Concurrent use of somatotopic and external reference frames in a tactile mislocalization task

Cost of Dual-Task Performance in Tactile Perception Is Greater for Competing Tasks of the Same Type

This study examines dual-task performance in the tactile modality and tests whether dual-task cost depends on task type. Experiment 1 involved competing tasks of the same type, using a primary localisation task on the left hand and a secondary localisation task on the right hand. In Experiment 2, the primary task on the left hand remained the same, while an intensity discrimination task was used as the secondary task on the right hand. Subjects in both experiments completed three conditions: the primary task alone, a dual-task condition, and the primary task with the secondary stimulus present but no response required. Across both experiments, performance on the primary task was best when it was presented alone, and there was a performance decrement when the secondary stimulus was present but not responded to. Performance on the primary task was further decreased when participants had to respond to the secondary stimulus, and the decrease was larger when the secondary task was localisation rather than discrimination. This result indicates that task type in the tactile modality may modulate the attentional cost of dual-task performance and implies partially shared resources underlie localisation and intensity discrimination.

Relative posture between head and finger determines perceived tactile direction of motion

The hand explores the environment for obtaining tactile information that can be fruitfully integrated with other functions, such as vision, audition, and movement. In theory, somatosensory signals gathered by the hand are accurately mapped in the world-centered (allocentric) reference frame such that the multi-modal information signals, whether visual-tactile or motor-tactile, are perfectly aligned. However, an accumulating body of evidence indicates that the perceived tactile orientation or direction is inaccurate; yielding a surprisingly large perceptual bias. To investigate such perceptual bias, this study presented tactile motion stimuli to healthy adult participants in a variety of finger and head postures, and requested the participants to report the perceived direction of motion mapped on a video screen placed on the frontoparallel plane in front of the eyes. Experimental results showed that the perceptual bias could be divided into systematic and nonsystematic biases. Systematic bias, defined as the mean difference between the perceived and veridical directions, correlated linearly with the relative posture between the finger and the head. By contrast, nonsystematic bias, defined as minor difference in bias for different stimulus directions, was highly individualized, phase-locked to stimulus orientation presented on the skin. Overall, the present findings on systematic bias indicate that the transformation bias among the reference frames is dominated by the finger-to-head posture. Moreover, the highly individualized nature of nonsystematic bias reflects how information is obtained by the orientation-selective units in the S1 cortex.

A common representation of fingers and toes

There are many similarities and differences between the human hands and feet. On a psychological level, there is some evidence from clinical disorders and studies of tactile localisation in healthy adults for deep functional connections between the hands and feet. One form these connections may take is in common high-level mental representations of the hands and feet. Previous studies have shown that there are systematic, but distinct patterns of confusion found between both the fingers and toes. Further, there are clear individual differences between people in the exact patterns of mislocalisations. Here, we investigated whether these idiosyncratic differences in tactile localisation are shared between the fingers and toes, which may indicate a shared high-level representation. We obtained confusion matrices showing the pattern of mislocalisation on the hairy skin surfaces of both the fingers and toes. Using a decoding approach, we show that idiosyncratic differences in individuals' pattern of confusions are shared across the fingers and toes, despite different overall patterns of confusions. These results suggest that there is a common representation of the fingers and toes.

A Conceptual Model of Tactile Processing across Body Features of Size, Shape, Side, and Spatial Location

The processing of touch depends of multiple factors, such as the properties of the skin and type of receptors stimulated, as well as features related to the actual configuration and shape of the body itself. A large body of research has focused on the effect that the nature of the stimuli has on tactile processing. Less research, however, has focused on features beyond the nature of the touch. In this review, we focus on some features related to the body that have been investigated for less time and in a more fragmented way. These include the symmetrical quality of the two sides of the body, the postural configuration of the body, as well as the size and shape of different body parts. We will describe what we consider three key aspects: (1) how and at which stages tactile information is integrated between different parts and sides of the body; (2) how tactile signals are integrated with online and stored postural configurations of the body, regarded as priors; (3) and how tactile signals are integrated with representations of body size and shape. Here, we describe how these different body dimensions affect integration of tactile information as well as guide motor behavior by integrating them in a single model of tactile processing. We review a wide range of neuropsychological, neuroimaging, and neurophysiological data and suggest a revised model of tactile integration on the basis of the one proposed previously by Longo et al.

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. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

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.

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.