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J Dent Res 83(5): 388-392, 2004
© 2004 International and American Associations for Dental Research

RESEARCH REPORT
Clinical

Relationship between Oral Sensitivity and Masticatory Performance

L. Engelen1,2,*, A. van der Bilt2, and F. Bosman2

1 Wageningen Centre for Food Sciences, 6700 AN Wageningen, the Netherlands; and
2 Department of Head and Neck, Oral Physiology Group, University Medical Centre Utrecht, PO Box 85060, 3508 AB Utrecht, the Netherlands;

* corresponding author, l.engelen{at}med.uu.nl


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The size of a bolus determines how it will be manipulated in the mouth and swallowed. We hypothesized that mucosal sensitivity would be important for masticatory function. The accuracy of solid object size perception, spatial acuity, and food particle size reduction during mastication were measured in 22 healthy adults with/without topical anesthesia of their oral mucosa. Topical anesthesia had no effect on the perception of sphere sizes, but significantly reduced spatial sensitivity. Without anesthesia, there was a correlation between an individual’s ability to perceive the sizes of steel spheres (diameter, 4–9 mm) and the sizes of food particles chewed for 15 cycles and at swallowing. There was no correlation between spatial sensitivity and food particle size. We suggest that the stimuli used to test two-point discrimination stimulates only superficial receptors, which involve light touch and are easily anesthetized, while the spheres might excite more deeply-set receptors. The latter appear to be more important for masticatory performance and swallowing.

KEY WORDS: oral sensitivity • human • size perception • topical anesthesia • masticatory performance


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Sensitivity of the mouth includes the ability to assess shape, size, and surface texture. Oral sensitivity has often been measured to track damage and rehabilitation after strokes (Pow et al., 2001), prosthodontic treatment (Berry and Mahood, 1966; Muller et al., 1995), and for speech disorders (Speirs and Maktabi, 1990). Various methods to measure oral sensitivity have been used, including oral form recognition (Berry and Mahood, 1966; Litvak et al., 1971; Landt and Fransson, 1975; Grasso and Catalanotto, 1979; Garrett et al., 1994), interdental size and weight discrimination tests (Williams and La Pointe, 1972), intra-oral size judgments of small holes (Anstis and Loizos, 1967; Lamey et al., 1996; Bittern and Orchardson, 2000; Melvin and Orchardson, 2001), cylinders (Dellow et al., 1970), spheres (Engelen et al., 2002), and two-point discrimination (Ringel and Ewanowski, 1965). The last-named method has been standard since the 1860s and remains the most commonly used method for determining a subject’s tactile spatial resolution.

Information on the significance of various oral components in oral size perception and sensitivity is required if we are to understand their role in controlling mastication and swallowing. In the present study, we were specifically interested in oral sensitivity to size. Size can be sensed by pressure and stretch receptors in tongue and palate mucosa, in addition to mechanoreceptors in the periodontal ligament (Jacobs and van Steenberghe, 1994). Oral perception of size does not depend on the density or material of an object, but solely on its actual size and shape, and results from a combination of sensory inputs from the tongue and palate (Engelen et al., 2002). In the present study, we excluded input from the superficial layers of mucosa by applying topical anesthesia to the tongue and palate.

Masticatory function has been studied in various groups of subjects, such as dentate subjects (van der Bilt et al., 1994), partial- and complete-denture wearers (Slagter et al., 1993), and subjects with implant-retained overdentures (Fontijn-Tekamp et al., 2000). Masticatory performance is significantly reduced when dentures replace natural teeth. The most common way to study masticatory function is to determine an individual’s capacity to grind or pulverize a test food by analyzing the chewed material (Lucas and Luke, 1983). For safe swallowing to be ensured, the particle sizes in the bolus of the chewed food need to be detected. It has been suggested that the main site for detecting food particles is not between the teeth, but on oral mucosa (Prinz and Lucas, 1995). In addition, tongue motor skill is significantly correlated with masticatory performance (Koshino et al., 1997). Thus, information from oral mucosa, e.g., oral sensitivity, may be related to measurements of masticatory performance. If median swallowing particle size is related to one’s ability to assess objective size, the question arises whether a subject with good discriminative abilities also chooses to swallow smaller particles.

The aim of the present study was three-fold: First, we were interested in how size is perceived in the mouth and whether one’s ability to assess size is related to spatial sensitivity. Second, we studied how these features are influenced by topical anesthesia. Finally, we wanted to study how the median particle size at swallowing and masticatory performance are related to the subject’s ability to assess the sizes of objects and sensitivity in the mouth.

We hypothesized that topical anesthesia would affect oral perception of size and spatial acuity, and that the particle size at swallowing would depend on oral mucosal sensitivity.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Twenty-two healthy individuals (13 female and nine male, with an average age of 27.6 yrs) participated in the study on two-point discrimination and masticatory performance. Fourteen of these subjects (eight female and six male) also participated in the study on oral size perception. The Ethics Committee of the University Medical Center approved the protocol. Written informed consent was obtained from each subject after a full explanation of the procedure. All treatments and stimuli were administered in random order within each part of the study.

Oral Perception of Object Size
The chosen stimuli for oral perception of size were steel spheres (Dejay, Wokingham, UK) in 5 sizes, varying from 4 to 9 mm. The spheres were given to the subjects, one at a time, in black cups. A reference set of 8 spheres numbered zero to 7 was displayed in front of the subjects. The reference set included the 5 test sphere sizes plus 2-, 12-, and 15-mm spheres. The extra sphere sizes allowed subjects to under- or overestimate perceived sizes. The subjects were not informed about the extra sizes in the reference set. The subjects were instructed to transfer the sphere from the cup directly into the mouth without any visual or tactile cues about the object’s size and to assess the size of the sphere between the tongue and palate. Then, with the sphere still in the mouth, the subject matched its size with the size of a sphere in the reference set. The sphere was then spat into a collection cup with the number corresponding to the size of the sphere in the reference set. The procedure was performed according to Engelen et al.(2002). The same procedure was then repeated after topical anesthesia was administered, by liquid lidocaine (Xylocaine 10 mg, AstraZeneca, Zoetermeer, Netherlands) sprayed onto a cotton wool roll which was then rubbed over the tongue or tongue and palate. The anesthetic was left for 2 min, after which the subjects rinsed out any surplus with water.

Two-point Discrimination Threshold
The minimum separation of 2 punctiform stimuli that can be discriminated as 2 distinct points was determined by lightly pressing 2 pins onto the anterior part the tongue, with and without local anesthesia. The separation of the pins ranged from 0 to 8 mm, and the staircase method was used, with steps of 1 mm. The subjects were instructed to indicate whether they felt 1 or 2 stimulus points. Topical anesthesia was applied as described above.

Masticatory Performance and Swallowing Threshold
In an initial test, we determined the masticatory performance of the subjects by quantifying the degree of fragmentation of an artificial test food, Optocal Plus (Fontijn-Tekamp et al., 2000), without local anesthesia. The subjects chewed on portions of 17 cube-shaped particles (edge size, 5.6 mm; total, approximately 3 cm3) for 15 chewing strokes. We determined the degree of fragmentation of the chewed food by sieving the food through a stack of 9 sieves with apertures decreasing from 5.6 to 0.7 mm and a bottom plate. The amounts of test food on each sieve and on the bottom plate were weighed. The distribution of particle sizes of the comminuted test food can be mathematically described by a cumulative function (van der Bilt et al., 1993a) with the degree of fragmentation of the food given by the median particle size, X50, which is the aperture of a theoretical sieve through which 50% of the weight of the comminuted food could pass. In a second test, the subjects were instructed to chew until they were ready to swallow, but instead of swallowing, they spat out the test food. We determined the number of chewing strokes until the urge to swallow and determined the degree of fragmentation of the chewed food at the swallowing threshold.

Data Analysis
Object size perception was compared with the standard line, and the effects of the different treatments (control, anesthetized tongue, and anesthetized tongue and palate) were analyzed with regression analysis. We performed a paired t test to analyze the effect of anesthesia on two-point discrimination performance and to analyze the difference between actual and perceived object size in the mouth. Bonferroni correction was made for multiple comparisons (N = 5; p < 0.05/N = 0.01). Pearson’s correlation coefficients were calculated for the relations among object size perception, two-point discrimination, and the different parameters of masticatory performance. All analyses were performed with SPSS (9.0 SP 4M, SPSS Inc., Chicago, IL, USA). P < 0.05 was considered significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Regression lines of perceived object size after different treatments were determined (Fig.Go). Anesthetizing either the tongue or the tongue and palate had no significant effect on the subject’s ability to assess size, and accordingly, the subsequent analysis included the mean of the two treatments and the untreated condition. The slope of the regression line for the perceived object size (1.37 ± 0.02, N = 280) was significantly steeper than 1.0 (p < 0.001). A comparison between the actual and subjective object sizes revealed that there were significant differences at the far ends of the line, where small spheres (4.0 mm) were underestimated (N = 15, p = 0.001), while the sizes of spheres of 9.4 mm were overestimated (N = 15, p = 0.01).



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  		Figure. The relation between actual and perceived sphere size during normal conditions, with anesthetized tongue and palate, and anesthetized tongue. The thin line through the origin depicts the standard line where the perceived size equals the actual particle size.

 
Means and standard deviations for the two-point discrimination test, median particle size (X50) after 15 chewing strokes (15x) and at swallowing, number of chewing strokes, and chewing time until swallowing are shown in Table 1Go. Topical anesthesia had a significant effect on the tactile spatial resolution of the tip of the tongue, where the threshold increased on average with 0.4 mm ± 0.7 (N = 22), as observed in the two-point discrimination test. The median particle size (X50) at swallowing varied between 1.0 and 2.5 mm (N = 22; mean, 1.7 ± 0.4 mm), and was related to the median particle size after 15 chewing cycles (N = 22; range, 1.6 to 4.9; mean, 3.5 ± 0.8 mm). The number of chewing cycles and the time until swallowing were strongly and positively correlated (N = 22, r = 0.88; p < 0.001).


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  		Table 1. Results of Oral Sensitivity Tests
 
The correlation coefficients of perceived object sizes, masticatory performance, and two-point discrimination thresholds are depicted in Table 2Go. The median particle size after 15 chewing strokes was positively correlated with the oral perception of object size for particles of 6.4 mm and larger (N = 14, p < 0.05). However, no such relation was observed for median particle size at swallowing (X50 [swallow]). There was no relation between the spatial resolution on the tongue and the oral ability to perceive object size, nor with the median particle sizes, chewing time, and number of strokes.


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  		Table 2. Correlations among Oral Sensitivity Tests and Masticatory Performance (N = 14)
 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
When the objects in the mouth were matched with a visual reference set, the size of small spheres was underestimated, medium spheres correctly estimated, and large spheres overestimated. These results are in conflict with those of previous studies (Anstis and Loizos, 1967) that found no such disparity when the sizes of holes in objects were visually matched to intra-oral determinations. The discordance between these studies can be explained by the difference in experimental design (Engelen et al., 2002). When a subject assesses a hole, the tip of the tongue can sense the full extent of the hole, including the edges. During the assessment of sphere size, the subject would manipulate the sphere in the mouth by pressing it against the palate by the tongue. The area of the sphere touching the palate would be much smaller than the actual diameter. The tongue senses the whole diameter, while the palate senses only a small part of it, resulting in conflicting information, with spheres being perceived in the mouth as smaller than they actually are. Covering the palate with an acrylic plate removes this effect.

The observation in the present study that topical anesthesia had no effect on size perception was surprising. An explanation for this could be found in the degree to which the mucosa was anesthetized. The result of two-point discrimination, where the anesthetized tongue was significantly less sensitive, shows that the anesthesia was effective. This is in line with results from other studies (Dahan et al., 2000) where superficially anesthetized subjects needed significantly longer to complete an oral discrimination task. We suggest that the two-point discrimination test stimulates only superficial receptors, which involve light touch and are easily anesthetized. During the manipulation of the spheres, however, the spheres could be pressed against the palate and excite sensors set more deeply into the tongue. In this way, the effect of topical anesthesia might exclude superficial receptors, but possibly still include deeper-situated receptors.

A positive correlation between the median particle size at swallowing and after 15 chewing strokes was observed. This is consistent with results of previous studies (Abel and Manly, 1954; Yurkstas, 1965; van der Bilt et al., 1993b) and suggests that good chewers (small median particle size) often swallow boluses containing smaller particles. In accordance with Yurkstas (1965), no correlation was found between masticatory performance and the number of chewing cycles to prepare food for swallowing. It follows that poor chewers do not compensate for their reduced chewing performance by using more chewing strokes. In this study, we observed a negative correlation between the number of chewing strokes until swallowing and median particle size at swallowing. This is consistent with results from an unpublished study (van der Bilt et al.) performed on 80 subjects, and it does seem logical that a larger number of chewing strokes results in smaller particles. The time until swallowing shows results similar to those found for the number of chewing cycles, due to the high correlation between these variables.

We found a positive correlation between size perception and median particle size after 15 chewing strokes. Poor chewers estimated the spheres to be larger than the good chewers and more often overestimated the sphere size. This implies that poor chewers swallow larger particles because their chewing ability, not their oral sensitivity, is reduced. It would appear that they are or have become even more sensitive to sizes of particles that would cause discomfort during swallowing. This suggests that poor chewers are more cautious about large food particles, and possibly that they would initiate deglutition even with a bolus containing particles up to 5 mm. Prinz and Lucas (1995) suggest that the upper size limit of particles that are swallowed is dictated by the individual’s tolerance of discomfort from distension of soft tissue in the pharynx and esophagus. Good chewers can easily comminute food to particles well below this upper tolerance level, and as a result they are less sensitive to the larger object sizes.

The fact that no correlation was found between spatial acuity and assessment of sphere size indicates that the two measurements are unrelated. Spatial acuity describes only a lower limit of sensitivity, which does not overlap with the sizes of the spheres used in this study. Hence, the subject’s ability to assess object size in the mouth cannot be predicted by the spatial acuity of the tongue.

In conclusion, oral perception of the sizes of small spheres is underestimated, and the sizes of large spheres are overestimated. Topical anesthesia reduces spatial acuity but does not affect the perception of sphere size. We suggest that two-point discrimination stimulates only superficial receptors, which involve light touch and are easily anesthetized, while the spheres might excite more deeply-set receptors. These receptors appear to be critical to masticatory performance and swallowing. Poor chewers are sensitive to sphere sizes that could cause them discomfort while swallowing. These results invite more research on oral sensitivity in healthy and orally impaired subjects, so that we can gain more insight into the mechanisms controlling chewing, swallowing, and object perception.


   ACKNOWLEDGMENTS
 
We thank Dr. Maria Schipper for her advice on the statistical analysis and David Punchard for revising the English of this text. This work was supported by the Wageningen Centre for Food Sciences, the University Medical Center Utrecht, and the Netherlands Institute for Dental Sciences.

Received June 24, 2003; Last revision February 25, 2004; Accepted February 25, 2004


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 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
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