By Rex Graham
Professor Edward M. Callaway’s lab at the Salk Institute for Biological Studies has reported the largest and most detailed activity map of the visual cortex of the mouse brain, providing a better understanding of how visual information is processed in all mammals, including humans.
“This gives us new ways to explore the neural underpinnings of consciousness and to identify what goes wrong in neural circuits in the case of diseases such as schizophrenia and autism,” Callaway said.
In a Dec. 22 report in the journal Neuron, Callaway’s group described a functional map of 4,609 fluorescent-dye-primed neurons in a mouse’s visual cortex. The dye in the neurons fluoresced when the cells became activated, creating waves of fluorescent light across the visual cortex as animals watched a bar of light move across a television monitor. The experiment produced what looks like a topographical map of mountains and valleys.
Indeed, the rainbow-colored contours of seven closely grouped regions of the visual cortex might be called the first of many geological surveys of the mouse brain.
While other groups have mapped the mouse visual cortex on smaller, less detailed scales, the Salk-led team used high-resolution photography to record the responses of neurons in 0.5-mm-sized regions. They stitched each region together to create a complete map.
“In the field of cognitive research, this puts the mouse on the map – by putting the map on the mouse,” James Marshel, a Salk research associate, said in a Salk news release. He and Marina Garrett, a graduate student at University of California San Diego, were lead authors on the paper in the Neuron paper. The fourth co-author was Salk scientist Ian Nauhaus.
Neurons, the cells that make up the nervous system of animals, transmit information via thin, branched filamentary extensions, including dendrites that bring information in, and axons, which send information out. Information is transmitted neuron to neuron across small gaps called synapses. Scientists want to piece together a finely detailed map that shows how some or all of the 75 million mouse neurons are connected. With such a map in hand, they will be able to understand how information processing works at the level of individual cells and neural circuits and how it guides behavior.
The mouse is an ideal experimental animal in which to study the mammalian brain: scientists can selectively perturb each type of neuron with genetic mutants, and determine the effect of individual genes in each type of mouse neuron.
The newest findings show that neurons in different parts of the mouse visual cortex respond differently to various combinations of position, speed and direction of movement of a bar of moving light. Such visual specialization indicates that neurons in some areas respond better to motion while others prefer particular spatial features.”
“The neurons, specialized for the location of objects, are useful as a mouse navigates in a maze,” said Callaway, a professor in Salk’s Systems Neurobiology Laboratory, where the research was conducted. “As a mouse runs through the world, objects move past and the animal must keep track. Certain neurons would respond on a left turn or right turn, or a specific pattern. It’s one thing to say it’s there, and they are connected in amazing ways, but how it all works – that’s what we want to understand. That’s what we don‘t know.”
To create the fine-scaled map, Callaway’s team first injected the mice with a fluorescent dye designed to glow when exposed to calcium. The amount of available calcium inside a neuron increases in tandem with its activity level: the Salk team recorded the changing activity of neurons simply by noting the intensity of their fluorescing.
While the computational model operating within any given neuron governs its all-or-nothing electrochemical responses, the circuitry of the entire visual cortex is capable of much greater complexity. The brain of mice, like the human brain, is subdivided into areas devoted to smell, hearing, touch, and other sensory functions. The human brain, with about 100 billion neurons and trillions of neuronal connections, devotes many more of its neurons to vision than does the mouse, which devotes a whopping 30 percent of its neurons to its whiskers.
Callaway said that the discovery of the principles and computational rules, even in the 5 to 10 percent of the mouse brain devoted to vision, is an enormous task, one that will require perturbing that system with precise genetic manipulations to “genetically dissect” the wiring diagram of the mouse brain.
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“While mice cannot replace the work that is being done in monkeys, these research techniques are much further along in mice than in monkeys,” Callaway said. “The ability to modify neural activity using genetic tools and to study the resulting changes in brain and nerve activity is revolutionizing neuroscience.”
The concept of consciousness includes a higher level of discrimination. It includes our ability to pay selective attention to a small fraction of the sensory information bombarding us. Understanding the principles and computational rules underlying visual neurons is expected to be an important step toward understanding consciousness and higher-order cognitive function in humans.
“We’ve learned a lot about how our eyes feed information to our brains, and a huge portion of our brain is devoted to processing this information,” Callaway said. “Vision is a terrific system for understanding how the brain works and, ultimately, for studying mental diseases and consciousness.”
Researchers studying the tiny roundworm called C. elegans have created the animal’s “connectome,” the wiring diagram of all of its 302 neurons. The connectome looks like a route map of all the world’s airlines combined.
Callaway says that what has been learned in the roundworm and fruit fly, and now expanded to the visual cortex of the mouse, will have practical payoffs.
“If we can understand just one area of the mouse brain, it will tell us a lot about all the other areas of its brain,” said Callaway in a telephone interview. “And what we learn in mice will provide insight into cognition and thinking processes in humans.”
A 2008 review in Neuron by Callaway, Liqun Luo, a Howard Hughes Medical Institute (HHMI) researcher at Stanford, and Karel Svoboda, an HHMI researcher at the Janelia Farm Research Campus in Virginia, describes the challenges in creating connectomes of animals more complex than a roundworm.
For example, neuronal extensions, densely packed in brain tissue, are too thin and transparent to be seen microscopically. Researchers have used a staining technique called the Golgi method to stain a small number of neurons in their entirety in frozen tissue samples.
“Genetic Golgi” stains can be developed to visualize individual neurons in living tissue. Such an advance, one of many that are hoped for, will help researchers methodically generate highly detailed circuit maps of the mouse, knowledge that can be applied to humans.
“Genetic analysis is promising to facilitate breakthroughs in our understanding of how neural circuits process information, and to establish causality between the activity in specific groups of neurons, the function of neural circuits, and animal behavior,” Luo, Callaway and Svoboda wrote in the Neuron review. “Examples are drawn largely from our areas of expertise, mainly the olfactory system in fruit flies and the cerebral cortex of mice and primates, but the concepts and techniques we discuss are applicable to other genetic or nongenetic model organisms.”