THE SENSE OF TASTE 



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Stimuli, l)ut the recovery curves tend to have the 

 same shape (92). Subthreshold stimuli may elevate 

 the threshold in a similar manner but to a lesser de- 

 cree. Adaptation by acid adapts the sour taste for 

 other acids, but in the case of bitter and sweet stimuli 

 cross adaptation within these respective qualities may- 

 occur only between some but not all stimuli. More 

 striking, however, is the case of salt where no cross 

 adaptation was found among 24 inorganic salts 

 studied (94). 



It has commonly been assumed that sensory adapta- 

 tion reflects the exhaustion of some receptive sub- 

 stance in the cell in a manner analogous to the 

 bleaching of visual purple by light. The combination 

 of the stimulus with such a receptive substance was 

 assumed to be necessary for stimulation (129), and 

 further that all stimuli eliciting the same quality 

 would be mediated by the same receptor substance 

 so that cross adaptation would result. Such a mecha- 

 nism for the salts would require 24 different receptive 

 substances in the taste cell. Hahn rejected this notion 

 and hypothesized a specific inhibition of the cell re- 

 ceptor membrane (change in permeability) for the 

 adapting stimulus only. The receptor cell itself was 

 not rendered inexcitable (94). Analogous results were 

 found in recent electrophysiological studies (20). No 

 cross adaptation between calciuin chloride and so- 

 dium chloride for example, was found, even though 

 the single fiber analysis shows that calcium chloride 

 and sodium chloride affected the same peripheral 

 fiber and receptor cell (75). 



Bujas has pointed out that the subjective intensity 

 of taste does not always parallel the peripheral process. 

 Maximal subjective intensity develops only after the 

 stimulus has been acting for some seconds. During 

 this 'buildup' period, however, the receptor sensi- 

 tivity is falling as shown by the rise in threshold. 

 Subjective intensity begins to fall off^ only later, 

 showing that the magnitude of sensation is probably 

 the result of a central and peripheral process working 

 in opposition (37). Beidler has noted that the main- 

 tained steady discharge for sodium chloride in the 

 electrophysiological record is at variance with the 

 complete disappearance of salt sensation reported 

 for all but the strongest concentrations in the human 

 observer (2, 39, 127). This points to a process of 

 central adaptation. 



Adaptation to sucrose or sodium chloride enhances 

 sensitivity to stimuli eliciting other qualities. Adapta- 

 tion by quinine enhances sensitivity to sour and salt, 

 but adaptation by hydrochloric acid does not affect 

 the other qualities (64, 140). It is well known that 



distilled water appears sweet following a weak acid. 

 The recent finding that water acts as a stimulus for 

 certain taste endings and that the magnitude of such 

 discharge can be modified by prior treatment with 

 acids or other chemical stimuli suggests a peripheral 

 locus for some of these effects (136, 164). 



A positive after taste, i.e. persistence of the same 

 taste quality after withdrawal of the stimulus, has 

 been attributed usually to residual taste particles in 

 the mouth or to slow dcsorption from the receptor 

 surface. 



Interaction when two disparate areas of the tongue 

 are stimulated has been reported. Weak acid or sugar 

 solutions were said to reduce the threshold for salt 

 on the opposite side (119). An enhancement of salt 

 sensitivity occurs with weak sugar solutions but 

 inhibition or elevation of salt threshold occurs with 

 stronger sugar concentrations. Such effects with 

 stimulation of disparate sensory surfaces must have a 

 central origin. Successive contrast effects of a similar 

 nature also have been described (36, 198). 



Unfortunately, there have been few systematic 

 studies of masking and interactions with taste mix- 

 tures, except for efforts to duplicate complex tastes 

 by mixing four components. One well-known inter- 

 action is the reduction of sourness by the addition of 

 sugar or other sweetener. This has been studied by 

 the electrophysiological method. The discharge of a 

 nerve strand to a mixture of 10 per cent sucrose and 

 an acid of pH 2.5 showed only an increase compared 

 to the response to the sugar or acid individually (10). 

 There was no peripheral inhibition. Such sour-sweet 

 interaction, therefore, must have a central locus. 



Intensitive Relations 



Differential sensitivity (A/ /) as found by different 

 investigators is summarized in table 9. Values from 

 i/io (10 per cent) to i i (100 per cent) with a 

 modal value of i '5 (20 per cent) have been reported 

 depending upon the intensity level, amount of stimu- 

 lus, criterion of judgment, etc., employed. Constancy 

 of A/// with intensity has been reported by some, 

 whereas others reported a decrease in differential 

 sensitivity at the high or low intensities, but the 

 change in these latter instances was relatively small 

 (approximately 10 times) compared with the too to 

 1000 times change found for vision and hearing. High 

 differential sensitivity for one taste quality is not 

 correlated with high sensitivity for others, and dif- 

 ferences between subjects may be greater than the 



