These pages of this recent book (The Brain That Changes Itself) by psychiatrist Norman Doidge are very relevant to porn addiction, and also, if you keep reading, to how and why oxytocin (connection with others) can help reset the brain:
For more on the book: http://books.google.com/books?id=XvPHiN61mg4C&pg=PA108&dq=%22the+brain+t...
102 The current porn epidemic gives a graphic demonstration that sexual tastes can be acquired. Pornography, delivered by high-speed Internet connections, satisfies every one of the prerequisites for neuroplastic change [forming new neural circuitry- a key piece in addiction].
Pornography seems, at first glance, to be a purely instinctual matter: sexually explicit pictures trigger instinctual responses, which are the product of millions of years of evolution. But if that were true, pornography would be unchanging. The same triggers, bodily parts and their proportions, that appealed to our ancestors would excite us. This is what pornographers would have us believe, for they claim they are battling sexual repression, taboo, and fear and that their goal is to liberate the natural, pent-up sexual instincts.
But in fact the content of pornography is a dynamic phenomenon that perfectly illustrates the progress of an acquired taste. Thirty years ago, "hardcore" pornography usually meant the explicit depiction of sexual intercourse between two aroused partners, displaying their genitals. "Softcore" meant pictures of women, mostly, on a bed, at their toilette, or in some semi-romantic setting, in various states of undress, breasts revealed.
Now hardcore has evolved and is increasingly dominated by the sadomasochistic themes of forced sex, ejaculations on women's faces, and angry anal sex, all involving scripts fusing sex with hatred and humiliation. Hardcore pornography now explores the world of perversion, while softcore is now what hardcore was a few decades ago,  explicit sexual intercourse between adults, now available on cable TV. The comparatively tame softcore pictures of yesteryear--women in various states of undress--now show up on mainstream media all day long, in the pornification of everything, including television, rock videos, soap operas, advertisements, and so on.
Pornography’s growth has been extraordinary; it accounts for 25 percent of video rentals and is the fourth most common reason people give for going online. An MSNBC.com survey of viewers in 2001 found that 80 percent felt they were spending so much time on pornographic sites that they were putting their relationships or jobs at risk. Softcore pornography’s influence is now most profound because, now that it is no longer hidden it influences young people with little sexual experience and especially plastic minds, in the process of forming their sexual tastes and desires. Yet the plastic influence of pornography on adults can also be profound, and those who use it have no sense of the extent to which their brains are reshaped by it.
During the mid- to late 1990s, when the Internet was growing rapidly and pornography was exploding on it, I treated or assessed a number of men who all had essentially the same story. Each had acquired a taste for a kind of pornography that, to a greater or lesser degree, troubled or even disgusted him, had a disturbing effect on the pattern of his sexual excitement, and ultimately affected his relationships and sexual potency.
None of these men were fundamentally immature, socially awkward, or withdrawn from the world into a massive pornography collection that was a substitute for relationships with real women. These were pleasant, generally thoughtful men, in reasonably successful relationships or marriages.
Typically, while I was treating one of these men for some other problem, he would report, almost as an aside and with telling discomfort, that he found himself spending more and more time on the Internet, looking at pornography and masturbating. He might try to  ease his discomfort by asserting that everybody did it. In some cases he would begin by looking at a Playboy-type site or at a nude picture or video clip that someone had sent him as a lark. In other cases he would visit a harmless site, with a suggestive ad that redirected him to risque sites, and soon he would be hooked.
A number of these men also reported something else, often in passing, that caught my attention. They reported increasing difficulty in being turned on by their actual sexual partners, spouses or girlfriends, though they still considered them objectively attractive. When I asked if this phenomenon had any relationship to viewing pornography, they answered that it initially helped them get more excited during sex but over time had the opposite effect. Now, instead of using their senses to enjoy being in bed, in the present, with their partners, lovemaking increasingly required them to fantasize that they were part of a porn script. Some gently tried to persuade their lovers to act like porn stars, and they were increasingly interested in “fucking” as opposed to “making love.” Their sexual fantasy lives were increasingly dominated by the scenarios that they had, so to speak downloaded into their brains, and these new scripts were often more primitive and more violent than their previous sexual fantasies. I got the impression that any sexual creativity these men had was dying and that they were becoming addicted to Internet porn.
The changes I observed are not confined to a few people in therapy. A social shift is occurring. While it is usually difficult to get information about private sexual mores, this is not the case with pornography today, because its use is increasingly public. This shift coincides with the change from calling it "pornography" to the more casual term "porn." For his book on American campus life, I Am Charlotte Simmons, Tom Wolfe spent a number of years observing students on university campuses. In the book one boy, Ivy Peters, comes into the male residence and says, "Anybody got porn?"
Wolfe goes on, "This was not an unusual request. Many boys  spoke openly about how they masturbated at least once every day, as if this were some sort of prudent maintenance of the psychosexual system."One of the boys tells Ivy Peters, "Try the third floor. They got some one-hand magazines up there." But Peters responds, "I've build up a tolerance to magazines...I need videos." Another boy says, "Oh, f'r Chrissake, I.P., it's ten o'clock at night. In another hour the cum dumpsters will start coming over here to spend the night...And you're looking for porn videos and a knuckle fuck." Then Ivy "shrugged and turned his palms up as if to say, 'I want porn. What's the big deal?'"
The big deal is his tolerance. He recognizes that he is like a drug addict who can no longer get high on the images that once turned him on. And the danger is that this tolerance will carry over into relationships, as it did in patients whom I was seeing, leading to potency problems and new, at times unwelcome, tastes. When pornographers boast that they are pushing the envelope by introducing new, harder themes, what they don't say is that they must, because their customers are building up a tolerance to the content. The back pages of men's risque magazines and Internet porn sites are filled with ads for Viagra-type drugs--medicine developed for older men with erectile problems related to aging and blocked blood vessels in the penis. Today young men who surf porn are tremendously fearful of impotence, or “erectile dysfunction” as it is euphemistically called. The misleading term implies that these men have a problem in their penises, but the problem is in their heads, in their sexual brain maps. The penis works fine when they use pornography. It rarely occurs to them that there may be a relationship between the pornography they are consuming and their impotence. (A few men, however, tellingly described their hours at computer porn sites as time spent "masturbating my brains out.")
One of the boys in Wolfe's scene describes the girls who are coming over to have sex with their boyfriends as "cum dumpsters." He too is influenced by porn images, for "cum dumpsters," like many  women in porn films, are always eager, available receptacles and therefore devalued.
The addictiveness of Internet pornography is not a metaphor. Not all addictions are to drugs or alcohol. People can be seriously addicted to gambling, even to running. All addicts show a loss of control of the activity, compulsively seek it out despite negative consequences, develop tolerance so that they need higher and higher levels of stimulation for satisfaction, and experience withdrawal if they can't consummate the addictive act.
All addiction involves long-term, sometimes lifelong, neuroplastic change in the brain. For addicts, moderation is impossible, and they must avoid the substance or activity completely if they are to avoid addictive behaviors. Alcoholics Anonymous insists that there are no "former alcoholics" and makes people who haven't had a drink for decades introduce themselves at a meeting by saying, "My name is John, and I am an alcoholic." In terms of [brain] plasticity, they are often correct.
In order to determine how addictive a street drug is, researchers at the National Institutes of Health (NIH) in Maryland train a rat to press a bar until it gets a shot of the drug. The harder the animal is willing to work to press the bar, the more addictive the drug. Cocaine, almost all other illegal drugs, and even nondrug addictions such as running make the pleasure-giving neurotransmitter dopamine more active in the brain. Dopamine is called the reward transmitter, because when we accomplish something--run a race and win--our brain triggers its release. Though exhausted, we get a surge of energy, exciting pleasure, and confidence and even raise our hands and run a victory lap. The losers, on the other hand, who get no such dopamine surge, immediately run out of energy, collapse at the finish line, and feel awful about themselves. By hijacking our dopamine system, addictive substances give us pleasure without our having to work for it.
 Dopamine, as we saw in Merzenick's work, is also involved in plastic change. The same surge of dopamine that thrills us also consolidates the neuronal connections responsible for the behaviors that led us to accomplish our goal. When Merzenick used an electrode to stimulate an animal's dopamine reward system while playing a sound, dopamine release stimulated plastic change, enlarging the representation for the sound in the animal's auditory map. An important link with porn is that dopamine is also released in sexual excitement, increasing the sex drive in both sexes, facilitating orgasm, and activating the brain's pleasure centers. Hence the addictive power of pornography.
Eric Nestler, at the University of Texas, has shown how addictions cause permanent changes in the brains of animals. A single dose of many addictive drugs will produce a protein, called delta FosB that accumulates in the neurons. Each time the drug is used, more delta FosB accumulates until it throws a genetic switch, affecting which genes are turned on or off. Flipping this switch causes changes that persist long after the drug is stopped, leading to irreversible damage to the brain’s dopamine system and rendering the animal far more prone to addiction. Non-drug addictions, such as running and sucrose drinking, also lead to the accumulation of deltaFosB and the same permanent changes in the dopamine system. [Note: Not sure these are permanent. The study we saw said 1-2 months for lingering deltaFosB. More on Delta FosB, and even more on DeltaFosB.]
Pornographers promise healthy pleasure and relief from sexual tension, but what they often deliver is addiction, tolerance, and an eventual decrease in pleasure. Paradoxically, the male patients I worked with often craved pornography but didn’t like it. The usual view is that an addict goes back for more of his fix because he likes the pleasure it gives and doesn't like the pain of withdrawal. But addicts take drugs when there is no prospect of pleasure, when they know they have an insufficient dose to make them high, and will crave more before they begin to withdraw. Wanting and liking are two different things.
 An addict experiences cravings because his plastic brain has become sensitized to the drug or the experience. Sensitization leads to increased wanting. It is the accumulation of deltaFosB, caused by exposure to an addictive substance or activity, that leads to sensitization.
Pornography is more exciting than satisfying because we have two separate pleasure systems in our brains, one that has to do with exciting pleasure and one with satisfying pleasure. The exciting system relates to the "appetitive" pleasure that we get imagining something we desire, such as sex or a good meal. Its neurochemistry is largely dopamine-related, and it raises our tension level.
The second pleasure system has to do with the satisfaction, or consummatory pleasure, that attends actually having sex or having that meal, a calming, fulfilling pleasure. Its neurochemistry is based on the release of endorphins, which are related to opiates and give a peaceful, euphoric bliss.
Pornography, by offering an endless harem of sexual objects, hyperactivates the appetitive system. Porn viewers develop new maps in their brains, based on the photos and videos they see. Because it is a use-it-or-lose-it brain, when we develop a map area, we long to keep it activated. Just as our muscles become impatient for exercise if we've been sitting all day, so too do our senses hunger to be stimulated.
The men at their computers 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 centers of the brain, with the rapt attention necessary for plastic change. They imagined these images when away from their computers, or while having sex with their girlfriends, reinforcing them. 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 behavior; it provoked none of the embarrassment they felt purchasing Playboy at a store. Here was a behavior with no “punishment,” only reward.
The content of what they found exciting changed as the Web sites introduced themes and scripts that altered their brains without their awareness. Because plasticity is competitive, the brain maps for new, exciting images increased at the expense of what had previously attracted them--the reason, I believe, they began to find their girlfriends less of a turn-on.
Until he happened upon the spanking pictures, which presumably tapped into some childhood experience or fantasy about being punished, the images he saw interested him but didn't compel him. Other people's sexual fantasies bore us. Thomas's experience was similar to that of my patients; without being fully aware of what they were looking for, they scanned hundreds of images and scenarios until they hit upon an image or sexual script that touched some buried theme that really excited them.
Once Thomas found that image, he changed. That spanking image had his focused attention, the condition for plastic change. And unlike a real woman, these porn images were available all day, every day on the computer.
Now Thomas was hooked. He tried to control himself but was spending at least five hours a day on his laptop. He surfed secretly, sleeping only three hours a night. His girlfriend, aware of his exhaustion, wondered if he was seeing someone else. He became so sleep deprived that his health suffered, and he got a series of infections that landed him in a hospital emergency room and finally caused him to take stock. He began inquiring among his male friends and found that many of them were also hooked.
Hardcore porn unmasks some of the early neural networks that formed in the critical periods of sexual development and brings all these early, forgotten or repressed elements together  to form a new network, in which all the features are wired together. Porn sites generate catalogs of common kinks and mix them together in images. Sooner or later the surfer finds a killer combination that presses a number of his sexual buttons at once. Then he reinforces the network by viewing the images repeatedly, masturbating, releasing dopamine and strengthening these networks. He has created a kind of "neosexuality," a rebuilt libido that has strong roots in his buried sexual tendencies. Because he often develops tolerance, the pleasure of sexual discharge must be supplemented with the pleasure of an aggressive release, and sexual and aggressive images are increasingly mingled--hence the increase in sadomasochistic themes in hardcore porn.
Here's another excerpt from Doidge's book, on how a taste for bondage can develop.
On how the brain can modify itself to permit us to fall in love:
Romantic love [ON THE OTHER HAND] triggers such powerful emotion that we can reconfigure what we find attractive, even overcoming “objective” beauty.”
113 Being in love triggers an emotional state so pleasurable that it can make even pockmarks attractive, plastically rewiring our aesthetic sense. … Falling in love also lowers the threshold at which the pleasure centers will fire.
114 The addict, the manic and the lover are increasingly filled with hopeful anticipation and are sensitive to anything that might give pleasure…. Because our brains are experiencing a surge of dopamine, which consolidates plastic change, any pleasurable experiences and associations we have in the initial state of love are thus wired into our brains. [It] not only allows us to take more pleasure in the world, it also makes it harder for us to experience pain and displeasure or aversion. … Things that normally bother us don’t. [Being in love] makes it harder for us to be unhappy.
116 A tolerance, akin to tolerance for a drug, can develop in happy lovers as they get used to each other. Dopamine likes novelty. [Advises lovers to surprise each other]
… Love creates a generous state of mind. Because love allows us to experience as pleasurable situations or physical features that we otherwise might not, it also allows us to unlearn negative associations, another plastic phenomenon.
117 Different chemistries are involved in learning that in unlearning … Unlearning and weakening connections between neurons is just as plastic a process, and just as important, as learning and strengthening them. …Evidence suggestions that unlearning existing memories is necessary to make room for new memories in our networks.
118 [Say goodbye to flashbacks…in a sort of grieving process? He says that releasing each intense memory of a loved one who is gone - in a mini ritual of farewell -, can clear the decks for new action. This *might* work with "favorite flashbacks, or even a porn habit.]
119 Freeman believes that when we commit in love, the brain neuromodulator  oxytocin is released, allowing existing neuronal connections to melt away so that changes on a large scale can follow. Oxytocin is sometimes called the commitment neurmodulator because it reinforces bonding in mammals. … Many young people who doubt they will be able to handle the responsibilities of parenting are not aware of the extent to which OT [oxytocin] may change their brains, allowing them to rise to the occasion.
Whereas dopamine induces excitement, puts us into high gear, and triggers sexual arousal, OT induces a calm, warm mood that increases tender feelings and attachment and may lead us to lower our guard.
120, [OT may “wipe” past loves in favor of current one.] Freeman proposes that OT melts down existing neuronal connections that underlie existing attachments, so new attachments can be formed. [OT makes it possible for them to learn new patterns.] … What nature provides, in a neuromodulator like OT, is the ability for two brains in love to go through a period of heightened plasticity, allowing them to mold to each other and shape each other’s intentions and perceptions. …As Freeman says, “The deepest meaning of sexual experience lies  not in pleasure, or even in reproduction, but in the opportunity it affords to [overcome self-absorption and build mutual trust].”