Neural signal for skin indentation depth. II. Steady indentations

The glabrous skin of the monkey's hand was stimulated with a waveform that indented the skin at a rate of 0.4 mm/set, held the skin steadily or nearly steadily indented for 12 set or longer, and then retracted back to the starting position. Recordings were made of activity in single afferent fibers in response to these stimuli. The average discharge frequency of 21 slowly adapting mechanoreceptors declined 38% during the first 12 set of a steady indentation when the amplitude of the displacement was 0.65 mm and 36% when the displacement was 1.3 mm. When the plateau was not steady but the indentation depth gradually decreased by 15% during the 12-set plateau period, the average decline was 47% for the 0.65-mm indentation and 46% for the 1.3-mm stimulus. When the indentation depth gradually increased by 15% during the 12-set plateau, the discharge declined an average of 26% during the 0.65-mm indentation and 22% during the 1.3-mm displacement. To determine the effect of receptor adaptation on the perception of skin indentation depth, 13 human subjects had the skin of their fingertips indented 1 mm with similar trapezoidal waveform and were asked whether the indentation depth increased or decreased during the plateau portion of the stimulus. Ten of the 13 subjects thought that the indentation depth was increasing when the plateau was steady. The method of limits was then used to determine how much the stimulus had to change for the subject to feel the depth during the plateau as unchanging; i.e., a "perceptual zero." The average perceptual zero for the entire group occurred when the stimulator steadily retracted by 14% during the plateau. The subject whose indentation depth sensation adapted the most felt the plataeu to be steady when the stimulator gradually advanced by 15% during the plateau. A different group of 10 subjects traced the perceived depth of steady fingertip indentations which were 1 and 2 mm deep while the stimuli were actually in progress, and more than half traced a sensation of gradually increasing depth. The subject in this group whose depth sensation adapted the most showed a decline of 13% during a steady plateau 18 set long, as compared with the average discharge of our sample of slowly adapting monkey mechanoreceptors, which declined 45% during a comparable stimulus. Tracings were also made by these subjects of waveforms that remained steady for 2 to 4 set, partially retracted by 5 to 30%, and then reindented to the same depth. When the partial retractions were slow, the subjects thought the indentation depth increased with each repetition of the stimulus although the stimulator actually reindented to the same depth each time. The fact that human subjects tend to feel the depth of a steady indentation as increasing at a time when the discharge of their glabrous skin mechanoreceptor is declining could be explained if the adapting discharge were to be integrated (in the mathametical sense) by the central neural circuitry responsible for judgments of skin indentation depth. Such an integration process could also account for the increase in perceived depth during reindentation since present evidence indicates that receptor discharge declines rather than increases during repeated reindentations to the same depth.