A comparison of tactile spatial sensitivity on the palm and fingerpad.

Studies of tactile spatial pattern perception have, for the most part, been carried out using the fingerpad. On the basis of these studies, models have been developed linking spatial pattern identification and resolution with underlying neural structures. It has been suggested that with appropriate scaling, these models would apply to the processing of spatial patterns presented to other sites on the body. Spatial sensitivity was examined on another site on the body, the palm, using two measures, letter identification and grating orientation. The results from these measures were compared with results from similar studies conducted on the fingerpad and with estimates of the density of innervation of the fingerpad and palm. To produce levels of performance similar to those on the fingerpad required letters on the palm 50 mm in height, seven to nine times larger than those used on the fingerpad. Gratings had to be six to more than seven times larger on the palm to produce the same levels of performance achieved on the fingerpad. For the two types of receptor systems sensitive to spatial information, the ratio of density of innervation between the fingerpad and the palm is estimated to be 5.7:1 and 8.8:1. Performance of spatial tasks on the palm can be predicted quantitatively from fingerpad data with a moderate degree of accuracy. Qualitative comparisons between the palm and fingerpad data indicate that spatial patterns are processed similarly at the two sites.