Neural mechanisms of scanned and stationary touch

Abstract

The neural mechanisms subserving the sense of touch set the limits for the acquisition of information regarding the spatial and temporal characteristics of stimuli impinging on the skin surface. The results of 3 different psychophysical experiments imply that the skin of the finger pad can resolve the elements of a stimulus separated by 0.9 mm when the stimulus is applied to the skin and held stationary. This resolution limit is only slightly improved (to about 0.7 mm) when movement between the stimulus and skin is allowed. Single-unit recordings from the 3 classes of primary mechanoreceptive afferents in anesthetized monkeys shows that only 1 class, the slowly adapting afferents, resolves spatial detail of stationary stimuli near the resolution limit. In addition, slow adaptors appear to resolve moving stimuli (e.g., Braille-dot patterns) more effectively than do the other 2 classes. Thee observations do not explain the extraordinary capacity of the finger-pad skin for discriminating between fine textures. Neurophysiological evidence suggests that information about such textures (i.e., surfaces with spatial details below the resolution limit) may be conveyed by a code based on the relative engagement of the 3 receptor populations.