Theory of Love

     This post has also been a long time in coming. I firmly reject the stupid notion that our emotions are anything other than chemical reactions occurring in the brain. So having all the hocus pocus of 'souls' and spirits etc out of the way, we can concentrate on the chemistry of neural activity. And when speaking of emotions, what better emotion than the most exalted and mysterious of these, 'love'. What the hell is it, by the way ? Hmm, its like being punched by the invisible man, you can feel it, but you cannot explain or predict it.

     The 'punch in the face by invisible man' hypothesis of love is very apt for a discussion of the neurochemistry of love. Because every emotion has to have a sensory input, and no emotion can exist in vacuum. So, the real question we must ask ourselves is this : "What is the mechanism of formation of 'love'-like emotions in the brain ?". Is it different for 'love' of a laddoo for example than 'love' for a person ? How about 'maternal love' and 'romantic love' ? Is it the same thing for all of these ? Or are there different senses involved ?

     The point of the thing is, if we understand the mechanisms involved, then we can perhaps make one of those ubiquitous 'love potions' of folklore. So, first of all, there is a difference between maternal love and romantic love and fMRI studies point out that although there are certain regions common, there are thankfully other regions which are quite unique to each (for example http://www.ncbi.nlm.nih.gov/pubmed/15006682). Now, the love for a food or activity is similarly rooted in different areas of the brain. In each case, the expectation of the input (maternal love/romantic love/food love) is different and one could hypothesize that perhaps it is these different expectations that give rise to the different areas of the brain being active.

     In case of maternal love, we expect unquestioning support and to some extent guidance, and this feeling underlies most of our interactions. By the way, I am including siblings and fathers and relatives in the 'maternal love' group. In case of food love, we expect our senses to be satiated. In case of romantic love, its a mixture of the senses being satiated and support encouragement etc. The classification is deliberately left vague since we dont really understand the neural chemistry of any of these processes in great detail. The crucial point that I would like to mention here is the fact that in each case, we get a 'positive feeling', which could be the excitation of the pleasure centers. The route taken to reach the pleasure center from various sensory and imaginary processes is unfortunately not known, but from fMRI studies we can extrapolate that the routes are different. Since society would not allow us to dissect human beings and study the neural connections formed or lost upon 'love'-like feelings formation in the brain, we have to resort to backtracking from behavioral analysis.

     As such, I think psychology is a lot of hokum, so I usually stay away from it, but unfortunately there is nothing else to be done in this case. So, first things first. I propose that 'romantic love' is strong direct, long term potentiation between the memory of a person and the pleasure center. What do I mean by this ? neural connections are made and broken all the time, so somebody makes you feel good one day and makes you feel bad another day would lead his/her memory to be associated with pleasure center one day and not on the other day. So, the long term potentiation is I propose usually very weak between the memory of a person and pleasure centers. When it becomes strong and direct, even remembering the person evokes a very strong 'positive feeling'. Now, most of the symptoms of being in love, can be related to this strong and direct LTP. Consider for example, that you start fantasizing about the person and it makes you happy, easy enough to understand since there is a direct LTP between the memory of the said person and pleasure center. Consider the adage "beauty lies in the eyes of the beholder", again easy to understand as the pleasure center excitation would motivate you to remodel the memory of the person so that the LTP is sustained for a longer time. In other words, after the LTP is formed, it forms a feedforward loop to further increase the LTP strength. What about "love is blind" ? Yes, if the fantasy continues for long then, for no particular reason, just remembering the love interest makes you feel positive, so what motivation is left to behave rationally ? In other words, given the LTP, and the resources that are directed in the feed forward loop, the other regions of the brain suffer.

     I know it does not sound too convincing, but bear with me. Now, I will try to classify the formation of 'romantic love' into various categories. If you can think of any other situation in which such emotions can be formed, let me know and I will try and interpret them using the new framework. The classes are:

1. Stress-induced love. It is a common observation that the probability of a person falling in love is indirectly proportional to the distance (both physical and emotional), between the person and his/her family. This is for example seen in places like IISc and IIT, where the proportion of 'love marriages' is far greater than 'arranged marriages'. There are a lot of other factors also involved, so this cant be the only reason. One way to test this would be to analyze office romances. Are people from far away places more likely to get into a romantic situation ? In any case, my major point here is not the separation itself, but rather the low level stress induced due to such separation. The classic "damsel in distress, and knight in shining armor". Or, "knight in distress, and loving damsel in frilly frock" situation. The point here is, stress increases LTP (for discussion of stress and LTP see http://www.ncbi.nlm.nih.gov/pubmed/18394472, especially http://www.ncbi.nlm.nih.gov/pubmed/17502912). The point made in the second paper is LTP increase or decrease depends a lot on the kind of stress etc. And also on how much corticosterone is present in the brain. Too much stress, as just before an exam, depresses LTP, and so no love during exams. Too little and no LTP and love either. Just the right amount that can be caused due to a low level hidden anxiety is perfect for induction of LTP. So, if we take LTP between memory of a person and pleasure centers to be 'love', then stress is a major inducer of 'love' formation. Typically, what is happening here is, you are under low level stress and somebody does a good thing to you. If you are stress free, you would not give it much importance. But if the right stress chemistry is going on in the brain, the good thing is immediately made permanent due to LTP. A few more encounters like that and bam!! you are in love. The key point here is the initial LTP is facilitated by low level stress. Makes sense from evolutionary perspective, since being with someone in a stressful period, is more advantageous than being alone.
2. Friend conversion love. Now this type is also fairly common. You are friends with a person of the opposite sex for some time, and you have a good time etc. And suddenly one fine day, you realize that you are in love with the person. Now, if you look at the chemistry of it, from the angle presented here, it is quite obvious. As I said earlier, a strong direct LTP needs to be formed. When you are friendly, the association is perhaps indirect via intermediate memories. For example, you share the same love for movies. So, thinking about the person makes you feel positive not directly but due to the anticipation of common interests which make you feel positive. Given long enough time, many such indirect LTPs could simulate the effect of a direct LTP. This requires friendship for a long time during which time, you realize all the common interests and the LTPs keep piling on, till they reach a threshold. Perhaps this kind of mechanism evolved so that nobody is left unmated. Move around with a person long enough, and you will mate with them and the species survives. 
3. Sexual encounter conversion love. This one I am sure happens a lot, and for good reason. For whatever reason, if you find yourself in bed with someone and it has not been unpleasant, it is bound to lead to 'falling in love'. The classic 'love at first sight' kind of thing, in which a sexual component is almost always implied. Now, there is some evidence for sexual activity affecting LTP. For example, oxytocin which is released at the time of orgasm is shown to affect LTP in mice (http://www.ncbi.nlm.nih.gov/pubmed/18468792). Moreover, in rats, sexual activity apparently decreases fear memory formation, so negative feelings are depressed, and positive feelings are evoked (http://www.ncbi.nlm.nih.gov/pubmed/18853437), a perfect recipe for falling in 'love. This makes evolutionary sense especially for monogamous societies where both parents are involved in raising a child.  Having a sexual encounter, could lead to pregnancy, so it is best to stay with the person.

    So, I hope my hypothesis made some sense to people. If you can think of any other situation which goes against LTP hypothesis of love, let me know. And coming back to the love potion thing, here's my recipe for it, a little bit more involved than a simple drop in the tea cup of folklore, but here it goes anyway:

• Inject right amount of corticosterone and adrenaline in the brain to pave the way for LTP to occur.
• At the right time (maybe 1 or 2 hours later), do something nice for the other person, while injecting them with mild tranquilizers such as Buspirone, which affect dopamine and serotonin levels, and put you in a good mood. If this is done cleverly, then the target would probably associate the good mood to you than the drugs, increasing chances of LTP !!!!
• Lastly, inject PKMzeta activators which are known to be involved in LTP maintenance  (http://www.ncbi.nlm.nih.gov/pubmed/18394466).
• Tada!!! a brand new love interest.