The neural signal for the intensity of a tactile stimulus.

The effect of indenting the skin at different rates on the perceived intensity of the stimulus was studied by indenting the skin of the fingertip with two triangular waveforms, given as a pair. The subjects were asked to judge which member of the pair was more intense. Perceived intensity was found to increase both with the depth and the speed of the indentation. In contrast, changes in the rate of skin indentation had little influence on perceived skin indentation depth. This suggests that intensity and depth are different attributes of tactile sensibility. Since the skin is viscous, a rapid indentation is more forceful than a slow indentation of the same depth, raising the possibility that perceived intensity is related to stimulus force. Even though intensity judgments were more closely correlated with the force of a stimulus than with the indentation it produced, a rapidly increasing force was felt as more intense than one that increased more slowly but attained the same final magnitude. When mechanoreceptors in the palmar aspect of the monkey's hand were excited with triangular stimuli like those used in the psychophysical studies, their discharge frequency increased with the rate of skin indentation. However, the receptors were distinctly more rate sensitive than the human judgments of stimulus intensity, suggesting that impulse summation in the central nervous system summates (integrates in the mathematical sense) the receptor input so as to enhance, relatively, the perceived intensity of the slower stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)