Posterior Hippocampus and Sexual Frequency

Fig. 2D (Acevedo et al., 2011). Image and scatter plot illustrating greater response to the Partner (vs. a highly familiar acquaintance) in the region of the posterior hippocampus is associated with higher sexual frequency.

Now there's an unexpected correlation suitable for Valentine's Day. How romantic! Actually, it is romantic because the neuroimaging study by Acevedo et al. (2011) is entitled "Neural correlates of long-term intense romantic love." How do you quantify long-term intense romantic love in an fMRI experiment?

Well, what the study really examined is the brain's hemodynamic response to viewing pictures of a spouse with whom participants were still "madly in love" after an average of 21 years. Over the course of the experiment, subjects repeatedly viewed four different digital photos: Partner, Close Friend (CF), Highly Familiar "Neutral" acquaintance (HFN), and a Low-Familiar Neutral acquaintance (LFN). Specifically,

The protocol implemented a block design of two 12-min sessions each consisting of six sets of four 30-s tasks in an alternating fashion, followed by stimulus ratings. Each session included two alternating images (starting image counterbalanced), interspersed with a count-back task. Duplicating procedures of Aron et al. (2005), Session 1 displayed Partner and HFN images. For the additional control comparisons, Session 2 displayed CF and LFN images. Participants were instructed to think about experiences with each stimulus person, nonsexual in nature.

Yeah, it might be a problem if the participants remembered bouts of sex when they viewed their partners... Fig. 2D shows that activation in a tiny area of the left posterior hippocampus correlated with sexual frequency. The two outliers who had sex every day (or nearly every day)¹ could be driving the correlation -- they certainly had a greater number of memories to choose from, and to suppress. In humans, activity in the posterior hippocampus is sensitive to the familiarity of stimuli that have behavioral relevance (Strange et al., 1999), and is associated with memory for repeated stimuli (Poppenk et al., 2010).

How do Acevedo et al. (2011) interpret this correlation?

Although little is known about the posterior hippocampal region [NOTE: untrue], some studies have shown increased activation in this area in association with hunger and food craving (LaBar et al., 2001; Pelchat et al., 2004), with particularly greater activity shown in obese individuals (Bragulat et al., 2010).

Craving, eh? Not memory? Although the authors would like to think they controlled for familiarity with the Close Friend contrast, it seems to me nearly impossible that a co-worker, sibling, cousin, or friend could fulfill all familiarity criteria except romantic relationship. Furthermore, most of the analyses focused on comparisons between Partner vs. Highly Familiar Neutral ² to match their previous paper (Aron et al., 2005) on the early stages of romantic love (1-17 months in duration).

I could go on about the analysis methods, and whether reporting the single voxel with highest activity is appropriate [see Voodoo Correlations]. Or I could go on about the subject selection criteria: the 17 heterosexual participants (10 women, 7 men, ages 39-67 yrs, married 10-29 yrs) had an annual household income ranging from $100,000-$200,000 (perhaps not representative of the general population).

But what about the main findings? Am I just being a cynic when it comes to love? It's true, some of the expected dopamine/reward areas [ventral tegmental area (VTA) and substantia nigra (SN)] showed greater activation when looking at the long-term Partner, which was very much like what was seen in the young lovers.

Fig. 2A (Acevedo et al., 2011). Individuals self-reporting intense love for a long-term spouse show significant neural activation in dopamine-rich, reward regions of the VTA/SN in response to images of their partner vs a highly familiar acquaintance.

Ultimately, the paper sends a positive message that in certain relationships, the exciting, obsessive, and rewarding period of intense romantic love can last for over 20 yrs, well beyond the typical and oft-cited (oprah.com)³ 18 month to 3 year duration:

IMPLICATIONS
Individuals in long-term romantic love showed patterns of neural activity similar to those in early-stage romantic love. These results support theories proposing that there might be mechanisms by which romantic love is sustained in some long-term relationships. For example, the self-expansion model suggests that continued expansion and novel, rewarding events with the beloved may promote increases in romantic love. Novel, rewarding experiences may use dopamine-rich systems (Schultz, 2001; Guitart-Masip et al., 2010) similar to those activated in this study.

Beyond reporting relationship length (and sexual frequency), the participants filled out questionnaires including the Passionate Love Scale, the Love Attitudes Scale, the inclusion of other in the self (IOS) Scale, and the friendship-based love scale. All indicators suggested that the subjects were still "madly in love" with their partners. Did we really need neuroimaging to tell us that? Maybe...

Image from The Science of Love♥

Footnotes

¹ The mean sexual frequency was 2.2 times a week.

² The HFN has been known about as long as the Partner, but is substantially less close than both the Partner and the Close Friend.

³ If anyone can find a better reference for this than oprah.com or Tennov, D., 1979. Love and limerence. The Experience of Being in Love. Stein and Day, New York -- let me know.

References

Acevedo BP, Aron A, Fisher HE, & Brown LL (2011). Neural correlates of long-term intense romantic love. Social cognitive and affective neuroscience PMID: 21208991

Aron A, Fisher H, Mashek DJ, Strong G, Li H, Brown LL. (2005). Reward, motivation, and emotion systems associated with early-stage intense romantic love. J Neurophysiol. 94:327-37.

Poppenk J, McIntosh AR, Craik FI, Moscovitch M. (2010). Past experience modulates the neural mechanisms of episodic memory formation. J Neurosci. 30:4707-16.

Strange BA, Fletcher PC, Henson RN, Friston KJ, Dolan RJ. (1999). Segregating the functions of human hippocampus. Proc Natl Acad Sci. 96:4034-9.