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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



One study among recent ones on the temperature 

 effect using an electrophysiological method reported 

 no change in response magnitude for sodium chloride 

 at temperatures of 20°, 25° and 30°C (19)- Another 

 study with the larger temperature range found only 

 a 10 per cent variation in magnitude response between 

 20° and 30°C, but larger effects outside these limits 

 with sodium chloride (i). There was a sharp rise 

 from 15° to an optimum of 22 °C with a gradual 

 fall from 22°C tb 37°C and a greater drop at 45 °C. 

 Since a fall in neural response magnitude is equiva- 

 lent to a rise in threshold, the electrophysiological 

 results are in agreement with the human data showing 

 that sodium chloride sensitivity falls off with tem- 

 perature rise, particularly above 22°C. 



In interpreting the temperature effects it should be 

 remembered that biological systems have a normal 

 functional range beyond which biological arrest usu- 

 ally occurs. With extreme cooling or excessive heating 

 there may be irreversible changes (22). Thus bio- 

 logical systems with a mid-temperature region of 

 optimal function, in general, yield U-shaped functions. 

 It was the optimal range with which earlier workers 

 were concerned. The more careful study of systematic 

 changes within normal limits shows clearly that 

 temperature increase does not increase all taste 

 sensitivity. There is no simple temperature coefficient 

 in the usual sense. 



AREA AND DURATION. Stimulation of single papillae 

 or of a limited area of the tongue by a single drop of 

 solution usually results in higher thresholds or less 

 intense suprathreshold tastes than does tasting by the 

 whole mouth (44, 198). The expression IS'^ = K 

 approximately describes the relation between thresh- 

 old intensity, /, and surface area, S, with exponents 

 of 0.73 for sodium chloride, 0.6 for citric acid, 0.93 

 for sucrose, and i .42 for quinine hydrochloride. 

 Threshold decreases with areas up to 60 to 90 mm-. 

 A similar relation also holds for the apparent in- 

 tensity of suprathreshold solutions (43)- 



The relation between threshold and stimulus dura- 

 tion can be expressed as t = C/i" where / is duration, 

 i is threshold and C is a constant. For sodium chloride 

 and citric acid, n equals 1.5, for sucrose n equals 2.0. 

 A similar relation with shorter durations holds for 

 the electric taste (33). Similarly the apparent in- 

 tensity of suprathreshold solutions depends upon 

 duration. With long durations, the sensation of taste 

 waxes slowly, reaching a maximum for quinine in '8 

 to 10 sec. and for salt in 4 to 5 sec. With electric 

 taste the 'build-up' time is i to 1.5 sec. C42). 



REACTION TIME. Most early workers report that bitter 

 yields the longest, and salty the shortest times with 

 sugar and acid intermediate (198). Because the 

 stimulus intensity influences reaction time (169), it is 

 necessary to specify this parameter. One recent study 

 (32, 34) utilized a flow system in conjunction with 

 an electrical measurement of the solution flow at the 

 tongue surface or onset of current, in the case of the 

 electric taste. Reaction times vary with different 

 qualities and within the same quality. Reaction time 

 is longest at threshold and shortest at the higher in- 

 tensities, often by a factor of three or more. It seems 

 reasonable, in \ icw of the uneven distribution of 

 sensitivity over the tongue surface, that reaction time 

 for different stimulus classes would vary with the 

 region stimulated. This parameter has not been in- 

 vestigated, however. An increase both in the area of 

 stimulated surface or of hydrostatic pressure of the 

 solution against the tongue surface decrea.ses reaction 

 time to some extent (108). 



Adaptation 



The continued flow of taste solution over the tongue 

 leads to a diminution in subjective intensity and an 

 elevation of the absolute threshold which is propor- 

 tional to the intensity of the adapting stimulus (see 

 fig. 15). The rate and form of the adaptation curve 

 within the same quality may vary with different 



FIG. 15. Adaptation and recovery curves for NaCl. The 

 ordinate indicates the threshold concentrations. The course of 

 adaptation to three concentrations of NaCl, 5, 10 and 15 per 

 cent, is shown for an adaptation period of 30 sec. and a re- 

 covery period of 30 sec. The unadapted threshold is 0.24 per 

 cent. [From Hahn (92). j 



