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… The Attic warbler pours her throat,

Responsive to the cuckoo's note,

The untaught harmony of spring:

While, whisp'ring pleasure as they fly,

Cool Zephyrs thro' the clear blue sky

Their gathered fragrance fling….

Thomas Gray, "Ode on the Spring"

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Forget about odes to spring. Neuroscience has taken the magic, not to mention the mystery, out of the poetry.

That surge of optimism? Merely the serotonergic response to increased daylight. The distraction and dreaminess? The neurotransmitter dopamine is responding to light and warmth. And what about the "gathered fragrance" of romance in the air? Hardly poetic, since the sensitivity of the olfactory system has been proven to directly relate to pheromones, the essential chemical ingredient of sexual attraction.

And here the poets thought the "untaught harmony of spring" was inexplicable.

While a formal scientific study of the delightful disorder known as spring fever is yet to be undertaken, the serious work of scientists looking at other maladies and other biological phenomena goes a long way in explaining why this particular season makes us feel the way we do.

For example, the study of seasonal affective disorder, or SAD, whose sufferers are morbidly depressed during the winter months and less so during the spring and summer, has shown how responsive our mood-defining neurotransmitters are to light. The intricacies of olfaction are intimately tied to human social, sexual and emotional responses. And the various chemicals involved in coupling, or what biologists call pair bonding — in the aching world of singles ads, the grimly unromantic "committed relationship" — seem to have special intensity in the spring.

In other words, the season's temperature, light and orgy of scents seemingly conspire to create a trifecta of feel-good stimuli. Interacting with those environmental stimuli, our hormones and neurotransmitters mix a heady cocktail for nearly everyone as the rain, chill and dark of winter give way to the warmth, sunshine and fecundity of spring.

"Anything that is novel and exciting drives up dopamine and norepinephrine in the brain," says Helen Fisher, author of "Why We Love: The Nature and Chemistry of Romantic Love." "In the spring we are more impulsive; we leap up and go into the park rather than drive. The novelty of spring, the warmth and the light all drive up our creativity, our impulsivity, our sex drive. And that sounds a lot like spring fever. "

The most obvious impetus for that sense of health and vitality can be found in two basic characteristics of spring: more daylight and warmer temperatures. Flowers need light to grow, and research has shown that humans need light to regulate the concentration of a number of chemicals, including those related to mood and emotion, among them serotonin, melatonin, dopamine and norepinephrine.

Although the precise causes of SAD are unclear, scientists have found that one explanation for the blank depression that afflicts those with the disorder is that these brain chemicals get out of balance in the winter months. Those with SAD often crave carbohydrates, which create a temporary but not very therapeutic surge of serotonin. At the same time, the brain's basic biological clock is governed by circadian rhythms, and when there is insufficient light, too much melatonin is released, also causing depression.

For those with seasonal depression, a dose of light therapy is often recommended by physicians to lift some of the melancholy created by the darkness of winter. While some doctors have been highly skeptical of light therapy, a comprehensive analysis published in the April issue of the American Journal of Psychiatry found not only that the treatment was effective for seasonal depression but that it also eased moderate depression nearly as effectively as the class of antidepressant drugs known as SSRIs, which focus on serotonin.

And if light affects those who are depressed, it also affects those who aren't. A study of 101 healthy men that was published in the British journal Lancet revealed that the turnover of serotonin in the brain was at its lowest during the dark winter months. The production of serotonin was "directly related to the prevailing duration of bright sunlight and rose rapidly with increased luminosity." Dazzling spring sunshine during daylight savings time can make even the healthiest people feel as if they have experienced the ideal response to Prozac.

"I feel so energetic in the spring," says Pamela C. Regan, professor of psychology at Cal State L.A. and author of "The Mating Game: A Primer on Love, Sex, and Marriage ." "And it makes sense. Spring is a time of new life and rebirth. We have this great cognitive capacity when we see all this new life; our serotonin levels increase and so does our sense of health and vitality."

Spending time outside in nice weather, studies have found, improves mood, memory, even creativity. But spending too much time outside when the weather is too warm makes people just as cranky as if they were outside when it rained. The optimal mood temperature: 72 degrees. "It looks like there is something uniquely uplifting about nice springtime weather that just doesn't occur in other seasons," says Matthew Keller, a post doctoral fellow at UCLA who led a series of three studies. "After people have been deprived of sunlight and sunny days, the body is just thirsting for it. So it makes sense to go outside and get a big gulp of a good mood."

Accompanying that big gulp with a deep whiff only increases the intoxicating effects of the weather. Smell is the only sense that does not make a brief stop at the brain's relay station, the thalamus, before going to the prefrontal cortex, the area of the brain that explains, reasons and rationalizes. Smell is unmediated, unfiltered; it hits the prefrontal cortex with a wallop.

Research has shown that many people with schizophrenia have an impaired sense of smell, and with that impairment comes dysfunction in higher brain centers, such as the parietal lobes. This part of the brain is responsible for integrating sensory output; it helps people read social cues and contextualize those cues. Increasingly, it appears that smell is a good window into the unconscious basis for sociability and social interest. One of the biggest problems for those with schizophrenia — and one that isn't helped by medication — is the inability to read social cues and interact with others in a healthy way.

For hamsters, elephants, mice and other mammals, smell plays a strong role in mating choices; they secrete pheromones indicating sexual readiness, and a member of the opposite sex responds. And while humans are far more complex in their choice of partners, what they smell does make a difference. As usual, men and women are not the same.

In 2002, researchers at the University of Chicago demonstrated that women prefer the scent of certain men because of genes the women inherited from their fathers. These genes somehow narrowed the choice of mates by eliminating the extremes: mates whose smell indicated a genetic profile that was too close to their own (and therefore dangerous in terms of procreation) were unappealing, as were potential mates whose smell indicated a genetic profile that was too disparate for healthy offspring. There is wisdom in our olfaction.

Warm weather, fewer garments, the gleam of perspiration — not to mention the heady odors of buds and blooms — run riot over the more than 1,000 different olfactory receptor proteins found on neurons in the nose. Our reactions to smells then commingle with other parts of the brain that govern reasoning, memory and social cues to create an often unconscious, but intensely powerful, emotional response. Part of the feverish quality of spring involves a greater desire to socialize and, perhaps, even to fall in love.

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The bonding chemical Oxytocin has many roles, but one of its most important is that it supports our feelings of attachment and social bonding. Nursing mothers are awash in oxytocin, and areas of the brain associated with memory and emotions have a high density of oxytocin receptors. Testosterone is the essential hormone for sexual attraction, but the relationship goes nowhere without the bonding that oxytocin provides. Add to this mix the feel-good neurotransmitter dopamine — which also surges in the spring, and which is involved with the release of oxytocin — and you have all the ingredients for social bonding.

"The same hormones that facilitate sex and long-term pair bonding also mediate trusting relationships among strangers," says Paul J. Zak, the director of the Center for Neuroeconomics Studies at Claremont Graduate University. When people feel that sense of camaraderie, he explains, or that surge of happy trust during a spontaneous conversation in the park or in a deepening romance, neurochemicals such as dopamine and oxytocin flood the brain. And with oxytocin come relationships that are more than just superficial.

To understand this, one need only look at the vole, a mouse-like rodent. While, to the unschooled, all voles may look the same, in fact the prairie vole is a monogamous creature, a good parent, an upstanding member of society — unlike its close relative, the solitary and promiscuous meadow vole.

Studies of these two voles have shown that the major difference between them can be found in the oxytocin and vasopressin pathways of the prairie vole brain, and in the ways that dopamine is processed. When researchers injected a drug to block dopamine in previously monogamous prairie voles, they became as fickle as Casanova. Conversely, when the vasopressin gene was transferred to the promiscuous meadow vole, the previous commitment-phobe became the doting mate.

Voles are also instructive about the sense of smell: When the olfactory bulbs of prairie voles were damaged, they were unable to form pair bonds.

"Human behavior is of course much more complex than rodent social behavior," cautions Miranda M. Lim, a researcher at the Yerkes National Primate Research Center at Emory University in Atlanta who was involved in the vole study. "However, a component of behavior is undeniably influenced by genes." And, she points out, human brain imaging studies show that being in a romantic relationship activates reward regions in the brain, much as pair bonding does in voles.

So if one's fancy turns to thoughts of love in the spring — if one finds oneself, in the words of Oscar Hammerstein, "as restless as a willow in a windstorm … as jumpy as a puppet on a string" — then the explanation can be found in our biology. And in the end, that very riot of biological and neurochemical responses may be spring fever's greatest mystery of all.

Copyright © 2014, Los Angeles Times
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