The Hydra: What the Brainless Can Teach Us About the Brain

By Shawn Radcliffe

Stefan Siebert and Charles David

Scientists have long described brains as being made up of neurons that work together to control the behavior of organisms. But recent research suggests that the functional unit of the brain is not a single neuron, but networks of neurons, also known as neural circuits.

Figuring out how the brain goes from neurons to neural circuits to behavior is not an easy task. The human brain has an estimated 100 billion neurons, with each neuron connecting to many others. So some scientists have turned to less “complicated” organisms, those with far fewer neurons. They hope that this will help them capture the entire neuronal activity of an organism, what’s known as a Brain Activity Map (BAM).

While understanding the brain is a key goal of this research, having a brain is not a prerequisite for taking part in laboratory experiments. At a lab at Columbia University, in New York City, scientists have enlisted the aid of Hydra vulgaris, a fresh-water polyp that is half an inch long or less. A recent story in Nautilus described their research.

The Hydra has many interesting features, including being able to reproduce asexually by budding off a genetically identical copy of themselves. But they are also thin and transparent, which allows scientists to easily study all of their neurons at once using a simple microscope. And they only have several hundred neurons — which is much easier to study than the human brain.

The Hydra’s neurons, though, are not organized in a typical brain. Instead, they form neural nets that span its body. By genetically engineering the Hydra so its neurons emit light in proportion to neuronal activity, researchers can see the glowing connection between “brain” activity and behavior.

Researchers at the Columbia lab have found that the Hydra has four networks of neurons, each of which fire during particular behaviors, such as contracting when disturbed, doing somersaults to move around, or nodding while exploring.

Seeing which neuron networks fire and when, though, is only the first step. Researchers will also need to come up with a computational model of how the Hydra’s nervous system works. From this, they can make predictions about how each neuron contributes to the animal’s behavior — and test those predictions in the lab.

Some researchers caution that the goal isn’t just to study networks of neurons in isolation, anymore than it’s about studying single neurons in isolation. Neurons and neuronal networks don’t exist for their own benefit. They are there to control an organism’s body and behavior.

That means that in order to understand how neural circuits work, you first have to understand an organism’s behaviors. This approach to brain research, though, has exposed a raw nerve in the field of neuroscience. Some researchers challenge that many in the field put too much emphasis on cool, new tools for studying the brain and not enough on understanding behavior.

One of these is John Krakauer, a neuroscientist at Johns Hopkins Hospital, who published a paper last year in the journal Neuron, calling for a shift in how brain research is done. Part of his argument is what he sees as the field’s overemphasis on technology.

”People think technology + big data + machine learning = science,” Krakauer told The Atlantic. “And it’s not.”

Others, though, argue that advances in technology are still needed for the field to advance. This is not just so we understand how neurons or neural circuits work, but also so we can see neural circuits in action.

This is the kind of work that is being done with the Hydra, which may lack a brain, but still has much to teach neuroscientists about neural circuits in action.

Krakauer, though, remains unconvinced. He told The Atlantic that this approach just swaps “neuron” for “neural circuit.”

Others agree, arguing that you still need to understand the behavior of an organism first before generating ideas about what neural circuits might be driving this behavior. Only then can you design experiments — with the cool, new technology — to test whether your hypotheses were correct.

As outlined in The Atlantic, this top-down approach has led to a deeper understanding of how the brain controls different behaviors — including why people with Parkinson’s move slowly, how owls localize sound when hunting for prey, and how marmosets interact vocally (which may also apply to turn-taking in human conversations).

It remains to be seen what the Hydra can teach us about how the human brain controls our behavior (maybe it has something to do with somersaults). But it’s clear that we have a long road to go before scientists will be able to visualize all of the human neural circuits at once with the glowing efficiency of a tiny polyp.

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