Transfer of Tactile Learning to Untrained Body Parts: Emerging Cortical Mechanisms.

Pioneering investigations in the mid-19th century revealed that the perception of tactile cues presented to the surface of the skin improves with training, which is referred to as tactile learning. Surprisingly, tactile learning also occurs for body parts and skin locations that are not physically involved in the training. For example, after training of a finger, tactile learning transfers to adjacent untrained fingers. This suggests that the transfer of tactile learning follows a somatotopic pattern and involves brain regions such as the primary somatosensory cortex (S1), in which the trained and untrained body parts and skin locations are represented close to each other. However, other results showed that transfer occurs between body parts that are not represented close to each other in S1-for example, between the hand and the foot. These and similar findings have led to the suggestion of additional cortical mechanisms to explain the transfer of tactile learning. Here, different mechanisms are reviewed, and the extent to which they can explain the transfer of tactile learning is discussed. What all of these mechanisms have in common is that they assume a representational or functional relationship between the trained and untrained body parts and skin locations. However, none of these mechanisms alone can explain the complex pattern of transfer results, and it is likely that different mechanisms interact to enable transfer, perhaps in concert with higher somatosensory and decision-making areas.

Olov Oscarsson (1931-1996) of Lund University, a Pioneer in Cerebellar Neurobiology.

The present Cerebellar Classic highlights the experimental work of the Swedish neurophysiologist Olov Oscarsson (1931-1996) on the afferent innervation of the cerebellum by axons emanating from neurons in the spinal cord and the inferior olive. Historically, the schemes of cerebellar division had been principally based on the external morphology of lobules and fissures. However, the macroscopic anatomical division of the cerebellum does not coincide with its pattern of functional organization. By defining a system of longitudinal somatotopy, Oscarsson contributed to the much needed plan of cerebellar division that correlates experimental information on axonal connections with physiology. His contribution has ultimately led to the currently accepted microzonal modular scheme of cerebellar corticonuclear microcomplexes.

Analyzing Thalamocortical Tract-Tracing Experiments in a Common Reference Space.

Current mesoscale connectivity atlases provide limited information about the organization of thalamocortical projections in the mouse brain. Labeling the projections of spatially restricted neuron populations in thalamus can provide a functionally relevant level of connectomic analysis, but these need to be integrated within the same common reference space. Here, we present a pipeline for the segmentation, registration, integration and analysis of multiple tract-tracing experiments. The key difference with other workflows is that the data is transformed to fit the reference template. As a test-case, we investigated the axonal projections and intranuclear arrangement of seven neuronal populations of the ventral posteromedial nucleus of the thalamus (VPM), which we labeled with an anterograde tracer. Their soma positions corresponded, from dorsal to ventral, to cortical representations of the whiskers, nose and mouth. They strongly targeted layer 4, with the majority exclusively targeting one cortical area and the ones in ventrolateral VPM branching to multiple somatosensory areas. We found that our experiments were more topographically precise than similar experiments from the Allen Institute and projections to the primary somatosensory area were in agreement with single-neuron morphological reconstructions from publicly available databases. This pilot study sets the basis for a shared virtual connectivity atlas that could be enriched with additional data for studying the topographical organization of different thalamic nuclei. The pipeline is accessible with only minimal programming skills via a Jupyter Notebook, and offers multiple visualization tools such as cortical flatmaps, subcortical plots and 3D renderings and can be used with custom anatomical delineations.

Extended source analysis of movement related potentials (MRPs) for self-paced hand and foot movements demonstrates opposing cerebral and cerebellar laterality: a preliminary study.

The cerebellum is known to have extensive reciprocal connectivity with the cerebral cortex, including with prefrontal and posterior parietal cortex, which play an important role on the planning and execution of voluntary movement. In the present article we report an exploratory non-invasive electrophysiological study of the activity of the cerebellum and cerebrum during voluntary finger and foot movements. In a sample of five healthy adult subjects, we recorded EEG and the electro-cerebellogram (ECeG) with a 10% cerebellar extension montage during voluntary left and right index finger and foot movements. EMG was recorded from finger extensors and flexors and from the tibialis anterior and soleus muscles and was used to generate triggers for movement related averaging (-2000 to +2000 ms). Source analysis was conducted over five epochs defined relative to EMG onset: whole epoch (-1000 to +1000 ms), pre-move 1000 (-1000 to 0 ms), pre-move 500 (-500 to 0 ms), post-move 500 (0 to +500 ms) and post-move 1000 (0 to +1000 ms). This yielded a total of 123 cerebral and 65 cerebellar dipole clusters from across all epochs, including the pre-movement epochs, which were then subject to statistical analysis. These demonstrated predominantly contralateral dominance for the cerebral clusters, but predominantly ipsilateral dominance for the cerebellar clusters. In addition, both cerebral and cerebellar clusters showed evidence of a somatotopic gradient, medially (X-axis) for the cerebral clusters, and medially and dorso-ventrally (Z-axis) for the cerebellar clusters. These findings support the value of recording cerebellar ECeG and demonstrate its potential to contribute to understanding cerebellar function.

Performing a vibrotactile discrimination task modulates finger representations in primary somatosensory cortex.

It is well established that vibrotactile stimuli are represented in somatotopic maps. However, less is known about whether these somatotopic representations are modulated by task demands and maybe even in the absence of tactile input. Here, we used a vibrotactile discrimination task as a tool to investigate these questions in further detail. Participants were required to actively perceive and process tactile stimuli in comparison to a no-task control condition where identical stimuli were passively perceived (no-memory condition). Importantly, both vibrotactile stimuli were either applied to the right index or little finger, allowing us to investigate whether cognitive task demands shape finger representations in primary somatosensory cortex (S1). Using multivoxel pattern analysis and representational similarity analysis, we found that S1 finger representations were more distinct during the memory than the no-memory condition. Interestingly, this effect was not only observed while tactile stimuli were presented but also during the delay period (i.e., in the absence of tactile stimulation). Our findings imply that when individuals are required to focus on tactile stimuli, retain them in their memory, and engage in active processing of distinctive stimulus features, this exerts a modulatory effect on the finger representations present in S1.NEW & NOTEWORTHY Using multivoxel pattern analysis, we found that discrimination task demands shape finger representations in the contralateral primary somatosensory cortex (S1), and that somatotopic representations are modulated by task demands not only during tactile stimulation but also to a certain extent in the absence of tactile input.

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.

Reorganization of sensorimotor representations of the intact limb after upper but not lower limb traumatic amputation.

It is becoming increasingly clear that limb loss induces wider spread reorganization of representations of the body that are nonadjacent to the affected cortical territory. Data from upper extremity amputees reveal intrusion of the representation of the ipsilateral intact limb into the former hand territory. Here we test for the first time whether this reorganization of the intact limb into the deprived cortex is specific to the neurological organization of the upper limbs or reflects large scale adaptation that is triggered by any unilateral amputation. BOLD activity was measured as human subjects with upper limb and lower limb traumatic amputation and their controls moved the toes on each foot, open and closed each hand and pursed their lips. Subjects with amputation were asked to imagine moving the missing limb while remaining still. Bayesian pattern component modeling of fMRI data showed that intact ipsilateral movements and contralateral movements of the hand and foot were distinctly represented in the deprived sensorimotor cortex years after upper limb amputation. In contrast, there was evidence reminiscent of contralateral specificity for hand and foot movements following lower limb amputation, like that seen in controls. We propose the cortical reorganization of the intact limb to be a function of use-dependent plasticity that is more specific to the consequence of upper limb loss of forcing an asymmetric reliance on the intact hand and arm. The contribution of this reorganization to phantom pain or a heightened risk of overuse and resultant maladaptive plasticity needs investigating before targeting such reorganization in intervention.

Protocol for mapping of the supplementary motor area using repetitive navigated transcranial magnetic stimulation.

Background: Damage to the supplementary motor area (SMA) can lead to impairments of motor and language function. A detailed preoperative mapping of functional boarders of the SMA could therefore aid preoperative diagnostics in these patients. Objective: The aim of this study was the development of a repetitive nTMS protocol for non-invasive functional mapping of the SMA while assuring effects are caused by SMA rather than M1 activation. Methods: The SMA in the dominant hemisphere of 12 healthy subjects (28.2 ± 7.7 years, 6 females) was mapped using repetitive nTMS at 20 Hz (120% RMT), while subjects performed a finger tapping task. Reductions in finger taps were classified in three error categories (≤15% = no errors, 15-30% = mild, >30% significant). The location and category of induced errors was marked in each subject's individual MRI. Effects of SMA stimulation were then directly compared to effects of M1 stimulation in four different tasks (finger tapping, writing, line tracing, targeting circles). Results: Mapping of the SMA was possible for all subjects, yet effect sizes varied. Stimulation of the SMA led to a significant reduction of finger taps compared to baseline (BL: 45taps, SMA: 35.5taps; p < 0.01). Line tracing, writing and targeting of circles was less accurate during SMA compared to M1 stimulation. Conclusion: Mapping of the SMA using repetitive nTMS is feasible. While errors induced in the SMA are not entirely independent of M1, disruption of the SMA induces functionally distinct errors. These error maps can aid preoperative diagnostics in patients with SMA related lesions.

Layer-specific vulnerability is a mechanism of topographic map aging.

Topographic maps form a critical feature of cortical organization, yet are poorly described with respect to their microstructure in the living aging brain. We acquired quantitative structural and functional 7T-MRI data from younger and older adults to characterize layer-wise topographic maps of the primary motor cortex (M1). Using parcellation-inspired techniques, we show that quantitative T1 and Quantitative Susceptibility Maps values of the hand, face, and foot areas differ significantly, revealing microstructurally distinct cortical fields in M1. We show that these fields are distinct in older adults and that myelin borders between them do not degenerate. We further show that the output layer 5 of M1 shows a particular vulnerability to age-related increased iron, while layer 5 and the superficial layer show increased diamagnetic substance, likely reflecting calcifications. Taken together, we provide a novel 3D model of M1 microstructure, where body parts form distinct structural units, but layers show specific vulnerability toward increased iron and calcium in older adults. Our findings have implications for understanding sensorimotor organization and aging, in addition to topographic disease spread.

Functional brainstem representations of the human trigeminal cervical complex.

BACKGROUND: The human in-vivo functional somatotopy of the three branches of the trigeminal (V1, V2, V3) and greater occipital nerve in brainstem and also in thalamus and insula is still not well understood. METHODS: After preregistration (clinicaltrials.gov: NCT03999060), we mapped the functional representations of this trigemino-cervical complex non-invasively in 87 humans using high-resolution protocols for functional magnetic resonance imaging during painful electrical stimulation in two separate experiments. The imaging protocol and analysis was optimized for the lower brainstem and upper spinal cord, to identify activation of the spinal trigeminal nuclei. The stimulation protocol involved four electrodes which were positioned on the left side according to the three branches of the trigeminal nerve and the greater occipital nerve. The stimulation site was randomized and each site was repeated 10 times per session. The participants partook in three sessions resulting in 30 trials per stimulation site. RESULTS: We show a large overlap of peripheral dermatomes on brainstem representations and a somatotopic arrangement of the three branches of the trigeminal nerve along the perioral-periauricular axis and for the greater occipital nerve in brainstem below pons, as well as in thalamus, insula and cerebellum. The co-localization of greater occipital nerve with V1 along the lower part of brainstem is of particular interest since some headache patients profit from an anesthetic block of the greater occipital nerve. CONCLUSION: Our data provide anatomical evidence for a functional inter-inhibitory network between the trigeminal branches and greater occipital nerve in healthy humans as postulated in animal work. We further show that functional trigeminal representations intermingle perioral and periauricular facial dermatomes with individual branches of the trigeminal nerve in an onion shaped manner and overlap in a typical within-body-part somatotopic arrangement.Trial registration: clinicaltrials.gov: NCT03999060.

Activation of Cerebellum, Basal Ganglia and Thalamus During Observation and Execution of Mouth, hand, and foot Actions.

Humans and monkey studies showed that specific sectors of cerebellum and basal ganglia activate not only during execution but also during observation of hand actions. However, it is unknown whether, and how, these structures are engaged during the observation of actions performed by effectors different from the hand. To address this issue, in the present fMRI study, healthy human participants were required to execute or to observe grasping acts performed with different effectors, namely mouth, hand, and foot. As control, participants executed and observed simple movements performed with the same effectors. The results show that: (1) execution of goal-directed actions elicited somatotopically organized activations not only in the cerebral cortex but also in the cerebellum, basal ganglia, and thalamus; (2) action observation evoked cortical, cerebellar and subcortical activations, lacking a clear somatotopic organization; (3) in the territories displaying shared activations between execution and observation, a rough somatotopy could be revealed in both cortical, cerebellar and subcortical structures. The present study confirms previous findings that action observation, beyond the cerebral cortex, also activates specific sectors of cerebellum and subcortical structures and it shows, for the first time, that these latter are engaged not only during hand actions observation but also during the observation of mouth and foot actions. We suggest that each of the activated structures processes specific aspects of the observed action, such as performing internal simulation (cerebellum) or recruiting/inhibiting the overt execution of the observed action (basal ganglia and sensory-motor thalamus).

"Neuro-Fiber Mapping": An Original Concept of Spinal Cord Neural Network Spatial Targeting Using Live Electrostimulation Mapping to (Re-)Explore the Conus Medullaris Anatomy.

Spinal cord (SC) anatomy is often assimilated to a morphologically encapsulated neural entity, but its functional anatomy remains only partially understood. We hypothesized that it could be possible to re-explore SC neural networks by performing live electrostimulation mapping, based on "super-selective" spinal cord stimulation (SCS), originally designed as a therapeutical tool to address chronic refractory pain. As a starting point, we initiated a systematic SCS lead programming approach using live electrostimulation mapping on a chronic refractory perineal pain patient, previously implanted with multicolumn SCS at the level of the conus medullaris (T12-L1). It appeared possible to (re-)explore the classical anatomy of the conus medullaris using statistical correlations of paresthesia coverage mappings, resulting from 165 different electrical configurations tested. We highlighted that sacral dermatomes were not only located more medially but also deeper than lumbar dermatomes at the level of the conus medullaris, in contrast with classical anatomical descriptions of SC somatotopical organization. As we were finally able to find a morphofunctional description of "Philippe-Gombault's triangle" in 19th-century historical textbooks of neuroanatomy, remarkably matching these conclusions, the concept of "neuro-fiber mapping" was introduced.

fNIRS is sensitive to leg activity in the primary motor cortex after systemic artifact correction.

BACKGROUND: functional near-infrared spectroscopy (fNIRS) is an increasingly popular tool to study cortical activity during movement and gait that requires further validation. This study aimed to assess (1) whether fNIRS can detect the difficult-to-measure leg area of the primary motor cortex (M1) and distinguish it from the hand area; and (2) whether fNIRS can differentiate between automatic (i.e., not requiring one's attention) and non-automatic movement processes. Special attention was attributed to systemic artifacts (i.e., changes in blood pressure, heart rate, breathing) which were assessed and corrected by short channels, i.e., fNIRS channels which are mainly sensitive to superficial scalp hemodynamics. METHODS: Twenty-three seated, healthy participants tapped four fingers on a keyboard or tapped the right foot on four squares on the floor in a specific order given by a 12-digit sequence (e.g., 434141243212). Two different sequences were executed: a beforehand learned (i.e., automatic) version and a newly learned (i.e., non-automatic) version. A 36-channel fNIRS device including 12 short channels covered multiple motor-related cortical areas including M1. The fNIRS data were analyzed with a general linear model (GLM). Correlation between the expected functional hemodynamic responses (i.e. task regressor) and the short channels (i.e. nuisance regressors), necessitated performing a separate short channel regression instead of integrating them in the GLM. RESULTS: Consistent with the M1 somatotopy, we found significant HbO increases of very large effect size in the lateral M1 channels during finger tapping (Cohen's d = 1.35, p<0.001) and significant HbO increases of moderate effect size in the medial M1 channels during foot tapping (Cohen's d = 0.8, p<0.05). The cortical activity differences between automatic and non-automatic tasks were not significantly different. Importantly, leg movements produced large systemic fluctuations, which were adequately removed by the use of all available short channels. DISCUSSION: Our results indicate that fNIRS is sensitive to leg activity in M1, though the sensitivity is lower than for finger activity and requires rigorous correction for systemic fluctuations. We furthermore highlight that systemic artifacts may result in an unreliable GLM analysis when short channels show signals that are similar to the expected hemodynamic responses.

The evidence against somatotopic organization of function in the primate corticospinal tract.

We review the spatial organization of corticospinal outputs from different cortical areas and how this reflects the varied functions mediated by the corticospinal tract. A long-standing question is whether the primate corticospinal tract shows somatotopical organization. Although this has been clearly demonstrated for corticofugal outputs passing through the internal capsule and cerebral peduncle, there is accumulating evidence against somatotopy in the pyramidal tract in the lower brainstem and in the spinal course of the corticospinal tract. Answering the question on somatotopy has important consequences for understanding the effects of incomplete spinal cord injury. Our recent study in the macaque monkey, using high-resolution dextran tracers, demonstrated a great deal of intermingling of fibres originating from primary motor cortex arm/hand, shoulder and leg areas. We quantified the distribution of fibres belonging to these different projections and found no significant difference in their distribution across different subsectors of the pyramidal tract or lateral corticospinal tract, arguing against somatotopy. We further demonstrated intermingling with corticospinal outputs derived from premotor and supplementary motor arm areas. We present new evidence against somatotopy for corticospinal projections from rostral and caudal cingulate motor areas and from somatosensory areas of the parietal cortex. In the pyramidal tract and lateral corticospinal tract, fibres from the cingulate motor areas overlap with each other. Fibres from the primary somatosensory cortex arm area completely overlap those from the leg area. There is also substantial overlap of both these outputs with those from posterior parietal sensorimotor areas. We argue that the extensive intermingling of corticospinal outputs from so many different cortical regions must represent an organizational principle, closely related to its mediation of many different functions and its large range of fibre diameters. The motor sequelae of incomplete spinal injury, such as central cord syndrome and 'cruciate paralysis', include much greater deficits in upper than in lower limb movement. Current teaching and text book explanations of these symptoms are still based on a supposed corticospinal somatotopy or 'lamination', with greater vulnerability of arm and hand versus leg fibres. We suggest that such explanations should now be finally abandoned. Instead, the clinical and neurobiological implications of the complex organization of the corticospinal tract need now to be taken into consideration. This leads us to consider the evidence for a greater relative influence of the corticospinal tract on upper versus lower limb movements, the former best characterized by skilled hand and digit movements.

Detailed organisation of the human midbrain periaqueductal grey revealed using ultra-high field magnetic resonance imaging.

The midbrain periaqueductal grey (PAG) is a critical region for the mediation of pain-related behavioural responses. Neuronal tract tracing techniques in experimental animal studies have demonstrated that the lateral column of the PAG (lPAG) displays a crude somatotopy, which is thought to be critical for the selection of contextually appropriate behavioural responses, without the need for higher brain input. In addition to the different behavioural responses to cutaneous and muscle pain - active withdrawal versus passive coping - there is evidence that cutaneous pain is processed in the region of the lPAG and muscle pain in the adjacent ventrolateral PAG (vlPAG). Given the fundamental nature of these behavioural responses to cutaneous and muscle pain, these PAG circuits are assumed to have been preserved, though yet to be definitively documented in humans. Using ultra-high field (7-Tesla) functional magnetic resonance imaging we determined the locations of signal intensity changes in the PAG during noxious cutaneous heat stimuli and muscle pain in healthy control participants. Images were processed and blood oxygen level dependant (BOLD) signal changes within the PAG determined. It was observed that noxious cutaneous stimulation of the lip, cheek, and ear evoked maximal increases in BOLD activation in the rostral contralateral PAG, whereas noxious cutaneous stimulation of the thumb and toe evoked increases in the caudal contralateral PAG. Analysis of individual participants demonstrated that these activations were located in the lPAG. Furthermore, we found that deep muscular pain evoked the greatest increases in signal intensity in the vlPAG. These data suggest that the crude somatotopic organization of the PAG may be phyletically preserved between experimental animals and humans, with a body-face delineation capable of producing an appropriate behavioural response based on the location and tissue origin of a noxious stimulus.

Cortical field maps across human sensory cortex.

Cortical processing pathways for sensory information in the mammalian brain tend to be organized into topographical representations that encode various fundamental sensory dimensions. Numerous laboratories have now shown how these representations are organized into numerous cortical field maps (CMFs) across visual and auditory cortex, with each CFM supporting a specialized computation or set of computations that underlie the associated perceptual behaviors. An individual CFM is defined by two orthogonal topographical gradients that reflect two essential aspects of feature space for that sense. Multiple adjacent CFMs are then organized across visual and auditory cortex into macrostructural patterns termed cloverleaf clusters. CFMs within cloverleaf clusters are thought to share properties such as receptive field distribution, cortical magnification, and processing specialization. Recent measurements point to the likely existence of CFMs in the other senses, as well, with topographical representations of at least one sensory dimension demonstrated in somatosensory, gustatory, and possibly olfactory cortical pathways. Here we discuss the evidence for CFM and cloverleaf cluster organization across human sensory cortex as well as approaches used to identify such organizational patterns. Knowledge of how these topographical representations are organized across cortex provides us with insight into how our conscious perceptions are created from our basic sensory inputs. In addition, studying how these representations change during development, trauma, and disease serves as an important tool for developing improvements in clinical therapies and rehabilitation for sensory deficits.

Cortical response variability is driven by local excitability changes with somatotopic organization.

Identical sensory stimuli can lead to different neural responses depending on the instantaneous brain state. Specifically, neural excitability in sensory areas may shape the brain´s response already from earliest cortical processing onwards. However, whether these dynamics affect a given sensory domain as a whole or occur on a spatially local level is largely unknown. We studied this in the somatosensory domain of 38 human participants with EEG, presenting stimuli to the median and tibial nerves alternatingly, and testing the co-variation of initial cortical responses in hand and foot areas, as well as their relation to pre-stimulus oscillatory states. We found that amplitude fluctuations of initial cortical responses to hand and foot stimulation - the N20 and P40 components of the somatosensory evoked potential (SEP), respectively - were not related, indicating local excitability changes in primary sensory regions. In addition, effects of pre-stimulus alpha (8-13 Hz) and beta (18-23 Hz) band amplitude on hand-related responses showed a robust somatotopic organization, thus further strengthening the notion of local excitability fluctuations. However, for foot-related responses, the spatial specificity of pre-stimulus effects was less consistent across frequency bands, with beta appearing to be more foot-specific than alpha. Connectivity analyses in source space suggested this to be due to a somatosensory alpha rhythm that is primarily driven by activity in hand regions while beta frequencies may operate in a more hand-region-independent manner. Altogether, our findings suggest spatially distinct excitability dynamics within the primary somatosensory cortex, yet with the caveat that frequency-specific processes in one sub-region may not readily generalize to other sub-regions.

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.

Automated intraoperative central sulcus localization and somatotopic mapping using median nerve stimulation.

Objective. Accurate identification of functional cortical regions is essential in neurological resection. The central sulcus (CS) is an important landmark that delineates functional cortical regions. Median nerve stimulation (MNS) is a standard procedure to identify the position of the CS intraoperatively. In this paper, we introduce an automated procedure that uses MNS to rapidly localize the CS and create functional somatotopic maps.Approach. We recorded electrocorticographic signals from 13 patients who underwent MNS in the course of an awake craniotomy. We analyzed these signals to develop an automated procedure that determines the location of the CS and that also produces functional somatotopic maps.Main results. The comparison between our automated method and visual inspection performed by the neurosurgeon shows that our procedure has a high sensitivity (89%) in identifying the CS. Further, we found substantial concordance between the functional somatotopic maps generated by our method and passive functional mapping (92% sensitivity).Significance. Our automated MNS-based method can rapidly localize the CS and create functional somatotopic maps without imposing additional burden on the clinical procedure. With additional development and validation, our method may lead to a diagnostic tool that guides neurosurgeons and reduces postoperative morbidity in patients undergoing resective brain surgery.

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.