SMELL IS THE underdog of the five senses. Critics expend vast amounts of energy explaining the look of paintings and movies; indie rock geeks quibble over the sound of two basses versus one. Meanwhile, the sense of taste is elevated to an art in gourmet restaurants, and touch is explored ad nauseam in countless sex-instruction guides.

But when was the last time you had an impassioned debate or even an in-depth discussion about your olfactory perceptions?

Unless you're a member of the Sense of Smell Institute (www.senseofsmell.org), it's probably been a long time. But according to Noam Sobel, director of UC Berkeley's Berkeley Olfactory Research Project (ist-socrates.berkeley.edu/~borp), "all answers are in the smell." Like many other researchers in his field, Sobel is convinced that the humble, oft-maligned nose is in fact the key to understanding human emotion.

"Smell is the earliest distal sensory system to evolve," he says, "and it's the one all the others are modeled after." A distal sense is one that picks up information away from the body, like a smell in the air or something you see across the street. Proximal senses, like touch and taste, require you to be in contact with whatever you're perceiving. Because the sense of smell developed so early in our evolution, "the way odors are identified and distinguished tells us about the basic structure of the brain," Sobel asserts. In January members of his lab collaborated with researchers at Stanford University on an article published in Nature Neuroscience that suggests humans distinguish between good and bad smells using the same parts of their brains that distinguish between pleasant and unpleasant emotions.

This could help explain why certain aromas elicit such vivid feelings and memories. When Proust described his trip down memory lane in Remembrance of Things Past after smelling and tasting a madeleine, he wasn't just employing a poetic conceit. He was recording a neurological phenomenon.

After hooking up volunteers to a computerized smell-dispensing tool called an olfactometer (psych.wlu.edu/cnl/olfactometer_construction.htm), researchers in Sobel's lab used functional magnetic resonance imaging (fMRI) to look at which parts of the brain responded to a nice lemony odor called citral, versus which parts responded to one called valeric acid that Sobel described as being "like dirty laundry." First, they verified that smells kick up electrical activity in the same spots that emotions do. Then, intriguingly, they discovered the intensity of a smell is measured by a different brain structure than the one that decides whether the odor is nasty or not.

What this might mean, among other things, is that our brains evolved two separate ways to process feelings. One part of our brain examines a feeling's intensity while another evaluates what kind of feeling it is.

Many researchers have tried to uncover the brain structures involved in emotions by exposing people to upsetting and happy pictures, but Sobel says frankly that these scientists are heading in the wrong direction. None of their results have revealed a difference between the way we process intensity and types of feelings. Possibly this is because what we see is influenced by so many factors – what one person considers fun to look at might be another person's worst nightmare.

Often, scientists use pictures of babies to elicit "nice feelings," but certainly there are many people who are indifferent or even hostile to images of babies. The same thing could be said of sexual images, which would make some people (such as this writer) pleased as punch but make others want to exit the room quietly (which is pretty hard to do when you're stuffed inside the MRI scanner). In the end what this means is that it's hard to know why certain parts of a person's brain light up when he or she looks at something.

Smells, however, most people can all agree on. Beauty may be in the eye of the beholder, but all answers are in the smell.

Cultural theorist Herbert Marcuse once made the weird assertion that humans became sexually and socially oppressed when they stopped smelling each other and started looking instead. What we see, he suggested, is easy for other people to manipulate. But what we smell – well, that's the truth. For once, it seems possible that the musings of a philosopher have been corroborated by scientific facts.