ABSTRACT
Stimulation of a pressure spot or, for that matter, of any spot on the skin, calls forth a spatially extended sensation. Our determination of this can be based only upon the direct impression, but the correctness of the observation can be confirmed by the following facts. What happens if we simultaneously stimulate several spots on the skin? If we place both points of a compass (it is best to use a specially constructed touch compass or aesthesiometer) close together on the skin, we experience a simple pressure as if only a single point had been stimulated. If we extend the compass points a little and again place them upon the skin, we perceive a somewhat broader point. With a greater extension of the points we perceive a surface, then a line, and finally when the points are at a certain distance apart we experience two separate points on the skin. Since the two simultaneously-effective touch stimuli must be kept just at this distance in order for us to perceive them as two, we call this distance the spatial threshold. On different parts of the body this threshold is of different magnitudes: on the point of the tongue, 1 mm.; on the tip of the finger, 2 mm. It is greatest on the back and upon the limbs, being perhaps 60 mm. on the upper arm. The spatial threshold is a measure of the fineness of our spatial sense—that is to say, of our capacity to recognize by our tactual sense the form, magnitude, number, and movement of the objects coming into contact with the surface of the skin. Furthermore, it should be emphasized that the touch sensation is only the palpable foundation of this knowledge and cannot mediate the knowledge itself.
