Abstract
Perceiving the external spatial location of touch requires that tactile information about the stimulus location on the skin be integrated with proprioceptive information about the location of the body in external space, a process called tactile spatial remapping. Recent results have suggested that this process relies on a distorted representation of the hand. Here, I investigated whether similar distortions are also found on the forearm and how they are affected by the presence of the wrist joint, which forms a categorical, segmental boundary between the hand and the arm. Participants used a baton to judge the perceived location of touches applied to their left hand or forearm. Similar distortions were apparent on both body parts, with overestimation of distances in the medio-lateral axis compared to the proximo-distal axis. There was no perceptual expansion of distances that crossed the wrist boundary. However, there was increased overestimation of distances near the wrist in the medio-lateral orientation. These results replicate recent findings of a distorted representation of the hand underlying tactile spatial remapping, and show that this effect is not idiosyncratic to the hand, but also affects the forearm. These distortions may be a general characteristic of the mental representation of the arms.
Highlights
Several classes of afferent signals provide information about the current postural configuration of the body, including receptors in joints, in muscle spindles, and in the skin signaling skin stretch (Burgess et al, 1982; Proske and Gandevia, 2009, 2012)
In three recent studies (Mattioni and Longo, 2014; Longo et al, 2015b; Longo and Morcom, 2016), we have extended this hand-mapping paradigm to investigate the integration of proprioceptive and tactile information involved in localizing touch in external space, a process known as tactile spatial remapping (Yamamoto and Kitazawa, 2001; Azañón and SotoFaraco, 2008; Bremner et al, 2008; Heed and Azañón, 2014; Heed et al, 2015)
Longo and Morcom (2016) replicated this result using a 4x4 grid of points. These results suggest that tactile spatial remapping relies on a distorted representation of the hand, wider and squatter than its actual shape, a pattern broadly similar to the pattern found for position sense alone, described in the previous paragraph
Summary
Several classes of afferent signals provide information about the current postural configuration of the body, including receptors in joints, in muscle spindles, and in the skin signaling skin stretch (Burgess et al, 1982; Proske and Gandevia, 2009, 2012). Efferent copies of motor commands provide information used to determine current limb position (Gandevia et al, 2006; Walsh et al, 2013). Each of these signals provides information about the angles of joints, rather than their absolute location in space. Raw afferent information specifying joint angles is converted into a representation of absolute position in egocentric space. Calculating the absolute spatial location of part of the body requires that these signals specifying angular information be combined with information about the size and shape of the body segments between
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