Discrimination

DISCRIMINATION (from Latin discriminatio - distinction), the ability to distinguish between two closely related stimuli, whether qualitative or quantitative. Discriminatory sensitivity is contrasted with protopathic sensitivity, i.e., the ability to perceive intense, extreme stimuli—painful, thermal, cold, etc. The investigation of discriminatory sensitivity consists of determining the differential threshold of stimulation in the field of tactile sensitivity, temperature, the sense of pressure, etc. Of particularly great importance is the study of discriminatory localization sensitivity, investigated with the help of Weber's compass (see Weber's tactile circles). In a normal state, discriminatory sensitivity presents large differences depending on the area being examined. It is very fine on the mucous membrane of the lips, on the tip of the tongue, and on the palmar surface of the fingers. The discrimination threshold here is 2-3 mm. Conversely, on the back and the lumbar region, it is very weak (discrimination threshold 50-70 mm). In other areas, the magnitude of the differential threshold fluctuates between these indicated limits. Under pathological conditions, discriminatory sensitivity weakens more or less significantly, and in cases of deep lesions, even the maximum separation of the compass points produces the sensation of a single touch. Discriminatory sensitivity in the field of pressure is investigated by so-called baresthesiometers (see Baresthesia). Here, too, the differential threshold, expressed by the ratio of the additional pressure to the initial pressure, proves to be different in different areas. In the area of the lips, forehead, and temple, it constitutes 1/10-1/30 of the initial pressure; in the area of the upper limb, 1/20-1/10, etc. Thermal discriminatory sensitivity is investigated by thermoesthesiometers. The Roth thermoesthesiometer consists of two hollow cylinders, into which water of the desired temperature is drawn with the help of bulbs. By alternately applying the cylinders with water to one and the same area and changing the temperature of the water in them, it is possible to determine the minimum difference in temperature that is still distinguishable by the subject. The discrimination threshold in a normal state averages 1-1.5°. Precise investigations, however, reveal even finer sensitivity in many areas (down to 0.2-0.3°).