Tactile distance anisotropy on the palm: A meta-analysis

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.