Finger representations in human primary somatosensory cortex as revealed by high-resolution functional MRI of tactile stimulation

The 3D Structural Architecture of the Human Hand Area Is Nontopographic

The functional topography of the human primary somatosensory cortex hand area is a widely studied model system to understand sensory organization and plasticity. It is so far unclear whether the underlying 3D structural architecture also shows a topographic organization. We used 7 Tesla (7T) magnetic resonance imaging (MRI) data to quantify layer-specific myelin, iron, and mineralization in relation to population receptive field maps of individual finger representations in Brodman area 3b (BA 3b) of human S1 in female and male younger adults. This 3D description allowed us to identify a characteristic profile of layer-specific myelin and iron deposition in the BA 3b hand area, but revealed an absence of structural differences, an absence of low-myelin borders, and high similarity of 3D microstructure profiles between individual fingers. However, structural differences and borders were detected between the hand and face areas. We conclude that the 3D structural architecture of the human hand area is nontopographic, unlike in some monkey species, which suggests a high degree of flexibility for functional finger organization and a new perspective on human topographic plasticity.SIGNIFICANCE STATEMENT Using ultra-high-field MRI, we provide the first comprehensive in vivo description of the 3D structural architecture of the human BA 3b hand area in relation to functional population receptive field maps. High similarity of precise finger-specific 3D profiles, together with an absence of structural differences and an absence of low-myelin borders between individual fingers, reveals the 3D structural architecture of the human hand area to be nontopographic. This suggests reduced structural limitations to cortical plasticity and reorganization and allows for shared representational features across fingers.

Topographic Mapping of the Primary Sensory Cortex Using Intraoperative Optical Imaging and Tactile Irritation

The determination of exact tumor boundaries within eloquent brain regions is essential to maximize the extent of resection. Recent studies showed that intraoperative optical imaging (IOI) combined with median nerve stimulation is a helpful tool for visualization of the primary sensory cortex (PSC). In this technical note, we describe a novel approach of using IOI with painless tactile irritation to demonstrate the feasibility of topographic mapping of different body regions within the PSC. In addition, we compared the IOI results with preoperative functional MRI (fMRI) findings. In five patients with tumors located near the PSC who received tumor removal, IOI with tactile irritation of different body parts and fMRI was applied. We showed that tactile irritation of the hand in local and general anesthesia leads to reliable changes of cerebral blood volume during IOI. Hereby, we observed comparable IOI activation maps regarding the median nerve stimulation, fMRI and tactile irritation of the hand. The tactile irritation of different body areas revealed a plausible topographic distribution along the PSC. With this approach, IOI is also suitable for awake surgeries, since the tactile irritation is painless compared with median nerve stimulation and is congruent to fMRI findings. Further studies are ongoing to standardize this method to enable a broad application within the neurosurgical community.

Resting-state abnormalities in functional connectivity of the default mode network in migraine: A meta-analysis

Migraine-a disabling neurological disorder, imposes a tremendous burden on societies. To reduce the economic and health toll of the disease, insight into its pathophysiological mechanism is key to improving treatment and prevention. Resting-state functional magnetic resonance imaging (rs-fMRI) studies suggest abnormal functional connectivity (FC) within the default mode network (DMN) in migraine patients. This implies that DMN connectivity change may represent a biomarker for migraine. However, the FC abnormalities appear inconsistent which hinders our understanding of the potential neuropathology. Therefore, we performed a meta-analysis of the FC within the DMN in migraine patients in the resting state to identify the common FC abnormalities. With efficient search and selection strategies, nine studies (published before July, 2022) were retrieved, containing 204 migraine patients and 199 healthy subjects. We meta-analyzed the data using the Anisotropic Effect Size version of Signed Differential Mapping (AES-SDM) method. Compared with healthy subjects, migraine patients showed increased connectivity in the right calcarine gyrus, left inferior occipital gyrus, left postcentral gyrus, right cerebellum, right parahippocampal gyrus, and right posterior cingulate gyrus, while decreased connectivity in the right postcentral gyrus, left superior frontal gyrus, right superior occipital gyrus, right orbital inferior frontal gyrus, left middle occipital gyrus, left middle frontal gyrus and left inferior frontal gyrus. These results provide a new perspective for the study of the pathophysiology of migraine and facilitate a more targeted treatment of migraine in the future.

Synaptic reshaping of plastic neuronal networks by periodic multichannel stimulation with single-pulse and burst stimuli

Synaptic dysfunction is associated with several brain disorders, including Alzheimer's disease, Parkinson's disease (PD) and obsessive compulsive disorder (OCD). Utilizing synaptic plasticity, brain stimulation is capable of reshaping synaptic connectivity. This may pave the way for novel therapies that specifically counteract pathological synaptic connectivity. For instance, in PD, novel multichannel coordinated reset stimulation (CRS) was designed to counteract neuronal synchrony and down-regulate pathological synaptic connectivity. CRS was shown to entail long-lasting therapeutic aftereffects in PD patients and related animal models. This is in marked contrast to conventional deep brain stimulation (DBS) therapy, where PD symptoms return shortly after stimulation ceases. In the present paper, we study synaptic reshaping by periodic multichannel stimulation (PMCS) in networks of leaky integrate-and-fire (LIF) neurons with spike-timing-dependent plasticity (STDP). During PMCS, phase-shifted periodic stimulus trains are delivered to segregated neuronal subpopulations. Harnessing STDP, PMCS leads to changes of the synaptic network structure. We found that the PMCS-induced changes of the network structure depend on both the phase lags between stimuli and the shape of individual stimuli. Single-pulse stimuli and burst stimuli with low intraburst frequency down-regulate synapses between neurons receiving stimuli simultaneously. In contrast, burst stimuli with high intraburst frequency up-regulate these synapses. We derive theoretical approximations of the stimulation-induced network structure. This enables us to formulate stimulation strategies for inducing a variety of network structures. Our results provide testable hypotheses for future pre-clinical and clinical studies and suggest that periodic multichannel stimulation may be suitable for reshaping plastic neuronal networks to counteract pathological synaptic connectivity. Furthermore, we provide novel insight on how the stimulus type may affect the long-lasting outcome of conventional DBS. This may strongly impact parameter adjustment procedures for clinical DBS, which, so far, primarily focused on acute effects of stimulation.

In the back of your mind: Cortical mapping of paraspinal afferent inputs

Topographic organisation is a hallmark of vertebrate cortex architecture, characterised by ordered projections of the body's sensory surfaces onto brain systems. High-resolution functional magnetic resonance imaging (fMRI) has proven itself as a valuable tool to investigate the cortical landscape and its (mal-)adaptive plasticity with respect to various body part representations, in particular extremities such as the hand and fingers. Less is known, however, about the cortical representation of the human back. We therefore validated a novel, MRI-compatible method of mapping cortical representations of sensory afferents of the back, using vibrotactile stimulation at varying frequencies and paraspinal locations, in conjunction with fMRI. We expected high-frequency stimulation to be associated with differential neuronal activity in the primary somatosensory cortex (S1) compared with low-frequency stimulation and that somatosensory representations would differ across the thoracolumbar axis. We found significant differences between neural representations of high-frequency and low-frequency stimulation and between representations of thoracic and lumbar paraspinal locations, in several bilateral S1 sub-regions, and in regions of the primary motor cortex (M1). High-frequency stimulation preferentially activated Brodmann Area (BA) regions BA3a and BA4p, whereas low-frequency stimulation was more encoded in BA3b and BA4a. Moreover, we found clear topographic differences in S1 for representations of the upper and lower back during high-frequency stimulation. We present the first neurobiological validation of a method for establishing detailed cortical maps of the human back, which might serve as a novel tool to evaluate the pathological significance of neuroplastic changes in clinical conditions such as chronic low back pain.

Diabetic Neuropathy in Hands: An Endemic Complication Waiting to Unfold?

In this cross-sectional study we aimed to quantify the somatosensory dysfunction in the hand in people with diabetes with distal symmetrical polyneuropathy (DSPN) in hands and explore early signs of nerve dysfunction in people with diabetes without DSPN in hands. The clinical diagnosis of DSPN was confirmed with electrodiagnosis and corneal confocal microscopy. Thermal and mechanical nerve function in the hand was assessed with quantitative sensory tests. Measurements were compared between healthy participants (n = 31), individuals with diabetes without DSPN (n = 35), individuals with DSPN in feet but not hands (DSPNFEET ONLY) (n = 31), and individuals with DSPN in hands and feet (DSPNHANDS & FEET) (n = 28) with one-way between-group ANOVA. The somatosensory profile of the hand in people with DSPNHANDS & FEET showed widespread loss of thermal and mechanical detection. This profile in hands is comparable with the profile in the feet of people with DSPN in feet. Remarkably, individuals with DSPNFEET ONLY already showed a similar profile of widespread loss of nerve function in their hands. People with diabetes without DSPN in feet already had some nerve dysfunction in their hands. These findings suggest that nerve function assessment in hands should become more routine in people with diabetes.

More comprehensive proprioceptive stimulation of the hand amplifies its cortical processing

Corticokinematic coherence (CKC) quantifies the phase coupling between limb kinematics and cortical neurophysiological signals reflecting proprioceptive feedback to the primary sensorimotor (SM1) cortex. We studied whether the CKC strength or cortical source location differs between proprioceptive stimulation (i.e., actuator-evoked movements) of right-hand digits (index, middle, ring, and little). Twenty-one volunteers participated in magnetoencephalography measurements during which three conditions were tested: 1) simultaneous stimulation of all four fingers at the same frequency, 2) stimulation of each finger separately at the same frequency, and 3) simultaneous stimulation of the fingers at finger-specific frequencies. CKC was computed between MEG responses and accelerations of the fingers recorded with three-axis accelerometers. CKC was stronger (P < 0.003) for the simultaneous (0.52 ± 0.02) than separate (0.45 ± 0.02) stimulation at the same frequency. Furthermore, CKC was weaker (P < 0.03) for the simultaneous stimulation at the finger-specific frequencies (0.38 ± 0.02) than for the separate stimulation. CKC source locations of the fingers were concentrated in the hand region of the SM1 cortex and did not follow consistent finger-specific somatotopic order. Our results indicate that proprioceptive afference from the fingers is processed in partly overlapping cortical neuronal circuits, which was demonstrated by the modulation of the finger-specific CKC strengths due to proprioceptive afference arising from simultaneous stimulation of the other fingers of the same hand as well as overlapping cortical source locations. Finally, comprehensive simultaneous proprioceptive stimulation of the hand would optimize functional cortical mapping to pinpoint the hand region, e.g., prior brain surgery.

NEW & NOTEWORTHY Corticokinematic coherence (CKC) can be used to study cortical proprioceptive processing and localize proprioceptive hand representation. Our results indicate that proprioceptive stimulation delivered simultaneously at the same frequency to fingers (D2–D4) maximizes CKC strength allowing robust and fast localization of the human hand region in the sensorimotor cortex using MEG.

Somatotopic Mapping of the Fingers in the Somatosensory Cortex Using Functional Magnetic Resonance Imaging: A Review of Literature

Multiple studies have demonstrated finger somatotopy in humans and other primates using a variety of brain mapping techniques including functional magnetic resonance imaging (fMRI). Here, we review the literature to better understand the reliability of fMRI for mapping the somatosensory cortex. We have chosen to focus on the hand and fingers as these areas have the largest representation and have been the subject of the largest number of somatotopic mapping experiments. Regardless of the methods used, individual finger somatosensory maps were found to be organized across Brodmann areas (BAs) 3b, 1, and 2 in lateral-to-medial and inferior-to-superior fashion moving from the thumb to the pinky. However, some consistent discrepancies are found that depend principally on the method used to stimulate the hand and fingers. Therefore, we suggest that a comparative analysis of different types of stimulation be performed to address the differences described in this review.

Tactile perception of right middle fingertip suppresses excitability of motor cortex supplying right first dorsal interosseous muscle

The present study examined whether tactile perception of the fingertip modulates excitability of the motor cortex supplying the intrinsic hand muscle and whether this modulation is specific to the fingertip stimulated and the muscle and hand tested. Tactile stimulation was given to one of the five fingertips in the left or right hand, and transcranial magnetic stimulation eliciting motor evoked potential in the first dorsal interosseous muscle (FDI) or abductor digiti minimi was given 200 ms after the onset of tactile stimulation. The corticospinal excitability of the FDI at rest was suppressed by the tactile stimulation of the right middle fingertip, but such suppression was absent for the other fingers stimulated and for the other muscle or hand tested. The persistence and amplitude of the F-wave was not significantly influenced by tactile stimulation of the fingertip in the right hand. These findings indicate that tactile perception of the right middle fingertip suppresses excitability of the motor cortex supplying the right FDI at rest. The suppression of corticospinal excitability was absent during tonic contraction of the right FDI, indicating that the motor execution process interrupts the tactile perception-induced suppression of motor cortical excitability supplying the right FDI. These findings are in line with a view that the tactile perception of the right middle finger induces surround inhibition of the motor cortex supplying the prime mover of the finger neighboring the stimulated finger.

Dissociative pseudo-ulnar sensory loss in a patient with a focal haemorrhage secondary to cerebral venous thrombosis - A case report

INTRODUCTION

Small cortical strokes can mimic weakness of peripheral nerve lesions. However, isolated sensory deficits involving the fingers due to cortical lesions are rare.

METHODS

We present a case of a 46 year old man with cerebral venous thrombosis, and a haemorrhage restricted to the postcentral gyrus, who reported numbness in an ulnar neuropathy-like distribution. Testing of somatosensory (SSEP) and pain-related evoked potential (PREP) was done, and the lesion location was mapped to the template brain.

RESULTS

The patient had impaired touch and pain but preserved proprioception. He had a normal SSEP response but a prolonged PREP. The lesion was mapped to Broadmann areas 1 and 3b of the postcentral gyrus.

DISCUSSION

Sensory cortical representation is such that, the ulnar fingers are medial, and the radial ones are lateral. Also, modality-specific organization is noted with tactile sensation being mapped to areas 1 and 3b, and proprioceptive sensation to area 3a and 2. Thus focal lesions involving the post central gyrus can have selective sensory loss over some fingers and can have selective impairment of some modalities.

CONCLUSIONS

We highlight the rare finding of an ulnar-like sensory loss in a patient with cerebral venous thrombosis and the dissociate nature of the sensory loss in isolated cortical lesions.

Quantifying the Generation Process of Multi-Level Tactile Sensations via ERP Component Investigation

Humans obtain characteristic information such as texture and weight of external objects, relying on the brain's integration and classification of tactile information; however, the decoding mechanism of multi-level tactile information is relatively elusive from the temporal sequence. In this paper, nonvariant frequency, along with the variant pulse width of electrotactile stimulus, was performed to generate multi-level pressure sensation. Event-related potentials (ERPs) were measured to investigate the mechanism of whole temporal tactile processing. Five ERP components, containing P100-N140-P200-N200-P300, were observed. By establishing the relationship between stimulation parameters and ERP component amplitudes, we found the following: (1) P200 is the most significant component for distinguishing multi-level tactile sensations; (2) P300 is correlated well with the subjective judgment of tactile sensation. The temporal sequence of brain topographies was implemented to clarify the spatiotemporal characteristics of the tactile process, which conformed to the serial processing model in neurophysiology and cortical network response area described by fMRI. Our results can help further clarify the mechanism of tactile sequential processing, which can be applied to improve the tactile BCI performance, sensory enhancement, and clinical diagnosis for doctors to evaluate the tactile process disorders by examining the temporal ERP components.

Population Receptive Field Characteristics in the between- and Within-Digit Dimensions of the Undominant Hand in the Primary Somatosensory Cortex

Somatotopy is an important guiding principle for sensory fiber organization in the primary somatosensory cortex (S1), which reflects tactile information processing and is associated with disease-related reorganization. However, it is difficult to measure the neuronal encoding scheme in S1 in vivo in normal participants. Here, we investigated the somatotopic map of the undominant hand using a Bayesian population receptive field (pRF) model. The model was established in hand space with between- and within-digit dimensions. In the between-digit dimension, orderly representation was found, which had low variability across participants. The pRF shape tended to be elliptical for digits with high spatial acuity, for which the long axis was along the within-digit dimension. In addition, the pRF width showed different change trends in the 2 dimensions across digits. These results provide new insights into the neural mechanisms in S1, allowing for in-depth investigation of somatosensory information processing and disease-related reorganization.

A touch of hierarchy: population receptive fields reveal fingertip integration in Brodmann areas in human primary somatosensory cortex

Several neuroimaging studies have shown the somatotopy of body part representations in primary somatosensory cortex (S1), but the functional hierarchy of distinct subregions in human S1 has not been adequately addressed. The current study investigates the functional hierarchy of cyto-architectonically distinct regions, Brodmann areas BA3, BA1, and BA2, in human S1. During functional MRI experiments, we presented participants with vibrotactile stimulation of the fingertips at three different vibration frequencies. Using population Receptive Field (pRF) modeling of the fMRI BOLD activity, we identified the hand region in S1 and the somatotopy of the fingertips. For each voxel, the pRF center indicates the finger that most effectively drives the BOLD signal, and the pRF size measures the spatial somatic pooling of fingertips. We find a systematic relationship of pRF sizes from lower-order areas to higher-order areas. Specifically, we found that pRF sizes are smallest in BA3, increase slightly towards BA1, and are largest in BA2, paralleling the increase in visual receptive field size as one ascends the visual hierarchy. Additionally, we find that the time-to-peak of the hemodynamic response in BA3 is roughly 0.5 s earlier compared to BA1 and BA2, further supporting the notion of a functional hierarchy of subregions in S1. These results were obtained during stimulation of different mechanoreceptors, suggesting that different afferent fibers leading up to S1 feed into the same cortical hierarchy.

Impaired Performance in Mental Rotation of Hands and Feet and Its Association with Social Cognition in Patients with Complex Regional Pain Syndrome

Objectives. 

So far, dysfunction in mental rotation has been assessed in relation to the left- or right-sided CRPS. Here we examined mental rotation in patients with upper or lower limb CRPS. Considering the potential role of socio-emotional functioning on the perception of body image, we further investigated the association between performance on mental rotation and socio-emotional characteristics.

Methods. 

We examined the performance of 36 patients with upper or lower limb CRPS on the limb laterality recognition. Accuracy and response times for pictures of hands and feet at 4 rotation angles were evaluated. Socio-emotional functioning was measured by the Interpersonal Reactivity Scale and the Toronto Alexithymia Scale.

Results. 

Patients with upper limb pain showed longer RTs to recognize the laterality of hands than feet (P = 0.002), whereas patients with lower limb pain showed longer RTs for feet than hands (P = 0.039). Exploratory correlation analyses revealed that RTs for feet were negatively correlated with the levels of empathic ability to take another’s perspective (P = 0.006) and positively correlated with the level of emotional difficulty in identifying feelings (P = 0.006).

Conclusions. 

This study is the first to report selectively impaired mental rotation of hands vs feet in patients with upper or lower limb CRPS. The findings suggest that impaired mental rotation derives from relative deficits in the representation of the affected limb. Correlations between impaired mental rotation and socio-emotional inability indicate that an altered body schema may be closely associated with impaired social cognitive aspects in CRPS patients.

The organizational principles of de-differentiated topographic maps in somatosensory cortex

Topographic maps are a fundamental feature of cortex architecture in the mammalian brain. One common theory is that the de-differentiation of topographic maps links to impairments in everyday behavior due to less precise functional map readouts. Here, we tested this theory by characterizing de-differentiated topographic maps in primary somatosensory cortex (SI) of younger and older adults by means of ultra-high resolution functional magnetic resonance imaging together with perceptual finger individuation and hand motor performance. Older adults' SI maps showed similar amplitude and size to younger adults' maps, but presented with less representational similarity between distant fingers. Larger population receptive field sizes in older adults' maps did not correlate with behavior, whereas reduced cortical distances between D2 and D3 related to worse finger individuation but better motor performance. Our data uncover the drawbacks of a simple de-differentiation model of topographic map function, and motivate the introduction of feature-based models of cortical reorganization.

Tactile acuity of fingertips and hand representation size in human Area 3b and Area 1 of the primary somatosensory cortex

Intracortical mapping in monkeys revealed a full body map in all four cytoarchitectonic subdivisions of the contralateral primary somatosensory cortex (S1), as well as positive associations between spatio-tactile acuity performance of the fingers and their representation field size especially within cytoarchitectonic Area 3b and Area 1. Previous non-invasive investigations on these associations in humans assumed a monotonous decrease of representation field size from index finger to little finger although the field sizes are known to change in response to training or in disease. Recent developments improved noninvasive functional mapping of S1 by a) adding a cognitive task during repetitive stimulation to decrease habituation to the stimuli, b) smaller voxel size of fMRI-sequences, c) surface-based analysis accounting for cortical curvature, and d) increase of spatial specificity for fMRI data analysis by avoidance of smoothing, partial volume effects, and pial vein signals. We here applied repetitive pneumatic stimulation of digit 1 (D1; thumb) and digit 5 (D5; little finger) on both hands to investigate finger/hand representation maps in the complete S1, but also in cytoarchitectonic Areas 1, 2, 3a, and 3b separately, in 21 healthy volunteers using 3T fMRI. The distances between activation maxima of D1 and D5 were evaluated by two independent raters, blinded for performance parameters. The fingertip representations showed a somatotopy and were localized in the transition region between the crown and the anterior wall of the post central gyrus agreeing with Area 1 and 3b. Participants were comprehensively tested for tactile performance using von Freyhair filaments to determine cutaneous sensory thresholds (CST) as well as grating orientation thresholds (GOT) and two-point resolution (TPD) for spatio-tactile acuity testing. Motor performance was evaluated with pinch grip performance (Roeder test). We found bilateral associations of D1-D5 distance for GOT thresholds and partially also for TPD in Area 3b and in Area 1, but not if using the complete S1 mask. In conclusion, we here demonstrate that 3T fMRI is capable to map associations between spatio-tactile acuity and the fingertip representation in Area 3b and Area 1 in healthy participants.

Relation between palm and finger cortical representations in primary somatosensory cortex: A 7T fMRI study

Many studies focused on the cortical representations of fingers, while the palm is relatively neglected despite its importance for hand function. Here, we investigated palm representation (PR) and its relationship with finger representations (FRs) in primary somatosensory cortex (S1). Few studies in humans suggested that PR is located medially with respect to FRs in S1, yet to date, no study directly quantified the somatotopic organization of PR and the five FRs. Importantly, the link between the somatotopic organization of PR and FRs and their activation properties remains largely unexplored. Using 7T fMRI, we mapped PR and the five FRs at the single subject level. First, we analyzed the cortical distance between PR and FRs to determine their somatotopic organization. Results show that PR was located medially with respect to D5. Second, we tested whether the observed cortical distances would predict the relationship between PR and FRs activations. Using three complementary measures (cross-activations, pattern similarity and resting-state connectivity), we show that the relationship between PR and FRs activations were not determined by their somatotopic organization, that is, there was no gradient moving from D5 to D1, except for resting-state connectivity, which was predicted by the somatotopy. Instead, we show that the representational geometry of PR and FRs activations reflected the physical structure of the hand. Collectively, our findings suggest that the spatial proximity between topographically organized neuronal populations do not necessarily predicts their functional properties, rather the structure of the sensory space (e.g., the hand shape) better describes the observed results.

Comparison of intrinsic brain activity in individuals with low/moderate myopia versus high myopia revealed by the amplitude of low-frequency fluctuations

Background

Previous neuroimaging studies demonstrated that individuals with high myopia are associated with abnormalities in anatomy of the brain.

Purpose

The purpose of this study was to explore alterations in the intrinsic brain activity by studying the amplitude of low-frequency fluctuations.

Material and Methods

A total of 64 myopia individuals (41 with high myopia with a refractive error <–600 diopter [D], 23 with low/moderate myopia with a refractive error between –100 and –600 D, and similarly 59 healthy controls with emmetropia closely matched for age) were recruited. The amplitude of low-frequency fluctuations method was conducted to investigate the difference of intrinsic brain activity across three groups.

Results

Compared with the healthy controls, individuals with low/moderate myopia showed significantly decreased amplitude of low-frequency fluctuation values in the bilateral rectal gyrus, right cerebellum anterior lobe/calcarine, and bilateral thalamus and showed significantly increased amplitude of low-frequency fluctuation values in left white matter (optic radiation), right prefrontal cortex, and left primary motor cortex (M1)/primary somatosensory cortex (S1). In addition, individuals with high myopia showed significantly decreased amplitude of low-frequency fluctuation values in the right cerebellum anterior lobe/calcarine/bilateral parahippocampal gyrus, bilateral posterior cingulate cortex, and bilateral middle cingulate cortex and significantly increased amplitude of low-frequency fluctuation values in left white matter (optic radiation), bilateral frontal parietal cortex, and left M1/S1. Moreover, we found that the amplitude of low-frequency fluctuation values of the different brain areas was closely related to the clinical features in the high myopia group.

Conclusion

Our results demonstrated that individuals with low/moderate myopia and high myopia had abnormal intrinsic brain activities in various brain regions related to the limbic system, default mode network, and thalamo-occipital pathway.

Surface-based analysis increases the specificity of cortical activation patterns and connectivity results

Spatial smoothing of functional magnetic resonance imaging (fMRI) data can be performed on volumetric images and on the extracted surface of the brain. Smoothing on the unfolded cortex should theoretically improve the ability to separate signals between brain areas that are near together in the folded cortex but are more distant in the unfolded cortex. However, surface-based method approaches (SBA) are currently not utilized as standard procedure in the preprocessing of neuroimaging data. Recent improvements in the quality of cortical surface modeling and improvements in its usability nevertheless advocate this method. In the current study, we evaluated the benefits of an up-to-date surface-based smoothing in comparison to volume-based smoothing. We focused on the effect of signal contamination between different functional systems using the primary motor and primary somatosensory cortex as an example. We were particularly interested in how this signal contamination influences the results of activity and connectivity analyses for these brain regions. We addressed this question by performing fMRI on 19 subjects during a tactile stimulation paradigm and by using simulated BOLD responses. We demonstrated that volume-based smoothing causes contamination of the primary motor cortex by somatosensory cortical responses, leading to false positive motor activation. These false positive motor activations were not found by using surface-based smoothing for reasonable kernel sizes. Accordingly, volume-based smoothing caused an exaggeration of connectivity estimates between these regions. In conclusion, this study showed that surface-based smoothing decreases signal contamination considerably between neighboring functional brain regions and improves the validity of activity and connectivity results.

A Data-Driven Multi-scale Technique for fMRI Mapping of the Human Somatosensory Cortex

A previously introduced Bayesian non-parametric multi-scale technique, called iterated Multigrid Priors (iMGP) method, is used to map the topographic organization of human primary somatosensory cortex (S1). We analyze high spatial resolution fMRI data acquired at ultra-high field (UHF, 7T) in individual subjects during vibrotactile stimulation applied to each distal phalange of the left hand digits using both a travelling-wave (TW) and event-related (ER) paradigm design. We compare the somatotopic digit representations generated in S1 using the iMGP method with those obtained using established fMRI paradigms and analysis techniques: Fourier-based analysis of travelling-wave data and General Linear Model (GLM) analysis of event-related data. Maps derived with the iMGP method are similar to those derived with the standard analysis, but in contrast to the Fourier-based analysis, the iMGP method reveals overlap of activity from adjacent digit representations in S1. These findings validate the use of the iMGP method as an alternative to study digit representations in S1, particularly with the TW design as an attractive means to study cortical reorganization in patient populations such dystonia and carpal tunnel syndrome, where the degree of spatial overlap of cortical finger representations is of interest.

Topographic Somatosensory Imagery for Real-Time fMRI Brain-Computer Interfacing

Real-time functional magnetic resonance imaging (fMRI) is a promising non-invasive method for brain-computer interfaces (BCIs). BCIs translate brain activity into signals that allow communication with the outside world. Visual and motor imagery are often used as information-encoding strategies, but can be challenging if not grounded in recent experience in these modalities, e.g., in patients with locked-in-syndrome (LIS). In contrast, somatosensory imagery might constitute a more suitable information-encoding strategy as the somatosensory function is often very robust. Somatosensory imagery has been shown to activate the somatotopic cortex, but it has been unclear so far whether it can be reliably detected on a single-trial level and successfully classified according to specific somatosensory imagery content. Using ultra-high field 7-T fMRI, we show reliable and high-accuracy single-trial decoding of left-foot (LF) vs. right-hand (RH) somatosensory imagery. Correspondingly, higher decoding accuracies were associated with greater spatial separation of hand and foot decoding-weight patterns in the primary somatosensory cortex (S1). Exploiting these novel neuroscientific insights, we developed-and provide a proof of concept for-basic BCI communication by showing that binary (yes/no) answers encoded by somatosensory imagery can be decoded with high accuracy in simulated real-time (in 7 subjects) as well as in real-time (1 subject). This study demonstrates that body part-specific somatosensory imagery differentially activates somatosensory cortex in a topographically specific manner; evidence which was surprisingly still lacking in the literature. It also offers proof of concept for a novel somatosensory imagery-based fMRI-BCI control strategy, with particularly high potential for visually and motor-impaired patients. The strategy could also be transferred to lower MRI field strengths and to mobile functional near-infrared spectroscopy. Finally, given that communication BCIs provide the BCI user with a form of feedback based on their brain signals and can thus be considered as a specific form of neurofeedback, and that repeated use of a BCI has been shown to enhance underlying representations, we expect that the current BCI could also offer an interesting new approach for somatosensory rehabilitation training in the context of stroke and phantom limb pain.

A common representation of fingers and toes

There are many similarities and differences between the human hands and feet. On a psychological level, there is some evidence from clinical disorders and studies of tactile localisation in healthy adults for deep functional connections between the hands and feet. One form these connections may take is in common high-level mental representations of the hands and feet. Previous studies have shown that there are systematic, but distinct patterns of confusion found between both the fingers and toes. Further, there are clear individual differences between people in the exact patterns of mislocalisations. Here, we investigated whether these idiosyncratic differences in tactile localisation are shared between the fingers and toes, which may indicate a shared high-level representation. We obtained confusion matrices showing the pattern of mislocalisation on the hairy skin surfaces of both the fingers and toes. Using a decoding approach, we show that idiosyncratic differences in individuals' pattern of confusions are shared across the fingers and toes, despite different overall patterns of confusions. These results suggest that there is a common representation of the fingers and toes.

Investigations on maladaptive plasticity in the sensorimotor cortex of unilateral upper limb CRPS I patients

BACKGROUND

Patients with a complex regional pain syndrome (CRPS) in the upper limb show a sensory and motor impairment of the hand. Decreased intra-cortical-inhibition (ICI) of the motor representation of the affected hand muscle and decreased somatosensory hand representation size were related to maladaptive plasticity.

OBJECTIVE

To achieve new insights about CRPS we examined whether these alterations were present in a single cohort.

METHODS

We used a multi-modal approach comprising behavioral testing, transcranial magnetic stimulation, and high resolution fMRI combined with a new analysis technique for improved neuronal specificity.

RESULTS

We found a decreased pinch-grip performance, two-point discrimination on the fingertips, ICI in the motor cortex, and representation size of the hand in Brodmann Area 3b (BA3b) in the somatosensory cortex. Our analysis further showed that correlations with ICI on the non-affected side were absent on the affected side.

CONCLUSIONS

This study is the first to gather behavioral, neurophysiologic and imaging measurements for one patient cohort and it therefore enables a comprehensive view of collapsed associations of function and representation focused on the hemisphere contralateral to the affected hand.

Comparison of fMRI Digit Representations of the Dominant and Non-dominant Hand in the Human Primary Somatosensory Cortex

The tactile digit representations in the primary somatosensory cortex have so far been mapped for either the left or the right hand. This study localized all ten digit representations in right-handed subjects and compared them within and across the left and right hands to assess potential differences in the functional organization of the digit map between hands and in the structural organization between hemispheres. Functional magnetic resonance imaging of tactile stimulation of each fingertip in BA 3b confirmed the expected lateral-anterior-inferior to medial-posterior-superior succession from thumb to little-finger representation, located in the post-central gyrus opposite to the motor hand knob. While the more functionally related measures, such as the extent and strength of activation as well as the Euclidean distance between neighboring digit representations, showed significant differences between the digits, no side difference was detected: the layout of the functional digit-representation map did not consistently differ between the left, non-dominant, and the right, dominant hand. Comparing the absolute spatial coordinates also revealed a significant difference for the digits, but not between the left and right hand digits. Estimating the individual subject's digit coordinates of one hand by within-subject mirroring of the other-hand digit coordinates across hemispheres yielded a larger estimation error distance than using averaged across-subjects coordinates from within the same hemisphere. However, both methods should only be used with care for single-subject clinical evaluation, as an average estimation error of around 9 mm was observed, being slightly higher than the average distance between neighboring digits.

Examining the Triple Code Model in numerical cognition: An fMRI study

The Triple Code Model (TCM) of numerical cognition argues for the existence of three representational codes for number: Arabic digits, verbal number words, and analog nonsymbolic magnitude representations, each subserved by functionally dissociated neural substrates. Despite the popularity of the TCM, no study to date has explored all three numerical codes within one fMRI paradigm. We administered three tasks, associated with each of the aforementioned numerical codes, in order to explore the neural correlates of numerosity processing in a sample of adults (N = 46). Independent task-control contrast analyses revealed task-dependent activity in partial support of the model, but also highlight the inherent complexity of a distributed and overlapping fronto-parietal network involved in all numerical codes. The results indicate that the TCM correctly predicts the existence of some functionally dissociated neural substrates, but requires an update that accounts for interactions with attentional processes. Parametric contrasts corresponding to differences in task difficulty revealed specific neural correlates of the distance effect, where closely spaced numbers become more difficult to discriminate than numbers spaced further apart. A conjunction analysis illustrated overlapping neural correlates across all tasks, in line with recent proposals for a fronto-parietal network of number processing. We additionally provide tentative results suggesting the involvement of format-independent numerosity-sensitive retinotopic maps in the early visual stream, extending previous findings of nonsymbolic stimulus selectivity. We discuss the functional roles of the components associated with the model, as well as the purported fronto-parietal network, and offer arguments in favor of revising the TCM.

Cry presence and amplitude do not reflect cortical processing of painful stimuli in newborns with distinct responses to touch or cold

OBJECTIVE

Newborns requiring hospitalisation frequently undergo painful procedures. Prevention of pain in infants is of prime concern because of adverse associations with physiological and neurological development. However, pain mitigation is currently guided by behavioural observation assessments that have not been validated against direct evidence of pain processing in the brain. The aim of this study was to determine whether cry presence or amplitude is a valid indicator of pain processing in newborns.

DESIGN

Prospective observational cohort.

SETTING

Newborn nursery.

PATIENTS

Healthy infants born at >37 weeks and <42 weeks gestation.

INTERVENTIONS

We prospectively studied newborn cortical responses to light touch, cold and heel stick, and the amplitude of associated infant vocalisations using our previously published paradigms of time-locked electroencephalogram (EEG) with simultaneous audio recordings.

RESULTS

Latencies of cortical peak responses to each of the three stimuli type were significantly different from each other. Of 54 infants, 13 (24%), 19 (35%) and 35 (65%) had cries in response to light touch, cold and heel stick, respectively. Cry in response to non-painful stimuli did not predict cry in response to heel stick. All infants with EEG data had measurable pain responses to heel stick, whether they cried or not. There was no association between presence or amplitude of cries and cortical nociceptive amplitudes.

CONCLUSIONS

In newborns with distinct brain responses to light touch, cold and pain, cry presence or amplitude characteristics do not provide adequate behavioural markers of pain signalling in the brain. New bedside assessments of newborn pain may need to be developed using brain-based methodologies as benchmarks in order to provide optimal pain mitigation.

Measuring the effects of attention to individual fingertips in somatosensory cortex using ultra-high field (7T) fMRI

Attention to sensory information has been shown to modulate the neuronal processing of that information. For example, visuospatial attention acts by modulating responses at retinotopically appropriate regions of visual cortex (Puckett and DeYoe, 2015; Tootell et al. 1998). Much less, however, is known about the neuronal processing associated with attending to other modalities of sensory information. One reason for this is that visual cortex is relatively large, and therefore easier to access non-invasively in humans using tools such as functional magnetic resonance imaging (fMRI). With high-resolution fMRI, however, it is now possible to access smaller cortical areas such as primary somatosensory cortex (Martuzzi et al., 2014; Sanchez-Panchuelo et al., 2010; Schweisfurth et al. 2014; Schweizer et al. 2008). Here, we combined a novel experimental design and high-resolution fMRI at ultra-high field (7T) to measure the effects of attention to tactile stimulation in primary somatosensory cortex, S1. We find that attention modulates somatotopically appropriate regions of S1, and importantly, that this modulation can be measured at the level of the cortical representation of individual fingertips.

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.

7-T MRI in Cerebrovascular Diseases: Challenges to Overcome and Initial Results

Magnetic resonance imaging (MRI) plays a key role in the investigation of cerebrovascular diseases. Compared with computed tomography (CT) and digital subtraction angiography (DSA), its advantages in diagnosing cerebrovascular pathology include its superior tissue contrast, its ability to visualize blood vessels without the use of a contrast agent, and its use of magnetic fields and radiofrequency pulses instead of ionizing radiation. In recent years, ultrahigh field MRI at 7 tesla (7 T) has shown promise in the diagnosis of many cerebrovascular diseases. The increased signal-to-noise ratio (SNR; 2.3x and 4.7x increase compared with 3 and 1.5 T, respectively) and contrast-to-noise ratio (CNR) at this higher field strength can be exploited to obtain a higher spatial resolution and higher lesion conspicuousness, enabling assessment of smaller brain structures and lesions. Cerebrovascular diseases can be assessed at different tissue levels; for instance, changes of the arteries feeding the brain can be visualized to determine the cause of ischemic stroke, regional changes in brain perfusion can be mapped to predict outcome after revascularization, and tissue damage, including old and recent ischemic infarcts, can be evaluated as a marker of ischemic burden. For the purpose of this review, we will discriminate 3 levels of assessment of cerebrovascular diseases using MRI: Pipes, Perfusion, and Parenchyma (3 Ps). The term Pipes refers to the brain-feeding arteries from the heart and aortic arch, upwards to the carotid arteries, vertebral arteries, circle of Willis, and smaller intracranial arterial branches. Perfusion is the amount of blood arriving at the brain tissue level, and includes the vascular reserve and perfusion territories. Parenchyma refers to the acute and chronic burden of brain tissue damage, which includes larger infarcts, smaller microinfarcts, and small vessel disease manifestations such as white matter lesions, lacunar infarcts, and microbleeds. In this review, we will describe the key developments in the last decade of 7-T MRI of cerebrovascular diseases, subdivided for these 3 levels of assessment.

The effect of motor familiarity during simple finger opposition tasks

Humans are more familiar with performing (and observing) index-thumb than with any other finger to thumb grasping and the effect of familiarity has not been tested specifically with simple and intransitive actions. The study of simple and intransitive motor actions (i.e. simple actions without need of object interaction) provides the opportunity to investigate specifically the brain motor regions reducing the effect of non-motor aspects that are related with more complex and/or transitive motor actions. The aim of this study is to evaluate brain activity patterns during the execution of simple and intransitive finger movements with different degrees of familiarity. With this in mind, a functional Magnetic Resonance Imaging (fMRI) study was performed in which participants were asked to execute finger to thumb opposition tasks with all the different fingers (index, middle, ring and little) with a fixed frequency (1 Hz) determined by a visual cue. This movement is considered as the pantomime of a precision grasping action. Significant activity was identified in the Sensory Motor Cortex (SMC), posterior parietal and premotor regions for all simple conditions (index-finger>control, middle-finger>control, ring-finger>control and little-finger>control). However, a linear trend contrast (index<middle<ring<little) demonstrated that there was a linear increase of activity in the SMC (mainly in the Precentral Gyrus) while the finger used to perform the action was further from the thumb. Therefore, the execution of less familiar simple intransitive movements seems to lead to a stronger activation of the SMC than familiar ones. Posterior parietal and premotor regions did not show the aforementioned stronger activation. The most important implication of this study is the identification of differences in brain activity during the execution of simple intransitive movements with different degrees of familiarity.

Somatotopic Map and Inter- and Intra-Digit Distance in Brodmann Area 2 by Pressure Stimulation

The somatotopic representation of the tactile stimulation on the finger in the brain is an essential part of understanding the human somatosensory system as well as rehabilitation and other clinical therapies. Many studies have used vibrotactile stimulations and reported finger somatotopic representations in the Brodmann area 3 (BA 3). On the contrary, few studies investigated finger somatotopic representation using pressure stimulations. Therefore, the present study aimed to find a comprehensive somatotopic representation (somatotopic map and inter- and intra-digit distance) within BA 2 of humans that could describe tactile stimulations on different joints across the fingers by applying pressure stimulation to three joints-the first (p1), second (p2), and third (p3) joints-of four fingers (index, middle, ring, and little finger). Significant differences were observed in the inter-digit distance between the first joints (p1) of the index and little fingers, and between the third joints (p3) of the index and little fingers. In addition, a significant difference was observed in the intra-digit distance between p1 and p3 of the little finger. This study suggests that a somatotopic map and inter- and intra-digit distance could be found in BA 2 in response to pressure stimulation on finger joints.

Investigating the Stability of Fine-Grain Digit Somatotopy in Individual Human Participants

Studies of human primary somatosensory cortex (S1) have placed a strong emphasis on the cortical representation of the hand and the propensity for plasticity therein. Despite many reports of group differences and experience-dependent changes in cortical digit somatotopy, relatively little work has considered the variability of these maps across individuals and to what extent this detailed functional architecture is dynamic over time. With the advent of 7 T fMRI, it is increasingly feasible to map such detailed organization noninvasively in individual human participants. Here, we extend the ability of ultra-high-field imaging beyond a technological proof of principle to investigate the intersubject variability of digit somatotopy across participants and the stability of this organization across a range of intervals. Using a well validated phase-encoding paradigm and an active task, we demonstrate the presence of highly reproducible maps of individual digits in S1, sharply contrasted by a striking degree of intersubject variability in the shape, extent, and relative position of individual digit representations. Our results demonstrate the presence of very stable fine-grain somatotopy of the digits in human S1 and raise the issue of population variability in such detailed functional architecture of the human brain. These findings have implications for the study of detailed sensorimotor plasticity in the context of both learning and pathological dysfunction. The simple task and 10 min scan required to derive these maps also raises the potential for this paradigm as a tool in the clinical setting.

SIGNIFICANCE STATEMENT

We applied ultra-high-resolution fMRI at 7 T to map sensory digit representations in the human primary somatosensory cortex (S1) at the level of individual participants across multiple time points. The resulting fine-grain maps of individual digits in S1 reveal the stability in this fine-grain functional organization over time, contrasted with the variability in these maps across individuals.

Inter- and Intradigit Somatotopic Map of High-Frequency Vibration Stimulations in Human Primary Somatosensory Cortex

Although more about the somatotopic mapping of fingers continues to be uncovered, there is lack of mapping attempts regarding the integration of within-finger and across-finger somatotopic coordinates in Broadmann area (BA) 3. This study aimed to address the issue by finding an inter-/intradigit somatotopic map with high-frequency (250 Hz) vibrotactile stimulation. Functional magnetic resonance imaging (fMRI) data were acquired while stimulation was applied to 3 phalanxes (distal [p1], intermediate [p2], and proximal [p3] phalanx) of 4 fingers (index, middle, ring, and little finger) for a total of 12 finger-phalanx combinations for a human. Inter-, intra-, and inter-/intradigit distances were calculated from peak activation coordinates in BA 3 for each combination. With regard to interdigit dimensions, the somatotopic coordinates proceeded in the lateral-to-medial direction for the index, middle, ring, and little fingers consecutively. This trend is comparable to that generated from low-frequency stimulation modalities (flutter stimulation). The somatotopic distances between fingers were greatest when p1 was compared across fingers. From an intradigit perspective, stimulation on p1, p2, and p3 yielded BA 3 peak coordinates aligned along the anterior-to-posterior and inferior-to-superior directions for all fingers. An inter-/intradigit map exhibited a radially propagating trend of distances calculated with respect to index p1 as a reference point; this provided an integrated view of inter- and intradigit somatotopies, which are traditionally discussed separately. We expect such an inter-/intradigit somatotopic map approach to contribute in generating a comprehensive somatotopic model of fingers.

Delineation of somatosensory finger areas using vibrotactile stimulation, an ECoG study

BACKGROUND

In surgical planning for epileptic focus resection, functional mapping of eloquent cortex is attained through direct electrical stimulation of the brain. This procedure is uncomfortable, can trigger seizures or nausea, and relies on subjective evaluation. We hypothesize that a method combining vibrotactile stimulation and statistical clustering may provide improved somatosensory mapping.

METHODS

Seven pediatric candidates for surgical resection underwent a task in which their fingers were independently stimulated using a custom designed finger pad, during electrocorticographic monitoring. A cluster-based statistical analysis was then performed to localize the elicited activity on the recording grids.

RESULTS

Mid-Gamma clusters (65-115 Hz) arose in areas consistent with anatomical predictions as well as clinical findings, with five subjects presenting a somatotopic organization of the fingers. This process allowed us to delineate finger representation even in patients who were sleeping, with strong interictal activity, or when electrical stimulation did not successfully locate eloquent areas.

CONCLUSIONS

We suggest that this scheme, relying on the endogenous neural response rather than exogenous electrical activation, could eventually be extended to map other sensory areas and provide a faster and more objective map to better anticipate outcomes of surgical resection.

Differences in Activation Area Within Brodmann Area 2 Caused by Pressure Stimuli on Fingers and Joints: In Case of Male Subjects

In this study, a constant pressure stimulus was applied on the 3 joints (first [p1], second [p2], and third [p3] joints) of 4 fingers (index, middle, ring, and little fingers), and the activation areas within Brodmann area 2 (BA 2) were compared for these different fingers and joints by using functional magnetic resonance imaging. Eight healthy male college students (25.4 ± 1.32 years) participated in the study. Each session was composed of 3 blocks, and each block was composed of a Control phase (30 seconds) and a Pressure phase (30 seconds). No pressure stimulus was applied in the Control phase, during which the subjects would simply lay comfortably with their eyes closed. In the Pressure phase, a pressure stimulus was applied onto one of the joints of the selected finger. For each finger and joint, BA 2 areas activated by the pressure stimulus were extracted by the region of interest method. There was a significant difference in the activation areas for the different fingers (P = .042) as well as for the different joints (P = .050). The activation area decreased in the order of the little, index, and middle fingers, as well as in the order of p1, p3, and p2.

Normalization in human somatosensory cortex

Functional magnetic resonance imaging (fMRI) was used to measure activity in human somatosensory cortex and to test for cross-digit suppression. Subjects received stimulation (vibration of varying amplitudes) to the right thumb (target) with or without concurrent stimulation of the right middle finger (mask). Subjects were less sensitive to target stimulation (psychophysical detection thresholds were higher) when target and mask digits were stimulated concurrently compared with when the target was stimulated in isolation. fMRI voxels in a region of the left postcentral gyrus each responded when either digit was stimulated. A regression model (called a forward model) was used to separate the fMRI measurements from these voxels into two hypothetical channels, each of which responded selectively to only one of the two digits. For the channel tuned to the target digit, responses in the left postcentral gyrus increased with target stimulus amplitude but were suppressed by concurrent stimulation to the mask digit, evident as a shift in the gain of the response functions. For the channel tuned to the mask digit, a constant baseline response was evoked for all target amplitudes when the mask was absent and responses decreased with increasing target amplitude when the mask was concurrently presented. A computational model based on divisive normalization provided a good fit to the measurements for both mask-absent and target + mask stimulation. We conclude that the normalization model can explain cross-digit suppression in human somatosensory cortex, supporting the hypothesis that normalization is a canonical neural computation.

Stroking or Buzzing? A Comparison of Somatosensory Touch Stimuli Using 7 Tesla fMRI

Studying body representations in the brain helps us to understand how we humans relate to our own bodies. The in vivo mapping of the somatosensory cortex, where these representations are found, is greatly facilitated by the high spatial resolution and high sensitivity to brain activation available at ultra-high field. In this study, the use of different stimulus types for somatotopic mapping of the digits at ultra-high field, specifically manual stroking and mechanical stimulation, was compared in terms of sensitivity and specificity of the brain responses. Larger positive responses in digit regions of interest were found for manual stroking than for mechanical stimulation, both in terms of average and maximum t-value and in terms of number of voxels with significant responses to the tactile stimulation. Responses to manual stroking were higher throughout the entire post-central sulcus, but the difference was especially large on its posterior wall, i.e. in Brodmann area 2. During mechanical stimulation, cross-digit responses were more negative than during manual stroking, possibly caused by a faster habituation to the stimulus. These differences indicate that manual stroking is a highly suitable stimulus for fast somatotopic mapping procedures, especially if Brodmann area 2 is of interest.

Distributed functions of detection and discrimination of vibrotactile stimuli in the hierarchical human somatosensory system

According to the hierarchical view of human somatosensory network, somatic sensory information is relayed from the thalamus to primary somatosensory cortex (S1), and then distributed to adjacent cortical regions to perform further perceptual and cognitive functions. Although a number of neuroimaging studies have examined neuronal activity correlated with tactile stimuli, comparatively less attention has been devoted toward understanding how vibrotactile stimulus information is processed in the hierarchical somatosensory cortical network. To explore the hierarchical perspective of tactile information processing, we studied two cases: (a) discrimination between the locations of finger stimulation; and (b) detection of stimulation against no stimulation on individual fingers, using both standard general linear model (GLM) and searchlight multi-voxel pattern analysis (MVPA) techniques. These two cases were studied on the same data set resulting from a passive vibrotactile stimulation experiment. Our results showed that vibrotactile stimulus locations on fingers could be discriminated from measurements of human functional magnetic resonance imaging (fMRI). In particular, it was in case (a) we observed activity in contralateral posterior parietal cortex (PPC) and supramarginal gyrus (SMG) but not in S1, while in case; (b) we found significant cortical activations in S1 but not in PPC and SMG. These discrepant observations suggest the functional specialization with regard to vibrotactile stimulus locations, especially, the hierarchical information processing in the human somatosensory cortical areas. Our findings moreover support the general understanding that S1 is the main sensory receptive area for the sense of touch, and adjacent cortical regions (i.e., PPC and SMG) are in charge of a higher level of processing and may thus contribute most for the successful classification between stimulated finger locations.

Estimation of the spatial profile of neuromodulation and the temporal latency in motor responses induced by focused ultrasound brain stimulation

This study investigates the spatial profile and the temporal latency of the brain stimulation induced by the transcranial application of pulsed focused ultrasound (FUS). The site of neuromodulation was detected using 2-deoxy-2-[¹⁸F]fluoro-d-glucose PET immediately after FUS sonication on the unilateral thalamic area of Sprague–Dawley rats. The latency of the stimulation was estimated by measuring the time taken from the onset of the stimulation of the appropriate brain motor area to the corresponding tail motor response. The brain area showing elevated glucose uptake from the PET image was much smaller (56±10% in diameter, 24±6% in length) than the size of the acoustic focus, which is conventionally defined by the full-width at half-maximum of the acoustic intensity field. The spatial dimension of the FUS-mediated neuromodulatory area was more localized, approximated to be full-width at 90%-maximum of the acoustic intensity field. In addition, the time delay of motor responses elicited by the FUS sonication was 171±63 (SD) ms from the onset of sonication. When compared with latencies of other nonultrasonic neurostimulation techniques, the longer time delay associated with FUS-mediated motor responses is suggestive of the nonelectrical modes of neuromodulation, making it a distinctive brain stimulation method.

Surgically relevant localization of the central sulcus with high-density somatosensory-evoked potentials compared with functional magnetic resonance imaging

BACKGROUND

Resection of abnormal brain tissue lying near the sensorimotor cortex entails precise localization of the central sulcus. Mapping of this area is achieved by applying invasive direct cortical electrical stimulation. However, noninvasive methods, particularly functional magnetic resonance imaging (fMRI), are also used. As a supplement to fMRI, localization of somatosensory-evoked potentials (SEPs) recorded with an electroencephalogram (EEG) has been proposed, but has not found its place in clinical practice.

OBJECTIVE

To assess localization accuracy of the hand somatosensory cortex with SEP source imaging.

METHODS

We applied electrical source imaging in 49 subjects, recorded with high-density EEG (256 channels). We compared it with fMRI in 18 participants and with direct cortical electrical stimulation in 6 epileptic patients.

RESULTS

Comparison of SEP source imaging with fMRI indicated differences of 3 to 8 mm, with the exception of the mesial-distal orientation, where variances of up to 20 mm were found. This discrepancy is explained by the fact that the source maximum of the first SEP peak is localized deep in the central sulcus (area 3b), where information initially arrives. Conversely, fMRI showed maximal signal change on the lateral surface of the postcentral gyrus (area 1), where sensory information is integrated later in time. Electrical source imaging and fMRI showed mean Euclidean distances of 13 and 14 mm, respectively, from the contacts where electrocorticography elicited sensory phenomena of the contralateral upper limb.

CONCLUSION

SEP source imaging, based on high-density EEG, reliably identifies the depth of the central sulcus. Moreover, it is a simple, flexible, and relatively inexpensive alternative to fMRI.

Optimized protocol for high resolution functional magnetic resonance imaging at 3T using single-shot echo planar imaging

BACKGROUND

To translate highly accelerated EPI-fMRI protocols as commonly used at ultra-high field strengths to clinical 3T settings.

NEW METHOD

EPI protocols with increasing matrix sizes and parallel imaging (PI) factors were tested in two separate fMRI studies, a simple motor-task and a complex motivation-task experiment with focus on the sensorimotor cortex (SMC) and the nucleus accumbens (NAcc), respectively.

RESULTS

By increasing the matrix size and the PI-factor simultaneously, BOLD-sensitivity in terms of maximal t-values and numbers of activated clusters was uncompromised in single individuals in both fMRI experiments. In the SMC, the multi-subject analysis revealed an increase of 66% of the maximal t-value whereby the number of activated clusters was increased by a factor of 3.3 when the matrix size (PI-factor) was increased from 96×96 (R=2) to 192×192 (R=4). In the NAcc, the number of activated clusters increased from 5 to 7 whereby the maximal t-value remained unaffected when the matrix size (PI-factor) was increased from 96×96 (R=2) to 160×160 (R=3).

COMPARISON WITH EXISTING METHOD

Using the proposed high-resolution EPI protocol, spatial blurring was clearly reduced. Further, BOLD sensitivity was clearly improved in multi-subject analyses and remained unaffected in single individuals compared to using the standard protocols.

CONCLUSIONS

Conventionally used matrix sizes (PI-factors) might be non-optimal for some applications sacrificing BOLD spatial specificity. We recommend using the proposed high-resolution protocols applicable in detecting robust BOLD activation in fMRI.

Individual fMRI maps of all phalanges and digit bases of all fingers in human primary somatosensory cortex

This study determined the individual maps of all fingers in Brodmann area 3b of the human primary somatosensory cortex in a single fMRI session by tactile stimulation at 19 sites across all phalanges and digit bases of the 5 right-hand digits. To quantify basic features of the digit maps within and across subjects, we applied standard descriptive measures, but also implemented a novel quantitative analysis. This so-called Direction/Order (DiOr) method tested whether subjects exhibited an ordering of peak fMRI representations along their individual direction of alignment through the set of analyzed phalanges and whether these individual directions were similar across subjects. Across-digit analysis demonstrated that for each set of homologous phalanges, the D5-to-D1 representations were successively represented along a common direction of alignment. Hence, the well-known mediolateral D5-to-D1 somatotopy was not only confirmed for the distal phalanges (p1), but could also be shown for the medial (p2) and proximal phalanges (p3). In contrast, the peak activation for the digit bases (p4) only partly elicited that digit succession. Complementary, intra-digit analysis revealed a divergent picture of map topography for the different digits. Within D5 (and in a trend: D4), an ordered p1-to-p3 succession was found across subjects, pointing to a consistent intra-digit somatotopy for D5, with p3 generally found medial-posterior to p1. In contrast, for D1, D2, and D3, most subjects did not present with ordered p1-to-p3 maps nor were directions of alignment similarly oriented between subjects. These digits therefore exhibited highly diverse representation patterns across subjects.