T
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he two hemispheres of our brain —
left and right — specialize in different tasks. A recent study asks how this
occurs and reaches a surprising conclusion.
Exactly how do the left and right
brain compete for dominance?
Hemispheric
dominance, also known as lateralization of brain function, describes the
tendency for either the left or the right side of the brain to carry out
specific brain activities.
Even
though both sides of the brain are almost identical, one hemisphere primarily
carries out some functions over others.
For
instance, the left hemisphere houses brain regions linked to speech (or the
right hemisphere in left-handed people).
Previously,
scientists thought humans were the only creatures to exhibit this phenomenon.
However, recent research has found lateralized brain function throughout the
animal kingdom — from insects, such as honeybees, to aquatic mammals, including
killer whales.
The corpus callosum — a thick tract of nerve cells,
known as commissural fibers — connects the two hemispheres. Exactly how
dominance is produced remains uncertain.
Recently,
researchers from Ruhr-Universität Bochum in Germany set out to investigate this
question. They chose to study the visual system of pigeons, and they have
published their findings in the journal Cell Reports.
Bird brains and old ideas
Previously,
scientists have theorized that one side of the brain simply inhibits the other,
allowing it to take dominance.
Co-lead
author Prof. Onur Güntürkün explains that "[i]n the past, it had been
assumed that the dominant hemisphere transmits inhibitory signals to the other
hemisphere via the commissures, thus suppressing specific functions in that
region."
In effect, the dominant hemisphere is thought to
overpower its neighbor. However, scientists have also noted that excitatory
messages run both ways, so there must be more to this interaction.
The
researchers decided to use a pigeon model because other studies have described
hemispheric dominance in this species in some detail over
recent years.
For
instance, in pigeon brains, the left hemisphere takes the lead when it comes to
visual processing of patterns and colors. Conversely, the right brain more
often deals with social or emotionally charged stimuli.
The
scientists trained the birds to perform a color differentiation task. In
particular, this challenge involves part of the brain that uses visual
information to guide motor activity. In this type of task, the left side of the
brain is dominant.
To
understand how cross-talk between hemispheres influences dominance, Prof.
Güntürkün and co-author Dr. Qian Xiao intermittently switched off some of the
neurons that run between the two sides of the brain.
Interfering in cross-talk
After
blocking specific neurons running from one side, they would observe the
activity of the neurons that usually receive their input on the opposite side.
In this way, they could pick apart the way in which the dominant hemisphere
exerts its control.
The
researchers showed that, rather than merely inhibiting the right side of the
brain during this task, the left brain could delay the response of the right
brain, so preventing it from getting involved.
As
Prof. Güntürkün explains, "The right hemisphere simply acts too late to control
the response."
Rather
than merely inhibiting the response, the right brain still operates, but its
signals are too late to the party to make a difference to the bird's behavior.
"These results show that
hemispheric dominance is based on a sophisticated mechanism. It does not hinge
on one general inhibitory or excitatory influence; rather it is caused by
minute temporal delays in the activity of nerve cells in the other hemisphere."
Prof. Onur
Güntürkün
The
findings provide an entirely new way to look at hemispheric dominance. Research
is bound to continue into this rather peculiar phenomenon that evolution has
lovingly conserved throughout many branches of life.
However,
it is likely to be quite some time before we understand why dividing tasks
between the hemispheres is so evolutionarily advantageous.
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