Clinical observations and lesion based studies have long shown that the brain is an important adjunct for sexual functions. Although the role of the brain in ejaculation and orgasmic sensations is not well understood, the impairment of these functions in patients with strokes or parkinsonism have long shown that the brain has some role to play in it.
In this study, the researchers used positron emission tomography to identify the areas of the brain which showed increased regional cerebral blood flow during ejaculation in 11 male subjects, who were brought to climax by manual penile stimulation by their female partners. I was of the opinion that an fMRI was a more sensitive way to understand which areas were getting activated/deactivated during the process of ejaculation, the authors make a compelling point in favor of using PET: the high sensitivity of the fMRI would itself be a bane in the study where there were too many extraneous motor noise sources anyways!
As expected, there are so many parameters that can cloud the results of the findings in such a study, given how multiple sensory stimulations are involved in the process of sexual arousal and ejaculation. However, the manner in which the researchers tried to control the experiment and minimize the “noise” in the data, is an interesting read in itself. Let us take a brief look at some of those measures:
- To minimize extraneous motor signals, the study volunteers were stimulated by their female partners.
- The tactile penile stimulation was continued even after ejaculation so that there were no confounding features.
- The volunteer’s head was restrained with an adhesive band.
- To minimize the visual inputs, the volunteer was asked to keep his eyes closed. How this works is difficult to gauge, since some form of visualization is bound to happen.
- The volunteers were extensively coached in the process and were asked to practice a week in advance. There can be few other incentives which are more appealing to man to give his self for the advancement of science.
- The volunteers said that there were no significant difference in the sexual pleasure they experienced under normal circumstances and under the scanner. (I can only assume they were habituated to get hand-j*bs while their heads were tied down, eyes shut, while a bunch of scienctists looked at colorful pictures of his brain and shouted out when he could have a go).
Anyways. Eventually, 5 of the volunteers ejaculated once, 3 twice, and 3 did not manage to make any positive contributions to the results of the experiment. The ejaculations were timed with respect to gaining the most out of the time frame for the PET scan to bring home the results and out of the 11 ejaculations, 8 could be timed and were used for the purposes of the study.
On analysis, the strongest activation was found in the Mesodiencephalic junction. One of the areas of the activated portions is the ventral tegmental area. It is this portion of the brain which is responsible for several reward mechanisms being set off in motion, including the orgasmic effects of cocaine or heroine (coke/heroin rush) (1). Hence, it would not be out of place for the same area to be activated during ejaculation following manual tactile sexual stimulation by a partner.
Another area which seemed to have been prominently lit up is the cerebellum. Although it is possible that the movements by the volunteers maybe responsible for part of the activation, it has also been implicated in being a part of the heroin rush phenomenon (1). It is also in line with lesion studies that have shown an emotional flattening and increased risk taking following cerebellar damage, which indicates that it does have a role to play beyond motor coordination.
Figure 8. Activations in the cerebellum, brainstem, and occipital cerebral cortex. Increases in rCBF are superimposed on the averaged MRI of the volunteers and are depicted in oblique (45°) sections (see the red lines on the glass brain on the left). Cerebellar activations can be observed in the vermis (v; sections b-h), the cerebellar hemispheres (ch; sections d-h), and the deep cerebellar nuclei (dcn; sections b-d). Note that activation in the cerebellar hemisphere is more pronounced on the left than on the right side. Brainstem activation is present in the medial pontine tegmentum (section a), the lateral pontine tegmentum (sections b and c), and in a region possibly involving the dorsal vagal nuclei and the solitary complex (sections f and g). pt, Pontine tegmentum; r, right side.
One more interesting finding was the medial preoptic area (MPOA), which has been seen to be critical for sexual activities in rodents, was not activated. Since a lot of early sexual neurobehavioral studies and lesion studies were conducted on the rodent model, this might be an interesting offshoot. Since the human nervous system is significantly more complicated than the rodent’s, this would mean that there may be areas which need reexploration if the MPOA inactivation is seen to be a prominent occurrence.
Notably, of the neocortical regions, only the right sided Brodmann’s area was activated. This is in line with the expectation that as a primal instinct, sexual stimulation would be more a matter of the older segments of the brain. And sometimes, it is small things like these which make me wonder at the marvel that is the human body. There can be no man made machine that can be as finely evolved and balanced, and of course, shrouded in mystery, as the human brain itself.
Another interesting finding, which was, however, more predictable, was the fact that there was significant deactivation of the amygdala and the entorhinal complex. It is med school neurophysiology MCQ knowledge that the amygdala is involved in the mediating fear and vigilance, and hence a deactivation of this area with sexual stimulation seems appropriate. Since the times immemorial, how many men were taken down in their vulnerable moments by women who were clever enough to get them to lower their amygdalic defenses? Understandably, this area is also deactivated during cocaine rush (1). Thus, mediation of euphoric states seems to be accompanied with an inactivation of the amygdala.
Although my understanding of the complex neurological response to psychological manipulation is little, it is safe to say that despite such a well controlled test, there still remain a lot of unanswered queries. For example, what exactly are the implications of the cerebellar activations? More details on the absence of activation of the MPOA and bed nucleus of the stria terminalis (BNST), which have been traditionally seen to be important areas mediating sexual activities in rats is needed. If there are differences in the functioning of this area between rats and humans, how significant is it? Would it mean that some older perspectives need to be reassessed in the light of this information?
I think this is an interesting study and for the first time, it lends some credibility to the long held female notion that all men have sex on their brains.
Study in Focus:
Holstege G, Georgiadis JR, Paans AM, Meiners LC, van der Graaf FH, & Reinders AA (2003). Brain activation during human male ejaculation. The Journal of neuroscience : the official journal of the Society for Neuroscience, 23 (27), 9185-93 PMID: 14534252