Seeing the whole picture

CAMBRIDGE, Mass.—A team of researchers at the Massachusetts Institute of Technology (MIT) and the University of Vienna has developed an imaging system that captures the activity of the entire nervous system of living animals. Because this is the first technique to be able to generate 3D movies of entire brains at the millisecond timescale, the researchers envision that it could help scientists to learn how the nervous system processes sensory information and generates behavior.

 

The team is using its invention “to enable systematic approaches to neuroscience, revealing how entire neural circuits operate to generate behavior and empowering new therapeutic strategies for neurological and psychiatric disorders,” said Ed Boyden, an associate professor of biological engineering and brain and cognitive sciences at MIT, a member of MIT’s Media Lab and McGovern Institute for Brain Research and one of the leaders of the research team. He added, “Looking at the activity of just one neuron in the brain doesn’t tell you how that information is being computed; for that, you need to know what upstream neurons are doing. And to understand what the activity of a given neuron means, you have to be able to see what downstream neurons are doing. In short, if you want to understand how information is being integrated from sensation all the way to action, you have to see the entire brain.”

 

Boyden and his team—which developed the brain-mapping method with researchers in the lab of Alipasha Vaziri, an associate professor at the University of Vienna—used the system to simultaneously image the activity of every neuron in the worm Caenorhabditis elegans, as well as the entire brain of a zebrafish larva. The research offers a more complete picture of nervous system activity than has been previously possible, Boyden said. Eventually, such an approach, which was described May 18 in Nature Methods, could help neuroscientists learn more about the biological basis of brain disorders and monitor the reaction of the nervous system to drugs and other substances in the body. The researchers believe that the “ability to survey activity throughout a nervous system may help pinpoint the cells or networks that are involved with a brain disorder, leading to new ideas for therapies.”

 

By engineering fluorescent proteins to glow when they bind calcium, scientists can visualize the electrical firing of neurons that encode information using electrical impulses called action potentials, which provoke calcium ions to stream into each cell as it fires. The new approach is designed to image this neural activity over a large volume in three dimensions at high speed.

 

While scanning the brain with a laser beam can produce 3D images of neural activity, it takes a long time to capture an image when scanning each point individually. By accelerating the process, the researchers could see neuronal firing, which takes only milliseconds, as it occurs.

 

The new method is based on a widely used technology known as light-field imaging, which creates 3D images by measuring the angles of incoming rays of light. Ramesh Raskar, an associate professor of media arts and sciences at MIT and an author of the paper, has worked extensively on developing this type of 3D imaging. Microscopes that perform light-field imaging have been developed previously by multiple groups. The MIT and University of Vienna researchers optimized the light-field microscope and applied it, for the first time, to imaging neural activity.

 

With this kind of microscope, the light emitted by the sample being imaged is sent through an array of lenses that refracts the light in different directions. Each point of the sample generates about 400 different points of light, which can then be recombined using a computer algorithm to recreate the 3D structure.

 

The researchers used this technique to image neural activity in the worm C. elegans, the only organism for which the entire neural wiring diagram is known. This 1-millimeter worm has 302 neurons, each of which the researchers imaged as the worm performed natural behaviors, such as crawling. They also observed the neuronal response to sensory stimuli, such as smells.

 

“The system is easy to set up and is cost-effective and compatible with standard microscopes,” Boyden summarized. “Both the temporal resolution and the obtainable fields of view make this an attractive technique for future combination with behavioral studies.”