Discoveries made in how the brain senses scent

Studies have revealed that mono-molecular odors activate distinct patterns of glomeruli — bundles of olfactory receptor axons on the main olfactory bulb of the brain — and that the reaction of two odors combined is equal to the sum of the individual components’ reactions.

Until recently, however, scientists had yet to learn whether those discoveries would be true for complex natural stimuli as well.

In a study reported June 15 in Neuron, scientists from Duke University Medical Center in Durham, N.C., compared the glomerular responses of natural odors with their fractionated components. Their results provide insight into the process by which the brain distinguishes scents.

Initially, they identified and separated volatile components in each odor with gas chromatography. They used the scents from food, nesting materials, rodent attractants and repellants, and urine from predatory animals. They exposed mice to the original odor and its individual components and measured the neural responses to each. They used an intrinsic signal imaging system from Optical Imaging Inc. and mapped brain activity by detecting changes in reflected light with a Sony CCD camera.

The scientists found that spatial maps of activated glomeruli act as identifiers for components, and that each component activates a fraction of the glomeruli that contribute to the whole natural scent’s representation.

To distinguish a scent, the brain compiles the signals of all of the odors’ individual chemical components. The researchers expect that the olfactory bulb transmits the information perceived from individual components to another part of the brain, where it is assembled and the scent identified.

Because humans and mice rely on the same smell-decoding system and have similar brain structures for scent, the scientists believe that the human scent identification process is similar.