# Exploring the Relationship Between Astringency and Lingual Tactile Sensitivity

Anaïs Lavoisier, Alix Rollinat, Christophe Martin, Rohit Srivastava, Adrien Izzet, Anne Saint-Eve, Carole Tournier, Vincent Mathieu, Hélène Labouré

► To cite this version:

Anaïs Lavoisier, Alix Rollinat, Christophe Martin, Rohit Srivastava, Adrien Izzet, et al. Exploring the Relationship Between Astringency and Lingual Tactile Sensitivity. Journal of Texture Studies, 2026, 57 (1), pp.e70062. 10.1111/jtxs.70062. hal-05495400

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# Exploring the relationship between astringency and lingual tactile sensitivity

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## Lingual tactile sensitivity and astringency

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Anaïs Lavoisier¹, Alix Rollinat¹,², Rohit Srivastava¹, Adrien Izzet³, Christophe Martin¹,², Anne Saint-Eve³,

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Carole Tournier¹,², Vincent Mathieu⁴, Hélène Labouré¹

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¹ Université Bourgogne Europe, Institut Agro, CNRS, INRAE, UMR CSGA, 21000 Dijon, France

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² PROBE research infrastructure, ChemoSens facility, France

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³ UMR SayFood, AgroParisTech, INRAE, Université Paris-Saclay, France

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⁴ INRAE, Institut Agro, STLO, 35042 Rennes, France

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Corresponding Author: Hélène Labouré

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Email: helene.laboure@agrosupdijon.fr

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# Graphical Abstract

<figure><img src="image_2.png" /></figure>

* ✓ Oral tactile sensitivity varies across individuals and involves distinct perceptual mechanisms.

* ✓ A significant correlation was found between astringency detection threshold and tongue pressure sensitivity, suggesting a tactile contribution to astringency perception.

* ✓ No relationship was observed with roughness sensitivity or tongue

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# Abstract

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When lingual tactile sensitivity has received less intention than taste, it plays a critical role in food

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textural preferences and eating behaviour. The anterior tongue, rich in specialized
mechanoreceptors, is particularly sensitive to tactile stimuli. This study investigated inter-individual variability in lingual tactile sensitivity and its potential relationship with astringency sensitivity.

To this end, three distinct aspects of lingual tactile sensitivity as well as tongue strength were assessed in 39 subjects (26 F, mean age = 35 ±5 years). The three tactile dimensions included: (i) light touch (assessed with Von Frey monofilaments in a one-point pressure test), (ii) spatial perception (two-point discrimination test), and (iii) roughness sensitivity (using paper coupons with varying grit sizes). Astringency detection thresholds, previously determined using tannic acid solutions, were available for all participants.

Results revealed significant inter-individual variability across all tactile measures. Notably, no significant correlations were found between the three different tactile sensitivity tests (light touch, spatial perception, roughness), suggesting that each one captures a distinct aspect of lingual tactile functions. Tongue strength was not associated with either tactile or astringency sensitivity.

A key finding was a significant positive correlation between the tannic acid astringency detection threshold and the pressure discrimination threshold, suggesting that astringency sensitivity may involve a tactile component possibly mediated by the slowly adapting mechanoreceptors (SAI and SAll types). However, no relationship was found between astringency sensitivity and roughness sensitivity. The study emphasizes the need for standardized methods to better understand distinct dimensions of lingual tactile sensitivity and their influence on food texture perception.

Keywords

Lingual tactile perception, Oral mechanoreceptors, Astringency, Food texture, Inter-individual variability

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**Introduction**

Oral tactile sensitivity may play a key role in understanding individual differences in textural preferences and eating behaviour but, compared to taste sensitivity, it remains relatively underexplored (Aktar, Chen, Ettelaie, & Holmes, 2015; Galler & Varela, 2023; Lv et al., 2020a; Miles, Van Simaeys, Whitecotton, & Simons, 2018; Zhou et al., 2021). In the oral cavity, the tongue is particularly sensitive to tactile stimulations and studies have shown that the anterior part of the tongue is more sensitive to touch than the lips, palate, cheeks, and gums (Bearelly & Cheung, 2017; Miles, Berkowitz, Mueller, & Simons, 2022).

Tactile stimuli are detected in the mouth through specialized mechanoreceptors and transmitted to the brain by the trigeminal nerve (Haggard & De Boer, 2014). The physiology and morphology of the mechanoreceptors on the tongue are similar to those on the glabrous skin (e.g., on the palms of the hands and soles of the feet) (Foegeding, Vinyard, Essick, Guest, & Campbell, 2015). Four types of mechanoreceptors have been identified, two with slowly adapting nerve fibers (SA) responding to sustained static stimulation and, two with rapidly adapting nerve fibers (RA) responding to fast changes in stimulation. They are subdivided according to the type of receptor cell encapsulating the nerve ending which responds to a specific type of surface stress: SAI fibers are associated with Merkel's disk receptors (edges and points), SAII with Ruffini ending receptors (skin stretch), RAI with Meissner corpuscles (skin motion), and RAII with Pacinian corpuscles (skin vibration) (Roudaut et al., 2012). The only difference between mechanoreceptors on the tongue and on the glabrous skin is the absence of RAII fibers afferent to Pacinian corpuscles in oral tissues (Trulsson & Essick, 1997). To date, no other structures have been identified in the lingual mucosa, and the pathway responsible for the tongue ability to perceive fine surface roughness remains unknown (Miles et al., 2018). Signal patterns detected by trigeminal free nerve endings and the different mechanoreceptors of the oral cavity are integrated in the brain through higher-order cognitive processes to express the perception of the specific texture attributes of a food, such as creaminess or grittiness for example (Foegeding et al., 2015).
Lingual tactile sensitivity seems to vary from one individual to another (Breen, Etter, Ziegler, & Hayes, 2019; Linne & Simons, 2017; Mani et al., 2022), but the physiological variables behind this inter-individual variability in young healthy adults are still unclear (Liu, Cattaneo, Papavasileiou, Methven, & Bredie, 2022). For example, fungiform papillae density (FPD) on the anterior part of the tongue seems to be related to tactile sensitivity, even though results available in the literature are contradictory. Zhou et al. (2021) reported a positive correlation between FPD and tactile sensitivity measured with a one-point pressure test (n = 94, 64 F, 30 M, age range = 18 – 70 y.), while Nachtsheim & Schlich (2013) did not observe a significant correlation between these two parameters using a similar test (n = 116, 84 F, 32 M, age range = 19 – 39 y.) Moreover, methodological differences between studies further complicate comparisons.

In addition, comparisons between studies are not always straightforward due to the diversity of techniques used to measure tactile sensitivity on the tongue.

Methodological approaches to assess the lingual tactile sensitivity have been recently reviewed by several authors (Galler & Varela, 2023; Liu et al., 2022; Riantiningtyas et al., 2024). A series of tests is available to assess the lingual's tactile sensitivity: one-point pressure test, two-point discrimination test, grating orientation task, letter recognition, stereognosis tests, etc. However, standardized methods are lacking, and there is currently no consensual recommendation for measuring lingual tactile sensitivity in the food context. Moreover, dynamic aspects of oral tactile perception remain largely under-investigated (Galler & Varela, 2023).

On another note, astringency, which is defined as "the complex of sensations due to the shrinking, drawing or puckering of the epithelium as a result of exposure to substances such as alums or tannins" by The American Society for Testing and Materials (ASTM, 2004), is often described in the literature as a tactile sensation (Bajec & Pickering, 2008a; De Wijk & Prinz, 2006b; Wang, Smyth, Olarte Mantilla, Stokes, & Smith, 2024a). However, the contribution of oral tactile sensitivity to the sensitivity of astringency has not been systematically studied. To our knowledge, only one study has been published on this subject: Linne & Simons (2017) investigated the influence of lingual sensitivity
91 to surface roughness on the sensitivity of two astringent polyphenolic compounds (i.e.,
92 epigallocatechin gallate (EGCG) and, tannic acid). In a group of 30 individuals (18 F, 12 M, age range =
93 21 - 60 y.), they observed a strong relationship between lingual sensitivity to roughness and the
94 perception threshold for astringency triggered by EGCG, but no correlation was observed when using
95 tannic acid. Their results suggest that the sensitivity of astringency could be, at least partially, linked
96 to the sensitivity of individuals to a purely mechanical stimulus (Linne & Simons, 2017). To date, no
97 study has simultaneously examined multiple tactile modalities of the tongue in relation to
98 astringency thresholds.

99 The aim of this study was, first, to assess inter-individual variability in lingual tactile sensitivity, and,
100 second, to explore the relationship between the tactile sensitivity of the lingual and the sensitivity of
101 astringency. We hypothesized that higher tactile sensitivity, particularly to pressure or roughness,
102 would be associated with greater astringency sensitivity. Tongue strength and three different
103 dimensions of lingual tactile sensitivity were quantified (i.e., sensitivity to light touch, spatial
104 accuracy, and roughness sensitivity) and analyzed in relation to astringency detection thresholds
105 assessed using tannic acid solutions.

106 **Materials & Methods**

107 **1. Evaluation** of lingual tactile sensitivity

108 **1.1. Ethics**

109 This study was approved by the Research Ethics Committee (CER) of Université Bourgogne Franche-
110 Comté (CER UBFC-2024-06-03-020). All subjects provided an informed written consent and received
111 financial compensation for their participation.

112 **1.2. Subjects**

113 Thirty-nine subjects (26 F and 13 M, aged 23-42 y., mean age = 35 ±5 y.) participated in the study. To
114 be included in the study, subjects had to meet the following criteria: not having any known food
115 allergies, not being subject to a chronic pathology, not having olfactory and/or taste perception
116 disorders, not being pregnant or breastfeeding, and not smoking or smoking less than once a week.
117 Astringency detection thresholds of these subjects were previously assessed by Martin, Rollinat,
118 Tournier, & Peltier, (2025) using tannic acid solutions following the Best Estimated Threshold (BET)
119 method (ASTM, 2011). These 39 participants constitute a subgroup of those included in the panel
120 described in Martin, Rollinat, Tournier, & Peltier (2025).

121 **1.3. Experimental conditions**

122 Evaluations took place in a private exam room and each subject participated in one individual
123 session. The mean duration of a session was 50 minutes. Participants were seated and blindfolded
124 with a sleep mask. All assessments were performed on the anterior part of the tongue, at the
125 midline, which has been shown to be one of the most sensitive regions to both mechanical and
126 electrotactile stimulation (Moritz Jr., Turk, Williams, & Stone-Roy, 2017). Subjects were instructed to
127 bring their tongue back into their mouth between each test in order to avoid tongue dryness. They
128 also had access to water to rinse their mouth according to their needs at any point of the session. No
129 feedback was given to participants during the session.

130 **1.4. Evaluation of sensitivity to light touch (one-point pressure test)**

131 **1.4.a. Material**

132 Sensitivity to light touch was measured using a set of calibrated monofilaments (Von Frey hairs,
133 Aesthesio® Precision Tactile Sensory Evaluators, DanMic Global LLC, San Jose, CA). Filaments ranging
134 from 0.02 g to 1 g were used. The amount of force applied from these filaments is discrete and the
135 progression is nonlinear, as the nominal pressure levels (in grams) are predetermined by the
136 manufacturer (0.02, 0.04, 0.07, 0.16, 0.4, 0.6, 1). A new set of filaments was purchased prior to
137 testing, and no attempt was made to verify manufacture provided values. Filaments were sanitized
138 immediately after the test.

139 **1.4.b. Estimation of the discrimination threshold**

140 First, we evaluated which size of filament needed to be used as a reference depending on subject
141 sensitivity. Subjects should be able to clearly perceive this light stimulus in order to compare it with
the following stronger stimuli. A two-interval-forced-choice (2-IFC) task was used to validate the
detection of the reference filament. Subjects heard the researcher say “trial one” and “trial two”,
with the stimulus being presented randomly in only one trial. The subject indicated verbally in which
trial they felt the stimulus; if subjects were unsure, they were told to make their best guess (i.e., a
forced choice). All subjects started at 0.02 g (0.196 mN) of nominal force. When a subject gave 3
correct answers, this stimulus level was validated and used as a reference for further testing. If the
subject was incorrect the researcher increased the stimulus to the next level up, until the subject was
able to give 3 correct answers. Subjects that were not able to perceive a nominal force < 0.16 g
(1.570 mN) were excluded from the study.

Then, discrimination threshold estimates were determined using a similar 2-IFC approach (Breen et
al., 2019; Etter, Miller, & Ballard, 2017). In this case the subjects received stimuli in both trials one
and two. They were asked to indicate in which trial they felt the “strongest or hardest” force. The
presentation order of the stimuli was randomised. Again, a staircase method was used.
Discrimination trials started at 3 levels above the reference filament previously validated. For
example, if a subject was able to perceive a nominal force of 0.02 g, this reference was compared to
a 0.16 g stimulus. When a subject gave 3 consecutive correct answers at a specific stimulus level, the
researcher decreased the “strongest force” filament to the next lower level. If the subject was
incorrect, the researcher increased the “greater pressure” stimulus to the next level up (i.e., 3 correct
answers = go down, 1 incorrect answer = go up). The test stopped after 3 incorrect answers at the
lowest level reached in the staircase, or after 5 consecutive correct answers at the lowest level of
stimulation.

The discrimination threshold of each subject was then estimated according to Etter, Breen, Alcala,
Ziegler, & Hayes (2020), as follows: all trials conducted between the first attempt at the lowest force
level and the final trial of the protocol were included. The corresponding filament forces were
averaged over these trials. All stimuli delivered between the first instance of the stopping point
167 pressure level and the last were added and then divided by the corresponding total number of trials.

168 This gives a threshold estimate mean that considers variability in the subject's answer sequence.

169 **1.5. Evaluation of spatial perception (two-point discrimination test)**

170 Spatial perception was studied using an ophthalmic Castroviejo compass (IC60100, Holtex, GSM-med, Aix-en Provence, France).

172 The minimum separation of two punctiform stimuli that can be discriminated by the subject as two distinct points was determined by lightly pressing the compass on the tongue. An active testing approach was used (Olarte Mantilla et al., 2022) and the compass was moved on the tongue in a forward direction for approx. 1 cm (i.e., light stroking action). The distance between the two punctate stimuli ranged from 4 to 0.5 mm. The largest distance was always used as the initial stimulus. A staircase method was used (i.e., 3 consecutive correct answers = go down to smaller distance between tips, 1 incorrect answer = go up to larger distance between tips), with steps of 0.5 mm. The subjects were instructed to indicate verbally whether they felt one or two stimulus points.
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179
180 The test stopped after 3 incorrect answers at the lowest level reached in the staircase, or after 5 consecutive correct answers at the lowest level of stimulation. The compass was sanitized immediately after the test. Threshold estimation was calculated as described in the previous section (cf. M&M 1.4.b).
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184 **1.6. Evaluation of roughness sensitivity (paper coupons of varying grit sizes)**

185 **1.6.a. Material**

186 The roughness sensitivity of the subjects was evaluated with a set of paper coupons (waterproof abrasive paper, electro-coated with silicon carbide, Dragon Abrasives Group Limited, Tianjin, China) with varying grit sizes. These paper coupons measured approx. 4x2 cm and were fixed with double-sided tape on wooden tongue depressors sticks. Seven coupons with different roughness were used.
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189
190 The surface roughness of the coupons was evaluated by optical profilometry (Altisurf 500, Altimet, Thonon-les-Bains, France). The arithmetic average roughness height (Ra) was derived as the average
of the deviation from the mean height of the profiles. Ra values of the coupons ranged from 3.7 to 7.9 µm and all the values are summarized in Table 1.

<table><caption>Table 1. Summary of the abrasive paper coupons used: coupon references, average grain size according to the classification of the Federation of European Producers of Abrasives, average and standard deviation of Ra from profilometry measurements.</caption><thead><tr><th>Coupon reference</th><th>P1000</th><th>P1200</th><th>P1500</th><th>P2000</th><th>P2500</th><th>P3000</th><th>P5000</th></tr></thead><tbody><tr><th>Grain diameter (µm)</th><td>18.3</td><td>15.3</td><td>12.6</td><td>10.3</td><td>8.4</td><td>7</td><td>5</td></tr><tr><th>Ra (µm)</th><td>7.9 ±<br>0.1</td><td>5.6 ±<br>0.4</td><td>5.4 ±<br>0.1</td><td>5.2 ±<br>0.4</td><td>4.6 ±<br>0.2</td><td>4.4 ±<br>0.5</td><td>3.7 ±<br>0.3</td></tr></tbody></table>

### 1.6.b. Estimation of the discrimination threshold

The roughness discrimination threshold was also studied with a 2-IFC approach, similarly to Miles et al., (2018). Subjects were presented with pairs of stimuli always containing the least rough stimulus (Ra = 3.7 µm), considered as the reference, and an additional rougher stimulus. Discrimination trials started at 3 levels above the reference with the coupon P-2000 (Ra = 5.2 µm). For each stimuli pair presented, subjects were asked to identify verbally which stimulus they perceived to be the “roughest”. The presentation order of the stimuli was randomised. The test was performed dynamically: while blindfolded, participants held one stimulus in each hand and successively placed each one in the mouth, actively moving it from the back to the front of the tongue to assess its roughness. Again, a staircase method was used, but the number of tests was reduced compared with previous tactile sensitivity tests (cf. M&M 1.4 & 1.5) to avoid irritations on the tongue (i.e., 2 consecutive correct answers to go down one level, 1 incorrect answer to go up, and stopping point after 2 incorrect answers at the lowest level reached in the staircase, or after 3 consecutive correct answers at the lowest level of stimulation).

Threshold estimation was calculated as described previously (cf. M&M 1.4.b).

### 1.7. Evaluation of tongue strength
213 Tongue strength was measured using the Iowa oral Performance Instrument (IOPI, IOPI Medical, 214 Redmond, WA).

215 Subjects were first instructed to place the IOPI air-filled bulb just behind their front teeth between 216 their tongue and palate. Then, they had to squeeze the bulb against their palate as hard as possible 217 by lifting their tongue for at least 2 seconds. Subjects were able to practice before performing the 218 actual experimental task. Pressures were recorded in kilopascals (kPa). Results were displayed in 219 real-time on the instrument and were visible to the subjects. This visual feedback allowed 220 participants to monitor their performance during each trial and encouraged them to adjust their 221 tongue posture or positioning in an effort to reach the highest possible pressure values

222 This task was repeated 5 times with short pauses between each trial (the IOPI bulb was removed 223 from the mouth so subjects could drink water and relax their tongue). The subject's maximum 224 tongue strength was calculated as the mean of the 3 highest peak pressures produced across this 225 series of 5 trials.

226 **1.8. Statistics**

227 Results are presented as mean values ± standard deviation. Statistical analysis was conducted using 228 XLSTAT Premium (version 2023.1.5 -1409 Addinsoft SARL, Paris, France). A significance level of 229 p<0.05 was applied to all statistical tests.

230 Unimodality of the data distributions was first assessed using Hartigan's dip test. The presence of 231 extreme values was tested according to Grubbs' test for outliers.

232 Normality of the datasets was tested using the Shapiro-Wilk test. When data followed a normal 233 distribution, comparison between two sample sets was performed using Student's t-tests; otherwise, 234 Mann-Whitney U tests were used.

235 Linear correlations were assessed with Pearson correlation coefficient or Spearman correlation when 236 the data did not follow a normal distribution.
## 237 Results

### 238 1. Sensitivity to light touch

239 All subjects were able to clearly perceive a nominal pressure < 0.16 g (Table 2). In most cases, a pressure of 0.02 g was used as a reference (32 subjects, 82% of total population).

<table><caption>241 Table 2: Filament force used as a reference depending on the sensitivity of the subjects</caption><thead><tr><td>Filament force (g)</td><td>Filament force (mN)</td><td>Number of subjects</td><td>% Total population</td></tr></thead><tbody><tr><td>0.02</td><td>0.196</td><td>32</td><td>82</td></tr><tr><td>0.04</td><td>0.392</td><td>6</td><td>15</td></tr><tr><td>0.07</td><td>0.686</td><td>1</td><td>3</td></tr></tbody></table>

242 With respect to the estimation of the discrimination threshold, 6 subjects (15% of total population) had a threshold < 0.02 g (0.0196.10⁻³ N) that was not quantifiable with this test. A pressure discrimination threshold of 0.010 g (0.0981 mN) was attributed to these subjects. According to Grubbs test for extreme values, results obtained for 3 subjects, who had pressure discrimination thresholds ≥ 0.16 g (1.57 mN), should be considered as outlier values (p = 0.014, α = 0.05) (Figure 1).

<figure><img src="image_3.png" /><figcaption>249 Figure 1: Thresholds estimated by the one-point pressure test</figcaption></figure>
250

For the remaining 36 subjects, the data followed a unimodal distribution, but did not meet the assumption of normality (W = 0.93; *p* = 0.024). The mean pressure discrimination threshold of the group (n = 36) was 0.041 ± 0.022 g.

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## 2. Spatial perception

254

The mean spatial perception threshold of the group (n = 36) was 1.61 ± 0.49 mm. Fifty percent of subjects had a spatial perception threshold between 1 and 1.5 mm, and only 2 subjects (6% of total population) had a spatial perception threshold ≥ 2.5 mm (Figure 2).

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256

<figure><img src="image_4.png" /><figcaption>Figure 2: Thresholds estimated by the two-point discrimination test</figcaption></figure>

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258

259

Nevertheless, the data distribution showed no extreme values according to Grubbs' test for outliers (G = 3.0, *p* = 0.05). The distribution was unimodal based on Hartigan's dip test (D = 0.05, *p* = 0.68), but did not follow a normal distribution as indicated by the Shapiro-Wilk test (W = 0.83, *p* < 0.001).

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## 3. Roughness sensitivity

263

A total of 15 subjects (42% of total population) had a roughness sensitivity threshold < 0.7 µm that was not quantifiable with this test. A roughness discrimination threshold of 0.6 µm was arbitrarily attributed to these subjects, but threshold test could have gone to a finer resolution (<0.5 µm). Distribution of roughness perception threshold values among this group of subjects was not

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265

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unimodal according to Hartigan's dip test (D = 0.13 *p* < 0.05). Fitting a two-component Gaussian mixture model does not provide a significantly better fit than the unimodal model (ΔBIC<0). The distribution, however, appears visually bimodal: 58% of the subjects had a threshold ≤ 1.1 µm and 42% of the subjects had a threshold ≥ 1.3 µm (Figure 3), although this difference is not statistically supported according to the Bayesian information criteria.

<figure><img src="image_5.png" /><figcaption>Figure 3: Discrimination thresholds estimated by the roughness sensitivity test</figcaption></figure>

Based on these results, two groups of subjects were identified: a high-sensitivity group (n = 21), with sensitivity values ranging from 0.6 to 1.3 µm, and a low-sensitivity group (n = 15), with values ranging from 1.58 to 2.28 µm. For both groups, no outliers were detected according to Grubbs' test (G = 3.9, *p* > 0.05 and G = 3.5, *p* > 0.05 for the low- and high-sensitivity groups, respectively). The distributions were unimodal according to Hartigan's dip test (D = 0.78, *p* > 0.05 and D = 0.05, *p* > 0.05, respectively), but did not follow a normal distribution as assessed by the Shapiro-Wilk test (W = 0.71, *p* < 0.0001 and W = 0.63, *p* < 0.0001, respectively).

The mean roughness sensitivity was 1.70 ± 0.20 µm for the low-sensitivity group and 0.69 ± 0.16 µm for the high-sensitivity group. The difference in roughness sensitivity between the two groups was statistically significant according to the Wilcoxon rank-sum test (W = 0, *p* < 0.0001) (Figure 4).
<figure><img src="image_6.png" /><figcaption>Figure 4: Discrimination thresholds for each group of roughness sensitivity test</figcaption></figure>

4. Tongue strength

In the group (n = 36), tongue pressure ranged from 34 to 85 kPa, with 2 subjects (5% of total population) under 40 kPa and 2 subjects (5% of total population) over 80 kPa (Figure 5). Tongue pressure data were unimodal according to Hartigan's dip test (D = 0.06, p > 0.05), contained no outliers as assessed by Grubbs' test (G = 4.6, p > 0.05), and followed a normal distribution according to Shapiro-Wilk normality test (W=0.97, p > 0.05). The mean tongue pressure score was 59 ± 11 kPa.

The expected normal range for this age group is 40-80 kPa, according to IOPI Medical (IOPI Medical, s. d.).
<figure><img src="image_7.png" /><figcaption>Figure 5: Pressure scores obtained with the IOPI test</figcaption></figure>

5. Astringency sensitivity

The detection threshold of tannic acid astringency for these subjects (n=36) was previously estimated by Martin et al. (2025) and ranged from 0.02 to 0.57g/L, except for one subject who had a detection threshold of 1.35 g/L. This subject was considered as an outlier and was excluded from the following steps of the analysis (Grubbs' test, G=4.5, *p* <0.0001). In the reduced sample (n=35), the astringency sensitivity data were unimodal (D=0.07, *p* >0.05), had no more outliers (G=3.3, *p* > 0.05) but did not follow a normal law (W=0.97, *p* <0.001). The mean astringency sensitivity threshold was 0.18 ± 0.16 g/L.

6. Correlations

Participants with high or low sensitivity to roughness did not differ significantly in astringency detection thresholds, tongue pressure, or any of the two other tests used to characterize lingual tactile sensitivity (light touch and spatial perception). Therefore, we examined the correlations across the entire group of 35 participants (Table 3).

<caption>Table 3: Spearman correlation value between each sensitivity parameters measured *p* <0.05</caption>
<table><thead><tr><th></th><th>Tongue Strength (kPa)</th><th>Pressure Discrimination Threshold (mN)</th><th>Spatial Sensitivity (mm)</th><th>Roughness Sensitivity (mm)</th><th>Astringency sensitivity (g/L)</th></tr></thead><tbody><tr><td>Tongue Strength (kPa)</td><td>1</td><td></td><td></td><td></td><td></td></tr><tr><td>Pressure Discrimination Threshold (mN)</td><td>0.043</td><td>1</td><td></td><td></td><td></td></tr><tr><td>Spatial Sensitivity (mm)</td><td>-0.135</td><td>0.239</td><td>1</td><td></td><td></td></tr><tr><td>Roughness Sensitivity (mm)</td><td>0.146</td><td>-0.220</td><td>0.162</td><td>1</td><td></td></tr><tr><td>Astringency sensitivity (g/L)</td><td>0.045</td><td><strong>0.382*</strong></td><td>0.305</td><td>0.018</td><td>1</td></tr></tbody></table>

311 No significant correlations were observed between threshold values obtained with the three different tests used to characterize lingual tactile sensitivity (light touch, spatial perception, and roughness). Subjects performed differently on each test, meaning, for example, that the same individual could be considered very sensitive with the two-point discrimination test and the roughness discrimination test, but had a relatively high sensitivity threshold in the one-point pressure test, and conversely. Moreover, tongue strength was not related to lingual tactile sensitivity or astringency sensitivity.

318 However, positive and significant correlations were observed between the tannic acid astringency detection threshold and the pressure discrimination threshold (Spearman's coefficient = 0.382, *p* =0.02) (Figure 6). Roughness discrimination thresholds, in contrast, were not related to astringency detection thresholds.

<figure><img src="image_8.png" /></figure>
Discussion

This study firstly assesses inter-individual variability in lingual tactile sensitivity, quantified through
measurements of sensitivity of light touch, spatial discrimination, and roughness. Secondly, it
explores the relationship between lingual tactile sensitivity and astringency detection thresholds, as
assessed using tannic acid solutions.

1. Inter-individual variability in tongue strength and lingual tactile sensitivity

a. Tongue strength

This group of subjects was representative of the physical capabilities of a group of healthy young
adults in terms of tongue strength. Results are in line with the literature, in terms of distribution and
mean pressure max scores (IOPI Medical, s. d.).

b. Sensitivity to light touch

In this study, we evaluated light touch pressure discrimination thresholds, meaning that participants
had to distinguish differences in intensity between two stimuli. This methodological approach was
chosen to avoid the “floor effect” (i.e., smallest filament size detected by a majority of the subjects)
often reported in the literature when measuring detection thresholds using Von Frey monofilaments
and the one-point pressure test (Breen et al., 2019; Etter et al., 2020; Lv et al., 2020; Santagiuliana et
al., 2019).

The mean pressure discrimination threshold observed in the group of subjects (0.04 ±0.02g) tended
to be lower than values estimated by others in similar experimental conditions (i.e., measurements
on the midline of the anterior part of the tongue with monofilaments and using a 2-AFC staircase
methodology). Etter et al. (2020) observed a mean discrimination threshold of 0.6 ± 1.8 g (range
0.02-15 g) in a group of 107 subjects (76 F, 31 M) with a mean age of 32 years (range 18-68 y.). This
discrepancy may be related to the age range of the subjects that were recruited in the present study
(i.e., 23-42 y.), since oral somatosensory acuity tends to decrease with age (Liu et al., 2022). Breen et
al. (2019) also reported higher discrimination threshold estimates: 0.22 ± 0.03 g (range 0.02-15.0 g) in a group of 47 subjects (no information on age and sex distribution available). However, they divided their participants into high and low sensitivity groups and estimated a discrimination threshold of 0.032 g for the high sensitivity group (against 0.957 g for the low sensitivity group), which is more consistent with the results obtained here. Moreover, with a slightly different methodology (i.e., did not use a reference filament but presented different pairs of stimuli: 0.16 g / 0.07 g, then 0.07 g / 0.04 g, etc.) (Mani et al., 2022) observed discrimination threshold values ranging from 0.008 to 0.160 g (mean 0.037 g), in a group of 37 F aged between 18 and 30 y., which is perfectly in line with the results observed in this study.

We did not observe a significant effect of sex on tactile sensitivity assessed by the one-point pressure test. Similarly, Appiani, Rabitti, Methven, Cattaneo, & Laureati (2020) did not observe any sex difference in tactile sensitivity assessed by Von Frey filaments (detection thresholds, n=70, 53% F, 19-33 y., mean age 22 ± 3 y.) However conflicting results are reported in the literature about sex differences in lingual mechano-sensations (Liu et al., 2022).

### c. Spatial perception

In this study, we assessed the two-point discrimination ability of the subjects with an active/moving approach, which might be more closely related to the dynamics of texture perception (Galler & Varela, 2023). The moving two-point discrimination test evaluates the function of rapidly adapting nerve fibers (RA) as detectors of transient touch (i.e., movement) (Won, Kim, Kim, & Kim, 2017).

The mean spatial perception threshold observed in this study (1.6 ± 0.5mm) was slightly lower than results reported in the literature. Using active measurements on the anterior midline part of the tongue, Olarte Mantilla et al. (2022) observed a mean threshold of 2.2 mm (n = 117, 54 F / 63 M, 18-65 y., mean age 34 ± 13 y.), Perez, Fuoco, Dorion, Ho, & Chen (2006) a mean threshold of 2.6 ± 1.2 mm (n=35, 14M/19F, mean age 35 ±11 y), and Won et al. (2017) a mean threshold of 2.4 ±0.5 mm (n = 40, 20 M / 20 F, 21-37 y., mean age 27 ± 3 y.) Methodological differences could perhaps explain the lower values observed in our study, and in particular the type of compass used. Won et al. (2017)
374 have shown that the results vary with the shape of the tips, but different materials are also often
375 used (i.e., plastic vs metallic tips) which are probably perceived differently (Howes, Wongsriruksa,
376 Laughlin, Witchel, & Miodownik, 2014). It should also be noted that the studies reporting thresholds
377 of 2.6 mm and 2.4 mm included relatively small sample sizes, whereas the study reporting 2.2 mm
378 included a larger sample. For this reason, differences in participant numbers were not initially
379 considered a likely explanation for the observed variations in threshold. Nevertheless, we
380 acknowledge that sample size could contribute to differences across studies. These results again
381 highlight the need for standardized and validated procedures to assess individual differences in
382 lingual tactile sensitivity (Galler & Varela, 2023).

383 d. Roughness sensitivity

384 In this study, we investigated roughness sensitivity measuring discrimination thresholds for relatively
385 large asperities (i.e., Ra from approximately 4 to 8 µm), while Just-Noticeable Difference (JND) values
386 of roughness may be < 0.05 µm, according to Linne & Simons (2017); Miles et al., (2022, 2018).
387 Asperities of this size (i.e., 1-10 µm) have been described elsewhere as moderately to strongly rough
388 (Linne & Simons, 2017). However, in the context of texture perception, it seemed more appropriate
389 since most food structures are in this order of magnitude.

390 This test clearly identified two sensitivity groups, in contrast to the two previous tactile tests. These
391 results are in agreement with Linne & Simons, (2017) who also observed a relatively large variability
392 in roughness detection thresholds and identified two levels of sensitivity (i.e., high and low) in a
393 group of 30 individuals (12 M / 18 F, 21-60 y.) According to these authors, roughness sensitivity was
394 not related to fungiform papillae density, salivary flow or age. In this study, it was not related to sex
395 or tongue strength. Further studies are necessary to understand whether these differences in
396 roughness perception are linked to physiological differences in the tongue (e.g., topography of the
397 surface of the tongue, lingual film composition and thickness, etc.)

398 2. Links between the three different tactile tests
In this study, no correlations were observed between results obtained with the different lingual tactile sensitivity tests. Each test characterises a specific aspect of lingual tactile sensitivity (i.e., different tactile functions underlined by different mechanoreceptors and different neural mechanisms), and it seems that they are not related to each other. But this subject has not been systematically studied and it is unclear if perception patterns could exist. Furukawa, Ito, Tanaka, Ito, & Hattori (2019) also found that results from the one-point pressure test and from the two-point discrimination test were not correlated. Similarly, Mani et al. (2022) and Appiani et al. (2020) did not find any correlation between results from the one-point pressure test and from the Grating Orientation Task (GOT).

On another note, no correlations were observed between tongue strength and tactile sensitivity, as previously reported by Steele, James, Hori, Polacco, & Yee (2014) using a letter recognition task.

### 3. Relationships between lingual tactile sensitivity and astringency sensitivity

The mean astringency detection threshold value observed in this group (0.18 ±0.16 g/L) was coherent with data reported in the literature for tannic acid astringency sensitivity (Martin et al., 2025; Nuvoli, Fillion, Lacoste Gregorutti, & Labbe, 2023; Valentová, Škrovánková, Panovská, & Pokorný, 2001).

Linne & Simons (2017) explored whether lingual tactile sensitivity to roughness would be an indicator of astringency sensitivity (n = 30, 12 M / 18 F, 21-60 y.). They reported a strong relationship between sensitivity to surface roughness and sensitivity to astringency when the astringent sensation was triggered by EGCG but not with tannic acid. In this study, a positive correlation was observed between tannic acid astringency sensitivity and light touch pressure sensitivity.

Mechanisms behind the sensitivity of astringency are still not clear. It has been demonstrated to be mediated via the trigeminal nerve (Nicole Schöbel et al., 2014), but different hypotheses have been proposed to explain how this nerve is stimulated. One of the main assumptions is that astringent compounds modify the lubrication properties of salivary proteins and/or the mucosal pellicle, leading to an increase of the friction forces at the surface of the oral mucosa, activating the mechanoreceptors in the oral epithelium (Canon et al., 2021; Canon, Neiers, & Guichard, 2018;
Nuvoli et al., 2023). However, it is not known whether such friction forces are sufficient to activate trigeminal mechanoreceptors. An alternative hypothesis is that trigeminal chemoreceptors could also be involved via molecular mechanisms leading to the release of neurotransmitters activating the free ends of the trigeminal nerve in the oral epithelium or via the activation of transient receptor potential (TRP) channels (Canon et al., 2021, 2018), or through G protein-coupled receptors (Espinase Nandorfy et al., 2025; N. Schöbel et al., 2014; Wang, Smyth, Olarte Mantilla, Stokes, & Smith, 2024b). The sensitivity of astringency could be related to different human factors, such as the fungiform papillae density (reflecting the density of the sensory innervation), the saliva flow rate and composition (related to aggregation and lubrication mechanisms (Ammam, 2024)) or the tactile sensitivity, but conflicting and/or inconclusive results have been reported in the literature (Wang et al., 2024a). For example, Linne & Simons (2017) observed that participants with higher salivary flow exhibited lower detection threshold for tannic acid astringency. This may result from higher salivary protein content in these individuals, which enhances protein-tannin interactions and reduces oral lubrication, thereby increasing astringency perception. However, this relationship was not consistently observed across all studies (Rollinat et al., 2025), suggesting that additional factors may also influence tannic acid sensitivity.

According to the results of this study, tannic acid astringency sensitivity may be related to the activation of the mechanoreceptors located in the anterior part of the tongue. Both superficial (i.e., SAI, with Merkel's disk receptors) and deeper (SAII, with Ruffini ending receptors) slowly adapting mechanoreceptors have been associated with the one-point pressure test (Tong, Mao, & Goldreich, 2013; Won et al., 2017). While astringency is generally considered an exteroceptive sensation resulting from lubrication loss at the mucosal surface, the mechanical stresses generated by increased friction may propagated into deeper tissue layers. Such stress transmission could recruit SAII afferents and thus contribute to the observed association between pressure sensitivity and astringency sensitivity. This potential involvement of deeper mechanoreceptive pathways remains speculative but deserve further investigation.
Given that changes in oral lubrication due to astringent compounds generate friction and vibration signals (Huang et al., 2025), we hypothesized that the sensitivity of astringency would be perceived similarly as the sensitivity of roughness. However, in this study, tannic acid astringency sensitivity was not related to roughness sensitivity. It is possible that the salivary film reduces the tongue's sensitivity to fine roughness, which could mask a potential association between astringency and roughness sensitivity. Vibration sensitivity was not evaluated in this study. It would be interesting to develop a method for assessing tongue sensitivity to vibration, to better understand the involvement of this sensation in the sensitivity of astringency and more generally, food texture. In addition, the pathway responsible for the tongue's ability to perceive fine surface roughness remains to be elucidated.

Our findings strengthen the hypothesis that a tactile dimension underlies astringency sensitivity, in line with the findings of Bajec & Pickering (2008); De Wijk & Prinz (2006); Wang et al. (2024). In the case of tannic acid in particular, this tactile component is likely mediated by the cross-linking of the lubrication salivary proteins pellicle, which alters the lubrication of the oral surface (De Wijk & Prinz, 2006). Our results therefore suggest that individuals with higher pressure sensitivity on the tongue may also be more capable of detecting changes in friction or tactile sensations induced by astringent compounds.

De Wijk & Prinz (2006) suggested that-particulate precipitates formed by protein aggregation might contribute to tactile sensation, potentially producing graininess-like cues rather than true surface roughness. However, this mechanism has not been validated, and such aggregates may be soft and unlikely to generate a strong tactile signal. In our study, we did not find a link between tannic acid-induced astringency sensitivity and roughness sensitivity. This discrepancy might also reflect our roughness sensitivity test setup, which used small abrasive particles fixed to a static support rather than free-moving particles, possibly limiting the ecological validity of the tactile experience and explaining the lack of correlation between roughness perception and astringency sensitivity.
476 Beyond astringency, our findings may have implications for understanding the perception of other
477 textural attributes that involve interactions with oral surfaces and mechanoreceptor stimulation. For
478 instance, creaminess and smoothness are often associated with low friction (De Wijk & Prinz, 2006).
479 Pressure sensitivity may also contribute to the perception of changes in lubrication underlying these
480 sensations. People more sensitive to pressure, may be more adept at detecting subtle differences in
481 creaminess or smoothness.

482 Although the study by Aktar et al. (2015) did not find a direct relationship between general tactile
483 sensitivity and viscosity discrimination, other studies, such as that by Lv et al. (2020), reported a
484 correlation between vibratory sensitivity of the tongue and viscosity discrimination. Our results
485 therefore suggest that different types of tactile sensitivity (e.g., pressure vs. vibration) may be
486 involved in the perception of different texture attributes. It should be noted, however, that vibratory
487 sensitivity was not assessed in the present study, representing a limitation and a potential avenue for
488 future research on oral texture and astringency perception.

489 Conclusions

490 In conclusion, the observed link between tongue pressure sensitivity and the sensitivity to tannic
491 acid-induced astringency supports the presence of a tactile component in this sensation, in line with
492 hypotheses suggesting the involvement of slowly adapting mechanoreceptors such as SAI and SAII
493 types. This finding enhances our understanding of astringency as a multisensory phenomenon and
494 suggests that individuals with heightened pressure sensitivity may be more attuned to frictional
495 changes caused by astringent compounds. Future studies could explore the differential roles of
496 pressure receptors and surface mechanoreceptors in perceived astringency, particularly regarding its
497 sub-quality (roughness, drying, and pucker), as different sub-qualities may be linked to different
498 degrees of activation of these receptor categories.
Beyond astringency, these observations open up promising perspectives for exploring how different forms of oral tactile sensitivity—whether to pressure, movement, or vibration—could influence the perception of other textural properties such as creaminess, smoothness, or roughness. The diversity of responses observed across the different tongue sensitivity tests further highlights the importance of better characterizing the distinct dimensions of lingual tactile sensitivity, each likely relying on specific physiological mechanisms.

Finally, these results underscore the need to develop standardized and complementary methods for assessing lingual tactile sensitivity in order to better capture individual differences in oral tactile sensitivity, with potential applications in sensory science, personalized nutrition, and the design of texture-optimized food products.

## 509 Acknowledgments

This research leading to these results was funded by the "FlaveurVeg" grant, from Carnot Qualiment© (DOI:10.17180/h5gd-gk88) supported by Agence Nationale de la Recherche (20 CARN 0026).

## 513 Data availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## 516 Ethical statement

## 517 Conflict of Interest Statement

The authors declare that they do not have any conflict of interest.
519 Ethical review

520 This study was approved by the Research Ethics Committee (CER) of Université Bourgogne

521 Franche-Comté (CER UBFC-2024-06-03-020).

522 Consent statement

523 Written informed consent was obtained from all study participants.

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