in language while the right is concerned with visuospatial tasks. How complex this
is, though, is reflected in the fact that language is exclusive to the right hemisphere
in less than 20% of left-handers and is bilateral - ie: in both hemispheres - in
another 20% of left-handers. However, in the majority of left-handers language is
found in the left hemisphere.
Generally speaking, male brains are said to be more lateralised - having a preference
for the right side. This would help explain why males are better at Mathematics and
map-reading.
Females are usually thought to use both hemispheres more equally than males. Females
often
have a larger corpus callosum, meaning their two hemispheres are better connected
so they can use the two halves together more. This could explain superior female
fluency in speech, thought, body language...and, of course, multi-
Katalin Vladar, Douglas Jones & Daniel Weinberger (1992) that, using MRI scans, the
left temporal plane was 38% longer than the right temporal plane in men but there
was no real difference between the 2 planes in women.
Bennett Shaywtiz, Sally Shaywitz, Ken Pugh, Todd Constable, Pawel Skudlarski, Robert
Fulbright, Richard Bronen, Jack Fletcher, Donald Shankweiler, Leonard Katz & John
Gore (1995) used functional scanning to record blood flow to different brain areas
during cognitive tasks. They found that, for a rhyming task, activity was localised
to the left hemisphere in males but the activity in females was symmetrical. Pugh,
Shaywitz, Shaywitz, Constable, Skudlarski, Fulbright,Bronen, Shankweiler, Katz, Fletcher
& Gore (1996), in a similar piece off research, got similar results. However, J A
Frost, J R Binder, J A Springer, T A Hammeke, P S Bellgowan, S M Rao & R W Cox (1999)
failed to replicate such findings In a task requiring recognition of psuedowords,Susan
Rossell, Edward Bullmore, Steve Williams & Anthony David (2002) found that men were
faster if the pseudoword was presented in the right of their visual field whereas
women were faster if it was presented in the left of their visual field. Brain scans
showed that the task generated activity mainly in the left hemispheres of the male
participants whereas activity was more evenly distributed between the two hemispheres
for the females. Similarly Jeri Jaeger, Alan Lockwood, Robert Van Valin, David Kemmerer,
Brian Murphy & David Wack (1998) found via PET scans that, when male participants
in a language task, generated past tenses of verbs, activity was lateralised to the
left whereas it was more bilateral in the female participants.
One problem in mapping brain activity to (self-reported) thoughts and (observable)
behaviour is that, although blood flow can be measured to indicate neural activity,
the activity may occur not specifically due to different brain structure but because
different strategies are employed. Also some procedures measure activities in which
men and women differ in performance capability while other procedures measure activities
in which men and women are recognised as being equally competent.
Another difficulty in attempting to attribute the causes of thought and behaviour
is that nurture is often involved as well as nature - see Nature-Nurture - with the
differences between male and female brains often being a combination of biology and
environment.
More Differences between Male & Female Brains
In their research Ruben Gur, Bruce Teretsky, Mae Matsui, Michelle Yan, Warren Bilker,
Paul Hughett & Raquel Gur (1999) identified several underpinning factors to the apparent
differences between male and female brains...
- The greater amount of the fatty ‘white matter’ throughout male brains gives them
superiority at spatial reasoning.
- The white matter also prevents ‘information spread’ in the cortex, resulting in males
tending to pay attention to only one thing at a time - supposedly why men proverbially
can’t walk and chew gum at the same time!
- The white matter in females is concentrated in the corpus callosum - which means
female brains are more balanced and better organised
- One result of more white matter in the female corpus callosum is that the right side
of the brain can join in language tasks – language areas of the brain being primarily
in the left hemisphere – associated with female superiority in language skills
Michael De Bellis, Matcheri Keshavan, Sue Beers, Julie Hall, Karin Frustaci, Azadeh
Masalehdan, Jessica Noll & Amy Boring (2001) studied 61 males and 57 female children
aged from 6.9 to 17 years. They used MRI scans to investigate how volumes of grey
and white matter and the corpus callosum change with age. They found that volume
of grey matter fell significantly with age more in males than females but that the
volumes of white matter and the corpus callosum both increased with age more in males
than females. Although the whole cerebral volume did not change significantly with
age, it was consistently larger in males than females. De Bellis et all concluded
from these results that boys’ brains mature faster. They at least partly attributed
these effects to sex hormones - with oestrogen delaying pruning while testosterone
promotes myelination.
Daniel Voyer (1996) carried out a meta-analysis, with 396 male-female comparisons
from 266 studies and concluded that, overall, males show greater lateralisation than
females. However, another recent meta-analysis did not find consistent gender differences
- especially in relation to the corpus callosum.
Child abuse and neglect affects the brain
It appears brain abnormalities can be caused by child abuse and neglect.
This is particularly so in the limbic system which is associated with emotions. Abuse
can cause disturbances that can lead to seizures and other abnormalities that show
in the electrical activity measured by an electroencephalogram (EEG). In abuse cases
investigated this way, the abnormality has been in the left hemisphere and has been
linked to Depression and memory deficits.
The corpus callosum is smaller in those who have been abused as children. A reduction
in size of 24%-42% has been found in boys who have suffered neglect. In girls who
have been sexually abused, a reduction of 18%-30% has been found. However, neglect
of girls appears to have no effect on the size of the corpus callosum. It is thought
that a reduction in size of the corpus callosum means less activity between the two
hemispheres and this has been associated with changes in mood.
Connectionism
More recently the idea of Connectionism (distributed function) has taken hold. This
developed in the first place from the work of Donald Hebb (1949) who suggested that
learning is consolidated in the brain by the activation of cell assemblies, groups
of neurons that fire systematically to respond to a specific stimulus. This causes
structural changes in the cells, possibly even the growth of new synapses. As this
reoccurs, the cell assembly ‘learns’ and responds each time, forming the neural basis
of memory.
Connectionism is a holistic perspective in which the brain functions as a whole.
All areas are inter-connected and have multiple tasks to carry out. Information is
distributed in networks made up of millions of neurons.
This concept allows for cortical specialisation but the interconnections mean no
one area has overall control.
However, even this attempt at an all-encompassing view of brain activity falls down
in face of the fact that some brain functions are strictly localised. Eg: no evidence
has yet been produced to suggest that temperature control is the responsibility
of any other part of the brain other than the hypothalamus.
Brain Lateralisation & Gender
The brain’s two hemispheres do, in fact, engage in some notable specialisations.
Which means, at times, one half will be ‘cerebrally dominant’. Eg: the left hemisphere
specialises
