[...]Discussion :
Yawns as Stereotyped Action Patterns
Yawning is a behavior of the type called
"fixed," "modal," or "stereoryped" by
ethologists (BARLOW 1977; HOYLF 1984; LORENZ
1932; SCHLEIDT 1974). (For simplicicy, the term
"stereotyped action pattern" is used here to
refer to such behavior; doing so does not deny
the legitimate concerris about behavioral
nomenclature raised in the analyses cited
above.) Consider the following properties of
yawning that are characteristic of commonly
described stereotyped motor patterns. Yawning is
consistent in duration, it occurs periodically,
and withinsubject stability in yawn duration and
frequency is maintained over intervals of
several weeks. Yawning is under a high degree of
genetic control because it is performed by
normal (GESELL 1928; PREYER 1923; TAYLOR-JONES
1927) and anencephalic human newborns (GAMPER,
CATEL, both cited in HEUSNER
1946); these populations are presumed to have
little opportunity to learn how to yawn. Yawns
are unitary, being performed at so-called
"typical intensity" fractional yawns detectable
as atypically short yawns in the analyses of
yawn duration were seldom if ever produced.
Yawns are "released" by witnessing yawns or
yawn-related stimuli. (The internal
physiological stimulus for "Spontaneous" yawning
is unknown.) The amplitude and duration of yawns
seem to be independent of the amplitude of the
releasing stimulus; yawns seem to be
"all-ornone" actions. Once initiated, yawns go
to completion with minimal influence of sensory
feedback; everyone is familiar with the
difficulty of trying to stifle a yawn. Yawns are
complex in spatio-temporal organization and have
facial, respiratory, and other components; yawns
are not simple reflexes of short duration.
Although not established empirically, the
components of a yawn seem to occur in only one
order and the timing of components is consistent
from yawn to yawn. This stability of sequence
contributes to the yawn's unmistakable
appearance. The finding that yawns are prominent
behavior of people who are "waiting" (KATAOKA
1975) or performing monotonous work (KISHIDA
1973), of fish that are changing from one
activity to another (MYRBERG 1972), and of dogs
on the threshold of aggression or being forced
to participate in an aversive activity (P.
BORCHELT, pers. comm.) is consistent with the
performance of yawns as "displacement acts" and
"vacuum activity" Stereotyped action patterns
often occur as displacement and vacuum activity.
Given the above properties and the frequency and
prominence of yawning in everyday life, it is
curlous that yawning has been almost totally
ignored by human ethologists (EiBL-EIBESFELDT
1975; MORRIS 1977). Indeed, yawning may be the
best example of a stereotyped action pattern and
releaser in humans. Specific properties of the
yawn as a motor act and stimulus are explored
below.
Once a yawn is initiated, it seems to go to
completion with the inevitability of a sneeze.
The procedure of yawning with clenched teeth was
instituted to test for effects of eliminating or
modifying movement-produced feedback associated
with the gaping component of the yawn. Normal
respiration was permitted through the clenched
teeth. The frequency and duration of normal and
clenched teeth yawns were similar, if not
identical. Thus, the respiratory component of
yawning, that which the subjects recorded, could
be performed in the grossly abnormal sensory
environment of the clenched-teeth condition.
Normal sensory feedback from the gaping
component of the yawn was not necessary for the
initiation, maintenance, or completion of a
yawn; the motor process responsible for yawning
operates with relative independence from sensory
input. However, subjective reports by subjects
that clenched-teeth yawning was unpleasant, did
not satisfy the urge to yawn and gave the
impression of being "stuck" in midyawn suggests
that yawn-produced sensory feedback plays some
role in normal yawning.
Yawn Function
Clenched-teeth yawning is useful in
evaluating accounts of yawn function that stress
a role for respiration or for "stretching" the
face. The facial stretch explanation assumes
that the contraction of the facial muscles
during a yawn forces blood through cerebral
blood vessels to the brain, presumably to
increase alertness (BARBIZET
1958; HEUSNER 1946).
A yawn also opens the eustachian tubes,
balancing atmospheric pressure between the
air-filled middle car and the environment
(LASKIEWICZ
1953). The respiratory explanations emphasize
the increased blood 02 levels that may occur
during the deeper-than-usual inspirations of the
yawn (BARNETT et al. 1971). The clenching of
teeth during yawns reduced or prevented
stretching movements of the law that might
normally have increased cerebral blood flow or
opened the eustachian tubes; only isometric
contractions of the jaw and facial muscles were
possible. Although the clenching of teeth did
not block yawns, the lack of satisfaction and
unpleasant sensation produced by clenched-teeth
yawning suggest that some function served by
normal yawning went unfulfilled in this
experimental condition. The respiration that was
possible during the clenched-teeth condition, by
itself, was insufficient to satisfy the urge to
yawn in most subjects. The inadequacy of the
respiratory hypothesis is also suggested by the
absence of a significant correlation between
spontaneous yawn duration and inter-yawn
interval (i.e., infrequent yawners did not
compensate by producing longer than normal
yawns). By elimination, the jaw stretching
movements of the facial muscles must play an
important, if not principal, physiological role
in normal yawning. A role for stretching is also
indicated by the involuntary stretching
movements of the paralyzed limbs of hemiplegics
during yawns (WALSHE 1923).
Yawns as Releasing Stimuli
The present study confirms the common
knowledge that yawns are "infectious" and
provides details about the nature of the
effective stimulus and the probability and
latency of the yawn response. Visually observed
yawns were potent yawn-producing stimufi. Most
of the subjects viewing a series of videotaped
yawns yawned within 5 mn of viewing the first
yawn. Non-visual stimuli were also effective in
evoking yawns. Reading about yawning produced
actual yawns in over 1/4 of subjects, and
approximately 3/4 of the subjects either yawned
or thought about yawning within 5 min of
starting to read a passage about yawning.
Readers of this paper have probably yawned.
Simply instructing subjects to "think about
yawning" was effective in evoking yawns in the
majority of cases; this was the technique used
to evoke yawns in the descriptive studies of the
yawning act. Thus, direct visual stimulation was
not a necessary characteristic of an effective
yawn stimulus. This conclusion is supported
further by the interesting although largely
anecdotal report by MOORE
(1942) that auditory recordings of yawns evoked
yawning in blind subjects. The wide variety of
yawnproducing stimuli contributes to the
"Infectiousness" of yawns.
Yawns are releasers of the stereotypic action
pattern that is the yawn. Releasers studied by
ethologists in non-humans range from the
simplicity of a red spot on the mandible of the
female herring gull to complex dances performed
by courting male avians (EIBL-EIBESFELD 1975).
Although we yawn in response to observed yawns,
yawn-induced yawning is not the result of a
conscious desire to imitate the yawner. An
observed yawn initiates a complex series of
neurobehavioral events that culminates in a yawn
in the observer.
Yawn-induced yawning may provide insights
into the recently reported ability of human
neonates to "imitate" facial expressions (Field
et al. 1982; MELTZOFF & MOORE 1977). Since
the newborns have not had an opportunity to
learn how to imitate facial expressions, a
releasing mechanism such as here proposed for
yawning may be involved. Indeed, yawn-induced
yawning may be an example of such "imitative"
behavior that has survived into adulthood.
Aithough yawning is performed by human newborns,
the presence of yawninduced yawning has not been
tested. Its detection would add to the
repertoire of newborn "imitative" responses and
provide further evidence of the innate nature of
the yawning act.
Conclusion
The present rescarch establishes the yawn as
a stereoryped action pattern and releasing
stimulus, both of which have been rarely
identified in human behavior. Yawn-evoked
yawning provides an excellent and perhaps unique
opportunity to observe in ourselves the
sensation of having a.stereotyped action pattern
released. Much additional work is needed to
define the possible physiological, social and
developmental roles of yawning in health and
disease. At present, yawning may have the
dubious distinction of being the least
understood, common human behavior. Yawning may
prove to be elther an unimportant or vestigial
act, or a maneuver with profound physiological
consequences that may provide important clues to
the mechanisms and evolution of human behavior.
The prescrit, incomplete, evidende favors the
later alternatives. The prominence of
spontaneous and yawn-induced yawning suggests
that yawning has, or once had, a major function;
yawning dots not deserve its current status as a
minor behavioral curiosity.
