Abstract
Indentations of the needle from the skin surface to the depth where prick pain sensation is first felt, indentation threshold, have been measured as a parameter of the threshold of pain sensation of human subjects.
When the skin is stimulated spatially at random, the distribution of indentation threshold becomes unimodal and of skewed distribution. Applying psychometrical technique, it was found that higher threshold points have weeker pain intensity. Experiments on various indentation velocities showed that faster indentations generate sharp pain sensation but higher threshold amplitude, while slower indentations generate weak pain sensation and lower threshold. This difference of threshold amplitude means that there must be some factors of lag time. In the case of stepwise indentation in short period, the influence of such indentation fluctuation was not significantly observed on thresholds. In order to see the dependency on the onset or stop of indentation movement, stepwise indentation in long period was also examined. Histograms of reaction time from the latest onset of indentation movement showed that the subjects was apt to feel pain 1-2 seconds after the movement was started.
A simple one-dimensional vasco-elastic model, including springs, a dashpot and a mass, of the skin was introduced to explain the lag time and the tendency of reaction time observed in the experiments. From this, pain sensation on the skin is considered to be related to the deformation of inner part of the skin.
