechanism in which stimulation of a given body segment triggered a gradient of activation with a significant deactivation in more remote areas. In the ipsilateral cortex, deactivation was not only located in the homolog area of the stimulated parts but rather was widespread across many parts of S1. Additionally, analysis of resting-state functional magnetic resonance imaging signal showed a gradient of connectivity to the neighboring contralateral body parts as well as to the ipsilateral homologous area for each body part. Taken together, our results indicate a complex pattern of baseline and activity-dependent responses in the contralateral and ipsilateral sides. Both primary sensory areas were characterized by unique negative BOLD responses, suggesting that they are an important component in topographic organization of sensory cortices.

Anatomical and functional neuroimaging in awake, behaving marmosets

The common marmoset (Callithrix jacchus) is a small New World monkey that has gained significant recent interest in neuroscience research, not only because of its compatibility with gene editing techniques, but also due to its tremendous versatility as an experimental animal model. Neuroimaging modalities, including anatomical (MRI) and functional magnetic resonance imaging (fMRI), complemented by two-photon laser scanning microscopy and electrophysiology, have been at the forefront of unraveling the anatomical and functional organization of the marmoset brain. High-resolution anatomical MRI of the marmoset brain can be obtained with remarkable cytoarchitectonic detail. Functional MRI of the marmoset brain has been used to study various sensory systems, including somatosensory, auditory, and visual pathways, while resting-state fMRI studies have unraveled functional brain networks that bear great correspondence to those previously described in humans. Two-photon laser scanning microscopy of the marmoset brain has enabled the simultaneous recording of neuronal activity from thousands of neurons with single cell spatial resolution. In this article, we aim to review the main results obtained by our group and by our colleagues in applying neuroimaging techniques to study the marmoset brain. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 373-389, 2017.

Quantitative sensory testing of temperature, pain, and touch in adults with Down syndrome

The spinothalamic pathway mediates sensations of temperature, pain, and touch. These functions seem impaired in children with Down syndrome (DS), but have not been extensively examined in adults. The objective of the present study was to compare the spinothalamic-mediated sensory functions between adults with DS and adults from the general population and to examine in the DS group the relationship between the sensory functions and level of intellectual functioning. Quantitative sensory testing (QST) was performed in 188 adults with DS (mean age 37.5 years) and 142 age-matched control participants (median age 40.5 years). Temperature, pain, and touch were evaluated with tests for cold-warm discrimination, sharp-dull discrimination (pinprick), and tactile threshold, respectively. Level of intellectual functioning was estimated with the Social Functioning Scale for Intellectual Disability (intellectual disability level) and the Wechsler Preschool and Primary Scale of Intelligence--Revised (intelligence level). Overall, the difference in spinothalamic-mediated sensory functions between the DS and control groups was not statistically significant. However, DS participants with a lower intelligence level had a statistically significant lower performance on the sharp-dull discrimination test than DS participants with higher intelligence level (adjusted p=.006) and control participants (adjusted p=.017). It was concluded that intellectual functioning level is an important factor to take into account for the assessment of spinothalamic-mediated sensory functioning in adults with DS: a lower level could coincide with impaired sensory functioning, but could also hamper QST assessment.

Age-Related Reduced Somatosensory Gating Is Associated with Altered Alpha Frequency Desynchronization

Sensory gating (SG), referring to an attenuated neural response to the second identical stimulus, is considered as preattentive processing in the central nervous system to filter redundant sensory inputs. Insufficient somatosensory SG has been found in the aged adults, particularly in the secondary somatosensory cortex (SII). However, it remains unclear which variables leading to the age-related somatosensory SG decline. There has been evidence showing a relationship between brain oscillations and cortical evoked excitability. Thus, this study used whole-head magnetoencephalography to record responses to paired-pulse electrical stimulation to the left median nerve in healthy young and elderly participants to test whether insufficient stimulus 1- (S1-) induced event-related desynchronization (ERD) contributes to a less-suppressed stimulus 2- (S2-) evoked response. Our analysis revealed that the minimum norm estimates showed age-related reduction of SG in the bilateral SII regions. Spectral power analysis showed that the elderly demonstrated significantly reduced alpha ERD in the contralateral SII (SIIc). Moreover, it was striking to note that lower S1-induced alpha ERD was associated with higher S2-evoked amplitudes in the SIIc among the aged adults. Conclusively, our findings suggest that age-related decline of somatosensory SG is partially attributed to the altered S1-induced oscillatory activity.

Increasing the reliability of data analysis of functional magnetic resonance imaging by applying a new blockwise permutation method

A recent paper by Eklund et al. (2012) showed that up to 70% false positive results may occur when analyzing functional magnetic resonance imaging (fMRI) data using the statistical parametric mapping (SPM) software, which may mainly be caused by insufficient compensation for the temporal correlation between successive scans. Here, we show that a blockwise permutation method can be an effective alternative to the standard correction method for the correlated residuals in the general linear model, assuming an AR(1)-model as used in SPM for analyzing fMRI data. The blockwise permutation approach including a random shift developed by our group (Adolf et al., 2011) accounts for the temporal correlation structure of the data without having to provide a specific definition of the underlying autocorrelation model. 1465 publicly accessible resting-state data sets were re-analyzed, and the results were compared with those of Eklund et al. (2012). It was found that with the new permutation method the nominal familywise error rate for the detection of activated voxels could be maintained approximately under even the most critical conditions in which Eklund et al. found the largest deviations from the nominal error level. Thus, the method presented here can serve as a tool to ameliorate the quality and reliability of fMRI data analyses.

Vibrotactile discriminative capacity is impacted in a digit-specific manner with concurrent unattended hand stimulation

A number of perceptual and neurophysiological studies have investigated the effects of delivering unilateral versus bilateral tactile sensory stimulation. While a number of studies indicate that perceptual discrimination degrades with opposite-hand stimulation, there have been no reports that examined the digit specificity of cross-hemispheric interactions to discriminative capabilities. The purpose of this study was to determine whether unattended hand (UH) stimulation significantly degraded or improved amplitude discriminative capacity on the attended hand (AH) in a digit-specific manner. The methods are based on a sensory perceptual task (vibrotactile amplitude discriminative capacity on the tips of the fingers D2 and D3 of the left hand) in the absence and presence of conditioning stimuli delivered to D2 and D3 of the right hand. Non-specific equal-amplitude stimulation to D2 and D3 of the UH significantly worsened amplitude discrimination (AD) performance, while delivering unequal-amplitude stimuli to D2 and D3 of the UH worsened task performance only under the condition in which the unattended stimuli failed to appropriately match the stimulus parameters on the AH. Additionally, delivering single-site stimuli to D2 or D3 of the UH resulted in degraded performance on the AD task when the stimulus amplitude did not match the amplitude of the stimulus applied to homologous digits of the AH. The findings demonstrate that there is a reduction in performance under conditions where UH stimulation least matched stimulation applied to the AH, while there was little or no change in performance when stimulus conditions on the homologous digit(s) of the contralateral sites were similar. Results suggest that bilateral interactions influence perception in a context-dependent manner that is digit specific.

Sensory and motor thresholds of transcutaneous electrical stimulation are influenced by gender and age

BACKGROUND

Transcutaneous electrical stimulation (ES) is a therapeutic procedure used in rehabilitation. However, the effectiveness of it depends on sensory responses to pain and motor control in neuromuscular recruitment, considering the differences related to gender and age of the subjects treated, as well as the intensity and frequency of ES.

OBJECTIVE

To determine the threshold of sensory perception (TSP) and the threshold of motor response (TMR) in young and elderly individuals of both genders.

DESIGN

Randomized controlled clinical trial.

SETTING

General community.

PARTICIPANTS

Eighty volunteers with no history of systemic diseases were selected to participate in the study: 40 men and 40 women were subdivided by convenience sampling and age group into young male and female (age 21.6 ± 2.4 years) groups as well as into elderly female and male groups (age 72.6 ± 6.1 years).

INTERVENTIONS

The participants received electrical stimulation (ES) at 5 and 50 Hz, with pulse durations of 20, 100, 400, 1000, and 3000 μs applied on the flexor muscle bellies of the wrist and fingers.

MAIN OUTCOME MEASURES

TSP was identified as the first sensation of increased current intensity and TMR as the minimum muscle contraction detected. The results were submitted to analysis of variance, followed by the Tukey's test, with a significance level of 5%.

RESULTS

TSP was lower than TMR for all pulse durations, regardless of gender and age. In women, TSP was lower than that in young and elderly men at both frequencies. However, TSP was higher in elderly subjects than in younger subjects at 50 Hz for both genders. Age also affected the TMR, presenting higher thresholds in elderly subjects of both genders at 50 Hz; however the same occurred only in male subjects at 5 Hz.

CONCLUSION

Age and gender interfere directly with ES. These variables should be considered during rehabilitation because they indicate that electrical stimulation in elderly women should be carefully performed, as they have lower thresholds than elderly men when polarized currents are used, and there is a risk of skin lesion because of their high thresholds.

Effects of aerobic fitness on aging-related changes of interhemispheric inhibition and motor performance

Physical fitness has been long associated with maintenance and improvement of motor performance as we age. In particular, measures of psychomotor speed and motor dexterity tend to be higher in physically fit aging adults as compared to their sedentary counterparts. Using functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS), we explored the patterns of neural activity that may, in part, account for differences between individuals of varying physical fitness levels. In this study, we enrolled both sedentary and physically fit middle age (40–60) and younger (18–30) adults and measured upper extremity motor performance during behavioral testing. In a follow-up session, we employed TMS and fMRI to assess levels of interhemispheric communication during unimanual tasks. Results show that increased physical fitness is associated with better upper extremity motor performance on distal dexterity assessments and increased levels of interhemispheric inhibition in middle age adults. Further, the functional correlates of changes of ipsilateral activity appears to be restricted to the aging process as younger adults of varying fitness levels do not differ in hemispheric patterns of activity or motor performance. We conclude that sedentary aging confers a loss of interhemispheric inhibition that is deleterious to some aspects of motor function, as early as midlife, but these changes can be mediated by chronic engagement in aerobic exercise.

Structural and functional changes across the visual cortex of a patient with visual form agnosia

Loss of shape recognition in visual-form agnosia occurs without equivalent losses in the use of vision to guide actions, providing support for the hypothesis of two visual systems (for "perception" and "action"). The human individual DF received a toxic exposure to carbon monoxide some years ago, which resulted in a persisting visual-form agnosia that has been extensively characterized at the behavioral level. We conducted a detailed high-resolution MRI study of DF's cortex, combining structural and functional measurements. We present the first accurate quantification of the changes in thickness across DF's occipital cortex, finding the most substantial loss in the lateral occipital cortex (LOC). There are reduced white matter connections between LOC and other areas. Functional measures show pockets of activity that survive within structurally damaged areas. The topographic mapping of visual areas showed that ordered retinotopic maps were evident for DF in the ventral portions of visual cortical areas V1, V2, V3, and hV4. Although V1 shows evidence of topographic order in its dorsal portion, such maps could not be found in the dorsal parts of V2 and V3. We conclude that it is not possible to understand fully the deficits in object perception in visual-form agnosia without the exploitation of both structural and functional measurements. Our results also highlight for DF the cortical routes through which visual information is able to pass to support her well-documented abilities to use visual information to guide actions.

fMRI in the awake marmoset: somatosensory-evoked responses, functional connectivity, and comparison with propofol anesthesia

Functional neuroimaging in animal models is essential for understanding the principles of neurovascular coupling and the physiological basis of fMRI signals that are widely used to study sensory and cognitive processing in the human brain. While hemodynamic responses to sensory stimuli have been characterized in humans, animal studies are able to combine very high resolution imaging with invasive measurements and pharmacological manipulation. To date, most high-resolution studies of neurovascular coupling in small animals have been carried out in anesthetized rodents. Here we report fMRI experiments in conscious, awake common marmosets (Callithrix jacchus), and compare responses to animals anesthetized with propofol. In conscious marmosets, robust BOLD fMRI responses to somatosensory stimulation of the forearm were found in contralateral and ipsilateral regions of the thalamus, primary (SI) and secondary (SII) somatosensory cortex, and the caudate nucleus. These responses were markedly stronger than those in anesthetized marmosets and showed a monotonic increase in the amplitude of the BOLD response with stimulus frequency. On the other hand, anesthesia significantly attenuated responses in thalamus, SI and SII, and abolished responses in caudate and ipsilateral SI. Moreover, anesthesia influenced several other aspects of the fMRI responses, including the shape of the hemodynamic response function and the interareal (SI-SII) spontaneous functional connectivity. Together, these findings demonstrate the value of the conscious, awake marmoset model for studying physiological responses in the somatosensory pathway, in the absence of anesthesia, so that the data can be compared most directly to fMRI in conscious humans.