Journal of Neurological Sciences (Turkish) 2017 , Vol 34 , Num 2
Activity of Facial and Swallowing Muscles During Water and Sour Bolus Deglutition in Healthy Adult Humans
Nevin GÜRGÖR,Yesim BECKMANN,Nazlı HASSANZADEH,Şehnaz ARICI,Tülay K. İNCESU,Yaprak SEÇİL,Cumhur ERTEKİN
1İzmir Katip Çelebi University Atatürk Training and Research Hospital , Neurology, İzmir, Turkey DOI : 10.24165/jns.9875.16

Summary

There are very close relationships among deglutition, taste sensations, and facial expressions in humans. However, the associations among mimicking, swallowing, and the suprahyoid/submental muscle group (SM) during deglutition events have yet to be investigated using electrophysiological techniques to elucidate taste sensation and facial expression further. Thus, this study used surface EMG to evaluate synchronous electromyography activity in the facial and swallowing muscles during the swallowing of water and sour boluses in 20 normal healthy adults. All participants were instructed to swallow four bolus volumes (5, 10, 15, and 20 ml) each of a water bolus and a sour bolus (lemon juice) in a neutral head position. EMG recordings were obtained from the orbicularis oculi (OC; a mimic muscle), orbicularis oris (OR; a mimic and swallowing muscle), and SM (deglutition muscles) to determine whether these muscles were synchronously activated during voluntary oropharyngeal swallowing. The mimic (OC and OR) and swallowing muscles (SM) were synchronously activated in more than 90% of participants during water swallowing. Swallowing lemon juice increased the number of spontaneous swallows after the first deglutition and significantly shortened the duration of a single swallowing apnea. The present findings indicate that both taste and the intraoral trigeminal afferents of water and sour boluses excite facial neurons via the nucleus tractus solitarius and/or a central pattern generator. Additionally, sour boluses produced stronger and safer oropharyngeal swallowing in conjunction with increases in facial expressions due to the activation of facial mimic muscles.

Introduction

Reflexive facial expressions in response to taste stimuli occur in both humans and rats such that stereotypical expressions are produced following different taste qualities (1-5). These reflexive facial expressions can even be elicited at birth prior to any experience with taste stimuli (1,3,6,7). Two types of studies have investigated the relationships among facial mimicry, taste, swallowing, and electromyography (EMG). Additionally, facial expressions in response to different taste stimuli have been investigated in subjects of different ages using EMG, but these studies did not use the EMG to measure the activities of swallowing muscles, such as the submental muscle. Conversely, other studies have investigated the effects of sour and other tastants on swallowing, but did not assess changes in facial expression, such as alterations of the orbicularis oculi (OC) muscle (8-14). Sour boluses alter oropharyngeal swallowing in humans, but no studies have associated this type of stimulus with changes in facial expression (14-16). Nonetheless, there seems to be very close relationships among deglutition, taste sensation, and facial expressions.

In healthy adults, the OC is a strong facial mimic muscle that contributes to oropharyngeal swallowing in conjunction with the synchronous EMG activation of the orbicularis oris (OR) and suprahyoid/submental muscle group (SM) (17). In humans, the cooperation of the OC and deglutition muscles may primarily occur in response to the taste and nociceptive aspects of intraoral boluses. For example, extremely sour, salty, and bitter foods and very hot water produce OC movements in clinical situations. The OC is a mimic muscle, the OR is both a mimic and a swallowing muscle, and the SM is a swallowing muscle; hence, it is important to determine how they are activated in synchrony during the swallowing of a water bolus or a sour bolus.

It was hypothesised in the present study that sour tastants would increase the excitability of related brainstem structures as well as the synchronous activity of facial and SM muscles to facilitate facial expressions and swallowing. In other words, the present study aimed to demonstrate the effects of the increased input of a sour taste on oropharyngeal swallowing behaviour and facial expressions using electrophysiological techniques in healthy adult humans. It was not possible to assess the suprasegmental aspects of facial expressions and oropharyngeal swallowing in the present study.

Methods

This study included 20 healthy adult participants (17 females and 3 males) with a mean age of 37.6 years (range: 26?62 years). All participants were volunteers selected from among our hospital staff who had no reported history of olfactory, gustatory, or swallowing disorders, and were not currently taking any concomitant pharmacological treatments that could influence taste, olfaction, or swallowing. A neurological examination confirmed that participants" facial nerves and mimic muscles were intact. All participants provided written informed consent according to Declaration of Helsinki, and the study was approved by the hospital ethical committee.

During the study measurements, each subject was instructed to sit on a chair and hold his or her head in a neutral upright position. Ambient conditions were as calm and relaxed as possible. Swallowing signals were recorded from the SM with an EMG (SM-EMG) that used bilateral silver chloride electroencephalography (EEG) electrodes (diameter: 10 mm, disc electrodes) taped under the chin over the submental muscle complex (mylohyoid geniohyoid and anterior digastricus) approximately 10 mm from the midline, as previously reported (18). The skin overlying the submental complex was cleaned, and electrogel (Lome EEG paste) was used for the bipolar surface electrodes. The EMG signals were band-pass filtered (100-10 kHz), amplified, rectified, and integrated.

EMG signals were also recorded using electrodes placed on the facial skin surface over the OR and OC muscles on one side (left) (17). To record OR activity, electrodes were attached to the upper and lower lips near the corner of the mouth with an interelectrode distance of approximately 0.6-0.9 cm. To record OC activity, Ag/AgCl bipolar surface electrodes were attached over the lower eyelid at an interelectrode distance of approximately 0.5-0.7 cm; the distance between the OC and OR recording electrodes (from the middle of the OC electrodes to the corner of the mouth on the same side) was >4 cm. The EMG activities of the OC and OR were electronically adjusted in the same manner as the SM-EMG activity except that the amplitude for each muscle was adjusted according to the magnitude of EMG activity. The amplitude was measured from the baseline to the peak, and the duration was measured from the onset to the end of deflexion according to the baseline. Care was taken to be aware of the short blinking artefact during the OC recording and other blinking artefacts during all recordings, particularly during deglutition. In some cases, there were EMG artefacts in the OR recording prior to the first deglutition, but this was likely due to facial movement intended to avoid leakage of fluid from the mouth.

The respiratory signal was obtained via a nasal cannula placed at the entrance of the nares, which was connected to an airflow sensor transducer (Sleep Sense ®, SLP; Tel Aviv, Israel) and interfaced with the EMG apparatus. Airflow direction was recorded with the negative polarity representing inspiration, and the positive polarity representing expiration. The phases of respiration were simultaneously recorded to determine the apnea period; the respiratory signals were also recorded during rest. A plateau in the respiratory signals along the baseline indicated the apnea period, and swallowing apnea was measured from the end of the last respiration cycle to the onset of the next respiration. Instances of respiration cessation due to the second or additional swallows were also measured, but the swallowing apnea period was calculated first. The signals were filtered (0.2-10 kHz), amplified, rectified, and then integrated. A four-channel EMG device (Nicolet-Viking Select V11.0, VIASIS Health Care, Medical Device Directive, Netherlands) was used to record muscle activity; the electrodes were connected to the four channels of the EMG apparatus.

To test the effects of increasing water volume on deglutition, four volumes of water were swallowed (5, 10, 15, and 20 ml) from a disposable cup positioned between the lips. Each participant began to swallow each bolus immediately after being instructed to do so by the examiner. The water temperature was approximately 23-25°C. For the sour stimulus, 100% lemon juice was freshly prepared from a lemon in the laboratory without water dilution. All participants received the boluses of lemon juice in a stepwise manner (5, 10, 15, and 20 ml) from a disposable cup positioned between lips. The intraoral cavity was rinsed with tap water after each trial, and there were intervals of several minutes between boluses.

The following parameters were measured for each volume of water and lemon juice: (1) number of swallows over 20 sec during the swallowing of 20 ml of liquid; (2) EMG amplitude of the investigated muscle (SM, OC, or OR) measured from baseline to peak during swallowing; (3) EMG duration of the investigated muscle (SM, OC, or OR) measured from the onset to the end; and (4) duration of swallowing apnea measured in the first swallowing. When any suspicion of an artefact occurred, the recording was repeated two or three times.

Statistical Analysis
All statistical analyses were conducted with the Medcalc V11.3.0 statistical package, and all measured quantities are expressed as mean ± standard deviation (SD). Wilcoxon tests were applied to compare the amplitudes and durations associated with the swallowing movements among the OC, OR, and SM muscles. For all analyses, p-values 0<0.05 were considered to indicate statistical significance.

Results

All 20 participants completed the experiment, and EMG activity was recorded from the OR, OC, and SM during movements related to deglutitional events in all. The participants averaged one swallow during water swallowing with increasing volumes. In contrast, the same procedure with lemon juice resulted in a mean of more than one swallow during the 20-sec analysis period (p < 0,0001; Fig. 1) and a greater number of swallows during the 1-min analysis period (p < 0.0001; Table 1). The first swallowing apnea was significantly shorter for the 15- and 20-ml lemon juice boluses compared to the respective water boluses (p < 0.05; Table 1, Fig. 2).

Figure 1: 5 ml water (A) and 5 ml lemon juice (B) swallowing SM: submental muscle, Resp:Breathing, OC: Orbicularis Oculi, OR: Orbicularis Oris Note: 5 ml lemon juice swallowing produce successive swallows. Duration of SM muscle is increase and in all water and lemon juice swallows, there are clear synchronisation (vertical dotted line). Time base of calibration mark was 1 second. The amplitude was written to right side of all traces.

Figure 2: The individual graphics compare the first swallowing apnea duration of SM EMG for each volumes of water and lemon juice swallowing.

Table 1: The summary of statistical results of comparision water-lemon juice drinking (mean+SD)

The amplitudes and durations of SM, OC, and OR activity were affected differently (Table 1). All muscles were synchronously activated for all swallowing volumes except for two cases (10%) in which OC activity was not clearly recordable (Fig. 3). The mean amplitudes of SM, OC, and OR activity tended to increase, although the values did not significantly differ, and the SM activity amplitude significantly increased while swallowing the 10-ml lemon juice bolus. The mean durations of SM, OR, and OC activity increased when lemon juice was swallowed, but the effects of the sour taste differed among the three muscles. The duration of OR activity was not significantly prolonged for any of the lemon juice boluses, the duration of OC activity was significantly prolonged after the 10- and 15-ml lemon juice bolus, and the duration of SM activity was significantly prolonged after the 15- and 20-ml lemon juice boluses (Table 1, Fig. 4). These findings suggest that lemon juice effectively activated the fundamental muscles involved in deglutition (SM) as well as the mimic muscles (OC).

Figure 3:In a normal subject without synchronisation between OC and swallowing muscles Note the question marks in the OC traces.(There is one spontaneous blinking denoted with #) Calibration mark is 1 second, the amplitude calibrations were written to the right side of all traces.

Figure 4: Duration of OC and SM muscles activities during 15 ml water and Lemonade(Lemon) juice on the individual bases.

Discussion

Studies using videofluoroscopy (14,16,19,20) and EMG have shown that the physiological link between a sour taste and swallowing alters and facilitates oropharyngeal swallowing in humans (8-14,21,22). In the present study, the SM exhibited greater activity during sour bolus swallowing than during water swallowing, but this change was only significant with the 10-ml lemon juice bolus (Table 1). However, the SM-EMG duration was significantly longer with larger volumes of the sour stimulus.

The importance of response duration to bolus stimuli has not been stressed in the relevant literature, except by Leow et al. who found that the duration of SM-EMG activity following a sweet tastant did not significantly differ from that after a sour tastant and that taste had no significant effect on the duration of swallowing or SM-EMG amplitude (12). This could be related to differences in the study methods. The present study used 100% lemon juice, which contains more citric acid, and it is possible that the higher concentration of citric acid remains in the oropharyngeal region for a longer period of time. Consequently, the longer duration of the sour taste would result in the repetitive stimulation of the taste receptors. Thus, citric acid may cause more effective and longer stimulation at the oropharyngeal receptor level and/or the brain stem level. In the brainstem, this likely occurs in the nucleus tractus solitarius (NTS). Increasing volumes of citric acid excite multiple peripheral sensory receptors in a dynamic fashion, activating premotor neurons in this region (23).

The majority of studies investigating swallowing have used the SM as the indicator muscle. Several studies have shown that sour tastants cause the greatest response amplitude of submental muscle contraction during swallowing and that a sour bolus results in stronger muscle contractions, as evidenced by greater electromyographic activity (8,11,12,21,22). Additionally, motor-evoked potentials evoked by transcranial magnetic stimulation and recorded from the submental muscles revealed that sour tastants increased neural excitability during swallowing (8). However, the increase in SM muscle contractions following simultaneous tastant stimulation was not significant compared to baseline (9).

In this study, with the exception of the amplitude of SM activity in response to a 10-ml sour bolus in healthy participants, the amplitudes of SM and OC activity in healthy human participants showed no significant increase with increased volumes of water or a sour bolus when the two were compared. More specifically, the mean amplitudes following each bolus increased, but not to a significant degree, which is in partial agreement with the abovementioned studies. It is possible that the significantly increased response duration observed in the OC following large volumes of lemon juice resulted in the spatial dispersion of activity in these muscles which, in turn, minimized any influence on amplitude. Similarly, OR activity may have exhibited higher amplitude and a longer duration to lemon juice without reaching significance in the present study because its function involves facial expressions as well as deglutition. Involvement in these two different functions may also influence OR activity because it is necessary to close one's mouth tightly while swallowing to prevent the escape of liquid (24,25). On the other hand, facial expressions might entail a more relaxed position for the OC (3,26-29).

Measurements of the first apnea period during the first swallow revealed that the apnea period was significantly shortened (Table 1, Fig. 4). This suggests that the sour taste increased oro-swallowing while the airway was protected due to the shorter apnea period, an important effect of a sour bolus. Strong and prolonged oropharyngeal swallowing, as measured from the OR and SM, can result in fast and short breathing. Spontaneous saliva swallows and their frequency have rarely been mentioned in previous studies (15,30). However, in the present study, the number of spontaneous swallows significantly increased, with individual variation among participants, after swallowing the lemon juice. Pelletier and Lawless observed a significant increase in spontaneous dry swallows after sour stimuli, and the number of subsequent spontaneous swallows after a citric acid preparation was applied to the oral cavity was higher than that after an acid sucrose mixture (20).The present study demonstrated that high numbers of spontaneous swallows can be recorded from the SM as well as from the SM, OR, and OC simultaneously. To our knowledge, this finding has not been described previously. Citric acid stimulation initiates high flow rates of saliva that is rich in bicarbonate, which acts as a buffer for the major salivary glands (6). Ongoing swallowing apnea should not be due to peripheral mechanisms and is likely based on dysfunction in the brainstem, perhaps related to the link between swallowing and respiration (23,24,25,31,32,33). For example, a sour taste modulates activity within central pattern generators (CPGs) in the medulla by enhancing the triggering of swallowing (34). However, sour tastes can play a role in the upregulation of CPGs in the brainstem as well as in cortical swallowing networks (35,36). Similarly, facial expressions can be modified by supramedullary centres. The synchronous activation of the OC with the OR and SM, the swallowing muscles, may be the last efferent destination of the bilateral linkage between the pontine facial nucleus and the NTS.

High levels of citric acid also elicit the so-called "chemesthesis" sensation, which is mediated by the trigeminal nerve and may play a crucial role in swallowing physiology (37,38,39). Chemesthesis may enhance gustatory input to the NTS, and this enhancement should be evoked and organised by the trigeminal nerve afferents of the oral cavity during the first step of swallowing. Additionally, the OC and OR are likely related to some degree due to connections in the brainstem; the trigeminal nerve afferents that arise from the face and intraoral cavity may represent this linkage. This may underlie the synchronous activity of the OC and OR during lemon juice deglutition.

In this study, 90% of the healthy normal participants exhibited clear synchronisation of the OC, OR, and SM during both water and lemon juice swallowing. Additionally, in the majority of cases, the lemon juice bolus enhanced the size and rate of the electrophysiological swallow responses. Facial expressions in response to different taste stimuli have been studied using participants of different ages (1,5,6,26,27,40,41,42) and the electrical activity of facial muscles has been used to investigate a variety of hedonic emotional and cognitive responses to taste stimuli (27,28,40,41). However, in the present study, the age range was rather large, and this limited any interpretation of age-related effects.

Individual differences in response to a sour bolus may also be related to the "genetic taster status" of an individual human subject (43,44,45). Two cases in the present study exhibited stimulation of the OC, but not synchronous activity of the OC, OR, and SM, after swallowing the lemon juice. This difference may indicate that the taste and mimic functions of some individuals do not cooperate at the medullary level.

From an anatomical point of view, little information is available regarding the functional connections between afferent taste inputs and motorneurons in the facial motor nucleus that are responsible for stereotypical responses to taste stimuli (6). Some projections to the facial motor nucleus originate in the trigeminal sensory complex, (46,47) and fewer originate in the NTS (48,49). Studies of lesioned decerebrated animals demonstrated that the circuits responsible for mimetic facial responses to taste stimuli are located in the brainstem (7). However, this function is likely to be more complex in adult humans.

This study has several general limitations that should be noted. First, the number of participants in the study was relatively small, and participants were not compared according to age groups. Additionally, although it could be easily identified, spontaneous blinking disturbed the regularity and clarity of the EMG data, and its effects on the OC could not be avoided. Fortunately, however, this type of bioelectric activity is clearly different from that evoked by the swallowing function. Second, it is difficult to draw precise conclusions about the nature of brainstem or other central nervous system activation using EMG methods. Thus, future studies should include neural assessments during swallowing. In conclusion, this study found that a sour tastant increased the excitability of brainstem structures and that the synchronous activity of facial and swallowing muscles facilitates facial expressions and oropharyngeal swallowing. Additionally, a water bolus produced synchronous activation of several facial muscles, including the OR and OC.

Author Contributions: Concept - M.G, E.A, B.G., Design - M.G, E.A, B.G; Supervision - M.G, E.A.; Analysis and/or Interpretation - M.G, E.A, B.G.; Literature Search - M.G, B.G.; Writing - M.G, E.A., Critical Reviews - M.G, E.A.

Conflict of Interest: No conflict of interest was declared by the authors.

Disclosure: The authors declared that this study has received no financial support.Received by: 10 December 2016
Revised by: 11 May 2017
Accepted: 20 May 2017

References

1) Armstrong JE, Hutchinson I, Laing DG,Jinks AL. Facial electromyography: responses of Children to odor and taste stimuli. Chem Senses 2007; 32: 611-621.

2) Boelhouwer AJ, Brunia CH. Blink reflexes and the state of arousal. J Neurol Neurosurg Psychiatry 1977;40:58-63.

3) Burrows AM. The facial expression musculature in primates and its evolutionary significance. Bioessays 2008;30:212-225.

4) Navarro L, Torero C, Salas M. Basic brainstem taste responsivity: effects of perinatal influences. Revista eNeurobiología 2011;2: 090511.

5) Tassinary LG, Cacioppo JT, Geen TRA. Psychometric study of surface electrode placements for facial electromyographic recording: I. The brow and cheek muscle regions. Psychophysiology 1989;26:1-16.

6) Bradley RM, Kim M. Reflex Connections. In: Bradley RM, editor. The Role of the Nucleus of the Solitary Tract in Gustatory Processing. Boca Raton FL: CRC Press/ Taylor & Francis; 2007: Chapter 4.

7) Grill HJ, Norgren R. The taste reactivity test. I. Mimetic responses to gustatory stimuli in neurologically normal rats. Brain Res 1978;143:, 263-279.

8) Abdul Wahab N, Jones RD,Huckabee ML. Effects of olfactory and gustatorystimuli on neural excitability for swallowing. Physiol Behav 2010;101:568-575.

9) Abdul Wahab N, Jones RD, Huckabee ML. Effects of olfactory and gustatory stimuli on the biomechanics of swallowing. Physiol Behav 2011;102:485-490.

10) Butler SG, Postma GN,Fischer E. Effects of viscosity, taste, and bolus volume on swallowing apnea duration of normal adults. Otolaryngol Head Neck Surg 2004;131: 860-863.

11) Ding R, Logemann JA, Larson CR,Rademaker AW. The effects of taste and consistency on swallow physiology in younger and older healthy individuals: a surface electromyographic study. J Speech Lang Hear Res 2003;46:977-989.

12) Leow LP, Huckabee ML, Sharma S,Tooley TP. The influence of taste on swallowing apnea, oral preparation time, and duration and amplitude of submental muscle contraction Chem Senses, 2007;32:119-128.

13) Mistry S, Rothwell JC, Thompson DG,Hamdy S. Modulation of human corticalswallowing motor pathways after pleasant and aversive taste stimuli. Am J PhysiolGastrointest Liver Physiol 2006;291: G666-671.

14) Sciortino K, Liss JM, Case JL, Gerritsen KG, Katz RC. Effects of mechanical, cold, gustatory, and combined stimulation to the human anterior facial pillars. Dysphagia 2003;18:16-26.

15) Nederkoorn C, Smulders FT, Jansen A. Recording of swallowing events using electromyography as a non-invasive measurement of salivation. Appetite 1999;33:361-369.

16) Logemann JA, Pauloski BR, Colangelo L, Lazarus C, Fujiu M,Kahrilas PJ . Effects of a sour bolus on oropharyngeal swallowing measures in patients with neurogenic dysphagia. J.Speech Hear Res 1995;38:556-563.

17) Ertekin C, Eryasar G, Gurgor N, Arici S, Secil Y, Kurt T . Orbicularis oculi muscle activation during swallowing in humans. Exp Brain Res 2013;224: 79-91.

18) Ertekin C, Pehlivan M, Aydogdu I,et al. An electrophysiological investigation of deglutition in man. Muscle Nevre 1995;18:1177-1186.

19) Hamdy S, Jilani S, Price V, Parker C, Hall N, Power M. Modulation of human swallowing behaviour by thermal and chemical stimulation in health and after brain injury. Neurogastroenterol Motil 2003;15:69-77.

20) Pelletier CA, Lawless HT. Effect of citric acid and citric acid-sucrose mixtures on swallowing in neurogenic oropharyngeal dysphagia. Dysphagia 2003;18:231-241.

21) Miura Y, Morita Y, Koizumi H,Shingai T . Effects of taste solutions, carbonation, and cold stimulus on the power frequency content of swallowing submental surface electromyography. Chem Senses 2009;34 :325-331.

22) Palmer PM, McCulloch TM, Jaffe D,Neel AT. Effects of a sour bolus on the intramuscular electromyographic (EMG) activity of muscles in the submental region. Dysphagia 2005;20:210-217.

23) Jean A. Brain stem control of swallowing: neuronal network and cellular mechanisms. Physiol Rev 2001;81:929-969.

24) Ertekin C,Aydogdu I. Neurophysiology of swallowing. Clin Neurophysiol 2003;114: 2226-2244.

25) Jean A, Dallaporta M. Electrophysiological characterization of the swallowing pattern generator in the brainstem. GI Motility online 2006; doi:10.1038/gmi09

26) Dimberg U. Facial reactions to facial expressions. Psychophysiology 1982;19:643-647.

27) Horio T. EMG activities of facial and chewing muscles of human adults in response to taste stimuli. Percept Mot Skills 2003;97:289-298.

28) Hu S, Luo YJ,Hui L. Preliminary study of associations between objective parameters of facial electromyography and subjective estimates of taste palatability. Percept Mot Skills 2000; 91: 741-747.

29) Vrana SR. The psychophysiology of disgust: differentiating negative emotional contexts with facial EMG. Psychophysiology 1993;30:279-286.

30) Watanabe S, Dawes C. The effects of different foods and concentrations of citric acid on the flow rate of whole saliva in man. Arch Oral Biol 1988;33:,1-5.

31) Hårdemark Cedborg AI, Sundman E, Boden K, et al.Co-ordination of spontaneous swallowing with respiratory airflow and diaphragmatic and abdominal muscle activity in healthy adult humans. Exp Physiol 2009; 94:, 459-68.

32) Martin-Harris B. Coordination of respiration and swallowing. GI Motility 2006; online doi:10.1038/gimo10.

33) Uysal H, Kızılay F, Ünal A, Güngör HA, Ertekin C. The interaction between breathing and swallowing in healthy individuals. J Electromyogr Kinesiol 2013;23: 659-663.

34) Doty RW, Richmond WH, Storey AT. Effect of medullary lesions on coordination of deglutition. Exp Neurol 1967;17: 91-106.

35) Humbert IA, Joel S. Tactile, gustatory, and visual biofeedback stimuli modulate neural substrates of deglutition. Neuroimage 2012);59:1485-1490.

36) Humbert IA, McLaren DG. Differential psychophysiological interactions of insular subdivisions during varied oropharyngeal swallowing tasks. Physiol Rep 2014;2:e00239. doi: 10.1002/phy2.239. Print 2014.

37) Bulow M, Olsson R,Ekberg O. Videoradiographic analysis of how carbonated thin liquids and thickened liquids affect the physiology of swallowing in subjects with aspiration on thin liquids. Acta Radiol 2003;44:366-372.

38) Green BG,Gelhard B. Salt as an oral irritant. Chem Senses 1989;14:259-271.

39) Pelletier CA,Dhanaraj GE. The effect of taste and palatability on lingual swallowing pressure. Dysphagia 2006;21:121-128.

40) Epstein LH,Paluch RA. Habituation of facial muscle responses to repeated food stimuli. Appetite 1997;29:213-224.

41) Hu S, Player KA, McChesney KA, Dalistan MD, Tyner CA, Scozzafava JE. Facial EMG as an indicator of palatability in humans. Physiol Behav 1999;68:31-35.

42) Kring AM, Kerr SL,Earnst KS. Schizophrenic patients show facial reactions to emotional facial expressions. Psychophysiology 1999;36:186-192.

43) Williams JA, Bartoshuk LM, Fillingim RB, Dotson CD. Exploring Ethnic Differences in Taste Perception.Chem Senses 2016;41:;449-456. .

44) Nagy A, Steele CM, Pelletier CA. Differences in swallowing between high and low concentration taste stimuli. Biomed Res Int 2014; 2014:813084.

45) Pelletier CA, Steele CM. Influence of the perceived taste intensity of chemesthetic stimuli on swallowing parameters given age and genetic taste differences in healthy adult women. J Speech Lang Hear Res 2014;57: 46-56.

46) Erzurumlu RS,Killackey HP. Efferent connections of the brainstem trigeminal complex with the facial nucleus of the rat. J Comp Neurol 1979; 188:75-86.

47) Pinganaud G, Bernat I, Buisseret P, Buisseret-Delmas C. Trigeminal projections to hypoglossal and facial motor nuclei in the rat. J Comp Neurol 1999;6;415:91-104.

48) Flynn FW, Grill HJ, Schulkin J, Norgren R. Central gustatory lesions: II. Effects on sodium appetite, taste aversion learning, and feeding behaviors. BehavNeurosci 1991; 105: 944-954.

49) Norgren R. Projections from the nucleus of the solitary tract in the rat. Neuroscience 1978;3: 207-218.