- ----- Original Message -----From: Jack DrescherTo: sexnet@...Sent: Sunday, April 30, 2000 5:26 PMSubject: FerretsFrom:
'Rewired' Ferrets Overturn Theories of Brain Growth
By SANDRA BLAKESLEE
Like inventive electricians rewiring a house, scientists at the
Massachusetts Institute of Technology have reconfigured newborn ferret
brains so that the animals' eyes are hooked up to brain regions where
hearing normally develops.
The surprising result is that the ferrets develop fully functioning
visual pathways in the auditory portions of their brains.
In other words, they see the world with brain tissue that was only
thought capable of hearing sounds.
The findings, reported by Dr. Mriganka Sur and his colleagues in the
April 20 issue of Nature magazine, contradict popular theories on how
animal brains develop specialized regions for seeing, hearing, sensing
touch and, in humans, generating language and emotional states.
Many scientists claim that genes operating before birth create these
specialized regions or modules, arguing for example that the visual
cortex is destined to process vision and little else. But the ferret
experiments show that brain regions are not set in stone at birth.
Rather, they develop specialized functions based on the kind of
information flowing into them after birth.
"Some scientists are going to have a hard time believing these
experiments," said Dr. Jon Kaas, a professor of psychology at
Vanderbilt University in Nashville. They demonstrate, Dr. Kaas said,
"that the cortex can develop in all sorts of directions."
"It's just waiting for signals from the environment and will wire
itself according to the input it gets," he said.
The findings may shed light on unusual brain patterns observed in
people who are born deaf or blind, he added.
"If you wanted to create a dream experiment, this would be it," said
Dr. Michael Merzenich, a neuroscientist at the University of California
at San Francisco and a leading authority on the brain's ability to
change and reorganize, a process known as plasticity. "It's about the
most compelling demonstration you could have that experience shapes the
The researchers are all members or former members of the department of
brain and cognitive sciences at M.I.T. The rewiring experiments began
more than 10 years ago, Dr. Sur said. He chose ferrets because their
brains are very immature at birth and undergo a late form of
development that the researchers can exploit.
As in humans, the ferret's optic and auditory nerves travel through a
way station called the thalamus before reaching areas in the higher
brain or cortex where vision and hearing are perceived.
In humans, this very basic wiring is present at birth, but in ferrets,
these important nerves grow into the thalamus after the animal is born.
Dr. Sur found that if he stopped the auditory nerve from entering the
thalamus, the optic nerve would arrive a few days later and make a
double connection. It would go on through the thalamus and connect
itself up to both seeing and hearing regions of the cortex.
The researchers then waited to see what would happen to the hearing
region of the brain once it was getting all its signals from the
After a ferret or human is born, cells in the brain's primary visual
area become highly specialized for analyzing the orientation of lines
found in images or shapes. Some cells fire only in response to vertical
lines. If presented with a horizontal or slanted line, they don't do
Other cells fire exclusively when a horizontal line falls on them and
yet others fire in response to lines slanted at various angles. These
specialized cells are draped across the primary visual area in a
somewhat splotchy fashion that resembles a bunch of pinwheels.
The hearing region of the brain is organized very differently, Dr. Sur
Each cell is connected to the next in a kind of single line. There are
no pinwheel shapes.
After the rewired ferrets matured, researchers looked at the auditory
region of their brains and found that cells were organized pinwheel
fashion. They found horizontal connections between cells responding to
The rewired map was less orderly than the maps found in normal visual
cortex, Dr. Sur said, but looked as if it might be functional.
The researchers then asked, What does the rewired ferret experience?
Does it see or does it hear with its auditory cortex?
Rewired ferrets were trained to turn their heads one way if they heard
a sound and in the other direction if they saw a flash of light. In
these experiments, one hemisphere was rewired and the other was left
normal as a control. Thus the animals could always hear with the intact
side of their brains and were deaf in the rewired side.
Not surprisingly, when the light was presented to the rewired side, the
animals responded correctly.
But when connections to visual areas were severed on the rewired side,
the animals still responded to the light. It meant that they were
seeing lights with their rewired auditory cortex, Dr. Sur said.
The research reopens the question of what are the relative
contributions of genes and experience in building brain structure,
according to Dr. Kaas.
Genes, Dr. Kaas suggests, create a basic scaffold but not much
Thus, in a normal human brain, the optic nerve is an inborn scaffold
connected to the primary visual area. But it is only after images pour
into this area from the outside world that it becomes the seeing part
of the brain. All the newborn cortex knows about the outside world is
from the electrical activity of these inputs, or images that fall on
the retina, sounds that reach the inner ear or touch sensations that
press on the skin, Dr. Kaas said.
As the inputs arrive, the cells organize themselves into circuits and
As these circuits grow larger and more complex, Dr. Kaas said, they
become less malleable and, probably with the help of changes in
neurochemistry, become stabilized. This is why a mature brain is less
able to recover from injury than a very young brain.
Young brains are astonishingly plastic, Dr. Kaas said. For example, he
said, children who suffer from a severe form of epilepsy that is
treatable only by removing one-half of their brains can learn to walk,
talk, throw balls and otherwise develop normally with only half a
brain, if operated on early in life, he said.
But in recent years, scientists are also discovering that adult brains,
as well, can undergo surprising changes in response to experience. For
example, imaging experiments carried out on blind people show that when
they learn to read Braille, "visual" areas of their brains light up.
Touch seems to be residing in visual areas. Similar experiments on deaf
people show that they use the auditory cortex to read sign language,
whereas people who can hear use the visual areas of the brain for this
Dr. Sur said his laboratory was now searching for molecules that help
produce these kinds of changes in mature and developing brains.
If the chemistry of regrowth and reorganization can be understood, he
said, it would offer new avenues for helping people recover from damage
caused by strokes, accidents and various brain diseases.
- Paul Okami forwarded a message:
> Like inventive electricians rewiring a house, scientists at theThis may be surprising for those who believe that the brain is
> Massachusetts Institute of Technology have reconfigured newborn ferret
> brains so that the animals' eyes are hooked up to brain regions where
> hearing normally develops.
> The surprising result is that the ferrets develop fully functioning
> visual pathways in the auditory portions of their brains.
> In other words, they see the world with brain tissue that was only
> thought capable of hearing sounds.
capable of cognitive operations and that it is the brain who sees or
hears. However, if we think that the brain is only a part of the
system of hearing, then this result can be predicted. I don't want to
be magisterial, but perhaps you allow a citation from a recent
"The question of subjectivity of consciousness is the question of the
contents of consciousness. What determines the content - the qualia -
of the conscious experience? It is nowadays usual in cognitive science
to answer this question by referring to a certain area of the brain: a
visual sensation differs qualitatively from a tactile one, because the
former is processed in the visual cortex and the latter in the
somatosensory one. Such an answer, however, is only a repetition of
Muller's law of specific nerve energies from the 19th century in a
modern form. Muller thought that each nerve contains its own specific
energy which makes it understandable why, for example, any stimulation
of a visual nerve produces a visual sensation. According to Muller,
the nerve contains a "specific energy" related to the evoked
sensation. Nowadays nobody believes in such energies, but in fact this
concept has been replaced by the concept of the locus of activation.
Visual sensation appears, because neurons in the visual cortex are
However, the neurons in the different parts of the brain function in a
similar way even if there are some anatomical differences in their
constitution. There is nothing "visual" in the visual cortex, or
nothing "tactile" in the tactile cortex, and there is no reason to
think that the quality of the sensation would be determined by the
locus of activation per se. Thus, Muller's original problem has still
not been adequately answered in cognitive science.
According to the present consideration the problem is wrongly
formulated. If I ask "What determines the content of my
consciousness?", the answer cannot deal only with the brain, but must
include much more. What makes me feel like a human being does? The
first condition for this possibility is precisely that I am a human
being, that I belong to the human species. I am a human being only if
the contents of my consciousness are typically human; if they are
something totally different then I am no more a human being. According
to the present formulation this is self-evident, because the content
of consciousness is determined by the common results, by the
possibility of co-operation with other human beings. Thus, the content
of consciousness is shared with other human beings; otherwise
co-operation would not be possible. "
(Jarvilehto, Integrative Physiological and Behavioral Science
Professor of psychology
University of Oulu
PB 2000, 90014 Oulun yliopisto, Finland