Cambridge, MA — March 21, 2006 — Finding a new map in the brain doesn't happen every day, especially an exquisite micro-map the size of the period at the end of this sentence. Neuroscientists at the McGovern Institute at MIT have discovered just such a precise map, but in a place that nobody predicted it, connected to the whiskers on a rat's face. Previously, scientists thought that highly refined maps representing the complexities of the external world were the exclusive domain of the visual cortex in mammals, based on discoveries in primates and cats. 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 principle investigator at the McGovern Institute. A paper describing this finding appears online in Nature Neuroscience on March 20.

Scientists are interested in studying how fine scale visual maps develop in the brain and process information in 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 in the dark.

"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."

'Roll In the Barrels'
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 ~4000 densely packed neurons. This map is the rat parallel to the human 'homunculus.' 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, despite the evidence to the contrary. However, the technology didn't yet exist to find it. After creating the world's most intricate whisker stimulator and a multi-neuron recording device, they indeed 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 multi-dimensional 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 brains 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 an 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 related. "It's a little like traveling to an unexplored island. It's literally charting new territory."

A. The whiskers (vibrissae) on a rat's face line up almost on a grid and can be referred to by coordinates like those used in the game Battleship, letters for 'rows' of whiskers and numbers for 'arcs'. B. and C. For each whisker on the face, there is a 'barrel' column in the cortex, a density of cells that form an anatomical topographic map of the whisker inputs, the rat equivalent of the human 'homunculus'. Note the scale bar: Each 'barrel' is ~500 microns across.

The primary finding of the Andermann and Moore paper is a micro-map within a barrel column, in which cells are systematically organized to indicate the direction of whisker deflection. This map is shown on the right, where color indicates which direction of whisker motion neurons like best, and it approaches the type of "pinwheel" map for orientation found in the visual cortex of cats and monkeys. As shown on the right, the position of a neuron within the barrel column predicts its information content: For example, cells in the upper right of a barrel prefer motion of a whisker in the dorsal/rostral direction (yellow/orange), and cells in the lower left part of a barrel column prefer the opposite motion, caudal/ventral (blue/purple).

Left: Moore and Andermann also recently discovered that arcs of barrels prefer similar frequencies of whisker motion, and a map of these columns, from low to high frequency, stretches across the barrel map (Andermann et al. (2004), Neuron 42:452-463). For example, the most posterior column of neurons (shown towards the left in dark red) prefers whiskers being vibrated at 50-100 cycles/second, while the most anterior neuron columns (yellow) responds best when vibrissae are vibrated at >500 cycles/second. This map covers the entire rat whisker map, several millimeters of cortical space. Right: An image of the idealized direction map, which requires a few hundred microns to represent all possible directions of a whisker's motion. These two recent discoveries-of a micro- direction map and a macro- frequency map-demonstrate that the tactile sensory system of a rat has the same kind of detailed, systematic and overlapping representations as the visual system of primates, reflecting the predominance of these two sensory systems in their respective species.

About the McGovern Institute at MIT

The McGovern Institute at MIT is a research and teaching institute committed to advancing human understanding and communications. Led by a team of world-renowned, multi-disciplinary scientists, The McGovern Institute was established in February 2000 by Lore Harp McGovern and Patrick McGovern to meet one of the great challenges of modern science - the development of a deep understanding of thought and emotion in terms of their realization in the human brain. Additional information is available at: http://web.mit.edu/mcgovern

Contacts
skyemedia, inc.
Lyn Chamberlin, 978-443-0400
lyn@skyepr.com

###