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Studies on the relationship between electrogustometry and sour taste perception
Auris Nasus Larynx 34 (2007) 477–480 www.elsevier.com/locate/anl
Studies on the relationship between electrogustometry and sour taste perception Eva K. Ellega˚rd a,*, David Goldsmith b, K. David Hay b, Randall P. Morton c a
Department of Otorhinolaryngology, Kungsbacka Hospital, S-434 80 Kungsbacka, Sweden b Oral Health Unit, Green Lane Hospital, Auckland, New Zealand c Department of Otorhinolaryngology Manukau Super Clinic, Manukau, New Zealand Received 15 June 2006; accepted 22 March 2007 Available online 27 April 2007
Abstract Objective: Electrogustometry is used as a measurement of taste perception. The prevailing theory is that the anodal current delivered to the tongue mucosa stimulates the sour taste receptors, but this is not universally accepted. Our aim was to evaluate to what extent electrogustometry relates to an ability to detect sour taste—rather than sweet, salt, or bitter. Methods: We compared automated electrogustometric thresholds with visual analogue scale (VAS) ratings of various tastant solutions in 114 subjects. The whole mouth, and each side of the tongue were tested separately. VAS scores from the strongest set of solutions, and the lowest electrogustometry thresholds for each location were used for statistics. Results: There was a significant correlation between electrogustometry threshold and the whole mouth perception of the salt taste solution. Electrogustometry correlated significantly but weakly for all taste qualities when testing was confined to left and right oral tongue. The positive predictive values of electrogustometry were no better in relation to sour taste perception than to the other taste qualities. Conclusions: Our results do not support the theory that electrogustometry is mediated by sour taste receptors or even that it reflects the sour taste quality. We postulate that electrogustometry measures a function of taste perception, which is different from that induced by chemical stimuli. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Electrogustometry; Taste; Visual analogue scale; Subjective; Method
1. Introduction Measurement of taste perception in human subjects can be performed by several methods with different types of stimuli: perceived intensity of solutions of different strengths and taste qualities, used as a whole mouth wash or applied on parts of the oral mucosa [1], thresholds to oral solutions [2], or to solutions instilled in filter paper disks [3], or in tablets [4]. Electric current is the stimulus in electrogustometry, and it has been used for the study of taste loss from various causes [5], like age [6], tonsillectomy and laryngomicrosurgery [7], extraction of molar teeth [8], drug side effects [3], and middle ear surgery [9]. These * Corresponding author. Tel.: +46 300 565284; fax: +46 300 565301. E-mail address: [email protected] (E.K. Ellega˚rd).
methods for measuring taste depend on the subject’s response. Involuntary measures of taste pathways involve gustatory evoked potentials, which can also be used together with positron emission tomography, and functional magnetic resonance imaging [10]. In our report of sensory disturbances after lower third molar tooth extraction, we noted that despite an almost total lack of qualitative taste sensation for each of the four basic tastes, automated electrogustometry unexpectedly showed normal thresholds [8]. This raised the question of what the electrogustometer actually does measure. The prevailing theory is that the weak anodal current delivered to the surface of the tongue elicits a sour taste experience [11]. The aim of this study was to evaluate to what extent electrogustometry relates to an ability to appreciate sour taste, rather than sweet, salt, or bitter.
0385-8146/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.anl.2007.03.004
478
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
2. Subjects and method Data from 66 women and 48 men aged 23–92 (mean 60 years) were included in the study. The subjects comprised healthy controls (N = 10), patients who had been treated with radiotherapy for head-and-neck cancer (N = 34), patients with Sjo¨gren’s syndrome (N = 31), and patients with burning mouth syndrome or oral dysesthesia (N = 39), in order to register a wide range of taste capacity. All subjects were tested by automated electrogustometry, using a computer-controlled two-alternative forcedchoice technique described earlier [12,13], with a stimulating anode diameter of 5 mm. This method can measure electrogustometric thresholds of less than 3 mA [14], and thresholds greater than 30 mA are considered to be pathological [13]. Solutions of different concentrations for each of the four basic qualities (sweet, sour, salt and bitter) were used to challenge the subjects’ perception of taste, using a method we have described elsewhere [8]. The solutions were sucrose (1, 0.3, 0.1, 0.03 M), citric acid (0.032, 0.01, 0.003, 0.001 M), sodium chloride (1, 0.3, 0.1, 0.03 M), and caffeine (0.1, 0.03, 0.01, 0.003 M). A water control was included in each set. The order of presentation, the solution strength for each taste quality and the side of the tongue to be tested first was randomised by drawing blindly from a container that held each option on separate pieces of paper. Cotton buds dipped in the respective solutions were applied to the lateral border of the anterior tongue. The subject was aware of which taste quality was being used. After each presentation and before withdrawing the tongue, the subject was asked to score the intensity of the taste by making a cross on a visual analogue scale (VAS), in the form of a simple line scale with word anchors ‘‘no taste’’ at the left end, and ‘‘greatest imaginable taste’’ at the right. After each VAS scoring, the mouth was rinsed with distilled water. The above process was also performed using whole mouth solutions in 4 ml samples, taken as mouth rinses and then expectorated. For the purposes of analysis, the VAS was considered to be 0 at the left end, and 100 at the right end. At the strongest whole mouth concentrations all subjects registered above the detection threshold (i.e. >0), so we used the results from those concentrations for statistical analyses with a cut-off level of VAS <50 for ‘‘poor tasters’’. An electrogustometric threshold of 30 mA was the upper limit of normal. Positive predictive values (PPV: the proportion of those with a high electrogustometric threshold who were ‘‘poor tasters’’) were calculated for each of the taste qualities for each side of the tongue separately, and for whole mouth. 95% confidence intervals (CI), taken from binominal distribution were calculated for the values of PPV. Spearman’s correlation was performed for VAS in each of the taste qualities against the lowest (i.e. best) electrogustometry value for each subject, for whole mouth and for each side separately. Friedman’s nonparametric test was performed on the VAS values for the different taste qualities for whole mouth, and each side of the
tongue separately. Wilcoxon’s signed rank test with Bonferroni correction for multiple comparisons was used pairwise on the different whole mouth taste qualities. P < 0.05 was considered statistically significant. All participants gave their informed consent, and the Auckland Ethics Committee ‘‘X’’ approved the study.
3. Results Five subjects did not perform the whole mouth tests. Three of the cancer patients were tested before radiotherapy. Individual registrations, and the correlations, are shown in figure and table. There was a significant correlation between electrogustometry thresholds and the whole mouth perceptions of the salt taste solution (Fig. 1). Electrogustometry correlated significantly but weakly for all taste qualities when testing was confined to left and right tongue (Table 1). The whole mouth and left and right tongue VAS values were significantly different between the four taste qualities (P < 0.001 for all). For whole mouth VAS, bitter scored significantly lower (i.e. less) than salt (P < 0.001), and sour (P = 0.024), and sweet scored lower than salt (P < 0.001) (Fig. 1). The 95% CIs for PPV of the various taste qualities for an abnormal electrogustometric threshold (i.e. >30 mA) were not significantly different.
4. Discussion Among our 114 subjects, some healthy but most with different conditions which could impair their taste ability, we could not find specific support for the theory that electrogustometry relates particularly to sour taste quality. Even though we found significant correlations between VAS and electrogustometry thresholds, the regression coefficients were relatively low and did not favour sour taste. Moreover, we found that the PPV figures for sour taste were no better than for the other taste qualities. The study included a comparatively large mixed group of subjects of different ages, with different taste abilities, and the chemical tests were performed on whole mouth, as well as on each side of the tongue separately. Some subjects may have an inability to distinguish sour from other taste qualities, but in our group we found significant differences Table 1 Spearman’s correlation on one-sided solutions VAS scores vs. automated electrogustometry threshold values in 114 subjects for each of the four basic taste qualities
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
479
Fig. 1. Whole mouth washes VAS scores 0–100 (y-axis) vs. automated electrogustometry threshold values on best side (x-axis) in 114 subjects for each of the four basic taste qualities (Spearman’s correlation).
between taste qualities in left, right, and whole mouth challenges in intra-subject comparison. It may be more relevant to use the threshold value for qualitative taste rather than to use supra-threshold VAS scores, as chemical thresholds may not correlate well with supra-threshold profiles [1]. However, in our evaluation we chose the strongest solutions, as that concentration was perceived by all subjects and provided a more complete comparison between subjects. Some of the lower concentrations exhibited less consistent results in our subjects and Bartoshuk has shown that loss of taste can occur over part of the concentration range to which an individual is sensitive [1]. Our results are consistent with the findings of others. Tomita and Ikeda found a group of 12 patients – from 129 with taste disorders – who had normal thresholds on electrogustometry, but no response to any of the qualitative filter paper disc tests of different tastes. They also reported that salt taste correlated best with electrogustometry using whole mouth detection and recognition thresholds [5]. Salata et al. proposed that electric taste sensation was different from a chemical stimulus, based on sensitivity patterns to supra-threshold electrogustometry and taste solutions. As a stimulation of tactile receptors does not occur at normal electrogustometry testing thresholds, they suggested that the electric current affects the taste cell membrane potential but not via the ion channels that the chemical stimuli use [15]. Murphy et al. found a poor correlation between electrogustometry thresholds and detection thresholds of citric acid and sodium chloride in 15 college students and 17 elderly adults. The dominant sensation produced
by electrogustometry at threshold levels was described as a ‘‘buzzing vibration’’ rather than any of the four basic taste qualities. They concluded that while electrogustometry can be used reliably for assessment of unilateral loss of taste, there is no correspondence with whole mouth assessment of chemical thresholds [6]. Murphy et al.’s study is comparable to ours, but there are several differences. They used the average threshold for left and right electrogustometry in the comparison, rather than the best (=lowest), which we used. They also had fewer subjects, many of whom had the same, low, electrogustometric threshold value. Their chemical tests were on the whole mouth only. We tested not only whole mouth but also each side of the tongue separately. A localised taste loss does not show in a whole mouth challenge, due to release of inhibition, taste phantoms, and localisation illusions [1]. We chose to use the VAS score as an indicator of ‘‘poor tasters’’, because the categorical data cannot be used for calculations as if they were continuous, and we set the arbitrary cut-off level for ‘‘poor tasters’’ to VAS 50 on the assumption that at the solution concentrations we were using, taste perception should have been quite strong. A lower VAS cut-off affects the figures of PPV, but only marginally, and in any event, too few subjects would have been in some of the groups to make relevant calculations, in spite of our large, varied study group. We also chose to define 30 mA as the normal cut-off for electrogustometry, rather than 40 mA as suggested earlier by Grant et al. [16], as we have previously shown 30 mA to be an appropriate level for the automated version of electrogustometry, in which the computer produces a repeatable,
480
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
predetermined level of performance with minimal subject bias [12]. The current density is thought to correlate better with the true electrical taste stimulus than current intensity, and therefore it is important to note the size of the stimulus electrode in the comparison of data obtained by electrogustometry [17]. There are two hypotheses for the mechanisms of electrical taste: (a) The electrolyte–chemical hypothesis, which proposes that the anode creates an acid solution by electrolysis, and the surplus of H+ ions then stimulates the taste cells specific for the sour quality. (b) The direct-effect hypothesis, which claims that electrical taste is the result of an action of the current on either nerve fibres or taste receptor cells [18]. Only the sour-sensitive taste receptor cells are thought to have low apical resistance, which permits the current to enter the cells [11].
5. Conclusion Our results infer that electrogustometry measures a function of taste perception somewhat different from that induced by chemical stimuli. We propose that there is a direct depolarising mode of action on the sensori-neural apparatus. There must be some involvement of taste cells, as current density is related to the number of fungiform papillae in the stimulated region [19]. Further, our results do not support the theory that electrogustometry specifically measures the sour taste quality, as we found weak correlations with all four taste qualities. Electrogustometry is a clinically useful tool in evaluation of taste nerve function but it cannot be interpreted as necessarily a measurement of sour taste perception.
Acknowledgements Financial assistance from the Head and Neck Trust, The Green Lane Research and Education Trust Fund, and the Maurice and Phyllis Paykel Trust was provided for D.G. to perform the taste testing. E.E.’s time in New Zealand was supported financially by the Swedish Association of Otorhinolaryngology Head and Neck Surgery, the Swedish Society of Medicine, the Go¨teborg Medical Society, and the Acta Otolaryngologica Foundation, Sweden.
References [1] Bartoshuk L. Clinical evaluation of the sense of taste. ENT J 1989;68:331–7. [2] Yamauchi Y, Endo S, Sakai F, Yoshimura I. A new whole-mouth gustatory test procedure. 1. Thresholds and principal components analysis in healthy men and women. Acta Otolaryngol Suppl 2002;39–48. [3] Tsuruoka S, Wakaumi M, Araki N, Ioka T, Sugimoto K, Fujimura A. Comparative study of taste disturbance by losartan and perindopril in healthy volunteers. J Clin Pharmacol 2005;45:1319–23. [4] Ahne G, Erras A, Hummel T, Kobal G. Assessment of gustatory function by means of tasting tablets. Laryngoscope 2000;110:1396– 401. [5] Tomita H, Ikeda M. Clinical use of electrogustometry: strengths and limitations. Acta Otolaryngol Suppl 2002;27–38. [6] Murphy C, Quinonez C, Nordin S. Reliability and validity of electrogustometry and its application to young and elderly persons. Chem Senses 1995;20:499–503. [7] Tomofuji S, Sakagami M, Kushida K, Terada T, Mori H, Kakibuchi M. Taste disturbance after tonsillectomy and laryngomicrosurgery. Auris Nasus Larynx 2005;32:381–6. [8] Morton RP, Hay KD, Goldsmith DB, Stillman JA. Patterns of sensory recovery in the lingual nerve after surgical trauma. N Z Dent J 2005;101:53–7. [9] Nin T, Sakagami M, Sone-Okunaka M, Muto T, Mishiro Y, Fukazawa K. Taste function after section of chorda tympani nerve in middle ear surgery. Auris Nasus Larynx 2006;33:13–7. [10] Ikui A. A review of objective measures of gustatory function. Acta Otolaryngol Suppl 2002;60–8. [11] Lindemann B. Taste reception. Physiol Rev 1996;76:718–66. [12] Stillman JA, Morton RP, Goldsmith D. Automated electrogustometry: a new paradigm for the estimation of taste detection thresholds. Clin Otolaryngol Allied Sci 2000;25:120–5. [13] Stillman JA, Morton RP, Hay KD, Ahmad Z, Goldsmith D. Electrogustometry: strengths, weaknesses, and clinical evidence of stimulus boundaries. Clin Otolaryngol Allied Sci 2003;28: 406–10. [14] Lobb B, Elliffe DM, Stillman JA. Reliability of electrogustometry for the estimation of taste thresholds. Clin Otolaryngol Allied Sci 2000;25:531–4. [15] Salata J, Raj J, Doty R. Differential sensitivity of tongue areas and palate to electrical stimulation: a suprathreshold cross-modal matching study. Chem Senses 1991;16:483–9. [16] Grant R, Ferguson MM, Strang R, Turner JW, Bone I. Evoked taste thresholds in a normal population and the application of electrogustometry to trigeminal nerve disease. J Neurol Neurosurg Psychol 1987;50:12–21. [17] Ajdukovic D. Electrical taste stimulus: current intensity or current density? Chem Senses 1990;15:341–7. [18] Bujas Z. Electrical taste. In: Autrum H, Jung R, Loewenstein W, MacKay D, Teuber H, editors. Handbook of sensory physiology. Berlin/Heidelberg/New York: Springer-Verlag; 1971. p. 180–99. [19] Miller SL, Mirza N, Doty RL. Electrogustometric thresholds: relationship to anterior tongue locus, area of stimulation, and number of fungiform papillae. Physiol Behav 2002;75:753–7.
Studies on the relationship between electrogustometry and sour taste perception Eva K. Ellega˚rd a,*, David Goldsmith b, K. David Hay b, Randall P. Morton c a
Department of Otorhinolaryngology, Kungsbacka Hospital, S-434 80 Kungsbacka, Sweden b Oral Health Unit, Green Lane Hospital, Auckland, New Zealand c Department of Otorhinolaryngology Manukau Super Clinic, Manukau, New Zealand Received 15 June 2006; accepted 22 March 2007 Available online 27 April 2007
Abstract Objective: Electrogustometry is used as a measurement of taste perception. The prevailing theory is that the anodal current delivered to the tongue mucosa stimulates the sour taste receptors, but this is not universally accepted. Our aim was to evaluate to what extent electrogustometry relates to an ability to detect sour taste—rather than sweet, salt, or bitter. Methods: We compared automated electrogustometric thresholds with visual analogue scale (VAS) ratings of various tastant solutions in 114 subjects. The whole mouth, and each side of the tongue were tested separately. VAS scores from the strongest set of solutions, and the lowest electrogustometry thresholds for each location were used for statistics. Results: There was a significant correlation between electrogustometry threshold and the whole mouth perception of the salt taste solution. Electrogustometry correlated significantly but weakly for all taste qualities when testing was confined to left and right oral tongue. The positive predictive values of electrogustometry were no better in relation to sour taste perception than to the other taste qualities. Conclusions: Our results do not support the theory that electrogustometry is mediated by sour taste receptors or even that it reflects the sour taste quality. We postulate that electrogustometry measures a function of taste perception, which is different from that induced by chemical stimuli. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Electrogustometry; Taste; Visual analogue scale; Subjective; Method
1. Introduction Measurement of taste perception in human subjects can be performed by several methods with different types of stimuli: perceived intensity of solutions of different strengths and taste qualities, used as a whole mouth wash or applied on parts of the oral mucosa [1], thresholds to oral solutions [2], or to solutions instilled in filter paper disks [3], or in tablets [4]. Electric current is the stimulus in electrogustometry, and it has been used for the study of taste loss from various causes [5], like age [6], tonsillectomy and laryngomicrosurgery [7], extraction of molar teeth [8], drug side effects [3], and middle ear surgery [9]. These * Corresponding author. Tel.: +46 300 565284; fax: +46 300 565301. E-mail address: [email protected] (E.K. Ellega˚rd).
methods for measuring taste depend on the subject’s response. Involuntary measures of taste pathways involve gustatory evoked potentials, which can also be used together with positron emission tomography, and functional magnetic resonance imaging [10]. In our report of sensory disturbances after lower third molar tooth extraction, we noted that despite an almost total lack of qualitative taste sensation for each of the four basic tastes, automated electrogustometry unexpectedly showed normal thresholds [8]. This raised the question of what the electrogustometer actually does measure. The prevailing theory is that the weak anodal current delivered to the surface of the tongue elicits a sour taste experience [11]. The aim of this study was to evaluate to what extent electrogustometry relates to an ability to appreciate sour taste, rather than sweet, salt, or bitter.
0385-8146/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.anl.2007.03.004
478
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
2. Subjects and method Data from 66 women and 48 men aged 23–92 (mean 60 years) were included in the study. The subjects comprised healthy controls (N = 10), patients who had been treated with radiotherapy for head-and-neck cancer (N = 34), patients with Sjo¨gren’s syndrome (N = 31), and patients with burning mouth syndrome or oral dysesthesia (N = 39), in order to register a wide range of taste capacity. All subjects were tested by automated electrogustometry, using a computer-controlled two-alternative forcedchoice technique described earlier [12,13], with a stimulating anode diameter of 5 mm. This method can measure electrogustometric thresholds of less than 3 mA [14], and thresholds greater than 30 mA are considered to be pathological [13]. Solutions of different concentrations for each of the four basic qualities (sweet, sour, salt and bitter) were used to challenge the subjects’ perception of taste, using a method we have described elsewhere [8]. The solutions were sucrose (1, 0.3, 0.1, 0.03 M), citric acid (0.032, 0.01, 0.003, 0.001 M), sodium chloride (1, 0.3, 0.1, 0.03 M), and caffeine (0.1, 0.03, 0.01, 0.003 M). A water control was included in each set. The order of presentation, the solution strength for each taste quality and the side of the tongue to be tested first was randomised by drawing blindly from a container that held each option on separate pieces of paper. Cotton buds dipped in the respective solutions were applied to the lateral border of the anterior tongue. The subject was aware of which taste quality was being used. After each presentation and before withdrawing the tongue, the subject was asked to score the intensity of the taste by making a cross on a visual analogue scale (VAS), in the form of a simple line scale with word anchors ‘‘no taste’’ at the left end, and ‘‘greatest imaginable taste’’ at the right. After each VAS scoring, the mouth was rinsed with distilled water. The above process was also performed using whole mouth solutions in 4 ml samples, taken as mouth rinses and then expectorated. For the purposes of analysis, the VAS was considered to be 0 at the left end, and 100 at the right end. At the strongest whole mouth concentrations all subjects registered above the detection threshold (i.e. >0), so we used the results from those concentrations for statistical analyses with a cut-off level of VAS <50 for ‘‘poor tasters’’. An electrogustometric threshold of 30 mA was the upper limit of normal. Positive predictive values (PPV: the proportion of those with a high electrogustometric threshold who were ‘‘poor tasters’’) were calculated for each of the taste qualities for each side of the tongue separately, and for whole mouth. 95% confidence intervals (CI), taken from binominal distribution were calculated for the values of PPV. Spearman’s correlation was performed for VAS in each of the taste qualities against the lowest (i.e. best) electrogustometry value for each subject, for whole mouth and for each side separately. Friedman’s nonparametric test was performed on the VAS values for the different taste qualities for whole mouth, and each side of the
tongue separately. Wilcoxon’s signed rank test with Bonferroni correction for multiple comparisons was used pairwise on the different whole mouth taste qualities. P < 0.05 was considered statistically significant. All participants gave their informed consent, and the Auckland Ethics Committee ‘‘X’’ approved the study.
3. Results Five subjects did not perform the whole mouth tests. Three of the cancer patients were tested before radiotherapy. Individual registrations, and the correlations, are shown in figure and table. There was a significant correlation between electrogustometry thresholds and the whole mouth perceptions of the salt taste solution (Fig. 1). Electrogustometry correlated significantly but weakly for all taste qualities when testing was confined to left and right tongue (Table 1). The whole mouth and left and right tongue VAS values were significantly different between the four taste qualities (P < 0.001 for all). For whole mouth VAS, bitter scored significantly lower (i.e. less) than salt (P < 0.001), and sour (P = 0.024), and sweet scored lower than salt (P < 0.001) (Fig. 1). The 95% CIs for PPV of the various taste qualities for an abnormal electrogustometric threshold (i.e. >30 mA) were not significantly different.
4. Discussion Among our 114 subjects, some healthy but most with different conditions which could impair their taste ability, we could not find specific support for the theory that electrogustometry relates particularly to sour taste quality. Even though we found significant correlations between VAS and electrogustometry thresholds, the regression coefficients were relatively low and did not favour sour taste. Moreover, we found that the PPV figures for sour taste were no better than for the other taste qualities. The study included a comparatively large mixed group of subjects of different ages, with different taste abilities, and the chemical tests were performed on whole mouth, as well as on each side of the tongue separately. Some subjects may have an inability to distinguish sour from other taste qualities, but in our group we found significant differences Table 1 Spearman’s correlation on one-sided solutions VAS scores vs. automated electrogustometry threshold values in 114 subjects for each of the four basic taste qualities
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
479
Fig. 1. Whole mouth washes VAS scores 0–100 (y-axis) vs. automated electrogustometry threshold values on best side (x-axis) in 114 subjects for each of the four basic taste qualities (Spearman’s correlation).
between taste qualities in left, right, and whole mouth challenges in intra-subject comparison. It may be more relevant to use the threshold value for qualitative taste rather than to use supra-threshold VAS scores, as chemical thresholds may not correlate well with supra-threshold profiles [1]. However, in our evaluation we chose the strongest solutions, as that concentration was perceived by all subjects and provided a more complete comparison between subjects. Some of the lower concentrations exhibited less consistent results in our subjects and Bartoshuk has shown that loss of taste can occur over part of the concentration range to which an individual is sensitive [1]. Our results are consistent with the findings of others. Tomita and Ikeda found a group of 12 patients – from 129 with taste disorders – who had normal thresholds on electrogustometry, but no response to any of the qualitative filter paper disc tests of different tastes. They also reported that salt taste correlated best with electrogustometry using whole mouth detection and recognition thresholds [5]. Salata et al. proposed that electric taste sensation was different from a chemical stimulus, based on sensitivity patterns to supra-threshold electrogustometry and taste solutions. As a stimulation of tactile receptors does not occur at normal electrogustometry testing thresholds, they suggested that the electric current affects the taste cell membrane potential but not via the ion channels that the chemical stimuli use [15]. Murphy et al. found a poor correlation between electrogustometry thresholds and detection thresholds of citric acid and sodium chloride in 15 college students and 17 elderly adults. The dominant sensation produced
by electrogustometry at threshold levels was described as a ‘‘buzzing vibration’’ rather than any of the four basic taste qualities. They concluded that while electrogustometry can be used reliably for assessment of unilateral loss of taste, there is no correspondence with whole mouth assessment of chemical thresholds [6]. Murphy et al.’s study is comparable to ours, but there are several differences. They used the average threshold for left and right electrogustometry in the comparison, rather than the best (=lowest), which we used. They also had fewer subjects, many of whom had the same, low, electrogustometric threshold value. Their chemical tests were on the whole mouth only. We tested not only whole mouth but also each side of the tongue separately. A localised taste loss does not show in a whole mouth challenge, due to release of inhibition, taste phantoms, and localisation illusions [1]. We chose to use the VAS score as an indicator of ‘‘poor tasters’’, because the categorical data cannot be used for calculations as if they were continuous, and we set the arbitrary cut-off level for ‘‘poor tasters’’ to VAS 50 on the assumption that at the solution concentrations we were using, taste perception should have been quite strong. A lower VAS cut-off affects the figures of PPV, but only marginally, and in any event, too few subjects would have been in some of the groups to make relevant calculations, in spite of our large, varied study group. We also chose to define 30 mA as the normal cut-off for electrogustometry, rather than 40 mA as suggested earlier by Grant et al. [16], as we have previously shown 30 mA to be an appropriate level for the automated version of electrogustometry, in which the computer produces a repeatable,
480
E.K. Ellega˚rd et al. / Auris Nasus Larynx 34 (2007) 477–480
predetermined level of performance with minimal subject bias [12]. The current density is thought to correlate better with the true electrical taste stimulus than current intensity, and therefore it is important to note the size of the stimulus electrode in the comparison of data obtained by electrogustometry [17]. There are two hypotheses for the mechanisms of electrical taste: (a) The electrolyte–chemical hypothesis, which proposes that the anode creates an acid solution by electrolysis, and the surplus of H+ ions then stimulates the taste cells specific for the sour quality. (b) The direct-effect hypothesis, which claims that electrical taste is the result of an action of the current on either nerve fibres or taste receptor cells [18]. Only the sour-sensitive taste receptor cells are thought to have low apical resistance, which permits the current to enter the cells [11].
5. Conclusion Our results infer that electrogustometry measures a function of taste perception somewhat different from that induced by chemical stimuli. We propose that there is a direct depolarising mode of action on the sensori-neural apparatus. There must be some involvement of taste cells, as current density is related to the number of fungiform papillae in the stimulated region [19]. Further, our results do not support the theory that electrogustometry specifically measures the sour taste quality, as we found weak correlations with all four taste qualities. Electrogustometry is a clinically useful tool in evaluation of taste nerve function but it cannot be interpreted as necessarily a measurement of sour taste perception.
Acknowledgements Financial assistance from the Head and Neck Trust, The Green Lane Research and Education Trust Fund, and the Maurice and Phyllis Paykel Trust was provided for D.G. to perform the taste testing. E.E.’s time in New Zealand was supported financially by the Swedish Association of Otorhinolaryngology Head and Neck Surgery, the Swedish Society of Medicine, the Go¨teborg Medical Society, and the Acta Otolaryngologica Foundation, Sweden.
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