|What occurs during sexual attraction, and what parts of the brain
are involved in such an important process in the human life cycle?
Sexual attraction is the first step in the process that leads to one of the fundamental instincts of any living organism, sexual reproduction. It is an essential step leading to reproduction and different organisms have different ways of making themselves seem more attractive to the other gender. There is evidence that attraction is gender dependent as there seems to be different regions of the brain that get activated when a person is attracted to someone else. Men seem to focus more on visual stimuli, such as physique and face, while women seems to focus more on emotional connections with their partner. However there are some areas that are activated in both genders, mainly the dopamine reward system. There are other factors that play a role in the process of sexual attraction, such as pheromones released by the other person or neurotransmitter ratios in the brain. Asexuality is most often described as the lack of sexual attraction, but can be described in multiple ways. There is limited research concerning asexuality, but the research that has been done have identified certain aspects of sexual life which can help define asexuality.
1. Perspectives on Sexual Attraction
Sexual attraction dependent on gender because of an evolutionarily stand point. Organisms evolve and adapt in order to increase their chances of reproduction, with a key feature being the ability to attract a mate. There are multiple factors which help determine if a person is attractive, with a lot of these factors being physical factors as humans are the eighth most visually sexually dimorphic primates. However, there are some common processes that occur when attracted to some. There is usually an increased energy and attention on the partner, increased heart rate, and sexual arousal.
1.1 Male Perspective
Visual stimuli seems to be a lot more important to males than females. Men seem to be attracted to femininity. Mainly, they prefer feminine faces as facial attractiveness is very important. Signs of youth and beauty are attractive to males, but physical attractively is not the only factor for attraction, the personality of the partner has also been found to be very important. 
1.2 Female Perspective
Females appear to be less concerned with the physical appearance of the partner than men, but are still attracted to physical features. For this reason, men have evolved features which help sexually attract women. Women tend to be attracted to men with more robust faces and facial hair. However, this has not been shown to be true for all women, as some prefer partners with less facial hair A males voice also affects how attractive he is perceived by females. Women prefer more masculinized pitch and timbre. This preference is thought to be due to the steroid androgen as these are androgen dependent traits, and high androgen levels indicate that the person is healthier. Women are more attracted to money, education, and position than physical attributes.
2. Brain During Sexual Attraction
2.1 Activated Regions
There are numerous brain regions that are activated during sexual attraction, with a lot of similarities between the two genders, and a few differences. Through evolution, humans have a set of common brain regions that are activated when attracted to someone. These regions are usually associated with the numerous feeling associated with attraction. The most important pathway for sexual attraction is the limbic reward pathway, but there are other areas involved. These areas include the ventral tegmental area (VTA), specifically, the A10 dopamine receptors in the VTA were activated, which are involved in the limbic system of the brain, caudate nucleus, anterior cingulate cortex, mid insular cortex, lateral frontal (prefrontal) cortex, left subgyrus, anterior lobe of the right cerebellum, and the amygdala. These activated regions are involved in pleasure, reward, and appetitive motivation. 
The VTA is an essential part of the brains reward pathway, necessary for pleasure, arousal, focused attention, and motivation. The VTA also has projections to another essential brain region for sexual attraction, the caudate nucleus. The caudate nucleus plays an important role in the reward detection process, as well as in the integration of sensory data to produce a behavioral response. The left cingulate gyrus plays a role in emotions such as happiness and pain. The anterior cingulate cortex also plays a role in conditional emotional learning.  The amygdala is a key region of the limbic system and plays a role in memory and emotions. It has been shown to play a role in visual appetitive stimuli for sexual attraction, and has also been shown to have a greater activation in men. Its activation has been shown to drive motivational behavior.
|Deactivated regions when shown a picture of their loved ones. |
2.2 Deactivated Regions
Deactivation of certain regions also occurs during sexual attraction, and their interaction with the activated regions are important for the overall process of sexual attraction. These areas include the posterior cingulate gyrus, part of the amygdala, the right prefrontal cortex, part of the parietal cortex, the middle temporal cortex, and parietotemporal junction. The deactivation of these areas cause a reduction in the ability to critically judge, reduced fear, and increases social trustworthiness. The deactivations are usually right lateralized. The amygdala is associated with fear, an its deactivation may be important for the lack of fear when sexual attraction occurs. The deactivations of the frontal cortex is correlated to the inability to properly judge the others character when sexually attracted. This ties in with deactivations in the temporal, and parietotemporal junctions, which are essential in evaluating feelings and intentions.
3 Chemical Signals
|The brain's reward pathway and dopamine pathway, as well as where
they project to and what behaviour they induce.
Neurotransmitters are the signaling mechanism for the brain. It allows for information to be transferred between the different brain regions, and are responsible for human behavior. Attraction is also a behavior mediated by the neurotransmitters released when an attractive stimuli is presented. The neurotransmitters involved in attraction include dopamine, serotonin, norepinepherine, oxytocin, vasopressin, and the nerve growth factor.  Dopamine is the main neurotransmitter in the brain’s reward pathway, and is associated with a lot of the behavioural changes associated with attraction. Dopamine is released by the hypothalamus, which is an important link between the nervous and endocrine systems. Increased levels of dopamine causes an increase in attention on the positive qualities of the attractant, euphoria, decreased anxiety and fear, and is important in the rewarding nature of sexual attraction.  Coupled with the increase in dopamine is the decrease in serotonin, which is important for appetite and mood. A decrease in serotonin is associated with multiple psychiatric disorders, such as OCD, depression and anxiety. This explains the striking similarities between sexual attraction and these disorders, such as anxiety, stress and obsessive thinking.3] Norepinephrine is important for the memory of new stimuli and in important for the memory of the other person when attracted to them. Oxytocin and vasopressin are closely linked and both are important for attachment and bonding. They are important for romantic attachment and pairing, and are important for the brain’s reward pathway. Oxytocin has also been shown to play an important role in the parasympathetic autonomic nervous system, as well as human trust and social cognition and fear. The nerve growth factor has been shown to be important in the intensity of attraction between two individuals.
|Mean Male Attractiveness Rating|
|Mean male attractiveness ratings for the three trials done, with females
with androstadienone, clove oil, and water.
Pheromones are chemicals that are released by an organism to affect other organisms of the same species, causing physiological and behavioral responses. There are four functions of pheromones which have been identified: mate attraction, same sex repellents, maternal bonding, and menstrual cycle modulation. Animal models have shown that without olfactory cues or the ability to detect olfactory cues, attraction between mice is significantly reduced. Human pheromones are mainly produced by the apocrine glands located in the axillae and pubic region. After the onset of puberty, these glands produce derivatives of the steroid 16-androstenes via testosterone. Pheromones are detected by a specific region of the olfactory system, the vomeronasal organ (VNO). The VNO is located superior to the nasal septum, on both sides of the nasal septum. It contains olfactory receptors which projects axons to the accessory olfactory bulb, which innervates the hypothalamic nuclei. One important pheromone is Androstadienone, which is a odorous axillary secretion. It has been shown to activate regions of the female brain associated with attention and sexual attraction. It has also been shown to increase a woman’s perspective of males. Women given androstadienone rated men more attractive than the control group, who were given water or clove oil. Overall attraction rating for men were higher by women in the androstadienone condition, but the selection rate was not different. Thus it was concluded that pheromones can enhance the attractiveness of the opposite sex.
4.1 Characteristics of Asexuality
There is limited research available in the field of asexuality. The most accurate definition of asexuality is the lack of sexual desire. Because asexuality is a very recent field of research, the data available is very limited. Data gathered from asexual subjects indicate that asexuality should not be based on sexual arousal or sexual activity as there is no definitive value to arousal or activity that may indicate asexuality. Data instead indicates that asexual people actually do engage in sexual activity with their partners, even though they themselves do not have any desire to engage in sexual activities. Some of the reasons given were curiosity and because they were in a relationship, he/she believed that the partner deserved sex. In a recent research done, it was found that most of the reasons for sexual activity was not related to sexual attraction. Instead, one common factor found is the lack of sexual attraction to other people. There also appears to be a decrease in the Dyadic sexual desire score for people who identify as asexual compared to those who do not. A small group of people identifying as asexual also identified themselves as heteroromantic, which indicates a focus on romance and not on sexual attraction and activity.