Neuroscientists at the McGovern Institute for Brain Research at MIT have discovered an exquisite micro-map of the brain. It's the size of the period at the end of this sentence, and it's in a most unexpected place -- connected to the whiskers on a rat's face.
Based on discoveries in primates and cats, scientists previously thought that highly refined maps representing the complexities of the external world were the exclusive domain of the visual cortex in mammals. This new map is a miniature schematic, representing the direction a whisker is moved when it brushes against an object.
"This study is a great counter example to the prevailing view that only the visual cortex has beautiful, overlapping, multiplexed maps," said Christopher Moore, a principal investigator at the McGovern Institute and an assistant professor in the Department of Brain and Cognitive Sciences, where he holds the Mitsui Career Development Chair. A paper on the work appeared online in Nature Neuroscience on March 20.
Scientists are interested in studying how fine-scale visual maps develop in the brain and process information as part of their effort to understand how mental illnesses, strokes or epilepsy affect the cortex. But the standard workhorse for studying how brains develop, the rodent, evolved to spend its time underground in dark tunnels, and it lacks such visual maps. Rats instead use their whiskers to navigate.
"Our finding suggests that high-resolution sensory maps that can quickly and accurately handle many different kinds of sensory features are an essential hallmark of high sensory acuity, in whatever mode of perception is most important to the animal," Moore said. "It makes sense that mammals develop intricate sensory maps in the sensory system that is crucial for them -- like vision is for us or the whisker system is for rodents."
The layout of whiskers on a rat's face creates a topographic map, with one-to-one correspondence between a whisker and a "barrel" of approximately 4,000 densely packed neurons. Like the grid coordinates in the game Battleship, stimulating one whisker barrel, say the third one in from row D, or D3, tells the brain exactly what's happening at that location.
Moore and his graduate student Mark Andermann, the first author of the paper, hypothesized that a directional map lay hidden within each barrel. However, the technology didn't yet exist to find it. After creating the world's most intricate whisker stimulator and a multineuron recording device, they discovered a micro-map for directional cues within one barrel. Interestingly, this map looks similar to the pinwheel layout of the visual map for line orientation, as if nature reused a similar layout for similar functions.
This directional map joins their recent discovery of a completely different kind of map spanning several whisker barrels, a frequency map reminiscent of the auditory system. Most likely, the frequency information from many barrels gets wired together with the directional cues from within a single barrel, giving the rat the multidimensional cues it needs to know how to negotiate the object before it.
The study also has important pragmatic implications. Rodents are an ideal model system for studying brain function for many reasons, including the ease of using cutting-edge genetic approaches, and they are currently used to study how mental illness, strokes and epilepsy affect the cortex. Discovering a highly refined sensory system in the rodents makes them even more appropriate as a model for studying precise sensory processing in mammals.
"Finding a new map in the brain is a truly exciting experience," Moore said. "It's a little like traveling to an unexplored island. It's literally charting new territory."
The work was funded by NIH, NSF and the Howard Hughes Medical Institute.