jeffl@ucbvax.Berkeley.EDU (Jeff Lichtman) (11/14/88)
Several days ago I wrote in this newsgroup that the dual brain theory
had been largely debunked. Russell Lawrence (wpg!russ) wrote to me
asking for references. Today I went to the biology library at the
University of California at Berkeley to look things up.
I found out I was wrong, at least according to the sources I found. One
source mentioned my objection to the theory, that it was based on experiments
on people with severed corpus callosums, and didn't extend to people with
"normal" brains. This source wrote off the objection out of hand, saying
that you can't just throw out data that are difficult to interpret.
According to the sources I read, however, many popularized theories about
the functions of the two hemispheres of the brain assume more than is in
evidence. From "Neurology and Neurobiology, Vol. 17" by Lepore', Ptito,
and Jasper (Alan R. Liss, Inc, ISBN 0-8451-2719-15):
"One must caution in this connection that the experimentally
observed polarity in right-left cognotive style is an idea in
general with which it is very easy to run wild. You can read
today that things such as intuition, the seat of the subconcious,
creativity, parapsychic sensitivity, the mind of the Orient,
ethnocultural disposition, hypnotic susceptibility, the roots
of counter-culture, altered states of conciousness - and what not -
all reside predominantly in the right hemisphere. The extent to
which extrapolations of this kind may eventually prove to be fact
or fancy will require many more years to determine. In the meantime
it is important to remember that the two hemispheres in the normal
intact brain tend regularly to function closely together as a unit
and that different states of mind are apt to involve different
hierarchical and organizational levels or front-back and other
differentiations as well as differences in laterality."
The book also cautions that every person's brain is wired differently, to
the extent that it is impossible to predict things about individuals.
Here are the specializations of the two brain hemispheres, according to what
I read:
Left:
speech, writing, and language
calculation
Right:
spatial construction
nonverbal ideation
Note how short the lists are. Also note that logic, emotion, and intuition
are not listed.
According to "The Dual Brain", edited by D. Frank Benson and Eran Zaidel
(The Guilford Press, ISBN 0-89862-643-9), some experiments have shown that
women and left-handed people have less hemispheric specialization and have
relatively larger corpus callosums than do men and right-handers. Some
subsequent experiments have duplicated these results for left- and right-
handers, but not for men and women. Some experiments have shown that women
have greater hemispheric specialization while performing "expressive
language tasks". Some experiments suggest that very young girls show
slightly *greater* hemispheric specialization than do very young boys.
Methodological problems make all of this hard to interpret. For example,
sex differences may be stronger when subjects are presented with unfamiliar
material. Also, very simple stimuli can produce greater effects than more
complex stimuli. Also, evidence exists that sex differences can be
influenced by the instructions given to the subjects.
What this means is that, even if differences is hemispheric specialization
exist, it is not true that women's brain hemispheres are simply less specialized
than men's. This may be true for some types of tasks. For other tasks,
women's brains have shown more lateral specialization than men's. For still
other tasks, no differences have been measured.
There is still controversy about whether sex differences exist at all. One
researcher estimates that, in 129 experiments using hemifield presentations
(whatever that is :-)), 103 of the experiments did not support the notion
that men and women differ in the degree of hemisphere specialization. The
tasks where there were significant gender differences were:
dot localization
perception of faces
perception of line orientation
line detection
Men showed a greater left-side advantage in verbal tasks, but this effect
was diminished in simple tasks or where there was no expressive component
(which seems to contradict some other findings about the effects of
complexity and expressive components, see above).
Finally, the differences between individuals are much greater than the
differences between groups. In the experiments which showed sex
differences, the differences were so small that they would only show
up in large samples.
I hope I have managed to confuse the issue for everyone. :-) :-) :-)
--
Jeff Lichtman at Sybase
{mtxinu,pacbell}!sybase!jeffl
"Saints should always be judged guilty until they are proved innocent..."