Vanilla, camphor, lavender and skunk are just a few of the tens of thousands of odors that humans can detect with exquisite sensitivity. Even a tiny change in chemistry can shift a smell from sweet to rancid, according to Nobel laureate Linda B. Buck, but how do mammals detect so many different odors with such precision, and how do their brains translate that information into behavior, emotions and actions?
Buck, a researcher with the Fred Hutchinson Cancer Research Center of Seattle, gave a keynote address, "Unraveling Smell," at the fifth annual Picower-RIKEN Symposium, "New Frontiers in Brain Science -- from Molecules to Mind," held March 26-28 at the Picower Institute for Learning and Memory.
Sponsored jointly by the Picower Institute and the RIKEN Brain Science Institute of Japan, the symposium brought together distinguished neuroscientists from around the world to present state-of-the-art knowledge on emerging brain research technologies, learning and memory, the mechanisms underlying the brain's ability to change in response to stimuli and how systems within the brain interact. In addition to 16 speakers from MIT and elsewhere, postdoctoral researchers and graduate students presented their work at poster sessions during the two-and-a-half-day event.
Buck shared the Nobel prize in 2004 for her work in discovering the odorant receptor (OR) family, which is how the nose detects odors. Humans have close to 350 different ORs, while mice have approximately 1,000. Olfactory sensory neurons of the ORs in the nose transmit odor signals to the brain's olfactory bulb, which relays those signals to the olfactory cortex. From there, odor information is sent to higher cortical areas as well as to limbic brain areas that control emotional, behavioral and physiological responses.
The OR family is used in different combinations to detect different odorants and encode their individual identities, Buck said. Each receptor recognizes multiple odorants, but each odorant is detected by a unique combination of receptors.
Odorants with nearly identical structures have different receptor codes, explaining how they produce different odors. Buck said that she recently found that mixing the scents of rose and clover in certain combinations, for instance, can generate the completely unrelated smell of a carnation.
Wolf Singer, director of the Max Planck Institute for Brain Research in Frankfurt, Germany, gave a second keynote address on "Time as Coding Space in Cortical Processing." Singer's hypothesis is that synchronized responses from neurons distributed throughout the brain through a "self-organized synchronization mechanism" allow a kind of parallel processing in the cerebral cortex that may be key to perception and memory.
Each neuron codes for tiny attributes of an object, and the brain is able to pick up inputs from these neurons firing in the right frequency and phase the way a radio is tuned to a station. Singer asked, "How are different attributes of unfamiliar objects bound together and represented as a coherent whole?" This mechanism could explain how we can pick out an animal or a face camouflaged by unrelated patterns: The neurons that "recognize" the relevant features create a code through precisely timed synchronization of a certain brain frequency.
A malfunction in this ability may be behind some of the short-term memory problems associated with schizophrenia, Singer said.
A version of this article appeared in MIT Tech Talk on April 5, 2006 (download PDF).