Positron Emission Tomography Scan Studies in Humans

Although male rat studies are of utmost importance for a better understanding of the neurobiology of ejaculation, brain imaging studies in humans are the tools which provide a better understanding of how the human brain mediates ejaculation and orgasm. Brain imaging studies will probably lead to a deeper insight into which parts of the brain mediate ejaculation and which parts are involved in the mechanism of orgasm, how these neural areas are linked to each other, and which parts are disturbed in the different ejaculatory and orgasm disturbances. The first Positron Emission Tomography (PET)-scan study during ejaculation has recently been conducted by Holstege et al. (14). Eleven healthy male volunteers were brought to ejaculation by manual stimulation of their female partner. The PET technique using radioactive water (H!5O) shows increases or decreases in blood flow in distinct parts of the brain, representing increases or decreases of activation of neurons in these areas. It was found that during ejaculation the strongest activation occurred in the so-called mesodiencephalic region, a brain area which comprises structures as the ventral tegmental area (VTA), also known as the "reward are" and the lateral central tegmental field. This area comprises dopa-minergic neurons. The activation of the VTA shows that ejaculation leads to rewarding processes in the brain. Increased activation was also observed in the lateral putamen and adjoining parts of the claustrum and insula, and certain parts of the prefrontal, temporal, parietal, and insular cortexes. The meaning of all the activations of these various cortical areas is not yet clear. Further studies need to clarify whether associated sensations of orgasm are mediated by these cortical areas. Unexpectedly, a very strong activation was found in parts of the cerebellum, the meaning of it remaining unclear. Decreased blood flow, and thus less activity was found in the amygdala during sexual arousal, erection, and ejaculation. This may perhaps indicate that the brain looses a state of anxiety or fear during sexual activity. Hypothalamic involvement, as has been demonstrated in animal sexual behavior, has not been found in these male volunteers. Obviously, further PET-scan studies are needed to unravel this human and animal dichotomy.

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