Spatial attention modulates the cortical somatosensory representation of the digits in humans

Experimental environment improves the reliability of short-latency afferent inhibition

Evidence indicates attention can alter afferent inhibition, a Transcranial Magnetic Stimulation (TMS) evoked measure of cortical inhibition following somatosensory input. When peripheral nerve stimulation is delivered prior to TMS, a phenomenon known as afferent inhibition occurs. The latency between the peripheral nerve stimulation dictates the subtype of afferent inhibition evoked, either short latency afferent inhibition (SAI) or long latency afferent inhibition (LAI). While afferent inhibition is emerging as a valuable tool for clinical assessment of sensorimotor function, the reliability of the measure remains relatively low. Therefore, to improve the translation of afferent inhibition within and beyond the research lab, the reliability of the measure must be improved. Previous literature suggests that the focus of attention can modify the magnitude of afferent inhibition. As such, controlling the focus of attention may be one method to improve the reliability of afferent inhibition. In the present study, the magnitude and reliability of SAI and LAI was assessed under four conditions with varying attentional demands focused on the somatosensory input that evokes SAI and LAI circuits. Thirty individuals participated in four conditions; three conditions were identical in their physical parameters and varied only in the focus of directed attention (visual attend, tactile attend, non- directed attend) and one condition consisted of no external physical parameters (no stimulation). Reliability was measured by repeating conditions at three time points to assess intrasession and intersession reliability. Results indicate that the magnitude of SAI and LAI were not modulated by attention. However, the reliability of SAI demonstrated increased intrasession and intersession reliability compared to the no stimulation condition. The reliability of LAI was unaffected by the attention conditions. This research demonstrates the impact of attention/arousal on the reliability of afferent inhibition and has identified new parameters to inform the design of TMS research to improve reliability.

Acute and long-lasting cortical thickness changes following intensive first-person action videogame practice

Recent evidence shows how an extensive gaming experience might positively impact cognitive and perceptual functioning, leading to brain structural changes observed in cross-sectional studies. Importantly, changes seem to be game-specific, reflecting gameplay styles and therefore opening to the possibility of tailoring videogames according to rehabilitation and enhancement purposes. However, whether if such brain effects can be induced even with limited gaming experience, and whether if they can outlast the gaming period, is still unknown. Here we quantified both cognitive and grey matter thickness changes following 15 daily gaming sessions based on a modified version of a 3D first-person shooter (FPS) played in laboratory settings. Twenty-nine healthy participants were randomly assigned to a control or a gaming group and underwent a cognitive assessment, an in-game performance evaluation and structural magnetic resonance imaging before (T0), immediately after (T1) and three months after the end of the experiment (T2). At T1, a significant increase in thickness of the bilateral parahippocampal cortex (PHC), somatosensory cortex (S1), superior parietal lobule (SPL) and right insula were observed. Changes in S1 matched the hand representation bilaterally, while PHC changes corresponded to the parahippocampal place area (PPA). Surprisingly, changes in thickness were still present at T2 for S1, PHC, SPL and right insula as compared to T0. Finally, surface-based regression identified the lingual gyrus as the best predictor of changes in game performance at T1. Results stress the specific impact of core game elements, such as spatial navigation and visuomotor coordination on structural brain properties, with effects outlasting even a short intensive gaming period.

An unavoidable modulation? Sensory attention and human primary motor cortex excitability

The link between basic physiology and its modulation by cognitive states, such as attention, is poorly understood. A significant association becomes apparent when patients with movement disorders describe experiences with changing their attention focus and the fundamental effect that this has on their motor symptoms. Moreover, frequently used mental strategies for treating such patients, e.g. with task-specific dystonia, widely lack laboratory-based knowledge about physiological mechanisms. In this largely unexplored field, we looked at how the locus of attention, when it changed between internal (locus hand) and external (visual target), influenced excitability in the primary motor cortex (M1) in healthy humans. Intriguingly, both internal and external attention had the capacity to change M1 excitability. Both led to a reduced stimulation-induced GABA-related inhibition and a change in motor evoked potential size, i.e. an overall increased M1 excitability. These previously unreported findings indicated: (i) that cognitive state differentially interacted with M1 physiology, (ii) that our view of distraction (attention locus shifted towards external or distant location), which is used as a prevention or management strategy for use-dependent motor disorders, is too simple and currently unsupported for clinical application, and (iii) the physiological state reached through attention modulation represents an alternative explanation for frequently reported electrophysiology findings in neuropsychiatric disorders, such as an aberrant inhibition.

Neural Correlates of Switching Attentional Focus during Finger Movements: An fMRI Study

Research on motor-related attentional foci suggests that switching from an internal to an external focus of attention has advantageous effects on motor performance whereas switching from an external to an internal focus has disadvantageous effects. We used functional magnetic resonance imaging to investigate the neural correlates of switching the focus of attention. Two experimental groups were trained to apply one focus direction – internal or external – on a previously learned finger tapping sequence. Participants with an internal focus training were instructed to attend to their moving fingers; those with an external focus training were instructed to attend to the response buttons. In the first half of the experiment, participants performed with their trained focus, in the second half, they were unexpectedly asked to switch to the untrained attentional focus. Our data showed that the switch from a trained internal to an unfamiliar external focus of attention elicited increased activation of the left lateral premotor cortex (PMC). We propose that this activation can be linked to the role of the PMC in action planning – probably indicating a facilitation effect on selectional motor processes. Switching from a trained external to an unfamiliar internal focus of attention revealed enhanced activation of the left primary somatosensory cortex and intraparietal lobule. We interpret these modulations as a result of the amplifying influence of afferent information on motor processing when asked to attend internally in a motor task after being trained with an external focus.

Interactions within the hand representation in primary somatosensory cortex of primates

Previous studies indicate that primary somatosensory cortical area 3b in macaques contains a somatotopic map of the hand, encompassing representations of each digit. However, numerous observations including recent findings in anesthetized New World monkeys indicate that that the digit representations within the map are not discrete. We assessed the generality and spatial extent of these effects in awake macaques. We show that, within a given digit representation, (1) there is response to stimulation of all other digits tested, extending across most or all of the digit map, and (2) response to stimulation of the locally preferred digit is modulated by concurrent stimulation of each of the other digits. Control experiments rule out effects of attention and mechanical spread of stimulation. We thus confirm that, even at the first level of somatosensory cortical processing, inputs from potentially all of the digits frame the context within which the input to a single digit is represented.

Tactile acuity in experienced Tai Chi practitioners: evidence for use dependent plasticity as an effect of sensory-attentional training

The scientific discovery of novel training paradigms has yielded better understanding of basic mechanisms underlying cortical plasticity, learning and development. This study is a first step in evaluating Tai Chi (TC), the Chinese slow-motion meditative exercise, as a training paradigm that, while not engaging in direct tactile stimulus training, elicits enhanced tactile acuity in long-term practitioners. The rationale for this study comes from the fact that, unlike previously studied direct-touch tactile training paradigms, TC practitioners focus specific mental attention on the body's extremities including the fingertips and hands as they perform their slow routine. To determine whether TC is associated with enhanced tactile acuity, experienced adult TC practitioners were recruited and compared to age-gender matched controls. A blinded assessor used a validated method (Van Boven et al. in Neurology 54(12): 2230-2236, 2000) to compare TC practitioners' and controls' ability to discriminate between two different orientations (parallel and horizontal) across different grating widths at the fingertip. Study results showed that TC practitioners' tactile spatial acuity was superior to that of the matched controls (P < 0.04). There was a trend showing TC may have an enhanced effect on older practitioners (P < 0.066), suggesting that TC may slow age related decline in this measure. To the best of our knowledge, this is the first study to evaluate a long-term attentional practice's effects on a perceptual measure. Longitudinal studies are needed to examine whether TC initiates or is merely correlated with perceptual changes and whether it elicits long-term plasticity in primary sensory cortical maps. Further studies should also assess whether related somatosensory attentional practices (such as Yoga, mindfulness meditation and Qigong) achieve similar effects.

Short-term modulation of the ipsilateral primary sensory cortex by nociceptive interference revealed by SEPs

We studied the modulation of the topographic arrangement of the human ipsilateral primary somatosensory cortex following interference of nociceptive stimuli by means of dipole source analysis. Multichannel somatosensory evoked potentials were obtained by electrical stimulation of digits 1 and 5 of the left hand before, during and after the application of pain to digits 2-4 of the right hand. The primary cortical response of the SEP (N20) was obtained for dipole localization of the representation of the primary sensory cortex receiving input from digits 1 to 5. The 3D-distance between these sides was calculated for further analysis. To account for possible attentional effects recordings were performed while simultaneously to this intervention subjects were asked to turn their attention to the right or left hand in a pseudorandom order. The application of pain induced an expansion of the 3D-distance between digits 1 and 5. Focusing attention to the stimulated limb or the site of the intervention did not yield to an additional effect. Our results provide further evidence for the presence of a quickly adapting interaction between primary somatosensory areas of both hemispheres following an interference of nociceptive stimulation in SEPs. This modifying process is probably mediated by interhemispheric and intercortical connections leading to hyperexcitability of the primary sensory cortex contralateral to that receiving nociceptive input. Spatial attention does not seem to have an impact on this kind of short-term intercortical plasticity.

Attentional modulation of spatial integration of pain: evidence for dynamic spatial tuning

In many sensory modalities, afferent processing is dynamically modulated by attention and this modulation produces altered sensory experiences. Attention is able to alter perceived pain, but the mechanisms involved in this modulation have not been elucidated. To determine whether attention alters spatial integration of nociceptive information, subjects were recruited to evaluate pain from pairs of noxious/innocuous thermal stimuli during different spatial attentional tasks. Divided attention was able to abolish spatial summation and produce inhibition of pain. In contrast, directed attention enhanced pain intensity by partially integrating both stimuli. This dynamic modulation of spatial integration indicates that attention alters spatial dimensions of afferent nociceptive processing to optimize the perceptual response to input from a particular body region or stimulus feature. This dynamic spatial tuning of nociceptive processing provides a new conceptual insight into the functional significance of endogenous pain inhibitory and facilitatory mechanisms.

Short-term cortical reorganization by deafferentation of the contralateral sensory cortex

The topographic arrangement of the human primary somatosensory cortex following deafferentation of the contralateral cortex has been investigated by means of dipole source analysis. Somatosensory-evoked potentials were obtained by electrical stimulation of digit 1 and digit 5 of the left hand before and after anesthesia of digits 2-4 of the right hand during different terms of attention. Anesthesia induced an expansion of the three-dimensional distance between digits 1 and 5. This suggests intercortical plasticity modulated between bilateral primary somatosensory cortical areas, which is unaffected by spatial attention. These changes occur rapidly and are probably mediated by disinhibition of intercortical connections, leading to hyperexcitability of the primary sensory cortex that is contralateral to the region undergoing deafferentation.

Morphing the body: Illusory feeling of an elongated arm affects somatosensory homunculus

Recent studies suggest that in contrast to traditional views of the body map the topographic representation in primary somatosensory cortex (SI) reflects the perceived rather than the physical aspects of peripheral stimulation. Here, we created a simple illusion of feeling an elongated arm by using the dominance of the visual domain over the tactile sense: employing an artificial hand and arm, which were connected to the body, subjects were given the visual impression that they had an extended arm. Since it is known from animal studies that tactile illusions alter early sensory processing in SI, we expected a modulation of the topography in SI corresponding to this illusion. Behavioral results showed that during the illusion the participants felt that their arm was elongated. Neuromagnetic source imaging of the functional organization in SI revealed that the cortical distance between first (D1) and fifth digit (D5) decreased when subjects felt the arm elongated. Since this modulation was significantly positively correlated with the illusionary feeling of an extended arm, the results suggest an involvement of SI during perceived changes in the size of body parts. We discuss the results as possible top-down modulations of SI by higher order somatosensory areas.

Tactile hyperacuity thresholds correlate with finger maps in primary somatosensory cortex (S1)

Behavioral tactile discrimination thresholds were compared with functional magnetic resonance imaging measurements of cortical finger representations within primary somatosensory cortex (S1) for 10 human subjects to determine whether cortical magnification in S1 could account for the variation in tactile hyperacuity thresholds of the fingers. Across 10 subjects, the increase in tactile thresholds from the index finger to the little finger correlated with the decrease in cortical representation across fingers in S1. Additionally, representations of the fingers within S1, in Brodmann areas 3b and 1, were also correlated with the thresholds. These results suggest that tactile hyperacuity is largely determined by the cortical representation of the fingers in S1.

A common framework for perceptual learning

In this review, we summarize recent evidence that perceptual learning can occur not only under training conditions but also in situations of unattended and passive sensory stimulation. We suggest that the key to learning is to boost stimulus-related activity that is normally insufficient exceed a learning threshold. We discuss how factors such as attention and reinforcement have crucial, permissive roles in learning. We observe, however, that highly optimized stimulation protocols can also boost responses and promote learning. This helps to reconcile observations of how learning can occur (or fail to occur) in seemingly contradictory circumstances, and argues that different processes that affect learning operate through similar mechanisms that are probably based on, and mediated by, neuromodulatory factors.

Cortical Dynamics As A Therapeutic Mechanism for Touch Healing

Touch Healing (TH) therapies, defined here as treatments whose primary route of administration is tactile contact and/or active guiding of somatic attention, are ubiquitous across cultures. Despite increasing integration of TH into mainstream medicine through therapies such as Reiki, Therapeutic Touch,(TM) and somatically focused meditation practices such as Mindfulness-Based Stress Reduction, relatively little is known about potential underlying mechanisms. Here, we present a neuroscientific explanation for the prevalence and effectiveness of TH therapies for relieving chronic pain. We begin with a cross-cultural review of several different types of TH treatments and identify common characteristics, including: light tactile contact and/or a somatosensory attention directed toward the body, a behaviorally relevant context, a relaxed context and repeated treatment sessions. These cardinal features are also key elements of established mechanisms of neural plasticity in somatosensory cortical maps, suggesting that sensory reorganization is a mechanism for the healing observed. Consideration of the potential health benefits of meditation practice specifically suggests that these practices provide training in the regulation of neural and perceptual dynamics that provide ongoing resistance to the development of maladaptive somatic representations. This model provides several direct predictions for investigating ways that TH may induce cortical plasticity and dynamics in pain remediation.

Cortical Characterization and Inter-Dipole Distance Between Unilateral Median Versus Ulnar Nerve Stimulation of Both Hands in MEG

Contralateral somatosensory evoked fields (SEF) by whole head MEG after unilateral median and ulnar nerve stimulation of both hands were studied in 10 healthy right-handed subjects. Major parameters describing cortical activity were examined to discriminate median and ulnar nerve evoked responses. Somatic sensitivity showed high similarity in the 4 study conditions for both hand and nerve. The brain SEFs consisted of 7-8 major peak stages with consistent responses in all subjects at M20, M30, M70 and M90. Comparable inter-hemispheric waveform profile but high inter-subject variability was found. Median nerve induced significantly shorter latencies in the early activities than those of the ulnar nerve. The 3D cortical maps in the post stimulus 450 ms timeframe showed for both nerves two polarity reversals, an early and a late one which is a new finding. Dipole characteristics showed differential sites for the M20 and M30 in the respective nerve. Higher dipole moments evoked by the median nerve were noticed when compared to the ulnar. Furthermore, the results of the dipole distances between both nerves for M20 were calculated to be at 11.17 mm +/- 4.93 (LH) and 16.73 mm +/- 5.66 (RH), respectively after right hand versus left hand stimulation. This study showed substantial differences in the cortical responses between median and ulnar nerve. Especially the dipole distance between median and ulnar nerve on the cortex was computed accurately for the first time in MEG. Little is known however of the cortical responses in chronic pain patients and the parameter(s) that may change in an individual patient or a group. These results provide precise basis for further evaluating cortical changes in functional disorders and disease sequelae related to median and ulnar nerves.

Cortical changes to experimental sensitization of the human esophagus

Topographical organization in the neocortex shows experience-dependent plasticity. We hypothesized that experimental sensitization of the esophagus results in changes of the topographical distribution of the evoked potentials and the corresponding dipole source activities to painful stimulation. An endoscopic method was used to deliver 35 electrical stimuli at the pain threshold to a fixed area of the mucosa in 10 healthy volunteer men and women. The stimulations were repeated after 30 min (reproducibility experiment), and after 60 min following perfusion of 200 ml 0.1 N hydrochloric acid (sensitization experiment). During stimulation the electroencephalogram was recorded from 64 surface electrodes. The sensitization resulted in a decrease in the pain threshold (F=6.2; P=0.004). The topographic distribution of the evoked potentials showed reproducible negative (N1, N2) and positive (P1, P2) components. After acid perfusion a reduced latency and a change in localization was seen for the P1 subdivided into frontal and occipital components (F=29.5, P<0.001; F=53.7, P<0.001). Furthermore the sensitization resulted in a reduction of the latency for P2 (F=6.2, P=0.009). The source analysis showed consistent dipolar activity in the bilateral opercular-insular cortex before and after acid perfusion. For the anterior cingulate dipole there was a reduction in latency (P=0.03) and a posterior shift (P=0.0002) following acid perfusion. The findings indicate that short-term sensitization of the esophagus results in central neuroplastic changes involving the cingulate gyrus, which also showed pathological activation in functional diseases of the gut, thus reflecting the importance of this region in visceral pain and hyperalgesia.

Effects of motor activity on the organization of primary somatosensory cortex

Recent studies have shown that adaptation of representational maps within the primary somatosensory cortex can be induced by task-related motor activity. Here, we explore the relationship between the complexity of the motor task and the extent of task-specific adaptation within the primary somatosensory cortex. We hypothesized that the extent of adaptation increases with the complexity of the motor task. Using neuromagnetic source imaging based on electrical stimulation of the thumb and ring finger, we demonstrate that cortical finger representations are more distant during performance of the pinch finger grip than in a rest condition. Our data suggest that somatosensory cortical maps undergo rapid modulation depending on the task-specific involvement of somatosensory feedback in movements.

Dynamic modulation of the primary somatosensory cortex during seeing and feeling a touched hand

Previous work has demonstrated cross-modal links between vision and somatosensation at an early stage of sensory processing. Furthermore, recent behavioral studies have shown that viewing the stimulated body part can enhance tactile discrimination ability at the stimulated site. This study aims to investigate the role of the primary somatosensory cortex (SI) during visuotactile integration processes. Subjects looked at a hand in a video being touched on the first digit (D1) in synchrony with felt touches on their real hidden hand as compared with watching a video with asynchronous touches. During synchronous stimulation, subjects reported to feel the tactile sensation on the video hand, thus indicating that in this condition the subjects regarded the video hand as their own touched hand. This feeling disappeared in the asynchronous condition. Using neuromagnetic source imaging, we assessed the topography of the functional organization of SI related to tactile stimulation of D1. The cortical representation of D1 moved to a more inferior location during synchronous in comparison to asynchronous stimulation and rest. This modulation of the map in SI was significantly positively correlated with the feeling that the seen touch in the video represented the touch on the real hand. Thus, only if the seen touch is attributed to the own body, SI seems to be modulated.

Spatial attention affects sensorimotor reorganisation in human motor cortex

It is well known that the somatotopic representation of sensory and motor cortices reorganises in response to sustained changes in sensory input. It is also clear that the extent of the effect depends on whether or not subjects pay attention during the procedure. Here we show that the pattern of reorganisation produced by sustained sensory input depends not only on the subjects' attention but also on the spatial focus of their attention on the body surface. Maximal effects are observed only when subjects pay attention to the site of the input; if they attend to an adjacent body part then the effects are reduced. These results may be relevant to rehabilitation procedures commonly used in patients after stroke.

Task-relevant modulation of primary somatosensory cortex suggests a prefrontal-cortical sensory gating system

Increasing evidence suggests that somatosensory information is modulated cortically for task-specific sensory inflow: Several studies report short-term adaptation of representational maps in primary somatosensory cortex (SI) due to attention or induced by task-related motor activity such as handwriting. Recently, it has been hypothesized that the frontal or prefrontal cortex may modulate SI. In order to test this hypothesis, we studied the functional organization of SI while subjects performed the Tower of Hanoi task. This task is known to be related to activation of frontal or prefrontal areas. The functional organization of SI while performing the Tower of Hanoi task was compared to the organization of SI during performing the same movements but without the Tower of Hanoi task and with rest. Topography of SI was assessed using neuromagnetic source imaging based on tactile stimulation of the first (D1) and fifth digits (D5). Performing the Tower of Hanoi task was accompanied by plastic changes in SI as indicated by significant shifts in the cortical representations of D1 and D5: They moved further apart during the Tower of Hanoi task compared to the control task containing the same movements but without the cognitive characteristic. Thus, we conclude that SI maps undergo dynamic modulation depending on motor tasks with different cognitive demands. The results suggest that this short-term plasticity may be regulated by a prefrontal-cortical sensory gating system.

Seeing the hand being touched modulates the primary somatosensory cortex

Recent behavioral studies have shown that viewing the stimulated body part can enhance tactile acuity. We examined effects of viewing the stimulated body part by using neuromagnetic source imaging. Study participants were covertly stimulated on their first digit (D1) while watching a video that showed a hand where D1 was stimulated. The video hand appeared in the region where the real hand would be expected. In addition, participants were stimulated on D1 without watching the video. The results demonstrated a shift of D1 to a more inferior position and an increase of the dipole strength of D1 during the observation of tactile stimulation. We discuss these cross-modal interactions with recent studies on visuotactile enhancement.

Perceptual context-dependent remodeling of the forepaw map in the SI cortex of rats trained on tactile discrimination

We combined behavioral assessment of texture discrimination and electrophysiological mapping of concomitant reorganization in the forepaw representation within the SI cortex. Rats were housed in enriched (EE) or impoverished (IE) environments which have been shown to remodel the forepaw map and possibly alter discriminative abilities. In addition, animals were trained to discriminate homogeneous floorboards of invariant roughness from heterogeneous floorboards of gradually decreasing roughness contrasts during locomotion. As reported recently, differences in perceptual abilities were not related to housing conditions, but to a predilection for a floorboard type [Bourgeon S, Xerri C, Coq JO. Abilities in tactile discrimination of textures in adult rats exposed to enriched or impoverished environments. Behav Brain Res 2004;153:217-231]. Consistently, the present study shows that cortical map remodeling resulting from short-duration daily experience can prevail over changes induced by housing conditions. The relative area of glabrous skin representation was related to the discrimination performance and learning abilities in the rats (H) with a predilection for heterogeneous floorboards, i.e. in the animals performing discrimination in the most challenging perceptual context. By contrast, this cortical area was influenced by the duration of sensory experience in rats (h) with a predilection for homogeneous floorboards. Both EE condition and training to discrimination selectively decreased the sizes of the SI neurons' receptive fields (RFs) located on glabrous skin. Smaller RFs and larger cortical areas serving glabrous skin were correlated with better perceptual performances and learning abilities in the H rats only. The present study shows that representational reorganization related to tactile discrimination performances depends upon the perceptual context.

Dynamic shifts in the organization of primary somatosensory cortex induced by bimanual spatial coupling of motor activity

Previous work has shown that training and learning can induce powerful changes in the homuncular organization of the primary somatosensory cortex (SI). Moreover, a number of studies suggest the existence of short-term adaptation of representational maps in SI. Recently, motor activity has been shown to induce rapid modulation of somatosensory cortical maps. It is hypothesized that there is a task-related influence of motor and premotor areas upon the organization of somatosensory cortex. In order to test this hypothesis, we studied the functional organization of somatosensory cortex by examining coupling effects in a bimanual movement task. Bimanual coupling is known to be related to an activation of the premotor cortex and the supplementary motor area. The functional organization of the somatosensory cortex for known bimanual coupling effects was compared to the organization of the somatosensory cortex during the same movements but with only a small effort in coupling. Topography of the functional organization of the somatosensory cortex was assessed using neuromagnetic source imaging based on tactile stimulation of the first (D1) and fifth digit (D5). We could show that the cortical representations of D1 and D5 moved further apart during the bimanual coupling task in comparison to the same task without coupling and rest. Our data suggest that somatosensory cortical maps undergo fast and dynamic modulation as a result of a task-related influence of motor or premotor areas.

Hand posture modulates cortical finger representation in SII

Somatosensory magnetic fields evoked by electrical stimuli of the thumb or the index finger were recorded using a whole head magnetoencephalography (MEG) system in 10 subjects performing different finger postures, open hand posture and close hand posture for picking up a small object. The mean Euclidean distances between the ECD (equivalent current dipole) locations for the thumb and index finger in the secondary somatosensory cortex (SII) across the subjects were 8.5 +/- 2.1 mm in the close hand posture and 11.2 +/- 2.6 mm in the open hand posture. The distance was significantly shorter in the close hand posture (paired t test, P = 0.002, n = 8). However, the distances of the P38m and P60m components in the primary somatosensory cortex (SI) were not significantly different between the two hand postures (P38m: 13.4 +/- 5.6 mm in the open and 13.5 +/- 3.9 mm in the close; P60m: 12.4 +/- 2.6 mm in the open and 16.2 +/- 5.3 mm in the close). This shortening of the spatial distance between the cortical finger representations suggests a similarity in humans of the rapid changes in the dynamics of cortical circuits reported in animal studies. In addition, the overlap of the cortical finger representations, which might be suggested by the shortening of the distance between the ECDs in SII, is likely to play a role in information integration between sensory inputs from the thumb and index finger.

Attention induces reciprocal activity in the human somatosensory cortex enhancing relevant- and suppressing irrelevant inputs from fingers

OBJECTIVE

We studied whether attention regulates information processing in the human primary somatosensory cortex (SI) by selective enhancement of relevant- and suppression of irrelevant information.

METHODS

Under successive and simultaneous electric stimuli to both the right index and middle fingers, tactile stimuli were randomly (20%) presented on one of the two fingers in separate two runs exchanging the finger. Subjects were requested to discriminate the tactile stimuli in an attention task to induce attention to one finger and to ignore the stimuli in a control task to avoid such an attention focus. Somatosensory evoked magnetic fields were measured only for the two-finger electric stimulation and an early component (M50) was analyzed.

RESULTS

In spite of the two-finger simultaneous stimulation, attention to either the index or middle finger lowered or heightened the M50-sourse location, respectively. The attention task did not increase the M50 amplitude.

CONCLUSIONS

Attention to a finger enhanced selectively the representation of the finger in the SI cortex. However, this SI activity did not increase the M50 amplitude, suggesting that the attention suppressed another finger region receiving the unattended inputs.

SIGNIFICANCE

Attention regulates the SI activity by selectively enhancing the task-relevant information and by filtering out other noise inputs.

The effect of sensory input and attention on the sensorimotor organization of the hand area of the human motor cortex

Sensory input can remodel representations in the sensory cortex, and this effect is heavily influenced by attention to the stimulus. Here we ask whether pure sensory input can also influence the spatial distribution of sensory effects on motor cortical hand area (sensorimotor organization) and whether this is modulated by attention. Sensorimotor organization was tested by applying short (1.5 s) periods of low amplitude vibration to single intrinsic hand muscles and measuring motor cortex excitability with transcranial magnetic stimulation (TMS). In healthy subjects, sensorimotor organization in the hand is focal, with input from one hand muscle increasing motor-evoked potentials (MEPs), decreasing short and increasing long-interval intracortical inhibition (SICI and LICI) in the vibrated muscle ('homotopic' effects) and having opposite effects on neighbouring muscles ('heterotopic' effects). Here we show that a 15 min intervention of vibration applied simultaneously to two hand muscles can lead to long-term (> 30 min) changes in the spatial pattern of sensorimotor interaction. The amount and direction of the effects depended on the subject's attention during the intervention: if subjects attended to both muscles when they were receiving simultaneous vibration, subsequent short-term vibration applied to one of them produced 'homotopic' effects on both muscles. 'Heterotopic' effects on a muscle not vibrated during the intervention were unaffected. If subjects did not attend to simultaneous vibration, subsequent short-term vibration of the muscles involved in the intervention no longer had any effect on them although the 'heterotopic' effects on a muscle not involved in the intervention were unchanged. We conclude that a 15 min period of pure sensory input can remodel the way that subsequent sensory inputs interact with motor output, that the effects are specific for the motor output to muscles involved in the intervention and that they are modulated by the subject's attention.

Short-term plasticity of the primary somatosensory cortex during tool use

Plastic changes within the primary somatosensory cortex (SI) related to tool use are reported. Subjects manipulated a small object with a pair of tongs or with their hand. Functional organization of SI during tool use was compared with that during executing the task with the fingers and during rest, respectively. Topography of SI was assessed using neuromagnetic source imaging based on tactile stimulation of the first (D1) and fifth digit (D5). We found that cortical representations of D1 and D5 are further apart during tool use than during non-tool use and rest. Our data suggest that somatosensory cortical maps are part of the neural network representing the modified schema of the hand in which the tool was incorporated.

Activation in SI and SII: the influence of vibrotactile amplitude during passive and task-relevant stimulation

Using functional magnetic resonance imaging (fMRI), responses in human somatosensory cortex were quantified in response to changes in the amplitude of peripheral stimulation during (1) passive vibration and (2) an attention-demanding tactile tracking task whereby changes in vibration amplitude were used to guide motor behaviour. Functional MRI was conducted using a scanner operating at 1.5 T, and vibration was delivered to the volar surface of the right index finger with a custom-built magnetomechanical device. Results showed that primary somatosensory cortex (SI) reliably reflects changes in vibration amplitude applied to the finger during passive vibration and also in the presence of a task that modulates the activity in SI. Secondary somatosensory cortex did not reveal any clear relationship with vibration amplitude but was more often activated during the attention demanding tracking task compared with passive vibration. The present study supports an increasing stimulus-response relationship between vibrotactile stimuli and activity in SI that persists during attentive, active states.

Functional organization of primary somatosensory cortex depends on the focus of attention

We used magnetic source imaging in human subjects to reveal within-subject variations of the homuncular hand representation within the primary somatosensory cortex modulated by attention. In one condition subjects were trained to detect sequential leftward or rightward stimulus motion across the fingers of the left hand ("hand" condition) and in a different condition to detect stimulus motion at a specific finger on this hand ("finger" condition). Afferent input was controlled by applying exactly the same stimulus pattern to the digits in the two tasks. Segregation of the somatotopic hand representation (an increase in the distance between the representations of digits 2 and 5) was observed, commencing with the onset of practice, in the finger relative to the hand condition. Subsequent training in the hand and finger conditions with feedback for correctness did not modify segregation, indicating that segregation was a task effect and not a training effect. These findings indicate that the hand representation within the primary somatosensory cortex is not statically fixed but is dynamically modulated by top-down mechanisms to support task requirements. A greater capacity for modulation of the functional cortical organization was positively correlated with superior learning and task performance.

Activation of somatosensory cortical areas varies with attentional state: an fMRI study

The differing roles of SI and SII areas in the somatosensory system have received relatively little interest in previous research. In the present study fMRI was applied to determine possible changes in activations of these areas as a function of attentional modulation (attending vs. not attending to the stimulation of a finger). The results showed that attention induced larger regional changes, mostly enlargements of activated areas, at SII than at SI. The number of instances where new, emerging activations, not present in the non-attend condition, were observed was larger at SII than at SI. These differential attentional effects indicate that SII areas may have a role in more complex tactile functions such as tactile working memory mechanisms.

The musician's brain as a model of neuroplasticity

Studies of experience-driven neuroplasticity at the behavioural, ensemble, cellular and molecular levels have shown that the structure and significance of the eliciting stimulus can determine the neural changes that result. Studying such effects in humans is difficult, but professional musicians represent an ideal model in which to investigate plastic changes in the human brain. There are two advantages to studying plasticity in musicians: the complexity of the eliciting stimulus music and the extent of their exposure to this stimulus. Here, we focus on the functional and anatomical differences that have been detected in musicians by modern neuroimaging methods.

Physiological correlates of perceptual learning in monkey V1 and V2

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation. During training the monkeys' discrimination thresholds gradually improved to much better than those of naive monkeys or humans. Although this improvement was specific to the trained orientation, it showed little retinotopic specificity. The receptive field properties of single neurons from regions representing the trained location and a location in the opposite visual hemifield were measured in V1 and V2. In most respects the receptive field properties in the representations of the trained and untrained regions were indistinguishable. However, in the regions of V1 and V2 representing the trained location, there were slightly fewer neurons whose optimal orientation was near the trained orientation. This resulted in a small but significant decrease in the V1 population response to the trained orientation at the trained location. Consequently, the observed neuronal populations did not exhibit any orientation-specific biases sufficient to explain the orientation specificity of the behavioral improvement. Pooling models suggest that the behavioral improvement was accomplished with a task-dependent and orientation-selective pooling of unaltered signals from early visual neurons. These data suggest that, even for training with stimuli suited to the selectivities found in early areas of visual cortex, behavioral improvements can occur in the absence of pronounced changes in the physiology of those areas.

Dynamic organization of the somatosensory cortex induced by motor activity

Intensive and long-lasting experience of altered sensory input induces permanent changes in the functional organization of the somatosensory cortex. In addition, an increasing body of evidence suggests the existence of dynamic, short-term and task-dependent adaptation of representational maps within somatosensory cortex. It is hypothesized that somatosensory maps can, not only, be acquired within a short period of time, but might also be set up during periods of training related to specific tasks and subsequently activated dynamically upon performance of that particular task. In order to test this hypothesis we studied the functional organization of somatosensory cortex for a heavily overlearned and frequently performed task for which no new acquisition of a sensory map had to be assumed. To this end, the functional organization of somatosensory cortex for handwriting was compared with the organization during rest in healthy humans. Functional organization of the somatosensory cortex was assessed using non-invasive, neuromagnetic source imaging based on tactile stimulation of the thumb (D1) and little finger (D5) during writing and rest. In different blocks, subjects wrote with their right, dominant and their left hand, respectively. During writing, D1 and D5 of the writing hand were stimulated. To test the reliability of our results all measurements were repeated after 1 week. It was found that amplitudes of somatosensory evoked magnetic fields with latencies of 45 ms were reduced during writing compared with rest. This finding is in accordance with the sensorimotor gating effect. Using source localization we could show that cortical representations of D1 and D5 are more distant during writing with either hand compared with rest. Our data suggest that somatosensory cortical maps undergo rapid modulation depending on task-specific involvement of sensory processing in daily-life overlearned movements. As it is unlikely that a new sensory map is always acquired when a frequently used task such as writing is performed, we suggest that somatosensory cortex switches between different, concurrently pre-existing maps depending on actual requirements. Task-dependent activation of pre-existing maps might be a powerful mechanism to optimize stimulus processing.

Selective spatial attention induces short-term plasticity in human somatosensory cortex

Early cognitive process in the primary somatosensory cortex (SI) was studied by measuring somatosensory evoked magnetic fields during selective attention tasks. We used vibratory stimuli to the index or middle finger with a frequency of 100 or 400 Hz for selective discrimination of spatial finger and non-spatial frequency attribute of stimuli. An early M50 component from the SI cortex indicated that the SI regions for the fingers were specifically segregated in a finger discrimination task but not in a frequency discrimination task or in a control condition. The task-dependent and immediate switchover of cortical finger representation demonstrates a dynamic SI activation for spatial information processing.

Human locognosic acuity on the arm varies with explicit and implicit manipulations of attention: implications for interpreting elevated tactile acuity on an amputation stump

In Experiment 1, normal subjects' ability to localize tactile stimuli (locognosia) delivered to the upper arm was significantly higher when they were instructed explicitly to direct their attention selectively to that segment than when they were instructed explicitly to distribute their attention across the whole arm. This elevation of acuity was eliminated when subjects' attentional resources were divided by superimposition of an effortful, secondary task during stimulation. In Experiment 2, in the absence of explicit attentional instruction, subjects' locognosic acuity on one of three arm segments was significantly higher when stimulation of that segment was 2.5 times more probable than that of stimulation of the other two segments. We surmise that the attentional mechanisms responsible for such modulations of locognosic acuity in normal subjects may contribute to the elevated sensory acuity observed on the stumps of amputees.

Functional reorganization of the human primary somatosensory cortex after acute pain demonstrated by magnetoencephalography

The somatosensory system is capable of functional reorganization following peripheral denervation or training. Studies on human amputees with phantom limb pain provided evidence that these reorganizational changes are modulated through nociceptive input. In the present study we used magnetoencephalographic recordings of six healthy volunteers to assess whether acute pain by itself causes a reorganization of the primary somatosensory cortex. After the induction of an intense experimental pain at the thenar of the left hand by intradermal injection of capsaicin, the extent of the cortical hand representation and the distance between the hand representation and the localization of the lip decreased. A likely mechanism for this acute reorganization is that pain induced hyperresponsiveness of the left thenar to tactile input from neighboring body sites.

Attending to and remembering tactile stimuli: a review of brain imaging data and single-neuron responses

Clinical and neuroimaging observations of the cortical network implicated in tactile attention have identified foci in parietal somatosensory, posterior parietal, and superior frontal locations. Tasks involving intentional hand-arm movements activate similar or nearby parietal and frontal foci. Visual spatial attention tasks and deliberate visuomotor behavior also activate overlapping posterior parietal and frontal foci. Studies in the visual and somatosensory systems thus support a proposal that attention to the spatial location of an object engages cortical regions responsible for the same coordinate referents used for guiding purposeful motor behavior. Tactile attention also biases processing in the somatosensory cortex through amplification of responses to relevant features of selected stimuli. Psychophysical studies demonstrate retention gradients for tactile stimuli like those reported for visual and auditory stimuli, and suggest analogous neural mechanisms for working memory across modalities. Neuroimaging studies in humans using memory tasks, and anatomic studies in monkeys support the idea that tactile information relayed from the somatosensory cortex is directed ventrally through the insula to the frontal cortex for short-term retention and to structures of the medial temporal lobe for long-term encoding. At the level of single neurons, tactile (such as visual and auditory) short-term memory appears as a persistent response during delay intervals between sampled stimuli.