As seen in the chapters of this book,
yawning has many aspects and it is very complex.
We need to try various approaches to study
yawing. I will suggest several points for future
research on yawning. Yawning can be seen in all
vertebrates and can be seen at the gestational
age. Neural structures involved in yawning may
be subcortical systems including the
hypothalamus. Thus, yawning is phylogenetically
and ontogenetically old behavior. However, in
this book, direct comparison of yawning behavior
among vertebrates is missing. I believe that it
is important to study primitive vertebrates, for
example reptiles. The primitive vertebrates
could be a good target to study the core neural
structures and the original function of yawning.
It is necessary to clarify the common parts and
difference of yawning behavior in lower and
higher vertebrates. The correlation with sleep
or sleepiness may be evident in human subjects
but this kind of correlation must be tested in
lower animals also. In higher animals, higher
brain functions or cortical functions can be
combined with the core function of yawning. It
is also necessary to compare yawning behavior
across species in higher animals. Contagious
yawning is one possible phenomenon that appeared
as a result of such integration. For this
purpose, as Campbell and de Waal suggested, we
need to pay attention to standardizing methods
to enable comparison of different studies. To
study brain functions in human subjects, it is
necessary to conduct imaging studies during
yawning behavior. Imaging studies using MRI,
PET, or MEG are not suitable for this purpose.
We should try other methods, for example EEG,
MRS, or SPECT for this purpose. Anyway, this
book is standing at the entrance of these
studies and is a good book for gaining insight
for future studies of yawning.
