Physiology and Behavior, Vol. 14, pp. 643-649. Brain Research Publications Inc., 1975. Printed in the U.S.A.

Taste Mixtures: Is Mixture Suppression Related to Compression? 1 LINDA M. B A R T O S H U K 2

John B. l~erce Foundation Laboratory and Yale University, N e w Haven, C T 06519

(Received 15 July 1974) BARTOSHUK, L. M. Taste mixtures, is mixture suppression related to compresston ~ PHYSIOL. BEHAV. 14(5) 643-649, 1975. - In mixtures of substances with different taste qualities, the components can still be recognized but are usually perceived as less intense than when unmixed. When the components are equally intense if unmixed, sourness is suppressed the least, if at all, and sweetness, saltiness, and bitterness are suppressed to greater extents. Psychophysical functions (relating taste intensity to stimulus concentration) are said to show compression when successive concentration increments produce progressively smaller psychological increments. Sour substances show the least compression and sweetness, saltiness, and bitterness are compressed to greater extents. That is, substances which show the least compression when added to themselves also show the least suppression when other substances are added to them. Antidromlc inhibition might mediate both compression and suppression. Taste

Antidromic inhibition

Mixtures

Mixture suppression

WHEN substances are mixed, their tastes change even when there are no chemical reactions among the substances. These changes were first studied thoroughly by Kiesow [12]. He found that the individual constituents could usually be identified in taste mixtures but their intensities were suppressed such that the intensity of the mixture was less than the sum of the intensities of the unmixed constituents. Some subsequent studies have reported enhancement in mixtures [2, 8, 11, 24, 25] but the quality " e n h a n c e d " is usually found to be associated with both substances. For example, weak NaC1 is said to enhance the sweetness of sucrose in mixtures [2,24] but weak NaC1 actually tastes sweet itself. In general, taste mixture research supports Kiesow's early conclusions [19, 22, 23]. Virtually all studies of taste mixtures have used twocomponent mixtures. The present experiment was designed to study 2, 3 and 4 component mixtures. Studies o f interactions in higher order mixtures offer a bridge between the simple stimuli of the laboratory and the complex mixtures actually encountered in the real world. In addition, important properties that might be overlooked in twocomponent mixtures may emerge in higher order mixtures. The major purpose of the present study is to suggest that the relative suppression that taste components undergo in mixtures can be predicted from the psychophysical functions produced by the unmixed components. Figure 1 shows 3 hypothetical psychophysical functions that relate taste intensity to stimulus concentration. The way in which

Compression

a substance adds to itself is shown by the shape of its function, Consider adding concentration 1.0 of Substance A to itself. The perceived intensity of the sum, that is, of concentration 2.0, is 1.4. The same operation on Substances B and C would produce perceived intensities of 2.0 and 2.8, respectively. The function for Substance A is said to show compression while that for C is said to show expansion [29]. The present study shows that the more compression shown by a substance when unmixed, the more that substance will be suppressed when other substances are added to it. METHOD

Subjects Nine paid volunteers, 3 females and 6 males (primarily Yale undergraduates), and the author served as subjects. The ages ranged from 20 to 35.

Procedure Stimuli were warmed to 34°C (approximately the temperature of the extended tongue) and delivered to the extended tongue by a modified McBurney flow system [15] at a rate of 4 ml/sec. Each stimulus was preceded by a 20 sec water rinse. Taste stimuli were evaluated by Steven's method of magnitude estimation [30]. That is, subjects assigned numbers proportional to the perceived intensities

1Supported by NIH Grant ES00880. These data were reported at the Eastern Psychological Association, Philadelphia, PA, April, 1974. The author thanks Charles M. Sommerfield, William S. Cam, and Lawrence E. Marks for helpful suggestions. 2 Reprint requests should be addressed to Lmda M. Bartoshuk, Pierce Foundation, 290 Congress Avenue, New Haven, CT 06519. 643

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t LO% CONCENTRATION ( ~ ) FIG. 1. Power functions showing perceived intensity as a function of concentration for three hypothetical taste substances A, B, and C. The function for A shows compression and the function for C shows expansion. of the stimuli. In addition, the total intensity was broken down into magnitudes of constituent qualities [28]. For example, a complex taste of salty-sweet of total intensity 100 might be judged 40 salty and 60 sweet. Judgments were written on sheets labeled Sweet, Salty, Sour, Bitter and Other Responses. The last category was rarely used. Subjects left columns blank when a particular quality was absent. The following concentrations of representatives of the 4 basic tastes were chosen because on the basis of an earlier study [1] they produced approximately equally intense

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tastes: 0.32 M sucrose, 0.32 M sodium chloride (NaC1), 0.013 M hydrochloric acid (HC1), and 0.001 M quinine hydrochloride (QHCI). In one session 16 stimuli were presented: the 4 stimuli mentioned above, the 6 possible two component mixtures of these, the 4 possible three component mixtures, the 1 four component mixture, and water. Each subject judged these 16 stimuli in 3 separate sessions. Additional testing was included m each session that was actually part of another experiment and will be discussed at another time. The additional testing consisted of repeating the mixture stimuli with an adapting solution preceding rather than water. The solutmns were made from water (Hydro Service and Supplies, Inc.) that was organicfree, as near neutral as possible, and had a resistance in excess of 18 M¢~ per cc. NaC1, sucrose, and HC1 were reagent grade and QHC1 was Baker grade. It should be noted that taste mixtures are not constructed by physically mixing two solutions. This would dilute both solutes. For example, 0.32 M NaC1 contains 18.7 g of NaC1 per liter of solution and 0.32 M sucrose contains 109.5 g of sucrose per liter of solution. The mixture of 0.32 M NaC1 and 0.32 M sucrose contains 18.7 g of NaC1 and 109.5 g of sucrose per liter of mixture. Since subjects were free to use numbers without regard to absolute size and were required only to make the ratios of the numbers reflect the ratios between taste intensities, the data were standardized in the following manner. Each subject's judgments were multiplied by a factor chosen to make the sum of the intensity of the four unmixed stimuh equal to 100. RESULTS AND DISCUSSION

Figure 2 shows l o g - l o g plots of intensity functions for HC1, sucrose, NaC1, and QHC1 scaled under conditions similar to those of the present study [ 1 ]. The functions are arranged in order of decreasing slope: HC1, sucrose, NaCI and QHC1. The sucrose function departs markedly from a power function at high concentrations as other have noted [19,20]. In the region around 0.32 M sucrose, the concen-

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CONCENTRATION

FIG. 2. Perceived intensity of HCI, sucrose, NaC1, and QHCI as a function of concentration The lowest concentrations are 0.003 M HC1, 0.03 M sucrose, 0.001 M NaC1, and 0.00001 M QHC1. Log units are marked along the abscissa. The slopes are: HC1, 0.89; sucrose, 0.67 (lower portion, 0.94); NaC1, 0.43; and QHC1, 0.32 (Modified from [1]). The slope of the HC1 function is sigmficantly different (two-tailed t, p0.05) from the slope of the lower portion of the sucrose function and this lower portion Is significantly different (p