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. Additional evidence in favor of this suggestion came from experiments in which the skin of the fingertip was indented with a stimulus that advanced to a depth of 1 or 2 mm at a rate of 0.4 mm/set, remained steady for 18 set, and then retracted. Ten subjects traced the perceived intensity of these stimuli while they were in progress. During the l-mm indentation, 8 of the 10 subjects felt the intensity to decline less during the steady phase of the stimulus than did the average discharge of slowly adapting receptors in monkey glabrous skin, and some subjects actually felt an increase in intensity during the stimulus plateau. The discrepancy between perceived intensity and nerve impulse activity was still more pronounced during the 2-mm indentation, when only 1 of the 10 subjects felt a decline in intensity comparable to receptor adaptation. Again, the discrepancy between nerve impulse activity and perceived intensity could be explained by impulse summation in central neural circuitry. To test this possibility further, the fingertips of the same 10 subjects were indented with a stimulus that advanced to a depth of 1 or 2 mm at a rate of 0.4 mm/set, remained steady for 2 to 4 set, and then partially retracted at 0.33 to 0.06 mm/set. When the partial retraction amounted to 5 to 30% of the original excursion, the fingertip was reindented to the same depth, and the sequence began again. Tracings made by 9 of the 10 subjects showed a systematic overestimation error (wind-up) in which they thought the intensity increased during reindentation, although the stimulator actually reindented to the same depth each time. Since cutaneous mechanoreceptors fatigue rather than increase their discharge when repeatedly stimulated, the occurrence of intensity wind-up appears to require a central integrator. However, this neural integration is less pronounced for judgments of intensity than for judgments of skin indentation depth, and intensity integration may be weak or absent in certain subjects.