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The Brain


The evolution of the nervous system.

The first single-celled creatures interacted with their environment by sensing the chemicals surrounding them. Specialised receptors developed for detecting food and possible harmful substances. They also were able to sense light and heat.

The most simple nervous process of all, one that evolved millions of years ago in simple organisms is the reflex. The favourite study animal for neurobiologists has been Aplysia, the sea slug. It obtains oxygen from the sea water in which it lives by means of large and beautiful gills. Needless to say, these are extremely vulnerable. The slightest disturbance in the water and sensory neurons in a simple nervous system cause the contractile filaments in the gills to react.

As species became more complex, they began to develop protective and supportive coverings, which became the shells of many invertebrates. Others, which depended on manoeuvrability and speed, developed bands of cartilage, which provided support and the ability to develop muscles for locomotion. Such creatures are the sharks and sting rays of today. In others, a bony spine developed, the teleosts, or bony fish.

At the same time, certain nerves cells became specialised for detecting light and heat, vibration, and chemicals in the water (smell). With increased complexity, groups of nerves evolved for comparing and organising these sensory inputs. In addition greater control over reflex responses developed.

A member of one the oldest groups of fish, in evolutionary terms, is the lamprey, which is a parasite on other fish, attaching itself for life, by its mouth part (It has no jaws as such). One can see four adjacent but virtually separate organs. The olfactory area deals with its sense of its smell, which it uses to detect its prey. The optic tectum processes information from its rudimentary eyes. The cerebellum and medulla control bodily processes and movement, and also detect vibration. It will be clear that, generally the brain is situated close to the main perceptual organs, those of vision, smell and sound.

As creatures developed more complex behaviours and perceptions, the various parts of the brain became larger in relation to each other, and began to overlap, in order to fit into a conveniently sized shape. The differences in brain structures between species is an accurate reflection of their lifestyles. For instance, sharks are extremely sensitive to chemical scents in the water, while in fishes a sensitive organ for detecting vibration has developed in the lateral line. Scent is also important to snakes, while eyesight is important to frogs. Smell is not important to most birds, but they have keen eyesight, and flying is a complex motor behaviour.

As a general rule, the mass of a particular brain component is proportional to the amount of processing it has to do, because of the increased number of neural cells and connections. It is not, of course, true that absolute size is a measure of mental capacity. Size in relation to body size is a rough guide, but the final criterion is the complexity of the neural connections inside. The cerebral cortex makes its appearance in mammals, becoming largest in relation to other areas in the primates and humans.

Overview of brain.

The inner part of the human brain is notionally divided into three main areas.

The forebrain has evolved from organs concerned with smell and also taste. The mid-brain is concerned with vision, while the hind-brain vibration, sound, and balance.

The aim in this section is not to describe the brain in detail, but to highlight areas that will figure in the discussions that follow. In the diagram the cerebral cortex can be seen almost covering the other component parts. We will return to this in the next section, though it is perhaps an artificial distinction to deal with it separately, when it is part of the brain as an integrated whole. The excuse is that, with the cortex, the sexually dimorphic debate takes a completely new direction.


This comes about because, like most of the body, the brain is divided, more or less, symmetrically about the front-to-back centre line. The Corpus Callosum, shown in the diagram is a band of thousands of nerve fibres which connects the two halves of the cortex. Although it is usual to write of the hypothalamus, the amygdala and so on, most of the other parts of the brain are similarly divided, being connected by bundles of fibres called commissures.

The cerebellum is concerned with fine motor behaviour, the co-ordination of automatic processes like walking and running. After much practice, learned complex behaviours like riding a bicycle are thought to become controlled by the cerebellum.

At the base of the brain is the brain stem, the medulla, previously mentioned, the pons and an area called the reticular activating system. These form the upper end of the spinal column. There are many nerve junctions (synapses) here and it is where many of the nerves cross over, since in general the left side of the brain controls and senses the right side of the body, and vice versa. This is also thought to be an evolutionary hang-over. In a very simple creature, it would make sense for a touch on the left to cause movement to the right, and vice versa, to take it way from possible danger.

The main feature below the cortex is the Limbic System, with a variety of different features. Within the limbic system is the amygdala (not shown in the diagram), the thalamus and the hypothalamus. Just below the last-named is the pituitary gland.

An important reason for relating behaviour to the hormone environment is the close coupling of the pituitary gland with the hypothalamus. They are very much the control centre for the body, regulating body chemicals, eating, drinking, breathing, body temperature, heart rate and sexual functioning.

In front of the hypothalamus is the optic chiasma, where the nerves from the eyes cross over on their way to the visual cortex at the rear of the brain. There are connections to the suprachiasmatic nucleus in the hypothalamus, to provide information about night and day, working in conjunction with pineal gland, generating circadian rhythms.

In humans the olfactory bulbs have virtually disappeared. They are part of the septum, which connects to the amygdala. Hearing is connected via various nuclei to the auditory cortex.

The hypothalamus.

The hypothalamus is a small organ, but one that affects practically every process in the body. It is difficult therefore, in animal studies, to investigate behaviour by surgical intervention.

Adams(1) compared the effect of removing the medial parts and the lateral parts of the hypothalamus in rats. In the first case, what he called defensive aggression, where rats fight on their hind legs, forepaw to forepaw, was reduced. In the second case, what he called territorial behaviour was reduced. Clearly, one has to be careful about what one means by aggression.

In another study, Kruk used electrodes placed in the hypothalami of a number of rats. Various parts seemed to be involved in various types of behaviour, some had no effect, others produced attack responses, others led to biting without any preliminary skirmishes.

Amygdala and septum.

Towards the rear are two prominences, called the mammillary bodies. In front of these is a bulge, called the median eminence.

In laboratory animals, removal of the amygdala generally renders them docile and difficult to provoke. Stimulation of different areas, in the rat, produces different behaviours. Either there is no effect, or there is defensive fighting, or there is biting attack without warning. However, stimulation in hamsters removes fighting behaviour, reaffirming the difficulty in generalising across species.

For a time, the experiment was tried of removing the amygdala in violent men. It was generally unsuccessful, and in those where it appeared to be successful, the behaviour returned within six months.

The septum is less well studied. Its removal makes rats more easily provoked, indicating a balancing relationship with the other centres. However, this complicated by the fact that, as we seen, having the ability to smell another male's scent affects the tendency to fight.

Testosterone: how it works.

During development, then, cells in the CNS might or might not be responsive to testosterone, estrogen, dihydroxytestosterone or, for that matter, a wide range of other chemicals. Those cells that are responsive might survive rather than die. They might start to divide or differentiate.

Hence the idea that they might become organised in ways which form the basis of future aggressive or sexual behaviour. Once organised, structures may be sensitised later on. Thus a given behaviour, or a particular response to a situation more likely.

This effect may also be indirect. Without testosterone, a male stickleback cannot make the glue that holds its nest together. Without a nest it has nothing to defend, so it is not likely to be aggressive towards intruders. As was mentioned in the previous chapter, the urine of castrated mice does not have the characteristic odour of male mice, so they are not attacked by other males. Since they also do not smell the odour of intact males they also do not attack other mice.

In passing, a comparison of the relative size of the olfactory centres in mice and humans, suggests that smell plays a much more important role for mice.

Fetal hormones and social behaviour.

Evidence has been presented, by various studies, that the hormonal balance in the mother's womb has an effect, not only on the genital system of the fetus, but on the development of the brain.

Given the physiological differences between males and females so far described, it would be surprising if there were no brain differences. To what extent, however, does they affect general social behaviour? In particular sexual preference and gender identity?

William Cookson, in The Gene Hunters,(2) recounts the study that involved inserting a piece of genetic material, the SRY gene, into the X chromosome of a mouse, making, not a homosexual or transsexual, but a transgenic mouse. (Why are there no newspaper reports about lesbian fruit- flies?) Then he committed a singular anthropomorphism, saying it "considered itself male . . . and that the female was obviously of the same opinion". Does a mouse consider, or have opinions? Does it need to stop and think "I'm a male, that's a female," or vice versa? All it needs are visual and olfactory cues, and the hypothalamus does the rest.

There are suggestions by some workers, from studies of rats, that sexual dimorphism is a gradual process through pregnancy. They propose three stages: the sex centres, then mating centres, then the gender role centres. This last stage determines personality traits such as aggressiveness, sociability or individualism, adventurousness or timidity. However, since many homosexual men demonstrate a very strong male identity, one might consider is that what is described is a transgendered, rather than a specifically homosexual personality. Clearly, too, these traits can only be assessed in relation to a particular social norm.

There is a study that discovered that a disproportionately large proportion of homosexual boys were born in Germany near the end of the war. Considering the lengths that Hitler had gone to in eliminating homosexuals, along with other 'undesirables' this seems particularly ironic. It was, at one time, theorised that their mothers would be likely to be under great stress and it seems that, under such conditions, the level of estrogens in women is often high. However, there are all sorts of situational explanations. The boys' fathers would be away fighting, or they were undermined by the grim necessity of survival. They might well have been dead, which might make a mother extremely protective. In any case, the whole social fabric of Germany, particularly Berlin, at that time, was distorted and unnatural.

More men report being homosexual than women, though whether there is an actual difference in the numbers is questionable. It has even been seriously suggested that homosexuality can be predicted by amniocentesis, and prevented by pre-natal injections.

Sexual dimorphism of hypothalamus.

Since it is the hypothalamus that, in conjunction with the pituitary, regulates the gonadotrophins, along with other hormones, it seems reasonable look for sex differences in it.

The problem with excising whole areas is that they are often concerned with a wide range of functions. Attention has shifted to groups of nerve cells, called nuclei - not to be confused with the nucleus of a cell.

Gorski and his colleagues found a group of cells, which they called the sexually dimorphic nucleus, that was much larger in male rats than in females.

Subsequently, Dutch workers found an area in humans, that they named the sexually dimorphic nucleus of the preoptic area. Unfortunately, it had already been named the nucleus intermedius but, in any case, no one else was able to replicate the findings.

LeVay(3) claims that various areas in the hypothalamus elicit different behaviours. The medial preoptic area leads to mounting behaviour, though not apparently controlling sexual appetite. That is monkeys damaged in this area would still masturbate. The event of ejaculation, however, seems to be triggered by another area, the dorsomedial nucleus. He adds that, in females, the region of note is the ventromedial nucleus, which is also involved in feeding behaviour. Some neurons control the likelihood of sexual signals, others control receptive behaviour, with others controlling cyclic hormone effect. In female primates removal of ovaries, and hence the supply of hormones, does not reduce sexual appetite. However damage to the adrenal glands which produce androgens in both sexes, does.

However Gorski and Allen discovered three nuclei in the preoptic area that they named the interstitial nuclei of the anterior hypothalamus - INAH 1 through 3. INAH1 was not sexually dimorphic but they claimed that the other two were.

This study was replicated, but I have to return to the point that runs through these essays, that we are dealing with statistical averages. Fausto-Sterling(4) points out that although there was a difference in the average between the sexes, the variation within the sexes was more than ten-fold

LeVay obtained a sample of 41 brains from autopsies of people who had mostly died from AIDS, 19 gay men, 16 heterosexual men and 6 women, and selected INAH3, for study. It turned out to be twice as big in his sample from 'straight' men than from gay men. He took into account that the changes could have been caused by AIDS, but the sizes were similar in his heterosexual men, whether they had died from AIDS or not. He admitted that his sample was small, but within his experiment, the results were statistically sound.

The BST-C - male/female differences.

Particular areas tend to be selected for study because they have large numbers of receptors for gonadotrophins, at least in the various laboratory animals that are studied.

Allen and Gorski(5) directed their attention to part of a group of cells called the bed nucleus of the stria terminalis.

The study involved taking post mortem sections of human brains, age-matched in pairs. That is, each of 13 pairs of roughly the same age was compared. A table of differences by age was compiled and the average taken, so that the result represented the average over a lifetime. The results claimed were that the nuclei of the females were significantly smaller than those of the males, with a volume difference of 2.47.

The relationship between age and the volume of the BST proved not to be significant. That doesn't mean there was no effect. In fact, there was an increase, especially among the male subjects.

The BST-C - transsexuals.

It was the foregoing study that persuaded Zhou(6) and colleagues to look for sexual dimorphism in the transsexual BST. Male to female transsexuals being still fairly rare, the six specimens had been collected over eleven years.

These were compared with specimens from heterosexual women and men, both homosexual and heterosexual. There was regret that specimens could not be obtained from lesbian women, or female to male transsexuals. The subjects were not age matched. Although it was not stated, all appeared to be middle aged, with the transsexuals around 10 years older. The report went to some length to eliminate possible sources of error, such as the effects of hormones during adulthood, and the fact that some subjects had died of AIDS.

The results obtained confirmed the volume difference between men and women, found in the previous study. Results for the homosexual men were comparable with those for the heterosexual men.

However the transsexual BST volumes were in the range of and even smaller those of the women. They, therefore, were also much smaller than those of the homosexual men. The authors concluded a strong relationship between male to female transsexuals and women, and a clear difference from homosexual men.

Once again there was considerable variability; two of the 16 putatively normal heterosexual men were within the transsexual range.

Breedlove, in the same issue of Nature,(6) comments on the complexity of a condition like transsexuality. He points out that it is becoming clear in neuroscience that experience can alter brain structures. We cannot be sure whether the small BSTc caused the people to be transsexual, or whether it was a result of the complexity of the transsexual life.

In all these studies sexual function is confounded with gender identity. Clearly humans, as much as other species, have an awareness of their genitalia and all it entails. However if transsexuals insist that 'who' they are is in their minds and not their gonads, they can hardly welcome this study as the basis of their identity.

Hormones can go
to your head

However, transsexuals have seized on the study as medical proof that it gives them a 'male' or 'female' brain. They should not have to rely on a study (that is likely to be brought into serious question by many researchers), in order to obtain fundamental human rights.

Conclusion from LeVay.

In spite of the reports that appear in the press and in books with a hidden agenda, Gorsky, Zhou and the others are careful to speak of their results being "in the range of" and Doctor LeVay is careful to speak of averages. In other words, people vary. I can do no better than end this chapter with his words:(7)

To many people, finding a difference in brain structure between gay and straight men is equivalent to proving that gay men are "born that way." Time and again I have been described as someone who "proved that homosexuality is genetic" or some such thing. I did not. My observations were made only on adults who had been sexually active for a considerable period of time. It is not possible, purely on the basis of my observations, to say whether the structural differences were present at birth, and later influenced the men to become gay or straight, or whether they arose in adult life, perhaps as a result of the men's sexual behaviour.

This review has necessarily been brief, but there is one more word I would add: If the brain is so susceptible to so much individual variability, there must be a good genetic reason for it.

Bibliography and good reading.

  1. Adams, D., (1971) Defence and territorial Behaviour dissociated by hypothalamic lesions in the rat, Nature, 232, (573-574) also Kruk, M.R., van der Poel, A.M., Meelis, W., Hermans, J., Mostert, P.G., Mos, J., Lohman, A.H.M., (1983) Discriminant analysis of the localisation of aggression inducing electrode placements in the hypothalamus of male rats, Brain Research, 260, (61-79) in Toates, F., (ed) (1992) Biology, Brain and Behaviour: Control of Behaviour, Milton Keynes: Open University Press.
  2. Cookson, W., (1994) The Gene Hunters: Adventures in the Genome Jungle London: Aurum Press
  3. LeVay,S., (1994) The Sexual Brain, (Page 72) Massachusetts Institute of Technology.
  4. Fausto Sterling, A., (1992) Myths of Gender, Biological Theories about Women and Men, (p244) New York: Basic Books
  5. Allen, L.S., Gorski, R.A., (1990) Sex Difference in the Bed Nucleus of the Stria Terminalis of the Human Brain, Journal of Comparative Neurology, 303:697-706
  6. Zhou, J.N., Hofman, M.A., Gooren, L.J.G., Swaab, D., (1995) A Sex Difference in the Human Brain and its Relation to Transsexuality, Nature 378, 68-70,
  7. Breedlove, M., in the same issue Another Important Organ, pp 15-16.
  8. LeVay,S., page 122,

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Bland, J., (1998) About Gender: The Brain
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Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 amended 09.07.98