Finger Representations Research Articles

Precise mapping of the somatotopic hand area using neuromagnetic steady-state responses

The body surface is represented in somatotopically organized maps in the primary somatosensory cortex. Estimating the size of the hand area with neuromagnetic source analysis has been used as a metric for monitoring neuroplastic changes related to training, learning, and brain injury. Commonly, results were significant as group statistics only because source localization accuracy was limited by factors such as residual noise and head motion. In this study we aimed to develop a robust method for obtaining the somatotopic map of the hand area in individuals using the bootstrap framework. Furthermore, a comprehensive analysis of the different factors affecting the accuracy of the obtained maps was provided. We applied vibrotactile touch stimuli to the tip of the index finger or the ring finger of the right hand and recorded the 22-Hz steady-state response using MEG. Single equivalent dipole sources were localized in contralateral left somatosensory cortex. Bootstrap resampling revealed the confidence intervals for the source coordinates using a single block of 5min MEG recording. Residual noise in the averaged evoked response predominantly affected source localization, and the related confidence interval was reciprocally related to the signal-to-noise ratio. Apparently, head movements within a block of MEG recording contributed less to the variability of source localization in cooperative volunteers. The results of the current study indicate that significant separations of index finger and ring finger representations along the somatotopic map can be revealed in an individual using bootstrap framework.

Perceiving numbers affects the internal random movements generator.

According to the evidence of direct relationships among space, numbers, and finger representations, a random movement generation (RMG) task was employed in order to investigate whether numerical exposure can influence the finger selection of healthy humans. To this purpose a group of participants were asked to generate random finger movements during the exposure to several numerical cues. Although participants were explicitly asked to move finger as random as possible, results showed that left-hand fingers were moved more frequently than right-hand fingers when low numerical cues (from 1 to 3) were presented, and, vice versa, right-hand fingers were moved more frequently than left-hand fingers when high numerical cues (ranged from 7 to 9) were presented. The current result suggests that spontaneous actions can be affected by abstract information, providing an evidence that numerical concepts can influence low-level, non-goal-directed behaviours.

Finger Counting and Numerical Cognition

Following a recent surge of interest in finger-based number knowledge, we invited empirical and conceptual contributions to assess the feasibility of a Research Topic on this issue. We received a considerable number of submissions, many of which were further improved by constructive and interactive peer-review and ultimately appeared as part of the Research Topic “Handy numbers: Finger counting and numerical cognition.” We wish to thank all authors and reviewers, as well as the publisher’s support team around Meghan Hodge, for their excellent work. This enthusiastic response from the research community confirmed our expectation that the time is ripe to consider the domain of number knowledge from the theoretical perspective of embodied cognition. This domain is particularly challenging for an embodied perspective on human cognition because mental arithmetic was thought to consist of abstract and amodal symbol manipulation. By disregarding the acquisition, implementation, or retrieval context of such knowledge, numerical cognition provided an ideal example of abstract information processing (e.g., Groen and Parkman, 1972). Yet in recent years a flurry of reports documented just such sensory and motor contributions to numerical cognition, and the contributions gathered for the present research topic on “handy numbers” provide an up-to-date survey of this development. The published contributions make clear that there is no agreement about the relevance of finger counting for numerical cognition. For example, finger associations might not be a necessary component of number knowledge acquisition (Crollen et al., 2011), they might merely reflect immature retrieval strategies (Kaufmann et al., 2011), and some aspects of finger-based number representation might actually hinder the initial learning process (Beller and Bender, 2011). Nevertheless, all empirical contributions to this research topic support a role of fingers in numerical cognition: Spatial–numerical associations, previously attributed to reading habits, may at least partly have their origin in finger counting routines (Fischer and Brugger, 2011; Riello and Rusconi, 2011). They are prevalent in finger counting systems of many cultures (Previtali et al., 2011; Domahs et al., 2012), affect a wide range of behaviors (Fischer and Brugger, 2011), depend on hand orientation (Previtali et al., 2011), and possibly on finger gnosia (Costa et al., 2011; Reeve and Humberstone, 2011). Furthermore, finger usage and finger-based number representations may vary considerably according to cultural influences (Bender and Beller, 2011; Domahs et al., 2012). Mental addition is selectively impaired by passive hand movements (Imbo et al., 2011) and shows sub-base five effects that can be attributed to hand-based representations (Klein et al., 2011). Addition also activates finger-related cortical structures (Krinzinger et al., 2011). Findings such as these highlight the special status of finger representations in numerical cognition (Di Luca and Pesenti, 2011) and require a conceptual rethinking. This can begin by aligning educational and neuroscientific perspectives (Moeller et al., 2011) or by contextualizing them within the embodied cognition framework (Fischer and Brugger, 2011).

Focal hand dystonia

To examine whether the corticospinal system emanating from the primary motor cortex may be organized to facilitate generation of dystonic movements. In this cross-sectional observational study, finger movement (FM) representations were assessed in 10 patients with focal hand dystonia (FHD) and 10 matched healthy controls by transcranial magnetic stimulation during rest. Evoked finger movements of the right hand were recorded using an instrumented data glove. Patterns of finger joint movements were analyzed using cluster analysis. Principal component analysis and centers of gravity for finger movement representations and motor evoked potentials recorded from the abductor pollicis brevis and abductor digiti minimi muscles were computed. For comparison, high-resolution somatosensory evoked potentials (SSEP) were recorded after electrical stimulation of the thumb (D1) or little finger (D5) in the same patients. Source reconstruction for the N20 SSEP component was performed using a dual-dipole model. Stimulation of the resting motor cortex did not reveal overt abnormalities in FHD, neither with respect to finger joint movement patterns nor with respect to the topologic organization of finger movements or intrinsic hand muscle representations. However, in line with previous reports, the distance between the dipole sources of D1 and D5 in the somatosensory cortex (S1) was smaller in patients with FHD, suggesting disruption of homuncular finger representations in S1. Our findings may imply that abnormality of motor organization in focal hand dystonia arises principally only during activation, when abnormal somatosensory representations are functionally integrated.

Somatosensory activation of two fingers can be discriminated with ultrahigh-density diffuse optical tomography

Topographic non-invasive near infrared spectroscopy (NIRS) has become a well-established tool for functional brain imaging. Applying up to 100 optodes over the head of a subject, allows achieving a spatial resolution in the centimeter range. This resolution is poor compared to other functional imaging tools. However, recently it was shown that diffuse optical tomography (DOT) as an extension of NIRS based on high-density (HD) probe arrays and supplemented by an advanced image reconstruction procedure allows describing activation patterns with a spatial resolution in the millimeter range. Building on these findings, we hypothesize that HD-DOT may render very focal activations accessible which would be missed by the traditionally used sparse arrays. We examined activation patterns in the primary somatosensory cortex, since its somatotopic organization is very fine-grained. We performed a vibrotactile stimulation study of the first and fifth finger in eight human subjects, using a 900-channel continuous-wave DOT imaging system for achieving a higher resolution than conventional topographic NIRS. To compare the results to a well-established high-resolution imaging technique, the same paradigm was investigated in the same subjects by means of functional magnetic resonance imaging (fMRI). In this work, we tested the advantage of ultrahigh-density probe arrays and show that highly focal activations would be missed by classical next-nearest neighbor NIRS approach, but also by DOT, when using a sparse probe array. Distinct activation patterns for both fingers correlated well with the expected neuroanatomy in five of eight subjects. Additionally we show that activation for different fingers is projected to different tissue depths in the DOT image. Comparison to the fMRI data yielded similar activation foci in seven out of ten finger representations in these five subjects when comparing the lateral localization of DOT and fMRI results.

Single trial discrimination of individual finger movements on one hand: A combined MEG and EEG study

It is crucial to understand what brain signals can be decoded from single trials with different recording techniques for the development of Brain-Machine Interfaces. A specific challenge for non-invasive recording methods are activations confined to small spatial areas on the cortex such as the finger representation of one hand. Here we study the information content of single trial brain activity in non-invasive MEG and EEG recordings elicited by finger movements of one hand. We investigate the feasibility of decoding which of four fingers of one hand performed a slight button press. With MEG we demonstrate reliable discrimination of single button presses performed with the thumb, the index, the middle or the little finger (average over all subjects and fingers 57%, best subject 70%, empirical guessing level: 25.1%). EEG decoding performance was less robust (average over all subjects and fingers 43%, best subject 54%, empirical guessing level 25.1%). Spatiotemporal patterns of amplitude variations in the time series provided best information for discriminating finger movements. Non-phase-locked changes of mu and beta oscillations were less predictive. Movement related high gamma oscillations were observed in average induced oscillation amplitudes in the MEG but did not provide sufficient information about the finger's identity in single trials. Importantly, pre-movement neuronal activity provided information about the preparation of the movement of a specific finger. Our study demonstrates the potential of non-invasive MEG to provide informative features for individual finger control in a Brain-Machine Interface neuroprosthesis.

FP04-4 Psychiatric Registrar's Perspective on Future Roles of their Career

During selective attention, ∼7–14 Hz alpha rhythms are modulated in early sensory cortices, suggesting a mechanistic role for these dynamics in perception. Here, we investigated whether alpha modulation can be enhanced by “mindfulness” meditation (MM), a program training practitioners in sustained attention to body and breath-related sensations. We hypothesized that participants in the MM group would exhibit enhanced alpha power modulation in a localized representation in the primary somatosensory neocortex in response to a cue, as compared to participants in the control group. Healthy subjects were randomized to 8-weeks of MM training or a control group. Using magnetoencephalographic (MEG) recording of the SI finger representation, we found meditators demonstrated enhanced alpha power modulation in response to a cue. This finding is the first to show enhanced local alpha modulation following sustained attentional training, and implicates this form of enhanced dynamic neural regulation in the behavioral effects of meditative practice.

An attentional approach to study mental representations of different parts of the hand

The aim of this study is to investigate whether the fingers are represented separately from the palm. An exogenous spatial orientation paradigm was used where participants had to detect a tactile stimulus that could appear on the palm, the middle finger or the ring finger of the left hand. The tactile target was preceded by a non-predictive cue using different stimulus-onset asynchronies (SOA). We observed a Facilitation Effect in the palm and inhibition of return (IOR) for fingers using a short cue-target SOA, whereas the IOR was found in fingers and palm in long cue-target SOA. Also we observed a 'Cue above Target' effect (facilitation effect when the Cue had appeared distal to the target location in a vertical line) at the long SOA. Together, we suggest that the general pattern of results supports the proposed hypothesis about the different mental representation of fingers and palms, but with a considerable and hierarchical interrelation between them.

Effects of mindfulness meditation training on anticipatory alpha modulation in primary somatosensory cortex

During selective attention, ∼7–14 Hz alpha rhythms are modulated in early sensory cortices, suggesting a mechanistic role for these dynamics in perception. Here, we investigated whether alpha modulation can be enhanced by “mindfulness” meditation (MM), a program training practitioners in sustained attention to body and breath-related sensations. We hypothesized that participants in the MM group would exhibit enhanced alpha power modulation in a localized representation in the primary somatosensory neocortex in response to a cue, as compared to participants in the control group. Healthy subjects were randomized to 8-weeks of MM training or a control group. Using magnetoencephalographic (MEG) recording of the SI finger representation, we found meditators demonstrated enhanced alpha power modulation in response to a cue. This finding is the first to show enhanced local alpha modulation following sustained attentional training, and implicates this form of enhanced dynamic neural regulation in the behavioral effects of meditative practice.

Integration of sensory and motor representations of single fingers in the human cerebellum

The cerebellum is thought to play a key role in the integration of sensory and motor events. Little is known, however, about how sensory and motor maps in the cerebellum superimpose. In the present study we investigated the relationship between these two maps for the representation of single fingers. Participants made isometric key presses with individual fingers or received vibratory tactile stimulation to the fingertips while undergoing high-resolution functional magnetic resonance imaging (fMRI). Using multivariate analysis, we have demonstrated that the ipsilateral lobule V and VIII show patterns of activity that encode, within the same region, both which finger pressed and which finger was stimulated. The individual finger-specific activation patches are smaller than 3 mm and only show a weak somatotopic organization. To study the superposition of sensory and motor maps, we correlated the finger-specific patterns across the two conditions. In the neocortex, sensory stimulation of one digit led to activation of the same patches as force production by the same digit; in the cerebellum, these activation patches were organized in an uncorrelated manner. This suggests that, in the cerebellum, a movement of a particular finger is paired with a range of possible sensory outcomes. In summary, our results indicate a small and fractured representation of single digits in the cerebellum and suggest a fundamental difference in how the cerebellum and the neocortex integrate sensory and motor events.

Functional Reorganization of the Brain in Humans Following Spinal Cord Injury: Evidence for Underlying Changes in Cortical Anatomy

Loss of somatosensory drive results in functional reorganization of the primary somatosensory cortex (SI). While the phenomenon of functional cortical reorganization is well established, it remains unknown whether in humans, functional reorganization results from changes in brain anatomy, or simply reflects an unmasking of already existing dormant synapses. In 20 subjects with complete thoracic spinal cord injuries (SCIs) and 23 controls, we used functional and structural magnetic resonance imaging to determine whether SI reorganization was associated with changes in SI anatomy. SCI resulted in a significant SI reorganization, with the little finger representation moving medially toward the lower body representation (i.e., area of sensory loss). Furthermore, although SCI was associated with gray matter volume loss in the lower body representation, this loss was minimized as reorganization increased. That is, the greater the medial shift in little finger representation, the greater the gray matter preservation in the lower body representation. In addition, in the region of greatest SI reorganization (little finger), fractional anisotropy was correlated with SI reorganization. That is, as SI reorganization increased, the extent of aligned structures decreased. Finally, although thalamocortical fibers remained unchanged, the ease and direction of water movement within the little finger representation was altered, being directed more toward the midline in SCI subjects. These data show that SI reorganization following SCI is associated with changes in SI anatomy and provide compelling evidence that SI reorganization in humans results from the growth of new lateral connections, and not simply from the unmasking of already existing lateral connections.

The Role of Finger Representations and Saccades for Number Processing: An fMRI Study in Children

A possible functional role of finger representations for the development of early numerical cognition has been the subject of recent debate; however, until now, only behavioral studies have directly supported this view. Working from recent models of number processing, we focused on the neural networks involved in numerical tasks and their relationship to the areas underlying finger representations and saccades in children aged 6–12 years. We were able to differentiate three parietal circuits that were related to distinct aspects of number processing. magnitude processing was subserved by an association area also activated by saccades and visually guided finger movements. Addition processes led to activation in an area only engaged during saccade encoding, whereas counting processes resulted in the activation of an area only activated during visually guided finger movements, namely in the anterior intraparietal sulcus. Apart from this area, a large network of specifically finger-related brain areas including the ventral precentral sulcus, supplementary motor area, dorso-lateral prefrontal cortex, insula, thalamus, midbrain, and cerebellum was activated during (particularly non-symbolic) exact addition but not during magnitude comparison. Moreover, a finger-related activation cluster in the right ventral precentral sulcus was only present during non-symbolic addition and magnitude comparison, but not during symbolic number processing tasks. We conclude that finger counting may critically mediate the step from non-symbolic to symbolic and exact number processing via somatosensory integration processes and therefore represents an important example of embodied cognition.

Five- to 7-Year-Olds? Finger Gnosia and Calculation Abilities

The research examined the relationship between 65 5- to 7-year-olds’ finger gnosia, visuo-spatial working memory, and finger-use in solving single-digit addition problems. Their non-verbal IQ and basic reaction time were also assessed. Previous research has found significant changes in children’s representational abilities between 5 and 7 years. One aim of the research was to determine whether changes in finger representational abilities (finger gnosia) occur across these ages and whether they are associated with finger-use in computation. A second aim was to determine whether visuo-spatial working memory is associated with finger gnosia and computation abilities. We used latent class profile analysis to identify patterns of similarities and differences in finger gnosia and computation/finger-use abilities. The analysis yielded four finger gnosia subgroups that differed in finger representation ability. It also yielded four finger/computation subgroups that differed in the relationship between finger-use and computation success. Analysis revealed associations between computation finger-use/success subgroups, finger gnosia subgroups, and visuo-spatial working memory. A multinomial logistic regression analysis showed that finger gnosia subgroup membership and visuo-spatial working memory uniquely contribute to a model predicting finger-use in computation group membership. The results show that finger gnosia abilities change in the early school years, and that these changes are associated with the ability to use fingers to aid computation.

Body posture affects tactile discrimination and identification of fingers and hands

It is an unresolved question whether the posture of single fingers relative to each other is represented in the brain within an external frame of reference. In two experiments, we investigated postural influences on the processing of tactile stimuli at fingers and hands. Healthy subjects received two simultaneous tactile stimuli at the fingertips while the fingers of both hands were either interleaved or not. In speeded response tasks, they were asked to discriminate (experiment 1) or to identify (experiment 2) the touched body parts, either regarding hand laterality or finger type. The results demonstrate that both finger discrimination and finger identification are influenced by body posture. We conclude that the assumption of a solely somatotopic representation of fingers is not tenable and that an external reference system must be available for the detection of single fingers. The results are discussed in terms of a mental segmentation of external space, based on body posture and task requirements.

Cortical processing of near-threshold tactile stimuli: An MEG study

In the present study we tested the applicability of a paired-stimulus paradigm for the investigation of near-threshold (NT) stimulus processing in the somatosensory system using magnetoencephalography. Cortical processing of the NT stimuli was studied indirectly by investigating the impact of NT stimuli on the source activity of succeeding suprathreshold test stimuli. We hypothesized that cortical responses evoked by test stimuli are reduced due to the preactivation of the same finger representation by the preceding NT stimulus. We observed attenuation of the magnetic responses in the secondary somatosensory (SII) cortex, with stronger decreases for perceived than for missed NT stimuli. Our data suggest that processing in the primary somatosensory cortex including recovery lasts for <200 ms. Conversely, the occupancy of SII lasts >/=500 ms, which points to its role in temporal integration and conscious perception of sensory input.

Somatotopic representation of finger in human primary motor cortex hand area

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High-resolution optical functional mapping of the human somatosensory cortex

Non-invasive optical imaging of brain function has been promoted in a number of fields in which functional magnetic resonance imaging (fMRI) is limited due to constraints induced by the scanning environment. Beyond physiological and psychological research, bedside monitoring and neurorehabilitation may be relevant clinical applications that are yet little explored. A major obstacle to advocate the tool in clinical research is insufficient spatial resolution. Based on a multi-distance high-density optical imaging setup, we here demonstrate a dramatic increase in sensitivity of the method. We show that optical imaging allows for the differentiation between activations of single finger representations in the primary somatosensory cortex (SI). Methodologically our findings confirm results in a pioneering study by Zeff et al. (2007) and extend them to the homuncular organization of SI. After performing a motor task, eight subjects underwent vibrotactile stimulation of the little finger and the thumb. We used a high-density diffuse-optical sensing array in conjunction with optical tomographic reconstruction. Optical imaging disclosed three discrete activation foci one for motor and two discrete foci for vibrotactile stimulation of the first and fifth finger, respectively. The results were co-registered to the individual anatomical brain anatomy (MRI) which confirmed the localization in the expected cortical gyri in four subjects. This advance in spatial resolution opens new perspectives to apply optical imaging in the research on plasticity notably in patients undergoing neurorehabilitation.

Decreased Cortical Somatosensory Finger Representation in X‐Linked Recessive Bulbospinal Neuronopathy (Kennedy Disease): A Magnetoencephalographic Study

Kennedy disease (KD) clinically presents as progressive lower motor neuron disease with minimal or no sensory impairment. However, electrophysiological studies found abnormal somatosensory-evoked potentials even in absence of clinical deficits. Little is known about possible influences of this sensory neuropathy on the central somatosensory processing. In this study, the cortical topography of index finger representation was studied in 7 patients with genetically proven KD compared to healthy control subjects by means of magnetoencephalography using an established stimulation paradigm. Data analysis was carried out with synthetic aperture magnetometry (SAM). Additionally, the latency and source amplitude of the earliest cortical somatosensory-evoked field (SEF) component were determined based on traditional single dipole source analysis. In KD patients the latency of the SEF was prolonged (48.6 vs. 37.4 ms, P < .005). There was no significant difference in dipole source amplitude, but stimulus-related SAM activation of the contralateral sensorimotor cortex (pseudo-t-values -.107 vs. -.199, P < .05), including maximum activity (53.5%), was reduced. These results implicate that even subclinical sensory neuropathy leads to possible functional reorganization of the sensorimotor cortex in KD patients and reinforces the view that in KD the somatosensory system is extensively involved.

Quantitative Analysis and Biophysically Realistic Neural Modeling of the MEG Mu Rhythm: Rhythmogenesis and Modulation of Sensory-Evoked Responses

Variations in cortical oscillations in the alpha (7-14 Hz) and beta (15-29 Hz) range have been correlated with attention, working memory, and stimulus detection. The mu rhythm recorded with magnetoencephalography (MEG) is a prominent oscillation generated by Rolandic cortex containing alpha and beta bands. Despite its prominence, the neural mechanisms regulating mu are unknown. We characterized the ongoing MEG mu rhythm from a localized source in the finger representation of primary somatosensory (SI) cortex. Subjects showed variation in the relative expression of mu-alpha or mu-beta, which were nonoverlapping for roughly 50% of their respective durations on single trials. To delineate the origins of this rhythm, a biophysically principled computational neural model of SI was developed, with distinct laminae, inhibitory and excitatory neurons, and feedforward (FF, representative of lemniscal thalamic drive) and feedback (FB, representative of higher-order cortical drive or input from nonlemniscal thalamic nuclei) inputs defined by the laminar location of their postsynaptic effects. The mu-alpha component was accurately modeled by rhythmic FF input at approximately 10-Hz. The mu-beta component was accurately modeled by the addition of approximately 10-Hz FB input that was nearly synchronous with the FF input. The relative dominance of these two frequencies depended on the delay between FF and FB drives, their relative input strengths, and stochastic changes in these variables. The model also reproduced key features of the impact of high prestimulus mu power on peaks in SI-evoked activity. For stimuli presented during high mu power, the model predicted enhancement in an initial evoked peak and decreased subsequent deflections. In agreement, the MEG-evoked responses showed an enhanced initial peak and a trend to smaller subsequent peaks. These data provide new information on the dynamics of the mu rhythm in humans and the model provides a novel mechanistic interpretation of this rhythm and its functional significance.

Cortical Reorganization in Primary Somatosensory Cortex in Patients With Unilateral Chronic Pain

Bodily representations of the primary somatosensory (SI) cortex are constantly modified according to sensory input. Increased input due to training as well as loss of input due to deafferentation are reflected as changes in the extent of cortical representations. Recent studies in complex regional pain syndrome (CRPS) patients have indicated that the chronic pain itself is associated with cortical reorganization. However, it is unclear whether the observed reorganization is specific for CRPS or if it can be detected also in other types of chronic pain. We therefore searched for signs of cortical reorganization in a group of 8 patients who suffered from chronic pain associated with herpes simplex virus infections. The pain was widespread but restricted to unilateral side of the body and included the upper limb. We recorded neuromagnetic responses to tactile stimulation of fingers of both hands in patients and in a group of healthy, matched control subjects. In the patients, the distance between the thumb (D1) and little finger (D5) representations in SI cortex was statistically significantly smaller in the hemisphere contralateral to painful side than in the hemisphere contralateral to healthy side. In the control subjects, the D1-D5 distance was the same in both hemispheres. The present results indicate that cortical reorganization occurs in chronic neuropathic pain patients even without peripheral nerve damage. It is possible that cortical reorganization is related to chronic pain, regardless of its etiology. Causality between reorganization and chronic pain should be examined further to develop therapeutic approaches for chronic pain.