Falling in love is not as simple as it seems, but it is very quick.
Those intense over-powering feelings of being truly, madly, deeply in love are the result of complex and rapid brain activity.
Being in love - or more precisely being in an emotional state of intense longing for union with another, involving chemical, cognitive, and goal-directed behavioural components - is a pretty complicated affair.
According to new research, it's not a basic emotion, as some thought, but a highly complex and businesslike process involving 12 areas of the brain working together to produce and sustain that magic moment.
And researchers have discovered that the first brain activity specific to love starts within one fifth of a second of being smitten.
According to a new study, The Neuroimaging of Love, brain regions with decidedly unromantic names, like the dorsolateral middle frontal gyrus and the anterior cingulate, as well as chemicals like nerve growth factor, dopamine and oxytocin, are all involved in orchestrating these feelings of love.
Some of these areas are those that are also active when people are under the influence of euphoria-inducing drugs - suggesting that falling in love may have a similar effect on the brain as using cocaine.
"Although many emotion theories have included love as a basic emotion, love is more than that," says Dr Stephanie Ortigue who led the study.
"Love includes basic emotions and also complex emotions, goal-directed motivations, body image, appraisal and cognition."
Passionate love, long the exclusive domain of poets, writers and artists, is increasingly being studied by scientists.
At the heart of the research is functional magnetic resonance imaging, or fMRI, a relatively new tool that is used to spot brain activity. The harder an area of the brain works, the greater the amount of oxygen it consumes.
The fMRI scan detects the increase in blood flow needed to supply that oxygen. It has been used in a number of areas, from the study of brain disorders to lie detection.
By identifying areas of the brain involved in, for example, pain or anxiety it can also help in the development of new therapies that target those areas.
One of the new growth areas is its use in pinpointing areas of the brain involved in particular mental processes, especially emotions and behaviour, including sex drive, and love.
Mapping the course of true love through the brain is not simply an academic exercise. Understanding the brain networks that are activated during love may help clinicians to better understand relationship problems and sexual behaviours.
It may, provide doctors, psychologists, and therapists with new treatments for couples suffering from love addiction, love deprivation, or rejection in love.
"The better our understanding of love, the greater our respect for the significance and potency of its role in mental and physical health," says Dr Ortigue.
In fMRI love research, scans are taken of the brains of men and women volunteers after they have been shown visual stimuli related to their partner or loved one. The results are then analysed to see where the action is within the brain.
Six fMRI studies have now been carried out on love, involving scans taken of 120 people, and Dr Ortigue and colleagues at Syracuse and Western Virginia universities, and the University Hospital of Geneva, have analysed the results to piece together a love map of the brain.
In one of the experiments, 17 men and women described as being truly, deeply, and madly in love with their partner had their brains scanned while looking at a picture of the partner for 17 seconds.
The scans showed that there was increased activity in the caudate nucleus and putamen areas of the brain, which are associated with the brain chemical dopamine and with sensations of euphoria and reward.
There was also activity in other dopamine areas, the same regions that are active in people using cocaine.
Increased activity was also seen in the posterior hippocampus, an area involved in memory and mental associations. There was activity too in areas processing emotions and rewards, but there was a drop in activity in areas associated with anxiety and fear.
Overall, Dr Ortigue's analysis shows that passionate love involves brain areas involved in emotion, motivation, reward, social cognition, attention, and self-representation or body image.
Activity in these areas leads to changes in the levels of a number of chemicals in the besotted brain, including increases in dopamine, oxytocin, adrenaline, vasopressin, and a decrease of serotonin, which results in the classic love symptoms, like obsessively thinking about the beloved, craving for a union with him or her, euphoria, and greater energy.
Dopamine is associated with feelings of euphoria, motivation, motor activity, desire, craving and addiction, while adrenaline heightens an indivual's attention, and boosts short-term memory, hyperactivity, and goal-oriented behaviour. It is also adrenaline which gets the heart racing.
The importance of dopamine has been shown in a number of animal studies. When a female prairie vole was paired with a male, dopamine levels went up 50 per cent. Levels of oxytocin, (the so-called "cuddle hormone", released in response to stimuli including skin-to-skin contact) likewise go up, as do levels of vasopressin, both promoting relationship bonding. When the female vole was injected with a drug that blocks the activity of dopamine, she lost interest in him.
Nerve growth factor of NGF is involved too. Researchers at the University of Pavia in Italy measured blood levels of NGF in 58 men and women who had recently fallen in love, and two control groups. Blood levels of NGF were significantly higher in those who were in love.
The highest levels were seen in men and women who had just fallen in love, compared to those in longer standing relationships.
The researchers also found that the higher the levels of NGF, the greater the intensity of the relationship. Both these findings suggest that NGF may be involved in the very early stages of the love.
"Our data demonstrate for the first time that circulating levels of nerve growth factor are elevated among people in love, suggesting an important role for this molecule in the 'social chemistry' of humans," say the researchers.
Researchers at Rutgers University in the US have also looked at romantic love and brain activity.
Their view is that romantic love is one of three primary brain systems that evolved in avian and mammalian species to direct reproduction.
The suggestion is that sex drive evolved to motivate individuals to seek a range of mating partners, while attraction and romantic love evolved to motivate them to prefer and pursue specific partners, and attachment evolved to motivate them to remain together long enough to complete parenting responsibilities.
The researchers, who have also carried out work on people rejected in love, say that the power of love is greater than sex drive alone.
Romantic love is evidently stronger than the sex drive, because when one's sexual overtures are rejected, people do not kill themselves or someone else. Instead, abandoned lovers sometimes stalk, commit suicide or homicide or fall into a clinical depression.
The collection of brain areas that are active in passionate or romantic love appear to be unique to that particular kind of love, with research showing that maternal and unconditional love involve other areas.
A study at the University of Montreal into unconditional love shows that brain regions not implicated in romantic and maternal love, including BA 13 and BA 32, were activated.
"As in the case of romantic love and maternal love, the rewarding nature of unconditional love facilitates the creation of strong emotional links between humans. Such robust emotional bonds may critically contribute to the preservation of the human species," say the researchers.
Dr Ortigue and her colleagues believe that there are 12 areas of the brain involved in passionate love - the caudate nucleus/putamen, thalamus, ventral tegmental area, insula, anterior cingulate, posterior hippocampus, occipital, occipito-temporal/fusiform region, angular gyrus/temporo-parietal junction, dorsolateral middle frontal gyrus, superior temporal gyrus, and the precentral gyrus.
But what happens first, what is the trigger for that torrent of cerebral activity?
A new study to be published shortly by Dr Ortigue offers some clues. The researchers used a high-density electroencephalogram or EEG to measure the volume of electrical activity among brain cells.
The results reveal that when people were shown names of loved ones, electrical activity was swift. Activity spiked very quickly at a pre-conscious level, or within 200 milliseconds, in one of those 12 separate areas of the brain, the angular gyrus.
This area is involved in the processing of visual images, sounds, language comprehension, metaphors, and bodily self-representation. People with brain damage to this area suffer a number of classic symptoms, including depression, poor memory, frustration, poor relationships, belligerence, difficulty with metaphors, and disorders of bodily self representation.
These tentative results suggest that that the brain responds to the stimulus of the soon-to-adored one in less time that it takes to blink.
Romantics, who have long propagated the idea of love at first sight, may have been right after all.