New research on the elephant-nose fish (Gnathonemus petersii) has brought neuroscientists a step closer to understanding how the brain processes vital sensory information.
Elephant-nose fish are in the family Mormyridae, and are known as weakly electric fish. They move slowly through murky streams and lakes in Africa, using an electrosensory system to detect the minute electrical pulses emitted by their invertebrate prey.
At the same time, its electric organ, located on the tail, emits larger pulses to aid navigation and communication with other members of the species.
Because the fish uses electrical signals for these purposes, it is essential that its brain can differentiate between pulses generated by its own body and those emitted by the environment, prey and other members of its species.
The elephant-nose fish thus represents an excellent model system for neuroscientists to study how humans filter out their own internally generated noises, such as heart beat, breathing, digestion, and movements of inner ear. This noise-cancelling mechanism is considered an essential part of perception and until now research had not been able to prove the link between the brain’s ability to tune out internal noise and effectively perceive the outside world.
“At its most fundamental, the brain’s purpose is to create an accurate and stable representation of the world around us, and we’ve long hypothesised that this noise-cancellation mechanism played a part in that,” says Nathaniel Sawtell, from Columbia University in the US, lead author of the latest research.
“With today’s study, we offer direct evidence that this mechanism is essential to improve and enhance the brain’s ability to sense its surroundings.”
Neuroscientists have dubbed the sum total of self-generated electrical signals the ‘negative image’ – because it is sensed and then subtracted from the elephant-nose fish’s environment.
“The brain is constantly receiving both sensory information along with internal signals related to an animal’s own behaviour,” says Sawtell.
“Those internal signals act as a sort of ID tag, clueing the brain into which components are self-generated and which are not.
“[The negative image] acts as a kind of subtraction mechanism, allowing the animal to focus on behaviourally important signals coming in from the outside world.”
The researchers used a combination of behavioural experiments and neural recordings in the lab to pinpoint just how essential this negative image is to elephant-nosed fish. Electrical signals of various amplitudes that imitate prey signals were played to the fish and behaviour assessed when the electric organ was momentarily impaired by paralysis, and thus unable to emit electrical pulses.
The team, which included a number of other researchers from Columbia University, used neural recordings to examine the response of the fish’s electroreceptors to these artificial prey signals. They found that neural coding – the processing of sensory information – improved when a negative image was used.
In the final series of experiments, specific neural pathways involved in the perception of the negative image were then disabled by administering a drug that blocked these receptors.
“The fish could no longer distinguish between electrical signals generated by their environment and the signals generated by their own actions,” says Sawtell.
“This means they were essentially blind to their surroundings at the most basic level.”
The elephant-nose fish could no longer detect prey, or navigate their surroundings.
The neural circuit responsible for the negative image, or noise cancelling mechanism, has been studied previously, but this is the first time researchers have been able to show its direct role in behaviour. Sawtell and the team are continuing efforts to build a model of how human brains use a negative image for our sensory processing, and what happens when this system breaks down.
“One example of this is tinnitus, which causes a ringing of the ears and which may begin in the dorsal cochlear nucleus — a brain region we study in mice and that has striking similarities to the human equivalent in a brain region called the cerebellum,” says Sawtell.
“What we learn in electric fish is likely to be relevant for understanding how the human brain distinguishes self from other.”
The research is published in the journal Neuron.