Tactile spatial acuity varies with site and axis in the human upper limb

Historically, beginning with Weber's [E.H. Weber, On the sensitivity of the tactile senses, in: H.E. Ross, D.J. Murray (Eds. and Trans.), E.H. Weber on the Tactile Senses, Erlbaum (UK) Taylor & Francis, Hove, 1996 (Original work published in 1834), pp. 21-136] classical studies, regional variations in the accuracy of localisation of tactile stimuli applied to a limb have been recognised. However, important questions remain concerning both the map of localisation resolution and its neuroscientific basis since methodological confounds have militated against an unambiguous, unified interpretation of the diverse findings. To test the hypotheses that localisation precision on the upper limb varies with site (hand, wrist, forearm) and limb axis (transverse, longitudinal), regional differences in locognosic acuity were quantified in psychophysical experiments. Participants identified the perceived direction (e.g. medial or lateral) relative to a central reference locus of brief tactile test stimuli applied to a cruciform array of loci. Acuity was greater in the transverse than longitudinal axis. This effect probably arises from the asymmetry of receptive fields of upper limb first-order sensory units and their higher-order projection neurons. Additionally, acuity was greater on the dorsal surface at the wrist than either the hand or forearm sites, in the longitudinal axis, supporting an enhancement of resolution at joints (anchor points). This effect may contribute to improved proprioceptive guidance of active wrist movements.