Subcortical structures

The ascending sensory inputs from the genitalia appear to relay at the venterolateral and intralaminar nuclei of the thalamus prior to following their cortical pathway to the paracentral lobule (Horn & Zasler, 1990). Stimulation of thalamic areas may also result in erection (MacLean, 1975). The medial dorsal and anterior thalamus are sites that may be involved in the production of an erection. In these cases, input probably relays via the efferent tracts from the cingulum and frontal systems. Lesions in the thalamus have been associated with hypersexuality (Miller et al., 1986).

Limbic and paralimbic structures, particularly the hippocampus, septal complex, amygdala and several of the hypothalamic nuclei, play a crucial role in sexuality. MacLean (1975) postulated that the hippocampus exerts a modulating effect on penile tumescence. In primates, electrical stimulation of the fimbria of the hippocampus may lead to erection. After-discharges have also been seen in the hippocampus following erection-producing stimulation of septal and anterior thalamic sites (Horn & Zasler, 1990). Ablative and stimulation studies of the amygdala have been explored in relation to sexual behaviour. In primates, ablative studies have bilaterally removed large portions of the anterior temporal lobes, including the amygdala. The characteristic syndrome observed in the monkey following these ablations (i.e., the Kluver-Bucy syndrome) often induces sexual and oral activity with inappropriate objects (see the discussion of the clinical features and origins of Kluver-Bucy syndrome in chapter 8).

The form of "hypersexuality" noted in the Kluver-Bucy syndrome in man probably represents a failure to discriminate and effectively channel otherwise intact basic drives in the external world (Horn & Zasler, 1990; Mesulam, 1985, 2000a, 2000b). Throughout phylogeny eating, sexual, and aggressive behaviours have been consistently linked to limbic structures (Horn & Zasler, 1990), and direct stimulation of the amygdala often results in aggressive behaviours in primates and other animals (Horn & Zasler, 1990). In humans, complex partial seizures of temporal lobe origin (i.e., temporal lobe epilepsy: TLE) have been associated with disturbances in sexual functioning and demonstrates the clinical effects of recurrent amygdalar stimulation (Horn & Zasler, 1990). TLE may display itself as a "sexual seizure" with sexual genital sensations and autonomic changes associated with arousal (Boller & Frank, 1982; Freemon & Nevis, 1969). In certain cases, sexual activity may precipitate a seizure. Remmilard, Anderman, and Testa (1983) have reported 12 cases of women who observed epileptic sensations of sexual excitement or orgasm. In most cases, the EEG changes indicated temporal lobe epilepsy, the majority of which had a right-sided focus. The inter-ictal state for some patients with TLE may be characterised by hyposexuality, loss of libido, and unusual sexual practices (for example, transvestism or fetishism) (Boller & Frank, 1982; Freemon & Nevis, 1969; Miller et al., 1986).

Lesions of the limbic system or dysfunction at the hypothalamus and temporal lobe have also been associated with change in sexual preference (Miller et al., 1986). The left limbic region has been described as the area most likely affected in sex offenders who have brain injury, and dysfunction in this area has been reported to cause sexual sadism (Money, 1990).

The hypothalamus has been described as the major effector organ of the limbic system (Mesulam, 1985, 2000a, 2000b). Lesions of the hypothalamus often result in disturbances of sexual behaviour (Horn & Zasler, 1990). The anterior hypothalamus appears to be involved in endocrine activity and associated copulatory behaviours. Lesions here commonly produce hypogonadism and hyposexuality (Horn &

Zasler, 1990). The posterior hypothalamus and mammillary bodies may be part of an inhibitory system, as lesions in these structure may produce increased copulation and precocious puberty in some species (Boller & Frank, 1982).

The septal complex is located anteriorly in the wall of the third ventricle, behind the frontal lobe. Stimulation of the lower septum produces erection (MacLean, 1975), and in humans using self-stimulation procedures, it has been associated with pleasurable sexual sensations somewhat like the feeling of impending orgasm (Geschwind & Galaburda, 1985; Heath, 1964; Mesulam, 1985). Lesions of the septum are also associated with exaggerated emotional responses to threatening or unfamiliar stimuli. It has been postulated that the neurons of the septal complex help to link objects in the extrapersonal world with their motivational value (Mesulam, 1985, 2000a, 2000b), operating as part of the "pleasure circuit" originally described by Olds and Milner (1954).

In both the human male and female, direct stimulation of the septal region of the brain produces sexual orgasm and orgasm in the human is accompanied by electrical discharges in the septal area (Heath, 1963, 1972, 1975). Stimulation of the lower part of the septum and the adjoining medial preoptic area elicited full erection (MacLean & Ploog, 1962). Stimulation in some instances within the dorsal psalterium or fimbria of the hippocampus resulted in recruitment of hippocampal potentials and penile erection (MacLean, Denniston, Dua, & Ploog, 1962). Positive loci for erection were also found in mammillary bodies, along the course of the mammillothalamic tract, in the anterior thalamus, in the pregenual cingulate, and subcallosal cortex, all of which are known to receive projections from the anterior thalamic nuclei (MacLean, 1975). Stimulation at positive sites in the septum and the anterior thalamus commonly led to the development of self-sustained hippocampal after-discharges associated with throbbing of the penis and an increase in size of the erection (MacLean & Ploog, 1962), suggesting that the hippocampus exerts a modulatory influence on penile tumescence (MacLean, 1975). While many subcortical regions have been demonstrated to be responsive to electrical stimulation by producing erection, many points within the caudate, putamen, and globus pallidus elicit no genital response when stimulated (MacLean & Ploog, 1962), indicating that the circuitry is specific and not generalized.

Gorman and Cummings (1992) have reported two patients who developed markedly increased sexual behaviour following placement of ventriculoperitoneal shunts (VP) for the treatment of hydrocephalus. In both cases the catheter tip was located in the dorsal septal region as determined by subsequent CT imaging. Neither patient evidenced atypical sexual behaviour before the brain injury, although the second patient had disinhibited sexual behaviour while in a postencephalitic confusional state prior to shunt placement. There was, however, a dramatic exaggeration of sexual activity following the shunt insertion.

Gorman and Cummings (1992) also noted that no similar cases of hypersexual-ity with VP shunts in the absence of septal injury were noted in their series, or had been reported by other investigators. They reviewed extensive literature indicating that damage to the septum in animals and man had previously been associated with hypersexuality, including the case described by Heath and Fitzjarrell (1984) in which these investigators attempt to control seizures in a woman with epilepsy by placing a chemically stimulating cannula directly into the septum. When acetylcholine was introduced into the septal region, the patient became euphoric and experienced sexual orgasm on each of the 12 applications. An autopsy several years later confirmed the septal location of the cannula. Sem-Jacobsen (1968) made similar observations in relation to electrical stimulation of the septal area that consistently elicited sexual interest or orgasm. Gorman and Cummings (1992) concluded that the relationship between septal injury and hypersexuality in the two patients they described indicates that the septal nuclei play an important role in mediating sexual behaviour in humans.

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