Brain Research, 565 (1991) 330-336 (~ 1991 Elsevier Science Publishers B.V. All rights reserved, 0006-8993/911503.50 ADONIS 0006899391171928

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Modulation of mechanically evoked perioral reflexes during active force S . M . Barlow Department of Speech and Heari;zg Sciences and Program it: Neural Science, indiana University, Bloomington, IN 47405 (U.S.A.) and Boys Town National Research Hospital, Omaha, NE 68131 (U.S.A.) (Accepted 9 July 1991)

Key words: Perioral reflex; Orbicularis oris; Mcntalis; Active Force; Mechanical stimulation; Skin contactor; Electromyography

Mechanically evoked activity in orbicularis oris inferior and mentalis muscles was studied in humans during active lip force generation. A spcc|aliy designed multipoint array skin contactor, coupled to a position servo-controlled linear motor, was used to deliver precise mechanical imputs to the lip vermilion. The array size of the skin eontactor was systematically varied to quantify the effects of spatial summation on the ~tmplitude and time course of the early component (RI) of the perioral reflex. For normal young adults, significant positive trends were found for the amplitude of R I sampled from orbieularis oris inferior and mentalis muscle recording sites as contactor array size increased. Increasing skin contactor size from 2 to 16 points was also effective in shortening the latency of RI by 3-5 ms.

INTRODUCTION

The evoked perioral EMG response is classically described as a two-component excitatory response consisting of an early (10-18 ms) discharge (RI) that is present on repeated stimulation, .'md a late (30-55 ms) component (R2) that adapts rapidly, Early investigations evoked the perioral reflex with a brisk mechanical tap or stretch applied to the perioral region, or by electrical stimulation of trigeminal nerve branchesZ~"t'~'tM'z'~,More recently, investigations of perioral sensorimotor integration have used controlled low level mechanical inputs to skin areas around the oral opening in an attempt to understand how stimulus parameters, skin site, and voluntary muscle activity influence perior~,l reflex modulation amoung muscles of the lower face ~'~''~-z':2':~':H, There are no known published reports on the effects of varying the size of the contactor on evoked muscle activity in the face. In psychophysical terms, the use of larger skin contactors for constant displacement inputs is associated with an increase in the perceived magnitude of that stimulus '~'~':'~''~",This increase in perceived magnitude of a stimulus associated with larger skin contactars is known as vibrotactile spatial summation and is thought to be due to an increase in the number of active primary afferents t4. Likewise, spatial summation may be a significant factor in determining the magnitude and

time course of the mechanically evoked perioral reflex. Given these considerations, the objective of the present study was to examine the hypothesis that under conditions of controlled voluntary lip EMG activation, the amplitude of the early component (RI) of the perioral reflex is positively related to the size of the skin contactor, thus manifesting the physiological property of spatial summation. MNI'ERIALS AND METHODS

Sllbjc¢l$ Normal human female subjects (n = 8, age 21-34) performed a lip force tracking task during the sampling of mechanically evoked perioral reflexes. Selection critera included (I) good physical health with no history of neurologie disorder, and (2) free of orofacial impairments, Subjects were seated comfortably in a dental chair with the h~ad supported for the mechanical stimulation and EMG recording procedures, The .t major components of the experimental protocol included the recording of electromyographic potentials, real-time display and sampling of subject generated perioral force, and stimulus delivery to skin sites on the lower face using a specially designed servo-controlled (position feedback) linear motor, EMG Recording The spatial distribution of reflex EMG responses was mapped bilaterally for the orbicularis oris inferior ( e e l ) and mentalis (MENT) muscle recording sites, Miniature (4 mm diameter) Ag/ AgC! surface electrodes were used in bipolar configurations with 5 mm separation, The OOi electrodes were placed within I mm of the lip vermilion while the MENT electrodes were placed on the skin overlying the chin some 15-20 mm from the OO! placement.

Correspondence: S.M. Barlow, Speech-Orofacial Physiology and Biomcchanics Laboratory, Department of Speech and llearing Sciences and Program in Neural Science, 3rd and Jordan, Indiana University, Bloomington, IN 47405, I~,S.A.

331 Previous work using both hookwire and the same miniature surface EMG electrodes has demonstrated the capability for sampling EMG activity from one electrode site with negligible contamination from the other, especially at low EMG activation levels s. The ground electrode was taped to the forehead. Biopotentials from each electrode pair were conditioned by a Grass P511 amplifier (gain = 5000, bandpass filtered 0.1-3 kHz). After electrode placement, subjects were required to perform movements of the lips independent of the jaw to assess if the myogenic and kinematic outputs were appropriate for the muscle recording site according to previously established criterias. Under these guidelines, the criterion for an acceptable e e l electrode placement is that the activity recorded during a maximal lip lowering gesture is less than 10% of the EMG level sampled from that site during a maximal lip rounding gesture. The criterion for an acceptable MENT electrode placement is that the activity sampled during a maximal lower lip retraction and depression gesture is less than 10% of the EMG level sampled from that site during a maximal lower lip compression gesture.

Force recording Subjects were required to generate a controlled level of interangle force to stabilize background activity levels during the course o[ the experiment. Previous work has shown that the magnitude and distribution of R! is highly dependent on subject state and activity levels of the facial muscles 4'22"2x. The device used to transduce compression force between the corners of the mouth (interangle) consisted of a pair of load sensitive cantilevers instrumented with strain gages. This transducer was supported by a rigid stainles~ steel platform via a pivot bearing. This permitted free rotation of the transducer unit and did not restrict lip posturing. The platform was encapsulated in a comfortable dental impression mold (Citricon), and placed between the jaws during stimulation. The analog lip force signal was conditioned by a bridge amplifier (DCcoupled) and low-pass filtered (-3 dB @ 50 Hz, 4 pole Butterworth).

to be well within the physiologic operating range of perioral forces (typically less than 10% of maximum voluntary contraction) used for speech, swallowing, and other skilled motor behaviors involving the lipss'~'23. The probe of the mechanical stimulator was positioned on one side of the lower lip vermilion while each subject generated the intended lip force. Previous work on orofacial cutaneous mechanical frequency detection thresholds3"2~and perioral reflex sensitivity22"27 has shown the glabrous skin surfaces of lip vermilion to be one of the most sensitive regions of the lower face. The duration of subject generated labial contractions required to complete each train of 32 mechanical taps was approx. 15 s.

Digital signal processing Signals associated with the linear motor (trigger pulses, probe displacement, probe force), active lip force, and electromyographic activity were digitized at 2000, 200, and 10,000 samples per second, respectively, using a specially configured laboratory data acquisition system consisting of a PDP-11183 and an 80386 microprocessor. The 80386 was programmed to provide the experimenter with on-line signal averages of evoked EMG activity for monitoring purposes. Following the acquisition, digitized EMG signals were demeaned, full wave rectified, integrated, and signal averaged over 100 ms windows, including a 5 ms pre-trigger buffer.

Data analysis Measures of perioral reflex amplitude ~ V ) and latency (ms) were indexed on the processed and averaged EMG waveforms across conditions using calibrated cursors and plotted using a specially written hidden-line graphics display and plotting program. In cases where RI onset latency could not be accurately determined (due ;o relatively high levels of background EMG associated with active lip force), measures of latency indexed to the peak amplitude of RI were obtained. Indexed measurements were automatically saved to disk as ASCII files. These parametric data were sub-

Mechanical stimulation A custom-designed linear motor operating under position feedback was used to deliver mechanical stimuli to the skin overlying the muscles of the lower filet, A 16-bit dcglltchcd digital-analog converter was programmed to generate the control signals for the linear motor to produce replicable monophasic inward displacements of the lip vermilion of approx. 1200 l~m at 2.85 taps per second. The rise/fall time (based on 10-90% intercepts) for the displacement waveform was approx. 4 ms. The array size of the skin contactor was systematically varied using a set of 8 interchangeable probes ranging from 2-16 contact points, Each contact point was approx. I(Xl~m in diameter. Individual point separation was 3 mm. The orientation of the mechanical stimulator to the lip vermilion, and the interangle force transducer is shown in Fig. 1.

Test sequence Subjects were instructed to generate sustained active lip forces at the 0.10 N level simultaneous with the presentation of the mechanical stimulus. This task was necessary to control subject state during the recording session. The averaged evoked perioral reflex was obtained for each contactor array size by using a stimulus train consisting of 32 taps with one replication. Test order was randomized for contactor probe array s',ze.

Experimental procedures The lip force transducer was positioned and subjects were trained to match their analog force signal as acccurately and as steady as possible to a horizontal target cursor displayed on a digital oscilloscope positioned eye level approx. 20 inches from the face. All subjects learned the task with only a few minutes of practice and were able to maintain the lip compression force to within -+ 5% of the 0.10 N target force. The 0.10 N force magnitude is considered

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Fig. I. A line drawing of the mechanical stimulator in relation to the lip vermilion and force transducer. The strain gage device was used to transduce compression force between the corners of the mouth (intcrangle). The stainless steel cantilevers were frec to pivot on a needle bearing that was mounted to a rigid non-yielding stainless steel yoke embedded within a dental impression mold and placed between the jaws. The linear motor, operating under position serve, was suspended in front of the perioral region by a Zciss articulating microscope arm. The skin contactor of the stimulator was placed against the vermilion skin of the lower lip. Approximate electrode locations are shown for orbicularis otis inferior (OOI), and mentalis (MT).

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Fig. 2. A representative example of the influenceof probe contactot array size (2, 6, 10, and 16 point contactors) on the modulation of RI during active lip force (0.10 N) obtained from one subject. Evoked EMG activity was sampled from OOl ipsilateral to the mechanicalstimulus. Ten individualsweepsof evoked raw (unprocessed) EMG activityare superimposedfor each stimuluscondition, The black vertical pointer represents stimulus onset. jetted to regn.ssion and analysisof variance to identify significant trends in the data (i.e. growthof RI as a function contactor probe array). RESULTS The amplitude and latency of the early component of the ~rioral reflex (RI) was found to be highly dependent upon the size of the contactor array when stimulus displacement was held constant under position servo. An example of the influence of probe contactor array size and active lip force on the modulation of R1 is shown in Figs. 2 and 3. Individual records (10 sweeps) of raw EMG evoked from the OO1 muscle recording site ipsilateral to the mechanical stimulus using a 2, 6, 10, and 16 point contactor are shown in Fig. 2. The black vertical pointers represent stimulus onset. The amplitude of the evoked EMG response is clearly dependent upon contactor array size. Fig. 3 includes 6 different panels

corresponding to the stimulus probe displacement (LVDT), skin contactor force transduced by the load cell (LOADCELL), and digitally processed electromyographic activity sampled from the orbicularis otis inferior (OOI-ipsi and OOI-contra), and bilaterally from the mentalis muscle (MENT-ipsi and MENT-contra). In this example, the mechanical input was applied to the left half of the lower lip vermilion simultaneous with 0.10 N of subject-generated active lip force. The size of the contactor array is represented along the z-axis. Each slice in the EMG displays represents the averaged activity resulting from 32 stimulus presentations. As shown in the upper left panel, the position servo was effective in generating a highly replicable displacement trajectory regardless of the contactor array size. The magnitude of the load presented to the vermilion surface increased as the size of the contactor array increased from 2 to 14 points. This is expected since larger contactors 'push' more tissue. R1 is primarily localized ipsilateral to the stimulus. For example, a well-defined R1 response is present in the OOI-ipsi recording site and poorly defined in the contralateral recording site, OOI-contra. The MENT-ipsi placement yielded a prominent RI response whereas the contralateral mentalis site yielded some reflex activity that was not as well organized and much lower in amplitude. Evidence of a systematic relation between contactor array size and the magnitude of the mechanically evoked R1 respc ~e is apparent in the averaged waveforms associated ,,th the muscle recording sites ipsilateral to the stimulus, For example, the RI activity in OOl-ipsi and MENT-ipsi evoked using a 2-point contactor is considerably lower in amplitude than the resulting RI evoked using a 14-point contactor. Examples of the influence of probe contactor array size on the modulation of R1 amplitude and latency for OOI-L and MT-L are shown in Fig. 4. For the OOI-L plot mechanical inputs were delivered to the left side of the upper lip, For the MT-L plot, the mechanical input was delivered to the left side of the lower lip, Each filled symbol corresponds to the averaged R1 potential obtained following the delivery 32 mechanical stimuli. The general pattern of R1 amplitude growth for larger contactor arrays is apparent for the two examples shown. Results from regression analysis indicated the slope coefficient of the growth function for each example shown in Fig, 4 was highly significant (OOI-L, bt = 1.70, P < 0.005; and biT-L, bt = 3.25, P < 0.001). The growth in amplitude was accompanied by decreases in R1 latency (measured from either the onset or peak of EMG activity) that were significantly related to contactor array size (dOS-L, b t - -0.17, P < 0.001; OOI-L, b t - -0.29, P < 0.005; and MT-L, bt = -0.18, P < 0.001). A summary of individual R1 amplitude growth func-

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Fig, 3, An example of the influence of probe contactor array size (z-axis) on the specificity and modulation of RI during active lip force (0.10 N). This display includes 6 different panels corresponding to the stimulus probe displacement (LVDT), skin contactor force transduced by the load cell (LOADCELL), and electromyographic activity sampled from orbicularis oris inferior (OOI-ipsi and OOl-contra), and mentalis muscle (MENT-ipsi and MENT-contra). The mechanical stimulus was applied to the left half of the lower lip vermilion. The EMG signals were rectified, integrated, and signal-averaged over the stimulus train of 32 taps.

tions for mentalis and orbicularis oris inferior muscles is shown in Fig. 5. The data were collected while subjects generated 0.10 N of active interangle lip force in an effort to control excitatory effects from descending inputs on the facial motor nucleus. Eight different growth functions were collected for the MT and OOI electrode placements. In all cases, there is a trend of increasing R! IEMG amplitude as the contactor size increased. Some multiunit recording sites exhibited greater levels of

evoked activity than others as inferred from the amplitude of the averaged IEMG signal. For example, the MT unit indicated by the filled circle had an average R1 IEMG amplitude of approx. 8 #V using a 2-point contactor while the MT unit indicated by the filled diamond had an average R1 IEMG amplitude of approx. 39 #V using the same probe. Linear regression analysis indicated a significant rate of R1 IEMG response growth for mentalis (slope coefficient, b~ = 1.50 ~V/contactor point,

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Fig, 4. Examples of the inlluenc¢ of probe contactor array size on the modulation of RI amplitude for OO! and MENT recording sites (left hand plots) and associated measures of RI latency (right hand plots) obtained from one subject. Each filled symbol corresponds to the averaged RI quantity following the delivery of 32 mechanical stimuli.

t-ratio - 2.89, P-value < 0.005), and orbicularis oris inferior (slope coefficient, bt - 2.12/~V/eontactor point, t-ratio -- 4.67, P-value < 0.001). Although the absolute level of excitability for the early component of the evoked perioral response for either recording site varied across subjects, the rates of response growth were similar among the group of subjects for an individual muscle recording site. Recasting the R I growth functions based on the actual change (Delta) in IEMO amplitude is shown in Fig. 6 for MT and OO!,

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Fig. 6. A summary of indivudual RI amplitude growth functions for mentalis and orbicularis oris inferior recording sites for each of the test subjects (n = 8). RI amplitude values are expressed in delta IEMG units to normalize differences in baseline activity levels.

This is an effective method for examining the pattern of response growth independent of baseline activity levels which varies among muscle recording sites. As expected, the results of regression analysis yielded similar slope coefficients for mentalis (slope coefficient, b I = 1.48 ltV/ contactor point, t-ratio = 6,64, /'-value < 0,001), and orbicularis oris inferior (slope coefficient, h l = 2.12 ~V/ contactor point, t-ratio = 7,69, P-value < 0,001), The significance levels of b I increased for both muscle sites as a result of hctoring out the baseline activity levels among subjects. For the most part, the MT and OO! recording sites yielded similar patterns of RI IEMG growth as a function of contactor array size during the lip force control task, The higher slope coefficient for OO! is largely due to the statistical leverage exerted by one of

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Fig. 7, A summary of RI latency modulation functions vs contacfor array size for mentalis and orbicularis oris inferior recording sites (left panel) and a leas! squares fit (right panel).

335 the 8 R1 growth functions (filled circles). The latency of RI generally decreased in a non-linear fashion as the size of the skin contactor array was increased from 2 to 16 points. The magnitude of the R1 latency shift versus contactor array size (points) for mentalis (n = 2) and orbicularis oris (n = 14) electrode sites is shown in the left panel of Fig. 7. The typical shift in latency was on the order of 3.5-4 ms for those units where accurate estimates of R1 latency could be made. Given the small sample size and apparent similarity, the mentalis and orbicularis oris R1 latency distributions were pooled and a least squares fit obtained (right panel of Fig. 7). The nature of this shift is adequately characterized (R ~' = 0.7955) by the exponential function given in equation 1, Y = A o . X H°

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where An = 7.8833 and Be - -0.8221. DISCUSSION The pattern of RI growth and latency shift for orbicularis oris and mentalis muscle recording sites associated with the delivery of mechanical stimuli to the lip using progressively larger contactor arrays is the first such demonstration of spatial summation for the perioral reflex in humans. The present findings are also consistent with earlier reports that found the mechanically evoked RI response was localized to muscle recording sites ipsilateral to the site of stimulation 4'~"'2°. Response specilicity of this nature is characteristic of the blink reflex as well unless a large stimulus is used 1'''17'24.

REFERENCES

I Auger, R.G., Hemifacial spasm: clinical and electrophysiologic observations, Neurology, 29 (1979) 1261-1272. 2 Auger, R.G., Brainstem disorders and cranial neuropathies, in W.F. Brown and C.E Bolton (Eds.), Clinical Eiectromyography, Butterworths, Boston, 1987, pp. 417-429. 3 Barlow, S.M., Mechanical frequency detection thresholds in the human face, Exp. Neuroi., 96 (1987) 253-261. 4 Barlow, S.M., Spatial summation of mechanically evoked perioral reflexes in human, Soc. Neurosci. Abstr., 16 (1990) 884. 5 Barlow, S.M. and Muller, E.M., The relation between interangle span and in rive resultant force in the perioral musculature, J. Spee~'h Hearing Res., 34 (1991) 252-259. 6 Barlow, S.M. and Netsell, R., Mechanically evoked responses of perioral muscles during fine force control, Soc. Neurosci. Abstr., 12 (1986) 1539. 7 Barlow, S.M. and Rath, E.M., Maximum voluntary closing forces in the upper and lower lips of humans, J. Speech Hearing Res., 28 (1985) 373-376. 8 Blair, C. and Smith, A., EMG recording in human lip muscles: can single muscles be isolated? J. Speech tlearing Res., 29 (1986) 256-266. 9 Bolanowski, S.J., Gescheider, G.A., Verrillo, R.T. and Chec-

Significant questions remain concerning the mechanosensitive properties of the perioral system and the potential importance of mechano-afferent input during speech and other skilled motor behaviors ~5a6. For example, microneurographic recordings from the human infraorbital nerve have shown that mechanoreceptors located in the upper lip appeared to be responsive to lip closure and deformation of facial skin 15.25. Under these conditions, some features of spatial summation, encoded from the activity of cutaneous mechanoreceptors located ~n the lip vermilion, may provide useful information about the area of contact between the upper and lower lip associated with certain speech sounds (Ipl,lbl, and Iml) and the anterior phase of swallowing. In summary, the present study has demonstrated that spatial summation, achieved through adjustments in the size of the skin contactor, is effective in modulating the amplitude and time course of the R1 component of the human perioral reflex in the orbicularis oris and mentalis muscles. Future studies are needed to control for contactor loading force, expand the range of active lip force, and vary the context and complexity of skilled motor tasks to delineate the specificity and modulation of the R1.

Acknowledgements. This study was supported in part by grants from the National Institute on Deafness and Other Communication Disorders R01 DC00365-05 and the Moody Foundation of Galveston, Texas MG.88-46. Special gratitude is expressed towards Mr. Tom Creutz, Mr. Greg Suing, and Mr. Ric Houghton for development of signal processing software and hidden line graphics routines, and Ms. Mary Burton and Mr. Paul Bradford for technical assistance, The helpful suggestions of the anonymous reviewers is also appreciated. kosky, C.M., Four channels mediate the mechanical aspects of touch, J. Acoust. Soc. Am., 84 (1988) 1680-1694. 10 Bratzlavsky, M., Feedback control of human lip muscle, Exp. Neurol., 65 (1979) 209-217. 11 Craig, J.C., Vibrotactile spatial summation, Perception Psycho. physics, 4 (1968) 351-354. 12 Ekbom, K., Jernelius, M. and Kugelberg, E., Perioral reflexes, Neurology, 2 (1952) 103-111. 13 Gandiglio, C. and Fra, L., Further observations on facial reflexes, J. Neurol. Sci., 5 (1967) 273-285. 14 Green, B.G. and Craig, J.C., The roles of vibration amplitude and static force in vibrotactile spatial summation, Perception l~s'ychophysics, 16 (1974) 503-507. 15 Johansson, R.S., Truisson, M., Olsson, K.A. and Abbs, J.H., Mechanoreceptive afferent activity in the infraorbital nerve in man during speech and chewing movements, Exp. Brain Res., 72 (1988) 209-214. 16 Johansson, R.S., Trulsson, M., OIsson, K.A. and Westberg, K.-G., Mechanorcceptor activity from the human face and oral mucosa, Exp. Brain Res., 72 (1988) 204-208. 17 Kimura, J., Rodnitzky, R.L. and Okawara, S.-H., Electrophysiologic analysis of aberrant regeneration after facial nerve paralysis, Neurology, 25 (1975) 989-993. 18 Kugelberg, E., Facial reflexes, Brain, 75 (1952) 385-396.

336 19 Larson, C.R., Folkins, J.W., McClean, M.D. and Muller, E.M., Sensitivity of the human perioral reflex to parameters of mechanical stretch, Brain Research, 146 (1978) 159-164. 20 Lund, J.P., Appenteng, K. and Seguin, J.J., Analogies and common features in the speech and masticatory control sytems. In S. GriUner, B. Lindblom, J. Lubker, and A. Persson (Eds.), Speech Motor Control, Oxford, Pergamon Press, 1982, pp. 231-245. 21 McClean, M.D., Gain and spatial characteristics of human lipmuscle reflexes, Brain Research, 503 (1989) 16-21. 22 McClean, M,D. and Smith, A., The reflex responses of single motor units in human lower lip muscles to mechanical stimulation, Brain Research, 251 (1982) 65-75. 23 Muller, E.M., Milenkovic, P.H. and Macleod, G.E., Perioral tissue mechanics during speech production. In C. De Lisi and J. Eisenfeld (Eds.), Proceedings of the Second IMAC lnternational Symposium on Biomedical Systems Modeling, North-Holland Publishers, Amsterdam, Netherlands. 1985, pp. 363-371. 24 Niei.~n, V.K., Pathophysiology of hemifacial spasm, ll. Lateral spread of the supraorbital nerve reflex, Neurology, 34 (1984)

427-431. 25 Nordin, M. and Thomander, L., Infrafascicular multi-unit recordings from the human infra-orbital nerve, Acta Physiol. Stand., 135 (1989) 139-148. 26 Rath, E.M. and Essick, G.K., Perioral somesthetic sensibility: do the skin of the lower face and midface exhibit comparable sensitivity? J. Oral Maxillofac. Surg., 48 (1990) 1181-1190. 27 Smith, A., McFarland, D.H., Weber, C.M. and Moore, C.A., Spatial organization of human perioral reflexes, Exp. NeuroL, 98 (1987) 233-248. 28 Smith, A., Moore, C.A., McFarland, D.H. and Weber, C.M., Reflex responses of human lip muscles to mechanical stimulation during speech, J. Motor Behav., 17 (1985) 148-167. 29 Verrillo, R.T., Effect of contactor area on the vibrotactile threshold, J. Acoust. Soc. Am., 35 (1963) 1962-1966. 30 Verrillo, R.T., A duplex mechanism of mechanoreccption. In D.R. Kenshalo (Ed.), The Skin Senses, Thomas, Springfield, IL, 1968 pp. 139-159.

Modulation of mechanically evoked perioral reflexes during active force.

Mechanically evoked activity in orbicularis oris inferior and mentalis muscles was studied in humans during active lip force generation. A specially d...
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