Effects of somatosensory stimulation and attention on human somatosensory cortex: an fMRI study

Simultaneous cortical, subcortical, and brainstem mapping of sensory activation

Nonpainful tactile sensory stimuli are processed in the cortex, subcortex, and brainstem. Recent functional magnetic resonance imaging studies have highlighted the value of whole-brain, systems-level investigation for examining sensory processing. However, whole-brain functional magnetic resonance imaging studies are uncommon, in part due to challenges with signal to noise when studying the brainstem. Furthermore, differentiation of small sensory brainstem structures such as the cuneate and gracile nuclei necessitates high-resolution imaging. To address this gap in systems-level sensory investigation, we employed a whole-brain, multi-echo functional magnetic resonance imaging acquisition at 3T with multi-echo independent component analysis denoising and brainstem-specific modeling to enable detection of activation across the entire sensory system. In healthy participants, we examined patterns of activity in response to nonpainful brushing of the right hand, left hand, and right foot (n = 10 per location), and found the expected lateralization, with distinct cortical and subcortical responses for upper and lower limb stimulation. At the brainstem level, we differentiated the adjacent cuneate and gracile nuclei, corresponding to hand and foot stimulation respectively. Our findings demonstrate that simultaneous cortical, subcortical, and brainstem mapping at 3T could be a key tool to understand the sensory system in both healthy individuals and clinical cohorts with sensory deficits.

Proprioceptive and tactile processing in individuals with Friedreich ataxia: an fMRI study

Objective

Friedreich ataxia (FA) neuropathology affects dorsal root ganglia, posterior columns in the spinal cord, the spinocerebellar tracts, and cerebellar dentate nuclei. The impact of the somatosensory system on ataxic symptoms remains debated. This study aims to better evaluate the contribution of somatosensory processing to ataxia clinical severity by simultaneously investigating passive movement and tactile pneumatic stimulation in individuals with FA.

Methods

Twenty patients with FA and 20 healthy participants were included. All subjects underwent two 6 min block-design functional magnetic resonance imaging (fMRI) paradigms consisting of twelve 30 s alternating blocks (10 brain volumes per block, 120 brain volumes per paradigm) of a tactile oddball paradigm and a passive movement paradigm. Spearman rank correlation tests were used for correlations between BOLD levels and ataxia severity.

Results

The passive movement paradigm led to the lower activation of primary (cSI) and secondary somatosensory cortices (cSII) in FA compared with healthy subjects (respectively 1.1 ± 0.78 vs. 0.61 ± 1.02, p = 0.04, and 0.69 ± 0.5 vs. 0.3 ± 0.41, p = 0.005). In the tactile paradigm, there was no significant difference between cSI and cSII activation levels in healthy controls and FA (respectively 0.88 ± 0.73 vs. 1.14 ± 0.99, p = 0.33, and 0.54 ± 0.37 vs. 0.55 ± 0.54, p = 0.93). Correlation analysis showed a significant correlation between cSI activation levels in the tactile paradigm and the clinical severity (R = 0.481, p = 0.032).

Interpretation

Our study captured the difference between tactile and proprioceptive impairments in FA using somatosensory fMRI paradigms. The lack of correlation between the proprioceptive paradigm and ataxia clinical parameters supports a low contribution of afferent ataxia to FA clinical severity.

Anodal Cerebellar Transcranial Direct Current Stimulation Reduces Motor and Cognitive Symptoms in Friedreich's Ataxia: A Randomized, Sham-Controlled Trial

BACKGROUND

Friedreich Ataxia is the most common recessive ataxia with only one therapeutic drug approved solely in the United States.

OBJECTIVE

The aim of this work was to investigate whether anodal cerebellar transcranial direct current stimulation (ctDCS) reduces ataxic and cognitive symptoms in individuals with Friedreich's ataxia (FRDA) and to assess the effects of ctDCS on the activity of the secondary somatosensory (SII) cortex.

METHODS

We performed a single-blind, randomized, sham-controlled, crossover trial with anodal ctDCS (5 days/week for 1 week, 20 min/day, density current: 0.057 mA/cm2 ) in 24 patients with FRDA. Each patient underwent a clinical evaluation (Scale for the Assessment and Rating of Ataxia, composite cerebellar functional severity score, cerebellar cognitive affective syndrome scale) before and after anodal and sham ctDCS. Activity of the SII cortex contralateral to a tactile oddball stimulation of the right index finger was evaluated with brain functional magnetic resonance imaging at baseline and after anodal/sham ctDCS.

RESULTS

Anodal ctDCS led to a significant improvement in the Scale for the Assessment and Rating of Ataxia (-6.5%) and in the cerebellar cognitive affective syndrome scale (+11%) compared with sham ctDCS. It also led to a significant reduction in functional magnetic resonance imaging signal at the SII cortex contralateral to tactile stimulation (-26%) compared with sham ctDCS.

CONCLUSIONS

One week of treatment with anodal ctDCS reduces motor and cognitive symptoms in individuals with FRDA, likely by restoring the neocortical inhibition normally exerted by cerebellar structures. This study provides class I evidence that ctDCS stimulation is effective and safe in FRDA. © 2023 International Parkinson and Movement Disorder Society.

Visual perceptual load and processing of somatosensory stimuli in primary and secondary somatosensory cortices

Load theory assumes that neural activation to distractors in early sensory cortices is modulated by the perceptual load of a main task, regardless of whether task and distractor share the same sensory modality or not. While several studies have investigated the question of load effects on distractor processing in early sensory areas, there is no functional magnetic resonance imaging (fMRI) study regarding load effects on somatosensory stimuli. Here, we used fMRI to investigate effects of visual perceptual load on neural responses to somatosensory stimuli applied to the wrist in a study with 44 participants. Perceptual load was manipulated by an established sustained visual detection task, which avoided simultaneous target and distractor presentations. Load was operationalized by detection difficulty of subtle or clear color changes of one of 12 rotating dots. While all somatosensory stimuli led to activation in somatosensory areas SI and SII, we found no statistically significant difference in brain activation to these stimuli under high compared to low sustained visual load. Moreover, exploratory Bayesian analyses supported the absence of differences. Thus, our findings suggest a resistance of somatosensory processing to at least some forms of visual perceptual load, possibly due to behavioural relevance of discrete somatosensory stimuli and separable attentional resources for the somatosensory and visual modality.

Somatosensory prediction in the premature neonate brain

Sensory prediction (SP) is at the core of early cognitive development. Impaired SP may be a key to understanding the emergence of neurodevelopmental disorders, however there is little data on how and when this skill emerges. We set out to provide evidence of SP in the brain of premature neonates in the fundamental sensory modality: touch. Using Diffuse Correlation Spectroscopy, we measured blood flow changes in the somatosensory cortex of premature neonates presented with a vibrotactile stimulation-omission sequence. When ISI was fixed, participants presented a decrease in blood flow during stimulus omissions, starting when a stimulus should begin: the expectation of a certain stimulus onset induced deactivation of the somatosensory cortex. When ISI was jittered, we observed an increase in blood flow during omissions: the expectation of a likely but not certain stimulus onset induced activation of the somatosensory cortex. Our results reveal SP in the brain as early as four weeks before term, based on the temporal structure of a unimodal somatosensory stimulation, and show that SP produces opposite regulation of activity in the somatosensory cortex depending on how liable is stimulus onset. Future studies will investigate the predictive value of somatosensory prediction on neurodevelopment in this vulnerable population.

Somatosensory Deviance Detection ERPs and Their Relationship to Analogous Auditory ERPs and Interoceptive Accuracy

Abstract. Automatic deviance detection has been widely explored in terms of mismatch responses (mismatch negativity or mismatch response) and P3a components of event-related potentials (ERPs) under a predictive coding framework; however, the somatosensory mismatch response has been investigated less often regarding the different types of changes than its auditory counterpart. It is not known whether the deviance detection responses from different modalities correlate, reflecting a general prediction error mechanism of the central nervous system. Furthermore, interoceptive functions have been associated with predictive coding theory, but whether interoceptive accuracy correlates with deviance detection brain responses has rarely been investigated. Here, we measured ERPs to changes in somatosensory stimuli’s location and intensity and in sound intensity in healthy adults ( n = 34). Interoceptive accuracy was measured with a heartbeat discrimination task, where participants indicated whether their heartbeats were simultaneous or non-simultaneous with sound stimuli. We found a mismatch response and a P3a response to somatosensory location and auditory intensity changes, but for somatosensory intensity changes, only a P3a response was found. Unexpectedly, there were neither correlations between the somatosensory location deviance and intensity deviance brain responses nor between auditory and somatosensory brain responses. In addition, the brain responses did not correlate with interoceptive accuracy. The results suggest that although deviance detection in the auditory and somatosensory modalities are likely based on similar neural mechanisms at a cellular level, their ERP indexes do not indicate a linear association in sensitivity for deviance detection between the modalities. Furthermore, although sensory deviance detection and interoceptive detection are both associated with predictive coding functions, under these experimental settings, functional relationships were not observed. These results should be taken into account in the future development of theories related to human sensory functions and in extensions of the predictive coding theory in particular.

Changes in the Organization of the Secondary Somatosensory Cortex While Processing Lumbar Proprioception and the Relationship With Sensorimotor Control in Low Back Pain

OBJECTIVES

Patients with nonspecific low back pain (NSLBP) rely more on the ankle compared with the lower back proprioception while standing, perform sit-to-stand-to-sit (STSTS) movements slower, and exhibit perceptual impairments at the lower back. However, no studies investigated whether these sensorimotor impairments relate to a reorganization of the primary and secondary somatosensory cortices (S1 and S2) and primary motor cortex (M1) during proprioceptive processing.

MATERIALS AND METHODS

Proprioceptive stimuli were applied at the lower back and ankle muscles during functional magnetic resonance imaging in 15 patients with NSLBP and 13 controls. The location of the activation peaks during the processing of proprioception within S1, S2, and M1 were determined and compared between groups. Proprioceptive use during postural control was evaluated, the duration to perform 5 STSTS movements was recorded, and participants completed the Fremantle Back Awareness Questionnaire (FreBAQ) to assess back-specific body perception.

RESULTS

The activation peak during the processing of lower back proprioception in the right S2 was shifted laterally in the NSLBP group compared with the healthy group (P=0.007). Moreover, patients with NSLSP performed STSTS movements slower (P=0.018), and reported more perceptual impairments at the lower back (P<0.001). Finally, a significant correlation between a more lateral location of the activation peak during back proprioceptive processing and a more disturbed body perception was found across the total group (ρ=0.42, P=0.025).

CONCLUSIONS

The results suggest that patients with NSLBP show a reorganization of the higher-order processing of lower back proprioception, which could negatively affect spinal control and body perception.

Enhanced positive functional connectivity strength in left-sided chronic subcortical stroke

Patients with stroke often exhibit evidence of abnormal functional connectivity (FC). However, whether and how anatomical distance affects FC at rest remains unclear in patients with chronic subcortical stroke. Eighty-six patients with chronic (more than six months post-onset) subcortical stroke (44 left-sided patients and 42 right-sided patients) with different degrees of functional recovery, and 75 matched healthy controls underwent resting-state functional magnetic resonance imaging scanning. Positive functional connectivity strength (FCS) was computed for each voxel in the brain using a data-driven whole-brain resting state FCS method, which was further divided into short- and long-range FCS. Compared with healthy controls, patients with left-sided infarctions exhibited stronger global- and long-range FCS in the left sensorimotor cortex (SMC), and no significant intergroup difference was found for short-range FCS. No significant differences were found between the patients with right-sided infarctions and healthy controls for global, long- and short-range FCS. These findings suggested that the positive FCS alteration was connection-distance dependent within patients with left-sided chronic subcortical stroke. Also, a positive correlation was found between the FCS in the left SMC and the accuracy of the Flanker test, reflecting a compensatory FCS alteration for altered attention and executive function abilities exhibited by those with left-sided stroke.

Effects of Acute Aerobic Exercise on Response Inhibition in Adult Patients with ADHD

Previous studies suggest beneficial effects of aerobic exercise on executive functions, which are a core deficit in ADHD. The aim of the present fMRI study was to investigate acute effects of aerobic exercise on inhibitory control and related brain activation in adult patients with ADHD. 23 patients and 23 matched healthy controls performed on a Go/No-go task in an MRI scanner, following both, an exercise condition involving 30 min of cycling at moderate intensity, and a control condition. ADHD patients compared to healthy controls showed increased brain activation during successful inhibition in the exercise compared to the control condition in parietal, temporal, and occipital regions. Exercise did not improve behavioral performance in either group, but in ADHD patients, exercise-related increases in brain activation and behavioral task performance (i.e., correct inhibition rate) negatively correlated with correct inhibition rate in the control condition. Thus, patients with worse inhibition performance showed stronger exercise-related enhancements, indicating that the lack of improvements on the behavioral level for the whole patient group could be due to ceiling effects. Our findings might be an important step in understanding the neural basis of exercise effects and could, in the long term, help in developing alternative treatment approaches for ADHD.

Activation of Bilateral Secondary Somatosensory Cortex With Right Hand Touch Stimulation: A Meta-Analysis of Functional Neuroimaging Studies

Background: Brain regions involved in processing somatosensory information have been well documented through lesion, post-mortem, animal, and more recently, structural and functional neuroimaging studies. Functional neuroimaging studies characterize brain activation related to somatosensory processing; yet a meta-analysis synthesis of these findings is currently lacking and in-depth knowledge of the regions involved in somatosensory-related tasks may also be confounded by motor influences. Objectives: Our Activation Likelihood Estimate (ALE) meta-analysis sought to quantify brain regions that are involved in the tactile processing of the right (RH) and left hands (LH) separately, with the exclusion of motor related activity. Methods: The majority of studies (n = 41) measured activation associated with RH tactile stimulation. RH activation studies were grouped into those which conducted whole-brain analyses (n = 29) and those which examined specific regions of interest (ROI; n = 12). Few studies examined LH activation, though all were whole-brain studies (N = 7). Results: Meta-analysis of brain activation associated with RH tactile stimulation (whole-brain studies) revealed large clusters of activation in the left primary somatosensory cortex (S1) and bilaterally in the secondary somatosensory cortex (S2; including parietal operculum) and supramarginal gyrus (SMG), as well as the left anterior cingulate. Comparison between findings from RH whole-brain and ROI studies revealed activation as expected, but restricted primarily to S1 and S2 regions. Further, preliminary analyses of LH stimulation studies only, revealed two small clusters within the right S1 and S2 regions, likely limited due to the small number of studies. Contrast analyses revealed the one area of overlap for RH and LH, was right secondary somatosensory region. Conclusions: Findings from the whole-brain meta-analysis of right hand tactile stimulation emphasize the importance of taking into consideration bilateral activation, particularly in secondary somatosensory cortex. Further, the right parietal operculum/S2 region was commonly activated for right and left hand tactile stimulation, suggesting a lateralized pattern of somatosensory activation in right secondary somatosensory region. Implications for further research and for possible differences in right and left hemispheric stroke lesions are discussed.

Cortical and Subcortical Neural Correlates for Respiratory Sensation in Response to Transient Inspiratory Occlusions in Humans

Cortical and subcortical mechanosensation of breathing can be measured by short respiratory occlusions. However, the corresponding neural substrates involved in the respiratory sensation elicited by a respiratory mechanical stimulus remained unclear. Therefore, we applied the functional magnetic resonance imaging (fMRI) technique to study cortical activations of respiratory mechanosensation. We hypothesized that thalamus, frontal cortex, somatosensory cortex, and inferior parietal cortex would be significantly activated in response to respiratory mechanical stimuli. We recruited 23 healthy adults to participate in our event-designed fMRI experiment. During the 12-min scan, participants breathed with a specialized face-mask. Single respiratory occlusions of 150 ms were delivered every 2–4 breaths. At least 32 successful occlusions were collected for data analysis. The results showed significant neural activations in the thalamus, supramarginal gyrus, middle frontal gyrus, inferior frontal triangularis, and caudate (AlphaSim corrected p < 0.05). In addition, subjective ratings of breathlessness were significantly correlated with the levels of neural activations in bilateral thalamus, right caudate, right supramarginal gyrus, left middle frontal gyrus, left inferior triangularis. Our results demonstrated cortical sources of respiratory sensations elicited by the inspiratory occlusion paradigm in healthy adults were located in the thalamus, supramarginal gyrus, and the middle frontal cortex, inferior frontal triangularis, suggesting subcortical, and cortical neural sources of the respiratory mechanosensation are thalamo-cortical based, especially the connections to the premotor area, middle and ventro-lateral prefrontal cortex, as well as the somatosensory association cortex. Finally, level of neural activation in thalamus is associated with the subjective rating of breathlessness, suggesting respiratory sensory information is gated at the thalamic level.

Top-down modulation of shape and roughness discrimination in active touch by covert attention

Due to limitations in perceptual processing, information relevant to momentary task goals is selected from the vast amount of available sensory information by top-down mechanisms (e.g. attention) that can increase perceptual performance. We investigated how covert attention affects perception of 3D objects in active touch. In our experiment, participants simultaneously explored the shape and roughness of two objects in sequence, and were told afterwards to compare the two objects with regard to one of the two features. To direct the focus of covert attention to the different features we manipulated the expectation of a shape or roughness judgment by varying the frequency of trials for each task (20%, 50%, 80%), then we measured discrimination thresholds. We found higher discrimination thresholds for both shape and roughness perception when the task was unexpected, compared to the conditions in which the task was expected (or both tasks were expected equally). Our results suggest that active touch perception is modulated by expectations about the task. This implies that despite fundamental differences, active and passive touch are affected by feature selective covert attention in a similar way.

Tactile learning transfer from the hand to the face but not to the forearm implies a special hand-face relationship

In the primary somatosensory cortex, large-scale cortical and perceptual changes have been demonstrated following input deprivation. Recently, we found that the cortical and perceptual changes induced by repetitive somatosensory stimulation (RSS) at a finger transfer to the face. However, whether such cross-border changes are specific to the face remains elusive. Here, we investigated whether RSS-induced acuity changes at the finger can also transfer to the forearm, which is the body part represented on the other side of the hand representation. Our results confirmed the transfer of tactile learning from the stimulated finger to the lip, but no significant changes were observed at the forearm. A second experiment revealed that the same regions on the forearm exhibited improved tactile acuity when RSS was applied there, excluding the possibility of low plastic ability at the arm representation. This provides also the first evidence that RSS can be efficient on body parts other than the hand. These results suggest that RSS-induced tactile learning transfers preferentially from the hand to the face rather than to the forearm. This specificity could arise from a stronger functional connectivity between the cortical hand and face representations, reflecting a fundamental coupling between these body parts.

Cervical dystonia: Normal auditory mismatch negativity and abnormal somatosensory mismatch negativity

OBJECTIVE

Previous electrophysiological and psychophysical tests have suggested that somatosensory integration is abnormal in dystonia. Here, we hypothesised that this abnormality could relate to a more general deficit in pre-attentive error/deviant detection in patients with dystonia. We therefore tested patients with dystonia and healthy subjects using a mismatch negativity paradigm (MMN), where evoked potentials generated in response to a standard repeated stimulus are subtracted from the responses to a rare "odd ball" stimulus.

METHODS

We assessed MMN for somatosensory and auditory stimuli in patients with cervical dystonia and healthy age matched controls.

RESULTS

We found a significant group ∗ oddball type interaction effect (F (1, 34) = 4.5, p = 0.04, ρ_I = 0.63). A follow up independent t-test for sMMN data, showed a smaller sMMN amplitude in dystonic patients compared to controls (mean difference control-dystonia: -1.0 µV ± 0.3, p < 0.00, t = -3.1). However the amplitude of aMMN did not differ between groups (mean difference control-dystonia: -0.2 µV ± 0.2, p = 0.24, t = -1.2). We found a positive correlation between somatosensory MMN and somatosensory temporal discrimination threshold.

CONCLUSION

These results suggest that pre-attentive error/deviant detection, specifically in the somatosensory domain, is abnormal in dystonia. This could underlie some previously reported electrophysiological and psychophysical abnormalities of somatosensory integration in dystonia.

SIGNIFICANCE

One could hypothesize a deficit in pre-conscious orientation towards potentially salient signals might lead to a more conservative threshold for decision-making in dystonia.

Weaker Functional Connectivity Strength in Patients with Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is related to cognitive impairments and increased risk for dementia. Neuroimaging studies have demonstrated T2DM-related brain structural and functional changes which are partly associated to the cognitive decline. However, few studies focused on the early neuroimaging findingsin T2DM patients. In this study, a data-driven whole-brain resting state functional connectivity strength (rsFCS) methodwas used to evaluate resting functional changes in 53 T2DM patients compared with 55 matched healthy controls (HCs), and to detect the associations between the rsFCSchanges and cognitive functions in T2DM patients. The T2DM patients exhibited weaker long-range rsFCS in the right insula and weaker short-range rsFCS in the right supramarginalgyrus (SG) compared with the HCs. Additionally, seed-based functional connectivity (FC) analysis revealed weaker FC between the right insula and the bilateral superior parietal lobule (SPL), and between the right SG and the bilateral supplementary motor area (SMA)/right SPL in T2DM patientscompared with the HCs. In T2DM patients, negative correlation was found between the long-range rsFCS in the right insula and HbA1c levels; and the FC between the right SG and the bilateral SMA negatively correlated with TMT-A scores. Our results indicated that the rsFCS alteration occurredbefore obvious cognitive deficits in T2DM patients, which might be helpful for understanding the neuromechanism of cognitive declines in T2DM patients.

Difference in Activity in the Supplementary Motor Area Depending on Limb Combination of Hand-Foot Coordinated Movements

Periodic interlimb coordination shows lower performance when the ipsilateral hand and foot (e.g., right hand and right foot) are simultaneously moved than when the contralateral hand and foot (e.g., right hand and left foot) are simultaneously moved. The present study aimed to investigate how brain activity that is related to the dependence of hand–foot coordination on limb combination, using functional magnetic imaging. Twenty-one right-handed subjects performed periodic coordinated movements of the ipsilateral or contralateral hand and foot in the same or opposite direction in the sagittal plane. Kinematic data showed that performance was lower for the ipsilateral hand–foot coordination than for the contralateral one. A comparison of brain activity between the same and opposite directions showed that there was a greater activation of supplementary motor area for ipsilateral hand–foot coordination as compared to that seen during contralateral hand–foot coordination. We speculate that this might reflect a difference in the degree of inhibition of the neural circuit that disrupts opposite directional movements between ipsilateral and contralateral hand–foot coordinated movements.

Hemispheric Lateralization of Resting-State Functional Connectivity of the Anterior Insula: Association with Age, Gender, and a Novelty-Seeking Trait

Resting-state functional connectivity (rsFC) is widely used to examine cerebral functional organization. The imaging literature has described lateralization of insula activations during cognitive and affective processing. Evidence appears to support a role of the right-hemispheric insula in attentional orientation to salient stimulus, interoception, and physiological arousal, and a role of the left-hemispheric insula in cognitive and affective control, as well as perspective taking. In this study, in a large data set of healthy adults, we examined lateralization of the rsFC of the anterior insula (AI) by computing a laterality index (LI) of connectivity with 54 regions from the Automated Anatomic Labeling atlas. At a corrected threshold (p < 0.001), the AI is left lateralized in connectivity with the dorsomedial prefrontal cortex, superior frontal gyrus, inferior frontal cortex, and posterior orbital gyrus and right lateralized in connectivity with the postcentral gyrus, supramarginal gyrus, and superior parietal lobule. In gender differences, women, but not men, showed right-lateralized connectivity to the thalamus. Furthermore, in a subgroup of participants assessed by the tridimensional personality questionnaire, novelty seeking is correlated with the extent of left lateralization of AI connectivity to the pallidum and putamen in men and with the extent of right lateralization of AI connectivity to the parahippocampal gyrus in women. These findings support hemispheric functional differentiation of the AI.

Neural mechanisms of selective attention in the somatosensory system

Selective attention allows organisms to extract behaviorally relevant information while ignoring distracting stimuli that compete for the limited resources of their central nervous systems. Attention is highly flexible, and it can be harnessed to select information based on sensory modality, within-modality feature(s), spatial location, object identity, and/or temporal properties. In this review, we discuss the body of work devoted to understanding mechanisms of selective attention in the somatosensory system. In particular, we describe the effects of attention on tactile behavior and corresponding neural activity in somatosensory cortex. Our focus is on neural mechanisms that select tactile stimuli based on their location on the body (somatotopic-based attention) or their sensory feature (feature-based attention). We highlight parallels between selection mechanisms in touch and other sensory systems and discuss several putative neural coding schemes employed by cortical populations to signal the behavioral relevance of sensory inputs. Specifically, we contrast the advantages and disadvantages of using a gain vs. spike-spike correlation code for representing attended sensory stimuli. We favor a neural network model of tactile attention that is composed of frontal, parietal, and subcortical areas that controls somatosensory cells encoding the relevant stimulus features to enable preferential processing throughout the somatosensory hierarchy. Our review is based on data from noninvasive electrophysiological and imaging data in humans as well as single-unit recordings in nonhuman primates.

Multilevel Cortical Processing of Somatosensory Novelty: A Magnetoencephalography Study

Using magnetoencephalography (MEG), this study investigates the spatio-temporal dynamics of the multilevel cortical processing of somatosensory change detection. Neuromagnetic signals of 16 healthy adult subjects (7 females and 9 males, mean age 29 ± 3 years) were recorded using whole-scalp-covering MEG while they underwent an oddball paradigm based on simple standard (right index fingertip tactile stimulation) and deviant (simultaneous right index fingertip and middle phalanx tactile stimulation) stimuli gathered into sequences to create and then deviate from stimulus patterns at multiple (local vs. global) levels of complexity. Five healthy adult subjects (3 females and 2 males, mean age 31, 6 ± 2 years) also underwent a similar oddball paradigm in which standard and deviant stimuli were flipped. Local deviations led to a somatosensory mismatch response peaking at 55-130 ms post-stimulus onset with a cortical generator located at the contralateral secondary somatosensory (cSII) cortex. The mismatch response was independent of the deviant stimuli physical characteristics. Global deviants led to a P300 response with cortical sources located bilaterally at temporo-parietal junction (TPJ) and supplementary motor area (SMA). The posterior parietal cortex (PPC) and the SMA were found to generate a contingent magnetic variation (CMV) attributed to top-down expectations. Amplitude of mismatch responses were modulated by top-down expectations and correlated with both the magnitude of the CMV and the P300 amplitude at the right TPJ. These results provide novel empirical evidence for a unified sensory novelty detection system in the human brain by linking detection of salient sensory stimuli in personal and extra-personal spaces to a common framework of multilevel cortical processing.

The Neural Correlates of Long-Term Carryover following Functional Electrical Stimulation for Stroke

Neurorehabilitation effective delivery for stroke is likely to be improved by establishing a mechanistic understanding of how to enhance adaptive plasticity. Functional electrical stimulation is effective at reducing poststroke foot drop; in some patients, the effect persists after therapy has finished with an unknown mechanism. We used fMRI to examine neural correlates of functional electrical stimulation key elements, volitional intent to move and concurrent stimulation, in a group of chronic stroke patients receiving functional electrical stimulation for foot-drop correction. Patients exhibited task-related activation in a complex network, sharing bilateral sensorimotor and supplementary motor activation with age-matched controls. We observed consistent separation of patients with and without carryover effect on the basis of brain responses. Patients who experienced the carryover effect had responses in supplementary motor area that correspond to healthy controls; the interaction between experimental factors in contralateral angular gyrus was seen only in those without carryover. We suggest that the functional electrical stimulation carryover mechanism of action is based on movement prediction and sense of agency/body ownership—the ability of a patient to plan the movement and to perceive the stimulation as a part of his/her own control loop is important for carryover effect to take place.

Anterior insular cortex activity to emotional salience of voices in a passive oddball paradigm

The human voice, which has a pivotal role in communication, is processed in specialized brain regions. Although a general consensus holds that the anterior insular cortex (AIC) plays a critical role in negative emotional experience, previous studies have not observed AIC activation in response to hearing disgust in voices. We used magnetoencephalography to measure the magnetic counterparts of mismatch negativity (MMNm) and P3a (P3am) in healthy adults while the emotionally meaningless syllables dada, spoken as neutral, happy, or disgusted prosodies, along with acoustically matched simple and complex tones, were presented in a passive oddball paradigm. The results revealed that disgusted relative to happy syllables elicited stronger MMNm-related cortical activities in the right AIC and precentral gyrus along with the left posterior insular cortex, supramarginal cortex, transverse temporal cortex, and upper bank of superior temporal cortex. The AIC activity specific to disgusted syllables (corrected p < 0.05) was associated with the hit rate of the emotional categorization task. These findings may clarify the neural correlates of emotional MMNm and lend support to the role of AIC in the processing of emotional salience already at the preattentive level.

Inhibition of somatosensory-evoked cortical responses by a weak leading stimulus

We previously demonstrated that auditory-evoked cortical responses were suppressed by a weak leading stimulus in a manner similar to the prepulse inhibition (PPI) of startle reflexes. The purpose of the present study was to investigate whether a similar phenomenon was present in the somatosensory system, and also whether this suppression reflected an inhibitory process. We recorded somatosensory-evoked magnetic fields following stimulation of the median nerve and evaluated the extent by which they were suppressed by inserting leading stimuli at an intensity of 2.5-, 1.5-, 1.1-, or 0.9-fold the sensory threshold (ST) in healthy participants (Experiment 1). The results obtained demonstrated that activity in the secondary somatosensory cortex in the hemisphere contralateral to the stimulated side (cSII) was significantly suppressed by a weak leading stimulus with the intensity larger than 1.1-fold ST. This result implied that the somatosensory system had an inhibitory process similar to that of PPI. We then presented two successive leading stimuli before the test stimulus, and compared the extent of suppression between the test stimulus-evoked responses and those obtained with the second prepulse alone and with two prepulses (first and second) (Experiment 2). When two prepulses were preceded, cSII responses to the second prepulse were suppressed by the first prepulse, whereas the ability of the second prepulse to suppress the test stimulus remained unchanged. These results suggested the presence of at least two individual pathways; response-generating and inhibitory pathways.

Re-thinking the role of motor cortex: context-sensitive motor outputs?

The standard account of motor control considers descending outputs from primary motor cortex (M1) as motor commands and efference copy. This account has been challenged recently by an alternative formulation in terms of active inference: M1 is considered as part of a sensorimotor hierarchy providing top–down proprioceptive predictions. The key difference between these accounts is that predictions are sensitive to the current proprioceptive context, whereas efference copy is not. Using functional electric stimulation to experimentally manipulate proprioception during voluntary movement in healthy human subjects, we assessed the evidence for context sensitive output from M1. Dynamic causal modeling of functional magnetic resonance imaging responses showed that FES altered proprioception increased the influence of M1 on primary somatosensory cortex (S1). These results disambiguate competing accounts of motor control, provide some insight into the synaptic mechanisms of sensory attenuation and may speak to potential mechanisms of action of FES in promoting motor learning in neurorehabilitation.

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Peripheral functional electrical stimulation provides altered proprioception.

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Altered proprioception and volitional movement interaction is shown in M1 and S1.

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M1–S1 connection is modulated by proprioception and therefore is context-sensitive.

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Context-sensitive M1–S1 pathway supports an active inference motor control account.

Impact of a single bout of aerobic exercise on regional brain perfusion and activation responses in healthy young adults

Purpose

Despite the generally accepted view that aerobic exercise can have positive effects on brain health, few studies have measured brain responses to exercise over a short time span. The purpose of this study was to examine the impact within one hour of a single bout of exercise on brain perfusion and neuronal activation.

Methods

Healthy adults (n = 16; age range: 20–35 yrs) were scanned using Magnetic Resonance Imaging (MRI) before and after 20 minutes of exercise at 70% of their age-predicted maximal heart rate. Pseudo-continuous arterial spin labeling (pcASL) was used to measure absolute cerebral blood flow (CBF) prior to exercise (pre) and at 10 min (post-10) and 40 min (post-40) post-exercise. Blood oxygenation level dependent (BOLD) functional MRI (fMRI) was performed pre and post-exercise to characterize activation differences related to a go/no-go reaction time task.

Results

Compared to pre-exercise levels, grey matter CBF was 11% (±9%) lower at post-10 (P<0.0004) and not different at post-40 (P = 0.12), while global WM CBF was increased at both time points post-exercise (P<0.0006). Regionally, the hippocampus and insula showed a decrease in perfusion in ROI-analysis at post-10 (P<0.005, FDR corrected), whereas voxel-wise analysis identified elevated perfusion in the left medial postcentral gyrus at post-40 compared to pre (p_corrected = 0.05). BOLD activations were consistent between sessions, however, the left parietal operculum showed reduced BOLD activation after exercise.

Conclusion

This study provides preliminary evidence of regionalized brain effects associated with a single bout of aerobic exercise. The observed acute cerebrovascular responses may provide some insight into the brain’s ability to change in relation to chronic interventions.

Set and setting: how behavioral state regulates sensory function and plasticity

Recently developed neuroimaging and electrophysiological techniques are allowing us to answer fundamental questions about how behavioral states regulate our perception of the external environment. Studies using these techniques have yielded surprising insights into how sensory processing is affected at the earliest stages by attention and motivation, and how new sensory information received during wakefulness (e.g., during learning) continues to affect sensory brain circuits (leading to plastic changes) during subsequent sleep. This review aims to describe how brain states affect sensory response properties among neurons in primary and secondary sensory cortices, and how this relates to psychophysical detection thresholds and performance on sensory discrimination tasks. This is not intended to serve as a comprehensive overview of all brain states, or all sensory systems, but instead as an illustrative description of how three specific state variables (attention, motivation, and vigilance [i.e., sleep vs. wakefulness]) affect sensory systems in which they have been best studied.

Involvement of the dorsal and ventral attention networks in oddball stimulus processing: a meta-analysis

The aim of this study was to provide the first, comprehensive meta-analysis of the neuroimaging literature regarding greater neural responses to a deviant stimulus in a stream of repeated, standard stimuli, termed here oddball effects. The meta-analysis of 75 independent studies included a comparison of auditory and visual oddball effects and task-relevant and task-irrelevant oddball effects. The results were interpreted with reference to the model in which a large-scale dorsal frontoparietal network embodies a mechanism for orienting attention to the environment, whereas a large-scale ventral frontoparietal network supports the detection of salient, environmental changes. The meta-analysis yielded three main sets of findings. First, ventral network regions were strongly associated with oddball effects and largely common to auditory and visual modalities, indicating a supramodal "alerting" system. Most ventral network components were more strongly associated with task-relevant than task-irrelevant oddball effects, indicating a dynamic interplay of stimulus saliency and internal goals in stimulus-driven engagement of the network. Second, the bilateral inferior frontal junction, an anterior core of the dorsal network, was strongly associated with oddball effects, suggesting a central role in top-down attentional control. However, other dorsal network regions showed no or only modest association with oddball effects, likely reflecting active engagement during both oddball and standard stimulus processing. Finally, prominent oddball effects outside the two networks included the sensory cortex regions, likely reflecting attentive and preattentive modulation of early sensory activity, and subcortical regions involving the putamen, thalamus, and other areas, likely reflecting subcortical involvement in alerting responses.

Bilateral plasticity of Vibrissae SII representation induced by classical conditioning in mice

The somatosensory cortex in mice contains primary (SI) and secondary (SII) areas, differing in somatotopic precision, topographic organization, and function. The role of SII in somatosensory processing is still poorly understood. SII is activated bilaterally during attentional tasks and is considered to play a role in tactile memory and sensorimotor integration. We measured the plasticity of SII activation after associative learning based on classical conditioning, in which unilateral stimulation of one row of vibrissae was paired with a tail shock. The training consisted of three daily 10 min sessions, during which 40 pairings were delivered. Cortical activation driven by stimulation of vibrissae was mapped with 2-[(14)C]deoxyglucose (2DG) autoradiography 1 d after the end of conditioning. We reported previously that the conditioning procedure resulted in unilateral enlargement of 2DG-labeled cortical representation of the "trained" row of vibrissae in SI. Here, we measured the width and intensity of the labeled region in SII. We found that both measured parameters in SII increased bilaterally. The increase was observed in cortical layers II/III and IV. Apparently, plasticity in SII is not a simple reflection of changes in SI. It may be attributable to bilateral integrative role of SII, its lesser topographical specificity, and strong involvement in attentional processing.