'598 



IIWDBOOK OF PHYSIOLOGY 



M I ROPHYSIOLOGY III 



tends to show thai Weber fractions arc only very 

 approximately equal; over a given ranee of stimulus 

 values, the Weber and Fechner expressions are usually 

 leasl adequate at the extremes of the range. Moreover, 

 recent developments in psychophysical scaling meth- 

 see Stevens (451)] suggest that Fechner's loga- 

 rithmic expression might have to be replaced by a 

 power function. 



V Stevens points out, the systematic measurement 

 ol sensation in relation to a given stimulus dimension 

 can he performed without recourse to the concept of 

 differential thresholds and without assuming the 

 constancy of such differences. When a picture in 

 black and white is viewed, first in the sunlight and 

 then in the shade, we ordinarily perceive little, if 

 any, change in the relative brightness of its features. 

 This could mean that the differences in brightness 

 have remained the same under changing illumina- 

 tion (a> Fechner might have said) or, alternatively, 

 that the ratios of light and dark portions have re- 

 mained unaltered. The assumption of constancy of 

 subjective differences would lead to the formulation 

 \f/ = kilogip (a restatement of Fechner's law) in which 

 the psychological magnitude \p is related to the 

 logarithm of a physical dimension ip, and a constant 

 ki. If we assume, instead, that the proper relation is 

 based on equal ratios, we obtain \p = k?<p" (power 

 law of Plateau and Stevens) where n, the exponent, 

 varies with sense modality and stimulus dimension. 4 



The procedures employed in verifying the power 

 law involve the use of a psychophysical method in 

 which the observer is asked 10 adjust a light (or a 

 sound 1 to half or one fifth, etc., the brightness (or 

 luiulne^, eic i of a standard. For over a dozen sensor) 

 continua, these ratio methods have resulted in power 

 functions, that is the subjective magnitude is roughly 

 proportion.il to the stimulus magnitude raised to a 



I In- classic methods employed in establishing quantitative 

 relations between stimulus dimensions and sensations are 

 reviewed and illustrated in Boring (53); more detailed de- 

 scriptions of recent developments in these psychophysical 

 methods are given in Woodum th \- s< hlosheri; (549, pp. i<u 

 also Stevens & Galantei (453). Stevens' methods 

 lii-en criticized by < • ai net 1 1 | 

 1 \n exponential, rather than logarithmic, relation ol 

 ilus dimension to sensor) dimension has been anticipated 

 l>s Plati iu ■;•'■ 1 who jectured thi 'powei law" after he had 



asked eight aitists to paint, independently, a hi ay halfway 

 I «>-i \s ■ i. in.- white and hl.uk The grays turned out 

 'presqui identiques and remained so undei different illumina- 

 tions I ..it. 1 in his career, Plateau retracted his views, 1 aril) 



mi. 1. 1 the influence ..1 certain psychophysical experiments bj 

 1 1 Ibo tl 99 s - . Stevi us 1 



power. The values for the exponent n cover a con- 

 siderable range. For loudness, the exponent is 0.3; 

 for the subjective intensity of electric shock to the 

 fingers, it is 3.5 (451 ). The apparent subjective magni- 

 tude of an artificial 'star' grows roughly as the square 

 root of the photometric level. 



These bare figures do tell us a good deal about the 

 differences between various sensory dimensions. For 

 apparent loudness, where the exponent is around 0.3, 

 there is enormous compression of the scale of magni- 

 tudes, since in order to double the apparent loud- 

 ness, we must multiplv the physical energy by 10 

 (or the sound pressure by the square root of 10). 

 For the 'unphysiologic' mode of stimulation by direct 

 electric shock to a subject's finger, the converse is 

 true; here, the subjective intensity shoots up as the 

 3.5 power of the current applied — a fact to bear in 

 mind when applying direct electrical stimulation to 

 peripheral nerves But whether the organism com- 

 presses or expands a given stimulus dimension, the 

 basic psychophysical relation would be simple: equal 

 stimulus ratios produce equal subjective ratios (451). 5 



Multidimensional Nature nj Sensory 'Attributes' 



Whether logarithmic or exponential, these psycho- 

 physical relations suggesl a simplicity of sensor) 

 processes which would seem to set them apart from 

 the complexity- of everyday perception of objects. 

 This impression of simplicity, however, is deceptive. 

 The classical attempt at restitution of sensory qualities, 

 as we termed it, presupposes the existence of inde- 

 pendent one-to-one relations between sensory and 

 stimulus dimensions. But il pitch is nothing else than 

 perception oi frequency, or hue of wavelength, it 

 would be difficult to understand in what sense Galileo 

 or Newton made discoveries about these physical 

 correlates of sensation. The fact is that psychophysie.il 

 relations need not be one-to-one, or isomorphic in the 

 sense that to each definable physical dimension there 

 corresponds one and onl) one sensory dimension, 

 litis lack oi parallelism has such far-reaching impli- 

 cations that it needs to be considered here in some 

 detail. 



For Wunclt, each sensation had two 'attributes,' 

 vi/. its specific qualitv and intensity (551). To this 

 meagei inventory, extension in space and duration in 



'Attempts at identifying electrophysiologic correlates ol 

 sensor) scales e.g Granit (168) must be reconsidered in the 

 light ni psy< hophysical evidence which shows the inadequai ies 

 oi the Weber-Fechner formulation. 



