mystery of yawning
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
 
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
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12 février 2012
Front Evol Neurosci.
2011;3(7):1-11
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Changes in Physiology
before, during, and after Yawning
Corey TP, Shoup-Knox ML, Gordis EB, Gallup GG Jr.
 
Department of Psychology, University at Albany, State University of New York Albany, NY, USA.

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The ultimate function of yawning continues to be debated. Here, we examine physiological measurements taken before, during, and after yawns in humans, in an attempt to identify key proximate mechanisms associated with this behavior. In two separate studies we measured changes in heart rate, lung volume, eye closure, skin conductance, ear pulse, respiratory sinus arrhythmia, and respiratory rate. Data were depicted from 75 s before and after yawns, and analyzed at baseline, during, and immediately following yawns. Increases in heart rate, lung volume, and eye muscle tension were observed during or immediately following yawning. Patterns of physiological changes during yawning were then compared to data from non-yawning deep inhalations. In one study, respiration period increased following the execution of a yawn. Much of the variance in physiology surrounding yawning was specific to the yawning event. This was not the case for deep inhalation. We consider our findings in light of various hypotheses about the function of yawning and conclude that they are most consistent with the brain cooling hypothesis.
 
INTRODUCTION
 
Yawning has been recorded in all five classes of vertebrates, and is phylogenically old, implying that it is an evolved mechanism that serves an important adaptive function (Baenninger, 1987). Yawning consists of opening the mouth, deep inspiration, a short period of apnea, followed by expiration (Walusinski and Deputte, 2004). In humans and other animals, yawning frequency has been shown to be dependent upon circadian rhythms (Zilli et al., 2007). Studies of yawning have generated varying explanations regarding its ultimate function and proximate mechanisms (Guggisberg et al., 2010).
 
One commonly held notion is that yawning functions to modify levels of oxygen and carbon dioxide in the blood. However, when measured in a controlled environment, yawning frequency was not affected by manipulating levels of oxygen and carbon dioxide (Provine et al., 1987). The same study demonstrated that while exercise doubled breathing rate, indicating a strong increase in oxygen requirements, yawning frequency remained unaffected.
 
A more recent hypothesis proposes that yawning facilitates arousal (Baenninger, 1997; Walusinski, 2006). Evidence for this hypothesis comes from the elevated occurrence of yawning before important events or during behavioral transitions (Baenninger, 1997). Matikainen and Elo (2008) proposed a proximate mechanism to support this theory, suggesting that yawning mechanically stimulates the carotid artery, promoting an increase in cortical arousal via neck compressions that accompany yawning. The carotid body is highly vascularized and compressions may increase circulation, resulting in stimulation by hormones such as adenosine or catecholamines (Matikainen and Elo, 2008).
 
The occurrence of contagious yawning has led some researchers to conclude that the primary purpose of yawning is to provide a means of inner-species social communication (Guggisberg et al., 2010), suggesting that yawning may be a catalyst for conveying empathetic feelings, or messages to a member of ones species. This hypothesis fails to account for several important aspects of yawning; including the proximate behaviors associated with yawning such as stretching, eye and mouth watering, mouth gaping, eye closure, or deep respiration, and the fact that contagious yawning occurs only in a few species, and frequently occurs in solitude. A recent review of this theory, suggests that the social implications of yawning are most likely a derived feature, and that the ultimate function is likely physiological due to its phylogenic history (Gallup, 2011).
 
Another hypothesis that has received recent support posits that yawning is a brain cooling mechanism (Gallup and Gallup, 2007, 2008). The brain cooling hypothesis stipulates that yawning is triggered by an increase in brain temperature, and that the physiological reactions following a yawn promote a return to brain thermal homeostasis. Many thermoregulatory mechanisms have been observed in animals, and possible routes of human brain cooling have been suggested (Zenker and Kubik, 1996). Recent research directly measured cortical temperature in rats and found a distinctive association between brain temperature and yawning (Shoup-Knox et al., 2010). By continuously monitoring cortical temperatures during the 3-min prior to and following a yawn, these researchers found a significant increase in temperature leading up to the onset of a yawn, followed by a significant decrease in temperature and return to baseline in the 3-min following the yawn.
 
While the ultimate function of yawning remains debated, the current study measured and evaluated the physiology associated with spontaneous yawning. Presented here are two studies which evaluated a variety of physiological measurements before, during, and after a yawn. Previous attempts to measure physiology include examinations of skin conductance (Baenninger and Greco, 1991; Greco and Baenninger, 1991), heart rate (Heusner, 1946; Greco and Baenninger, 1991), and vasoconstriction (Heusner, 1946).
 
Greco and Baenninger (1991) found increased variability in heart rate and inconclusive skin conductance changes associated with yawning. Heusner (1946), however, found an increase in beats per minute and an accompanying vasoconstriction, but her results were gathered from a small number of subjects and were not tested statistically. Our goal is to identify replicable patterns of physiological change associated with yawning to better inform theories of an ultimate function. Our first study examined archival physiological data, and focused on the impact of yawning on heart rate, eye closure, lung volume, and respiration rate. Additionally, we examined the effects of yawning on sympathetic and parasympathetic activity by measuring skin conductance and respiratory sinus arrhythmia (RSA). The second study more closely controlled measurements of heart rate, skin conductance, lung volume, respiration rate, and facial temperature, and provided a reliable control variable, in the form of deep inhalations.
 
 
GENERAL DISCUSSION
 
Together, these two studies demonstrate and replicate unique physiological changes associated with yawning. These physiological responses constitute clues as to the ultimate function of yawning. Yawning, as well as deep inhalation increased facial temperature, lung volume, and heart rate. Yawning, but not deep inhalation caused a transient increase in sympathetic nervous system arousal, and a temporary decrease in Eamp. Also a decrease in breathing rate appeared to be associated with deep inhalation behaviors rather than yawning specifically. The current findings suggest an acute sympathetic nervous system response concurrent with increased heart rate and tidal volume. These data also dispute previous findings that claim yawning did not produce autonomic nervous system arousal (Guggisberg et al., 2007).
 
Our results also differ from previous studies that observed an increased variability in heart rate following a yawn (Greco and Baenninger, 1991; Guggisberg et al., 2007). We observed a significant increase in heart rate which was surrounded by less variability than was observed surrounding deep inhalation. This discrepancy may be due to the large amount of yawns that Greco and Baenninger (1991) analyzed, and the awareness of their subjects to the research topic. They observed an average of 18.58 yawns an hour per person, across 30 subjects (4.59 per person during a 15-min trial). This is a much higher rate of yawning than previous studies have reported, suggesting that because they knew they were in a yawning study their subjects were displaying aberrant amounts of yawning. Furthermore, their data should not be considered indicative of spontaneous yawns, but rather contagious yawns. In contrast, both studies herein examined spontaneous yawning from individuals with no knowledge of being observed or participating in a yawning study.
 
In Study 2, the magnitude of increase in heart rate immediately following yawning (from 82.71 at baseline to 90.59 at its highest point) replicated the findings from Study 1, but surprisingly the difference from baseline did not occur until PostS as opposed to our previous effect at Peak. Heusner (1946) also reported similar cardiac acceleration: from 80 to 90 beats per minute, with variations depending on the strength of yawning. The acceleration reported by Heusner accompanied vasoconstriction in the finger. Changes in both heart rate and vasoconstriction began 4-5 s following the initiation of inhalation and were maximal at ri9lOs after initiation. These results temporally mimic the present results, confirming an increase in heart rate following yawning. Heart rate findings coupled with previous reports of vasoconstriction clearly reveal circulatory changes associated with yawning. Also, the observed increase in facial-eye temperature suggests an increase in blood flow. Local skin temperature has been shown to rise with increases in blood flow (for review see Charkoudian, 2003). With an increase in heart rate and skin temperature, we feel confident that there is an increase in blood flow associated with yawning.
 
The rate of blood flow to the brain is one of the key factors in determining the temperature of the brain. Blood flow rate, blood supply temperature, and metabolic thermogenesis as a result of neural activity, are the three primary determinates of temperature in the brain (Kiyatkin et al., 2002). Kiyatkin et al. (2002) also observed that the temperature of the arterial blood supplying the brain is consistently lower than the temperature of the brain during neuronal activation. The increase in heart rate observed in our study would enable this cooler blood to pass through the warmer brain tissue more rapidly, increasing convective cooling.
 
Because of the observed changes in physiology, we believe that yawning serves a physiological purpose. The changes lend support to multiple physiological hypotheses, specifically giving evidence for changes in arousal, cognitive state, and brain temperature. An increase in heart rate, and sympathetic nervous system activity could be causing increases in arousal as well as facilitating changes in cognitive state. Further, the increased heart rate strongly suggests increased blood flow, and thus increased convective cooling This suggests that proposed changes in arousal following a yawn (Baenninger, 1997; Walusinski and Deputte, 2004; Matikainen and Elo, 2008) may be caused by increased blood flow. The same could be true for the proposed state-change hypothesis (Provine, 1986,2005), which suggests that yawning is a vigorous, widespread behavior that stirs up our physiology and thus facilitates transitions to different cognitive states. Because of the increase in blood flow, we believe that our physiological data best match the brain cooling hypothesis (Gallup and Gallup, 2007). The regulation of brain temperature could explain observed changes in arousal levels, and would also support cognitive state-changes.
 
Consistent with the brain cooling hypothesis, arousal, and statechange hypotheses, it is known that human brain temperature and yawning co-vary with circadian sleep-wake cycles. Yawning occurs most frequently in the morning after waking, and in the evening just prior to sleep (Zilli et al., 2007). Brain temperature is highest before sleep, while lowest during sleep. Yawning stops during sleep, but yawning upon waking may be due to an increase in metabolic activity. The state-change associated with waking, and the increased arousal that accompanies waking may require an immediate change in blood flow, and temperature regulation as well.
 
Neither the brain cooling, arousal, state-change hypothesis, nor our current findings that heart rate increased during yawning are mutually exclusive of the hypothesis that yawning promotes alertness and wakefulness by stimulating the carotid artery (Matikainen and Elo, 2008). The carotid body is the main oxygen sensing organ in the body, its primary function is to mediate cardiorespiratory reflexes in response to systemic hypoxia and hypercapnia (Prabhakar et al., 2005; Kumar and Bin-Jaliah, 2007; Matikainen and Elo, 2008). A combination of increasing blood flow, inhaling cool air, or a transient spike in oxygen may cause the carotid body to release physiology-regulating hormones. This hypothesis needs to be empirically tested, and revised to take into account the mounting physiological evidence associated with yawning observed in this paper as well as others.
 
Some hypotheses regarding yawning stress that its primary significance is to promote social or empathetic communication (Guggisberg et al., 2010). The suggestion that the ultimate function of yawning is a social one fails to explain the changes in physiology associated with yawning. While social communication may be an additional/derived function among humans and a limited number of other species (Gallup, 2011), it is important to note the prevalence of yawning across species, including those that live in solitude and the majority of species which lack cognitive empathy and communicative understanding, such as fish and rats. Among humans yawning begins prenatally (Walusinski, 2010), yet contagious yawning does not begin until age 4 or 5 (Anderson and Meno, 2003), suggesting that social or communicative effects of yawning are a derived feature in a very small number of species.
 
These studies document the physiological mechanisms that occur during and following a yawn. While our findings show changes in physiology that are consistent with the concept that yawning acts as a thermoregulatory regulator, future studies need to examine this concept more closely. Further, we understand that a behavior as complex and phylogenetically salient as yawning may be amenable to numerous hypotheses regarding its ultimate function. We stress the importance of proximate mechanisms that have been documented to occur in tandem with yawning when when thinking about the adaptive function of yawning. The physiological changes we have identified are the measurable, salient, and important features of yawning.
 
In conclusion, we believe that our data are most consistent with the brain cooling hypothesis, and demonstrate an increase in blood flow; one of several physiological mechanisms by which yawning could induce brain cooling. The increase in heart rate and sympathetic nervous system activity associated with yawning also needs to be considered when dealing with cases of excessive yawning, and yawning related medical symptoms. There is evidence linking painful headaches (Jacome, 2001) and a variety of thermoregulatory disorders (Sato-Suzuki et al., 1998; Gallup and Gallup, 2008) with excessive yawning. The yawning experienced during these times may be due to circulatory dysfunction. This coupled with evidence that yawning has medical implications for a variety of disorders (Sato-Suzuki et al., 1998; Gallup and Gallup, 2008), suggests that aberrant yawning is symptomatic of thermoregulatory dysfunction. Thus, excessive yawning could be clinically associated with thermoregulatory/circulatory distress/dysfunction, and could be used as a diagnostic indicator. Further research needs to be done to examine cranial-facial blood circulation, as well as take direct thermal measurements during yawning to define pathways by which yawning influences brain temperature.