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On the self-perception of jaw positions in man
ON THE
SELF-PERCEPTION OF JAW POSITIONS IN MAN J. D. VAN WILLIG~N* and M. L. BROIXHUIJSENP
*Department of Oral Biology (Oral Physiology), TDepartment
University of Groningen. Antonius Deusinglaan I, 97 I3 AV Groningen and of Neurophysiology, University of Groningen. Bloemsingel IO. 9712 KZ Groningen, The Netherlands
Summary-A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined. verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.
INTRODUCTION Edentulous subjects can specify an occlusal position which is the most comfortable (Lytle, 1964; Timmer, 1964; Tryde et u/., 1974. 1977a. b; Van Willigen. De Vos and Broekhuijsen, 1976; Fujii rf (I/.. 1977). We have called this particular jaw separation the preferred vertical dimension of occlusion (PVDO). Broekhuijsen and Van Willigen (1982) showed that edentulous subjects can be subdivided into two groups: a stable group that shows instability of the PVDO which is nest greater than can be expected from the imprecision of the PVDO determinations and another group that shows instability of the PVDO. They concUed that the subjects in the stable group, about 50 per cent of all subjects tested. must have referred to a reasonably fixed internal standard during their PVDO determinations. Furthermore. variation in precision of PVDO determination was rare and differed from subject to subject in the range 0.25 to 1.5 mm. Although there are many studies on position sense, that is the ability of 21subject to perceive position and movement, there are few numerical data available on the precision and the stability by which a position is adopted or reproduced. There are thus no known norms of performance against which the PVDO measurements can bc: judged. Merton (1961) showed that a closely similar precision exists in directing the eyes and the hand in the dark. In both cases, errors were made such that the standard deviation of successive attempts around their mean position was about 1’~in both horizontal and vertical directions. Thilander (1961) assessed qualitatively the overall accuracy by which mandibular positions are remembered. Several studies (Manly et uI., 1952; Kawamura and Watanabe, 1960; Siirill and Laine. 1972; Christensen and Morimoto, 1977; Morimoto and Kawamura, 1978) investigated the limits of ability to dis-
criminate the size of objects placed successively between the teeth. In general. the smallest value for this limit was about 0.2 mm but it is dependent on many factors, including the degree of mouth opening. There is some disagreement between the various studies which can be attributed to methodological differences. The test procedures for successive comparison experiments, such as these. are designed to minimize the influence of memory. However, this factor is shown in the experiments of Merton (1961). Thilander (1961) and Van Willigen er ctl. (1976) to be very important. For this reason. we undertook an investigation into the ability of dentate subjects to estimate jaw positions when referring to a given memorized standard. This standard took three different forms. which in order of decreased abstractions were: a sclfgenerated imagined standard (preferred jaw separation experiments). a verbally-specified number of millimeters (verbal standard experiments) and the remembered size of a standard gauge which aas placed between the teeth for 20 min (real standard experiments), Such an investigation would provide a norm of performance. against which the PVDO values could be judged. The verbal standard experiments were done because Gibbs and Anderson (1977) found that, in estimating the length of lines, many subjects show that they have a good notion of the magnitude of the unit of the system with which they are familiar. This phenomenon could also influence the judgement of jaw separation, although direct visual feedback (looking into one’s own mouth) is absent.
MATERIALS AND METHODS The subjects were 1I healthy adults with natural dentition and no sign of mandibular joint disorders coded A-C (female) and D-K (male). From this
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J. D. van Willigen and M. L. Broekhuijsen
group, 4 subjects participated in the preferred jaw separation experiments and the same group, together with another subject. in the verbal standard experiments. Four subjects were involved in the real standard experiments. 2 in a long-term real standard experiment.
In all experiments, the method of forced choice was used. A collection of acrylic resin gauges. ranging from I to 30 mm, with increments of 0.5 mm. was available. For each experiment, an appropriate subset from this collection was used. During the experiment, the subjects sat in a chair with their eyes closed. They were instructed to imagine, or to recall, the standard chosen for that experiment (see below). Then a gauge was inserted into the mouth between the upper and lower incisors by the experimenter in as standardized a manner as possible. The subjects were instructed to touch the test gauge with their teeth once only. No tapping on the gauge was allowed. They then had to report whether the test gauge felt thicker or thinner than the standard under test. Each experiment started with an initial stage during which the subjects were made conversant with a standard, either by imagining the jaw separation that is easiest to reproduce (preferred jaw separation expcriments), or by imagining a jaw separation of 4.75, 7.75 or 17.75 mm (verbal standard experiment), or by learning the.size of a reference gauge of 4.75, 7.75 and 24.75 mm for 20 min (real standard experiments). In each test run, each stimulus value was presented IO or 20 times in random sequence. During each test run. the interval between the presentation of two gauges was about IO s. The subjects were not informed of their results. The test sequence in the preferred jaw separation experiments was 4. 5 or 6 test runs per subject, each on a different day, for the verbal standard experiments 4 or 5 test runs per subject. again each on a different day. Sessions of the preferred jaw separation and the verbal standard experiments were sometimes taken within one day. Care was taken that the time interval between the sessions was more than 30 min. In the real standard experiments, after the initial learning a 20 min pause was given. This pause was given to minimize short-term memory effects such as those reported by Morimoto and Kawamura (1978). After this pause, two test runs interspaced by a 20 min pause were performed. The procedure (20 min learning. 20 min pause, test run I. 20 min pause. test run 2) was repeated 5 times over 5 weeks. Hotelling’s Tz-test (Moroney, 1956) was applied to test whether differences existed between the first and second test run of each pair. No differences were found at a 5 per cent level of significance. Two subjects were tested for their performance in matching a standard gauge over IO weeks without repeating the learning procedure. The protocol for testing was: IO test runs (sample range I-IO mm: increments 0.5 mm) interspaced with one-week intervals. Preceding the first test run, a standard gauge of 4.75 mm was explored for 20 min. The sample range of the test gauges in the preferred jaw separation experiments and in the verbal standard experiments w’as dependent on the subject’s per-
formance. With the 4.75 and 7.75 verbal standard experiments, an increment of 0.5 mm between the test gauges was chosen and with the 17.75 mm experiments, an increment of I .O mm. The set of test gauges did not contain the value of the standard, In the real standard experiments. the sample range of the gauges W;IS I- IO mm (increments 0.5 mm) for the 4.75 and 7.75 mm standard experiments and 20 30 mm (increments I.0 mm) for the 24.75 mm standard experiments. The set of test gauges did not contain the standard under test.
The procedure of analysis was described by Broekhuijsen and Van Willigen (1982). In summary, by the method of probit analysis, we calculated the subject’s match for each test run. The subject’s match is that gauge size that the subject would report to be too thick or too thin in 50 per cent of the occasions. We calculated also the imprecision of the match, being the intra test-run standard deviation. These values were calculated under the conditions of the following 4 mutually-exclusive hypotheses: H,: neither the matches nor the imprecision of the matches derived from each test-run differ significantly; H,,: only the imprecision of the matches do not differ significantly; H,,: only the matches do not differ significantly: H,: all matches differ and so do their imprecisions. The tests were performed by a likelihood ratio test (5 per cent level of rejection). Where applicable, we tested whether the matches equalled the standard (Student’s t-test 5 per cent level of rejection). If the match was different from the standard. we called it inaccurate. If the match was greater than the standard, we called the inaccuracy positive, and if less, negative. Finally. we noted whether a trend was present in the values of the matches and/or in the values of imprecision of the matches by calculating their regression coefficient (two-sided r-test, 5 per cent level of significance). Acceptance of the H, hypothesis implies that the memorized image of the standard (the engram) does not change. Acceptance of the H,, hypothesis indicates long-term instability in the comparison mechanism or slow change in the engram, but the matches have the same precision. (The inter testrun standard deviation is a measure of the long-term instability.) Acceptance of the H, ,, hypothesis implies that the engram and the comparison mechanism are reasonably fixed in the long-term but the precision of the match is variable. Acceptance of the H, hypothesis indicates that the engram or the comparison mechanism is unstable in the long-term and the precision of the match is variable. RESULTS The data are summarized in Table I. Two subjects showed significant changes of preferred jaw separation, their precision being constant (H,, class). Two were unstable in their preferred jaw separation, these separations being judged with variable precision as well (H, class). In Fig. 1, the match of the preferred jaw separation is shown for 4 subjects. Precision when matching an internal standard was of the order of 0.5 mm. Numerical comparison between standard and performance is not possible.
Perception of jaw positions Table
Sub
1. Conden&
Standard (mm)
119
data of the results of the matches of jaw separations made by subjects self-generated internal standard (preferred jaw separation) Vo. of test runs
n
&m)
6 5 4 5
20 20 20 20
7.31 5.01 5.41 7.70
G H J K
(:A)
(i&)
0.46 0.42 0.48 0.64
0.87 0.20 0.41 0.23
Accepted hypothesis 2 la la 2
SEM (mm) 0.36 0.10 0.21 0.11
staEd:rd? ~ ~
referring
to a
Trend in the matches?
Trend in the imprecision?
No No No No
No
No
Each row represents a single subject. The columns show successively: the subject’s code: the standard; the number of test runs (I‘); II. the number of presentations of each stimulus value per test run; /,I. the match (i.e. the estimated match in the case of acceptance of the H, or the H,, hypothesis, or the mean value of the estimated matches, weighted by the inverse of their variances, in the case of acceptance of the H,, or the H, hypothesis); (T,, the imprecision ~i.e. the estimated intra test-run standard deviation in the case of acceptance of the H, or the H, a hypothesis, or the mean value of the estimated intra test-run standard deviations, weighted by the inverse of their variances, in the case of acceptance of H,, or the Hz hypothesis); oD, the instability (i.e. the estimated inter test-run standard deviation in the case of acceptance of ‘he__v, or the Hz hypothesis); the accepted hypothesis; SEM, the standard error of the mean (where SEM = Jvar p in the case of acceptance of the Ho or the H,, hypothesis, and SEM = Jv%> -4 ozdJ in the case of acceptance of the H,, or the H, hypothesis); the result of testing the hypothesis that the match equals the standard (Student’s t-test, 5 per cent level of rejection); if the match is greater than the standard it is called positive inaccuracy (No +) and if the match is less than the standard it is called negative inaccuracy (No -); the result of testing for the existence of a trend in the sequence of the values of the matches [hypothesis: the regression coefficient of the matches weighted by the inverse of the variances, vanishes (2-sided t-test, 5 per cent level of rejection; Yes’ or Yes1 mean an upward or downward drift in the match)]; similarly, for the values of the imprecisions.
Verbal standard rvprrimrnts The results of the experiments are summarized in Table 2 and Fig. 2. With these standards, 3, 4 and 4 subjects respectively showed significant changes of the match to the standard, over several weeks, their preclslon being constant (H,, class); 2, I and 0 subjects respectively were unstable in their match to the standard, these standards being matched with variable precision as well (H, class); 1 subject was stable in his
heferred
jaw separation
Real standard experiments
matchbnm)
I
~~imprecision
match to the verbal standard (17.75 mm), his precision being constant (Ho class). In matching jaw separations with a small verbal standard, the inaccuracy was positive (thick gauges are reported to be thin). With 7.75 and 17.75 mm standards, matches were generally accurate, any inaccuracy tending to be negative. Fourteen of the 15 verbal standard experiments were classified under the H,, or the Hz hypothesis. This means that in the long-term the comparison process of the engram was unstable. In 12 of the 15 experiments precision did not change. No individual systematic changes in the performance of the subjects were found over the different experiments.
:
; Av3tability A
Fig. I. Histogram of the matches of jaw separations made by subjects referring to a self-generated internal standard. The solid bars indicate the imprecision of the match; the dashed bars indicate the instability of the matches; the horizontal dashed lines on the top of the columns show limits of the SEM.
The results of the real standard experiments are summarized in Table 3 and Fig. 3. With these standards, 2, 3 and 2 subjects respectively showed significant changes of their match to the standard over several weeks, their precision being constant (H,, class); 2, 1 and 2 subjects respectively were unstable in their match to the standard, these standards being matched with variable precision as well (HZ class). The 4.75 mm standard was matched inaccurately. The inaccuracy was positive in all subjects. Furthermore, 2 out of 4 showed a trend in their matches. The 7.75 mm standard was matched accurately by 2 subjects; the other 2 showed positive inaccuracy. In 2 subjects, a trend in the matches was found. The 24.75 mm standard was also matched accurately by 2 subjects, the other 2 showed negative inaccuracy. No trend was detected. All subjects tested showed longterm instability in their matches. Two of the 4 subjects were classified under the HZ hypothesis in the successive experiments, the other 2 subjects matched with constant precision.
I20
J. D. van Willigen
and
M. L.
Broekhuijsen
Table 2. Condensed data of the results of the matches of jaw separations made by subjects, referring to a verbally specified number of millimeters (standard: 4.75, 7.75 and 17.75 mm). The rows and columns have the same meaning as in Table 1
Sub
Standard (mm)
No. of test runs
n
G H
4.75 4.75
5 4
20 20
I J K
4.75 4.75 4.75
5 4 5
20 20 20
G H I J K
7.75 7.75 7.75 7.75 7.75
5 4 5 4 5
20 20 20 20 20
G H
17.75 17.75
5 4
I J K
17.75 17.75 17.75
5 4 5
Trend in the matches?
Accepted (t%~) hypothesis
SEM (mm)
staid;rd?
8.20 0.76 5.24 0.44
1.75 0.29
la la
0.78 0.15
No + No +
Yes’ Yesf
5.70 0.39 4.72 0.39 6.46 0.70
0.89 0.39 0.62
2 la 2
0.40 0.20 0.28
Yes Yes No +
No No No
0.92 0.39 0.70 0.68 0.92
2.29 0.20 1.10 1.07 0.71
2 la la la la
1.03 0.11 0.49 0.54 0.32
Yes No Yes Yes No +
No No No No No
20 20.88 1.51 20 13.16 0.79 20 17.50 1.12 20 15.46 0.96 20 15.66 1.69
2.53 ~
la 0
1.13 0.05
Yes No -
No
1.14 1.67 2.57
la la la
0.51 0.84 1.15
Yes Yes Yes
No No No
&m)
10.42 6.93 8.49 8.25 8.76
(nr&
Long-term experiments
Trend in the imprecision?
No No No
-
-
DISCUSSION
Two subjects were tested for their performance in matching a standard gauge over 10 weeks without repeating the learning procedure. The results are shown in Table 3. We see that the subjects tested over long periods of time behave like subjects tested in the short-term.
From our experiments, the following statements can be made about the memory for jaw separations and its precision: Dentate subjects are able to match a self-generated standard; precision is comparable with that of the PVDO of edentulous subjects (Broek-
Verbal Standard standard 17.75 mm
standard 7.75 mm
standard 4.75 mm
match(standardliOO)
match(standard-lO0)
I
I
200.
200.
150,
150,
I
loo.--
loo-
50
50.
0
0
.impreciskn
:
iSinstability
Fig. 2. Histograms of the matches of jaw separations made by subjects referring to a verbally-specified number of millimetres (standard = 100). The standards were: 4.75, 7.75 and 17.75 mm. The bars and the horizontal dashed lines on top of the columns have the same meaning as in Fig. 1.
Perception
121
of jaw positions
Table 3. Condensed data of the results of the matches of jaw separations made by subjects, referring to a remembered size of a standard gauge (gauge sizes: 4.75, 7.75 and 24.75 mm; learning period about 20 min). Furthermore, condensed data of the results of long-term matches of jaw separations for 2 subjects are shown. These subjects referred to a standard gauge (4.75 mm), given only at the start of the test run sequence No. of
Sub
Standard (mm)
A C E F
4.75 4.75 4.75 4.75
A C E F
test runs
II (mm) (nk)
(r%)
Accepted hypothesis
10
10 10 10 10
5.47 5.95 5.05 5.04
0.38 0.43 0.48 0.43
0.62 0.37 0.19 0.19
2 2 la la
0.20 0.12 0.07 0.07
7.75 7.15 7.75 1.75
IO IO IO 10
10 10 10 10
7.55 8.23 8.41 7.93
0.40 0.44 0.48 0.61
0.58 0.33 0.26 0.36
2 la la la
A c E F
24.75 24.75 24.15 24.75
10 10 10 10
10 10 10 10
24.86 1.33 0.73 24.67 0.90 0.55 24.23 1.81 0.32 24.40 1.37 0.39
2 2 la la
B D
4.75 4.75
10 IO
10 IO
10 10
IO
The rows and columns
Real standard 5.50 0.39 0.41 6.17 0.54 0.57
have the same meaning
SEM (mm)
stagd;rd? No + No +
Trend in the matches? Yes 1 No
No +
No
No +
Yes’
0.19 0.1 I 0.09 0.12
Yes No + No + Yes
Yes’
0.25 0.19 0.15 0.15
Yes Yes No No -
No No No No
(long-term) la 0.13 2 0.19
No + No +
No Yesi
Trend in the imprecision? No No
No
No
Yes t No
No No
No
as in Table 1.
huijsen and Van Willigen, 1982). This means that periodontal sensors are not the prime sensors for sensing jaw separations. This is in accord with the observations of Christensen and Morimoto (1977). In half of the experiments, the subjects showed a match that equals the verbal standard or the real
standard. This means that over longer periods of time these subjects are able to match accurately a verbally specified number of millimetres or the remembered size of a standard gauge. Inaccuracies tend to be positive when matching small standards and negative when matching larger
Real Standard standard 4.75mm match(standard.100)
standard Z75mm
1match(standardllOO)
standard 24.75mm ~match(standard,lOO)
Axtability
Fig. 3. Histogr.ims of the matches of jaw separations made by subjects referring to a remembered size of a standard gauge (standard = 100). The standard gauges were: 4.75, 7.75 and 24.75 mm. The bars and the hcrizontal dashed lines on top of the columns have the same meaning as in Fig. I.
122
J. D. van Wilhgen
and M. L. Broekhuijsen
ones. This is in agreement with classical psychophysical experiments (e.g. Fullerton and Cattell, 1892; Needham. 1935). Over longer periods of time, subjects tend to give more accurate matches when they are referring to a verbal standard than when they are referring to a real standard. This can be caused by the fact that in normal life reinforcement of the internal representation of the common metric value often occurs. The relative imprecision (expressed as imprecision/standard) of the matches is nearly independent of the size of the standard. This is in agreement with the general characteristics of sensors. Matches are more precise. when referring to a real standard. than when referring to a verbal standard; one of our subjects had a rather distorted conception of the magnitude of a millimetre (Table 2. subject G). All subjects are classified under the H,, or the Hz hypothesis. This means that all our subjects are unstable in their matches over a longer period. This phenomenon can be caused by long-term instability in the comparison mechanism or slow changes in the engram of the standard. About 67 per cent of the results were classified under the H ,a hypothesis. which means that, in the long-term, matches were unstable but performed with the same precision. The remaining 33 per cent showed variable precision. Subjects tested over a long period of time, referring to a real standard that is given only once at the start of the sequence of test runs, behave in the same way as subjects who were matching a standard that they have explored on the same day. Some subjects showed a trend in the sequence of matches, No causal relation between the experimental parameters and this trend could be detected. No trend was detected in the precision. We conclude that dentate subjects recognize jaw separations fairly well. This holds whether the subject refers to an imagined, a verbally-imposed or a physically-imposed (real) standard. Over longer periods, about half of the subjects matched accurately. The relative precision of the match is independent of the size of the standard and the precision of matching in dentate subjects is similar to that in edentulous subjects All dentate subjects are unstable in their matches over a longer period of time. Some edentulous subjects showed that their performances of PVDO determinations were stable in the long term (Broekhuijsen and Van Willigen, 1982). However, we have to bear in mind that the subdivision into a stable and unstable class depends on the internal parameters of the experiments, especially the number of presentations of each stimulus value per test run. This means that, after increasing the number of presentations per test run, the probability that the subject will be classified as unstable is greater. Ack,lo~~ledgenlrrtrs~We gratefully thank Professors C. Rashbass and F. Veringa for helpful discussion and comments on the manuscript.
REFERENCES M. L. and Van Willigen J. D. 1982 The precision of measurement and the stability of the preferred vertical dimension of occlusion in man. Archs orul Biol. 27, 487749 I. Christensen J. and Morimoto T. 1977. Dimension discrimination at two different degrees of mouth opening and the effect of anaesthesia applied to the periodontal ligaments. J. orul Rehuh. 4, 157-164. Fujii H., Stoltze K., Tryde G.. McMillan D. R. and Brill N. 1977. A comparative study of three different approaches to the comfortable zone. J. oral Rrhah. 4, 125-130. Fullerton G. S. and Cattell J. M. 1892. On tltr Perc,ep(iort o/ Snrtrll Difl~&rrc~r.s. Univ. of Pennsylvania Press. Pa. Gibbs L. L. and Anderson R. 1977. Effect of weieht and sex on perception of line length. Prrwpt. Morr Skills 44, 783-786. Kawamura Y. and Watanabe M. 1960. Studies on oral sensory thresholds: The discrimination of small differences in thickness of steel wires in persons with natural and artificial dentitions. 1Llcd. J. O.wku Uric. IO. 291-301. Lytle R. B. 1964. Vertical relation of occlusion by the patient’s neuromuscular perception. J. prmsrhrt. Dent. 14, 12-21. Manley R. S., Pfaffman C., Lathrop D. D. and Keyser J. 1952. Oral sensorv thresholds of persons with natural and artificial dentitions. J. denr. Rrs. 31. 305-3 12. Merton P. A. 1961. The accuracy of directing the eyes and the hand in the dark. J. Physiol., Lo/d. 156, 555577. Morimoto T. and Kawamura Y. 1978. Interdental thtckness discrimination and position sense of the mandible. In: Oral Physiology md Occlusior~ (Edited by Perryman J. H.) pp. 1499169. Pergamon Press. New York. Moroney M. J. 1956. Facts,from FiCquws, 3rd edn. Penguin. Harmondsworth. Needham J. G. 1935. The effect of the time interval upon the time-error at different intensive levels, J. esp, Psyc/d. 18, 53C-543. Siirill H. S. and Laine P. 1972. Sensory thresholds in discriminating differences in thickness between the teeth, by different degrees of mouth opening. Proc. Finn drr~t. Sot. 68, 134139. Thilander B. 1961. Innervation of the temporomandibular joint capsule in man. Trtlns. R. Sch. Dwt. Stock/~. Umeu No. 7. Timmer L. H. 1964. De dynamische beetbepaling. ,ved. Tijdschr. Tundhrelk. 71. 174-177. Tryde G., McMillan D. R., Stoltze K., Morimoto T., Spanner 0. and Brill N. 1974. Factors influencing the determination of the occlusal vertical dimension by means of a screw jack. J. or-cl/ Rehub. I. 2333244. Tryde G., Stoltze K.. Morimoto T.. Salk D. and Brill N. i977a. Long-term changes in the perception of comfortable mandibular occlusal oositions. J. ord Rehuh. 4. 9-15. Tryde G.. Stoltze K., Fujii H. and Brill N. 1977b. Shortterm changes in the perception of comfortable mandibular occlusal positions. J. oral Rrhah. 4, 17-21. Van Willigen J. D., De Vos A. L. and Broekhuijsen M. L. 1976. Psychophysical investigations of the preferred vertical dimension of occlusion in edentulous patients. J. prosrlret. Dent. 35, 2599266.
Broekhuijsen
SELF-PERCEPTION OF JAW POSITIONS IN MAN J. D. VAN WILLIG~N* and M. L. BROIXHUIJSENP
*Department of Oral Biology (Oral Physiology), TDepartment
University of Groningen. Antonius Deusinglaan I, 97 I3 AV Groningen and of Neurophysiology, University of Groningen. Bloemsingel IO. 9712 KZ Groningen, The Netherlands
Summary-A longitudinal study shows that, in the short-term, dentate subjects are able to match fairly well the magnitude of jaw separations to any of three types of memorized standards (imagined. verbally imposed and physically imposed) of various dimensions. All subjects showed instability of the comparison mechanism or long-term changes in the engram. About 66 per cent of the matches were performed with the same precision. The relative precision of the matches is almost independent of the size of the standard. The absolute precision of matching of dentate subjects is similar to that of edentulous subjects. In half of the experiments, the subjects showed a match that equals the magnitude of the verbal standard or the real standard. The subjects tend to give more accurate matches when they refer to a verbal standard than when they refer to a real standard, but the long-term instability of matching is much greater when the subjects refer to a verbal standard than when the subjects refer to a real standard.
INTRODUCTION Edentulous subjects can specify an occlusal position which is the most comfortable (Lytle, 1964; Timmer, 1964; Tryde et u/., 1974. 1977a. b; Van Willigen. De Vos and Broekhuijsen, 1976; Fujii rf (I/.. 1977). We have called this particular jaw separation the preferred vertical dimension of occlusion (PVDO). Broekhuijsen and Van Willigen (1982) showed that edentulous subjects can be subdivided into two groups: a stable group that shows instability of the PVDO which is nest greater than can be expected from the imprecision of the PVDO determinations and another group that shows instability of the PVDO. They concUed that the subjects in the stable group, about 50 per cent of all subjects tested. must have referred to a reasonably fixed internal standard during their PVDO determinations. Furthermore. variation in precision of PVDO determination was rare and differed from subject to subject in the range 0.25 to 1.5 mm. Although there are many studies on position sense, that is the ability of 21subject to perceive position and movement, there are few numerical data available on the precision and the stability by which a position is adopted or reproduced. There are thus no known norms of performance against which the PVDO measurements can bc: judged. Merton (1961) showed that a closely similar precision exists in directing the eyes and the hand in the dark. In both cases, errors were made such that the standard deviation of successive attempts around their mean position was about 1’~in both horizontal and vertical directions. Thilander (1961) assessed qualitatively the overall accuracy by which mandibular positions are remembered. Several studies (Manly et uI., 1952; Kawamura and Watanabe, 1960; Siirill and Laine. 1972; Christensen and Morimoto, 1977; Morimoto and Kawamura, 1978) investigated the limits of ability to dis-
criminate the size of objects placed successively between the teeth. In general. the smallest value for this limit was about 0.2 mm but it is dependent on many factors, including the degree of mouth opening. There is some disagreement between the various studies which can be attributed to methodological differences. The test procedures for successive comparison experiments, such as these. are designed to minimize the influence of memory. However, this factor is shown in the experiments of Merton (1961). Thilander (1961) and Van Willigen er ctl. (1976) to be very important. For this reason. we undertook an investigation into the ability of dentate subjects to estimate jaw positions when referring to a given memorized standard. This standard took three different forms. which in order of decreased abstractions were: a sclfgenerated imagined standard (preferred jaw separation experiments). a verbally-specified number of millimeters (verbal standard experiments) and the remembered size of a standard gauge which aas placed between the teeth for 20 min (real standard experiments), Such an investigation would provide a norm of performance. against which the PVDO values could be judged. The verbal standard experiments were done because Gibbs and Anderson (1977) found that, in estimating the length of lines, many subjects show that they have a good notion of the magnitude of the unit of the system with which they are familiar. This phenomenon could also influence the judgement of jaw separation, although direct visual feedback (looking into one’s own mouth) is absent.
MATERIALS AND METHODS The subjects were 1I healthy adults with natural dentition and no sign of mandibular joint disorders coded A-C (female) and D-K (male). From this
11X
J. D. van Willigen and M. L. Broekhuijsen
group, 4 subjects participated in the preferred jaw separation experiments and the same group, together with another subject. in the verbal standard experiments. Four subjects were involved in the real standard experiments. 2 in a long-term real standard experiment.
In all experiments, the method of forced choice was used. A collection of acrylic resin gauges. ranging from I to 30 mm, with increments of 0.5 mm. was available. For each experiment, an appropriate subset from this collection was used. During the experiment, the subjects sat in a chair with their eyes closed. They were instructed to imagine, or to recall, the standard chosen for that experiment (see below). Then a gauge was inserted into the mouth between the upper and lower incisors by the experimenter in as standardized a manner as possible. The subjects were instructed to touch the test gauge with their teeth once only. No tapping on the gauge was allowed. They then had to report whether the test gauge felt thicker or thinner than the standard under test. Each experiment started with an initial stage during which the subjects were made conversant with a standard, either by imagining the jaw separation that is easiest to reproduce (preferred jaw separation expcriments), or by imagining a jaw separation of 4.75, 7.75 or 17.75 mm (verbal standard experiment), or by learning the.size of a reference gauge of 4.75, 7.75 and 24.75 mm for 20 min (real standard experiments). In each test run, each stimulus value was presented IO or 20 times in random sequence. During each test run. the interval between the presentation of two gauges was about IO s. The subjects were not informed of their results. The test sequence in the preferred jaw separation experiments was 4. 5 or 6 test runs per subject, each on a different day, for the verbal standard experiments 4 or 5 test runs per subject. again each on a different day. Sessions of the preferred jaw separation and the verbal standard experiments were sometimes taken within one day. Care was taken that the time interval between the sessions was more than 30 min. In the real standard experiments, after the initial learning a 20 min pause was given. This pause was given to minimize short-term memory effects such as those reported by Morimoto and Kawamura (1978). After this pause, two test runs interspaced by a 20 min pause were performed. The procedure (20 min learning. 20 min pause, test run I. 20 min pause. test run 2) was repeated 5 times over 5 weeks. Hotelling’s Tz-test (Moroney, 1956) was applied to test whether differences existed between the first and second test run of each pair. No differences were found at a 5 per cent level of significance. Two subjects were tested for their performance in matching a standard gauge over IO weeks without repeating the learning procedure. The protocol for testing was: IO test runs (sample range I-IO mm: increments 0.5 mm) interspaced with one-week intervals. Preceding the first test run, a standard gauge of 4.75 mm was explored for 20 min. The sample range of the test gauges in the preferred jaw separation experiments and in the verbal standard experiments w’as dependent on the subject’s per-
formance. With the 4.75 and 7.75 verbal standard experiments, an increment of 0.5 mm between the test gauges was chosen and with the 17.75 mm experiments, an increment of I .O mm. The set of test gauges did not contain the value of the standard, In the real standard experiments. the sample range of the gauges W;IS I- IO mm (increments 0.5 mm) for the 4.75 and 7.75 mm standard experiments and 20 30 mm (increments I.0 mm) for the 24.75 mm standard experiments. The set of test gauges did not contain the standard under test.
The procedure of analysis was described by Broekhuijsen and Van Willigen (1982). In summary, by the method of probit analysis, we calculated the subject’s match for each test run. The subject’s match is that gauge size that the subject would report to be too thick or too thin in 50 per cent of the occasions. We calculated also the imprecision of the match, being the intra test-run standard deviation. These values were calculated under the conditions of the following 4 mutually-exclusive hypotheses: H,: neither the matches nor the imprecision of the matches derived from each test-run differ significantly; H,,: only the imprecision of the matches do not differ significantly; H,,: only the matches do not differ significantly: H,: all matches differ and so do their imprecisions. The tests were performed by a likelihood ratio test (5 per cent level of rejection). Where applicable, we tested whether the matches equalled the standard (Student’s t-test 5 per cent level of rejection). If the match was different from the standard. we called it inaccurate. If the match was greater than the standard, we called the inaccuracy positive, and if less, negative. Finally. we noted whether a trend was present in the values of the matches and/or in the values of imprecision of the matches by calculating their regression coefficient (two-sided r-test, 5 per cent level of significance). Acceptance of the H, hypothesis implies that the memorized image of the standard (the engram) does not change. Acceptance of the H,, hypothesis indicates long-term instability in the comparison mechanism or slow change in the engram, but the matches have the same precision. (The inter testrun standard deviation is a measure of the long-term instability.) Acceptance of the H, ,, hypothesis implies that the engram and the comparison mechanism are reasonably fixed in the long-term but the precision of the match is variable. Acceptance of the H, hypothesis indicates that the engram or the comparison mechanism is unstable in the long-term and the precision of the match is variable. RESULTS The data are summarized in Table I. Two subjects showed significant changes of preferred jaw separation, their precision being constant (H,, class). Two were unstable in their preferred jaw separation, these separations being judged with variable precision as well (H, class). In Fig. 1, the match of the preferred jaw separation is shown for 4 subjects. Precision when matching an internal standard was of the order of 0.5 mm. Numerical comparison between standard and performance is not possible.
Perception of jaw positions Table
Sub
1. Conden&
Standard (mm)
119
data of the results of the matches of jaw separations made by subjects self-generated internal standard (preferred jaw separation) Vo. of test runs
n
&m)
6 5 4 5
20 20 20 20
7.31 5.01 5.41 7.70
G H J K
(:A)
(i&)
0.46 0.42 0.48 0.64
0.87 0.20 0.41 0.23
Accepted hypothesis 2 la la 2
SEM (mm) 0.36 0.10 0.21 0.11
staEd:rd? ~ ~
referring
to a
Trend in the matches?
Trend in the imprecision?
No No No No
No
No
Each row represents a single subject. The columns show successively: the subject’s code: the standard; the number of test runs (I‘); II. the number of presentations of each stimulus value per test run; /,I. the match (i.e. the estimated match in the case of acceptance of the H, or the H,, hypothesis, or the mean value of the estimated matches, weighted by the inverse of their variances, in the case of acceptance of the H,, or the H, hypothesis); (T,, the imprecision ~i.e. the estimated intra test-run standard deviation in the case of acceptance of the H, or the H, a hypothesis, or the mean value of the estimated intra test-run standard deviations, weighted by the inverse of their variances, in the case of acceptance of H,, or the Hz hypothesis); oD, the instability (i.e. the estimated inter test-run standard deviation in the case of acceptance of ‘he__v, or the Hz hypothesis); the accepted hypothesis; SEM, the standard error of the mean (where SEM = Jvar p in the case of acceptance of the Ho or the H,, hypothesis, and SEM = Jv%> -4 ozdJ in the case of acceptance of the H,, or the H, hypothesis); the result of testing the hypothesis that the match equals the standard (Student’s t-test, 5 per cent level of rejection); if the match is greater than the standard it is called positive inaccuracy (No +) and if the match is less than the standard it is called negative inaccuracy (No -); the result of testing for the existence of a trend in the sequence of the values of the matches [hypothesis: the regression coefficient of the matches weighted by the inverse of the variances, vanishes (2-sided t-test, 5 per cent level of rejection; Yes’ or Yes1 mean an upward or downward drift in the match)]; similarly, for the values of the imprecisions.
Verbal standard rvprrimrnts The results of the experiments are summarized in Table 2 and Fig. 2. With these standards, 3, 4 and 4 subjects respectively showed significant changes of the match to the standard, over several weeks, their preclslon being constant (H,, class); 2, I and 0 subjects respectively were unstable in their match to the standard, these standards being matched with variable precision as well (H, class); 1 subject was stable in his
heferred
jaw separation
Real standard experiments
matchbnm)
I
~~imprecision
match to the verbal standard (17.75 mm), his precision being constant (Ho class). In matching jaw separations with a small verbal standard, the inaccuracy was positive (thick gauges are reported to be thin). With 7.75 and 17.75 mm standards, matches were generally accurate, any inaccuracy tending to be negative. Fourteen of the 15 verbal standard experiments were classified under the H,, or the Hz hypothesis. This means that in the long-term the comparison process of the engram was unstable. In 12 of the 15 experiments precision did not change. No individual systematic changes in the performance of the subjects were found over the different experiments.
:
; Av3tability A
Fig. I. Histogram of the matches of jaw separations made by subjects referring to a self-generated internal standard. The solid bars indicate the imprecision of the match; the dashed bars indicate the instability of the matches; the horizontal dashed lines on the top of the columns show limits of the SEM.
The results of the real standard experiments are summarized in Table 3 and Fig. 3. With these standards, 2, 3 and 2 subjects respectively showed significant changes of their match to the standard over several weeks, their precision being constant (H,, class); 2, 1 and 2 subjects respectively were unstable in their match to the standard, these standards being matched with variable precision as well (HZ class). The 4.75 mm standard was matched inaccurately. The inaccuracy was positive in all subjects. Furthermore, 2 out of 4 showed a trend in their matches. The 7.75 mm standard was matched accurately by 2 subjects; the other 2 showed positive inaccuracy. In 2 subjects, a trend in the matches was found. The 24.75 mm standard was also matched accurately by 2 subjects, the other 2 showed negative inaccuracy. No trend was detected. All subjects tested showed longterm instability in their matches. Two of the 4 subjects were classified under the HZ hypothesis in the successive experiments, the other 2 subjects matched with constant precision.
I20
J. D. van Willigen
and
M. L.
Broekhuijsen
Table 2. Condensed data of the results of the matches of jaw separations made by subjects, referring to a verbally specified number of millimeters (standard: 4.75, 7.75 and 17.75 mm). The rows and columns have the same meaning as in Table 1
Sub
Standard (mm)
No. of test runs
n
G H
4.75 4.75
5 4
20 20
I J K
4.75 4.75 4.75
5 4 5
20 20 20
G H I J K
7.75 7.75 7.75 7.75 7.75
5 4 5 4 5
20 20 20 20 20
G H
17.75 17.75
5 4
I J K
17.75 17.75 17.75
5 4 5
Trend in the matches?
Accepted (t%~) hypothesis
SEM (mm)
staid;rd?
8.20 0.76 5.24 0.44
1.75 0.29
la la
0.78 0.15
No + No +
Yes’ Yesf
5.70 0.39 4.72 0.39 6.46 0.70
0.89 0.39 0.62
2 la 2
0.40 0.20 0.28
Yes Yes No +
No No No
0.92 0.39 0.70 0.68 0.92
2.29 0.20 1.10 1.07 0.71
2 la la la la
1.03 0.11 0.49 0.54 0.32
Yes No Yes Yes No +
No No No No No
20 20.88 1.51 20 13.16 0.79 20 17.50 1.12 20 15.46 0.96 20 15.66 1.69
2.53 ~
la 0
1.13 0.05
Yes No -
No
1.14 1.67 2.57
la la la
0.51 0.84 1.15
Yes Yes Yes
No No No
&m)
10.42 6.93 8.49 8.25 8.76
(nr&
Long-term experiments
Trend in the imprecision?
No No No
-
-
DISCUSSION
Two subjects were tested for their performance in matching a standard gauge over 10 weeks without repeating the learning procedure. The results are shown in Table 3. We see that the subjects tested over long periods of time behave like subjects tested in the short-term.
From our experiments, the following statements can be made about the memory for jaw separations and its precision: Dentate subjects are able to match a self-generated standard; precision is comparable with that of the PVDO of edentulous subjects (Broek-
Verbal Standard standard 17.75 mm
standard 7.75 mm
standard 4.75 mm
match(standardliOO)
match(standard-lO0)
I
I
200.
200.
150,
150,
I
loo.--
loo-
50
50.
0
0
.impreciskn
:
iSinstability
Fig. 2. Histograms of the matches of jaw separations made by subjects referring to a verbally-specified number of millimetres (standard = 100). The standards were: 4.75, 7.75 and 17.75 mm. The bars and the horizontal dashed lines on top of the columns have the same meaning as in Fig. 1.
Perception
121
of jaw positions
Table 3. Condensed data of the results of the matches of jaw separations made by subjects, referring to a remembered size of a standard gauge (gauge sizes: 4.75, 7.75 and 24.75 mm; learning period about 20 min). Furthermore, condensed data of the results of long-term matches of jaw separations for 2 subjects are shown. These subjects referred to a standard gauge (4.75 mm), given only at the start of the test run sequence No. of
Sub
Standard (mm)
A C E F
4.75 4.75 4.75 4.75
A C E F
test runs
II (mm) (nk)
(r%)
Accepted hypothesis
10
10 10 10 10
5.47 5.95 5.05 5.04
0.38 0.43 0.48 0.43
0.62 0.37 0.19 0.19
2 2 la la
0.20 0.12 0.07 0.07
7.75 7.15 7.75 1.75
IO IO IO 10
10 10 10 10
7.55 8.23 8.41 7.93
0.40 0.44 0.48 0.61
0.58 0.33 0.26 0.36
2 la la la
A c E F
24.75 24.75 24.15 24.75
10 10 10 10
10 10 10 10
24.86 1.33 0.73 24.67 0.90 0.55 24.23 1.81 0.32 24.40 1.37 0.39
2 2 la la
B D
4.75 4.75
10 IO
10 IO
10 10
IO
The rows and columns
Real standard 5.50 0.39 0.41 6.17 0.54 0.57
have the same meaning
SEM (mm)
stagd;rd? No + No +
Trend in the matches? Yes 1 No
No +
No
No +
Yes’
0.19 0.1 I 0.09 0.12
Yes No + No + Yes
Yes’
0.25 0.19 0.15 0.15
Yes Yes No No -
No No No No
(long-term) la 0.13 2 0.19
No + No +
No Yesi
Trend in the imprecision? No No
No
No
Yes t No
No No
No
as in Table 1.
huijsen and Van Willigen, 1982). This means that periodontal sensors are not the prime sensors for sensing jaw separations. This is in accord with the observations of Christensen and Morimoto (1977). In half of the experiments, the subjects showed a match that equals the verbal standard or the real
standard. This means that over longer periods of time these subjects are able to match accurately a verbally specified number of millimetres or the remembered size of a standard gauge. Inaccuracies tend to be positive when matching small standards and negative when matching larger
Real Standard standard 4.75mm match(standard.100)
standard Z75mm
1match(standardllOO)
standard 24.75mm ~match(standard,lOO)
Axtability
Fig. 3. Histogr.ims of the matches of jaw separations made by subjects referring to a remembered size of a standard gauge (standard = 100). The standard gauges were: 4.75, 7.75 and 24.75 mm. The bars and the hcrizontal dashed lines on top of the columns have the same meaning as in Fig. I.
122
J. D. van Wilhgen
and M. L. Broekhuijsen
ones. This is in agreement with classical psychophysical experiments (e.g. Fullerton and Cattell, 1892; Needham. 1935). Over longer periods of time, subjects tend to give more accurate matches when they are referring to a verbal standard than when they are referring to a real standard. This can be caused by the fact that in normal life reinforcement of the internal representation of the common metric value often occurs. The relative imprecision (expressed as imprecision/standard) of the matches is nearly independent of the size of the standard. This is in agreement with the general characteristics of sensors. Matches are more precise. when referring to a real standard. than when referring to a verbal standard; one of our subjects had a rather distorted conception of the magnitude of a millimetre (Table 2. subject G). All subjects are classified under the H,, or the Hz hypothesis. This means that all our subjects are unstable in their matches over a longer period. This phenomenon can be caused by long-term instability in the comparison mechanism or slow changes in the engram of the standard. About 67 per cent of the results were classified under the H ,a hypothesis. which means that, in the long-term, matches were unstable but performed with the same precision. The remaining 33 per cent showed variable precision. Subjects tested over a long period of time, referring to a real standard that is given only once at the start of the sequence of test runs, behave in the same way as subjects who were matching a standard that they have explored on the same day. Some subjects showed a trend in the sequence of matches, No causal relation between the experimental parameters and this trend could be detected. No trend was detected in the precision. We conclude that dentate subjects recognize jaw separations fairly well. This holds whether the subject refers to an imagined, a verbally-imposed or a physically-imposed (real) standard. Over longer periods, about half of the subjects matched accurately. The relative precision of the match is independent of the size of the standard and the precision of matching in dentate subjects is similar to that in edentulous subjects All dentate subjects are unstable in their matches over a longer period of time. Some edentulous subjects showed that their performances of PVDO determinations were stable in the long term (Broekhuijsen and Van Willigen, 1982). However, we have to bear in mind that the subdivision into a stable and unstable class depends on the internal parameters of the experiments, especially the number of presentations of each stimulus value per test run. This means that, after increasing the number of presentations per test run, the probability that the subject will be classified as unstable is greater. Ack,lo~~ledgenlrrtrs~We gratefully thank Professors C. Rashbass and F. Veringa for helpful discussion and comments on the manuscript.
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Broekhuijsen