Brain activation during human male ejaculation

Abstract

Brain mechanisms that control human sexual behavior in general, and ejaculation in particular, are poorly understood. We used positron emission tomography to measure increases in regional cerebral blood flow (rCBF) during ejaculation compared with sexual stimulation in heterosexual male volunteers. Manual penile stimulation was performed by the volunteer's female partner. Primary activation was found in the mesodiencephalic transition zone, including the ventral tegmental area, which is involved in a wide variety of rewarding behaviors. Parallels are drawn between ejaculation and heroin rush. Other activated mesodiencephalic structures are the midbrain lateral central tegmental field, zona incerta, subparafascicular nucleus, and the ventroposterior, midline, and intralaminar thalamic nuclei. Increased activation was also present in the lateral putamen and adjoining parts of the claustrum. Neocortical activity was only found in Brodmann areas 7/40, 18, 21, 23, and 47, exclusively on the right side. On the basis of studies in rodents, the medial preoptic area, bed nucleus of the stria terminalis, and amygdala are thought to be involved in ejaculation, but increased rCBF was not found in any of these regions. Conversely, in the amygdala and adjacent entorhinal cortex, a decrease in activation was observed. Remarkably strong rCBF increases were observed in the cerebellum. These findings corroborate the recent notion that the cerebellum plays an important role in emotional processing. The present study for the first time provides insight into which regions in the human brain play a primary role in ejaculation, and the results might have important implications for our understanding of how human ejaculation is brought about, and for our ability to improve sexual function and satisfaction in men.

Figure 1.

Protocol for the ejaculation condition. The bold black line shows a typical time-activity curve. Vertical lines indicate time frames of 10 sec. Ejaculation took place within the early phase of the time-activity curve, as indicated by gray shading. kcps, Kilocounts per second.

Figure 2.

So-called glass brains for ejaculation minus stimulation. Activations are shown in a sagittal, coronal, and horizontal orientation.

Figure 3.

Strong activation in the mesodiencephalic transition zone. Increased rCBF is represented in coronal sections (a-h) through the brain. The red lines on the glass brain on the left indicate the orientation and location of the sections. Activations are superimposed on the averaged MRI of the volunteers. The activated cluster contains the VTA (sections a-d). The midline thalamic nuclei are located slightly more caudally (sections d-f). The lateral central tegmental field (lctf; sections c-f) and the zona incerta are located lateral to this area. The activated region extends dorsally into the intralaminar nuclei (intralam. nucl.; sections d-h) and the ventroposterior thalamus. Note also the activation in the medial pontine tegmentum (pt; sections g and h). y = -14 (means 14 mm posterior to the anterior commissure). r, Right side.

Figure 4.

Increased rCBF is represented in oblique (45°) sections (a-d) through the brain, to enable comparisons with sections obtained in experimental animals. The red lines on the glass brain on the left indicate the orientation and location of the sections. Activations are superimposed on the averaged MRI of the volunteers. The activated cluster contains the VTA (sections a-d). r, Right side.

Figure 5.

Activations in the basal ganglia and the anterior nucleus of the thalamus and absence of activation in the hypothalamus. Increases in rCBF are superimposed on the averaged MRI of the volunteers and are depicted in coronal sections (see the red lines on the glass brain on the left). Activations are found in the lateral putamen and perhaps the laterally adjoining claustrum and insula (cp; sections a-c). Note that in sections c and d, the anterior nucleus (na) of the thalamus is on the right side. Sections (b-d) demonstrate that in the hypothalamus (hyp) no activation is found. y = -6 (means 6 mm posterior to the anterior commissure). r, Right side.

Figure 6.

Activations in the cerebral cortex rendered onto a standard anatomical template (SPM99). Note that the cortical activations are almost exclusively on the right side.

Figure 7.

Sagittal view of the activations in midline cortical structures, the mesodiencephalic transition zone, and the cerebellum. Increased rCBF is represented in sagittal sections, of which the location is indicated by the red lines on the glass brain on the left. Activations are superimposed on the averaged MRI of the volunteers. In b and c, activation in the secondary visual cortex (BA 18) and posterior cingulate/precuneus (BA 23/31), respectively, can be found. Note in sections a-c that the activation in the mesodiencephalic transition zone (mes-di) extends from the rostral midbrain into the ventral parts of the caudal thalamus. x = -12 (means 12 mm left to the intercommissural line).

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.

Figure 9.

Deactivation in the anterior part of the left medial temporal lobe, comprising parts of the amygdala and entorhinal cortex (entorhin.). Decreases in rCBF are superimposed on the averaged MRI of the volunteers and are depicted in coronal sections (see the red lines on the glass brain on the left). The significance threshold was p < 0.001 (uncorrected for multiple comparisons). y = -2 (means 2 mm caudal to the anterior commissure).