Thursday, July 08, 2010

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.


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.


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

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At July 08, 2010 5:33 AM, Blogger Disgruntled PhD said...

Thats quite an interesting study.

However, a number of cavaets. What was the size of the negative correlation between disgust ratings and brain activation?

What statistical approach did they use for determining brain regions involved?

What was the threshold for significance?

At July 09, 2010 7:17 AM, Blogger K Ginger said...

I am more curious about how this will be used. I understand why it is important to study and learn the world and the way it works, but I also wonder a sort of "why" to this study. I guess if we study disgust in heterosexuals, we should also do it in homosexuals, but I worry these studies can be used for evil, like all those neurological essays in the 80's/90's about lesbians.

Another thing, I was wondering if perhaps (and this is a big perhaps) homosexual men rated neutral on F-M porn is because of the fact that homosexuals (along with many other minority groups) are not prevalent in mainstream modern American cinema, and when they are, the characters are maybe so stereotypical that they are hard to identify with on a personal level. So, in connection to the heterosexual porn, these homosexual men were perhaps able to "queer" the porn and make it palatable, just because they do the same thing with every movie, television show, and ad that they see in their lives.

Just a theory, but it seemed accurate?

At July 09, 2010 8:17 PM, Anonymous Anonymous said...

The gays were probably neutral on the straight porn because they were paying attention to the guy, while being slightly distracted by the presence of the woman.

At July 10, 2010 1:29 PM, Anonymous Anonymous said...

Hairiness, like wearing glasses, is not sexually appealing because it's the sign of a high IQ geek. Ron Jeremy's mother was a cryptanalyst and his father is a physicist

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

Considering the extent to which anal sex is the main event in porn today I'm not surprised.

At July 11, 2010 1:09 PM, Blogger The Neurocritic said...

Disgruntled PhD - yes, there are a lot of caveats. Left medial frontal gyrus (x =−1; y = 39; z =−12) and left cuneus (x =−1; y =−81; z = 36) were defined as regions of interest after group differences were found (p = 0.002 uncorrected and p < 0.001 uncorrected, respectively):

"ROI analyses were performed by extracting peak values of parameter estimates (betas) from the sgnificant clusters of activation. Correlations between brain activation (peak beta values)and individual scores of disgust were calculated. Only negative correlations were found between the magnitude of MRI signals in the left medial frontal gyrus and scores of disgust in homosexual subjects(p = 0.035); in contrast,no such correlation was found
in heterosexual subjects(p = 0.768)."

At July 11, 2010 1:22 PM, Anonymous Katharine said...

Er, I don't think hairiness indicates anything more than an excess of expression of testosterone in the skin.

But regarding this study, have they shown the same thing to women? What were the reactions of the women shown these films?

At July 11, 2010 1:33 PM, Blogger The Neurocritic said...

Kelly Ginger - I was also interested in how the results would be used, and why the study was conducted (in China) in the first place.

As to the "queering" of the F-M clips and paying attention only to the guy, that seems quite plausible.

Anonymous #2 - the authors didn't describe the types of sexual acts that were depicted in the films.

At July 11, 2010 1:47 PM, Blogger The Neurocritic said...

Katharine - The study did not include women, unfortunately. The authors didn't seem to have any interest in conducting such a study.

At July 12, 2010 2:07 AM, Anonymous Anonymous said...

I'll grant Dr Alias' hypothesis that IQ correlates with hairiness (and DHT) remains unproven.

But it is true that like baldness hairiness is down to DHT not T and women do not like hairy men or bald ones either. Wearing glasses certainly correlates with IQ if you need to be told that and the stereotype of an intelligent person wears glasses

Good head of hair, muscular, and little body hair means high T /low DHT activity and = hunk stereotype while bald = scientist stereotype.

At July 15, 2010 1:16 AM, Blogger Neuroskeptic said...

Do they refer to where they got the porn? How do we know it was any good?...maybe they should have provided it as a Supplementary Video.

At July 15, 2010 2:12 AM, Blogger The Neurocritic said...

A separate group of 16 participants viewed and rated the films in advance. But no word on where the authors found the stimulus materials. This is all they say:

"The type of erotic film representing explicit sexual activity defined three experimental conditions: (1) heterosexual couples (F–M); (2) male homosexual couples (M–M); and (3) female homosexual couples (F–F)."

Perhaps one could write to the authors and ask for a sample? [corresponding author]

At July 19, 2010 3:03 AM, Anonymous Finn Årup Nielsen said...

Here is my critique of the study:

1) Isn't 3 minutes a fairly big block size for a BOLD fMRI block design? I should think they run into all sorts of confounds at that frequency.

2) Their P-values on the Z=3.79 coordinate differs between the text and the table. (<0.001 / 0.003).

3) Z=2.93 is a quite low Z-score. The P-value is 0.002, but in the text they write that they set the statistical threshold on <0.001. Perhaps there is some fiddling around with voxel and cluster uncorrected thresholds?

I would be interested in comments on these issue. I have put them in the Brede Database.

At July 23, 2010 12:09 PM, Blogger Whining Procrastinator said...

Would it be possible to control for empathy? Without even looking at the results, I would have predicted a less intense homosexual disgust response to F-F vs heterosexual disgust response to M-M simply due to empathy. M-M is going to be experienced as more personal and immediate when the audience is male.


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