Unterschiede im Gehirn von Männern und Frauen: Warum sie in Bedeutung haben

Ein Artikel In Nature Review Neuroscience stellt dar, warum das Geschlecht wichtig ist für die Neurowissenschaften:

Abstract | A rapidly burgeoning literature documents copious sex influences on brain  anatomy, chemistry and function. This article highlights some of the more intriguing recent discoveries and their implications. Consideration of the effects of sex can help to explain seemingly contradictory findings. Research into sex influences is mandatory to fully understand a host of brain disorders with sex differences in their incidence and/or nature. The striking quantity and diversity of sex-related influences on brain function indicate that the still widespread assumption that sex influences are negligible cannot be justified, and probably retards progress in our field.

Quelle: Why Sex matters for Neuroscience (PDF)

In dem Artikel finden sich einige interessante Punkte. Zunächst erfolgt eine Darstellung von typischen Mißverständnissen:

To best appreciate the evidence for sex influences on the brain, it is helpful to first consider some common misconceptions about the topic. Although not often
stated in print, investigators commonly encounter these misconceptions in their neuroscience colleagues.

The first misconception is that sex influences are small and unreliable. Although there is some evidence for, and some against, this view when purely behavioural studies are considered, there is no evidence to support it regarding sex differences in the brain3. No evidence of which I am aware suggests that the average effect size in the domain of sex influences on brain function differs from the average effect size found in other domains of neuroscience3.

Also durchaus deutlichere Unterschiede. Es zeigen sich vielmehr auch die ansonsten bestehenden durchschnitteln Effektstärken aus anderen Bereichen.

A second, and related, misconception holds that average differences between the sexes result from a few extreme cases in a distribution. Again, I know of no
evidence to support this general conclusion.

Das Argument, dass einige Ausreißer dazu führen, dass Geschlechterunterschiede angenommen werden, würde im Umkehrschluss bedeuten, dass ansonsten die Geschlechter relativ gleich wären. Auch dies ist aber nach dem Artikel nicht der Fall.

A third, also related, misconception holds that the differences within a sex are much more substantial than those between the sexes, the implication being that sex influences can therefore be dismissed as trivial. It is ironic that this view is advanced so often by scientists who use the statistical analyses (for example, ANOVA) that reveal sex influences and that specifically compare within versus between group variance to detect significant differences.

Das dieses Argument in der Tat wenig Aussagekraft hat, dass hatte ich hier auch schon ausgeführt.

A fourth widespread misconception is that all sex differences, once established, can be completely explained by the action of sex hormones, typically oestrogen. The unstated assumption underlying this view is that male and female brains are identical except for fluctuating (and unnecessarily complicating) sex hormone influences. Sex hormones are crucial for many sex differences (BOX 1), but, equally, cannot explain all observed sex differences. For example, a recent study reported several sex differences in cocaine-seeking behaviour in rats and, in addition, found that these differences were unaffected by oestrus state7. The view that circulating sex hormones in adult animals fully account for all sex differences in the brain also ignores the pronounced organizational effects of these hormones on brain development, as well as rapidly growing evidence for genetic mechanisms that induce sex differences in the brain independently of hormone action (reviewed in REFS 8,9).

Das Mißverständnis wäre also, dass die Unterschiede im Gehirn nur auf die gerade zirkulierenden Effekte von Hormonen zurückzuführen sind, während tatsächlich diese einen Teil der Effekte ausmachen, insbesondere aber auch pränatale Hormone erhebliche Veränderungen hervorrufen und auch genetische Einflüsse bestehen

A final misconception holds that if no sex difference exists in a particular behaviour, it can be assumed that the neural substrates underlying that behaviour are identical for both sexes. However, numerous studies report sex differences in neural activity despite no behavioural difference between the sexes. For example, Piefke and colleagues10 examined the neural correlates of retrieval of emotional, autobiographical memories in men and women. Memory performance did not differ between the sexes, nor did the degree of emotion induced by retrieval. However, brain regions associated with retrieval in the two sexes differed significantly. As a second example, Grabowski et al. examined the neural correlates of naming images11. Men and women performed the task equally well, but the patterns of brain activity associated with their performance differed significantly. Findings such as these indicate that isomorphic performance between the sexes does not necessitate isomorphic neural mechanisms. Indeed, as De Vries12 has effectively argued, neural sex differences can, in some cases, create behavioural sex differences, but might, in other cases, prevent them (when, for instance, they would be maladaptive) by compensating for sex differences in other physiological conditions, such as sex hormone levels.

Ein interessanter Punkt. Auch bei gleichem Verhalten kann also die nurologische Grundlage dieses Verhaltens anders ausgeprägt sein. Das macht insofern Sinn, weil bestimmte Verhalten auch für Männer und Frauen in bestimmten Situationen gleich vorteilhaft sind bzw. es nur eine sinnvolle Lösung gibt. Der Teufel kann dann eben im Detail stecken oder dann zum tragen kommen, wenn bestimmte Situationen vorliegen oder höhere Anforderungen gestellt werden.

Functional and structural dimorphisms Sex differences exist in every brain lobe, including in many ‘cognitive’ regions such as the hippocampus, amygdala and neocortex13 (FIG. 1). Sex differences can also be relatively global in nature. For example, widespread areas of the cortical mantle are significantly thicker in women than in men14. Ratios of grey to white matter also differ significantly between the sexes in diverse regions of the human cortex15. In many cases, the differences are not evident in overt anatomical structure, but in some type of functional dimension (hence the distinction above between ‘functional’ and ‘structural’ dimorphisms). For example, a region may differ between the sexes in aspects of its neurotransmitter function, or in its genetic or metabolic response to experience. Furthermore, new methodological approaches — from gene modification in mice to voxel-based morphometry analyses of human imaging data — are revealing previously undetected sexual dimorphisms16–18. It seems that the sexual dimorphisms uncovered so far, abundant as they may be, represent only a fraction of the sexual dimorphisms that are likely to exist in the brain.

Es liegen also in allen Bereichen des Gehirns Unterschiede vor. Dabei muss dies nicht nur in der Struktur geschehen, sondern kann auch in der Funktion deutlich werden. Der Forscher geht dabei davon aus, dass eher der kleinere Teil dieser Unterschiede bisher bekannt ist.

The hippocampus. One region that is evidently sexually dimorphic in its structure and function is the hippocampus, a region perhaps most associated with learning and memory. Extensive evidence demonstrates that male and female hippocampi differ significantly in their anatomical structure, their neurochemical make-up and their reactivity to stressful situations19. Imaging studies consistently show, for example, that the hippocampus is larger in women than in men when adjusted for total brain size20.

Der Hippodampus wird immer wieder gerne auch in der „feministischen Biologie“, also etwa von Cordelia Fine etc erwähnt, weil die Frage, wie er genau zu vermessen ist, schwierig zu beantworten ist und das Ergebnis, dass es hier Geschlechterunterschiede gibt, immer wieder in Zweifel gezogen worden ist. Hieran wird oft exemplarisch angeführt, dass das dann auf die gesamte Gehirnforschung zu übertragen ist, auch wenn andere Gehirnunterschiede wesentlich deutlicher sind

In dem Artikel werden dann auch weitere Unterschiede in der Funktion aufgelistet, die zumindest in Tierstudien festgestellt worden sind. Hier steht wohl weitere Forschung beim Menschen aus.

The amygdala. The medial nucleus of the amygdala has long been known to be sexually dimorphic, a fact that is easily accepted given its role in reproductive behaviour33. However, it is now clear that sexual dimorphism encompasses most, if not all, of the amygdaloid nuclei (…)

Die Amygdala findet hingegen eine weitaus geringere Beachtung in der „feministischen Biologie“. Hier sind die Unterschiede eben recht deutlich

A rapidly growing body of evidence also documents the sexually dimorphic nature of the human amygdala35,36. For example, it is significantly larger in men
than in women (adjusted for total brain size)20. Sex differences also exist in its structural relationship with the rest of the brain. In a study of a large sample of men and women, the patterns of covariance in the size of many brain structures were ‘remarkably consistent’ between men and women, with one exception — the amygdala (in particular, the left hemisphere amygdala), which showed several marked sex differences18.

Several studies now report sex influences on amygdala function, including in the context of its well-known role in memory for emotional events. (…)

Another, more recent study44 used fMRI to examine amygdala responses to fearful faces in men and women. (…)

There are also intriguing parallels between this functional lateralization of amygdala function in healthy individuals and amygdala dysfunction in certain disease
states. For example, women with Turner syndrome (who lack an X chromosome) show reduced responsiveness of the left hemisphere amygdala to emotional material46, whereas mainly female samples of patients with depression show heightened left hemisphere amygdala activity47, as do women with irritable bowel syndrome48.
In summary, the available evidence indicates that studies of amygdala function risk conclusions that are incomplete at best, and wrong at worst, if they fail to address potential influences of both sex and hemisphere

Es zeigen sich hier also deutliche Unterschiede sowohl in Struktur als auch Funktion und dies auch gerade in dem Bereich Emotionen.

Im Ganzen zeigt der Artikel, dass die Gehirne von Männern und Frauen erhebliche Unterschiede aufweisen und weitere Entdeckungen dieser Art zu erwarten sind. Das Gehirn von Männern und Frauen unterscheidet sich dabei auf vielen Ebenen, die nicht nur die reine Form betreffen.