Sex = Mirror Neurons!

We already know that mirror neurons are responsible for social interaction (except when they're not), meaning, art, religion, sports, dinosaurs, sun spots, Marxism, post-it notes, freeze-dried fruit, Harleys, and and Firefox 3.0, so it's not at all surprising that we're now learning that they're responsible for sex as well. Oh, I know, I know, we'd already learned that mirror neurons were responsible for sexual orientation, as I mentioned like two years ago, but we're just now learning that they're responsible for all sex. But we should have known already, right?

Let's start with the sexual orientation paper(1), since it's older. This paper started from the premise that viewing sexually arousing material would activate the reward system (really? who'da thunk?) and the motor system (apparently you have to move to have sex). The authors, Ponseti et al., then hypothesized that, if this premise is true, reward system and motor areas will preferentially respond to stuff that fits with your sexual preferences. That is, if you're heterosexual, heterosexual stuff will produce more activity, and if you're homosexual, then homosexual stuff will produce more activity. So they showed 26 women (12 heterosexual, 14 homosexual) and 27 men (12 heterosexual and 15 homosexual) pictures of sexually aroused male and female genitalia, along with control pictures (from the thoroughly normed International Affective Picture System), while they were hanging out in an fMRI machine. Each participant got to view 30 male genitalia, 30 female genitalia, and 30 control images for 300 ms each. The participants also had to rate the pleasantness of each image.

For all four groups (heterosexual and homosexual males and females), pictures of their preferred sex were rated as more pleasant than either the control pictures of the pictures of the non-preferred sex, and the control pictures were rated as more pleasant than non-preferred sex pictures. This suggests that the manipulation worked. Consistent with their hypothesis, activity in the reward system (specifically the ventral striatum and the thalamus) was greater when the participants looked at their preferred sexual pictures than when they looked at the other pictures. This means that viewing sexual images that you like is rewarding. They also found increased activity in the ventral premotor cortex when viewing the preferred sexual stimuli. This is (thought to be) where motor neurons live, so it stands to reason that we'd conclude from increased blood flow to it that the mirror neurons are involved, as it couldn't possibly be any other type of cell in that region.

Now, if you're like me, you're probably thinking: wait a minute, if mirror neurons respond when you perform an action or see a similar action, wouldn't they respond to pictures of same-sex genitalia, regardless of sexual orientation (I know the Neurocritic is wondering just this)? But hey, it could be that heterosexuals have "other sex's sex organs" mirror neurons, and homosexuals have only "same sex's sex organs" mirror neurons. Hey, that might be what makes us straight or gay! Or it could be, as Ponseti et al. argue, that in order to think about sex, we have to simulate sexual behavior in our brains (or as they put it, activate "action representations of sexual behavior"). My own interpretation is that sex involves moving, and so it's only natural that the motor system would get all hyped up, but perhaps I'm naive.

So, after Ponseti et al., we can conclusively say that sexual orientation is (at least partially) mirorr neurons. Now let's move on to sex in general. With a paper by Mouras et al.(2), we have data that will allow us to do that. In their study, Mouras et al. had ten heterosexual males (I know, I know, this study just uses guys, but gals and gays have mirror neurons too; didn't you read the part about the Ponseti et al. study?) watch porn and control flicks while lying in an fMRI machine, and with a doohickey attached to their penises to measure their erections. Mouras et al. then correlated blood flow in various brain regions with the level of arousal. They found correlations between BOLD signals and erection size in the following regions: right frontal operculum, right middle frontal gyrus (primary motor), postcentral gyri (sensory face), inferior parietal lobules (eye movements and stuff), left supramarginal gyrus (spatial orientation and other stuff), and bilateral insulae (body representation, emtion, etc.).

Of these regions, I know that some people have argued (though nobody really knows) that there are mirror neurons in the insula, somebody (as far as I can tell, without actual evidence) has hypothesized that they're also in the supramarginal gyrus(), and the authors suggest that they're also in the frontal operculum (a suggestion at which the Neurocritic scoffs), so it stands to reason (through rampant speculation, and knowledge that mirror neurons = everything) that this data "suggests the concept of a positive feedback loop, as motivational state may increase the response of mirror neurons, while the response of these neurons could further increase the level of motivation" (4). Furthermore, Mouras et al. argue:

we suggest that a... perception-action coupling mechanism mediated by the mirror-neuron system prompts the observers to resonate with the motivational state of other individuals appearing in visual depictions of sexual interactions, with observers activating motor representations and associated erectile responses associated with the observed depictions. (p. 22)

This would explain why men like strip clubs, 'cause we all know that men who go to them get aroused by "resonat[ing] with the motivational state" of the exotic dancers. My own theory is that what happens is, we see sex, we get aroused, causing our mirror system to simulate sex in our brains, causing us to get more aroused, causing more simulations, and so on. Sounds perfectly reasonable.

Anyway, there you have it. Never mind that we have no idea what's going on in those regions when they're active during sexual arousal, or in at least some cases, whether those regions really are part of the mirror system; I think we can safely conclude that sex = mirror neurons.

¹Ponsetia, J., Bosinkia, H.A., Wolff, S., Peller, M., Jansen, O., Mehdorn, H.M., Büchel, & Siebner, H.R. (2006). A functional endophenotype for sexual orientation in humans. NeuroImage, 33, 825-833.
²Mouras, H., Stoléru, S., Moulier, V., Pélégrini-Issac, M., Rouxel, R., Grandjean, B., Glutron, D., & Bittoun, J. (In Press). Activation of mirror-neuron system by erotic video clips predicts degree of induced erection: an fMRI study. NeuroImage.
³McGeoch, P.D., Brang, D., & Ramachandran, V.S. (2007). Apraxia, metaphor and mirror neurons. Medical Hypotheses, 69(6), 1165-1168.
⁴Mouras et al., p. 17.

Schematic representation of the mirror neuron/simulation theory of sex.