I hi id 



IIWDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY III 



-40 



•30 -20 -10 10 20 30 



Intensity in db (zero db = 1 dyne/cm 7 ) 



40 



no. a < mitours for constant pitch. A given contour describes 

 the combinations of frequency and intensity of a pure tone 

 which yield impressions of equal pitch. The ordinate was 

 arranged so that a contour with positive slope shows how 

 pitch increases with intensity (for high frequency tones). 

 [Based on data from Stevens '450).] 



and intensity. The equal-loudncss contour (449) indi- 

 cates, for a standard tone of low frequency, how 

 intensity has to be changed to balance changes in 

 frequency in a comparison tone so that the compari- 

 son tone appears to equal the standard in loudness 

 (see fig. 5). In addition, figure j shows that at least 

 two other psychological dimensions, 'volume' and 

 density,' can lie obtained by appropriate covaria- 

 tion of frequency and intensity. 



It is apparent from these sets of contours that two 

 dimensions on the physical (stimulus) side can yield 

 more than two perceptual dimensions. Moreover, a 

 perceptual dimension is not established by independ- 

 ent variation, as used to be believed in the classical 

 psychophysics "i the nineteenth century i Yii). It is 

 actually impossible to vary any of the dimensions de- 

 picted in figure ; in isolation, that is without varying 

 the others What we have, inste.nl, is .1 ease of 'de- 

 pendent constancy,' or in variance; we can maintain 

 a percepl l>\ appropriate variation of all of the other 

 relevam perceptual dimensions. The concept of in- 



64 



62 - 



60 



58 - 



bb 



400 



450 500 550 



Frequency in cycles per second 



600 



fig. 3. Isophonic contours for pitch, loudness, volume and 

 density. F.ach contour defines the combinations of frequency 

 and intensity at which a comparison tone will he perceived as 

 equal in pitch or loudness, or volume, etc., to the standard 

 tone of 500 cps and 60 db. [From Stevens (449).] 



variance will assume increasing significance as we 

 progress to other problems of perception in this 

 chapter. 



Implications jo Neurophysiologic Studies of 

 Pin, jitual Processes 



There are at least three lessons that may be de- 

 rived from this brief review of nineteenth- and twen- 

 tieth-century psychophysics. They concern a) the 

 stage in the nervous system where sensors differentia- 

 tion occurs; b) the complcxitv of sensory processes 

 which makes their separation from perception prob- 

 lematic; and 1 ) the need for uniform methods — an 

 enlarged psvehophysics — to attack, together what 

 used to be treated separately as sensory and per- 

 ceptual phenomena. 



We have seen that the organism can respond dif- 

 ferentially to sensory stimulation in such a way that 

 the responses can have more dimensions than the 

 stimulus itself. Different functions relating frequency 

 and inteusitv of sound yield at least four sensory 

 attributes, and perhaps more. If this is true, then our 

 search for neural liases of such differentiation should 

 proceed with yreat caution; clearly, the differentia- 

 tions iiiav occur at auv stage, from the periphery 

 (the receptors) to the final paths antecedent to the 

 discriminating motor responses. Thus, it should not 

 be surprising to find an organism making discrimina- 

 tions which appear to go beyond the differentiation 

 in its peripheral receptors. Nor need one assume that 



