Is the Y chromosome behind the masculinisation of the brain?

“It is interesting to study individuals with CAIS in an effort to find out more about the role of sex hormones in the differentiation of the brain”, emphasises the scientist.  “Studies on rodents have revealed that besides sex hormones, the genes situated on the sex chromosomes play an important role in the sexual differentiation of the brain. People affected by CAIS can help us to determine whether the Y chromosome leads to a masculinisation of the brain despite the absence of the influence of androgen hormones in this process”, Julie Bakker specifies.  In other words, this syndrome can help scientists to ascertain whether it is the hormones or the sex chromosomes that are responsible for the masculinisation of the human brain.

To find out, Julie Bakker and her colleagues used the mental rotation technique. This particular type of imagery provides indications on the ability of those tested to mentally turn a two-dimensional or three-dimensional image. “We used this technique because there are very clear differences between the capacities of men and women in this type of exercise”, the researcher explains. Indeed, men and women perform in a relatively characteristic manner – for instance, men are faster and make fewer mistakes – but they also use different regions of the brain during this exercise. “Women tend to activate the frontal and temporal lobes while men activate the parietal lobe”, the specialist continues. Both sexes therefore have their own strategy to solve the task!

Typically female brain activity

Three categories of people took part in the mental rotation exercises: 'control’ men and women as well as people suffering from CAIS. “The analyses confirmed the differences in the areas of the brain activated in men and women when they performed the exercise”, Julie Bakker reveals. “And we noticed the same results in people suffering from CAI syndrome as the control women”. Consequently, this research confirms that the differences in brain activity observed in the three categories of subjects result from the effect of sex hormones and not the presence/absence of the Y chromosome.

People affected by androgen insensitivity syndrome therefore have a female reaction to mental rotation tests. Their genetic gender is less important than hormonal exposure during their development in the definition of their sexual identity. While these results already provide a good indication concerning the sexual differentiation of the brain in these people, other tests will help confirm that the overall functioning of the latter is similar to that of women. “We are also studying the volume of grey matter because it is different in men and women. We would like to see what happens in people suffering from CAIS ”, Julie Bakker continues. “In addition, we are exploring the emotional responses of these three categories of people in an effort to understand to which gender CAIS individuals are similar in this respect”.