Until recently, the function of yawning
has been the subject of very little empirical
research, but there has been no dearth of
speculation. The most widely held hypothesis is
that yawning serves as a form of respiration
induced by high levels of carbon dioxide in the
blood. Barbizet
surveyed the existing literature on yawning and
found no evidence for this view. The fact that
fish yawn also makes the respiration hypothesis
less tenable. Lehman pointed out that yawning is
unlikely to increase oxygen levels because the
deep inspiration is normally followed by a
period of apnea.
The strongest evidence against a simple
respiration hypothesis was provided by Provine,
Tate and Geldmacher who systematically varied
carbon dioxide levels in compressed air that
human subjects were breathing. They found that
increasing the percentage of carbon dioxide did
not cause the subjects, to yawn more, but did
increase their rate of breathing; when their
subjects breathed 100% oxygen their rate of
yawning did not change.
We hypothesized that yawning is likely in
situations where arousal is low, but in which
there is some reason to remain awake and
alert. The function of yawning would be to
increase arousal when the environment provides
inadequate stimulation, but where the
consequences of low arousal could be hazardous.
One implication of our hypothesis is that
organisms in unstimulating or boring
environments should yawn in anticipation of
important events, a prediction previously
confirmed.
When human subjects are alone they are
accurate in reporting their own yawns, and their
rate of yawning is dramatically higher than in
the presence of an observer. In the present
study, we used self-reported yawns to examine
effects of yawning on galvanic skin response
(skin conductance) and heart rate, two
widely-used correlates of arousal.
Method
Subjects : A total of 50 male and female
undergraduate psychology students served as
subjects.
Apparatus : Data were recorded using HRM
Biofeedback Microlab software with appropriately
attached sensors for skin conductance (SC),
electromyogram (EMG), and heart rate (HR). These
measures were portrayed before, during. and
after each yawn.
Procedure : Subjects in the Yawn group
participated in three separate trials, each of
which lasted 15 minutes. In each trial, the
subject was seated alone in a room and recorded
his or her own yawns on an Esterline Angus
recorder while GSR, HR, or EMG was recorded (in
counterbalanced order) for 15 min by equipment
in an adjacent room. Electromyogram was recorded
using electrodes on the subject's masseter
muscle and was used to verify that they \vere
actually yawning when they so indicated, and
that movement artefacts were not producing any
spurious changes; skin conductance was recorded
by electrodes on the fingertips, and heart rate
was recorded using a gel-free sensor clipped to
the ear.
Subjects were told that they were
participating in a study concerning the
physiological correlates of yawning. They were
instructed to relax, to think about yawning, and
to depress the button in front of them when they
began to yawn, keeping it depressed until the
yawn was finished. After a 3-min baseline
reading was taken, subjects recorded their yawns
for 12 min for each trial.
Subjects in the Control group each
participated in two 15-min trials. In one trial
they were instructed to take deep breaths and in
the other trial they were instructed to open
their mouths wide while breathing normally
through the nose. Skin conductance and heart
rate were recorded in different trial
segments.
Results
Experimental Group Data : Of the 30
subjects in the Yawn group, 24 emitted at least
one yawn for a total of 418 yawns, a rate of
18.58 yawns/person-hour, considerably higher
than most rates reported by Baenninger. The mean
number of yawns emitted by a subject in a 15-min
trial was 4.59, with a mean duration of 6.53
seconds. The skin conductance measure increased
in all 21 subjects who yawned at least once. A
Friedman ANOVA performed on the first yawn
emitted by subjects showed a significant
difference between the mean pre-, during, and
postyawn skin conductance values (Fr= 14.90,
p<0.01). Post hoc analysis indicated that the
mean during and postyawn conductance values were
significantly higher than the mean preyawn
values. This pattern was consistent for the
conductance data from the second yawns as well
(Fr= 11.54, p<0.01). By the third yawns, only
the postyawn conductance was higher than the
preyawn value (Fr=7.09, p<0.05), and by the
last yawn given by subjects, the three
conductance values were not significantly
different from each other. But there was a clear
increase in the summed conductance values
between the first and last yawns (sign test,
p<0.01), indicating that successive yawns
were accompanied by increased arousal. Thus the
pattern of skin changes both within and between
yawns showed some changes over successive
yawns.
During the 15-min heart rate (HR) trial, 20
subjects yawned at least once, A Friedman ANOVA
performed on their first yawns indicated that
there were no significant changes in heart rate
during the course of a yawn. This lack of effect
was also true for the second, third, and fourth
yawns emitted during the HR trial; no effects
became apparent over successive yawns, from
first to last yawns.
During the electromyogram (EMG) trial, 22
subjects emitted at least one yawn. The EMG
values during and after yawns were significantly
higher than the mean preyawn EMG value
(Fr=33.09, p<0.01).
Control Data : In both the first and
second mouth opening, the mean conductance
values during and after were higher than the
conductance value before opening the mouth,
indicating an increase in arousal associated
with opening the mouth (F,=9.10, p<0.05;
F,=9.79, p<0.01, respectively). No
differences were apparent by the third or final
mouth opening.
Heart rate control data were also consistent
with this pattern. There was a significant
difference between the mean pre-, during. and
postopen mouth values during the HR trial, but
only for the first mouth opening (F,=9.70,
p<0.01). Post hoc analysis indicated a
decrease in HR as a result of wide opening of
the mouth.
A Friedman ANOVA performed on the mean pre-,
during and postbreathe skin conductance values
indicated significant differences for all three
deep breaths performed (F,=22.30, p<0.0 1; Fr
= 12.48, p<0.0 1; F, = 13.58, p<0.0 1,
respectively). Post hoc analysis showed that the
postbreathing conductance values were increased.
The HR values for the first deep breath
performed also showed a significant difference
between the three values (F,=9.10. p<0.05).
Post hoc analysis revealed that the HR values
belote and during a deep breath were higher than
the postbreath HR value. There were no
significant differences of these values for the
second. or final deep breaths performed. The
overall skin conductance and HR associated with
the first mouth opening and deep breath did not
differ froin the t%vo subsequent mouth openings
and deep breaths.
Discussion : Some progress is being
made in understanding hormonal and
neurotransmitter mechanisms of yawning. The
present study indicates that both yawning and
two of its major components (opening the mouth
and taking a deep breath) initially increased
skin conductance, partially supporting our
arousal hypothesis. Successive yawns accompanied
progressively increased skin conductance only,
and had only a slight cumulative effect on HR.
Opening the mouth or taking a deep breath also
was followed by skin conductance and HR changes
consistent with increased arousal. Thus it
appears that these two components of yawning
might increase arousal at least as much as
yawning itself.
