September 26, 2012 (TSR) – In the quest to understand how the brain turns sensory input into behavior, scientists have crossed a major threshold.
Using precisely-targeted lasers, Harvard researchers have taken over an animal’s brain, instructing miniature nematode worms Caenorhabditis elegans (C. elegans) to turn in any direction they choose by manipulating neurons in the worms’ “brain.”
They even implanted false sensory information, fooling the animal into thinking food was nearby.
The work, said team leader Sharad Ramanathan, an Assistant Professor of Molecular and Cellular Biology and of Applied Physics, is important because by taking control of complex behaviors in a relatively simple animal — C. elegans have just 302 neurons — we can understand how its nervous system functions.
“If we can understand simple nervous systems to the point of completely controlling them, then it may be a possibility that we can gain a comprehensive understanding of more complex systems,” Ramanathan said. “This gives us a framework to think about neural circuits, how to manipulate them, which circuit to manipulate and what activity patterns to produce in them.
“Extremely important work in the literature has focused on ablating neurons, or studying mutants that affect neuronal function and mapping out the connectivity of the entire nervous system. ” he added.
“Most of these approaches have discovered neurons necessary for specific behavior by destroying them. The question we were trying to answer was: Instead of breaking the system to understand it, can we essentially hijack the key neurons that are sufficient to control behavior and use these neurons to force the animal to do what we want?”
As the worm turns
But first, they needed to overcome a number of technical challenges.
Using genetic tools, researchers engineered worms whose neurons gave off fluorescent light, allowing them to be tracked during experiments.
Researchers also altered genes in the worms that made neurons sensitive to light, meaning they could be activated with pulses of laser light (using optogenetics).
They discovered that controlling the dynamics of activity in just one interneuron pair (AIY) was sufficient to force the animal to locate, turn towards, and track virtual light gradients.
The largest challenges, though, came in developing the hardware necessary to track the worms and target the correct neuron in a fraction of a second.
“The goal is to activate only one neuron,” he explained. “That’s challenging because the animal is moving, and the neurons are densely packed near its head, so the challenge is to acquire an image of the animal, process that image, identify the neuron, track the animal, position your laser and shoot the particularly neuron — and do it all in 20 milliseconds, or about 50 times a second.
“The engineering challenges involved seemed insurmountable when we started. But Askin Kocabas, [a Post-Doctoral Fellow in Molecular and Cellular Biology] found ways to overcome these challenges.”
The researchers developed a movable table to keep the crawling worm centered beneath a camera and laser. They also custom-built computer hardware and software, Ramanathan said, to ensure the system works at the split-second speeds they need.
The end result, he said, was a system capable of not only controlling the worms’ behavior, but their senses as well. In one test described in the paper, researchers were able to use the system to trick a worm’s brain into believing food was nearby, causing it to make a beeline toward the imaginary meal.
More complex animals next
Going forward, Ramanathan and his team plan to explore what other behaviors the system can control in C. elegans. Other efforts include designing new cameras and computer hardware with the goal of speeding up the system from 20 milliseconds to one millisecond. The increased speed would allow them to test the system in more complex animals, like zebrafish.
“By manipulating the neural system of this animal, we can make it turn left, we can make it turn right, we can make it go in a loop, we can make it think there is food nearby,” Ramanathan said. “We want to understand the brain of this animal, which has only a few hundred neurons, completely and essentially turn it into a video game, where we can control all of its behaviors.”
Funding for the research was provided by the Human Frontier Science Program, the NIH Pioneer Award and the National Science Foundation.