The neurobiology of love S. Zeki Minireview
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The neurobiology of love S. Zeki Minireview
The neurobiology of love S. Zeki*
FEBS Letters 581 (2007) 2575–2579
University College, Department of Anatomy, Gower Street, London WCIE 6BT, United Kingdom Received 24 November 2006; accepted 13 March 2007
Available online 8 May 2007
Edited by Veli-Pekka Lehto
Abstract Romantic and maternal love are highly rewarding experiences. Both are linked to the perpetuation of the species and therefore have a closely linked biological function of crucial evolutionary importance. The newly developed ability to study the neural correlates of subjective mental states with brain imag ing techniques has allowed neurobiologists to learn something about the neural bases of both romantic and maternal love. Both types of attachment activate regions specific to each, as well as overlapping regions in the brain’s reward system that coincide with areas rich in oxytocin and vasopressin receptors. Both deac tivate a common set of regions associated with negative emo tions, social judgment and ‘mentalizing’ that is, the assessment of other people’s intentions and emotions. Human attachment seems therefore to employ a push–pull mechanism that over comes social distance by deactivating networks used for critical social assessment and negative emotions, while it bonds individ uals through the involvement of the reward circuitry, explaining the power of love to motivate and exhilarate. Yet the biological study of love, and especially romantic love, must go beyond and look for biological insights that can be derived from studying the world literature of love, and thus bring the output of the human ities into its orbit.
2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.
Keywords: Romantic love; Maternal love; Brain imaging; Cortical deactivation; Oxytocin; Vasopressin;
Brain’s reward system
It is only relatively recently that neurobiologists have started to probe into the neural basis of one of the most powerful and exhilarating states known to humans, namely love. In this, they have been aided by the advent of imaging techniques which allows them to ask questions about the neural correlates of subjective mental states which, given their subjectivity, had been impervious to objective scientific investigation. What we can say today about those neural correlates is therefore, of necessity, limited and sketchy but it is almost certain that rapid advances in this field of research will be made in the coming years. In probing the neurobiology of love, neurobiologists of the future will also be looking into evidence derived from the world literature of love, since that literature is itself a prod uct of the brain and its careful study gives strong hints about how the romantic system in the brain is organized. But here I restrict myself more to considering the neural correlates of love derived from experimental studies.
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E-mail address: zeki.pa@ucl.ac.uk
More often than not, romantic love is triggered by a visual input, which is not to say that other factors, such as the voice, intellect, charm or social and financial status do not come into play. It is not surprising therefore that the first studies to inves tigate the neural correlates of romantic love in the human should have used a visual input. These studies showed that, when we look at the face of someone we are deeply, passion ately and hopelessly in love with, a limited number of areas in the brain are especially engaged. This is true regardless of gender. Three of these areas are in the cerebral cortex itself and several others are located in subcortical stations. All con stitute parts of what has come to be known as the emotional brain, which is not to say that they act in isolation. Romantic love is of course a complex emotion that includes, and cannot be easily separated from, other impulses such as physical desire and lust, although the latter can be loveless and therefore distinguishable from the sentiment of romantic love. This is not surprising and is consistent with a simple neurobiological rule – that if one can tell the difference it is because different brain areas, or cells, are involved. Consistent with this rule, ner vous structures that correlate with romantic love in all its com plexity are very distinctive even if they share brain areas with other, closely linked, emotional states.
- Cortical de-activations and the madness of love
It may seem surprising that the face that launched a thou sand ships did so through this limited set of areas. But the story of Paris and Helen of Troy should in itself be enough to tell us that these neurobiological results, viewed on their own, can lead to deceptive interpretations. For romantic love is all-engaging, transforming people’s lives and inducing them to both heroic and evil deeds. It is not surprising to find there fore that this core of brain areas that become engaged during romantic love has rich connections with other sites in the brain, both cortical and sub-cortical. Among these are connec
Fig. 3. Cortical deactivations in the cortex (shown in yellow and red) produced when subjects viewed pictures of their loved partners.
and artists have celebrated, Plato considering it in Phaedrus as a productive, desirable state because this kind of ‘‘madness comes from God, whereas sober sense is merely human’’. But of course if it comes from God, it transcends the world of rationality and is beyond the grasp of the intellect or logos. Perhaps the neurological explanations, of a de-activation of those parts of the brain that are involved in the making of judgments, makes the frequent apparent irrationality of love more comprehensible. As Nietszche once wrote, ‘‘There is al ways some madness in love. But there is always some reason in madness’’, the reason to be sought in the pattern of neuro biological activation and deactivation that romantic love en tails, which serves the higher purpose of uniting for biological purposes even unlikely pairs, and thus enhancing variability. If ‘‘the heart has its reasons of which reason knows nothing’’, it is quite literally, because reason is sus pended. When Blaise Pascal uttered these words he could not have known that reason is suspended because the frontal lobes are (temporarily at least) also suspended. In fact, we can draw a neurobiological lesson from this selective suspen sion of judgment. For, if those in love suspend judgment about their lovers, they do not necessarily as well suspended judgment about other things. They could, for example, be perfectly able to judge the quality of a book or of a scientific work. They could as well be perfectly able to have a theory of mind regarding persons other than the one they love. The suspension of judgment is selective, and argues for a very specific set of connections and brain operations when it comes to love.
- Neural correlates of maternal love
Equally interesting is that this pattern of areas activated by romantic yearnings shares parts of the brain that also become active when mothers view pictures of their own children, as op posed to other children (Fig. 4). Maternal and romantic love share a common and crucial evolutionary purpose, that of maintaining and promoting the species. They also share a functional purpose, in that both require that individuals stay together for a period of their lives. Both are thus calculated by nature to ensure the formation of firm bonds between indi viduals, by making of them rewarding experiences. It is not surprising to find that both sentiments share common brain areas. But, given the neurological axiom stated above, that if you can tell the difference it is because different brain areas are involved, it is also not surprising to find that the pattern of brain activation that correlates with maternal love is not identical to the one that correlates with romantic love. An interesting difference lies in the strong activation of parts of the brain that are specific for faces in maternal love. This may be accounted for by the importance of reading children’s facial expressions, to ensure their well being, and therefore the constant attention that a mother pays to the face of her child. Another interesting difference is that the hypothalamus, which is associated to sexual arousal, is only involved in romantic love. The commonly activated regions between the two types of love are located in the striatum, part of the reward system of the human brain. It is also true that in maternal love, no less than in romantic love, judgment is somewhat suspended, in that mothers are a good deal more indulgent with their chil dren and perhaps less likely to fault them.
Fig. 4. Brain activity produced by maternal love and romantic love (in both males and females) (shown in red and yellow). Note that there are considerable areas of overlap, although there are as well regions that are activated only by maternal or romantic love. Abbreviations: aC, anterior cingulate cortex; aCv, ventral aC; C, caudate nucleus; I, insula; S, striatum (consisting of putamen, caudate nucleus, globus pallidus); PAG, periaqueductal (central) gray; hi, hippocampus.
Fig. 5. Deactivated regions with maternal and romantic love, shown in red and yellow. Abbreviations: mt, middle temporal cortex; op, occipitoparietal junction; tp, temporal pole; LPF, (ventral) lateral prefrontal cortex.
that there is a pattern of cortical de-activation produced by maternal love which is remarkably similar to the one produced by romantic love and in particular the frontal cortex that is in volved in the formation of judgments (Fig. 5).
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how things work out and injection of these hormones into montane voles does not render them monogamous. This may seem at first paradoxical, but there is a simple biological way of accounting for it and it is of substantial interest in the con text of concept formation.
Once secreted by the pituitary, these neuro-hormones can only act if there are receptors for them. In the prairie vole there is an abundance of receptors for vasopressin and oxytocin in the reward centres of the brain. These centres are not clearly defined as yet but include many structures that have been found to be active in reward conditions. Many are located in the sub-cortex. Receptors for oxytocin and vasopressin are missing or not as abundant in the reward centres of the mon tane voles. Hence injecting montane voles with a surplus of these two neuro-hormones does not make them monogamous, since there are not sufficient receptors for them in the reward centres. It is as if these two hormones, strongly implicated from other evidence with bonding, are the ones that keep voles faithful and monogamous and as if the absence of receptors for them makes of their relatives promiscuous animals. There is no evidence that these two neuro-hormones act in the same way in humans; it would be surprising if they did, given the infinitely more complicated structure of the human brain. But it would not be surprising if we find in the vole a vestigial system to ac count for the sexual and romantic nature of humans. Mankind is often, but very mistakenly, considered to be monogamous. The evidence from divorce rates, adultery and other more or less clandestine and casual encounters, as well as the flourish ing trade in prostitution and pornography, suggests otherwise, which is not to say that many among the human race do not maintain monogamous, or serially monogamous, relation ships. It would be highly interesting to learn whether monog amous humans have a higher concentration of oxytocin and vasopressin, as well as a richer concentration of receptors for them in the reward centres of the human brain compared to their more promiscuous counterparts. One might even find that humans can be divided into three or more categories – ranging from the extremely promiscuous to the strictly mono gamous, and that this distribution reflects the distribution of receptors for vasopressin and oxytocin, which is known to vary in species as far apart as voles and humans.
Oxytocin and vasopressin seem to play a crucial role in forming a concept of the kind of partner that an organism wants to be with, at least in the world of vole ideas. They ap pear to do so by building a strong profile of the mating partner through odour and, once they do so, the odour-derived con cept seems to be very stable. The odour comes to be associated with a pleasurable and rewarding encounter with a particular partner. The same works in the visual domain, as has been shown in sheep – once oxytocin is released in the presence of a baby, the sheep will visually recognise the baby and behave in a motherly way toward it until it is grown up. If the gene
for either of these two neuro-modulators is disabled before birth by genetic engineering in a mouse, the mouse will no longer be able to form a profile – or a concept – of the mice that it meets. It becomes totally amnesic in this regard and hence promiscuous. It is not outrageous to suggest that this neurochemically mediated experience has all the hallmarks of concept formation, though concept formation at a very elementary, chemical, level. The concept formed is that of an individual; it is based on an encounter and sexual experi ence, is acquired postnatally and is associated with a pleasur able, rewarding, experience with a partner of a particular odour.