The ventromedial nucleus is crucial for female sexual behaviors in all species studied. Enhanced activity in this nucleus facilitates the occurrence and/or enhances the intensity of sexual behaviors. The medial preoptic area may have the opposite effects, but data on this are either not entirely consistent (rodents) or too scarce (primates) for any final conclusion. The many other brain structures now and then suggested to be of importance may perhaps be so in some particular contexts, but their role is probably marginal.
It might be interesting to speculate about the exact function of the crucial brain sites mentioned above. It is evident that the ventromedial nucleus is important for lordosis. Without it, this response to tactile stimulation is severely reduced. Proceptive behaviors, frequently also a response to tactile stimulation from the male, are also reduced when ventromedial nucleus function is impaired. Thus, it can be proposed that the ventromedial nucleus is important for responses to proximal stimuli, mainly tactile. Such stimuli activate the motor pattern of lordosis as well as those patterns forming the proceptive behaviors. On the other hand, the preoptic area is important for the activation of approach behaviors to distant, sexually relevant stimuli, like odor, sound or sight of a male.
We must recall that the abstract mechanism determining an individual's responsivity to an incentive stimulus is called the central motive state. Since the preoptic area is necessary for the approach response to a sexual incentive stimulus, it can be suggested that the central motive state regulating the probability of response to a remote sexual incentive is materialized in this structure. Some lines ago I suggested that the central motive state regulating the display of lordosis and perhaps also of proceptive behaviors in response to proximate sexual incentive stimuli, notably tactile, may be located in the ventromedial nucleus. The combined action of the preoptic area and the ventromedial nucleus, then, controls all aspects of female sexual behavior, from the approach to a mate to the execution of copula-tory reflexes. It is obvious that other brain regions are involved in the sensory input to these structures and the motor output from them. What I propose is not that the rest of the brain could be eliminated as far as sex behavior is concerned. What I suggest is that the transmission of sensory input to motor output takes place in these structures. If active, then appropriate sensory input excites motor output. If not, nothing happens.
The sensory input generated by distant stimuli needs to be elaborated somewhere else and given sexual significance before it is directed to the preoptic area. I have suggested that the medial prefrontal cortex may be the site where this stimulus elaboration occurs (Agmo et al., 1995). Proximal sensory input, like tactile stimulation of the flanks or the vaginocervical region, may arrive directly at the ventromedial nucleus and at the preoptic area without any elaboration. These speculative proposals coincide with all available data. It is not seriously contradicted by any known observations. However, it is undoubtedly in need of some sophistication and some unequivocal experimental data before it can be transformed from speculation into fact. Only further, systematic research can provide the basis for such an endeavor.
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