Whether you’re straight, gay, bisexual or something in between, sex means different things to different people. Regardless, it activates brain systems for romantic love and attachment.
Although most animal research is done in males, it turns out that sexual responses are actually quite different at a molecular level between the sexes.
Pain and Pleasure
Many brain regions are involved in the pleasure/pain experience (Georgiadis & Kringelbach, 2012). For example, the amygdala is an almond-shaped group of nuclei deep within the medial temporal lobes of complex vertebrates including humans. Bilateral lesions of the amygdala are associated with abnormal sexual behaviors (“hypersexed states”) in humans and other animals.
Another key area is the cingulate cortex. This area is divided into an anterior, middle, and dorsal subregions; fMRI studies have shown that activation of the dorsal cingulate cortex (dACC) occurs in response to sexual stimuli, suggesting that it plays a role in processing these cues in erotic contexts, enhancing decision-making, and facilitating behavioral responses through its outputs to motor-related areas and the periaqueductal gray (Leknes et al., 2008; Oei et al., 2016).
A study of eight volunteers undergoing brain scanning found that when their hands were gently warmed to a level that felt painful, but did not cause skin damage, they activated circuits in the center of the brain that are known to respond to pain and reward (see image below). However, the same neurons also switched to fire in a different way to signal pleasure, such as receiving money or good food. Researchers speculate that this is why sex has been referred to as pleasurable pain, and why it feels so good when you have an orgasm. Oxytocin, the hormone released during sex, has also been shown to act as a natural painkiller.
The facial expressions we make communicate a lot. They can convey happiness, surprise, anger, fear and sadness. In fact, Charles Darwin claimed that humans and other animals display universal emotions like smiles, scowls and gasps in response to different situations. And he believed that this universal communication of emotion helped us survive.
It’s also known that certain facial movements trigger a natural stimulant in the brain called dopamine. This stimulant is then used by other brain regions to signal sexual arousal. But what does the face actually do to trigger these responses? And how does it differ between men and women?
Picard’s lab recruited Affectiva cofounder Baba el Kaliouby, whose technology uses computer-vision and machine learning to detect facial expressions. The researchers asked participants to watch a series of short videos containing both positive and negative content, and then analyzed the facial responses.
They found that, on average, participants spent more time in positive valence states than in negative ones. And the number of times that a participant moved in and out of these states was predictive of their psychological view of sex (PSI).
They further discovered that BOLD activity in the bilateral hypothalamus was associated with this PSI. This suggests that the hypothalamus may play a role in how men and women perceive sex, and what type of sexual arousal is required for each.
During sexual activity, hormones such as testosterone and oxytocin affect the synchronization of brain waves in both partners. This leads to feelings of empathy and connection. Studies suggest that these emotional states help promote communication and understanding during sexual intercourse, as well as other types of interactions. For example, researchers at Princeton University found that rats who were allowed to have sex daily had more neurons in their hippocampus, the region of the brain associated with memory and learning, than did the rats who were permitted to be intimate only once every 14 days.
As sex progresses, oxytocin increases in your body and the hypothalamus goes into overdrive. As a result, you start to feel less pain, and your sexy feeling intensifies. Your brain’s reward center, the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA), also start to kick in. This explains why sex feels so good.
During orgasm, researchers have recorded a decrease in blood flow to the amygdala, which is linked to anxiety disorders. This is accompanied by an increase in the insula and secondary somatosensory cortex in the right hemisphere of the brain, which are associated with processing emotions and sensations. Research shows that sexually active older women have better cognitive function, and this may be a result of oxytocin, testosterone and other hormones that increase during sex.
Some sex differences in the brain are not what you would expect. Take, for example, long-term potentiation, or LTP, a brief pattern of neural activity that strengthens connections between neurons. LTP was discovered in 1973 and is known to underlie memory formation. However, scientists were not aware of a fundamental sex difference in how the activity induces LTP until recently. Females require a specific enzyme for LTP to occur, while males do not.
This finding could have broad implications for neuroscience, since many pharmaceuticals target molecular pathways and rely on the formation of new memories to work properly. If these molecular pathways differ between sexes, the drugs might not be effective or might even worsen conditions such as Alzheimer’s disease.
Sexual cues are processed in a complex circuit that involves decoding and integration of sensory stimuli, comparison to past experiences, and modulation of autonomic responses and complex cognitive behavior (Georgiadis & Kringelbach, 2012). Several of these structures are involved in the human sexual pleasure cycle: olfactory lobes that process pheromones and smell; the amygdala, which processes emotions; brainstem nuclei that regulate the heart and gastrointestinal tract; and dopamine-releasing neurons of the reward system, located primarily in the midbrain (substantia nigra pars compacta-SNc, ventral tegmental area-VTA) that trigger feelings of reward when engaging in sexual behaviors.
A few studies on human sex differences have been published, with one finding that frequent penile-vaginal intercourse is associated with higher neurogenesis in the hippocampus and boosts memory function for abstract words, but not faces, as reported by McGill University researchers in 2017. The study may be an indirect result of sex, but it supports the idea that sex is beneficial for brain health.