Scientists at the Salk Institute for Biological Studies are using new tools they developed to chart that world, a key step in revolutionizing research into the neurological basis of vision.
For the first time, they have produced neuron-by-neuron maps of the regions of the mouse brain that process different kinds of visual information, laying the groundwork for decoding the circuitry of the brain using cutting-edge, genetic research techniques only possible in mice.
"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," said Edward Callaway, a professor in Salk's Systems Neurobiology Laboratory, whose laboratory conducted the research.
"Vision is a terrific system for understanding how the brain works and, ultimately, for studying mental diseases and consciousness," he stated.
Until now, little was known about what areas of the mouse visual cortex - the high-level brain region that computes the meaning of signals from the eyes - were responsible for processing different elements of the visual information.
To find this, Callaway and his colleagues set out to chart a map of the mouse's visual processing system. They injected mice with a calcium-sensitive fluorescent dye that glows when exposed to a certain colour of light. The amount of calcium in nerve cells varies depending on the activity level of the neurons, so the scientists could measure the activity of brain cells based on how brightly they glowed.
The scientists then displayed different types of visual stimulus on a television monitor and recorded what parts of the brain glowed. To make the recordings, they used a high-resolution camera capable of discerning the activity of individual nerve cells.
They found that a mouse's visual field, the area of three-dimensional space visible through its eyes, is represented by a corresponding collection of neurons in its brain. The researchers precisely recorded which neurons were associated with which area of the animal's visual field.
The scientists studied seven different areas of the animal's visual cortex containing full neuronal "maps" of the visible outside world, and found that each area has a specialized role in processing visual information.
With these maps of brain function in hand, the Salk researchers and others now have a baseline against which they can compare the brain function of mice in which circuit function is manipulated using genetic methods.
Ultimately, Callaway said, understanding in detail how the mouse brain works will illuminate the workings of the human mind.
"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.
The results were reported in the December 22 issue of Neuron.