Time-shifted inhibition helps electric fish ignore their own signals


Date:
August 12, 2020
Source:
Washington University in St. Louis
Summary:
African fish called mormyrids communicate using pulses of
electricity.

New research shows that a time-shifted signal in the brain helps
the fish to ignore their own pulse. This skill has co-evolved with
large and rapid changes in these signals across species.



FULL STORY ========================================================================== Electric fish generate electric pulses to communicate with other fish and
sense their surroundings. Some species broadcast shorter electric pulses,
while others send out long ones. But all that zip-zapping in the water
can get confusing. The fish need to filter out their own pulses so they
can identify external messages and only respond to those signals.


==========================================================================
A solution to this problem is a brain function called a corollary
discharge.

It's sort of like a negative copy of the original message -- something
that tells the fish: Ignore this.

But an animal's brain doesn't have to block sensory inputs during the
entire message to effectively ignore its own signal, according to new
research from biologists at Washington University in St. Louis.

Instead, the inhibitory signal -- that call to ignore -- is delayed in
fish that communicate using longer electric pulses, versus those using
shorter pulses.

"In fish that communicate with longer pulses, sensory responses to
their own pulse are delayed," said Bruce Carlson, professor of biology
in Arts & Sciences. "Thus, a delayed corollary discharge optimally
blocks electrosensory responses to the fish's own signal." Carlson and Matasaburo Fukutomi, a postdoctoral fellow in his laboratory, published
their new research on African mormyrid weakly electric fish in the
Journal of Neuroscience.



==========================================================================
A brief, well-defined period of inhibition keeps electric fish from
missing out on other important external signals, Carlson said.

Time-shifted tune-out Scientists have known about corollary discharges
since the 1950s. In the decades since, corollary discharges have been
found in many different species and sensory systems, but it remained
unknown how corollary discharges were modified as communication signals evolved.

Previous work on corollary discharge in electric fish had been done
with species that communicate using short-duration electric pulses,
those lasting less than 1 millisecond.

For their new study, Carlson and Fukutomi included these fish and five additional species that communicate using electrical pulses ranging in
duration from 0.1 to 10 milliseconds.



==========================================================================
"We found the sensory neurons respond with spikes in a narrow time window regardless of pulse duration," Fukutomi said. "These spikes occurred
in a specific part of the self-generated pulse, the first peak of the
pulse. In addition, we compared the time courses between the corollary discharge inhibition and the pulse and found that the time-shifted
inhibition overlapped the first peak of the electric pulse.

"Time-shifted inhibition is a reasonable change because longer-lasting inhibition would result in an unnecessarily long insensitive period,"
he said.

"I am impressed that there is a solution that makes more sense in real organisms than we might have expected." The new findings have broader implications for understanding the evolution of brains.

"Despite the complexity of sensory and motor systems working together
to deal with the problem of separating self-generated from external
signals, it seems like the principle is very simple," Carlson said. "The systems talk to each other. Somehow, they adjust to even widespread,
dramatic changes in signals over short periods of evolutionary time."
As part of continuing research, Carlson and Fukutomi are working to
pinpoint the place in the brain circuit where the delay is adjusted,
and how that adjustment is made. They are also investigating how the
inhibition delay changes over the individual lifetime of a fish.

The researchers also recently co-authored a new review paper on the contributions of electric fish to the study of corollary discharge in
Frontiers in Integrative Neuroscience.

Even though humans aren't able to generate electric fields, research
on corollary discharge in electric fish has provided insights that
are important in medical science as well as basic science. Dysfunction
of corollary discharge may be related to psychiatric diseases such as schizophrenia in humans, for example.

"I love strange creatures, including electric fish," Fukutomi said. "We
can only feel electricity as pain, but we never sense electricity as
the fish does.

"Surprisingly, electrosensory systems share a lot of general features
with other sensory systems," he said. "I am very excited to be studying
these fish."

========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Original written by Talia
Ogliore. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Matasaburo Fukutomi, Bruce A. Carlson. Signal Diversification Is
Associated with Corollary Discharge Evolution in Weakly Electric
Fish.

The Journal of Neuroscience, 2020; 40 (33): 6345 DOI: 10.1523/
JNEUROSCI.0875-20.2020 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200812161320.htm

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