INTENSITY OF SENSATION (from Lat. intensus. p.p. of intenders, to stretch out. from in, in + tendert-. to stretch. (.k. 7E1PFIP, t, i nein.
Skt. tan, to stretch). One of the four attrihntes of sensation, the being quality, extent. and duration: The intensive aspect of sensation is that "property which enables its to compare it with others in respect to vividness" (Kuelpe). Theoretic-ally every sensation must possess a cer tain intensity in order to enter In the sphere of vision, however. there exist cer tain facts which make it advisable, according, to some psychologists, to eliminate the attribute of intensity. Variation of the amplitude of vibration of the stimulus acting upon the retina effects a variation not only in the luminosity. hut also, under most in the color-tone and the saturation of the correlated sensation. Here, then, intensity seems reducible to quality. This is the position of Iferimc and Tlillebraml. On the other hand. K.?;ni[c maintains that there are two places in the spectrum (the one in the red and the other in the violet I where this does not hold true. G. E. Miiller has. furthermore. to overcome the difficulty by defining in tensity as the distance of a sensation from the zero-point (apart from qualitative change). If two colors are both reduced, that which reaches the zero-point first has the less intensity. If we add to or subtract from the brightness of a given color quality, we change its intensity, but tie do not interpret the alteration simply a-. a change in intensity, because there i• alviass in operation a centrally excited sensation of gray. (See Vnststu. SENsATIox. ) If we could change the intensity of a color•tone without hav ing this central process in operation, the change would be correctly noted as only.
The main problems of intensity are ( 1 That of the minimal intensity of sensation. and (2) that of the in intensity of stimulus which is just noticeable in sensation. There must he. accordingly, a determination for each sense department of the intensive limen (q.v.), and of the intensive sensible discrimination. The im portance of these problems is attested by the fact that all the classics of the science of psyelio physics (q.v.) have been concerned with the re lation obtaining between intensity of stimulus and intensity of sensation rather than with the more intricate dependencies between the other attributes of stimulus and sensation.
The liminal intensity of pre,:sure depends upon the place stimulated. With eork pellets V'eights, it has found to be 2 mg. upon the forehead. wise, and cheeks: .1 upon the lips, upper arm, and nape of the neck; 10 mg. upon the fingers; and 1 g. on the nails. By stimulat ing individual pressure spots with fine hairs. Von Frey obtained much lower values, averaging, for the calf 1.44 gr. min_ and for the wrist 1.28 gr. (where gr. denotes the pressure exerted by the hair. and denotes it, sect ion). The pain lintel'. as recorded by the hair riethod. gives the following values in gr/mm% tban that for warmth; that the is greater over the lateral surfaces of the body than in the median plane; and that, as a rule, it in creases from the periphery toward the trunk.
The liminal intensity tor task- depends not only 441 tho e011eelltillt i011 of the MO1110011, hut the area stimulated, the amount of movement ithin the buccal eavity, and the general condi tion of excitability of the taste nerves prevailing at the time. Furtlwrmore, the tip of the tongue is most sensitive to sweet, the base to bitter, the sides to sour, and the body to salt. The average limina have been found to he represented by the where nun' denotes the area of the eross-seetion of the hair: over the epicondyle of the humerus, over the radius. 20; over the knee-pan, •9; over the olecrancon of the ulna, 40 : on the con junetiva of the eye, 2 to 7. (In account of the variation of the actual temperature of the skin, or the 'phys.iologieal zero-point.' the examination of the intensive sensibility of temperature sensa tions exhibits peculiar ditlienities. We can mere ly say that the sensitivity for cold is greater following figures, based upon a solution in 100 parts of distilled water: salt, 0.24; saccharine. 0.49; hydrochloric. arid. 0.0003; sulphate of quinine, 0.00005. The intensity of olfactory sen sations is likewise dependent upon the rate and manner of breathing. the rate of diffusion of the odorous vapor, and the general conditions of the smell organs, as 1%411 as upon the amount of saturation of the air with the minute particles of the odorous substance. Current figures show that the linen is very low for some qualities; e.g. musk, 1/200000 mg.: sulphureted hydrogen, 1;5000 mg.; bromine, 1/000 rag. 1n f/ million, the existene• of absolute silence is to be doubted. Most authorities agree that we can never get free from the noises caused by the pumping of blood through the ears. Closing the ears, or standing with (ien cars near any reflecting bur face, only intensifies the sound. The liminal in tensity for noise may, then. be regarded as that intensity which is just perceptible above this internal sound. „A cork pellet weighing I nig. and falling through 1 unit. upon a glass plate can just be heard at 91 nun. from the ear. .A tone from a pipe a ltil vs. per second is audible when the air•particles set in motion by it pos sess an amplitude of excursion of only 0.00001 and the mechanical work dime by it upon the ear is one three-billionth kilogram-meter. Other investigations hays placed the latter value as low as 0.00:22 mg.-nun, The sensitivity for high tones is in general greater than that for kw tones. The investigation of the InightnesA linen reveals a difficulty analogous to that cited in connection with the auditory linen. The existence of the 'idio-retinal light' precludes abso lute sightlessness, as the `idio-aural noise' pre cludes absolute silence. For external stimulation, the brightness limen may be put at the illu mination of black velvet by a stearin candle nine meters distant. it is sometimes estimated (though too high) at l/:300 the light of the full moon. In one-half second this light exerts upon the retina the energy required to raise 1/15 nig. of water one billionth of a degree C. For the liminal seasitivity to articular sensations, see „MOVEMENT, PERCEPTION OF.