Tactile Localization Research Articles

Reaching with one arm to the other: Coordinating touch, proprioception, and action during infancy

Reaching to target locations on the body has been studied little despite its importance for adaptive behaviors such as feeding, grooming, and indicating a source of discomfort. This behavior requires multisensory integration given that it involves coordination of touch, proprioception, and sometimes vision as well as action. Here we examined the origins of this skill by investigating how infants begin to localize targets on the body and the motor strategies by which they do so. Infants (7–21 months of age) were prompted to reach to a vibrating target placed at five arm/hand locations (elbow, crook of elbow, forearm, palm, and top of hand) one by one. To manually localize the target, infants needed to reach with one arm to the other. Results suggest that coordination increases with age in the strategies that infants used to localize body targets. Most infants showed bimanual coordination and usually moved the target arm toward the reaching arm to assist reaching. Furthermore, intersensory coordination increased with age. Simultaneous movements of the two arms increased with age, as did coordination between vision and reaching. The results provide new information about the development of multisensory integration during tactile localization and how such integration is linked to action.

Thermal-Tactile Integration in Object Temperature Perception.

The brain consistently faces a challenge of whether and how to combine the available information sources to estimate the properties of an object explored by hand. While object perception is an inference process involving multisensory inputs, thermal referral (TR) is an illusion demonstrating how the interaction between thermal and tactile systems can lead to deviations from physical reality-when observers touch three stimulators simultaneously with the middle three fingers of one hand but only the outer two stimulators are heated (or cooled), thermal uniformity is perceived across three fingers. Here, we used TR of warmth to examine the thermal-tactile interaction in object temperature perception. We show that TR is consistent with precision-weighted averaging of thermal sensation across tactile locations. Furthermore, we show that prolonged contact with TR stimulation results in adaptation to the local variations of veridical temperatures instead of the thermal uniformity perceived across three fingers. Our results illuminate the flexibility of processing that underlies thermal-tactile interactions and serve as a basis for thermal display design.

Dynamic Displacement Vector Interacts with Tactile Localization

SummaryLocating a tactile stimulus on the body seems effortless and straightforward. However, the perceived location of a tactile stimulation can differ from its physical location [1, 2, 3]. Tactile mislocalizations can depend on the timing of successive stimulations [2, 4, 5], tactile motion mechanisms [6], or processes that “remap” stimuli from skin locations to external space coordinates [7, 8, 9, 10, 11]. We report six experiments demonstrating that the perception of tactile localization on a static body part is strongly affected by the displacement between the locations of two successive task-irrelevant actions. Participants moved their index finger between two keys. Each keypress triggered synchronous tactile stimulation at a randomized location on the immobilized wrist or forehead. Participants reported the location of the second tactile stimulation relative to the first. The direction of either active finger movements or passive finger displacements biased participants’ tactile orientation judgements (experiment 1). The effect generalized to tactile stimuli delivered to other body sites (experiment 2). Two successive keypresses, by different fingers at distinct locations, reproduced the effect (experiment 3). The effect remained even when the hand that moved was placed far from the tactile stimulation site (experiments 4 and 5). Temporal synchrony within 600 ms between the movement and tactile stimulations was necessary for the effect (experiment 6). Our results indicate that a dynamic displacement vector, defined as the location of one sensorimotor event relative to the one before, plays a strong role in structuring tactile spatial perception.

Biases in tactile localization by pointing: compression for weak stimuli and centering for distributions of stimuli.

Weak electrocutaneous stimuli applied to the forearm are erroneously localized toward its middle (Steenbergen P, Buitenweg JR, Trojan J, Veltink PH. Exp Brain Res 232: 597-607, 2014). We asked whether mechanical touch stimuli exhibit a similar bias and whether the bias is toward the middle of the forearm or toward the middle of the recent stimulus distribution. In experiments 1 and 2, participants ( n = 12 and n = 10) localized by pointing von Frey filaments applied to four locations on the dorsal forearm. Individually adjusted weak and strong stimuli ( experiment 1) or two levels of strong stimuli ( experiment 2) were presented in single sessions in random order. Weaker stimuli were localized with greater variability than the strong, with compression toward the middle of the forearm. Responses to the two levels of strong stimuli did not differ. In experiment 3, participants ( n = 16) were presented two spatially offset stimulus distributions (8 cm center-to-center), each offset from the forearm middle, on 2 different days. Out of four target locations comprising each distribution, two were shared. Responses to weak stimuli were compressed compared with responses to strong stimuli. Importantly, biases for the common targets had opposite directions, each being toward the middle of the distribution within which targets were presented. Responses to strong stimuli also exhibited a distribution-dependent bias, a 2-cm overall shift across the forearm midpoint. We conclude that touch localization is subject to intensity-dependent biases determined by the recent history of stimulation and possibly also by the available or perceived response space. NEW & NOTEWORTHY Recent findings show that weak electrical stimuli applied to the forearm are mislocalized toward the forearm middle, relative to strong stimuli. We found that weak mechanical stimuli are similarly mislocalized. The bias changed if, as a group, stimuli were not centered on the forearm middle: weak stimuli gravitated toward the center of prior stimulation. Localization of strong stimuli was also biased, consistent with the tendency to center responses within the available response space.

Implied tactile motion: Localizing dynamic stimulations on the skin.

We report two experiments designed to investigate how the implied motion of tactile stimuli influences perceived location. Predicting the location of sensory input is especially important as far as the perception of, and interaction with, the external world is concerned. Using two different experimental approaches, an overall pattern of localization shifts analogous to what has been described previously in the visual and auditory modalities is reported. That is, participants perceive the last location of a dynamic stimulus further along its trajectory than is objectively the case. In Experiment 1, participants judged whether the last vibration in a sequence of three was located closer to the wrist or to the elbow. In Experiment 2, they indicated the last location on a ruler attached to their forearm. We further pinpoint the effects of implied motion on tactile localization by investigating the independent influences of motion direction and perceptual uncertainty. Taken together, these findings underline the importance of dynamic information in localizing tactile stimuli on the skin.

Altered tactile localization and spatiotemporal integration in complex regional pain syndrome patients

In complex regional pain syndrome (CRPS), altered perception of the affected hand and neglect-like symptoms of the affected body side are common features. In this study, we presented tactile stimuli to the affected hands in CRPS patients and matched healthy controls. The participants' task was to point at the perceived positions of the stimuli using a tracking device. The spatial coordinates of the perceived positions were analysed for accuracy and consistency. We also presented patterns consisting of two stimuli at distinct positions with a delay of 100ms. These patterns are known to induce spatiotemporal integration ("sensory saltation"). CRPS patients were less accurate and less consistent in the spatial perception of tactile stimuli on their hands. Furthermore, they showed increased spatiotemporal integration, although these effects were smaller than expected. These deficiencies were related to the clinically assessed intensity of recurrent pain episodes. Surprisingly, the intensity of clinically assessed ongoing pain was associated with increased precision. In line with earlier reports, our findings indicate that the representation of the affected hands in CRPS patients is less accurate than in healthy people, probably reflecting reorganization in somatosensory cortices. The exact relationships between these findings and other aspects of CRPS remain to be elucidated. CRPS patients performed poorly in localizing positions on their affected hands via pointing and exhibited increased spatiotemporal integration. The presented method may prove useful in diagnostics as well as psychophysical and neurofunctional research on CRPS and other chronic pain disorders.

More than skin-deep: Integration of skin-based and musculoskeletal reference frames in localization of touch.

The skin of the forearm is, in one sense, a flat 2-dimensional (2D) sheet, but in another sense approximately cylindrical, mirroring the 3-dimensional (3D) volumetric shape of the arm. The role of frames of reference based on the skin as a 2D sheet versus based on the musculoskeletal structure of the arm remains unclear. When we rotate the forearm from a pronated to a supinated posture, the skin on its surface is displaced. Thus, a marked location will slide with the skin across the underlying flesh, and the touch perceived at this location should follow this displacement if it is localized within a skin-based reference frame. We investigated, however, if the perceived tactile locations were also affected by the rearrangement in underlying musculoskeletal structure, that is, displaced medially and laterally on a pronated and supinated forearm, respectively. Participants pointed to perceived touches (Experiment 1), or marked them on a (3D) size-matched forearm on a computer screen (Experiment 2). The perceived locations were indeed displaced medially after forearm pronation in both response modalities. This misperception was reduced (Experiment 1), or absent altogether (Experiment 2) in the supinated posture when the actual stimulus grid moved laterally with the displaced skin. The grid was perceptually stretched at medial-lateral axis, and it was displaced distally, which suggest the influence of skin-based factors. Our study extends the tactile localization literature focused on the skin-based reference frame and on the effects of spatial positions of body parts by implicating the musculoskeletal factors in localization of touch on the body.

Women with Anorexia Nervosa do not show altered tactile localization compared to healthy controls

Body image disturbance is a key symptom of Anorexia Nervosa (AN). Previous studies found that women with AN overestimate their body size in comparison with healthy controls (HC), at least for unimodal measures involving either only visual input (e.g. distorted photographs technique) or only tactile input (e.g. tactile distance tasks). Distorted body representations are hypothesized to cause this misperception in AN. We here tested whether this overestimation remains present in a novel one-point-localization (OPL) task involving the mapping of a tactile stimulus onto a visual image. Two experiments compared the ability of 27 women with AN and 40 HC to accurately localize a tactile stimulus on a live image of their body. Women with AN and HC did not differ in their performance. Instead, participants in both groups showed systematic distortions in their localization performance. This study suggests that the mapping of a tactile stimulus does not involve a distorted body representation in women with AN compared to HC.

The Roles of Attentional Shifts and Attentional Reengagement in Resolving The Spatial Compatibility Effect in Tactile Simon-like Tasks

The Simon effect refers to the acceleration of choice responses when the target position and response location are consistent compared with scenarios in which they are inconsistent, even if the target position is not relevant to the response. Here, we provide the first demonstration that the tactile Simon-like effect operates in an attention-shifting manner. In unimodal scenarios (Experiments 1–4), for the tactile direction task, the spatial compatibility effect was absent in the focused-attention condition but maintained in the divided-attention condition. For the tactile localization task, this pattern was reversed: the spatial compatibility effect occurred for the focused-attention condition but was reduced/absent in the divided-attention condition. In the audiotactile interaction scenario (Experiment 5), the reaction times (RTs) for discriminating the tactile motion direction were prolonged; however, a spatial compatibility effect was not observed. We propose that the temporal course of resolving conflicts between spatial codes during attentional shifts, including attentional reengagement, may account for the tactile Simon-like effect.

A mechanism for spatial perception on human skin

Our perception of where touch occurs on our skin shapes our interactions with the world. Most accounts of cutaneous localisation emphasise spatial transformations from a skin-based reference frame into body-centred and external egocentric coordinates. We investigated another possible method of tactile localisation based on an intrinsic perception of ‘skin space’. The arrangement of cutaneous receptive fields (RFs) could allow one to track a stimulus as it moves across the skin, similarly to the way animals navigate using path integration. We applied curved tactile motions to the hands of human volunteers. Participants identified the location midway between the start and end points of each motion path. Their bisection judgements were systematically biased towards the integrated motion path, consistent with the characteristic inward error that occurs in navigation by path integration. We thus showed that integration of continuous sensory inputs across several tactile RFs provides an intrinsic mechanism for spatial perception.

A Multi-Axis Tactile Sensor Array for Touchscreen Applications

Touchscreens have been prevalent in daily life and ubiquitously applied in consuming electronics, industrial control systems, and other applications as human computer interfaces (HCIs), which offers a convenient way for the human to interact with the smart devices. However, the lack of tactile force feedback from these conventional touchscreens draws limitations on the dexterity and intuitiveness of those devices, which results in multi-level menus, waiting, multi-finger gestures, and so on. To enhance and diversify functions of touchscreens, this paper presents a multi-axis tactile sensor array prototype with a unique layered structure, which is capable of sensing both 3-directional tactile force and location information over an area of 60 mm $\times60$ mm by utilizing only $2\times2$ piezoresistive MEMS force sensors. This paper integrated the sensibility of tangential force and normal force within one system, which sheds light on multi-axis tactile applications and dramatically reduced the number of tactile cells. The sensor array design, fabrication and packaging, and test approaches have been discussed in this paper. Qualitative and quantitative analysis have been conducted to evaluate the performance of the tactile sensor array with tested force range of 0.1 ~ 0.5 N. The results demonstrated the functionality of proposed sensor array, which exhibited promising potential for touchscreen applications. [2017-0176]

Mind the Gap: The Effects of Temporal and Spatial Separation in Localization of Dual Touches on the Hand.

In this study, we aimed to relate the findings from two predominantly separate streams of literature, one reporting on the localization of single touches on the skin, and the other on the distance perception of dual touches. Participants were touched with two points, delivered either simultaneously or separated by a short delay to various locations on their left hand dorsum. They then indicated on a size-matched hand silhouette the perceived locations of tactile stimuli. We quantified the deviations between the actual stimulus grid and the corresponding perceptual map which was constructed from the perceived tactile locations, and we calculated the precision of tactile localization (i.e., the variability across localization attempts). The evidence showed that the dual touches, akin to single touch stimulations, were mislocalized distally and that their variable localization error was reduced near joints, particularly near knuckles. However, contrary to single-touch localization literature, we observed for the dual touches to be mislocalized towards the ulnar side of the hand, particularly when they were presented sequentially. Further, the touches presented in a sequential order were slightly “repelled” from each other and their perceived distance increased, while the simultaneous tactile pairs were localized closer to each other and their distance was compressed. Whereas the sequential touches may have been localized with reference to the body, the compression of tactile perceptual space for simultaneous touches was related in the previous literature to signal summation and inhibition and the low-level factors, including the innervation density and properties of receptive fields (RFs) of somatosensory neurons.

Task demands affect spatial reference frame weighting during tactile localization in sighted and congenitally blind adults.

Task demands modulate tactile localization in sighted humans, presumably through weight adjustments in the spatial integration of anatomical, skin-based, and external, posture-based information. In contrast, previous studies have suggested that congenitally blind humans, by default, refrain from automatic spatial integration and localize touch using only skin-based information. Here, sighted and congenitally blind participants localized tactile targets on the palm or back of one hand, while ignoring simultaneous tactile distractors at congruent or incongruent locations on the other hand. We probed the interplay of anatomical and external location codes for spatial congruency effects by varying hand posture: the palms either both faced down, or one faced down and one up. In the latter posture, externally congruent target and distractor locations were anatomically incongruent and vice versa. Target locations had to be reported either anatomically (“palm” or “back” of the hand), or externally (“up” or “down” in space). Under anatomical instructions, performance was more accurate for anatomically congruent than incongruent target-distractor pairs. In contrast, under external instructions, performance was more accurate for externally congruent than incongruent pairs. These modulations were evident in sighted and blind individuals. Notably, distractor effects were overall far smaller in blind than in sighted participants, despite comparable target-distractor identification performance. Thus, the absence of developmental vision seems to be associated with an increased ability to focus tactile attention towards a non-spatially defined target. Nevertheless, that blind individuals exhibited effects of hand posture and task instructions in their congruency effects suggests that, like the sighted, they automatically integrate anatomical and external information during tactile localization. Moreover, spatial integration in tactile processing is, thus, flexibly adapted by top-down information—here, task instruction—even in the absence of developmental vision.

Tactile localization biases are modulated by gaze direction.

Identifying the spatial location of touch on the skin surface is a fundamental function of our somatosensory system. Despite the fact that stimulation of even single mechanoreceptive afferent fibres is sufficient to produce clearly localised percepts, tactile localisation can be modulated also by higher level processes such as body posture. This suggests that tactile events are coded using multiple representations using different coordinate systems. Recent reports provide evidence for systematic biases on tactile localisation task, which are thought to result from a supramodal representation of the skin surface. While the influence of non-informative vision of the body and gaze direction on tactile discrimination tasks has been extensively studied, their effects on tactile localisation tasks remain largely unexplored. To address this question, participants performed a tactile localization task on their left hand under different visual conditions by means of a mirror box; in the mirror condition, a single stimulus was delivered on participants' hand, while the reflexion of the right hand was seen through the mirror; in the object condition, participants looked at a box through the mirror, and in the right hand condition, participants looked directly at their right hand. Participants reported the location of the tactile stimuli using a silhouette of a hand. Results showed a shift in the localization of the touches towards the tip of the fingers (distal bias) and the thumb (radial biases) across conditions. Critically, distal biases were reduced when participants looked towards the mirror compared to when they looked at their right hand suggesting that gaze direction reduces the typical proximo-distal biases in tactile localization. Moreover, vision of the hand modulates the internal configuration of points' locations, by elongating it, in the radio-ulnar axis.

Memory Unscented Particle Filter for 6-DOF Tactile Localization

This paper addresses 6-DOF (degree-of-freedom) tactile localization, i.e. the pose estimation of tridimensional objects given tactile measurements. This estimation problem is fundamental for the operation of autonomous robots that are often required to manipulate and grasp objects whose pose is a-priori unknown. The nature of tactile measurements, the strict time requirements for real-time operation and the multimodality of the involved probability distributions pose remarkable challenges and call for advanced nonlinear filtering techniques. Following a Bayesian approach, this paper proposes a novel and effective algorithm, named Memory Unscented Particle Filter (MUPF), which solves the 6-DOF localization problem recursively in real-time by only exploiting contact point measurements. MUPF combines a modified particle filter that incorporates a sliding memory of past measurements to better handle multimodal distributions, along with the unscented Kalman filter that moves the particles towards regions of the search space that are more likely with the measurements. The performance of the proposed MUPF algorithm has been assessed both in simulation and on a real robotic system equipped with tactile sensors (i.e., the iCub humanoid robot). The experiments show that the algorithm provides accurate and reliable localization even with a low number of particles and, hence, is compatible with real-time requirements.

Evidence for frequency-dependent cortical plasticity in the human brain

Frequency-dependent plasticity (FDP) describes adaptation at the synapse in response to stimulation at different frequencies. Its consequence on the structure and function of cortical networks is unknown. We tested whether cortical "resonance," favorable stimulation frequencies at which the sensory cortices respond maximally, influenced the impact of FDP on perception, functional topography, and connectivity of the primary somatosensory cortex using psychophysics and functional imaging (fMRI). We costimulated two digits on the hand synchronously at, above, or below the resonance frequency of the somatosensory cortex, and tested subjects' accuracy and speed on tactile localization before and after costimulation. More errors and slower response times followed costimulation at above- or below-resonance, respectively. Response times were faster after at-resonance costimulation. In the fMRI, the cortical representations of the two digits costimulated above-resonance shifted closer, potentially accounting for the poorer performance. Costimulation at-resonance did not shift the digit regions, but increased the functional coupling between them, potentially accounting for the improved response time. To relate these results to synaptic plasticity, we simulated a network of oscillators incorporating Hebbian learning. Two neighboring patches embedded in a cortical sheet, mimicking the two digit regions, were costimulated at different frequencies. Network activation outside the stimulated patches was greatest at above-resonance frequencies, reproducing the spread of digit representations seen with fMRI. Connection strengths within the patches increased following at-resonance costimulation, reproducing the increased fMRI connectivity. We show that FDP extends to the cortical level and is influenced by cortical resonance.

Radial Distance Estimation with Tapered Whisker Sensors.

Rats use their whiskers as tactile sensors to sense their environment. Active whisking, moving whiskers back and forth continuously, is one of prominent features observed in rodents. They can discriminate different textures or extract features of a nearby object such as size, shape and distance through active whisking. There have been studies to localize objects with artificial whiskers inspired by rat whiskers. The linear whisker model based on beam theory has been used to estimate the radial distance, that is, the distance between the base of the whisker and a target object. In this paper, we investigate deflection angle measurements instead of forces or moments, based on a linear tapered whisker model to see the role of tapered whiskers found in real animals. We analyze how accurately this model estimates the radial distance, and quantify the estimation errors and noise sensitivity. We also compare the linear model simulation and nonlinear numerical solutions. It is shown that the radial distance can be estimated using deflection angles at two different positions on the tapered whisker. We argue that the tapered whisker has an advantage of estimating the radial distance better, as compared to an untapered whisker, and active sensing allows that estimation without the whisker’s material property and thickness or the moment at base. In addition, we investigate the potential of passive sensing for tactile localization.

The Sensitive Period for Tactile Remapping Does Not Include Early Infancy.

Visual input during development seems crucial in tactile spatial perception, given that late, but not congenitally, blind people are impaired when skin-based and tactile external representations are in conflict (when crossing the limbs). To test whether there is a sensitive period during which visual input is necessary, 14 children (age=7.95) and a teenager (LM; age=17.38) deprived of early vision by cataracts, and whose sight was restored during the first 5months and at age 7, respectively, were tested. Tactile localization with arms crossed and uncrossed was measured. Children showed a crossing effect indistinguishable from a control group (Ns=28, age=8.24), whereas LM showed no crossing effect (Ns controls=14, age=20.78). This demonstrates a sensitive period which, critically, does not include early infancy.

Afferent motor feedback determines the perceived location of tactile stimuli in the external space presented to the moving arm.

People make systematic errors when localizing a brief tactile stimulus in the external space presented on the index finger while moving the arm. Although these errors likely arise in the spatiotemporal integration of the tactile input and information about arm position, the underlying arm position information used in this process is not known. In this study, we tested the contributions of afferent proprioceptive feedback and predictive arm position signals by comparing localization errors during passive vs. active arm movements. In the active trials, participants were instructed to localize a tactile stimulus in the external space that was presented to the index finger near the time of a self-generated arm movement. In the passive trials, each of the active trials was passively replayed in randomized order, using a robotic device. Our results provide evidence that the localization error patterns of the passive trials are similar to the active trials and, moreover, did not lag but rather led the active trials, which suggests that proprioceptive feedback makes an important contribution to tactile localization. To further test which kinematic property of this afferent feedback signal drives the underlying computations, we examined the localization errors with movements that had differently skewed velocity profiles but overall the same displacement. This revealed a difference in the localization patterns, which we explain by a probabilistic model in which temporal uncertainty about the stimulus is converted into a spatial likelihood, depending on the actual velocity of the arm rather than involving an efferent, preprogrammed movement.NEW & NOTEWORTHY We show that proprioceptive feedback of arm motion rather than efferent motor signals contributes to tactile localization during an arm movement. Data further show that localization errors depend on arm velocity, not displacement per se, suggesting that instantaneous velocity feedback plays a role in the underlying computations. Model simulation using Bayesian inference suggests that these errors depend not only on spatial but also on temporal uncertainties of sensory and motor signals.

Tactile localization performance in children with developmental coordination disorder (DCD) corresponds to their motor skill and not their cognitive ability

When localizing touches to the hands, typically developing children and adults show a “crossed hands effect” whereby identifying which hand received a tactile stimulus is less accurate when the hands are crossed than uncrossed. This demonstrates the use of an external frame of reference for locating touches to one’s own body. Given that studies indicate that developmental vision plays a role in the emergence of external representations of touch, and reliance on vision for representing the body during action is atypical in developmental coordination disorder (DCD), we investigated external spatial representations of touch in children with DCD using the “crossed hands effect”. Nineteen children with DCD aged 7–11years completed a tactile localization task in which posture (uncrossed, crossed) and view (hands seen, unseen) were varied systematically. Their performance was compared to that of 35 typically developing controls (19 of a similar age and cognitive ability, and 16 of a younger age but similar fine motor ability). Like controls, the DCD group exhibited a crossed hands effect, whilst their overall tactile localization performance was weaker than their peers of similar age and cognitive ability, but in line with younger controls of similar motor ability. For children with movement difficulties, these findings indicate tactile localization impairments in relation to age expectations, but apparently typical use of an external reference frame for localizing touch.