Researchers with the University of California, Berkeley, have for the first time reconstructed the whisker map a mouse creates of its surroundings in order to navigate its world, catch insects and avoid cats.
Unlike cats and dogs, mice are able to move their whiskers to map out their surroundings, much as humans use their fingers to build a 3D picture of a darkened room.
The sensory nerves at the base of each rodent whisker connect to well-defined structures in the cortex or outer layer of the brain, so-called barrel columns, because they're shaped like a barrel. Each of the mouse's 24 large facial whiskers has an associated barrel column that's activated when the whisker encounters an object. The entire barrel cortex is between 2.5 and 3 millimeters across.
With these columns thoroughly mapped over the past decades, neuroscientists try to discover how brain circuits process, store and use sensory information.
However, Hillel Adesnik, a UC Berkeley assistant professor of molecular and cell biology, was more interested in how the brain uses whisker information to construct a picture of the world around the head.
"Beyond the reach of the whiskers, rodents use vision as well as auditory and olfactory cues to explore the world, but because their near vision is very blurry, close to the face rodents use touch," he was quoted as explaining in a news release. "With their whiskers they scan the world the way we would scan with our hand at night on the night table to look for our cellphone."
His question was: "How does an animal represent space in its cortex so it can localize and then identify an object so that it can execute an action, like picking up the phone, or, for rodents, identifying something it wants to eat?"
The answer lies in a layer of cells in the outermost thickness of the mouse's somatosensory cortex. While the barrel cells in the preceding cortical layer, layer 4, are patterned according to the whiskers, neurons in layers 2 and 3 receive input from several barrel cells at once and process this information. Layer 4 is about 400 microns deep, while layers 2 and 3 start at about 100 microns deep from the cortical surface. The outermost layer of the cortex, layer 1, can range between 100 and 150 microns thick.
"The hypothesis was that layers 2 and 3, which are one step away from layer 4, are not just encoding whiskers; maybe they are encoding space, that is, the space the whiskers are currently scanning," Adesnik said. "You can think of it as the mouse's field of view as it's moving its whiskers. Maybe there is a map that represents that space, and not the whiskers."
In the study published in the current issue of the journal Neuron, using two-photon calcium imaging, which allows more precise localization of fluorescently tagged nerve cells deeper inside brain tissue, Adesnik and his colleagues were able to map how the cells in layers 2 and 3 respond when the mice whisked their whiskers and encountered objects near their head.
His recordings of cell activation through a window in the skull revealed a smooth map of the physical space the mouse was exploring with its whiskers, not a map of specific whisker activity. And the map changes continually as the animal moves its head.
"We think that in layer 4, the neurons really care about whiskers, but not the scanned space," he said, "while in 2 and 3 they are integrating over whiskers, perhaps even performing a mathematical operation something like smoothing over multiple whiskers, that encodes the absolute horizontal location of the object in the reference frame of the animal's head."