938 CUTANEOUS SENSATIONS. 



subliminal electric current between points only little above liminal dis- 

 tance apart, causes simultaneous touch sensations from those two points 

 to lose their separateness, and fuse. 1 Narcotics markedly increase the 

 average liminal distance. 2 



The sense of touch locality is disturbed and injured, disproportionately 

 to damage of other tactile qualities, by lesions of the cortex of the cerebral 

 hemisphere. In a hemiplegic patient the sense of touch in the affected 

 limb is sometimes but little impaired, when tested by the liminal 

 intensity or liminal difference of intensity ; but when the patient is 

 required to localise, great, often enormous, deficiency is found (Westphal), 

 so that a touch on the hand may be perceived as though located on the 

 upper arm. 



Theory of "localising" power of "touch." A something attaches 

 to or is inherent in a " touch " which gives it place, establishing it 

 within ideal space. There is reason to think (see "Muscular Sense" 

 and " Spinal Cord ") that something analogous may attach to even very 

 simple reactions not involving consciousness, i.e. to what are sometimes 

 called neuroses in distinction from psychoses. 



This quality of "place" is by some regarded as native to and intrinsic 

 in touch (Czermak, Vierordt), which thus possesses a simple spatial quale. 

 Others (Lotze, Meissner, etc.) have regarded it as the outcome of experience 

 of association, of sensations concomitant with each touch sensation ; the 

 associate groups centred on touch sensations generated at sufficiently separated 

 spots of skin are, it is argued, sufficiently different to be perceptibly distinct. 

 Each such little compound system of sensation serves as differentiating and 

 recognisable sign, labelling the " touch " attached to it with " locality," or, 

 in Lotze's language, giving it " local sign." Lotze's scheme of local signs was 

 based mainly on muscular sense ; Meissner attributed it in tactual sense to 

 physiological differences in the structure of the skin of different regions. 



A difficulty in the way of accepting locality as a native quality of simple 

 touch, is its relativity. At the root of locality lies discrimination of place, and 

 to discriminate in place between even only two touches implies perception 

 not alone of the excited points, but of the extensiveness of an unexcited 

 interval. Thus localisation would commence in duality. Weber insisted from 

 the outset on the perception being not only of two points, but always also of the 

 interval between. How is it, then, that a single touch comes to be localised ? 

 Previous experience has taught the recipient subject that when either point 

 was impressed by an object, the same object usually touched also certain 

 immediately neighbouring points. Each point becomes thus associated with 

 an area of circumjacent points, the associated fading in strength as the 

 distance from the point increases. The point about which the associated area 

 thus centres has its own peculiarity or shade of feeling. Wundt writes : " If 

 we touch first the cheek and then the palms, each time with the same pressure, 

 the sensation is, notwithstanding the similar stimulation, markedly different in 

 the two. So if we compare the feeling of the palm Avith that of the back of 

 the hand, in short, of any two distant parts of the skin with each other. 

 Moreover, we easily remark, by attentively observing, that spots even near 

 together differ in respect of quality of feeling." Each has its "local sign." 

 But the feeling of each point is, as above said, associated by experience with 

 the feeling of the circumjacent. Its sensation revives consciousness of the 

 whole region round about. The " local sign " of the central point, therefore, 

 suggests the feeling of the entire district, and its locality is given by judgment 

 of the relation between it and other points in the revived regional perception. 



1 Suslowa, Ztschr.f. rat. Med., 1863 (3), Bd. xvii. S. 155. 



2 Lichtenfels, Sitzungsb. d. k. ATcad. d. Wissensch., Wien (2), Bd. xvi. S. 3. 



