First View Of Many Neurons Processing Information In Living Brain

Reporting in this week's online issue of Nature, the research team used the technique to obtain the first close-up look at the neural circuits that produce vision in cats and rats.

"Put simply, this technique allows us to see the brain seeing," said R. Clay Reid, HMS professor of neurobiology, a member of the HMS Systems Neuroscience initiative, and principal investigator on the project. "It's an entirely new way of looking at brain function."

The method, the first to track the responses of all the neurons in a visual circuit simultaneously, promises to rapidly advance our understanding of how the brain is wired for complex image processing. Lessons learned by studying the visual system may eventually apply to other brain functions like movement, thinking, and learning, as well as neurodegenerative diseases.

The applications of single-neuron functional imaging will be plentiful, according to Alzheimer's disease researcher Bradley Hyman, HMS professor of neurology at Massachusetts General Hospital. Hyman, who was not involved with this research, says, "We have rodent models of Alzheimer's disease, Huntington's disease, and Parkinson's disease, and this imaging will be a powerful tool to dissect the cellular basis for the cognitive problems we see in these diseases."

To get a higher resolution picture of how visual cortex neurons are organized, Reid, research fellow Kenichi Ohki, and their colleagues used a recently developed technique to fill neurons in cats or rats with a dye that glows brightly when calcium rises, a tipoff that the nerves are firing. They then illuminated the cells with a high-powered laser and used a sophisticated microscope to make time-lapse images of hundreds of neurons blinking on and off while the animals viewed images, black and white bars moving in various directions, on a computer screen.

The research team captured pictures of nerve cells firing in the visual cortex, a well-studied region of the brain that processes neuronal input from the eye into the images we see. Decades of work by Harvard neurobiologists and Nobel laureates David Hubel and Torsten Wiesel revealed how neurons in the visual cortex respond to image fragments: some fire only when they see horizontal lines and some for vertical lines, others react specifically to leftward or rightward movement. But a deeper understanding of how the neurons coordinate to process a complex image has been elusive, partly because techniques to examine neural circuits were limited to tapping into just a few cells among many, or making fuzzy pictures of many cells at once.

In the past few years, research teams around the world have been attempting new methods for visualizing neurons at work. A team from Munich, Germany, led by Arthur Konnerth recently developed the technique for staining many neurons in the living cerebral cortex with calcium-sensitive molecules. The Harvard Medical School team used this information and the knowledge of the visual system to produce their results and, for the first time, to watch large ensembles of neurons in action.

"The ability to visualize what individual neurons in a circuit are doing while that circuit is functioning opens up new roads to understanding the neural basis of visual perception," said David Fitzpatrick, a professor of neurobiology at Duke University who is not an author on the paper but also studies visual cortex function. "By combining markers for different types of neurons with this calcium imaging technique to look at their activity, we will have a powerful approach to 'circuit breaking' in the visual cortex. The same principle will undoubtedly be applied to cortical areas responsible for other sensory modalities, as well as motor functions and higher cognitive processes."

Having a sharper view of the visual cortex revealed a precision of brain cell organization that was unexpected. In the cat, neurons that share a function, like sensitivity to the same direction of movement, are seen to associate more faithfully than previously appreciated. The new pictures show clearly segregated groups of neurons tiling the cortex with narrow borders separating nerve types. "When we are able to see every single neuron, we see not a neuron is out of place," Reid explained.

The result surprised researchers, since the fine mosaic of functional segregation in cats looks more precise than expected according to current models of how the circuit works. While the bodies of nerves that respond together are seen bunched tightly, their dendrites, the long arms that pick up incoming signals, branch out to cover a much larger surrounding area, overlapping into other neurons' territories. And in the rat, the observed microarchitecture was completely different than in the cat. Instead of being segregated, neurons that recognized different stimuli were all mixed together, suggesting that nature has managed to find different solutions to the same computational problem.