Male
and Female Brains
Summary of Lecture for Women's Forum West
Annual Meeting,
San
Francisco, California 2003
Sex
differences and the brain. What does it matter, you say? I think it does.
Through such knowledge we will eventually be better able to understand the
basis for behaviors that many now perceive as entirely rooted in social custom
or familial history. From that understanding, we will gain the acceptance,
patience, and respect so vital to all human endeavor.
Interestingly,
people who see a human brain for the first time often ask, "Is it male or
female?" Yet, for many millennia no one, even scientists, thought about
sex-related differences and similarities in the human brain. A brain was just a
brain. Now hardly a year goes by that we don't read authoritative studies
showing these differences. I was taken aback just a few months ago when, at a
Ph.D. examination dealing with Magnetic Resonance Imaging of human brains, the
student reported having pooled the data from both sexes. Even if the intent was
not to explore male-female differences, one can hardly expect to make accurate
interpretations from such mixed data.
Obviously,
no single factor accounts for the gender-related differences we are finding. We
are slowly, one by one, unraveling the various integrative factors involved in
this mystery. A basic question being asked is whether the differences between
male and female brains outweigh the similarities or vice versa. Some
researchers report finding more differences within the sexes than between the
sexes. Please understand that the objective of my talk is not to discuss
whether the brain of one sex is superior to the brain of the other but to explore
the significance of the differences we are discovering in the brains of males
and females. As you might imagine, to conduct these studies, we need brain
samples so that we can make our comparisons. So far, no live human beings,
males or females, have been willing to give us their brain tissue to use in our
experiments. But all is not lost: The rat brain, oddly enough, has the basic
components and major structures in its little pecan-size brain that we humans
have in our large cantaloupe-size brain. In general terms, what we have learned
about the anatomy of the rat brain has later been replicated by studies in
higher mammals including humans. What is particularly important, of course, is
that using the laboratory rat allows us to control many variables--the sex, the
age, the living conditions, the diet, the water intake, the environment, and so
forth, thus assuring clear comparisons.
To
appreciate the work we do, let me take a moment to give you some fundamentals
of the brain's anatomy. In the embryo our nervous system starts as a simple
tube, the head end forming the brain and the remainder forming the spinal cord.
The brain is divided into three parts: the hind brain, midbrain and forebrain.
Our interest is primarily in the forebrain, which expands tremendously over the
course of its development to form about 85% of our total brain, called the
cerebral hemispheres. These two large hemispheres are familiar to anyone who
has seen a picture of the brain The outer layers of the cerebral hemispheres
are called the cerebral cortex. (Cortex means bark.) With the use of a light
microscope we can easily measure the thickness of this cortex in the rat
because it is smooth and does not have folds as do more highly evolved brains.
Factors
affecting cortical thickness are the main interest in our gender studies
because the cerebral cortex is the most highly evolved part of the brain and
deals with higher cognitive processing. The cerebral cortex, like other parts
of the brain, consists of nerve cells with branches and functional connections
called synapses; glial cells, the metabolic and structural support cells for
the nerve cells; and blood vessels. Cortical thickness is a key factor; it
gives us an overall indication of what is happening collectively to these
structures within the cortex.
Table 1
Statistical significance of differences between
right and left cerebral cortical thickness in male and female rats (
S=statistically significant; NS=nonstatistically significant)
|
|
|
|
|
Cortical Areas
|
|
|
|
|
|
Age (days)
|
N
|
Frontal
|
|
Parietal
|
|
|
Occipital
|
|
|
|
|
10
|
4
|
3
|
2
|
18
|
17
|
18A
|
|
6
|
13
|
S
|
S
|
S
|
NS
|
S
|
S
|
S
|
|
14
|
17
|
S
|
S
|
S
|
NS
|
S
|
S
|
S
|
Males
|
20
|
15
|
S
|
S
|
S
|
NS
|
S
|
S
|
NS
|
|
90
|
15
|
NS
|
S
|
S
|
NS
|
NS
|
S
|
NS
|
|
185
|
15
|
S
|
NS
|
S
|
S
|
S
|
S
|
S
|
|
400
|
15
|
S
|
NS
|
S
|
S
|
S
|
S
|
NS
|
|
900
|
8
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
All
Ss R>L
|
|
|
|
|
|
|
|
|
|
|
7
|
15
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
|
14
|
15
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
Females
|
21
|
15
|
NS
|
NS
|
S#
|
NS
|
NS
|
NS
|
NS
|
|
90
|
19
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
|
180
|
11
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
NS
|
|
390
|
17
|
NS
|
NS
|
S#
|
NS
|
NS
|
NS
|
NS
|
S#=L>R
|
|
|
|
|
|
|
|
|
|
The
cortical areas sampled are seen in Figure 1. As the name implies, the frontal
area is in the front of the brain, the parietal is in the middle and the
occipital is at the back of the hemisphere. In very simple terms, the frontal
area deals with motor behavior and planning for action, the parietal area with
general sensory functions, and the occipital cortex with visual functions.
By
measuring the thickness in the frontal, parietal and occipital cortex in our
experimental rats, we can begin to assemble important information and to ask
such questions as:
1. Are
there sex-related differences in the growth of the cerebral cortex at birth?
The female cortex shows some areas are more highly developed than others at
birth compared to the male. Her motor cortex (frontal) shows the highest
development with her sensory cortex (parietal) next and visual cortex
(occipital) least developed. In the male brain, the motor, sensory and visual
cortical all show a similar degree of development at birth; differences in
growth rates appear soon after birth.
2. Is
the thickness of the right and left cerebral cortex different between male and
female animals? The answer is decidedly "yes" as revealed by a glance
at Table 1. "N" shows the number of brains sampled in each age group
from shortly after birth to well into adulthood and for males into very old
age. "S" means there is a statistically significant difference
between the cortical thickness in the right and left hemispheres.
"NS" means there is no statistically significant difference between
the thickness in the hemispheres.
In the
female brain,we observe no statistically significant differences in cortical
thickness between the right and left hemispheres from birth well into
adulthood. We found nonsignificant differences in 41 of the 43 regions measured;
in other words, in 95% of the cases she displays a symmetrical cortex
It has
been commonly stated that the female cortex is symmetrical and the male cortex
is asymmetrical. Turning again to Table 1, this time to assess the development
of the male cortex, we find that the hemispheric thickness differences from
birth to old age are definitely not as consistent as in the female brain. In
fact, in the male cortex the right hemisphere is significantly thicker than the
left in 31 of the 49 regions measured. In other words, in 60% of the cases, the
cortex of the male rat brain is significantly asymmetric.
With
the data in Table 1, we now need to state more accurately that parts of the
male cortex are asymmetrical and parts are not. Two consistent findings in the
male rat brain were the following: (1) area 2 in the parietal cortex showed
nonsignificant findings or symmetry from birth to 90 days of age; differences
in cortical thickness were seen only after 90 days of age. (2) In the
900-day-old male rats, all areas of the cortex showed nonsignificant
differences between the hemispheres. At this very old age, the male cortex
appeared similar to that of the female cortex in terms of its symmetry.
One
obvious question to ask when we assess our findings in the female brain is:
What role do the sex steroid play in establishing cortical thickness ? As we
would expect, in those animals with ovaries, there is no significant difference
in the thickness of the hemispheres, but in those without ovaries, two areas of
the occipital cortex show significant differences in thickness between the
right and left hemispheres. It seems that the visual cortex in female animals
without ovarian hormones is more like that of the normal male cortex. (Though
not shown, in our 800 day old females, we also found this pattern was similar
in the occipital cortex.)
In
summary, the female animal, with or without ovaries, shows no significant
difference in the thickness of her right and left cerebral cortices except in
part of the visual cortex where those without ovaries develop right dominance.
Other researchers have reported that two major connecting fiber tracts between
the two hemispheres are larger in females than in males, a finding that
supports the notion that the female exhibits symmetrical cortical patterns.
What might be the advantage of such symmetry in cortical morphology?
For the
female animal, the main functions in life are to bear, protect and raise her
offspring. These roles challenge her to go in many directions, both geographical
and conceptual, something that may be more accessible and readily achieved with
a symmetrical brain. We might conjecture that the trend to right dominance in
the older brain of the female without ovarian hormones suggests a shift to the
more visual focus demanded of the male.
Now we
need to ask the same question we asked of the female brain: What role do sex
steroid hormones play in determining cortical thickness patterns in the male?
In rats without testes some cerebral cortical areas show significant differences
and some do not. Of interest to me is that areas 17 and 18a dealing with visual
processing in both males and females devoid of sex steroid hormones showed
statistically significant differences between the right and left hemispheres.
Area 17 in the male also showed statistically significant differences in the
cortical thickness of the right and left hemispheres among animals with
circulating sex hormones, but area 18 a did not
In
summary, the male cerebral cortex displays both symmetrical and nonsymmetrical
right/left patterns in cortical thickness with the nonsymmetrical pattern being
slightly more anatomically frequent (60%). and in turn suggesting functionally
more frequent. What might be the advantage of having some cortical areas
asymmetrical in the male? In general, male behavior involves finding and
defending his territory and finding his female, all rather focused functions,
possibly benefiting from an asymmetrical cortex.
Another
consideration is the similarity between male and female; in these studies, the
question is in what areas do males and females have the same right/left
pattern, whether symmetrical or nonsymmetrical? In area 10 (motor behavior and
planning for action) at 90 days of age both are nonsymmetrical; in area 2
(general sensory functions) from birth to 90 days of age both are
nonsymmetrical; in area 18A (visual functions) from 20-21 to 90 days of age
both are nonsymmetrical. In area 3 (general sensory functions) at 2-21 days of
age both are symmetrical and at 400-390 days of age both are symmetrical.
Needless
to say, these data further emphasize the necessity of considering the numerous
variables that contribute to anatomical and in turn physiological development
generally and specifically to the growth of the cerebral cortex. Furthermore,
wresting meaning from the multiplicity of similarities and differences between
male and female brains presents a considerable challenge in the decades ahead,
but a challenge that those of us who dedicate our professional lives to such
research anticipate with relish.