Quality specific differences in human taste detection thresholds as a function of stimulus volume

Physiology & Behavior, Vol. 45, pp. 15-20. Copyright©Pergamon Press plc, 1989. Printed in the U.S.A.

0031-9384/89 $3.00 + .00

Quality Specific Differences in Human Taste Detection Thresholds as a Function of Stimulus Volume I G A R Y M, B R O S V I C ~ A N D W I L L I A M

W. McLAUGHLIN

Glassboro State College

R e c e i v e d 15 A p r i l 1988 BROSVIC, G. M. AND W. W. McLAUGHLIN. Quality specific differences in human taste detection thresholds as a function of stimulus volume. PHYSIOL BEHAV 45(1) 15--20, 1989.--Taste detection thresholds for sodium chloride, sucrose, citric acid and quinine sulfate were determined with the Henkin three drop forced-choice method at stimulus volumes 0.05 ml, 0.50 ml, and 0.90 ml, with and without water rinses. Taste thresholds were inversely related to stimulus volume (median rs=-.68 and, within each volume, thresholds did not differ as a function of water rinsing. The detection thresholds for sodium chloride (range: 15.06 mM to 6.7 mM), sucrose (range: 24.22 mM to 14.13 mM), citric acid (range: 1.47 mM to 0.5 mM) and quinine sulfate (range: 0.35 mM to 0.12 raM) were similar to those of other investigators using considerably larger stimulus volumes and different psychophysical procedures. The present results demonstrate that the Henkin three drop method provides a more optimal measure of changes in taste sensitivity when stimulus volumes of approximately 1 ml are used in place of the standard 0.05 ml stimulus volume. Taste

Detection threshold

Stimulus volume

Rinse

IN clinical investigations of human taste dysfunction and treatment effectiveness the psychophysical paradigm commonly used to measure the detection and recognition of tastants is the Henkin three drop forced-choice method (8, 13, 14, 16, 18, 31, 40, 44). The three drop method was developed for use in clinical practice (15,22) and has proven advantageous for the rapid determination of taste thresholds (40). Changes in taste sensitivity in patients with adrenal cortical insufficiency (15,22), zinc insufficiency (10, 20, 2l), cystic fibrosis (19), hypertension (13) and after d-penicillamine administration (17) have been reported in clinical studies in which the three drop method was used. The measurement strategy underlying the three drop method is quite simple and, in each trial, is based upon the discrimination of tastantcontaining from nontastant-containing stimuli (3 l, 40, 44). During a three drop test, the subject extends his/her tongue and the experimenter places a stimulus sample on the dorsal surface of the extended tongue. The subject samples three stimuli during each trial, and is allowed to retract his/her tongue and to swallow after tasting each sample (40,44). Two o f the stimulus samples contain distilled water and one contains a tastant dissolved in distilled water. The three stimulus samples are presented in a random order and the subject reports which sample tastes different from the other two (44). The molar concentration of the initial tastant-containing stimulus is usually well-above normal de-

tection limens (i.e., 60 mM for sodium chloride) and, in successive trials, is lowered after a correct response and increased after an incorrect response. Testing is continued until the subject fails to identify the stimulus sample that contains the tastant in two out of three trials. At this point, testing is discontinued and the molar concentration of the last correctly identified stimulus is designated as the detection threshold (15, 17, 20-22). The termination of testing, however, does not indicate statistical significance. As calculated by Weiffenbach et al. (44) the probability of succeeding by chance on a given trial is 1 in 3 and, given the requirement of correct detection on two out of three consecutive trials, criterion can be met by chance in 7 out of 27 cases. In comparison, the probability of chance success on the HarrisKalmus (12) test which requires the correct identification o f four tastant-containing solutions from a group of eight test solutions is only 1 out of 70 cases. In a recent test of the reliability and validity of the three drop method, Slotnick et al. (40) reported sodium chloride (NaCI) detection threshold values comparable to those of other investigators (14-22), as well as an inverse relationship between NaCI threshold and stimulus volume. The standard 0.05 ml stimulus volume of the three drop method produced a NaCI detection threshold of 4.95 mM, while stimulus volumes of 0.50 ml and 0.90 ml decreased threshold to 2.55 mM and 2.23 mM, respectively, for the same subjects. The in-

~Preliminary results of this study were presented at the 1987 Annual Meeting of the Eastern Psychological Association, Arlington, VA. 2Requests for reprints should be addressed to Dr. Gary M. Brosvic, Department of Psychology, Glassboro State College, Glassboro, NJ 08028-1763.

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BROSVIC AND M c L A U G H L I N

verse relationship between stimulus volume and taste thresholds suggests that thresholds determined by a smallvolume no rinse technique such as the Henkin three drop method could be significantly lowered by minor increases in stimulus volume (5, 6, 31, 40, 44). This suggestion is, in fact, supported by Smith's (41) power function which relates taste intensity to stimulus intensity and area. The present study examined the relationship of taste detection threshold and stimulus volume for representatives of the four taste qualities and whether the magnitude of these relationships could be influenced by water rinses. METHOD

Subjects Twenty-six women and 14 men served as volunteer subjects. Their ages ranged from 17 to 43 and each reported no known health problems or the use of any medication at the time of testing. Subjects were instructed to refrain from eating, smoking, and drinking anything other than tap water for at least two hours prior to testing.

Stimuli All solutions were prepared with reagent grade chemicals and triple-deionized water. The solutions were stored in 100-ml flasks at 8°C and brought to room temperature prior to use. Each series of tastant solutions was made by diluting a 1 molar solution of NaC1, sucrose, citric acid or quinine sulfate. Each stimulus in a given series was successively more dilute by 0.25 log units. The molar equivalents for the strongest and weakest solutions of each series were as follows: 5.6x10 -1 M and 1.0×10 -5 M for NaCl and for sucrose, 1.0× 10-2 and 1.0× 10-6 M for citric acid, and 1.0× l0 -2 and 3.0× 10-6 for quinine sulfate.

Calibration of Stimulus Delivery Disposable 5-ml pipets were used to deliver stimulus samples to the dorsal surface of the subject's extended tongue. The 0.05 ml stimulus volume was delivered by depressing the pipet bulb until one fluid drop was expelled, while the larger stimulus volumes, 0.50 ml and 0.90 ml, were dispensed by preloading the pipets. In tests conducted to determine the consistency in the volume of fluid expelled (weight at 22°C) by randomly selected pipets the mean fluid volume dispensed did not differ from the desired volume (t-tests, all p>0.50, two-tailed).

General Procedures The present study's procedures were identical to those used by Slotnick et al. (40) to examine the reliability and validity of the three drop method. Accordingly, an ascending method of limits procedure was used to determine detection thresholds at each stimulus volume, with and without the use of water rinses. At the beginning of the test session and upon completion of testing with each stimulus volume, subjects rinsed their mouths with 25 ml of triple-deionized water and then expectorated. The order of presentation of the stimulus volumes was randomized. Half of the subjects were first tested without intertrial water rinses and, within 7 days, retested using 1.5 ml intertrial water rinses. In the rinse condition, the subject was asked to rinse his/her mouth with 1.5 ml of triple-deionized water after answering and before the experimenter proceeded to the next trial. In the no-rinse con-

dition, the experimenter proceeded to the next trial immediately after the subject reported his/her decision. Each trial consisted of three stimulus samples (two of triple-deionized water and one tastant dissolved in tripledeionized water) being presented in random order. For each stimulus presentation, the subject was asked to extend his/her tongue out of the mouth. The stimulus sample was placed on the appropriate area of the tongue (i.e., NaCI solutions were placed on the anterior dorsal lingual surface), and the subject was allowed to withdraw his/her tongue into the mouth before receiving the next stimulus sample. After being presented with all three stimuli, the subject was asked to identify which stimulus sample tasted different from the other two and was required to guess when uncertain. Immediately after responding, the subject reported confidence in his/her judgement on a scale of 0 (no confidence) to I00 (absolute confidence). For each tastant, the session began with the lowest concentration of the stimulus series. The concentration of the tastant-containing stimulus was increased in successive trials until the tastant was correctly identified as the "different" stimulus in three consecutive trials. Threshold was defined as the molar concentration of the tastant-containing stimulus for the first trial of the three-correct trial sequence. For each subject, threshold was determined six times and a mean threshold was calculated. Seven women and five men were tested on the NaCI detection task and an equal number were tested on the sucrose detection task. Six women and two men were tested on the citric acid detection task and an equal number were tested on the quinine sulfate detection task.

Statistical Analysis For each tastant, a two-way analysis of variance (ANOVA) was calculated to examine differences in mean detection thresholds as a function of stimulus volume (0.05 ml, 0.50 ml, and 0.90 ml) and rinse procedure (none, intertrial). Regression analyses (expressed in mM) and Spearman correlation coefficients (rs) were calculated to summarize the relationshp between stimulus volume and detection threshold. Practice effects were examined by comparing mean detection thresholds on the first and the last three determinations for each tastant (one-sample t-tests). RESULTS As seen in Tables 1-4, an inverse monotonic relationship between stimulus volume and detection threshold was found for each tastant (median rs=-.68). Detection thresholds did not differ as a function of sex of subject (t-tests, all p >0.25, two-tailed) and, for each tastant, the lowest thresholds were attained in the last three determinations (t-tests, all p<0,05, two-tailed). For each tastant, a mean percent reduction (% RED) in threshold was calculated by dividing mean thresholds for the 0.05 ml volume by mean thresholds for the 0.50 ml and 0.90 ml volumes.

Taste Detection Thresholds Sodium chloride. A 2 (rinse procedure) x 3 (stimulus volume) ANOVA on sodium chloride detection thresholds yielded a significant main effect for stimulus volume, F(2,66)=7.62, p<0.001. Seheffe comparisons i n d i e a t ~ that thresholds for the 0.90 ml volume were ~ a n t l y lower than those for the 0.05 ml volume. NaCI detection thresholds

STIMULUS VOLUME

17

TABLE 1 MEAN NaC! DETECTION THRESHOLD (inM) AND PERCENT (%) REDUCTION IN THRESHOLD AS A FUNCTION OF STIMULUS VOLUME Sodium Chloride Detection Thresholds (mM) No Rinse

Mean SEM % RED

Intertrial Rinse

0.05 ml

0.50 ml

0.90 ml

0.05 ml

0.50 ml

0.90 ml

15.06" 1.76

13.26"t 2.01 -14%

8.87t 1.19 -35%

14.98" 2.37

10.81*t 2.05 -39%

6.70t 1.55 -124%

*tMeans with the same symbol are not significantly different (0>0.05).

TABLE 2 MEAN SUCROSE DETECTION THRESHOLD (raM) AND PERCENT (%) REDUCTION IN THRESHOLD AS A FUNCTION OF STIMULUS VOLUME Sucrose Detection Thresholds (mM) No Rinse

Mean SEM % RED

Intertrial Rinse

0.05ml

0.50 ml

0.90 ml

0.05 ml

0.50 ml

0.90 ml

22.42* 7.72

22.29"t 5.80 -9%

14.13t 4.63 -71%

22.42* 6.59

19.56"t 5.65 -17%

14.25t 4.11 -61%

*tMeans with the same symbol are not significantly different (0>0.05).

TABLE 3 MEAN CITRIC ACID DETECTION THRESHOLD (raM) AND PERCENT (%) REDUCTION IN THRESHOLD AS A FUNCTION OF STIMULUS VOLUME Citric Acid Detection Thresholds (mM) No Rinse

Mean SEM % RED

Intertrial Rinse

0.05 ml

0.50 mi

0.90 ml

0.05 ml

0.50 ml

0.90 ml

1.25* 0.22

1.09*¢ 0.21 -15%

0.50t 0.15 -150%

1.47" 0.30

0.97*# 0.22 -52%

0.62t 0.24 -137%

*tMeans with the same symbol are not significantly different (0>0.05).

TABLE 4 MEAN QUININE SULFATE DETECTION THRESHOLD (raM) AND PERCENT (%) REDUCTION IN THRESHOLD AS A FUNCTION OF STIMULUS VOLUME Quinine Sulfate Detection Thresholds (raM) No Rinse

Mean SEM % RED

Intertriai Rinse

0.05 ml

0.50 ml

0.90 ml

0.05 ml

0.50 ml

0.90 ml

0.35* 0.09

0.18*t 0.06 -94%

0.12t 0.04 -192%

0.30* 0.09

0.24"t 0.08 -25%

0.13t 0.06 -137%

*tMeans with the same symbol are not significantly different (0>0.05).

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BROSVIC A N D M c L A U G H L I N

were inversely related to stimulus volume (r~=-.72). The regression equation for this relationship is Y = ! 8.84X - 3.16, r=.66, and is significant, F(2,69)= 15.6, p<0.001. S u c r o s e . A 2 (rinse procedure) x 3 (stimulus volume) A N O V A on sucrose detection thresholds yielded a significant main effect for stimulus volume, F(2,66)=10.74, p<0.001. Scheffe comparisons determined that mean thresholds for the 0.90 ml volume were significantly lower than those for the 0.05 ml volume. Sucrose detection thresholds were inversely related to stimulus volume (r~= -.65). The regression equation for this relationship is Y = 35.28X - 7.07, r=.69, and is significant, F(2,69)=10.58, p<0.01. Citric A c i d . A 2 (rinse procedure) x 3 (stimulus volume) A N O V A on citric acid detection thresholds yielded a significant main effect for stimulus volume, F(2,42)=6.37, p<0.001. Scheffe comparisons determined that thresholds for the 0.90 ml volume were significantly lower than those for the 0.05 ml volume. Citric acid detection thresholds were inversely related to stimulus volume (r~=-.67). The regression equation for this relationship is Y = 1.68X - .403, r =.69, and is significant, F(2,45)=6.73, p<0.003. Quinine S u l f a t e . A 2 (rinse procedure) x 3 (stimulus volume) A N O V A on quinine sulfate detection thresholds yielded a significant main effect for stimulus volume, F(2,42)=4.23, p<0.021. Scheffe comparisons determined that thresholds for the 0.90 ml volume were significantly lower than those for the 0.05 ml volume. Quinine sulfate detection thresholds were inversely related to stimulus volume (rs= -.69). The regression equation for this relationship is Y =0.414X - .104, r=.64, and is significant, F(2,45)=4.43, p<0.018. DISCUSSION

The present results demonstrate that detection thresholds for representatives of the four taste qualities are inversely related to stimulus volume. As noted by other investigators, the lowering of detection thresholds is due to the greater number of tastant molecules contained in the larger stimulus volumes and available to receptors throughout the oral cavity, the potential for a longer dwell time of tastants on their respective receptors, and the decreased likelihood of stimulus dilution by saliva (4, 5, 11,25, 31, 40, 43, 44). While the specific causal factors were not identified in the present study, the results demonstrate that stimulus volume influences threshold outcomes. These results are comparable to those of O ' M a h o n y (31) and Slotnick et al. (40) and suggest that the three drop method provides a more optimal measure of the detection of differences in taste sensitivity when stimulus samples of approximately 1 ml in volume were used in place of the standard 0.05 ml stimulus volume. It should be noted that the present study provides a reasonable measure of detection but not recognition thresholds (9, 30, 31, 34, 36, 37). The three drop test employs a forcedchoice paradigm which requires subjects to state which sample contains tastant or, alternatively, which samples do not. As noted by O ' M a h o n y (30,31) this procedure reduces response bias for detection thresholds by encouraging subjects capable of distinguishing between the stimulus samples to set their criterion at a given level between the concentrations of the tastant and water samples. However, the three drop paradigm does not control for response bias when used to measure recognition thresholds because subjects are required to determine not only how different the stimulus samples must be to be reported as such but also to name the

sample which contains tastant (30, 31, 34). While special procedures may partially reduce criterion variation (30,31). they are unlikely to eliminate the bias, especially in clinical settings in which multiple within-subject measurements of tastant recognition are made (17, 19-22). In the present study, NaC1 detection thresholds range from 15.06 mM to 6.70 mM. The obtained NaCI threshold values compare favorably with the median 1 mM value of Weiffenbach et al. (44), the 1.8 to 5.6 mM values of Slotnick et al. (40), the median 6 mM value of Henkin et al. (22), the median 12 mM values of Henkin and associates (14-21), the median 12 mM value of Carson and Gormican (6), the t4 mM value of Wotman et al. (45,46), the 16.2 mM value of Lauer et al. (23), the 28.6 mM value of Richter and MacLean (39), and the 30 mM value of Weiffenbach et al. (44). The mean percent reduction in NaCI thresholds as a function of stimulus volume was 26.5%, for the 0.50 ml volume and 79.5% for the 0.90 ml volume. Sucrose detection thresholds in the present study range from 24.22 mM to 14.13 mM. The obtained sucrose threshold values compare favorably with the 6 mM value of Henkin and Christiansen (14), the l0 mM value of Weiffenbach et ul. (44), the median 12 mM values of Henkin and associates (15, 17-21), the 14 mM value of Wotman et al. (45,46), the 15 mM and 20 mM values of Zengo and Mandel (47), the 18.33 mM value o f C a s p a r et al. (7), and the 30 mM value of Weiffenbach et al. (44). The mean percent reduction in sucrose thresholds as a function of stimulus volume was 13% for the 0.50 ml volume and 66% for the 0.90 ml volume. Citric acid detection thresholds range from 1.47 mM to 0.50 mM. The mean detection threshold for citric acid has not been previously reported for the three drop method and is somewhat higher than the 0.1 mM value of Weiffenbach et al. (42) and the 0.01 mM value of Pfaffmann (38). The mean percent reduction in citric acid thresholds as a function of stimulus volume was 33.5% for the 0.50 ml volume and 143.5% for the 0.90 ml volume. Quinine sulfate detection thresholds range from 0.35 mM to 0.12 mM. The mean detection threshold for quinine sulfate has not been previously reported for the three drop method and is somewhat higher than the 0.00124 mM value of Weiffenbach et al. (42) and the 0.01 mM value of Pfaffmann (38). The mean percent reduction in quinine sulfate thresholds as a function of stimulus volume was 59.5% for the 0.50 ml volume and 164.5% for the 0.90 ml volume. However, these values were determined with differing psychophysical techniques, relatively larger stimulus volumes, and the use of interstimulus or intertrial water rinses. Such differences are relevant to the second question addressed in this study: the effect of intertrial water rinsing on taste detection thresholds. Although comparisons between studies are difficult because of differences in protocol, studies using relatively large stimulus volumes and water rinses have demonstrated significantly lower NaCI thresholds and somewhat lower sucrose thresholds (31, 32, 35, 43, 44). In the absence of rinsing, saliva provides an adapting medium which raises NaCI thresholds and, when rinses are used, opposite results are obtained because residuals are decreased which allows for greater sensitivity to weak concentrations of NaCI. Similar effects have been reported using interstimulus rinses or pauses of 15 to 30 seconds duration (32,35). In contrast, the adapting medium provides minimal stimulation to the sweet receptors and thus adaptation is comparatively nominal (1,3, 24, 26, 28, 30, 32, 33). In the present study detection thresholds were not differentially lowered by the use of intertrial water rinses. However, the size of the stimulus samples

S T I M U L U S VOLUME

19

(0.05 ml to 0.90 ml) and the water rinse (1.5 ml) may not have been sufficient to produce the "rinse effect" commonly reported when relatively large stimulus volumes are used even though the rinse volume was approximately 2 to 30 times as large as the stimulus volume (2, 26, 27, 31, 32, 25). As noted by O'Mahony (29,30), there are no standardized procedures for the use of water rinses and, in the present study, the requirement that the presentation of tastant-containing samples were separated by at least one water sample may have been sufficient to minimize the buildup of residuals and subsequent adaptation (27, 30, 31, 33). However, this conclusion seems specific to the NaC1 tests because a review of posttest debriefings indicated that water rinsing during the NaCI tests increased subjects' concentration and confidence in their judgements, but decreased subjects' concentration and confidence during the sucrose, citric acid, and quinine sulfate tests. The present results support previous demonstrations that the detection of NaCl and other tastants is inversely related to stimulus volume (11, 31, 40, 41, 44). However, these earlier investigations used considerably larger stimulus volumes and differing psychophysical methods. In the present study, detection thresholds for representatives of the four taste qualities were determined with the three drop method at stimulus volumes 0.05 ml, 0.50 ml, and 0.90 ml, with and

without water rinses. As noted by Slotnick et al. (40), the use of small stimulus samples (i.e., 0.05 ml) may result in increased task difficulty, but is compensated for by the rapid estimation of taste thresholds and the relative absence of adaptation. The present results and those of O'Mahony (31) and Slotnick et al. (40) demonstrate that minor increases in stimulus volume significantly increases taste sensitivity, as determined by the three drop method. This method was developed exclusively for the rapid and reliable determination of taste thresholds in clinical settings (15, 17, 22, 31, 44). In fact, the three drop method using a 1 ml stimulus sample was recently determined to be a more appropriate measure of changes in taste sensitivity than a sip-wise procedure using 10 ml stimulus samples (31). Thus, the present results and those of O'Mahony et al. (31) and Slotnick et al. (40) suggest that the Henkin three drop method provides a more optimal measure of the detection of changes in taste sensitivity when stimulus volumes of approximately 1 ml are used in place of the standard 0.05 ml stimulus volume. ACKNOWLEDGEMENTS The authors are grateful to Judith M. Risser, Scott Parker, John Frisone and Burton M. Slotnick for their helpful comments on earlier drafts.

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