Mirror Neurons Control Hard-ons?

from MR BEAN IN TOILET

Everyone knows that mirror neurons control the universe. Now, a study by Mourus and colleagues supposedly tells us that mirror neurons control the most important thing in the universe!

Mirror neurons control erection response to porn

14:15 16 June 2008
NewScientist.com news service
Alison Motluk

. . .

Harold Mouras, at University of Picardie Jules Verne in Amiens, France, and his colleagues wanted to understand the cerebral underpinnings of visually-induced erections.

They suspected there might be a role for mirror neurons, a special class of brain cell that fires both when people perform an action and when they observe it being performed.

The researchers invited eight young men into the lab and asked them to view three types of video clips. Along with late-night fishing documentaries and snippets of Mr Bean, the volunteers got to see erotic videos of [XXX]...¹

This isn't the first study to invoke the specter of mirror neurons as a critical aspect of responsiveness to viewing porn. Ponsetti et al. (2006) showed pictures of male and female sexually aroused genitals to gay and straight male and female participants [see An "Endophenotype" For Sexual Orientation? for a full description of that study]. The authors summarized their results as follows:

Consistent with our prediction, the ventral striatum and the centromedian thalamus, showed a stronger neuronal response to preferred relative to non-preferred stimuli. Likewise, the ventral premotor cortex which is a key structure for imitative (mirror neurons) and tool-related (canonical neurons) actions showed a bilateral sexual preference-specific activation, suggesting that viewing sexually aroused genitals of the preferred sex triggers action representations of sexual behavior.

At the time, I said:

Here's a thought. According to Wikipedia,

a mirror neuron is a neuron which fires both when an animal performs an action and when the animal observes the same action performed by another (especially conspecific) animal. Thus, the neuron "mirrors" the behavior of another animal, as though the observer were himself performing the action.

The Neurocritic is as skeptical as anyone about the mirror neuron craze, but if the PMv activity in this experiment is really imitative in nature, or even "empathetic" (instead of motor imagery or motor preparation), then wouldn't same-sex genitals elicit greater activity than opposite-sex genitals, regardless of sexual orientation?

Anyway. Now on to the present study. Here, Mourus et al. went further by using penile plethysmography:

To investigate the hypothesis that the activation of the mirror-neuron system could be part of the neural mechanisms regulating visually-induced sexual arousal, including the erectile response, we examined whether the response of the mirror-neuron system to sexually stimulating video clips is correlated with the erectile response of healthy volunteers.

The participants were ten healthy heterosexual males somewhere between the ages of 18 and 60. The authors hypothesized that

a neural pathway linking the mirror-neuron system to neural structures controlling erection could be represented by the efference of the frontal operculum – a region containing mirror neurons - to the insula.²

The ventral premotor cortex in Brodmann area 6 is posterior to the frontal operculum (Tomassini et al., 2007). Do we really know that the frontal operculum contains mirror neurons? The most [only] definitive evidence for mirror neuron-type activity is from single-unit recording, not from fMRI. Do monkeys even have a frontal operculum? Yes, but it seems to be mostly gustatory.

Mirror Neuron Erection Study a "BOLD" One

...at least, according to Ramachandran:

Vilayanur Ramachandran, at the University of California at San Diego, who also studies mirror neurons, calls it a "bold" study, and congratulates the group on defying the taboo on studying human sexual physiology.

While he thinks it is perfectly plausible that mirror neurons play a role in how porn turns us on, he says more needs to be done to understand what that role is. For a start, he says, a large number of the brain's structures seem to be involved, not just the pars opercularis, and the interaction between these regions in response to porn is unclear.

"It doesn't give you an experimental lever into the problem," he adds.

And while Ramachandran agrees that the timing of mirror neuron activation and erection is probably critical, fMRI isn't accurate enough to show clearly what is going on with these brain regions over such short time frames.

Ramachandran isn't usually one to show restraint in interpreting data, but here he's right that the BOLD signal was correlated positively with the plethysmographic signal in multitude of brain regions. The authors focus on Frontal operculum, Precentral gyrus, Middle frontal gyrus, Postcentral gyrus, Inferior parietal lobule, Postcentral sulcus, Supramarginal gyrus, Anterior insula, and Posterior insula in the main body of the text, but one can...

(see exhaustive list of regions in Tables 1 and 2 of the online electronic supplementary material)

...once they're online (they're not yet).

ADDENDUM: My criticism of this statement, "Pars opercularis (Brodmann area [BA] 44) is a likely homologue of a subdivision of area F5 of monkeys" appears to be justified. In a recent review of 24 fMRI studies, Morin and Grèzes (2008) discovered that

Observing biological actions with a physical target, compared to a visual control showing no action at all, consistently activated the ventral premotor cortex (BA 6), and did so significantly more than observing target-less actions (with the same control). In contrast, the activity in BA 44 ("Broca’s area") was not modulated by the presence or absence of targets. We propose that the ventral precentral gyrus, and not BA 44, shares the visual properties of "mirror" neurons found in area F5 of the macaque brain.

Footnotes

¹ Edited to avoid search engine hits to this blog from terms like "s*roking n**ed women, enjoying f***atio and engaging in interco**se."

² The insula -- it's not just for the concept of fairness any more!

References

MOURAS H, STOLERU S, MOULIER V, PELEGRINI-ISSAC M, ROUXEL R, GRANDJEAN B, GLUTRON D, BITTOUN J. (2008). Activation of mirror-neuron system by erotic video clips predicts degree of induced erection: an fMRI study. NeuroImage DOI: 10.1016/j.neuroimage.2008.05.051.

Although visually-induced erection is a common occurrence in human male behaviour, the cerebral underpinnings of this response are not well-known. We hypothesized that the magnitude of induced erection would be linearly correlated with the activation of the mirror-neuron system in response to sexually explicit films. When presented with sexual video clips, eight out of ten healthy subjects had an erectile response demonstrated through volumetric penile plethysmography. The level of activation of the left frontal operculum and of the inferior parietal lobules, areas which contain mirror neurons, predicted the magnitude of the erectile response. These results suggest that the response of the mirror-neuron system may not only code for the motor correlates of observed actions, but also for autonomic correlates of these actions.

Morin O, Grèzes J. (2008). What is "mirror" in the premotor cortex? A review. Neurophysiol Clin. 38:189-195.

Ponseti J, Bosinski HA, Wolff S, Peller M, Jansen O, Mehdorn HM, Buchel C, Siebner HR. (2006). A functional endophenotype for sexual orientation in humans. Neuroimage 33:825-833.

Tomassini V, Jbabdi S, Klein JC, Behrens TE, Pozzilli C, Matthews PM, Rushworth MF, Johansen-Berg H. (2007). Diffusion-weighted imaging tractography-based parcellation of the human lateral premotor cortex identifies dorsal and ventral subregions with anatomical and functional specializations. J Neurosci. 27:10259-69.