Bare hands and attention: evidence for a tactile representation of the human body

If brain lesions impair the allocation of attention to a representation of the body surface and the hand may serve as an attentional focus or “wand”, one might expect that somatosensory deficits caused by cerebral lesions would be ameliorated by contact with the ipsilesional hand.

To test this prediction, tactile detection tasks were administered to two subjects with right hemisphere lesions. Subject CB’s left tactile extinction was investigated in conditions in which the degree of contact between the right and left hands and the spatial relationship between his hands was systematically varied. His left tactile extinction was significantly reduced by touch of the right hand. Similarly, extinction at the left knee was ameliorated by touch of the knee by the right hand; touch of the right foot had no effect. Subject NC’s ability to detect a tactile stimulus delivered to the left side was systematically assessed in conditions in which the hands touched and the spatial relationship between the hands was varied. His ability to detect a touch on the left hand improved in conditions in which the left hand was touched by the right hand. This effect was not observed if direct contact between the two hands was prevented by inserting a thin cloth between the hands. For both subjects, placing the right hand in close proximity to the left hand or altering the spatial location of the hands relative to the body did not influence performance. These data demonstrate that the hand may serve as a conduit for attention and provide strong evidence for a distinct representation of the body surface that is at least in part independent of spatial representations.     

Dyslexia and across-hands finger localization deficits

Two experiments on finger localization are reported. Experiment 1 compared children who were poor readers with two groups of children matched to poor readers for sex and either age (CA controls) or reading age (RA controls). The participant's hands were kept out of his or her sight in a semi-open box while the fingers of one hand were lightly touched by the experimenter. The participant's task was to indicate, using the thumb of either the same hand (within-hand condition) or the opposite hand (across-hands condition) to respond, the finger(s) which had been touched by the experimenter. Performance was significantly impaired in the across-hands condition compared with the within-hand condition. Experiment 2 was carried out with dyslexic adults and a control group of normal readers. Using the same method of responding as in Experiment 1, a significant deficit in the across-hands condition compared to the within-hand condition was found for both groups. The effect was also obtained for both groups when participants were required to point to the relevant fingers on a photograph of a hand rather than use the thumb of the opposite hand to respond. The results are discussed in relation to the hypothesis of a deficit in inter-hemispheric transfer of tactile information in dyslexia.     

The electrophysiology of tactile extinction: ERP correlates of unconscious somatosensory processing

We examined the electrophysiological correlates of left-sided tactile extinction in a patient with right-hemisphere damage. Computer-controlled punctate touch was presented to the left, right or both index fingers in an unpredictable sequence. The patient reported his conscious tactile percept (“left”, “right” or “both”). He showed extinction on 75% of bilateral trials, reporting only right stimulation for these. Somatosensory evoked potentials for unilateral stimulation showed early components over contralateral somatosensory areas (P60 and N110) for either hand. In contrast to the results observed for age-matched controls, the patient’s P60 was smaller in amplitude for left-hand touch over the right hemisphere than for right-hand touch over the intact hemisphere. Bilateral trials with extinction revealed residual P60 and N110 components over the right hemisphere in response to the extinguished left touch. These results demonstrate residual unconscious somatosensory processing of extinguished touch. They also suggest that tactile extinction can be caused by attenuation rather than elimination of somatosensory responses in the damaged hemisphere, with an underlying deficit even on unilateral trials.     

Functional anatomy of GO/NO-GO discrimination and response selection - a PET study in man

The purpose of this study was to identify the functional fields activated in relation to the NO-GO decision. Nine healthy subjects participated in the study which consisted of two test positron emission tomography (PET) scans (GO/NO-GO task and response selection task) and one control scan. In the response selection task, subjects were asked to flex their thumb of the right hand when a light emitting diode (LED) placed 60 cm frorn their eyes turned on red and to flex their index finger of the right hand when LED turned on green. In the GO/NO-GO task, subjects were asked to flex their thumb when the LED turned on red, however, they were asked not to move their fingers when LED turned on green. In the control state, they were asked simply to look at the LED without any movement of finger during the course of the scan. The mean regional cerebral blood flow (rCBF) change images for each task minus control and task minus task were calculated and fields of significant rCBF changes were identified.Several fields in the prefrontal cortex of the right hemisphere were specifically activated in relation to the GO/NO-GO task. The results indicate that the prefrontal cortex of the right hemisphere may be a key structure to make a decision not to move.     

Differences in finger localisation performance of patients with finger agnosia

Several neuropsychological studies have suggested parallel processing of somatosensory input when localising a tactile stimulus on one's own by pointing towards it (body schema) and when localising this touched location by pointing to it on a map of a hand (body image). Usually these reports describe patients with impaired detection, but intact sensorimotor localisation. This study examined three patients with a lesion of the angular gyrus with intact somatosensory processing, but with selectively disturbed finger identification (finger agnosia). These patients performed normally when pointing towards the touched finger on their own hand but failed to indicate this finger on a drawing of a hand or to name it. Similar defects in the perception of other body parts were not observed. The findings provide converging evidence for the dissociation between body image and body schema and, more importantly, reveal for the first time that this distinction is also present in higher-order cognitive processes selectively for the fingers.     

Binding personal and peripersonal space: evidence from tactile extinction

Behavioral and neurophysiological studies suggest that the brain constructs different representations of space. Among these representations are personal and peripersonal space. Personal space refers to the space occupied by our bodies. Peripersonal space refers to the space surrounding our bodies, which can be reached by our limbs. How these two representations are bound to give a unified sense of space in which humans act is not clear. We tested 10 patients with tactile extinction to investigate this issue. Tactile extinction is an attentional disorder in which patients are unaware of being touched on their contralesional limb if they are also touched simultaneously on their ipsilesional limb. We hypothesized that mechanisms that bind personal and peripersonal representations would improve these patients' awareness of being touched on their contralesional limbs. Visual--tactile integration and intentional movements were considered candidate mechanisms. Patients were more likely to be aware of contralesional touch when looking towards their contralesional limb than when looking towards their ipsilesional limb, and when actively moving on tactile probes than when receiving tactile stimuli passively. The improved awareness of being touched on the contralesional limb under these conditions suggests that cross-sensory and sensorimotor integration help bind personal and peripersonal space.     

Visuo-tactile integration in personal space

Integration of information across sensory modalities is enhanced when stimuli in both modalities are in the same location. This "spatial rule" of multisensory integration has been primarily studied in humans by comparing stimuli located either in the same versus opposite side of the body midline or in peripersonal versus extrapersonal space, both of which involve large, categorical differences in spatial location. Here we used psychophysics and ERPs to investigate visuo-tactile integration in personal space (i.e., on the skin surface). We used the mirror box technique to manipulate the congruence of visual and tactile information about which finger on either the right or left hand had been touched. We observed clear compatibility effects for both visual and tactile judgments of which finger on the left hand had been touched. No such effects, however, were found for judgments about the right hand. ERP data showed a similar pattern. Amplitude of the vertex P200 potential was enhanced and that of the N2 was reduced for congruent visuo-tactile events on the left, but not the right, hand. Similarly, a later positivity over posterior parietal cortices (P300) showed contralateral enhancement for congruent visuo-tactile events on both the left and right hands. These results provide clear evidence for spatial constraints on visuo-tactile integration defined in personal space and also reveal clear lateralization of these effects. Furthermore, these results link these "ultraprecise" spatial constraints to processing in the right posterior parietal cortex.     

Hemispatial somatosensory and motor extinction after stereotactic thalamic lesions

Abstract

We report the case of a man (DN) who suffered a right hemisphere stroke which resulted In left hemiparesis, and in a dense left hemisensory loss when he was tested without visual feedback. By contrast, when DN saw and ‘believed’ that he was being touched, he invariably reported that he felt the appropriate tactile sensation. In this latter condition, DN could reliably estimate the pressure with which light touch was applied to his left hand. Furthermore, there was reliable transfer of tactile sensation from the left to the right hand without conscious awareness of touch on the left. Some of these findings are akin to previous reports of acquired synaesthesia; one dissimilarity is that in both developmental and acquired synaesthesia the relationship between the perceptual content experienced in the two modalities is arbitrary. An alternative Interpretation of DN's performance is that it is determined by the output of bimodal visual-somatosensory cells that have been partially deprived of their somatosensory inputs. When limited tactile information is available to DN, correlated visual information may boost subthreshold tactile stimulation Into conscious awareness; in some conditions, vision alone produces reports of tactile sensation from DN.     

Anatomical localization revealed by MEG recordings of the human somatosensory system.

A 14-channel cryogenic magnetometer system (BTi) was used to record the magnetic fields over the left hemisphere of 3 human subjects in order to locate the sources of responses to tactile stimulation of the index, the thumb and the little finger of the right hand. The locations of the active dipole sources determined using the spherical model were then projected onto the magnetic resonance image (MRI) of the individual subjects providing an anatomical localization. The MRI slices were also used to construct a 3-dimensional image to enhance visualization of the area of the calculated sources. The locations of the dipole sources from the 3 fingers were distinct from one another in all subjects. An analysis of variance ('ANOVA') showed the most significant (P less than 0.05) difference in source location between the little finger and the thumb with the former being superior to the sources of the other 2 fingers in all of the subjects. In all cases, the sources were found to be located on the postcentral gyrus. The strength of the equivalent dipole sources and the amplitudes of the responses to stimulation for all 3 fingers showed a consistent trend among all of the 3 subjects, with the thumb having the largest response. In general, no signs of habituation were found.     

The spatial distribution of attentional selectivity in touch: evidence from somatosensory ERP components.

OBJECTIVE: Somatosensory event-related brain potentials (ERPs) were measured to investigate the spatial distribution of selective attention in touch, and whether the focus of tactile attention can be split between non-contiguous areas of the body surface. METHODS: On each trial, vibratory tactile stimuli were delivered to one of 4 possible locations of the right hand. Participants had to attend to either one or two locations in order to detect infrequently presented target stimuli there. ERPs were recorded to tactile non-targets at attended and unattended locations. RESULTS: Attention directed to one finger versus another was reflected by amplitude modulations of the sensory-specific P100 component and a subsequent attentional negativity (Nd). These effects were smaller for within-finger as compared to between-finger selection. When attention was directed simultaneously to non-adjacent fingers, ERPs in response to stimuli delivered to spatially and anatomically intervening fingers showed no attentional modulations whatsoever. CONCLUSIONS: Allocating tactile-spatial attention to one finger versus another affects early modality-specific somatosensory processing stages, and these effects of within-hand attentional selectivity decrease gradually with increasing distance from the current attentional focus. Unlike vision, the focus of tactile attention can be split, and directed simultaneously to non-adjacent areas, thus excluding spatially and anatomically intermediate regions from attentional processing.     

Viewing touch improves tactile sensory threshold

Previous research on multisensory integration has demonstrated that viewing the stimulated body part enhances discrimination ability. Participants in this experiment watched a video showing a hand being touched by a stick and a second video showing the stick touching the space beneath the hand. Sensory thresholds of the index fingers were tested with von Frey filaments. We found significant enhancements of the sensory threshold after showing the video with the touched hand but not after showing the video with no touch of the hand. This enhancement was specific for the index finger shown in the video. The results link the visuotactile enhancement of this study to the observation of touch rather than to the simple depiction of the body part.     

Sensory and response interference by ipsilesional stimuli in tactile extinction

Extinction is thought to be due to a pathologically limited attentional capacity in which multiple stimuli cannot be processed simultaneously to conscious awareness. Patients with tactile extinction are aware of being touched on a contralesional limb, but seem unaware of similar contralesional touch if touched simultaneously on their ipsilesional limb. The ipsilesional stimulus interferes and competes with the processing of the contralesional stimulus. Most theorists assume that the ipsilesional stimulus affects the sensory processing of the contralesional stimulus, although the precise functional level at which this interference occurs is not clear. We report a series of experiments using signal detection analyses to investigate tactile extinction in one patient (DC). These analyses revealed that ipsilesional stimuli, in addition to interfering with processing of contralateral sensations, also interfere with verbal reports of those sensations. This influence on responses suggests that interference in tactile extinction can occur at a post-perceptual level, further 'downstream' than previously thought.     

Manual approach during hand gesture imitation

BACKGROUND: Patients' tendency to draw near or into the target when copying figures, a phenomenon termed closing-in, has been previously described. That the closing-in could occur when copying hand gestures has also been noted. OBJECTIVES: To study a patient with corticobasal degeneration to quantify his manual approach behavior and to test a possible working memory hypothesis. METHODS: The subject of this study is a patient with severe ideomotor apraxia from probable corticobasal degeneration. Fluorine 18-labeled deoxyglucose-positron emission tomographic findings revealed a hypometabolism involving the bilateral parietotemporal and the right frontal lobes. When asked to copy an examiner's (J.C.K.) hand gesture, the patient approached, touched, or grasped the examiner's hand, a behavior mostly consistent with the closing-in behavior previously proposed. To investigate the frequency and severity of closing-in, the patient was asked to copy 20 meaningless hand gestures (10 simple and 10 complex). Copying the 20 hand gestures was performed with either the left or the right hand while the patient was seated opposite the examiner (across condition) or on the same side of the examiner (lateral condition). RESULTS: Of the 80 trials, closing-in occurred in 43 (53.8%) (35 with approaching, 6 with touching, and 2 with grasping). The closing-in was more frequent and more severe when gesturing with the left than the right hand, but it did not differ between the lateral and across conditions and between simple and complex gestures. CONCLUSIONS: Corticobasal degeneration might be associated with aberrant manual approach behavior. Although our results do not support the working memory hypothesis, frontal dysfunction might have led to a loss of voluntary control of ontologically primitive propensity to move the forelimb in the direction to which one attends.     

Finger pressure adjustments to various object configurations during precision grip in humans and monkeys

In this study, we recorded the pressure exerted onto an object by the index finger and the thumb of the preferred hand of 18 human subjects and either hand of two macaque monkeys during a precision grasping task. The to‐be‐grasped object was a custom‐made device composed by two plates which could be variably oriented by a motorized system while keeping constant the size and thus grip dimension. The to‐be‐grasped plates were covered by an array of capacitive sensors to measure specific features of finger adaptation, namely pressure intensity and centroid location and displacement. Kinematic measurements demonstrated that for human subjects and for monkeys, different plate configurations did not affect wrist velocity and grip aperture during the reaching phase. Consistently, at the instant of fingers‐plates contact, pressure centroids were clustered around the same point for all handle configurations. However, small pressure centroid displacements were specifically adopted for each configuration, indicating that both humans and monkeys can display finger adaptation during precision grip. Moreover, humans applied stronger thumb pressure intensity, performed less centroid displacement and required reduced adjustment time, as compared to monkeys. These pressure patterns remain similar when different load forces were required to pull the handle, as ascertained by additional measurements in humans. The present findings indicate that, although humans and monkeys share common features in motor control of grasping, they differ in the adjustment of fingertip pressure, probably because of skill and/or morphology divergences. Such a precision grip device may form the groundwork for future studies on prehension mechanisms.     

A new means of measuring index/ring finger (2D:4D) ratio and its association with gender and hand preference

Relative finger length can predict a person's gender or their hand preference. We measured finger length using a new "tubes" test, which required participants to slide a clear plastic tube over their fingers and read the length from an attached mm scale. Data collected from 600 students demonstrated that the right fingers are longer than the left for dextrals, but not for non-dextrals. Examination of the relative length of the index and ring fingers revealed a clear gender effect. There was also an effect of hand preference on index/ring finger ratio whereby non-dextrals showed a more masculine pattern compared to dextrals. For non-dextrals, prenatal exposure to high testosterone levels may have caused both a shift away from dextrality and a more masculine pattern of finger ratio. In the second experiment, finger length was measured by the tubes test and by photocopying the hands in 124 undergraduates. The tubes test yielded a longer estimate of ring finger length compared to the photocopy method. Despite this, there was a strong correlation between the tests and both showed an association with gender. Finally, test-retest scores for 45 participants showed a high level of reliability for absolute and relative finger measures. We conclude that the tubes technique provides an effective and easy-to-use means of measuring finger length, which can be administered in a classroom setting.     

Integration of tactile input across fingers in a patient with finger agnosia

Finger agnosia has been described as an inability to explicitly individuate between the fingers, which is possibly due to fused neural representations of these fingers. Hence, are patients with finger agnosia unable to keep tactile information perceived over several fingers separate? Here, we tested a finger agnosic patient (GO) on two tasks that measured the ability to keep tactile information simultaneously perceived by individual fingers separate. In experiment 1 GO performed a haptic search task, in which a target (the absence of a protruded line) needed to be identified among distracters (protruded lines). The lines were presented simultaneously to the fingertips of both hands. Similarly to the controls, her reaction time decreased when her fingers were aligned as compared to when her fingers were stretched and in an unaligned position. This suggests that she can keep tactile input from different fingers separate. In experiment two, GO was required to judge the position of a target tactile stimulus to the index finger, relatively to a reference tactile stimulus to the middle finger, both in fingers uncrossed and crossed position. GO was able to indicate the relative position of the target stimulus as well as healthy controls, which indicates that she was able to keep tactile information perceived by two neighbouring fingers separate. Interestingly, GO performed better as compared to the healthy controls in the finger crossed condition. Together, these results suggest the GO is able to implicitly distinguish between tactile information perceived by multiple fingers. We therefore conclude that finger agnosia is not caused by minor disruptions of low-level somatosensory processing. These findings further underpin the idea of a selective impaired higher order body representation restricted to the fingers as underlying cause of finger agnosia.     

Spatial coordinate systems for tactile spatial attention depend on developmental vision: evidence from event-related potentials in sighted and congenitally blind adult humans

Changes in limb posture (such as crossing the hands) can impair people's performance in tasks such as those involving temporal order judgements, when one tactile stimulus is presented to either hand. This crossed hands deficit has been attributed to a conflict between externally and anatomically anchored reference systems when people localize tactile stimuli. Interestingly, however, the performance of congenitally blind adults does not seem to be affected by crossing the hands, suggesting a default use of an anatomically rather than an externally anchored reference system for tactile localization. In the present study, 12 congenitally blind and 12 sighted adults were instructed to attend to either the left or the right hand on a trial-by-trial basis in order to detect rare deviants (consisting of a double touch) at that hand, while ignoring both deviants at the other hand and frequent standard stimuli (consisting of a single touch) presented to either hand. Only the sighted participants performed less accurately when they crossed their hands. Concurrent electroencephalogram recordings revealed an early contralateral attention positivity, followed by an attention negativity in the sighted group when they adopted the uncrossed hands posture. For the crossed hand posture, only the attention negativity was observed with reduced amplitude in the sighted group. By contrast, the congenitally blind group displayed an event-related potential attention negativity that did not vary when the posture of their hands was changed. These results demonstrate that the default use of an external frame of reference for tactile localization seems to depend on developmental vision.     

Evaluation of the dynamics of sensory improvement in the hand after surgical treatment of carpal tunnel syndrome

BACKGROUND AND PURPOSE: The aim of the study was to evaluate the dynamics of improvement of tactile sensory threshold and two-point discrimination in fingers innervated by the median nerve after surgical treatment of carpal tunnel syndrome (CTS), to assess differences between improvements of both aforementioned sensory measures, as well as to estimate the time taken to achieve ultimate sensory improvement in the hand. MATERIAL AND METHODS: The study included 87 patients (78 females and 9 males) operated on at the Department of Traumatology and Hand Surgery, Medical Academy in Wroclaw between July 2002 and August 2004 because of carpal tunnel syndrome. Mean age of patients was 51.8 years. The study included evaluation of tactile sensory (pressure) threshold and two-point discrimination. Measurements were performed seven times in each patient, that is, before the surgery, and then on the second, fifth and tenth postoperative day as well as one, three and six months after the surgery. RESULTS: There were only minor differences in improvement of tactile sensory threshold and two-point discrimination between particular fingers, but in each case less favourable results were achieved in the thumb and the middle finger and the best results in the ring finger. Over time the observed degree of sensory improvement decreased, and during the last follow-up mean values of tactile sensory threshold and two-point discrimination in each of the studied fingers were within the normal range. CONCLUSIONS: After surgical treatment of CTS normalisation of two-point discrimination occurs earlier than normalisation of tactile sensory threshold. The highest degree of improvement of two-point discrimination is observed during the first two days after surgery, while the highest degree of improvement of tactile sensory threshold is noted between the 10th and 30th postoperative day. No further significant improvement of tactile sensory threshold or two-point discrimination is observed following an initial period of three months after surgery.     

Residual tactile sensitivity with self-directed stimulation in hemianaesthesia.

A case of hemianaesthesia caused by right hemisphere stroke was found to have good sensitivity and cutaneous localisation when she touched her impaired hand with her good contralateral hand or with a probe held by that hand. The capacity did not depend solely upon movement or positioning of her right arm, and she could also discriminate whether her own left fingers, as opposed to someone else's inter-digitated fingers, were being touched, but only when she did the touching. Thus, the threshold of the impaired hand appeared to be markedly affected by active involvement of the contralateral limb in delivering the stimulus.