Processing Of Tactile Stimuli Research Articles

Brain–computer interface for simultaneous dual‐region spatial coding in hippocampal and somatosensory cortex of freely behaving rats

AbstractDecoding of spatial information from place cells is currently limited to individual brain regions, severely constraining the understanding of the brain's spatial navigation mechanisms. In this study, synchronized detection of dual‐brain region spatial encoding was conducted using brain–computer interface (BCI). Therefore, an implantable microelectrode array (MEA) was developed tailored for the simultaneous detection of neuronal activities in the CA1 of the hippocampus and the Barrel Cortex (BC) of the somatosensory cortex in rats as BCI. The MEA was improved by modifying the nanocomposite PtNP/PEDOT:PSS to improve their electrical properties (reducing impedance from 1.82 ± 0.17 MΩ to 3.4 ± 0.3 kΩ), facilitating neural recordings in freely behaving rats. The neural activities were obtained from the CA1 and the BC simultaneously and validated the existence of place cells in both brain regions. This study highlighted the potential of PtNP/PEDOT:PSS‐modified MEA as BCI in concurrently detecting neural activity associated with spatial encoding in two brain regions, offering novel perspectives on the processing of tactile stimuli and the formation of cognitive maps for navigation.

Tactile Processing and Quality of Sleep in Autism Spectrum Disorders.

Individuals with autism spectrum disorders (ASDs) commonly experience problems with the processing of tactile stimuli and poor quality of sleep. The aim of the present study was to analyze whether tactile stimuli modulation (TSM) disorders might be linked to insomnia prevalence in ASD individuals. We hypothesized that sleep disorders in children with ASD may result from improper tactile under/over responsivity. The study included 27 children diagnosed with ASD, aged 6.8 (±2.9 years) with male dominance (n = 22, 81.5%). To evaluate the pattern of TSM we used a clinical interview with a parent, and guided and spontaneous observation of the patients. Sleep disorders were diagnosed using the Athens Insomnia Scale. Of all the children diagnosed with TSM, 20 patients (74.1%) had an over-responsivity pattern and 7 children (25.9%) had an under-responsivity pattern. Of the patients, 11 children (40.7%) met the diagnostic criteria for insomnia. The data indicated a statistical tendency for higher prevalence of insomnia in individuals diagnosed with tactile under-responsivity (p = 0.051). We concluded that under-responsivity toward tactile stimuli may be partly responsible for poor sleep quality in ASD. There is an urgent need to treat sleep and sensory disruptions which may intensify behavioral difficulties in ASD.

Disinhibition of Human Primary Somatosensory Cortex After Median Nerve Transection and Reinnervation.

Despite state-of-the-art surgical and postoperative treatment, median nerve transection causes lasting impaired hand function due to limitations in the nerve’s reinnervation ability. The defective innervation and thus controllability of the affected hand can shape the brain’s control of manual behaviors. Earlier studies of changes in the processing of tactile stimuli have focused mainly on stimulation of the reinnervated hand and lack sufficient control over the brain’s use of the tactile input in perceptual terms. Here we used fMRI to measure brain activity (BOLD-signal) in 11 people with median nerve injury and healthy controls (N = 11) when performing demanding tactile tasks using the tip of either the index or little finger of either hand. For the nerve-injured group, the left median nerve had been traumatically transected in the distal forearm and surgically repaired on average 8 years before the study. The hand representation of their contralesional (right) primary somatosensory cortex (S1) showed greater activity compared to controls when the left reinnervated index finger was used, but also when the left-hand little finger and the fingers of the right hand innervated by uninjured nerves were used. We argue that the overall increase in activity reflects a general disinhibition of contralesional S1 consistent with an augmented functional reorganizational plasticity being an ongoing feature of chronic recovery from nerve injury. Also, the nerve-injured showed increased activity within three prefrontal cortical areas implicated in higher-level behavioral processing (dorsal anterior cingulate cortex, left ventrolateral prefrontal and right dorsolateral prefrontal cortex), suggesting that processes supporting decision-making and response-selection were computationally more demanding due to the compromised tactile sensibility.

Properties of lower level processing modulate the actions of the norepinephrine system during response inhibition

We ask whether actions of the norepinephrine (NE) system during response inhibition are affected by properties of lower level sensory stimulus processing. We used a somato-sensory Go/Nogo task and combined ERP recordings with pupil diameter recordings as an index of NE system activity. The Go/Nogo task was designed to achieve processing of tactile stimuli predominantly over primary somatosensory (SI) and secondary somatosensory (SII) areas. The data show that response inhibition was better when stimuli were processed via SII, compared to SI areas. This was reflected by variations in the Nogo-N2/P3 associated with anterior cingulate structures. Correlations with the pupil diameter data, indicting modulations of the NE system during inhibitory control processes, were only evident when SI sensory areas were involved. These dissociable modulatory effects were associated with activations in the superior frontal gyrus. Actions of the NE system during response inhibition are modulated by properties of lower level processing.

Higher Tactile Temporal Resolution as a Basis of Hypersensitivity in Individuals with Autism Spectrum Disorder

Many individuals with autism spectrum disorder (ASD) have symptoms of sensory hypersensitivity. Several studies have shown high individual variations in temporal processing of tactile stimuli. We hypothesized that these individual differences are linked to differences in hyper-reactivity among individuals with ASD. Participants performed two tasks as to vibrotactile stimuli: One is a temporal order judgement task, and another is a detection task. We found that individuals with ASD with higher temporal resolution tended to have more severe hypersensitivity symptoms. In contrast, the tactile detection threshold/sensitivity were related to the severities of stereotyped behaviour and restricted interests, rather than to hypersensitivity. Our findings demonstrate that higher temporal resolution to sensory stimuli may contribute to sensory hypersensitivity in individuals with ASD.

Altered Neurocognitive Processing of Tactile Stimuli in Patients with Complex Regional Pain Syndrome

Chronic pain in complex regional pain syndrome (CRPS) has been linked to tactile misperceptions and deficits in somatotopic representation of the affected limb. In this study, we identify altered cognitive processing of tactile stimuli in CRPS patients that we propose marks heterogeneity in tactile decision-making mechanisms. In a case-control design, we compared middle- and late-latency somatosensory evoked potentials in response to pseudorandomized mechanical stimulation of the digits of both hands (including CRPS-affected and nonaffected sides) between 13 CRPS patients and 13 matched healthy controls. During a task to discriminate the digit simulated, patients (compared with controls) had significantly lower accuracy and slowed response times but with high between-subject variability. At middle latencies (124–132 ms), tactile processing in patients relative to controls showed decrements in superior parietal lobe and precuneus (that were independent of task demands) but enhanced activity in superior frontal lobe (that were task-dependent). At late latencies, patients showed an augmented P300-like response under task demands that localized to the supplementary motor area. Source activity in the supplementary motor area correlated with slowed response times, although its scalp representation intriguingly correlated with better functioning of the affected limb, suggesting a compensatory mechanism. Future research should investigate the clinical utility of these putative markers of tactile decision-making mechanisms in CRPS. PerspectiveWe present evidence of altered but highly variable cognitive processing (124–268 ms latency) in response to mechanical tactile stimuli in patients with CRPS compared with healthy controls. Such mid- to late-latency responses could potentially provide convenient and robust biomarkers of abnormal perceptual decision-making mechanisms in CRPS to aid in clinical detection and treatment.

Unconscious integration of multisensory bodily inputs in the peripersonal space shapes bodily self-consciousness

Recent studies have highlighted the role of multisensory integration as a key mechanism of self-consciousness. In particular, integration of bodily signals within the peripersonal space (PPS) underlies the experience of the self in a body we own (self-identification) and that is experienced as occupying a specific location in space (self-location), two main components of bodily self-consciousness (BSC). Experiments investigating the effects of multisensory integration on BSC have typically employed supra-threshold sensory stimuli, neglecting the role of unconscious sensory signals in BSC, as tested in other consciousness research. Here, we used psychophysical techniques to test whether multisensory integration of bodily stimuli underlying BSC also occurs for multisensory inputs presented below the threshold of conscious perception. Our results indicate that visual stimuli rendered invisible through continuous flash suppression boost processing of tactile stimuli on the body (Exp. 1), and enhance the perception of near-threshold tactile stimuli (Exp. 2), only once they entered PPS. We then employed unconscious multisensory stimulation to manipulate BSC. Participants were presented with tactile stimulation on their body and with visual stimuli on a virtual body, seen at a distance, which were either visible or rendered invisible. We found that participants reported higher self-identification with the virtual body in the synchronous visuo-tactile stimulation (as compared to asynchronous stimulation; Exp. 3), and shifted their self-location toward the virtual body (Exp.4), even if stimuli were fully invisible. Our results indicate that multisensory inputs, even outside of awareness, are integrated and affect the phenomenological content of self-consciousness, grounding BSC firmly in the field of psychophysical consciousness studies.

Olfactory modulation of affective touch processing — A neurophysiological investigation

Touch can be highly emotional, and depending on the environment, it can be perceived as pleasant and comforting or disgusting and dangerous. Here, we studied the impact of context on the processing of tactile stimuli using a functional magnetic resonance imaging (fMRI) paradigm. This was achieved by embedding tactile stimulation in a variable olfactory environment.Twenty people were scanned with BOLD fMRI while receiving the following stimulus blocks: Slow stroking Touch, Civette odor (feces like), Rose odor, Touch+Civette, and Touch+Rose. Ratings of pleasantness and intensity of tactile stimuli and ratings of disgust and intensity of olfactory stimuli were collected. The impact of the olfactory context on the processing of touch was studied using covariance analyses. Coupling between olfactory processing and somatosensory processing areas was assessed with psychophysiological interaction analysis (PPI).A subjectively disgusting olfactory environment significantly reduced the perceived pleasantness of touch. The touch fMRI activation in the secondary somatosensory cortex, operculum 1 (OP1), was positively correlated with the disgust towards the odors. Decreased pleasantness of touch was related to decreased posterior insula activity. PPI analysis revealed a significant interaction between the OP1, posterior insula, and regions processing the disgust of odors (orbitofrontal cortex and amygdala).We conclude that the disgust evaluation of the olfactory environment moderates neural reactivity in somatosensory regions by upregulation of the OP1 and downregulation of the posterior insula. This adaptive regulation of affective touch processing may facilitate adaptive reaction to a potentially harmful stimulus.

Modulation of Specific Sensory Cortical Areas by Segregated Basal Forebrain Cholinergic Neurons Demonstrated by Neuronal Tracing and Optogenetic Stimulation in Mice.

Neocortical cholinergic activity plays a fundamental role in sensory processing and cognitive functions. Previous results have suggested a refined anatomical and functional topographical organization of basal forebrain (BF) projections that may control cortical sensory processing in a specific manner. We have used retrograde anatomical procedures to demonstrate the existence of specific neuronal groups in the BF involved in the control of specific sensory cortices. Fluoro-Gold (FlGo) and Fast Blue (FB) fluorescent retrograde tracers were deposited into the primary somatosensory (S1) and primary auditory (A1) cortices in mice. Our results revealed that the BF is a heterogeneous area in which neurons projecting to different cortical areas are segregated into different neuronal groups. Most of the neurons located in the horizontal limb of the diagonal band of Broca (HDB) projected to the S1 cortex, indicating that this area is specialized in the sensory processing of tactile stimuli. However, the nucleus basalis magnocellularis (B) nucleus shows a similar number of cells projecting to the S1 as to the A1 cortices. In addition, we analyzed the cholinergic effects on the S1 and A1 cortical sensory responses by optogenetic stimulation of the BF neurons in urethane-anesthetized transgenic mice. We used transgenic mice expressing the light-activated cation channel, channelrhodopsin-2, tagged with a fluorescent protein (ChR2-YFP) under the control of the choline-acetyl transferase promoter (ChAT). Cortical evoked potentials were induced by whisker deflections or by auditory clicks. According to the anatomical results, optogenetic HDB stimulation induced more extensive facilitation of tactile evoked potentials in S1 than auditory evoked potentials in A1, while optogenetic stimulation of the B nucleus facilitated either tactile or auditory evoked potentials equally. Consequently, our results suggest that cholinergic projections to the cortex are organized into segregated pools of neurons that may modulate specific cortical areas.

Independent effects of eye gaze and spatial attention on the processing of tactile events: Evidence from event-related potentials

Directing one’s gaze at a body part reduces detection speed and enhances the processing of tactile stimuli presented at the gazed location. Given the close links between spatial attention and the oculomotor system it is possible that these gaze- dependent modulations of touch are mediated by attentional mechanisms. To investigate this possibility, gaze direction and sustained tactile attention were orthogonally manipulated in the present study. Participants covertly attended to one hand to perform a tactile target-nontarget discrimination while they gazed at the same or opposite hand. Spatial attention resulted in enhancements of the somatosensory P100 and Nd components. In contrast, gaze resulted in modulations of the N140 component with more positive ERPs for gazed than non gazed stimuli. This dissociation in the pattern and timing of the effects of gaze and attention on somatosensory processing reveals that gaze and attention have independent effects on touch.

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

Sensory perception, including 2-point discrimination (2ptD), 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 2ptD 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. 2ptD correlated with higher ReHo values in the representational areas of the contralateral S1 cortex (left hand: p = .028; right hand: p = .049). 2ptD 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.

Spatio-temporal processing of tactile stimuli in autistic children.

Altered multisensory integration has been reported in autism; however, little is known concerning how the autistic brain processes spatio-temporal information concerning tactile stimuli. We report a study in which a crossed-hands illusion was investigated in autistic children. Neurotypical individuals often experience a subjective reversal of temporal order judgments when their hands are stimulated while crossed, and the illusion is known to be acquired in early childhood. However, under those conditions where the somatotopic representation is given priority over the actual spatial location of the hands, such reversals may not occur. Here, we showed that a significantly smaller illusory reversal was demonstrated in autistic children than in neurotypical children. Furthermore, in an additional experiment, the young boys who had higher Autism Spectrum Quotient (AQ) scores generally showed a smaller crossed hands deficit. These results suggest that rudimentary spatio-temporal processing of tactile stimuli exists in autistic children, and the altered processing may interfere with the development of an external frame of reference in real-life situations.

Vision affects tactile target and distractor processing even when space is task-irrelevant

The human brain is adapted to integrate the information from multiple sensory modalities into coherent, robust representations of the objects and events in the external world. A large body of empirical research has demonstrated the ubiquitous nature of the interactions that take place between vision and touch, with the former typically dominating over the latter. Many studies have investigated the influence of visual stimuli on the processing of tactile stimuli (and vice versa). Other studies, meanwhile, have investigated the effect of directing a participant’s gaze either toward or else away from the body-part receiving the target tactile stimulation. Other studies, by contrast, have compared performance in those conditions in which the participant’s eyes have been open versus closed. We start by reviewing the research that has been published to date demonstrating the influence of vision on the processing of tactile targets, that is, on those stimuli that have to be attended or responded to. We outline that many – but not all – of the visuotactile interactions that have been observed to date may be attributable to the direction of spatial attention. We then move on to focus on the crossmodal influence of vision, as well as of the direction of gaze, on the processing of tactile distractors. We highlight the results of those studies demonstrating the influence of vision, rather than gaze direction (i.e., the direction of overt spatial attention), on tactile distractor processing (e.g., tactile variants of the negative-priming or flanker task). The conclusion is that no matter how vision of a tactile distractor is engaged, the result would appear to be the same, namely that tactile distractors are processed more thoroughly.

Somatosensory Demands Modulate Muscular Beta Oscillations, Independent of Motor Demands

Neural oscillations in the beta band (15-30 Hz) occur coherently throughout the primate somatomotor network, comprising somatomotor cortices, basal ganglia, thalamus, cerebellum, and spinal cord, with the latter resulting in beta oscillations in muscular activity. In accordance with the anatomy of this network, these oscillations have traditionally been associated strictly with motor function. Here we show in humans that somatosensory demands, both in anticipation and during the processing of tactile stimuli, also modulate beta oscillations throughout this network. Specifically, somatosensory demands suppress the degree to which not only cortical activity but also muscular activity oscillates in the beta band. This suppression of muscular beta oscillations by perceptual demands is specific to demands in the somatosensory modality and occurs independent of movement preparation and execution: it occurs even when no movement is required at all. This places touch perception as an important computation within this widely distributed somatomotor beta network and suggests that, at least in healthy subjects, somatosensation and action should not be considered as separable processes, not even at the level of the muscles.

It feels like it’s me: Interpersonal multisensory stimulation enhances visual remapping of touch from other to self.

Understanding other people's feelings in social interactions depends on the ability to map onto our body the sensory experiences we observed on other people's bodies. It has been shown that the perception of tactile stimuli on the face is improved when concurrently viewing a face being touched. This Visual Remapping of Touch (VRT) is enhanced the more similar others are perceived to be to the self and is strongest when viewing one's face. Here, we ask whether altering self-other boundaries can in turn change the VRT effect. We used the enfacement illusion, which relies on synchronous interpersonal multisensory stimulation (IMS), to manipulate self-other boundaries. Following synchronous, but not asynchronous, IMS, the self-related enhancement of the VRT extended to the other individual. These findings suggest that shared multisensory experiences represent one key way to overcome the boundaries between self and others, as evidenced by changes in somatosensory processing of tactile stimuli on one's own face when concurrently viewing another person's face being touched.

Fingers Crossed! An Investigation of Somatotopic Representations Using Spatial Directional Judgements

Processing of tactile stimuli requires both localising the stimuli on the body surface and combining this information with a representation of the current posture. When tactile stimuli are applied to crossed hands, the system first assumes a prototypical (e.g. uncrossed) positioning of the limbs. Remapping to include the crossed posture occurs within about 300 ms. Since fingers have been suggested to be represented in a mainly somatotopic reference frame we were interested in how the processing of tactile stimuli applied to the fingers would be affected by an unusual posture of the fingers. We asked participants to report the direction of movement of two tactile stimuli, applied successively to the crossed or uncrossed index and middle fingers of one hand at different inter-stimulus intervals (15 to 700 ms). Participants almost consistently reported perceiving the stimulus direction as opposite to what it was in the fingers crossed condition, even with SOAs of 700 ms, suggesting that on average they did not incorporate the unusual relative finger positions. Therefore our results are in agreement with the idea that, by default, the processing of tactile stimuli assumes a prototypical positioning of body parts. However, in contrast to what is generally found with tactile perception with crossed hands, performance did not improve with SOAs as long as 700 ms. This suggests that the localization of stimuli in a somatotopic reference and the integration of this representation with postural information are two separate processes that apply differently to the hands and fingers.

Inter-hemispheric interaction of touches at the fingers: A combined psychophysics and TMS approach

Many studies have investigated interactions in the processing of tactile stimuli across different fingers. However, the precise time-scale of these interactions when stimuli arrive on opposite sides of the body remains uncertain. Specifically, it is not clear how tactile stimulation of different fingers of the same and different hands can interact with each other. The aim of the present study was to address this issue using a novel approach combining the QUEST threshold estimation method with single pulse TMS (spTMS). First, QUEST was used in a two-interval forced-choice design in order to establish threshold for detecting a 200 ms, 100 Hz sinusoidal vibration applied to the index fingertip (target finger threshold). This was done either when the target was presented in isolation, or concurrently with a distractor stimulus on another finger of the same or a different hand. Second, the same participants underwent a series of MRI scans (localisers) to produce somatotopic maps of SI and SII cortices. These maps were used to stimulate over SI with spTMS during a subsequent behavioural task, with the aim of modulating the behavioural interactions between the different fingers. The results showed that the threshold for detecting the target was lower when it was presented in isolation, as compared to when a concurrent distractor was present. Moreover, detection thresholds varied as a function of the distractor finger stimulated. The differential effect of the distractor finger on target detection thresholds is consistent with the segregation of different fingers in early somatosensory processing, from the periphery to SI.

Representation of dynamic mechanical allodynia in the ventral medial prefrontal cortex of trigeminal neuropathic rats

Trigeminal neuropathic pain is due to lesion or dysfunction of the nervous system. Dynamic mechanical allodynia is a widespread symptom of neuropathic pain for which mechanisms are still poorly understood. Recent studies demonstrate that forebrain neurons, including neurons in the medial prefrontal cortex (mPFC) are important for the perception of acute and chronic pain. Using the phosphorylation of the extracellular-signal regulated kinase (pERK-1/2) as an anatomical marker of neuronal activation, the present study investigated how dynamic mechanical allodynia is processed in the rat ventral mPFC (prelimbic and infralimbic cortex) after chronic constriction injury to the infraorbital nerve (IoN-CCI). Two weeks after unilateral IoN-CCI, rats showed a dramatic bilateral trigeminal dynamic mechanical allodynia. Light, moving stroking of the infraorbital skin resulted in strong, bilateral upregulation of pERK-1/2 in the ventral mPFC of IoN-CCI animals. pERK-1/2 was located in neuronal cells only. Stimulus-evoked pERK-1/2 immunopositive cell bodies displayed a rostrocaudal gradient and layer-selective distribution in the ventral mPFC, being predominant in the rostral ventral mPFC and in layers II–III and V–VI of the ventral mPFC. In layers II–III, intense pERK-1/2 also extended into distal dendrites, up to layer I. These results demonstrate that trigeminal nerve injury induces a significant alteration in the ventral mPFC processing of tactile stimuli and suggest that ERK phosphorylation contributes to the mechanisms underlying abnormal pain perception under this condition.

Sustained Spatial Attention in Touch: Modality-Specific and Multimodal Mechanisms

Sustained attention to a body location results in enhanced processing of tactile stimuli presented at that location compared to another unattended location. In this paper, we review studies investigating the neural correlates of sustained spatial attention in touch. These studies consistently show that activity within modality-specific somatosensory areas (SI and SII) is modulated by sustained tactile-spatial attention. Recent evidence suggests that these somatosensory areas may be recruited as part of a larger cortical network,also including higher-level multimodal regions involved in spatial selection across modalities. We discuss, in turn, the following multimodal effects in sustained tactile-spatial attention tasks. First, cross-modal attentional links between touch and vision, reflected in enhanced processing of task-irrelevant visual stimuli at tactuallyattended locations, are mediated by common (multimodal) representations of external space. Second, vision of the body modulates activity underlying sustained tactile-spatial attention, facilitating attentional modulation of tactile processing in between-hand (when hands are sufficiently far apart) and impairing attentional modulation in within-hand selection tasks. Finally, body posture influences mechanisms of sustained tactile-spatial attention, relying, at least partly, on remapping of tactile stimuli in external, visuallydefined, spatial coordinates. Taken together, the findings reviewed in this paper indicate that sustained spatial attention in touch is subserved by both modality-specific and multimodal mechanisms. The interplay between these mechanisms allows flexible and efficient spatial selection within and across sensory modalities.

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.