Two-point discrimination values vary depending on test site, sex and test modality in the orofacial region: a preliminary study

Abstract The two-point discrimination (TPD) test is one of the most commonly used neurosensory tests to assess mechanoperception in the clinical settings. While there have been numerous studies of functional sensibility of the hand using TPD test, there have been relatively not enough reports on TPD in the orofacial region. Objective The aims of the present study were to determine the normal values of TPD in the six trigeminal sites (the forehead, cheek, mentum, upper lip, lower lip, and the tongue tip) and to investigate the effect of the site, sex, and test modality on the TPD perception. Material and Methods Forty healthy volunteers consisting of age-matched men (20) and women (20) with a mean age of 27.1 years were recruited. One examiner performed the TPD test using a simple hand-operated device, i.e., by drawing compass with a blunt or sharp-pointed tip. The static TPD with a blunt-pointed tip (STPDB), moving TPD with a blunt-pointed tip (MTPDB), and static TPD with a sharp-pointed tip (STPDS) were measured. The predictors were the site, sex, and test modality, and the outcome variable was the TPD value. Three-way ANOVA was used for statistics. Results The analysis showed a significant effect of the site, sex and test modality on the TPD values. Significant differences between the test sites were observed with the descending order from the forehead and cheek>mentum>upper lip and lower lip>tongue tip and index finger. Women showed lower TPD values than those of men. The STPDS measurements were consistently lower than those of the STPDB and MTPDB. Conclusions The normal values of TPD in this study suggest that the cheek and forehead were less sensitive than other regions evaluated and women were more sensitive than men. The STPDS was the most sensitive test modality.


Introduction
as the lip and tongue and finger have superior spatial acuity, i.e., the sensory neural pathways innervating these regions are specialized for spatial information processing 24 . Therefore, it is no wonder that damage on these sensory nerves is likely to bring a prominent loss of sensory acuity 24 . Accordingly, accurate measurement of orofacial spatial resolution deserves the attention of clinicians.
The modality of touch in TPD could be classified into three: static two-point discrimination with blunt tip, moving two-point discrimination with blunt tip, and static two-point discrimination with sharp tip 12,13,26 .
Static and moving TPD with blunt tip is usually tested using the Disk-Criminator and the Aesthesiometer is used for static TPD with sharp tip 7

Test sites and modality
The test sites were defined as the three major sensory branches of the trigeminal nerve region corresponding to the ophthalmic branch (V1), the maxillary branch (V2) and the mandibular branch (V3).
In these three branches, six coordinates were selected for the experiment. These were the mentum (above the mental foramen); the vermilion of the lower lip; the vermilion of the upper lip; the tip of the tongue; the mid-point of the cheek and the forehead (2 cm above the midpoint of the brow). The index fingertip was chosen randomly between the left and right side and was tested to examine the sensory sensitivity of the subjects and for comparison with the orofacial region. The testing was performed starting with the index finger, then proceeding to the six orofacial test sites in random order, selecting alternatively from the right and left side. To select the test site randomly, the examiner put the papers on which test sites were written in a box and picked a paper before the test.
The test sites were chosen according to the site written on the selected paper.
Three modalities of TPD tests were performed bilaterally at randomly selected trigeminal test sites. Two-point discrimination (TPD) sensory testing procedures The two-point test was performed using a simple hand-operated device, i.e., by drawing compass with blunt or sharp-pointed tip ( Figure 1). The interval between the two metal tips of this simple instrument was continuously adjustable and was measured in mm.
The two-point test was performed by applying the two tips of the device to the test site.
The stimulus intensity was chosen to be that which the subject could perceive as constant touching or moving without the perception of discomfort or pain for the STPDB and MTPDB tests; i.e., skin blanching itself was not used as control. The STPDS test was performed using a similar procedure to the STPDB and MTPDB tests, but the subjects could perceive mild discomfort due to the sharpness of the tip applied. The first distance of the tips, which was large enough for the subject to clearly perceive correctly, was determined at the preliminary testing. The initial TPD test distance was 20 mm for the forehead and A; drawing compass with sharp-pointed tip B; drawing compass with blunt-pointed tip cheek, 10 mm for the mentum, 6 mm for both lips and 5 mm for the tongue tip and the index finger tip. If the subject could not correctly perceive the initial distance, a longer distance was set for the initial distance. A threshold was determined using a descending stimulus magnitude and one point was inserted intermittently during the descending series to avoid the subject's expectation of the continuous decrease in distance between the two points. If the subject answered correctly in response to these changes, the distance decreased in intervals of 1 mm. This testing pattern was continued until the subject answered incorrectly, and the experimenter returned to the next longer distance. The series was terminated when a correct answer for the next longer distance was followed by two incorrect answers on two subsequently shorter distances. This final correct answer was chosen as the end-point for the TPD test.
When the subjects continuously had inconsistent responses with the repeated measure of the TPD tests at the given test site, the subjects were excluded from the corresponding test.
Two series of TPD testing for three modalities were performed to determine each TPD value and the mean values of two consecutive measurements were calculated. The subjects were given three alternatives for the answers; i.e., the subject was asked to say "one" if the subject felt one point and "two" if two separate points were felt. If the subject said "I can't discriminate one or two", it was regarded as an incorrect answer.
The above tests were carried out in a quiet room at room temperature by one investigator. The orofacial sensitivity tests were conducted with the subjects in the supine position in the dental chair. The subjects were asked to keep their eyes closed throughout the test procedure.

Data analysis
We defined the test site, sex, and test modality as predictors and the two-point discrimination values as the outcome. Before the data analysis, the normality of the data was evaluated using the Kolmogorov-Smirnov test and data were not normally distributed. Thus, log-transformation of data was applied to perform the further statistic calculation and correct the possible heteroscedasticity. All variables were continuous, and the mean threshold values and standard deviations were calculated from the raw data. The side differences at each test site were analyzed by paired t-test.
To test for the effects of site, sex, and test modality on two-point discrimination, data were analyzed using three-way analysis of variance (ANOVA). When the differences were significant, Tukey post hoc analysis was calculated for multiple comparison. The upper limits of normality for a given sample were calculated using the 95% prediction interval (1.96 SD). A

Subjects
Healthy young adults consisting of 20 men and 20 women were tested. There were no significant age differences between male and female subjects (Independent t test, P=0.327). In the STPDB test, inconsistent responses were recorded for the forehead and cheek for two men and one woman and the tests were excluded (Table 1). Tests performed in four men and one woman on the forehead, two men and one woman on the cheek, and one woman on the mentum were also excluded for the MTPDB test due to inconsistent responses ( Table 1). The results of the STPDS tests performed on four men and one woman on the forehead, two men and one woman on the cheek, and one woman on the mentum were excluded due to their inconsistent responses (

Discussion
The main findings of this study are as follows.
(1) This study showed that there is superior-inferior gradient for spatial acuity in the orofacial region; (2) Women were more sensitive than men in the TPD perception; (3) The static TPD with sharp tip seemed to be the most sensitive modality for TPD test.
Two-point perception tests typically express spatial acuity and reflect the density and receptive field size of the low-threshold mechanoreceptors 9 . It is well known that the spatial discrimination ability for touch varies according to the body location 6 There are still two unsolved major problems.
First, the TPD test with a handheld instrument does not control for applied pressure 13  Abbreviations: df=degree of freedom; ηp 2 =partial eta squared Table 2-Results of site×sex×modality three-way ANOVA for two-point discrimination Two-point discrimination values vary depending on test site, sex and test modality in the orofacial region: a preliminary study (1987) used another method: "just sufficient pressure is utilized for the subject to assess the stimulus".
In fact, Bell-Krotoski and Buford 2 (1997) indicated that application of force with handheld instrument produces variations and needs to be controlled for test reliability. This lack of repeatability of the force applied may inhibit the reliability of the TPD test. On the other hand, previous studies showed that spatial discrimination in the skin is relatively insensitive to the force applied 16,24,25 . Vriens and van der Glas 25 (2002) reported that the force levels observed were always at an extremely suprathreshold stimulus intensity and, The lack of a standardized protocol to perform TPD tests is another major problem 13 . For example, should the test start with the smallest distance using an increasing method or the widest distance using a decreasing method from the initial distance, and how many correct answers should be used for the value of the TPD? It is widely known that the method of limits leads to systemic errors in estimating thresholds due to response biases, i.e., habituation and expectation 20 .
Thus, this study adopted the descending method of limit with intermittent and random insertion of testing stimuli from one to two points as previously reported in Dellon's study 7 to reduce the subject biases.