Arts & Sciences

Zeroing out their own zap

African fish called mormyrids communicate using pulses of electricity. To distinguish their own signal from those of neighboring fish, their brains inhibit sensory responses using a corollary discharge, which is an internal copy of their own motor command. New research from biologists in Arts & Sciences shows that this corollary discharge has co-evolved with large and rapid changes in these signals across species. (Photo: Tsunehiko Kohashi)

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.

Bruce Carlson

“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.

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