Individual left-hand and right-hand intra-digit representations in human primary somatosensory cortex

Tactile stimulation designs adapted to clinical settings result in reliable fMRI-based somatosensory digit maps

Movement constraints in stroke survivors are often accompanied by additional impairments in related somatosensory perception. A complex interplay between the primary somatosensory and motor cortices is essential for adequate and precise movements. This necessitates investigating the role of the primary somatosensory cortex in movement deficits of stroke survivors. The first step towards this goal could be a fast and reliable functional Magnetic Resonance Imaging (fMRI)-based mapping of the somatosensory cortex applicable for clinical settings. Here, we compare two 3 T fMRI-based somatosensory digit mapping techniques adapted for clinical usage in seven neurotypical volunteers and two sessions, to assess their validity and retest-reliability. Both, the traveling wave and the blocked design approach resulted in complete digit maps in both sessions of all participants, showing the expected layout. Similarly, no evidence for differences in the volume of activation, nor the activation overlap between neighboring activations could be detected, indicating the general feasibility of the clinical adaptation and their validity. Retest-reliability, indicated by the Dice coefficient, exhibited reasonable values for the spatial correspondence of single digit activations across sessions, but low values for the spatial correspondence of the area of overlap between neighboring digits across sessions. Parameters describing the location of the single digit activations exhibited very high correlations across sessions, while activation volume and overlap only exhibited medium to low correlations. The feasibility and high retest-reliabilities for the parameters describing the location of the single digit activations are promising concerning the implementation into a clinical context to supplement diagnosis and treatment stratification in upper limb stroke patients.

Two-Dimensional Population Receptive Field Mapping of Human Primary Somatosensory Cortex

Functional magnetic resonance imaging can provide detailed maps of how sensory space is mapped in the human brain. Here, we use a novel 16 stimulator setup (a 4 × 4 grid) to measure two-dimensional sensory maps of between and within-digit (D2-D4) space using high spatial-resolution (1.25 mm isotropic) imaging at 7 Tesla together with population receptive field (pRF) mapping in 10 participants. Using a 2D Gaussian pRF model, we capture maps of the coverage of digits D2-D5 across Brodmann areas and estimate pRF size and shape. In addition, we compare results to previous studies that used fewer stimulators by constraining pRF models to a 1D Gaussian Between Digit or 1D Gaussian Within Digit model. We show that pRFs across somatosensory areas tend to have a strong preference to cover the within-digit axis. We show an increase in pRF size moving from D2-D5. We quantify pRF shapes in Brodmann area (BA) 3b, 3a, 1, 2 and show differences in pRF size in Brodmann areas 3a-2, with larger estimates for BA2. Generally, the 2D Gaussian pRF model better represents pRF coverage maps generated by our data, which itself is produced from a 2D stimulation grid.

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.

Case Report: Plasticity in Central Sensory Finger Representation and Touch Perception After Microsurgical Reconstruction of Infraclavicular Brachial Plexus Injury

After brachial plexus injury (BPI), early microsurgery aims at facilitating reconnection of the severed peripheral nerves with their orphan muscles and sensory receptors and thereby reestablishing communication with the brain. In order to investigate this sensory recovery, here we combined functional magnetic resonance imaging (fMRI) and tactile psychophysics in a patient who suffered a sharp, incomplete amputation of the dominant hand at the axilla level. To determine somatosensory detection and discomfort thresholds as well as sensory accuracy for fingers of both the intact and affected hand, we used electrotactile stimulation in the framework of a mislocalization test. Additionally, tactile stimulation was performed in the MRI scanner in order to determine the cortical organization of the possibly affected primary somatosensory cortex. The patient was able to detect electrotactile stimulation in 4 of the 5 fingertips (D1, D2, D4, D5), and in the middle phalanx in D3 indicating some innervation. The detection and discomfort threshold were considerably higher at the affected side than at the intact side, with higher detection and discomfort thresholds for the affected side. The discrimination accuracy was rather low at the affected side, with stimulation of D1/D2/D3/D4/D5 eliciting most commonly a sensation at D4/D1/D3/D2/D5, respectively. The neuroimaging data showed a mediolateral succession from D2 to D5 to D1 to D4 (no activation was observed for D3). These results indicate a successful regrowth of the peripheral nerve fibers from the axilla to four fingertips. The data suggest that some of the fibers have switched location in the process and there is a beginning of cortical reorganization in the primary somatosensory cortex, possibly resulting from a re-education of the brain due to conflicting information (touch vs. vision).

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.

Evoking highly focal percepts in the fingertips through targeted stimulation of sulcal regions of the brain for sensory restoration

Background:

Paralysis and neuropathy, affecting millions of people worldwide, can be accompanied by significant loss of somatosensation. With tactile sensation being central to achieving dexterous movement, brain-computer interface (BCI) researchers have used intracortical and cortical surface electrical stimulation to restore somatotopically-relevant sensation to the hand. However, these approaches are restricted to stimulating the gyral areas of the brain. Since representation of distal regions of the hand extends into the sulcal regions of human primary somatosensory cortex (S1), it has been challenging to evoke sensory percepts localized to the fingertips.

Objective/hypothesis:

Targeted stimulation of sulcal regions of S1, using stereoelectroencephalography (SEEG) depth electrodes, can evoke focal sensory percepts in the fingertips.

Methods:

Two participants with intractable epilepsy received cortical stimulation both at the gyri via high-density electrocorticography (HD-ECoG) grids and in the sulci via SEEG depth electrode leads. We characterized the evoked sensory percepts localized to the hand.

Results:

We show that highly focal percepts can be evoked in the fingertips of the hand through sulcal stimulation. fMRI, myelin content, and cortical thickness maps from the Human Connectome Project elucidated specific cortical areas and sub-regions within S1 that evoked these focal percepts. Within-participant comparisons showed that percepts evoked by sulcal stimulation via SEEG electrodes were significantly more focal (80% less area; p = 0.02) and localized to the fingertips more often, than by gyral stimulation via HD-ECoG electrodes. Finally, sulcal locations with consistent modulation of high-frequency neural activity during mechanical tactile stimulation of the fingertips showed the same somatotopic correspondence as cortical stimulation.

Conclusions:

Our findings indicate minimally invasive sulcal stimulation via SEEG electrodes could be a clinically viable approach to restoring sensation.

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.

A probabilistic atlas of finger dominance in the primary somatosensory cortex

With the advent of ultra-high field (7T), high spatial resolution functional MRI (fMRI) has allowed the differentiation of the cortical representations of each of the digits at an individual-subject level in human primary somatosensory cortex (S1). Here we generate a probabilistic atlas of the contralateral SI representations of the digits of both the left and right hand in a group of 22 right-handed individuals. The atlas is generated in both volume and surface standardised spaces from somatotopic maps obtained by delivering vibrotactile stimulation to each distal phalangeal digit using a travelling wave paradigm. Metrics quantify the likelihood of a given position being assigned to a digit (full probability map) and the most probable digit for a given spatial location (maximum probability map). The atlas is validated using a leave-one-out cross validation procedure. Anatomical variance across the somatotopic map is also assessed to investigate whether the functional variability across subjects is coupled to structural differences. This probabilistic atlas quantifies the variability in digit representations in healthy subjects, finding some quantifiable separability between digits 2, 3 and 4, a complex overlapping relationship between digits 1 and 2, and little agreement of digit 5 across subjects. The atlas and constituent subject maps are available online for use as a reference in future neuroimaging studies.

Pneumatical-Mechanical Tactile Stimulation Device for Somatotopic Mapping of Body Surface During fMRI

BACKGROUND

There is a need for devices that allow reproducible stimulation of skin areas of humans for investigating somatosensory mapping of the whole-body surface. However, their design is not simple, due to the magnetic field of MRI scanners.

PURPOSE

To accurately characterize the mapping of somatosensory presentation of the whole-body surface of subjects during functional (f)MRI scans.

STUDY TYPE

Prospective.

POPULATION

A water phantom and six healthy participants (age 23-27 years; two males) were recruited for the fMRI experiment.

FIELD STRENGTH/SEQUENCE

T₁ -weighted magnetization-prepared rapid acquisition gradient-echo, T₂ *-weighted gradient echo sequence at 3T.

ASSESSMENT

The stimulation device for somatotopic mapping was composed of three units: an air-generating unit, a control unit, and an execution unit. The fMRI in response to tactile stimulation was measured to characterize somatotopic mapping of the right-side body consisting of hand, arm, and leg in six healthy subjects.

STATISTICAL TESTS

Pared-samples t-test for the conditions in SII.

RESULTS

The pneumatical-mechanical tactile stimulation offered a wide range of stimulation intensities (0-400 g) in each channel. The predetermined physical pressure was successfully reached within ~5 msec and returned to baseline within 5 msec after the end of stimulation. With this tactile device, the digressive rate of the signal-to-noise ratio (SNR) (271.44 without the device, 269.68 with the device) was 0.65% in the magnetic field environment. For the fMRI experiment, the primary somatosensory activation contralateral to the stimulation site was detected in response to spatial task and attentive task.

DATA CONCLUSION

This stimulation device characterized the mapping of somatosensory representation of the whole-body surface in individual participants during fMRI scans.

LEVEL OF EVIDENCE

2.

TECHNICAL EFFICACY STAGE

1. J. Magn. Reson. Imaging 2020;52:1093-1101.

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.

Is Empathy for Pain Unique in Its Neural Correlates? A Meta-Analysis of Neuroimaging Studies of Empathy

Empathy is an essential component of our social lives, allowing us to understand and share other people's affective and sensory states, including pain. Evidence suggests a core neural network—including anterior insula (AI) and mid-cingulate cortex (MCC)—is involved in empathy for pain. However, a similar network is associated to empathy for non-pain affective states, raising the question whether empathy for pain is unique in its neural correlates. Furthermore, it is yet unclear whether neural correlates converge across different stimuli and paradigms that evoke pain-empathy. We performed a coordinate-based activation likelihood estimation (ALE) meta-analysis to identify neural correlates of empathy, assess commonalities and differences between empathy for pain and for non-pain negative affective states, and differences between pain-empathy evoking stimuli (i.e., facial pain expressions vs. acute pain inflictions) and paradigms (i.e., perceptual/affective vs. cognitive/evaluative paradigms). Following a systematic search, data from 128 functional brain imaging studies presenting whole-brain results of an empathy condition vs. baseline/neutral condition were extracted. Synthesizing neural correlates of empathy confirmed a core network comprising AI, MCC, postcentral gyrus, inferior parietal lobe, thalamus, amygdala, and brainstem. There was considerable overlap in networks for empathy for pain and empathy for non-pain negative affective states. Important differences also arose: empathy for pain uniquely activated bilateral mid-insula and more extensive MCC. Regarding stimuli, painful faces and acute pain inflictions both evoked the core empathy regions, although acute pain inflictions activated additional regions including medial frontal and parietal cortex. Regarding paradigms, both perceptual/affective and cognitive/evaluative paradigms recruited similar neural circuitry, although cognitive/evaluative paradigms activated more left MCC regions while perceptual/affective paradigms activated more right AI. Taken together, our findings reveal that empathy for pain and empathy for non-pain negative affective states share considerable neural correlates, particularly in core empathy regions AI and MCC. Beyond these regions, important differences emerged, limiting generalizability of findings across different affective/sensory states. Within pain-empathy studies, the core regions were recruited robustly irrespective of stimuli or instructions, allowing one to tailor designs according to specific needs to some extent, while ensuring activation of core regions.