THE SENSE OF TASTE 



529 



The role of taste or other head receptor stimulation 

 can be uncovered if the postingestion factors can be 

 eliminated or minimized. In the brief exposure be- 

 havioral test which permits little ingestion, rats show 

 a preference for the higher of two concentrations of 

 sugar solution over a wide range of pairs. But such 

 equally accepted solutions are not equally ingested 

 in continuous drinking periods as brief as 20 min. in 

 which the higher concentrations are usually con- 

 sumed in lesser amounts (204). McCleary (^142) has 

 clearly demonstrated the role of intragastric osmotic 

 pressure in this effect. 



In another behavioral test, the Skinner box with 

 sugar solutions as reinforcers, the rate of bar pressing 

 on an aperiodic reinforcement schedule is faster, the 

 higher the concentration of sugar (87). This schedule 

 provides relatively little drinking per response and 

 apparently minimizes postingestion factors. Further- 

 more, the concentrations of two different sugars, 

 glucose and sucrose found to give equal rates of re- 

 sponse, i.e. to have equal reinforcing value, corre- 

 spond to the equally preferred concentrations in the 

 short exposure test and to the equally sweet concen- 

 trations of these sugars for man. Thus, the direct 

 sensory taste effect appears not only to instigate in- 

 gestion but to be capable of reinforcing the acquisi- 

 tion of other responses leading to ingestion. The 

 usual measures of intake obscure the relation to 

 sensory stimulation because of postingestion effects. 



The nutritional consequences of sugar stimulation 

 do not appear to be essential for such reinforcement. 

 Nonnutritive saccharin solutions can also serve as 

 reinforcers for the acquisition of a maze-running 

 habit (189). The degree of reinforcement appears to 

 be correlated with the amount of consummatory be- 

 havior elicited. Whether reinforcement power is de- 

 termined by the magnitude of the afferent excitation 

 per se or by the magnitude of the consummatory be- 

 havior elicited, is not yet clear. 



The fact that certain taste stimuli control ingestion 

 directly appears to be biologically determined, for 

 nearly all organisms accept sugar solutions (78). 

 Although there is evidence that direct injection of 

 nutrient sugar into the blood stream may serve as a 

 reinforcer of learning, there is no evidence that the 

 "sweet tooth' depends upon the concomitant nourish- 

 ment. The drinking of nonnutritive saccharin solu- 

 tions shows no sign of extinction which would be ex- 

 pected if the preference for saccharin were acquired 

 by the association of the sweet taste with nourishment 



(189). 



The aversion to certain stimuli like bitter appears 



to be relatively unmodifiable by experience. In one 

 experiment, guinea pigs two days post jiartum were 

 provided with a nontoxic but normally avoided 

 solution as the only source of fluid until three weeks 

 of age. This substance has an extremely bitter taste 

 for man. Following this early exposure, preference 

 tests showed that the avoided stimulus had been 

 rendered somewhat more palatable, but in a retest 

 three months later the effect had dissipated so that 

 there was no difference between the control and ex- 

 perimental animals. The aversion had not been 

 moderated by the early experience (168). 



Thus the factors that control behavior in the taste 

 preference and related situations are becoming 

 clearer. Taste may trigger ingestion behavior but 

 alone does not accoimt for it. In many instances, feed- 

 ing behavior is directed toward the physiological 

 well-being of the organisms; but situations and 

 habits may exist which are contrary to the physiologi- 

 cal well-being. According to Young, "New habits 

 tend to form in agreement with bodily needs, but 

 established habits tend to persist as regulators of 

 food selection even when the food selections are out 

 of line with bodily needs." The limitations of self- 

 selection are well documented (85, loi, 204). 



Certain of these basic principles appear to be valid 

 for man. Instances of enhanced salt craving reported 

 by Richter included the case of a small boy who 

 apparently compensated for adrenal insufficiency 

 with an excessive intake of table salt (177). Patients 

 in whom hypoglycemia had been produced for 

 therapeutic reasons were reported to find strong 

 sugar solutions more palatable than when blood sugar 

 levels were normal (141). The bizarre taste cravings 

 of pregnant women are well-known. The change is 

 not one of taste sensitivity but one of changed likes 

 and di.slikes (97). The metabolic disequilibria of 

 diabetic patients are often associated with strong 

 cravings for sweet, although to satisfy this would 

 run counter to the individual's well-being (i 77). Thus, 

 although metabolic changes may be important factors 

 in determining the hedonic value of a taste stimulus, 

 such changes do not always automatically lead to 

 self-corrective behavior. 



The well-known fact that human subjects can taste 

 certain substances when injected intravenously ap- 

 pears to support the view that taste sensiti\ity can 

 be influenced by constituents in the blood stream 

 (102). As already noted, neither the adrenalectomized 

 nor hypoglycemic animal shows evidence of a change 

 in taste sensitivity when studied electrophysiologically. 

 In another study with this method, preliminary re- 



