Prof Dr Jan Hindrik Ravesloot (Amsterdam 1961) studied medicine (Leiden University 1986) and did his Ph.D work at Leiden University (1991). After a three-year post-doctoral period at Yale University (New Haven, CT, USA) he became assistant-professor at the Academic Medical Center-University of Amsterdam (AMC-UvA) Department of Physiology in 1993 and was promoted to full professor in 2001. He and his staff conduct research in ion transport processes in cardiac cells. He is actively involved in advancing (bio)medical (physiology) higher education. (LinkedIn)

This is the second half of a conversation we were honoured to have with Prof Dr Jan Hindrik Ravesloot, if you are interested in reading the first half where we discuss pleasure and assigned female at birth (AFAB) genital anatomy click here.

Conversation:

One of the things I find really interesting is how gender is seen in the brain and how the genitals form quicker than the brain in the womb. So I was thinking about trans people and if there have been studies on the brains of trans people.

Absolutely. So the neuroscience of sexual behaviour is very complex, and, in the context of our conversation, it's very difficult to summarise. It's a field that started maybe 10 years ago, and we now have piecemeal fashion observations on the brains. So people have started to compare cis individuals with trans individuals, and they see all sorts of differences in terms of connectivity in the brain, white matter, thickness, cortical thickness.

There are observable differences in reflexes, very elementary reflexes, between heterosexuals and homosexuals, and it is only as of now that we try to start making sense out of it. The thing is further complicated by the fact that hormone affirmative therapy, so trans woman receives oestrogen and a trans male receives testosterone. Also the administration of hormones start to make changes in the brain, believe it or not. So it is not a sitting duck we are looking at, the flexibility of the brain amazes us for many centuries and also in this field people do not know whether the difference in brain structure arises from an innate differences in brain or the result of the gender affirmative hormone therapy received by the trans person.

I don't know if there's been studies on it. But can they see differences within, like, the foetal brain?

We don't know. That's that's an area of research that has not been trotted.

It is very difficult to study the foetal brain. For obvious reasons, of course, never disturb, growing foetus.

And the other thing is, of course, we have prematurely foetal death. We can study the foetus, and their brains but the speed of development is so quick that you hardly can make inferences about the observations you make. There is some powerful research on the difference between homosexual and heterosexual men, and there we observe systematic differences in some brain nuclei, but we do not observe those differences in female heterosexuals and homosexuals. It's a novel field and it is unclear. We had hoped to find regions in the brain where you can pinpoint where your sexual appetite is located, but I think most scientists have abandoned the idea that there is one single brain portion that determines your sexual appetite.

As of now, there are 9 or 10 or so nuclei, that are associated with sexual behaviour and gender identity but it's a very fascinating field.

So interesting. Especially because, to see the, like, tangible physical evidence of stuff that people, socially say is made up.

Yes. So there is a biological foundation for orientation and gender identity. Absolutely.

It's hardwired in your brain. And, we see homosexuality also in animals. So in the UK, you have this western breed domestic rams, and about 10% of the male western breed domestic rams is homosexual and they have a nucleus in the base of their brain that is called the Sexually Dimorphic Nucleus (SDN) and their Sexually Dimorphic Nucleus is large in heterosexual rams, and it's a small in heterosexual ewes (female sheep).

But these homosexual male rams have a SDN size as large as the ewe’s SDN size. So it turns out that in homosexual male rams, the SDN size is as small as in heterosexual ewe's.

So interesting.

So as of today, we can only look at the size of this nuclei in animal brain preparations, and the same observation was made in heterosexual males and homosexual males. So in heterosexual males, a similar nucleus is large and in homosexual males it is much smaller.

And that led researchers to believe that an equivalent is also present in humans. Its an innate property of the brain.

Is that changeable or that's just what it is? Like, you can't change the size?

In 2024 I would say it's not changeable but you never can exclude what science comes up with. And what and whether that will change the sexual orientation, I don't know.

One of the biggest surprises I got was when I learned about how hormonal therapy changed actually the morphology and anatomy of the brain. And we're doing this sort of “experiments”, with transgender people. So, because transgender people want to resemble, the normal anatomies, they're given hormonal therapy. And as a byproduct, their brain changes. I find it highly interesting.

Do they see changes in sexual behaviour before and after doing hormonal therapy?

I don't know. I'm sure it's been researched, but I haven't looked at it.

Of course, it's, I mean, motivation for sex is multifactorial and I think most current researchers will tell you that the hormones prepare a body for sexual activity. They make the mind responsive to sexual cues, and they make sure that you are capable of conducting successful sexual activity. So hormones set a stage, they provide appetite, they provide lubrication, they provide mature sexual organs. But the next level is actually engaging in sexual activity, and that is a mindset that that has to do with your brain. And in trans individuals, you administer gender affirming hormones. But if that changes their sexual behaviour, I don't know.

You would expect that, but neither, cisgenders nor transgenders are slave of their hormones. So what I would expect is that the answer is presumably very complex. So if I take a trans male, so born female given testosterone, would that increase their sexual activity? Because testosterone is normally related to sex.

I don't know. Because many factors apart from your testosterone level determines whether you're engaged in sexual activity or not. So it's very hard to think of experiments to sort that out. But, the link between testosterone and sexual activity is weak at best. We know if you take it away, if there's no testosterone, there's no sexual activity.

But, what the minimal amount of testosterone should be for successful, normal sexual activity is undetermined. We know there's some sort of threshold level, guys go way up and females probably also surpass that threshold, not as high as the as the guys, but what the threshold exactly is, we don't know.

So there's a lot of uncertainty about the relationship between the testosterone level and sexual activity. The only thing we do know is that in when you castrate or remove the testicles from males or give therapy that is anti-testosterone, the sexual appetite goes to zero. That's one thing. So we know there is a relationship. We also know that in females testosterone, which is coming from the ovaries and the adrenal gland, testosterone stimulates, energises, females and makes them feel good and when you are energetic and you feel good, then you are much more likely to engage in sexual activity. So even there, there is reason, it's circumstantial the evidence that testosterone is related to sexual activty is circumstantial and may not even be directly related to the testosterone level itself.

Finally, regarding the science behind falling in love, as an individual, are there physical aspects we can tangibly see that show the difference between being in love and infatuation?

Oh, yeah. So, in your terminology, infatuation is what we would call the first acute phase of love. So you're extremely absorbed by the person you are in love with.

You call it infatuation, and that is usually a period of a couple of months to maybe a year where you're just madly in love and afterwards, you evolve into a less intense romantic love stage of a long term relationship. Yes. You you can observe difference in the brain, in their behaviour, and in the emotions of people who are infatuated.

So their emotions are different, their behaviour is different, and their cognition is different. So if you are a good observer, you can observe, for example, their behaviour is in the first phase or lust phase or acute phase or infatuation. In the literature, there are many terms for this first period. So their behaviour is always ignoring their normal responsibilities and their behaviour is always looking for contact with the person you are in love with.

So the behaviour is very much goal directed towards the other one. They want to maintain closeness. If situation is bilateral, so there you have two individuals, I mean, you must have seen it in your own network and your friends, you know, that fall in love, you lose them for a couple of months that they are together.

Does it differ between men and women? And what were the main differences?

I think there are no differences. When they are together, their dopamine goes through the roof, and the dopamine is actually stimulating their hedonistic pleasure centre and that's how we start it. So orgasm is the, number one stimulator of the pleasure centre, but when you are in love, when you're infatuated, when you're in acute phase of love, being in the vicinity of your the person you are in love with is giving you a huge dopamine rush, and that gives pleasure. So that that behaviour is perpetuated. So there's a reward on seeking contact and finding contact and being in the vicinity and seeking closeness to your partner.

Is it possible for people to form an addiction to a person if they're getting a dopamine rush every time they are with them?

It's addiction. So the same system that is hijacked by illicit drugs is also used in the acute phase of infatuation. Absolutely.

So if that's suddenly cut off can people exhibit symptoms of withdrawal?

Yes of course. That's what they call the heartache of love is exactly that. They feel terrible and bad, there's a yearning, there's a longing for the other one, but it is frustrating because they do not respond. People just go to pieces, they are absolutely devastated, and it takes time to recover from that.

And it has many parallels. In fact, I had a few students who looked up for the neuroscience between addiction and infatuation, and it's the same system, same neuro-circuitry. So it's all dopamine. It's all, related and of course that makes sense.

So nature is very, economical with their systems. So we have the same system for pleasure of listening to music, pleasure of eating, pleasure of drinking, pleasure of sex, it's all the same, pleasure of alcohol, it's all the same system. And people look for pleasure so they perpetuate the behaviour that leads to pleasure. And in the acute phase of love, pleasure is derived from the fact that you are in a vicinity or you are near the person you are in love with. So that's the behaviour.

Emotions, of course, emotions are very euphoric. So your brain suppresses areas that give you depression and anxiety and fear. The brain up scales the centres that make you happy and euphoric and you are experiencing one of the best times of your life and that is the emotional change in your brain and it can all be recorded in very expensive machines. You can look at the brains of people without opening the skulls and you can see, blood flowing through areas when they look at pictures of the person who they are in love with, acutely in love with, you can see these brain areas light up like a Christmas tree, and these are the areas that normally are associated with euphoria, excitement and happiness.

In cognition, they change because you think of your partner. The cognitive processes are not about arithmetic, not about responsibilities, not about school, not about work, but the cognitive processes are about the other one. So in infatuation phase, you change emotionally, behaviourally, and in your cognition and it's all recordable, and it's all reproducible, and it's in all cultures. It's a very basic biological mechanism.

We are so grateful to Prof Dr Jan Hindrik Ravesloot for sitting down with us and answering our questions.

click here to read the first half of the conversation where we discuss pleasure and assigned female at birth genital anatomy