Erotic or Disgusting?

What's hot? What's not? What do you consider unappealing?

A greater understanding of people different from ourselves makes for a more accepting and tolerant populace. Are attempts to deliberately evoke disgust by the sexual practices of "others" an important and worthy step towards achieving this goal? Or does it further stigmatize the minority "outgroup"? What if the "outgroup" is disgusted by the practices of the majority?

Different strokes for different folks
And so on and so on and scooby dooby dooby

Everyday People
------Sly & The Family Stone

Brain Responses to Erotic Films

What are the neural correlates of sexual arousal and disgust in heterosexual men and homosexual men viewing various types of porn (Zhang et al., 2010)? "Where can I sign up?" you say, both as a participant and a researcher. Or maybe you're horrified that such an experiment would be conducted by the scientific establishment. Pornography is a hot-button topic, and a discussion of its potential harms and merits is well beyond the scope of this post.¹

Disgust is considered to be one of the six basic emotions (Ekman, 1992). Given that disgust is a response to things that are physically distasteful or morally repugnant, this emotion has been examined in a specific evolutionary framework: "from oral to moral" (Rozin et al., 2009):

According to the principle of preadaptation, a system that evolves for one purpose is later used for another purpose. From this viewpoint, disgust originates in the mammalian bitter taste rejection system, which directly activates a disgust output system. This primal route (e.g., bitter and some other tastes) evokes only the output program, without a disgust evaluation phase. During human evolution, the disgust output system was harnessed to a disgust evaluation system that responded not to simple sensory inputs (such as bitter tastes) but to more cognitively elaborated appraisals (e.g., a cockroach). ... Later, through some combination of biological and cultural evolution, the eliciting category was enlarged to include reminders of our animal nature, as wel [sic] as some people or social groups.

In a rationale that is simple yet puzzling, Zhang et al. wished to see if the brains of gay men process disgust in a different manner from those of straight men.²

To our knowledge, there have been few studies concerning the [sic] disgust in homosexual men. Whether the patterns of disgust differ between homosexual and heterosexual men is unknown.

The participants were 16 heterosexual and 16 homosexual men (as identified by self-report). Bisexuals were excluded. The stimuli were 3 minute long film clips depicting explicit sexual activity between two men (M-M), two women (F-F), or a woman and a man (F-M). "Each type of erotic film was montaged with attractive short films." Subjects passively watched the films during scanning, then rated their levels of sexual arousal and sexual disgust after the fMRI portion had finished (shown below).


Fig 1 (Zhang et al., 2010). Mean scores of the sexual films showing F–F, F–M and M–M in the two groups. F–F and M–M stimuli induce sexual disgust, respectively. Results of two independent samples test comparisons (homosexual versus heterosexual) are displayed. Blue indicates homosexual men; green, heterosexual men; the asterisk, p less than 0.01. Error bars equal 1 SD. NOTE: the level of sexual disgust was assessed by scores from 1 (extremely high) to 4 (extremely low), and the level of sexual arousal was rated from 6 (extremely low) to 9 (extremely high).

It was no surprise to anyone that straight men were most turned on by F-M film clips and turned off by the M-M films. Straight guys were also a bit turned on by F-F (also not surprising given the popularity of girl-on-girl p0rn), although there was a great deal of variability. Also as expected, gay men were most aroused by M-M films. They rated their disgust as highest for F-F clips but were close to neutral for heterosexual p0rn (interestingly).³

The neuroimaging data were analyzed using Disgust versus Rest as the comparison of interest.

In the homosexual group, the F–F stimulus identified great activity in a large number of brain regions, including the left superior frontal gyrus, right and left medial frontal gyrus, left and right cerebellum, left middle occipital gyrus (BA 19), right lingual gyrus (BA 18), left precuneus, right middle temporal gyrus, left superior temporal gyrus (BA 38), left thalamus, and left supplementary motor area.

In the heterosexual group, M–M stimuli elicited great activations in the left middle frontal gyrus, right middle frontal gyrus (BA 6), left inferior frontal gyrus (BA 45), right inferior frontal gyrus (BA 47), left middle temporal gyrus, right middle temporal gyrus (BA 37, BA 39), left superior temporal gyrus (BA 13), right superior temporal gyrus (BA 38), left inferior occipital gyrus (BA 18), bilateral caudate, bilateral thalamus, bilateral insula, left putamen, right parahippocampal gyrus, right cerebellum, right anterior cingulate (BA 42), and right amygdala.

OK, so that's a bunch of areas that are activated relative to doing nothing (instead of relative to watching a neutral film). I won't try to interpret those results. How about comparing the Disgust vs. Rest responses of the gay and straight men? There was one region of the brain more active in each of the groups: left ventromedial prefrontal cortex for gay men (Fig. 4), and left cuneus [visual cortex] for straight men (Fig. 5).


Fig. 4 (modified from Zhang et al., 2010). Aversive sexual stimuli compared to rest: stronger brain activation in homosexual men compared to heterosexual men in the left medial frontal gyrus (maximum at −1, 39, −12).



Fig. 5 (modified from Zhang et al., 2010). Aversive sexual stimuli compared to rest: stronger brain activation in heterosexual men compared to homosexual men in the left cuneus (maximum at −1, −81, 36). [two-sample t-test, p less than 0.001 uncorrected, extend 5 voxels]


Why?? What does it mean?

The MPFC [media prefrontal cortex] engages in a number of processes, which are potentially common to various emotional tasks (e.g. appraisal/evaluation of emotion, emotional regulation, and emotion-driven decision-making). Notably, ROI analysis showed that the negative correlation was found between magnitude of MRI signals in the left media frontal gyrus and the level of disgust in homosexual men...

Why were the gay men better able to downregulate their emotional responses to unpleasant p0rn [if you believe that negative correlation]? Why would the heterosexual males engage imagery processes to a greater extent than homosexual males when viewing sexual acts they find distasteful? (as suggested by the authors suggest below).

...Activity in this occipital region [cuneus] might reflect more vivid mental imagery induced by viewing disgusting stimuli, specifically when videos were watched... Thus, in the present study, the activation of the left cuneus in heterosexual men may reflect a combination of external viewing and internal generation of disgusting images.

In the absence of a full report on the data that includes additional comparisons (disgusting versus neutral and arousing films), I'll refrain from commenting further. Perhaps other published studies will be able to shed light on the matter, as we'll see in the next post.


Footnotes

¹ For further reading (and a range of viewpoints), see the following:

THIRD of children have seen online porn by the time they are 10, shocking study reveals

Think critically when looking at statistics (A valuable lesson, now with porn!)

Porn: Good for us?

Scientific examination of the subject has found that as the use of porn increases, the rate of sex crimes goes down.

The truth about the porn industry

Gail Dines, the author of an explosive new book about the sex industry, on why pornography has never been a greater threat to our relationships

Finally, anything by Dr Petra on the topic would be recommended...

² Although not explicitly stated, it seems that they wanted to determine whether gay men respond more like straight men or straight women, since the latter have stronger reactions to disgust.

³ Therefore, the gay men in this study were not disgusted by the practices of the majority.


References

Ekman P. (1992). Are there basic emotions? Psychol Rev. 99:550-3.

Rozin P, Haidt J, Fincher K. (2009). Psychology. From oral to moral. Science 323:1179-80.

Zhang, M., Hu, S., Xu, L., Wang, Q., Xu, X., Wei, E., Yan, L., Hu, J., Wei, N., & Zhou, W. (2010). Neural circuits of disgust induced by sexual stimuli in homosexual and heterosexual men: An fMRI study. European Journal of Radiology DOI: 10.1016/j.ejrad.2010.05.021

Mary Millington & co-star Alan Lake share a private moment
in 1978's The Playbirds