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: (a) how and at which stages tactile information is integrated between different parts and sides of the body; (b) how tactile signals are integrated with online and stored postural configurations of the body, regarded as priors; (c) 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 behaviour 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 and colleagues (2010).

/pdf/a-conceptual-model-of-tactile-processing-across-body-3wuno7zp5g.pdf

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

Illusions of the perception of distance between two touches on the skin have been described since the classic work of Weber in the 19th century. The perceptual mechanisms underlying such spatial distortions, however, remain poorly understood. One potential interpretation is that the representational space of touch is related to the true structure of the skin by a geometrically simple stretch. If distortions of tactile distance perception reflect a simple stretch of tactile space, perceived distance should vary predictably as a function of the orientation of the stimulus on the skin, showing a sinusoidal pattern. Here, we tested this prediction by obtained judgments of perceived tactile distance for pairs of touches aligned with eight orientations on the skin. Across four experiments, the results were highly consistent with this prediction, showing no apparent deviation from a model of simple stretch of tactile space. Similar results were apparent on both the dorsum and palm of the hand, as well as the forehead. These results show that spatial distortions of touch are well characterized by a geometrically simple stretch of tactile space.

https://www.pnas.org/content/pnas/115/6/1238.full.pdf

Tactile distance illusions reflect a coherent stretch of tactile space

Two decades of research have demonstrated that using a tool modulates spatial representations of the body. Whether this embodiment is specific to representations of the tool-using limb or extends to representations of other body parts has received little attention. Several studies of other perceptual phenomena have found that modulations to the primary somatosensory representation of the hand transfers to the face, due in part to their close proximity in primary somatosensory cortex. In the present study, we investigated whether tool-induced recalibration of tactile perception on the hand transfers to the cheek. Participants verbally estimated the distance between two tactile points applied to either their hand or face, before and after using a hand-shaped tool. Tool use recalibrated tactile distance perception on the hand-in line with previous findings-but left perception on the cheek unchanged. This finding provides support for the idea that embodiment is body-part specific. Furthermore, it suggests that tool-induced perceptual recalibration occurs at a level of somatosensory processing, where representations of the hand and face have become functionally disentangled.

The recalibration of tactile perception during tool use is body-part specific

Illusions of the perceived distance between two touches on the skin have been studied since the classic work of Weber in the 19th century. For example, anisotropies of perceived tactile distance have been consistently found on several body parts, including the hand dorsum, the forearm, and the face. In each case, tactile distances that are oriented across body width are perceived as being larger than those oriented along body length. Several studies have investigated tactile distance anisotropy on the glabrous skin of the palm of the hand, but they have reached inconsistent conclusions-with some studies finding no anisotropy, and others finding an anisotropy analogous to that found on the dorsum. Given these inconsistencies, the aim of this study was to conduct a systematic meta-analysis of the existing data regarding anisotropy on the palm. A total of ten experiments were identified, which overall provided strong evidence for an anisotropy on the palm (Hedges's g = 0.521), with distances aligned with hand width being perceived as approximately 10% bigger than distances aligned with hand length. While this anisotropy is analogous to that found on the hand dorsum, it is substantially smaller in magnitude, and the two biases appear to be uncorrelated. The present results show that, despite inconsistent results across studies, the existing data do indicate an anisotropy of tactile distance on the palm of the hand.

/pdf/tactile-distance-anisotropy-on-the-palm-a-meta-analysis-1rcgl99ut8.pdf

Tactile distance anisotropy on the palm: A meta-analysis.

A growing literature shows that body posture modulates the perception of touch, as well as somatosensory processing more widely. In this study, I investigated the effects of changes in the internal postural configuration of the hand on the perceived distance between touches. In two experiments participants positioned their hand in two postures, with the fingers splayed (Apart posture) or pressed together (Together posture). In Experiment 1, participants made forced-choice judgments of which of two tactile distances felt bigger, one oriented with the proximal-distal hand axis (Along orientation) and one oriented with the medio-lateral hand axis (Across orientation). In Experiment 2, participants made verbal estimates of the absolute distance between a single pair of touches, in one of the two orientations. Consistent with previous results, there was a clear bias to perceive distances in the across orientation as larger than those in the along orientation. Perceived tactile distance was also modulated by posture, with increased judgments in both orientations when the fingers were splayed. These results show that changes in the internal posture of the hand modulate the perceived distance between touches on the hand, and add to a growing literature showing postural modulation of touch.

/pdf/hand-posture-modulates-perceived-tactile-distance-4c5qarl3ne.pdf

Hand Posture Modulates Perceived Tactile Distance

The stage at which processing of tactile distance occurs is still debated. We addressed this issue by implementing an adaptation-aftereffect paradigm with passive touch. We demonstrated the presence of a strong aftereffect, induced by the simultaneous presentation of pairs of tactile stimuli. After adaptation to two different distances, one on each hand, participants systematically perceived a subsequent stimulus delivered to the hand adapted to the smaller distance as being larger. We further investigated the nature of the aftereffects, demonstrating that they are orientation- and skin-region–specific, occur even when just one hand is adapted, do not transfer either contralaterally or across the palm and dorsum, and are defined in a skin-centered, rather than an external, reference frame. These characteristics of tactile distance aftereffects are similar to those of low-level visual aftereffects, supporting the idea that distance perception arises at early stages of tactile processing.

https://www.pnas.org/content/pnas/114/17/4555.full.pdf

Matthew Danvers

Papers

Adaptation aftereffects reveal that tactile distance is a basic somatosensory feature