From Holstege et al. "Brain activation during human male ejaculation."

"Manual penile stimulation was performed by the volunteer's female partner. Primary activation was found in...zona incerta"

Well, where else would you expect activation?!?

And from the Materials and Methods:

"Tasks. The volunteers were asked to perform the following tasks twice: rest, erection, sexual stimulation, and ejaculation induced by sexual stimulation. To minimize motor activity by the volunteer during the scan, sexual stimulation was provided by his female partner by means of manual penile stimulation in the tasks stimulation and ejaculation. Manual stimulation was continued throughout ejaculation. The volunteer's head was maintained in position with a head-restraining adhesive band, and, to minimize visual input, volunteers were asked to keep their eyes closed.

In the week before the experiments, the volunteers and their female partners were informed about how the experiments would be conducted, and they were asked to practice at home, especially regarding minimizing head and limb movements. Before the experiment, the precise procedure was again extensively discussed with the volunteers and their female partners. Great effort was made to let the volunteers feel relaxed during the experiments. When asked for their emotional experiences during the tasks, the volunteers did not report important differences between their sexual experience under normal circumstances and in the scanner. All volunteers reported to have used visual imagery to better perform the tasks, and that stimulation and ejaculation were accompanied by pleasurable sensations. Eventually, five of them ejaculated once, three others ejaculated twice, and three volunteers did not succeed."

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