Sex and the Single Amygdala
Part 2 of Your Sexiest Brain Bits
Posted Nov 05, 2019
For my Ph.D. research, I spent four years investigating changes in sex drive and behaviour after neurosurgery for epilepsy. Some patients reported hypersexuality, or a dramatic increase in sex drive and behaviour after their operations, and even expressed a wish for their partners to have the same operation to ‘keep up with them’ sexually. The purpose of my Ph.D. was to explore why this was happening. Was it because they were having fewer seizures, so they were more confident, more ‘in the mood’ for sex? Or was it to do with the part of the brain that had been removed? Was it perhaps a combination of both these reasons?
I interviewed over 70 people who had undergone epilepsy surgery to find out about their sex lives. More than half had experienced a post-operative change in sex drive. Not all of them had dramatic hypersexual changes; most experienced a subtler increase from their pre-operative hyposexuality, or low sex drive, to what they considered to be a normal frequency of sexual activity.
Sexual changes were more common in people who had temporal lobe surgery than in those who had surgery to other parts of the brain. This was what we had predicted, given that we already knew that the temporal lobe was crucial to the sexual neural network. (Read more here.)
Sexual changes were more commonly reported if the surgery was to the right temporal lobe rather than the left. This supported earlier research on the role of the right side of the brain in emotional processing: Sex involves complex emotions, so it is no surprise that there may be right-sided dominance in mediating sexual function, making sexual changes more likely when there is a dysfunction in the right temporal region. Women are more likely to report hypersexual changes after neurosurgery for epilepsy, and seizures that cause sexual arousal or orgasm are also more common in women.
I also looked at their brain scans before they had surgery. I was particularly focused on their amygdalae. I wanted to know if the size of their amygdalae had any impact on what happened to their sex lives after surgery. Incredibly, I found a significant relationship between the size of the amygdala in the healthy temporal lobe of patients – that is, the opposite or ‘contralateral’ lobe to the one where the seizures were coming from – and the person’s sex drive after they had a unilateral temporal lobectomy.
There was also a significant positive relationship between the volume of the contralateral amygdala and the maximum degree of sexual change. Patients who reported a post-operative increase in sex drive had a significantly larger amygdala in their healthy temporal lobe compared to patients who reported a sexual decrease or no change, and when compared with the control group of healthy people.
These findings provided the first evidence ever recorded for an association between amygdala size and human sexual behaviour. When it comes to amygdala size and sexual outcome after temporal lobectomy, bigger is better.
So why would a bigger amygdala be better? One possible explanation is based on the amygdala’s role in emotional processing. It is thought the amygdala regulates the attachment of emotional significance to things and applies reinforcing or discriminative properties to sensory stimuli. In other words, it helps us decide what or who we should or shouldn’t approach, feel attracted to or run away from, love or hate.
This theory suggests that rather than playing an inhibitory role, the amygdala has a positive effect on sexual behaviour by allowing the appropriate attachment of emotional significance to sexual cues.
A larger amygdala may function better in its role in processing emotional and specifically sexual information and the attachment of significance to it, which would increase the likelihood of a sexual response, resulting in an increased sex drive. A bigger amygdala would help you to notice and feel aroused by a sexual cue and make you more likely to accept that cue and go for it.
What about the cases of hypersexuality after temporal lobe surgery, when some patients’ partners complained that they couldn’t keep up with their loved one’s newfound post-surgical sexual desires? Could that be human Klüver-Bucy syndrome? This is the syndrome that animals who have bilateral amygdala damage show, and includes hypersexuality involving indiscriminate sexual behaviour towards inappropriate objects; for example, they will attempt to mount inanimate objects
Similarly, hypersexual changes in human Klüver-Bucy syndrome involve indiscriminate sexual behaviours, ranging from sexual advances (verbal or physical) toward strangers to homosexual advances that were previously uncharacteristic in the patient. (See here.)
I don’t think the dramatic sexual increase that some people experience after temporal lobe surgery for epilepsy is Klüver-Bucy syndrome, because the the post-operative sexual increase described by the patients who had undergone temporal lobectomy mostly involved an increase in sex drive without indiscriminate sexual behaviour.
Using the term ‘hypersexuality’ in relation to Klüver-Bucy syndrome confuses these two kinds of sexual behaviour – increased sex drive and indiscriminate sexual behaviour – and my research suggests that these are two distinct outcomes that may be caused by distinct neurological mechanisms. The fact that I found a positive relationship between amygdala size and sexual outcome also supports the notion that the patients I studied did not have Klüver-Bucy syndrome; if they did, I would have found a smaller dysfunctional amygdala associated with increased sexual behaviour.
Curing people of their seizures is the main purpose of neurosurgery for epilepsy, but this is not the only outcome relevant to people. Their sex lives matter too, and they need to know that their surgery could change the way they behave sexually. Typically, these changes are for the better, but patients need to be prepared for the possibility of more dramatic changes.
This is an adapted excerpt from Sex in the Brain: How your brain controls your sex life (NewSouth Publishing, 2019; and forthcoming Columbia University Press, 2020)
Baird, A.D., Wilson, S.J., Bladin, P.B., Saling, M.M., & Reutens, D.C. (2004). The amygdala and sexual drive: Insights from temporal lobe epilepsy surgery. Annals of Neurology, 55(1), 87–96.
Baird, A.D., Wilson, S.J., Bladin, P.B., Saling, M.M., & Reutens, D.C. (2002). Hypersexuality after temporal lobe resection. Epilepsy & Behavior, 3(2), 173–181.
Baird, A.D., Wilson, S.J., Bladin, P.B., Saling, M.M., & Reutens, D.C. (2003). Sexual outcome after epilepsy surgery. Epilepsy & Behavior, 4(3), 268–278.