3. Discussion
While male sexual response is complex, several key brain regions are critical for achieving and maintaining erections . Hypothalamic nuclei play an important role in regulating sexual behavior and erections by acting as an integration center for brain and peripheral input . The hypothalamic nuclei that facilitate erections receive pro-erectile input from the mesolimbic dopamine pathway, which comprises the ventral tegmental area (VTA) and the nucleus accumbens (NAc) . The VTA-NAc circuit is a key detector of rewarding stimuli, and forms the core of a broader, more complex set of integrated circuits commonly called the “reward system” . An individual’s response to natural rewards, such as sex, is largely regulated by the mesolimbic dopamine pathway, which receives excitatory and inhibitory input from other limbic structures and the prefrontal cortex . Erections are dependent upon activation of dopaminergic neurons in VTA and dopamine receptors in the NAc [65,66]. Excitatory glutamate inputs from other limbic structures (amygdala, hippocampus) and the prefrontal cortex facilitate dopaminergic activity in the VTA and NAc . Reward responsive dopamine neurons also project into the dorsal striatum, a region activated during sexual arousal and penile tumescence . Dopamine agonists, such as apomorphine, have been shown to induce erection in men with both normal and impaired erectile function . Thus, dopamine signaling in the reward system and hypothalamus plays a central role in sexual arousal, sexual motivation and penile erections [65,66,69].
We propose that chronic Internet pornography use resulted in erectile dysfunction and delayed ejaculation in our servicemen reported above. We hypothesize an etiology arising in part from Internet pornography-induced alterations in the circuits governing sexual desire and penile erections. Both hyper-reactivity to Internet pornography cues via glutamate inputs and downregulation of the reward system’s response to normal rewards may be involved. These two brain changes are consistent with chronic overconsumption of both natural rewards and drugs of abuse, and are mediated by dopamine surges in the reward system [70,71,72].
3.2. Internet Pornography as Supernormal Stimulus
Arguably, the most important development in the field of problematic sexual behavior is the way in which the Internet is influencing and facilitating compulsive sexual behavior . Unlimited high-definition sexual videos streaming via “tube sites” are now free and widely accessible, 24 h a day via computers, tablets and smartphones, and it has been suggested that Internet pornography constitutes a supernormal stimulus, an exaggerated imitation of something our brains evolved to pursue because of its evolutionary salience [74,75]. Sexually explicit material has been around for a long time, but (1) video pornography is significantly more sexually arousing than other forms of pornography [76,77] or fantasy ; (2) novel sexual visuals have been shown to trigger greater arousal, faster ejaculation, and more semen and erection activity compared with familiar material, perhaps because attention to potential novel mates and arousal served reproductive fitness [75,79,80,81,82,83,84]; and (3) the ability to self-select material with ease makes Internet pornography more arousing than pre-selected collections . A pornography user can maintain or heighten sexual arousal by instantly clicking to a study assessing Internet pornography’s effects on delay discounting (choosing immediate gratification over delayed rewards of greater value) states, “The constant novelty and primacy of sexual stimuli as particularly strong natural rewards make internet pornography a unique activator of the brain’s christian mingle reward system. . It is therefore important to treat pornography as a unique stimulus in reward, impulsivity, and addiction studies” (pp. 1, 10).
Novelty registers as salient, enhances reward value, and has lasting effects on motivation, learning and memory . Like sexual motivation and the rewarding properties of sexual interaction, novelty is compelling because it triggers bursts of dopamine in regions of the brain strongly associated with reward and goal-directed behavior . While compulsive Internet pornography users show stronger preference for novel sexual images than healthy controls, their dACC (dorsal anterior cingulate cortex) also shows more rapid habituation to images than healthy controls , fueling the search for more novel sexual images. As co-author Voon explained about her team’s 2015 study on novelty and habituation in compulsive Internet pornography users, “The seemingly endless supply of novel sexual images available online [can feed an] addiction, making it more and more difficult to escape” . Mesolimbic dopamine activity can also be enhanced by additional properties often associated with Internet pornography use such as, violation of expectations, anticipation of reward, and the act of seeking/surfing (as for Internet pornography) [88,89,90,91,92,93]. Anxiety, which has been shown to increase sexual arousal [89,94], may also accompany Internet pornography use. In short, Internet pornography offers all of these qualities, which register as salient, stimulate dopamine bursts, and enhance sexual arousal.