526 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOG\' I 



TABLE 9. Differential Thresholds {Al/I) 

 in Taste Modalities 



* a, Beidler (20); b, Bujas (35); c, Fodor & Happisch 

 (76); d, Holway & Hurvich (107); e, Kopera (126); f, Lem- 

 berger (133); g, SanduUah (185); h, Schutz & Pilgrim (186). 



t Saccharin. | Quinine. 



§ Krogh & Jensen cited in (126). Values obtained by 

 Keppler (117) were much lower than most values obtained 

 by above authors and have been omitted from the table. 



differences in the same subject for different intensities 

 or qualities (186). 



Attempts to measure or scale subjective taste in- 

 tensity have employed different methods. One is the 

 summation of just noticeable difference steps (JND's). 

 In one study, successive JND steps were determined 

 for two sweet substances, crystallose (sodium sac- 

 charin) and sucrose so that stimuli falling at equal 

 JND steps abo\e threshold could be specified (133). 

 These concentrations, however, were not equallv 

 sweet when directly compared. At high concentra- 

 tions, saccharin became relatively less sweet than 

 sucrose of an equal JND scale \alue. As in the case 

 of other modalities, the JND summation scale is not 



a valid scale for subjecti\e magnitude (193). This is 

 also true when cross quality comparisons, e.g. salti- 

 ness versus sweetness, are carried out (35). 



Another method, analogous to the equal loudness 

 measurement in hearing, utilizes the direct match 

 between one solution and an arbitrarily selected 

 set of standards (44, 158). For sweetness, a series of 

 sucrose solutions is often used; for sourness, hydro- 

 chloric acid, etc. This does not give taste intensity 

 directly, onK the relati\e taste effectiveness of dif- 

 ferent substances in eliciting equal taste intensities. 



In recent years, a number of direct scaling methods 

 have been developed, stemming from the work in 

 audition. In one series of studies (17, 134) the frac- 

 tionation method showed that subjectise magnitude 

 increased directK with the physical concentration, a 

 special case of Stevens' general psychophysical law, 

 ^ = s", where ^ is sensation, s is stimulus intensity, 

 and n is an exponent, the exponent in this case being 

 equal to one (193). Other studies using similar 

 methods found that taste intensity increased as an 

 exponential function of stimulus concentration, so 

 that the exact relation between taste intensity and 

 stimulus concentration is yet to be established (187). 



Glucose is less sweet, molecule for molecule, than 

 sucrose. Furthermore the ratios of concentrations for 

 equal sweetness of the two sugars change with concen- 

 tration. Sweetness does not increase equally with 

 concentration for both (137). That this depends upon 

 some basic receptor mechanism is suggested by the 

 fact that the electrophysiological response in the 

 chorda t\nipani nerve of at least one species, the rat, 

 follows a rather different course for glucose than for 

 sucrose as shown in figure 16 (89). 



These relations also bear upon another effect, the 

 so-called 'supplemental action' in mixtures of two or 

 inore sweetening agents. When glucose and sucrose 

 solutions are mixed, for example, the sweetness of 

 the mixture is greater than would be predicted by the 

 simple addition of the equivalent sweetness values of 

 each component stated in terms of the equisweet 

 sucrose solution. W hen such mixtures are computed 

 in terms of the equLsweet glucose concentrations, 

 however, the sweetness of the mixture is the sum of 

 the components. There is simple additixity with no 

 supplementary action (44). If it is assumed that the 

 magnitude of nerve impulse discharge determines 

 directly the magnitude of taste, i.e. sweetness, we 

 note that the sensory effect for glucose is nearly 

 linearly proportional to concentration, but for sucrose 

 it is cur\ ilinear, i.e. negatively accelerated. A graphi- 

 cal solution of the addition of 0.2 m glucose and 0.2 



