The Brain That Changes Itself by Norman Doidge is a book that I love to give away, a peek under the hood of the mind itself. Chapter 4 leads the reader down paths that help him or her better understand their own relationship to love and sexuality through the chemistry within the brain and the functions that shape it.

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"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 imaging 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."

– The Neurobiology of Love, Zeki S. (2007)

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The activation of the pleasure centers increases the level of dopamine in the brain, which is a neurotransmitter linked deeply to seeking rewards, rather than reward itself, with novelty providing motivation to seek a new experience. Dopamine is stimulated by the "chase" aspect of love particularly in the stage of early attraction. 

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As the relationship moves from attraction to attachment, the neurotransmitters Oxytocin and Vasopressin comes into play. In the paper "Love is more than just a kiss: a neurobiological perspective on love and affection", female prairie voles were injected with oxytocin into their brains and demonstrated bonding with nearby males. When Vasopressin was injected into the male prairie voles brains, they cuddled with nearby females. 

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These neurological functions do not occur in isolation. While an hourglass figure is a sign a woman is fertile, the biological argument for sexual attraction is limited according to Norman. He correctly explains that sexual tastes have varied through different cultures over time.

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"Rubens’s beauties were large by current standards, and over the decades the vital statistics of Playboy centrefolds and fashion models have varied from voluptuous to androgynous."    

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His work builds on Freud's discovery of critical periods for sexual plasticity. “The sexual instincts”, wrote Freud, “are noticeable to us for their plasticity, their capacity for altering their aims.” These behaviours are often acquired and then wired into the brain from experiences throughout life. 

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"Freud argued that an adult’s ability to love intimately and sexually unfolds in stages, beginning in the infant’s first passionate attachments to its parents. He learned from  from observing children, that early childhood, not puberty, was the first critical period for sexuality and intimacy, and that children are capable of passionate, proto-sexual feelings – crushes, loving feelings, and in some cases even sexual excitement. " 

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The understanding of sexual plasticity and the role of neurotransmitters become more important in the era of online pornography. Without taking a moral stance on porn itself Norman looks into the effect that the advent of high speed porn has has created among people through the critical periods of development of their lives. 

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"The men at their computers, who I and others were treating in the 1990s, looking at porn were uncannily like the rats in the cages of the NIH, pressing the bar to get a shot of dopamine or its equivalent. Though they didn’t know it, they had been seduced into pornographic training sessions that met all the conditions required for plastic change of brain maps. Since neurons that fire together wire together, these men got massive amounts of practice wiring these images into the pleasure centres of the brain, with the rapt attention necessary for plastic change."

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The implication that Norman stresses is that the rewiring of our pleasure systems, and the sexual tastes that come with them can be dramatically influenced by pornography. This is not to cast any moral judgement on the content itself, but its widespread use and effects are not generally discussed. In extreme cases, this chase for dopamine creates addictive behaviour. 

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Through repetitive behaviour, habits become engrained in the mind through neuro-plasticity, through the growth of a protein called Delta-FosB. This a gene transcriptor that floods into the reward pathways when you engage in dopamine inducing behaviours, remaining in the Nucleus Accumbens (a pleasure center of the brain). Tolerance gradually builds up, reinforcing the behaviour further. 

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"Each time they felt sexual excitement and had an orgasm when they masturbated, a “spritz of dopamine”, the reward neurotransmitter, consolidated the connections made in the brain during the sessions. Not only did the reward facilitate the behaviour; it provoked none of the embarrassment they might have felt purchasing Playboy at a store. Here was a behaviour with no “punishment”, only reward."

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The debate on if porn reveals kinks or creates them is still up for debate. What is certain is that as tolerance reduces once powerful dopamine hit, the novelty afforded by high speed internet pornography can influence sexual tastes and become unconsciously  reinforced: 

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"Does the net simply reveal quirks and kinks, or does it also help create them? I think it creates new fantasies out of aspects of sexuality that have been outside the surfer’s conscious awareness, bringing these elements together to form new networks. Freud discovered that such fantasies take hold of the mind because of the individual components in them. For instance, some heterosexual men are interested in porn scenarios where older, dominant women initiate younger women into lesbian sex. This may be because boys in early childhood often feel dominated by their mothers, who are the “boss”, and dress, undress and wash them."

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If an addicted individual wanted to gain some agency, the only way out is a sustained abstinence period. However, as the evolutionary function of DeltaFosB is to make sure that the brain remembers behaviour which leads to reward, the sensitisation will remain for some time after. 

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References: 
 

The Brain That Changes Itself - Norman Doidge

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https://www.ncbi.nlm.nih.gov/pubmed/22119059

de Boer, A., Van Buel, E. M., & Ter Horst, G. J. (2012). Love is more than just a kiss: a neurobiological perspective on love and affection. Neuroscience, 201, 114-124.