Tactile spatial resolution. II. Neural representation of Bars, edges, and gratings in monkey primary afferents

Abstract

Investigation of the manner in which stationary bars, edges and gratings are represented within the spatial-response profiles of mechanoreceptive afferents innervating monkey finger pad skin are presented. The data were obtained by stepping these stimuli in 200-.mu.m increments across the most sensitive spot in the receptive fields of 31 mechanoreceptive afferents. At each point the stimulus was indented into the skin to a depth of 1000 .mu.m for 1.0 s while the evoked response was recorded. The slowly adapting afferents were very sensitive to spatial discontinuities such as edges and their responses were suppressed by surrounding stimuli. The quickly adapting afferents showed none of these complex response features. Their responses were roughly proportional to the surface skin-displacement profile but they preserved only the coarsest spatial details. The responses to periodic gratings showed that only the slowly adapting afferents could account for the human ability to resolve fine spatial detail.