PERCEPTION 



'599 



Invariable points 



"i n 1 1 1 1 r 



675 650 625 600 575 550 525 500 475 450 



Millimicrons 



fig. I. Contours for constant hue. A given contour describe' 

 the combinations of wavelength and intensity of a mono" 

 chromatic light which yield impressions of equal hue. Only 

 three points within the visible spectrum are 'invariable,' i.e. 

 show no shift in apparent hue with changing intensity. Except 

 for these invariable points (a yellow at 572 m/i, a green at 503 

 itui and a blue at 478 m/il , all other points vary in a predictable 

 fashion according to the Bezold-Briicke effect. There is a 

 fourth invariable point which is not shown, since it lies outside 

 the spectrum, being constituted by a specific mixture of long 

 and short waves, in the purple. [From Purdy (391 I. 



time were added b) Kiilpe (296), and his four at- 

 tributes of sensory quality, intensity, extensity and 

 protensity (duration) have guided the search for 

 neural correlates of perception for more than half a 

 century. There is, however, no reason why this should 

 be so. Consider the case for vision. Here, a basic 

 sensory quality would be color (perceived hue), 

 ordinarily thought .is corresponding to wavelength 

 (or mixture of wavelengths) ; the intensity dimension 

 would be represented as perceived brightness. Yet, 

 with few exceptions, hue is not only dependent upon 

 wavelength, but upon intensity; except for certain 

 invariable points in the spectrum, all colors shift in 

 hue towards either yellow or blue as the intensity is 

 increased. This phenomenon of shift in hue with 

 changing intensity — the Bezold-Briicke effect — can 

 be quantified by plotting contours of equal hue (see 

 fig, 1) which describe the combinations of intensity 

 (in photons) and wavelength (in millimicrons) which 

 will yield equal-appearing; hues [see Purdv (391)]. 

 Perceived hue, thus, is a joint function of wavelength 

 and intensity. Correspondingly, perceived brightness 

 is a joint function (although a different function) of 

 the same physical dimensions; even a casual glance at 

 a spectrum will reveal that different wavelengths, 

 although energy is constant, have different-appear- 

 ing brightness within the spectrum. Moreover, when 



over-all energy is reduced, the relative brightness of 

 the different colors shifts in accordance with the 

 Purkinje phenomenon. (Characteristically, this Pur- 

 kinje shift fails to appear in the rod-free part of the 

 retina.) 



We thus have ample evidence for the dependence 

 of hue on both wavelength and energy (as the Bezold- 

 Briicke effect shows) and for brightness, again, on 

 energy and wavelength (as the Purkinje effect shows), 

 although two different functional relations establish 

 these two psychological dimensions, hue and bright- 

 ness. Lest there be a suspicion of some lingering 

 pre-established harmony (two physical for two psycho- 

 logical dimensions), we might consider a third psycho- 

 logical dimension of colors, saturation. Saturation, 

 too, varies with wavelength (being minimal in the 

 spectral yellow and violet) and with encrgv (being 

 maximal at intermediate energy levels I, so that we 

 can get three systematically discriminable aspects of 

 color sensations out of the appropriate combinations 

 of only two physical 1 stimulus 1 dimensions [see 

 Boring (-,ti . 



I In- situation has considerable generality. In audi- 

 tion, perceived pitch is easilv identified with fre- 

 quency, and loudness with intensity of sound waves. 

 Yet lor pure tones, one finds an analogue of the 

 Bezold-Briicke effect since perceived pitch changes as 

 intensity is changed, even though frequency is held 

 constant. Low-frequenc) tones, when raised in in- 

 tensity, sound lower in pitch, while high-frequencv 

 tones sound higher in pitch when their intensity 

 increases I iv>, V" !• There even is an 'invariable 

 point' which falls roughlv into the region of maxim. il 

 sensitivity along the frequency spectrum. As a result, 

 we can plot contours of equal pitch (fig. 2) which 

 indicate the changes in frequency that need to be 

 made in order to counteract the alterations in pitch 

 produced by a given change in intensity. 8 



But loudness, too, is ,1 joint product of frequency 



6 Happily for instrumental music, the shifts depicted in 

 fig. 2 are found in this pronounced fashion only for pure tones, 

 but much less for the complex 'timbered' tones produced by 

 musical instruments It is not clear whether this relative 

 constancy of pitch for complex tones is due to the presence of 

 harmonics or whether other factors play a role. However, the 

 phenomenon of greater pitch constancy of these complex 

 tones belongs to the large group of effects called 'perceptual 

 constancies' discussed below. It should be remembered, in 

 this context, that the statements about color sensations made 

 in the preceding sections are similarly restricted in scope; they 

 apply primarily to 'film' colors, i.e. colors which do not in any 

 obvious way belong to the surface of seen objects. [See Katz 



(249. 25" ; 



