A new method for imaging the areas of the brain that process visual information builds a detailed map which allows for more accurate experimenting.
“Mine is one of many labs interested in how cortical circuits process information,” says Edward Callaway of the Salk Institute for Biological Studies, California. Callaway and his team recently published a step-by-step guide that describes their method for imaging the areas of the brain that process visual information.
Callaway’s team attached a head frame to mice, allowing them to hold their heads in place in relation to their camera apparatus and visual stimuli. They also exposed a clean region of the skull over the visual cortex for imaging purposes. The researchers moved a bright checkerboard bar across the mouse’s visual field while recording the exposed skull, measuring changes in specific wavelengths of light that signal the presence of oxygen-carrying blood. “The visual stimulus results in metabolically demanding neuronal activity, which increases blood flow and changes the amount of long-wave red light that’s absorbed,” explains Callaway.
Capturing images of changes in cerebral blood flow is not new. The technique, known as intrinsic signal optical imaging (ISI), was first described in the 1980s and has since been common in neuroscience. Callaway and his team, however, have shown how ISI can be applied to map the visual-processing areas of the mouse brain in a far more precise way than by using physical landmarks — as the precise location and size of these areas differs between individuals of any species. The technique can be adapted to other species depending on the needs of an experiment. The team’s particular system is quicker than traditional ISI and allows for many repetitions, which minimizes the effect of breathing and heart rate on recordings.
From the absorption data acquired, the researchers produced visual maps of the sections of the visual cortex that were activated as the stimulus moved horizontally and vertically. The maps were then converted to a distinct chart of known brain areas, using open-source computer code produced by Calloway’s lab. The completed map shows the precise boundaries of specific brain areas, allowing for more accurate experimentation on the brain.
The team’s experiment offers a useful and flexible tool to neuroscience researchers working with small animals, in which these brain areas may be less than a quarter of a millimetre wide.
“We do this practically every day because we’re now studying in detail the circuitry between these areas ... We make these maps regularly to guide our subsequent experiments,” says Callaway.