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| Fish's Sixth Sense May Decipher Human Brain 6:00
a.m. ET (1000 GMT) April 28, 2000 By Dan Stimson
Many people probably picture electric fish as dangerous creatures
that can deliver painful shocks, creatures best observed from
a distance.
The hippocampus, responsible for memories of people, places and
events, is in the center of the brain near the rear. Seen here
is a cross section of the brain
Actually, only some produce the strong currents used to stun prey.
Most produce weak currents — undetectable to humans — that
are used primarily for hunting and finding potential mates. It
is these "weakly electric fish" that have sparked neuroscientists'
interest with what they may be able to tell us about how learning
and memory work in the human brain.
Since the fish brain lacks many structures found in the mammalian
brain — including one called the hippocampus, known to be critical
for our memories of people, places and events — it might seem
like a strange place to look for insights into human learning
and memory. But electric fish have a brain wired to provide a
"sixth sense" that we do not possess: electroreception, the ability
to perceive electric currents in the environment. It is this unique
feature that could be key to revealing universal mechanisms of
learning and memory.
It is widely accepted that learning and memory involve experience-driven
changes at the brain's synapses, the sites of communication between
nerve cells. Changing the relative strengths of synaptic connections,
the idea goes, changes the brain's output, producing memories
and learned behaviors.
This concept is supported by studies of fruit flies and sea slugs,
but in vertebrate animals, the intricacy of synaptic circuits
and the complexity of learned behaviors make it difficult to firmly
link synaptic change with the formation of memory. Stimulation
of synapses in the mammalian hippocampus can lead to an enhancement
of synaptic strength called long-term potentiation (LTP), and
there is evidence that LTP is a cellular mechanism for learning.
But "there's no absolutely definitive proof that LTP is involved
in learning," says psychology professor Lynn Nadel, who studies
the hippocampus's role in spatial learning at the University of
Arizona.
In the brains of electric fish, the relationship between synaptic
change and learned behavior is much more straightforward. In order
for these fish to interpret electrical signals, they must "remember"
sensations they have experienced in the past. Curtis Bell, a senior
scientist at the Neurological Sciences Institute of Oregon Health
Sciences University, has identified synaptic changes that allow
the electric fish, Gnathonemus petersii, to form these "perceptual"
memories.
This sixth sense of electroreception requires the fish to perform
a kind of mental gymnastics, explains Bell, because the fish produce
their own electrical signals that must be distinguished from the
signals caused by other animals.
From the point of view of the fish, the origin of electrical signals
depends on whether he is just hanging around or actively searching
for food or a mate. In the former case, the fish passively "listens"
to electrical signals caused by the movement of other animals,
right down to the beating of a heart or the flapping of gills.
In the latter, the fish uses a specialized muscle to fire an electrical
discharge that works like radar: When the discharge hits another
animal, it bounces back to the fish distorted.
But in either case, homing in on these external signals is tricky,
because — whether firing off discharges, swimming or just breathing
— the electric fish is constantly giving off its own signals.
Memories of these self-generated signals allow the fish to subtract
them, leaving just the external signals that carry the important
information about where to find dinner or a dinner date.
Unlike LTP in the mammalian hippocampus, the synaptic changes
underlying the fish's perceptual memories involve a decline of
synaptic function. During electroreception, synapses that carry
information about the fish's own electrical signals are relatively
quiet when compared to the synapses that carry information about
external signals.
In the absence of new electrical signals, this synaptic quiescence
persists for up to a half an hour. That is much shorter than the
duration of LTP, which can last for days. But, says Nadel, the
synaptic changes Bell has identified might still "serve the same
purpose for the [electric fish] that LTP serves in mammals."
What that purpose is exactly is at the crux of Bell's work.
His studies of electric fish might provide the key for linking
persistent synaptic changes to learning and memory. Bell says
his work makes it easier to establish that link because he is
examining the relatively simple synapses required for simple behaviors.
These synapses process information about electrical signals, and
changing the way the synapses work should modify the fish's responses
to those signals. In the hippocampus, however, much less is known
about the types of information being processed.
"At the hippocampus," says Bell, "we don't know exactly what the
incoming signals mean, and ... we don't know the role or the effects
of [synaptic] changes at the [behavioral] level."
The ultimate goal of his studies, Bell asserts, "is to try to
make the correlation between ... synaptic level functions and
some higher level functions, such as memory or perceptual learning."
By finding synaptic changes that allow electric fish to form perceptual
memories, Bell has taken a first step toward this goal.
Next, he plans to directly manipulate these synaptic changes and
alter the fish's behavior. Bell believes that by preventing the
synaptic quiescence that normally occurs during electroreception,
he'll be able to interfere with the fish's perceptual memories
so that it can't make sense of electrical signals. If this happens
he says, it could establish a strong connection between synaptic
change and behavioral change and provide a powerful jolt to our
understanding of learning and memory. Full text with illustrations:
http://foxnews.com/science/042800/electricfish.sml |
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