THE EFFECTS OF SIDE-PRESSURE ON PRESSURE SENSATION

Pressure and touch are often described as the same kind of sensation because they are elicited by similar mechanical stimulation. But they are quite different subjectively as well as electro-physiologically. The author discriminated between them by the difference of subjective sensation, and studied the effects of additional side-pressure applied to relatively broad surface of the body upon pressure sensation. The sensitivity of pressure sensation was measured by the number of pressure spots within a given area of the skin. In normal resting subjects, the localization of pressure spots changes from one examination to another (Fig. 1), but the number is always nearly constant (Table 1). The author studied how side-pressure by a weight given to various parts of the body surface influenced upon the pressure sensitivity in the dorsal side of right forearm, and obtained the following results: 1) Side-pressure caused the decrease of pressure sensitivity in general. 2) The nearer was the location of side-pressure to the test area, the greater was it's decreasing effect (Table 2). 3) However, the decreasing effect of side-pressure applied to the dorsal side of right hand (distal from the test area) was less than that to the dorsal side of right forearm (proximal from the test area), even if both distances from the test area were nearly equal. This difference of the effect may be explained by the dermatome: the skin of the hand was innervated by the spinal nerve differing from that of the test area, whereas the forearm was innervated by the same nerve. But there is a problem on such simplified explanation. 4) The effect increased as the size and intensity of side-pressure increased. Side-pressure given on the forearm by a cuff also depressed the pressure sensitivity. In this procedure, the asphyxia took place and the sensation subjectively differed from the former case. So, in the latter case another mechanism might be considered. 5) The effect of side-pressure showed the maximum value within 1min. after pressure application (Fig. 2), regardless of intensity and location of applied pressure. Consequently, it may not be considered that the effect of side-pressure spreads around from the pressed area. 6) The after-effect was recognized and it disappeared within 1min. (Fig. 3). The obtained results can not be simply explained by the pressure block of the peripheral nerve trunks. The sensitivity of pressure sensation is depressed also by the other stimulation of the ce