Tactile discrimination of grating orientation: fMRI activation patterns

A bioinspired tactile scanner for computer haptics

Computer haptics (CH) is about integration of tactile sensation and rendering in Metaverse. However, unlike computer vision (CV) where both hardware infrastructure and software programs are well developed, a generic tactile data capturing device that serves the same role as what a camera does for CV, is missing. Bioinspired by electrophysiological processes in human tactile somatosensory nervous system, here we propose a tactile scanner along with a neuromorphically-engineered system, in which a closed-loop tactile acquisition and rendering (re-creation) are preliminarily achieved. Based on the architecture of afferent nerves and intelligent functions of mechano-gating and leaky integrate-and-fire models, such a tactile scanner is designed and developed by using piezoelectric transducers as axon neurons and thin film transistor (TFT)-based neuromorphic circuits to mimic synaptic behaviours and neural functions. As an example, the neuron-like tactile information of surface textures is captured and further used to render the texture friction of a virtual surface for "recreating" a "true" feeling of touch.

Tactile adaptation to orientation produces a robust tilt aftereffect and exhibits crossmodal transfer when tested in vision

Orientation processing is one of the most fundamental functions in both visual and somatosensory perception. Converging findings suggest that orientation processing in both modalities is closely linked: somatosensory neurons share a similar orientation organisation as visual neurons, and the visual cortex has been found to be heavily involved in tactile orientation perception. Hence, we hypothesized that somatosensation would exhibit a similar orientation adaptation effect, and this adaptation effect would be transferable between the two modalities, considering the above-mentioned connection. The tilt aftereffect (TAE) is a demonstration of orientation adaptation and is used widely in behavioural experiments to investigate orientation mechanisms in vision. By testing the classic TAE paradigm in both tactile and crossmodal orientation tasks between vision and touch, we were able to show that tactile perception of orientation shows a very robust TAE, similar to its visual counterpart. We further show that orientation adaptation in touch transfers to produce a TAE when tested in vision, but not vice versa. Additionally, when examining the test sequence following adaptation for serial effects, we observed another asymmetry between the two conditions where the visual test sequence displayed a repulsive intramodal serial dependence effect while the tactile test sequence exhibited an attractive serial dependence. These findings provide concrete evidence that vision and touch engage a similar orientation processing mechanism. However, the asymmetry in the crossmodal transfer of TAE and serial dependence points to a non-reciprocal connection between the two modalities, providing further insights into the underlying processing mechanism.

Unraveling tactile categorization and decision-making in the subregions of supramarginal gyrus via direct cortical stimulation

OBJECTIVE

This study aims to investigate the potential of direct cortical stimulation (DCS) to modulate tactile categorization and decision-making, as well as to identify the specific locations where these cognitive functions occur.

METHODS

We analyzed behavioral changes in three epilepsy patients with implanted electrodes using electrocorticography (ECoG) and a vibrotactile discrimination task. DCS was applied to investigate its impact on tactile categorization and decision-making processes. We determined the precise location of the electrodes where each cognitive function was modulated.

RESULTS

This functional discrimination was related with gamma band activity from ECoG. DCS selectively affected either tactile categorization or decision-making processes. Tactile categorization was modulated by stimulating the rostral part of the supramarginal gyrus, while decision-making was modulated by stimulating the caudal part.

CONCLUSIONS

DCS can enhance cognitive processes and map brain regions responsible for tactile categorization and decision-making within the supramarginal gyrus. This study also demonstrates that DCS and the gamma activity of ECoG can concordantly identify the detailed brain mapping in a tactile process compared to other functional neuroimaging.

SIGNIFICANCE

The combination of DCS and ECoG gamma activity provides a more nuanced and detailed understanding of brain function than traditional neuroimaging techniques alone.

Pneumatic tactile stimulus delivery system for studying brain responses evoked by active finger touch with fMRI

BACKGROUND

Primates use their hands to actively touch objects and collect information. To study tactile information processing, it is important for participants to experience tactile stimuli through active touch while monitoring brain activities.

NEW METHOD

Here, we developed a pneumatic tactile stimulus delivery system (pTDS) that delivers various tactile stimuli on a programmed schedule and allows voluntary finger touches during MRI scanning. The pTDS uses a pneumatic actuator to move tactile stimuli and place them in a finger hole. A photosensor detects the time when an index finger touches a tactile stimulus, enabling the analysis of the touch-elicited brain responses.

RESULTS

We examined brain responses while the participants actively touched braille objects presented by the pTDS. BOLD responses during tactile perception were significantly stronger in a finger touch area of the contralateral somatosensory cortex compared with that of visual perception.

CONCLUSION

The pTDS enables MR studies of brain mechanisms for tactile processes through natural finger touch.

Sensorimotor performance after high-definition transcranial direct current stimulation over the primary somatosensory or motor cortices in men versus women

The primary somatosensory (S1) cortex is a central structure in motor performance. However, transcranial direct current stimulation (tDCS) research aimed at improving motor performance usually targets the primary motor cortex (M1). Recently, sex was found to mediate tDCS response. Thus, we investigated whether tDCS with an anodal electrode placed over S1 improves motor performance and sensation perception in men versus women. Forty-five participants randomly received 15-min high-definition tDCS (HD-tDCS) at 1 mA to S1, M1, or sham stimulation. Reaching performance was tested before and immediately following stimulation. Two-point orientation discrimination (TPOD) of fingers and proprioception of a reaching movement were also tested. Although motor performance did not differ between groups, reaching reaction time improved in the M1 group men. Reaching movement time and endpoint error improved in women and men, respectively. Correct trials percentage for TPOD task was higher in the S1 compared to the M1 group in the posttest and improved only in the S1 group. Reaching movement time for the proprioception task improved, overall, and endpoint error did not change. Despite the reciprocal connections between S1 and M1, effects of active tDCS over S1 and M1 may specifically influence sensation perception and motor performance, respectively. Also, sex may mediate effects of HD-tDCS on motor performance.

Effect of Transcranial Electrical Stimulation over the Posterior Parietal Cortex on Tactile Spatial Discrimination Performance

The intraparietal sulcus region, which is part of the posterior parietal cortex (PPC), has been shown to play an important role in discriminating object shapes using the fingers. Transcranial random noise stimulation (tRNS) and anodal transcranial pulsed current stimulation (tPCS) are noninvasive strategies widely used to modulate neural activity in cortical regions. Therefore, we investigated the effects of tRNS and anodal tPCS applied to left or right PPC on the tactile discrimination performance of the right index finger in 20 neurologically healthy subjects. A grating orientation task (GOT) was performed before and immediately after delivering tRNS (stimulus frequency 0.1-640 Hz) in Experiment 1 or anodal tPCS (pulse width 50 ms and inter-pulse interval 5 ms) in Experiment 2. Performing tRNS over the right PPC significantly improved discrimination performance on the GOT. Subjects were classified into low and high baseline performance groups. Conducting tRNS over the left PPC significantly reduced the GOT discrimination performance in the high-performance group. By contrast, anodal tPCS delivered to the PPC of the left and right hemispheres had no significant effect on the tactile GOT discrimination performance of the right hand. We show that transcranial electric stimulation over the PPC may improve tactile perception but the effect depends on stimulus modality, parameters, and on the stimulated hemisphere.

Neural Basis of Somatosensory Spatial and Temporal Discrimination in Humans: The Role of Sensory Detection

While detecting somatic stimuli from the external environment, an accurate determination of their spatial and temporal properties is essential for human behavior. Whether and how detection relates to human capacity for somatosensory spatial discrimination (SD) and temporal discrimination (TD) remains unclear. Here, participants underwent functional magnetic resonance imaging scanning when simply detecting vibrotactile stimuli of the leg, judging their location (SD), or deciding their number in time (TD). By conceptualizing tactile discrimination as consisting of detection and determination processes, we found that tactile detection elicited activation specifically involved in SD within the right inferior and superior parietal lobules, 2 regions previously implicated in the control of spatial attention. These 2 regions remained activated in the determination process, during which functional connectivity between these 2 regions predicted individual SD ability. In contrast, tactile detection produced little activation specifically related to TD. Participants' TD ability was implemented in brain regions implicated in coding temporal structures of somatic stimuli (primary somatosensory cortex) and time estimation (anterior cingulate, pre-supplementary motor area, and putamen). Together, our findings indicate a close link between somatosensory detection and SD (but not TD) at the neural level, which aids in explaining why we can promptly respond toward detected somatic stimuli.

α-tACS over the somatosensory cortex enhances tactile spatial discrimination in healthy subjects with low alpha activity

INTRODUCTION

Spontaneous oscillations in the somatosensory cortex, especially of the alpha (8 - 14 Hz) and gamma (60 - 80 Hz) frequencies, affect tactile perception; moreover, these oscillations can be selectively modulated by frequency-matched transcranial alternating current stimulation (tACS) on the basis of ongoing oscillatory brain activity. To examine whether tACS can actually improve tactile perception via alpha and gamma modulation, we measured the effects of 10-Hz and 70-Hz tACS (α- and γ-tACS) on the left somatosensory cortex on right-finger tactile spatial orientation discrimination, and the associations between performance changes and individual alpha and gamma activities.

METHODS

Fifteen neurologically healthy subjects were recruited into this study. Electroencephalography (EEG) was performed before the first day, to assess the normal alpha- and gamma-activity levels. A grating orientation discrimination task was performed before and during 10-Hz and 70-Hz tACS.

RESULTS

The 10-Hz tACS protocol decreased the grating orientation discrimination threshold, primarily in subjects with low alpha event-related synchronization (ERS). In contrast, the 70-Hz tACS had no effect on the grating orientation discrimination threshold.

CONCLUSIONS

This study showed that 10-Hz tACS can improve tactile orientation discrimination in subjects with low alpha activity. Alpha-frequency tACS may help identify the contributions of these oscillations to other neurophysiological and pathological processes.

Detection of Simulated Tactile Gratings by Electro-Static Friction Show a Dependency on Bar Width for Blind and Sighted Observers, and Preliminary Neural Correlates in Sighted Observers

The three-dimensional micro-structure of physical surfaces produces frictional forces that provide sensory cues about properties of felt surfaces such as roughness. This tactile information activates somatosensory cortices, and frontal and temporal brain regions. Recent advances in haptic-feedback technologies allow the simulation of surface micro-structures via electro-static friction to produce touch sensations on otherwise flat screens. These sensations may benefit those with visual impairment or blindness. The primary aim of the current study was to test blind and sighted participants' perceptual sensitivity to simulated tactile gratings. A secondary aim was to explore which brain regions were involved in simulated touch to further understand the somatosensory brain network for touch. We used a haptic-feedback touchscreen which simulated tactile gratings using digitally manipulated electro-static friction. In Experiment 1, we compared blind and sighted participants' ability to detect the gratings by touch alone as a function of their spatial frequency (bar width) and intensity. Both blind and sighted participants showed high sensitivity to detect simulated tactile gratings, and their tactile sensitivity functions showed both linear and quadratic dependency on spatial frequency. In Experiment 2, using functional magnetic resonance imaging, we conducted a preliminary investigation to explore whether brain activation to physical vibrations correlated with blindfolded (but sighted) participants' performance with simulated tactile gratings outside the scanner. At the neural level, blindfolded (but sighted) participants' detection performance correlated with brain activation in bi-lateral supplementary motor cortex, left frontal cortex and right occipital cortex. Taken together with previous studies, these results suggest that there are similar perceptual and neural mechanisms for real and simulated touch sensations.

GABA Concentration in the Left Ventral Premotor Cortex Associates With Sensory Hyper-Responsiveness in Autism Spectrum Disorders Without Intellectual Disability

Individuals with autism spectrum disorder (ASD) often exhibit abnormal processing of sensory inputs from multiple modalities and higher-order cognitive/behavioral response to those inputs. Several lines of evidence suggest that altered γ-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, is a central characteristic of the neurophysiology of ASD. The relationship between GABA in particular brain regions and atypical sensory processing in ASD is poorly understood. We therefore employed ¹H magnetic resonance spectroscopy (¹H-MRS) to examine whether GABA levels in brain regions critical to higher-order motor and/or multiple sensory functions were associated with abnormal sensory responses in ASD. We evaluated atypical sensory processing with a clinically-validated assessment tool. Furthermore, we measured GABA levels in four regions: one each in the primary visual cortex, the left sensorimotor cortex, the left supplementary motor area (SMA), and the left ventral premotor cortex (vPMC). The latter two regions are thought to be involved in executing and coordinating cognitive and behavioral functions in response to multisensory inputs. We found severer sensory hyper-responsiveness in ASD relative to control participants. We also found reduced GABA concentrations in the left SMA but no differences in other regions of interest between ASD and control participants. A correlation analysis revealed a negative association between left vPMC GABA and the severity of sensory hyper-responsiveness across all participants, and the independent ASD group. These findings suggest that reduced inhibitory neurotransmission (reduced GABA) in a higher-order motor area, which modulates motor commands and integrates multiple sensory modalities, may underlie sensory hyper-responsiveness in ASD.

Cortical Representation of Tactile Stickiness Evoked by Skin Contact and Glove Contact

Even when we are wearing gloves, we can easily detect whether a surface that we are touching is sticky or not. However, we know little about the similarities between brain activations elicited by this glove contact and by direct contact with our bare skin. In this functional magnetic resonance imaging (fMRI) study, we investigated which brain regions represent stickiness intensity information obtained in both touch conditions, i.e., skin contact and glove contact. First, we searched for neural representations mediating stickiness for each touch condition separately and found regions responding to both mainly in the supramarginal gyrus and the secondary somatosensory cortex. Second, we explored whether surface stickiness is encoded in common neural patterns irrespective of how participants touched the sticky stimuli. Using a cross-condition decoding method, we tested whether the stickiness intensities could be decoded from fMRI signals evoked by skin contact using a classifier trained on the responses elicited by glove contact, and vice versa. Our results found shared neural encoding patterns in the bilateral angular gyri and the inferior frontal gyrus (IFG) and suggest that these areas represent stickiness intensity information regardless of how participants touched the sticky stimuli. Interestingly, we observed that neural encoding patterns of these areas were reflected in participants’ intensity ratings. This study revealed common and distinct brain activation patterns of tactile stickiness using two different touch conditions, which may broaden the understanding of neural mechanisms related to surface texture perception.

Comparison of transcranial electrical stimulation regimens for effects on inhibitory circuit activity in primary somatosensory cortex and tactile spatial discrimination performance

Transcranial electrical stimulation (tES) can be used to modulate inhibitory circuits in primary somatosensory cortex, resulting in improved somatosensory function. However, efficacy may depend on the specific stimulus modality and patterns. For instance, transcranial alternating current stimulation (tACS), transcranial random noise stimulation (tRNS), and transcranial pulsed current stimulation (tPCS) were found to stably and effectively modulate neuronal excitability, while anodal transcranial direct current stimulation (tDCS) appeared less effective overall but with substantial response heterogeneity among subjects. Therefore, we compared the effects of tES applied to primary somatosensory cortex on somatosensory evoked potential paired-pulse depression (SEP-PPD) and tactile discrimination performance in 17 neurologically healthy subjects. In Experiment 1, somatosensory evoked potential N20/P25_SEP-PPD, N20_SEP-PPD, and P25_SEP-PPD responses were assessed before and immediately after anodal tDCS, tACS (stimulation frequency, 140 Hz), tRNS (stimulation frequency, 0.1-640 Hz), anodal tPCS (pulse width, 50 ms; inter-pulse interval, 5 ms), and sham stimulation applied to primary somatosensory cortex. In Experiment 2, a grating orientation task (GOT) was performed before and immediately after the same anodal tDCS, tRNS, anodal tPCS, and sham stimulation regimens. Anodal tDCS and anodal tPCS decreased N20_SEP-PPD, and tRNS increased the first N20 SEP amplitude. Furthermore, tRNS and anodal tPCS decreased GOT discrimination threshold (improved performance). These results suggest that tRNS and anodal tPCS can improve sensory perception by modulating neuronal activity in primary somatosensory cortex.

Behavioral and electrophysiological correlates of interactions between grouping principles in touch: Evidence from psychophysical indirect tasks

In two experiments we investigated the behavioral and brain correlates of the interactions between spatial-proximity and texture-similarity grouping principles in touch. We designed two adaptations of the repetition discrimination task (RDT) previously used in vision. This task provides an indirect measure of grouping that does not require explicit attention to the grouping process. In Experiment 1, participants were presented with a row of elements alternating in texture except for one pair in which the same texture was repeated. The participants had to decide whether the repeated texture stimuli (similarity grouping) were smooth or rough, while the spatial proximity between targets and distractors was varied either to facilitate or hinder the response. In Experiment 2, participants indicated which cohort (proximity grouping) contained more elements, while texture-similarity within and between cohorts was modified. The results indicated additive effects of grouping cues in which proximity dominated the perceptual grouping process when the two principles acted together. In addition, the independent component analysis (ICA) performed on electrophysiological data revealed the implication of a widespread network of sensorimotor, prefrontal, parietal and occipital brain areas in both experiments.

Impact of pressure as a tactile stimulus on working memory in healthy participants

Studies on cross-modal interaction have demonstrated attenuated as well as facilitated effects for both neural responses as well as behavioral performance. The goals of this pilot study were to investigate possible cross-modal interactions of tactile stimulation on visual working memory and to identify possible neuronal correlates by using functional magnetic resonance imaging (fMRI). During fMRI, participants (n = 12 females, n = 12 males) performed a verbal n-back task (0-back and 2-back tasks) while tactile pressure to the left thumbnail was delivered. Participants presented significantly lower behavioral performances (increased error rates, and reaction times) during the 2-back task as compared to the 0-back task. Task performance was independent of pressure in both tasks. This means that working memory performance was not impacted by a low salient tactile stimulus. Also in the fMRI data, no significant interactions of n-back x pressure were observed. In conclusion, the current study found no influence of tactile pressure on task-related brain activity during n-back (0-back and 2-back) tasks.

Interdependences between finger movement direction and haptic perception of oriented textures

Although the natural haptic perception of textures includes active finger movements, it is unclear how closely perception and movements are linked. Here we investigated this question using oriented textures. Textures that are composed of periodically repeating grooves have a clear orientation defined by the grooves. The direction of finger movement relative to texture orientation determines the availability of temporal cues to the spatial period of the texture. These cues are absent during movements directed in line with texture orientation, whereas movements orthogonal to texture orientation maximize the temporal frequency of stimulation. This may optimize temporal cues. In Experiment 1 we tested whether texture perception gets more precise the more orthogonal the movement direction is to the texture. We systematically varied the movement direction within a 2IFC spatial period discrimination task. As expected, perception was more precise (lower discrimination thresholds) when finger movements were directed closer towards the texture orthogonal as compared to in parallel to the texture. In Experiment 2 we investigated whether people adjust movement directions to the texture orthogonal in free exploration. We recorded movement directions during free exploration of standard and comparison gratings. The standard gratings were clearly oriented. The comparison gratings did not have a clear orientation defined by grooves. Participants adjusted movement directions to the texture orthogonal only for clearly oriented textures (standards). The adjustment to texture orthogonal was present in the final movement but not in the first movement. This suggests that movement adjustment is based on sensory signals for texture orientation that were gathered over the course of exploration. In Experiment 3 we assessed whether the perception of texture orientation and movement adjustments are based on shared sensory signals. We determined perceptual thresholds for orientation discrimination and computed 'movometric' thresholds from the stroke-by-stroke adjustment of movement direction. Perception and movements were influenced by a common factor, the spatial period, suggesting that the same sensory signals for texture orientation contribute to both. We conclude that people optimize texture perception by adjusting their movements in directions that maximize temporal cue frequency. Adjustments are performed on the basis of sensory signals that are also used for perception.

Touch engages visual spatial contextual processing

The spatial context in which we view a visual stimulus strongly determines how we perceive the stimulus. In the visual tilt illusion, the perceived orientation of a visual grating is affected by the orientation signals in its surrounding context. Conceivably, the spatial context in which a visual grating is perceived can be defined by interactive multisensory information rather than visual signals alone. Here, we tested the hypothesis that tactile signals engage the neural mechanisms supporting visual contextual modulation. Because tactile signals also convey orientation information and touch can selectively interact with visual orientation perception, we predicted that tactile signals would modulate the visual tilt illusion. We applied a bias-free method to measure the tilt illusion while testing visual-only, tactile-only or visuo-tactile contextual surrounds. We found that a tactile context can influence visual tilt perception. Moreover, combining visual and tactile orientation information in the surround results in a larger tilt illusion relative to the illusion achieved with the visual-only surround. These results demonstrate that the visual tilt illusion is subject to multisensory influences and imply that non-visual signals access the neural circuits whose computations underlie the contextual modulation of vision.

Alpha and beta band correlates of haptic perceptual grouping: Results from an orientation detection task

Behavioral and neurophysiological findings in vision suggest that perceptual grouping is not a unitary process and that different grouping principles have different processing requirements and neural correlates. The present study aims to examine whether the same occurs in the haptic modality using two grouping principles widely studied in vision, spatial proximity and texture similarity. We analyzed behavioral responses (accuracy and response times) and conducted an independent component analysis of brain oscillations in alpha and beta bands for haptic stimuli grouped by spatial proximity and texture similarity, using a speeded orientation detection task performed on a novel haptic device (MonHap). Behavioral results showed faster response times for patterns grouped by spatial proximity relative to texture similarity. Independent component clustering analysis revealed the activation of a bilateral network of sensorimotor and parietal areas while performing the task. We conclude that, as occurs in visual perception, grouping the elements of the haptic scene by means of their spatial proximity is faster than forming the same objects by means of texture similarity. In addition, haptic grouping seems to involve the activation of a network of widely distributed bilateral sensorimotor and parietal areas as reflected by the consistent event-related desynchronization found in alpha and beta bands.

Correlation of neural activity with behavioral kinematics reveals distinct sensory encoding and evidence accumulation processes during active tactile sensing

Many real-world decisions rely on active sensing, a dynamic process for directing our sensors (e.g. eyes or fingers) across a stimulus to maximize information gain. Though ecologically pervasive, limited work has focused on identifying neural correlates of the active sensing process. In tactile perception, we often make decisions about an object/surface by actively exploring its shape/texture. Here we investigate the neural correlates of active tactile decision-making by simultaneously measuring electroencephalography (EEG) and finger kinematics while subjects interrogated a haptic surface to make perceptual judgments. Since sensorimotor behavior underlies decision formation in active sensing tasks, we hypothesized that the neural correlates of decision-related processes would be detectable by relating active sensing to neural activity. Novel brain-behavior correlation analysis revealed that three distinct EEG components, localizing to right-lateralized occipital cortex (LOC), middle frontal gyrus (MFG), and supplementary motor area (SMA), respectively, were coupled with active sensing as their activity significantly correlated with finger kinematics. To probe the functional role of these components, we fit their single-trial-couplings to decision-making performance using a hierarchical-drift-diffusion-model (HDDM), revealing that the LOC modulated the encoding of the tactile stimulus whereas the MFG predicted the rate of information integration towards a choice. Interestingly, the MFG disappeared from components uncovered from control subjects performing active sensing but not required to make perceptual decisions. By uncovering the neural correlates of distinct stimulus encoding and evidence accumulation processes, this study delineated, for the first time, the functional role of cortical areas in active tactile decision-making.

Impact of transcranial direct current stimulation on structural plasticity of the somatosensory system

While there is a growing body of evidence regarding the behavioral and neurofunctional changes in response to the longitudinal delivery of transcranial direct current stimulation (tDCS), there is limited evidence regarding its structural effects. Therefore, the present study was intended to investigate the effect of repeatedly applied anodal tDCS over the primary somatosensory cortex on the gray matter (GM) and white matter (WM) compartment of the brain. Structural tDCS effects were, moreover, related to effects evidenced by functional imaging and behavioral assessment. tDCS was applied over the course of 5 days in 25 subjects with concomitant assessment of tactile acuity of the right and left index finger as well as imaging at baseline, after the last delivery of tDCS and at follow-up 4 weeks thereafter. Irrespective of the stimulation condition (anodal vs. sham), voxel-based morphometry revealed a behaviorally relevant decrease of GM in the precuneus co-localized with a functional change of its activity. Moreover, there was a decrease in GM of the bilateral lingual gyrus and the right cerebellum. Diffusion tensor imaging analysis showed an increase of fractional anisotropy exclusively in the tDCS_anodal condition in the left frontal cortex affecting the final stretch of a somatosensory decision making network comprising the middle and superior frontal gyrus as well as regions adjacent to the genu of the corpus callosum. Thus, this is the first study in humans to identify structural plasticity in the GM compartment and tDCS-specific changes in the WM compartment in response to somatosensory learning.

Transcranial direct current stimulation enhances foot sole somatosensation when standing in older adults

Foot-sole somatosensation is critical for safe mobility in older adults. Somatosensation arises when afferent input activates a neural network that includes the primary somatosensory cortex. Transcranial direct current stimulation (tDCS), as a strategy to increase somatosensory cortical excitability, may, therefore, enhance foot-sole somatosensation. We hypothesized that a single session of tDCS would improve foot-sole somatosensation, and thus mobility, in older adults. Twenty healthy older adults completed this randomized, double-blinded, cross-over study consisting of two visits separated by one week. On each visit, standing vibratory threshold (SVT) of each foot and the timed-up-and-go test (TUG) of mobility were assessed immediately before and after a 20-min session of tDCS (2.0 mA) or sham stimulation with the anode placed over C3 (according to the 10/20 EEG placement system) and the cathode over the contralateral supraorbital margin. tDCS condition order was randomized. SVT was measured with a shoe insole system. This system automatically ramped up, or down, the amplitude of applied vibrations and the participant stated when they could or could no longer feel the vibration, such that lower SVT reflected better somatosensation. The SVTs of both foot soles were lower following tDCS as compared to sham and both pre-test conditions [F_(1,76) > 3.4, p < 0.03]. A trend towards better TUG performance following tDCS was also observed [F_(1,76) = 2.4, p = 0.07]. Greater improvement in SVT (averaged across feet) moderately correlated with greater improvement in TUG performance (r = 0.48, p = 0.03). These results suggest that tDCS may enhance lower-extremity somatosensory function, and potentially mobility, in healthy older adults.

Interoceptive dimensions across cardiac and respiratory axes

Interoception refers to the sensing of signals concerning the internal state of the body. Individual differences in interoceptive sensitivity are proposed to account for differences in affective processing, including the expression of anxiety. The majority of investigations of interoceptive accuracy focus on cardiac signals, typically using heartbeat detection tests and self-report measures. Consequently, little is known about how different organ-specific axes of interoception relate to each other or to symptoms of anxiety. Here, we compare interoception for cardiac and respiratory signals. We demonstrate a dissociation between cardiac and respiratory measures of interoceptive accuracy (i.e. task performance), yet a positive relationship between cardiac and respiratory measures of interoceptive awareness (i.e. metacognitive insight into own interoceptive ability). Neither interoceptive accuracy nor metacognitive awareness for cardiac and respiratory measures was related to touch acuity, an exteroceptive sense. Specific measures of interoception were found to be predictive of anxiety symptoms. Poor respiratory accuracy was associated with heightened anxiety score, while good metacognitive awareness for cardiac interoception was associated with reduced anxiety. These findings highlight that detection accuracies across different sensory modalities are dissociable and future work can better delineate their relationship to affective and cognitive constructs.This article is part of the themed issue 'Interoception beyond homeostasis: affect, cognition and mental health'.

Analysis of haptic information in the cerebral cortex

Haptic sensing of objects acquires information about a number of properties. This review summarizes current understanding about how these properties are processed in the cerebral cortex of macaques and humans. Nonnoxious somatosensory inputs, after initial processing in primary somatosensory cortex, are partially segregated into different pathways. A ventrally directed pathway carries information about surface texture into parietal opercular cortex and thence to medial occipital cortex. A dorsally directed pathway transmits information regarding the location of features on objects to the intraparietal sulcus and frontal eye fields. Shape processing occurs mainly in the intraparietal sulcus and lateral occipital complex, while orientation processing is distributed across primary somatosensory cortex, the parietal operculum, the anterior intraparietal sulcus, and a parieto-occipital region. For each of these properties, the respective areas outside primary somatosensory cortex also process corresponding visual information and are thus multisensory. Consistent with the distributed neural processing of haptic object properties, tactile spatial acuity depends on interaction between bottom-up tactile inputs and top-down attentional signals in a distributed neural network. Future work should clarify the roles of the various brain regions and how they interact at the network level.

Non-optic vision: Beyond synesthesia?

Patient NS is a 28year-old female who went blind in her early twenties as a result of S-cone syndrome, a degenerative retinal disorder. A few years after losing her vision, she started experiencing visual perceptions of her hands as she moved them and objects that came into contact with her hands. Over the course of a year, these cross-modal sensations evolved to become veridical visual experiences accurately representative of her hands, objects she touched, and to some degree, objects she could infer from her immediate surroundings. We argue that these experiences are distinct from mental imagery as they occurred automatically, remained consistent over time, and were proprioceptively mediated by her head position much like normal optical vision. Moreover, she could neither consciously force these visual experiences to occur without sensory inference nor prevent them from happening when haptically exploring an object. Her previous visual experiences contributed to a strong influence of top-down processing in her perceptions. Though individuals have previously been able to develop limited veridical acquired synesthesia following extensive practice over many years with the use of a special sensory device, none reported experiencing the richness of complexity or degree of top-down processing exhibited by NS. Thus, we posit that NS's case may represent a phenomenon beyond synesthesia altogether.

Plasticity During Short-Term Visual Deprivation

Abstract. During everyday experiences, people sometimes close their eyes to better understand spoken words, to listen to music, or when touching textures and objects. A plausible explanation for this observation is that a reversible loss of vision changes the perceptual function of the remaining non-deprived sensory modalities. Within this work, we discuss general aspects of the effects of visual deprivation on the perceptual performance of the non-deprived sensory modalities with a focus on the time dependency of these modifications. In light of ambiguous findings concerning the effects of short-term visual deprivation and because recent literature provides evidence that the act of blindfolding can change the function of the non-deprived senses within seconds, we performed additional psychophysiological and functional magnetic resonance imaging (fMRI) analysis to provide new insight into this matter. Eye closure for several seconds led to a substantial impact on tactile perception probably caused by an unmasking of preformed neuronal pathways.

Transcranial Direct Current Stimulation Over the Primary and Secondary Somatosensory Cortices Transiently Improves Tactile Spatial Discrimination in Stroke Patients

In healthy subjects, dual hemisphere transcranial direct current stimulation (tDCS) over the primary (S1) and secondary somatosensory cortices (S2) has been found to transiently enhance tactile performance. However, the effect of dual hemisphere tDCS on tactile performance in stroke patients with sensory deficits remains unknown. The purpose of this study was to investigate whether dual hemisphere tDCS over S1 and S2 could enhance tactile discrimination in stroke patients. We employed a double-blind, crossover, sham-controlled experimental design. Eight chronic stroke patients with sensory deficits participated in this study. We used a grating orientation task (GOT) to measure the tactile discriminative threshold of the affected and non-affected index fingers before, during, and 10 min after four tDCS conditions. For both the S1 and S2 conditions, we placed an anodal electrode over the lesioned hemisphere and a cathodal electrode over the opposite hemisphere. We applied tDCS at an intensity of 2 mA for 15 min in both S1 and S2 conditions. We included two sham conditions in which the positions of the electrodes and the current intensity were identical to that in the S1 and S2 conditions except that current was delivered for the initial 15 s only. We found that GOT thresholds for the affected index finger during and 10 min after the S1 and S2 conditions were significantly lower compared with each sham condition. GOT thresholds were not significantly different between the S1 and S2 conditions at any time point. We concluded that dual-hemisphere tDCS over S1 and S2 can transiently enhance tactile discriminative task performance in chronic stroke patients with sensory dysfunction.

Feeling form: the neural basis of haptic shape perception

The tactile perception of the shape of objects critically guides our ability to interact with them. In this review, we describe how shape information is processed as it ascends the somatosensory neuraxis of primates. At the somatosensory periphery, spatial form is represented in the spatial patterns of activation evoked across populations of mechanoreceptive afferents. In the cerebral cortex, neurons respond selectively to particular spatial features, like orientation and curvature. While feature selectivity of neurons in the earlier processing stages can be understood in terms of linear receptive field models, higher order somatosensory neurons exhibit nonlinear response properties that result in tuning for more complex geometrical features. In fact, tactile shape processing bears remarkable analogies to its visual counterpart and the two may rely on shared neural circuitry. Furthermore, one of the unique aspects of primate somatosensation is that it contains a deformable sensory sheet. Because the relative positions of cutaneous mechanoreceptors depend on the conformation of the hand, the haptic perception of three-dimensional objects requires the integration of cutaneous and proprioceptive signals, an integration that is observed throughout somatosensory cortex.

Behavioural and neurofunctional impact of transcranial direct current stimulation on somatosensory learning

We investigated the effect of repeated delivery of anodal transcranial direct current stimulation (tDCS) on somatosensory performance and long-term learning. Over the course of five days, tDCS was applied to the primary somatosensory cortex (S1) by means of neuronavigation employing magnetencephalography (MEG). Compared to its sham application, tDCS promoted tactile learning by reducing the two-point discrimination threshold assessed by the grating orientation task (GOT) primarily by affecting intersessional changes in performance. These results were accompanied by alterations in the neurofunctional organization of the brain, as revealed by functional magnetic resonance imaging conducted prior to the study, at the fifth day of tDCS delivery and four weeks after the last application of tDCS. A decrease in activation at the primary site of anodal tDCS delivery in the left S1 along retention of superior tactile acuity was observed at follow-up four weeks after the application of tDCS. Thus, we demonstrate long-term effects that repeated tDCS imposes on somatosensory functioning. This is the first study to provide insight into the mode of operation of tDCS on the brain's response to long-term perceptual learning, adding an important piece of evidence from the domain of non-invasive brain stimulation to show that functional changes detectable by fMRI in primary sensory cortices participate in perceptual learning.

Eye closure enhances dark night perceptions

We often close our eyes when we explore objects with our fingers to reduce the dominance of the visual system over our other senses. Here we show that eye closure, even in complete darkness, results in improved somatosensory perception due to a switch from visual predominance towards a somatosensory processing mode. Using a tactile discrimination task and functional neuroimaging (fMRI) data were acquired from healthy subjects with their eyes opened and closed in two environments: under ambient light and in complete darkness. Under both conditions the perception threshold decreased when subjects closed their eyes, and their fingers became more sensitive. In complete darkness, eye closure significantly increased occipital blood-oxygen-level-dependent (BOLD) activity in the somatosensory and secondary visual processing areas. This change in brain activity was associated with enhanced coupling between the sensory thalamus and somatosensory cortex; connectivity between the visual and somatosensory areas decreased. The present study demonstrates that eye closure improves somatosensory perception not merely due to the lack of visual signals; instead, the act of closing the eyes itself alters the processing mode in the brain: with eye closure the brain switches from thalamo-cortical networks with visual dominance to a non-visually dominated processing mode.

Altered baseline brain activity in experts measured by amplitude of low frequency fluctuations (ALFF): a resting state fMRI study using expertise model of acupuncturists

It is well established that expertise modulates evoked brain activity in response to specific stimuli. Recently, researchers have begun to investigate how expertise influences the resting brain. Among these studies, most focused on the connectivity features within/across regions, i.e., connectivity patterns/strength. However, little concern has been given to a more fundamental issue whether or not expertise modulates baseline brain activity. We investigated this question using amplitude of low-frequency (<0.08 Hz) fluctuation (ALFF) as the metric of brain activity and a novel expertise model, i.e., acupuncturists, due to their robust proficiency in tactile perception and emotion regulation. After the psychophysical and behavioral expertise screening procedure, 23 acupuncturists and 23 matched non-acupuncturists (NA) were enrolled. Our results explicated higher ALFF for acupuncturists in the left ventral medial prefrontal cortex (VMPFC) and the contralateral hand representation of the primary somatosensory area (SI) (corrected for multiple comparisons). Additionally, ALFF of VMPFC was negatively correlated with the outcomes of the emotion regulation task (corrected for multiple comparisons). We suggest that our study may reveal a novel connection between the neuroplasticity mechanism and resting state activity, which would upgrade our understanding of the central mechanism of learning. Furthermore, by showing that expertise can affect the baseline brain activity as indicated by ALFF, our findings may have profound implication for functional neuroimaging studies especially those involving expert models, in that difference in baseline brain activity may either smear the spatial pattern of activations for task data or introduce biased results into connectivity-based analysis for resting data.

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.

Resting BOLD fluctuations in the primary somatosensory cortex correlate with tactile acuity

Sensory perception, including 2-point discrimination (2 ptD), is tightly linked to cortical processing of tactile stimuli in primary somatosensory cortices. While the role of cortical activity in response to a tactile stimulus has been widely investigated, the role of baseline cortical activity is largely unknown. Using resting state fMRI we investigated the relationship between local BOLD fluctuations in the primary somatosensory cortex (the representational field of the hand) and 2 ptD of the corresponding index finger (right and left). Cortical activity was measured using fractional amplitudes of the low frequency BOLD fluctuations (fALFF) and synchronicity using regional homogeneity (ReHo) of the S1 hand region during rest. 2 ptD correlated with higher ReHo values in the representational areas of the contralateral S1 cortex (left hand: p = .028; right hand: p = .049). 2 ptD additionally correlated with higher fALFF in the representational area of the left hand (p = .007) and showed a trend for a significant correlation in the representational area of the right hand (p = .051). Thus, higher BOLD amplitudes and synchronicity at rest, as measures of cortical activity and synchronicity, respectively, are related to better tactile discrimination abilities of the contralateral hand. Our findings extend the relationship seen between spontaneous BOLD fluctuations and sensory perception.

You'd better think twice: post-decision perceptual confidence

Current findings suggest that confidence emerges only after decision making. However, the temporal and neural dynamics of the emergence of post-decision confidence--a metacognitive judgement--are not fully explored. To gain insight into the dynamics of post-decision confidence processing and to disentangle the processes underlying confidence judgements and decision making, we applied a tactile discrimination task during functional magnetic resonance imaging (fMRI). Our results revealed that reaction times to post-decision confidence depend on the level of confidence, suggesting that post-decision confidence in a perceptual choice is not processed in parallel to perceptual decision making. Moreover, we demonstrated by the parametric analysis of fMRI data that post-decisionally modelled confidence processing can be distinguished from processes related to decision making through anatomical location and through the pattern of neural activity. In contrast to perceptual decision making, post-decision confidence appears to be strictly allocated to a prefrontal network of brain regions, primarily the anterior and dorsolateral prefrontal cortex, areas that have been related to metacognition. Moreover, the processes underlying decision making and post-decision confidence may share recruitment of the dorsolateral prefrontal cortex, although the former probably has distinct functions with regard to processing of perceptual choices and post-decision confidence. Thus, this is the first fMRI study to disentangle the processes underlying post-decision confidence and decision making on behavioural, neuroanatomical, and neurofunctional levels. With regard to the temporal evolution of post-decision confidence, results of the present study provide strong support for the most recent theoretical models of human perceptual decision making, and thus provide implications for investigating confidence in perceptual paradigms.

Characteristics of bilateral hand function in individuals with unilateral dystonia due to perinatal stroke: sensory and motor aspects

The authors assessed bilateral motor and sensory function in individuals with upper limb dystonia due to unilateral perinatal stroke and explored interrelationships of motor function and sensory ability. Reach kinematics and tactile sensation were measured in 7 participants with dystonia and 9 healthy volunteers. The dystonia group had poorer motor (hold time, reach time, shoulder/elbow correlation) and sensory (spatial discrimination, stereognosis) outcomes than the control group on the nondominant side. On the dominant side, only sensation (spatial discrimination, stereognosis) was poorer in the dystonia group compared with the control group. In the dystonia group, although sensory and motor outcomes were uncorrelated, dystonia severity was related to poorer stereognosis, longer hold and reach times, and decreased shoulder/elbow coordination. Findings of bilateral sensory deficits in dystonia can be explained by neural reorganization. Visual compensation for somatosensory changes in the nonstroke hemisphere may explain the lack of bilateral impairments in reaching.

Altered activation in visual cortex: Unusual functional magnetic resonance imaging finding in early Parkinson’s disease

Objective

Movement dysfunction in Parkinson’s disease (PD) is well documented but its cause remains unclear. This study examined whether motor symptoms are caused by sensory impairment, executive dysfunction or other factors.

Methods

We examined the integration of tactile and motor programmes using functional magnetic resonance imaging (fMRI) in early stage PD patients compared with neurologically normal controls, to investigate neural mechanisms underlying movement dysfunction.

Results

Twenty-two control participants exhibited activation in an extensive network of cortical regions involved in tactile–motor integration. Decreased activation in somatosensory and motor regions during performance of passive tactile and movement tasks was found in 21 PD patients. The extrastriate visual cortex exhibited greater activation in controls during performance of the tasks. In contrast, greater activation in bilateral prefrontal regions was found in PD patients.

Conclusions

These results indicate that the extrastriate visual cortex is a multisensory region playing an important role in sensorimotor integration. The findings suggest that PD patients exhibit decreased activation in the extrastriate visual cortex, possibly related to dysfunctional integrative processing, and compensatory activity in the prefrontal cortex.

A physical constraint on perceptual learning: tactile spatial acuity improves with training to a limit set by finger size

In touch as in vision, perceptual acuity improves with training to an extent that differs greatly across people; even individuals with similar initial acuity may undergo markedly different improvement with training. What accounts for this variability in perceptual learning? We hypothesized that a simple physical characteristic, fingertip surface area, might constrain tactile learning, because previous research suggests that larger fingers have more widely spaced mechanoreceptors. To test our hypothesis, we trained 10 human participants intensively on a tactile spatial acuity task. During 4 d, participants completed 1900 training trials (38 50-trial blocks) in which they discriminated the orientation of square-wave gratings pressed onto the stationary index or ring finger, with auditory feedback provided to signal correct and incorrect responses. We progressively increased task difficulty by shifting to thinner groove widths whenever participants achieved ≥90% correct block performance. We took optical scans to measure surface area from the distal interphalangeal crease to the tip of the finger. Participants' acuity improved markedly on the trained finger and to a lesser extent on the untrained finger. Crucially, we found that participants' tactile spatial acuity improved toward a theoretical optimum set by their finger size; participants with worse initial performance relative to their finger size improved more with training, and posttraining performance was better correlated than pretraining performance with finger size. These results strongly support the hypothesis that tactile perceptual learning is limited by finger size. We suspect that analogous physical constraints on perceptual learning will be found in other sensory modalities.

Length of Acupuncture Training and Structural Plastic Brain Changes in Professional Acupuncturists

Background

The research on brain plasticity has fascinated researchers for decades. Use/training serves as an instrumental factor to influence brain neuroplasticity. Parallel to acquisition of behavioral expertise, extensive use/training is concomitant with substantial changes of cortical structure. Acupuncturists, serving as a model par excellence to study tactile-motor and emotional regulation plasticity, receive intensive training in national medical schools following standardized training protocol. Moreover, their behavioral expertise is corroborated during long-term clinical practice. Although our previous study reported functional plastic brain changes in the acupuncturists, whether or not structural plastic changes occurred in acupuncturists is yet elusive.

Methodology/Principal Findings

Cohorts of acupuncturists (N = 22) and non-acupuncturists (N = 22) were recruited. Behavioral tests were delivered to assess the acupuncturists’ behavioral expertise. The results confirmed acupuncturists’ tactile-motor skills and emotion regulation proficiency compared to non-acupuncturists. Using the voxel-based morphometry technique, we revealed larger grey matter volumes in acupuncturists in the hand representation of the contralateral primary somatosensory cortex (SI), the right lobule V/VI and the bilateral ventral anterior cingulate cortex/ventral medial prefrontal cortex. Grey matter volumes of the SI and Lobule V/VI positively correlated with the duration of acupuncture practice.

Conclusions

To our best knowledge, this study provides first evidence for the anatomical alterations in acupuncturists, which would possibly be the neural correlates underlying acupuncturists’ exceptional skills. On one hand, we suggest our findings may have ramifications for tactile-motor rehabilitation. On the other hand, our results in emotion regulation domain may serve as a target for our future studies, from which we can understand how modulations of aversive emotions elicited by empathic pain develop in the context of expertise. Future longitudinal study is necessary to establish the presence and direction of a causal link between practice/use and brain anatomy.

Expertise modulates local regional homogeneity of spontaneous brain activity in the resting brain: an fMRI study using the model of skilled acupuncturists

Studies on training/expertise-related effects on human brain in context of neuroplasticity have revealed that plastic changes modulate not only task activations but also patterns and strength of internetworks and intranetworks functional connectivity in the resting state. Much has known about plastic changes in resting state on global level; however, how training/expertise-related effect affects patterns of local spontaneous activity in resting brain remains elusive. We investigated the homogeneity of local blood oxygen level-dependent fluctuations in the resting state using a regional homogeneity (ReHo) analysis among 16 acupuncturists and 16 matched nonacupuncturists (NA). To prove acupuncturists' expertise, we used a series of psychophysical tests. Our results demonstrated that, acupuncturists significantly outperformed NA in tactile-motor and emotional regulation domain and the acupuncturist group showed increased coherence in local BOLD signal fluctuations in the left primary motor cortex (MI), the left primary somatosensory cortex (SI) and the left ventral medial prefrontal cortex/orbitofrontal cortex (VMPFC/OFC). Regression analysis displayed that, in the acupuncturists group, ReHo of VMPFC/OFC could predict behavioral outcomes, evidenced by negative correlation between unpleasantness ratings and ReHo of VMPFC/OFC and ReHo of SI and MI positively correlated with the duration of acupuncture practice. We suggest that expertise could modulate patterns of local resting state activity by increasing regional clustering strength, which is likely to contribute to advanced local information processing efficiency. Our study completes the understanding of neuroplasticity changes by adding the evidence of local resting state activity alterations, which is helpful for elucidating in what manner training effect extends beyond resting state.

Crossmodal interactions of haptic and visual texture information in early sensory cortex

Both visual and haptic information add to the perception of surface texture. While prior studies have reported crossmodal interactions of both sensory modalities at the behavioral level, neuroimaging studies primarily investigated texture perception in separate visual and haptic paradigms. These experimental designs, however, only allowed to identify overlap in both sensory processing streams but no interaction of visual and haptic texture processing. By varying texture characteristics in a bimodal task, the current study investigated how these crossmodal interactions are reflected at the cortical level. We used fMRI to compare cortical activation in response to matching versus non-matching visual-haptic texture information. We expected that passive simultaneous presentation of matching visual-haptic input would be sufficient to induce BOLD responses graded with varying texture characteristics. Since no cognitive evaluation of the stimuli was required, we expected to find changes primarily at a rather early processing stage. Our results confirmed our assumptions by showing crossmodal interactions of visual-haptic texture information in early somatosensory and visual cortex. However, the nature of the crossmodal effects was slightly different in both sensory cortices. In early visual cortex, matching visual-haptic information increased the average activation level and induced parametric BOLD signal variations with varying texture characteristics. In early somatosensory cortex only the latter was true. These results challenge the notion that visual and haptic texture information is processed independently and indicate a crossmodal interaction of sensory information already at an early cortical processing stage.

Cortical plasticity and preserved function in early blindness

The "neural Darwinism" theory predicts that when one sensory modality is lacking, as in congenital blindness, the target structures are taken over by the afferent inputs from other senses that will promote and control their functional maturation (Edelman, 1993). This view receives support from both cross-modal plasticity experiments in animal models and functional imaging studies in man, which are presented here.

Neural basis of contagious itch and why some people are more prone to it

Watching someone scratch himself can induce feelings of itchiness in the perceiver. This provides a unique opportunity to characterize the neural basis of subjective experiences of itch, independent of changes in peripheral inputs. In this study, we first established that the social contagion of itch is essentially a normative response (experienced by most people), and that the degree of contagion is related to trait differences in neuroticism (i.e., the tendency to experience negative emotions), but not to empathy. Watching video clips of someone scratching (relative to control videos of tapping) activated, as indicated by functional neuroimaging, many of the neural regions linked to the physical perception of itch, including anterior insular, primary somatosensory, and prefrontal (BA44) and premotor cortices. Moreover, activity in the left BA44, BA6, and primary somatosensory cortex was correlated with subjective ratings of itchiness, and the responsivity of the left BA44 reflected individual differences in neuroticism. Our findings highlight the central neural generation of the subjective experience of somatosensory perception in the absence of somatosensory stimulation. We speculate that the habitual activation of this central "itch matrix" may give rise to psychogenic itch disorders.

Transcranial direct current stimulation ameliorates tactile sensory deficit in multiple sclerosis

BACKGROUND

Deficit of tactile sensation in patients with MS is frequent and can be associated with interference with daily life activities. Transcranial direct current stimulation (tDCS) showed to increase tactile discrimination in healthy subjects.

OBJECTIVE

In the present study, we investigated whether tDCS may be effective in ameliorating tactile sensory deficit in MS patients.

METHODS

Patients received sham or real anodal tDCS of the somatosensory cortex for 5 consecutive days in a randomized, double blind, sham-controlled study. Discrimination thresholds of spatial tactile sensation were measured using the grating orientation task (GOT). As secondary outcomes we also measured subjective perception of tactile sensory deficit through a visual analog scale (VAS), quality of life and overall disability to evaluate the impact of the treatment on patients daily life. Evaluations were performed at baseline and during a 4-week follow-up period.

RESULTS

Following anodal but not sham tDCS over the somatosensory cortex, there was a significant improvement of discriminatory thresholds at the GOT and increased VAS for sensation scores. Quality of life, and disability changes were not observed.

CONCLUSION

Our results indicate that a five day course of anodal tDCS is able to ameliorate tactile sensory loss with long-lasting beneficial effects and could thus represent a therapeutic tool for the treatment of tactile sensory deficit in MS patients.

Improved acuity and dexterity but unchanged touch and pain thresholds following repetitive sensory stimulation of the fingers

Neuroplasticity underlies the brain's ability to alter perception and behavior through training, practice, or simply exposure to sensory stimulation. Improvement of tactile discrimination has been repeatedly demonstrated after repetitive sensory stimulation (rSS) of the fingers; however, it remains unknown if such protocols also affect hand dexterity or pain thresholds. We therefore stimulated the thumb and index finger of young adults to investigate, besides testing tactile discrimination, the impact of rSS on dexterity, pain, and touch thresholds. We observed an improvement in the pegboard task where subjects used the thumb and index finger only. Accordingly, stimulating 2 fingers simultaneously potentiates the efficacy of rSS. In fact, we observed a higher gain of discrimination performance as compared to a single-finger rSS. In contrast, pain and touch thresholds remained unaffected. Our data suggest that selecting particular fingers modulates the efficacy of rSS, thereby affecting processes controlling sensorimotor integration.