The following page deals with some micropsychological aspects of human sexuality. It is a pre-print of a paper published in: Med Hypotheses 76 (2010) 384-387 D.O.I. 10.1016/j.mehy.2010.10.048
The Roots of Sexual Arousal and Sexual Orientation
Yehuda Salu, Ph.D.
Department of Physics and Astronomy, Howard University, 2235 6th Street NW, Washington, DC 20059. 202-806-6025, fax: 202-806-5830, ysalu@howard.edu
Abstract
Unlike members of other species that are genetically wired to be attracted to their sexual partners, humans learn the cues that guide them in choosing their sexual partners and that trigger sexual arousal. Genetically wired mechanisms must be directing the acquisition of those cues and organizing them in information structures that underlie human sexual behavior. Individual sexuality is a combination of the genetic mechanisms and information learned through personal experiences. This article focuses on the roots of human sexuality –on genetically embedded mechanisms, common to all humans, around which the wide variety of sexual behaviors is built. It proposes a model that defines the basic mechanisms and their role in developing individual sexuality. It is suggested that three brain areas host the root of human sexuality: the auditory area, which provides stimuli that serve as cues for the identification of a mate; an emotional area, which provides cues for emotional arousal; and a corporal area, which controls the physiological expressions of arousal. The amygdala is a main candidate for the emotional area, and the hypothalamus for the corporal area, but other areas may also provide those inputs. Experimental observations that support this model are discussed, and an outline of additional experiments for validating the model is proposed. If validated, the model would provide knowledge that fills a gap in the understanding of human sexuality –knowledge that would benefit individuals, the medical profession, and society as a whole.
Introduction
Throughout their lives, individuals develop their behavioral and emotional patterns from roots that are genetically embedded in their brains. This article proposes a theory about the roots of sexual orientation and arousal, and outlines how innate learning mechanisms and individual experiences expand those roots to create the wide spectrum of human sexual emotions and behaviors.
At puberty, many people discover that their sexuality has emerged, without them even noticing how it was evolving. Asexual experiences during childhood have been processes by the brain and formed the adult sexual phenotype. This article focuses on pre-pubertal developmental mechanisms. The same general mechanisms continue also after puberty, but by then, additional factors, which depend on the mature sexual system, affect the outcome.
The sexual system of the newborn consists of "hardware" and "software". The hardware is the immature sex organs, and the software is the immature brain programs that activate that hardware. Both parts evolve with time and experience, and at puberty the entire system becomes functional. Although it is not known how the software is encoded, it is apparently realized as synaptic weights between neurons that form neural networks. Neural networks process external stimuli and activate the physiological and mental components of the sexual system.
Classical conditioning is one of the innate mechanisms that the brain uses for acquiring and recording information in its neural networks. Three factors participate in classical conditioning (e.g. Pavlov's experiment): the unconditioned stimulus US(taste of food), the unconditioned response UR (salivation), and the conditioned stimulus CS (bell's ring). The unconditioned stimulus US and its response UR are already a part of the brain's information system. Then, the unconditioned stimulusUS triggers the unconditioned response UR in the presence of the conditioned stimulus CS. After the learning is complete, the CS too becomes a trigger of the UR. The UR is now called the conditioned response CR of the CS.
The Model
The root of sexual orientation
In many species, pheromones drive sexual attraction. In one mode of operation, a female releases a pheromone, and a male detects it. That triggers a sequence of activities in the male that leads him to the female. All this is possible because the releasing organs of the female and the detecting organs of the male are genetically designed to work together. In analogy, in order to uncover the roots of human sexual orientation, it is needed to identify a genetic human system that emits signals that depend on the sex of the emitter, and a receiving system that responds to those signals according to the sex of the receiver.
The auditory system fits these specifications. The voice of men is distinct from the voice of women, and this distinction is easily detected by the auditory system. Voice is one of the most reliable cues that humans use in order to recognize the sex of the speaker. Voice is a genetic, robust, universal cue that is not susceptible to surrounding factors. Therefore, it is suggested that voice is the enigmatic root US, around which sexual orientation is built by conditioning.
Evolvement of arousal cues
Identifying a potential mate of the desired sex is one goal of the sexual software. Another goal is triggering arousal towards that mate. The arousal software generates emotions that help trigger the physiological expressions of sexual arousal. It is plausible that the root arousal software, around which the mature software is built, deals with genetic emotions.
Fear is a genetic innate emotion that participates as the US and the UR in conditioning. It is the activated state of its default opposite, feeling safe. These two opposites can happen simultaneously, and when they do, an overall feeling of arousal may result. For example, fear and feeling-safe get mixed in a roller-coaster ride. The result is a kind of arousal. When watching a favorite team playing a close match, the fear of losing and the feeling of personal safety create a state of arousal. Such arousal feelings are different from pure fear and from pure safety. It has been suggested that a combination of two feelings: fear of another person and, at the same time, feeling safe is a root of sexual arousal. This combination is called SWAP, for Safe With Another Person.
The Loci
The roots of sexual arousal and orientation reside in inter-related brain areas that handle three kinds of information: sensory, emotional, and corporal. According to the proposed model, the auditory system is a main provider of sensory stimuli that identify the sex of a potential partner. The amygdala and the hypothalamus are two of the centers that are involved in generating the characteristic feelings and in triggering the bodily responses of sexual activity. The details of the innate connectivity between those three areas determine the root US's and UR's around which individual sexuality is developed.
Feasibility of the Model
Several observations support the feasibility of the model. The basilar membrane creates a tonotopical representation of the incoming sound. High pitch components cause stronger vibrations at the narrow end of the membrane. As the frequency of the sound decreases, the stronger vibrations shift towards the wide end of the membrane. Hair cells translate the vibrations of the membrane into electrical signals that propagate to the brain. The auditory tract and parts of the auditory cortex are also organized tonotopically. Thus, various brain areas receive information about the spectrum of the sound, and this information could be used for identifying the sex of a speaker.
The inner ear shows sexual dimorphism. The cochleae of human females are 8-13% shorter than those of males. Otoacoustic emission (OAE) is sound generated by the cochlea in response to external sound. It enhances features if the incoming sound. It was found that there are sex differences in OAE even in newborns, and that in adult women there is a correlation between such differences and sexual orientation.
According to the model, an amygdala, a hypothalamus, or any area of a similar role that is innervated heavier by high-pitch neurons would respond stronger to women. Areas that are innervated heavier by low pitch neurons would respond stronger to men. Such connectivity dimorphism may be created by pre- and post-natal genetic and hormonal factors that regulate neurogenesis, cell migration, cell differentiation, cell death, axon guidance and synapsogenesis.
Testing the Model
The model suggests that the auditory system provides the US's stimuli around which individual sexuality evolves. However, people born with dysfunctional inner ear and auditory neural networks still develop their sexuality before puberty. This is an indication that the model describes a sufficient, but not necessary condition for the development of individual sexuality. Combinations of other factors, such as social, visual, olfactory and tactile, may act in parallel to the auditory stimuli.
Off-the-shelf brain imaging, EEG, MEG and histological methodologies and techniques could be used for validating the model. The correlation between the frequency of sound and activated cortical and sub cortical brain areas could be further explored using those and other methods. In particular, responses of brain areas to men's and women's voice could be mapped and sequenced. This includes responses to voice in both asexual and sexual contexts. The role of different emotions in triggering brain sexual response centers could also be further investigated. If the model is validated, it would shed light on one of the oldest, fundamental, unresolved questions: why humans do it as they do it?
For the complete paper with full references, please see the published version: Med Hypotheses 76 (2010) 384-387 D.O.I. 10.1016/j.mehy.2010.10.048