Monday, August 10, 2009

A New Clitoral Homunculus?

Homunculus image from Reinhard Blutner.

OK kids, let's start today's lesson by viewing the G-Rated [i.e., genital-less] flash explanation of homunculus.

The neuroanatomical definition of homunculus is a "distorted" representation of the sensorimotor body map (and its respective parts) overlaid upon primary somatosensory and primary motor cortices. The above figure illustrates the sensory homunculus, where each body part is placed onto the region of cortex that represents it, and the size of the body part is proportional to its cortical representation (and sensitivity). It's rare to see the genitals represented at all. And if they are present, they are inevitably male genitals. To remedy this puritanical and androcentric situation, Swiss scientists at University Hospital in Zurich conducted a highly stimulating study in 15 healthy women to map the somatosensory representation of the clitoris (Michels et al., 2009).

The authors begin by reviewing the work of Wilder Penfield et al.:
During the last 70 years the description of the sensory homunculus has been virtually a standard reference for various somatotopical studies (Penfield and Boldrey 1937; PDF). This map consists of a detailed description of the functional cortical representation of different body parts obtained via electrical stimulation during open brain surgery. In their findings they relied on reported sensations of different body parts after electrical stimulation of the cortex. Assessment of the exact location was generally difficult and sometimes led to conflicting results. The genital region was especially hard to assess due to difficulties with sense of shame.
Recent studies have tried to map the somatosensory represenation of the human penis using neuroimaging methods, but there has been disagreement over whether it shows the classic medial representation seen in the figure above, or a more laterally located representation in the postcentral gyrus. For example, Kell et al. (2005) noted that...
...classical and [some] modern findings appear to be at odds with the principle of somatotopy,1 often assigning it to the cortex on the mesial wall. Using functional neuroimaging, we established a mediolateral sequence of somatosensory foot, penis, and lower abdominal wall representation on the contralateral postcentral gyrus in primary sensory cortex and a bilateral secondary somatosensory representation in the parietal operculum.
But there are no comparable fMRI studies of female genitalia. So how is such a study conducted, methodologically speaking? Electrical stimulation of the dorsal clitoral nerve was compared to electrical stimulation of the hallux (big toe). It was all very clinical, no sexual arousal involved. Here's the experimental protocol:
Prior to the imaging session, two self-attaching surface disc electrodes (1 × 1 cm) were placed bilaterally next to the clitoris of the subjects so that we were able to stimulate the fibers of the dorsal clitoral nerve. Before the start of the experiment, electrical test stimulation was performed to ensure that subjects could feel the stimulation directly at the clitoris. In addition, the strength of electrical stimulation was adjusted to a subject-specific level, i.e. that stimulation was neither felt [as] painful nor elicited – in case of clitoris stimulation – any sexual arousal (see below). Functional imaging was performed in a block design with alternating rest and stimulation conditions, starting with a rest condition. ... In addition to the clitoris stimulation, we performed in eight of the recorded subjects a second experimental session, in which we applied electrical stimulation of the right hallux using the same type of electrodes, stimulation and scan paradigm.
If you "see below" in the Methods you'll discover that after the fMRI session, participants rated their level of sexual arousal and discomfort on a visual analogue scale that ranged from -10 (unbearable pain or strong sexual arousal) to 10 (pleasure or no arousal at all/sleepiness). The median score for sexual arousal was zero with some variability [range: −7.5 to 8; −2 (25% percentile) and 2.5 (75% percentile)]. The median score for comfortableness was −2 [range: −7 to 9; −2.5 (25% percentile) and 0 (75% percentile)]. C'est la vie.

The neuroimaging results revealed that compared to the rest blocks,
Electrical clitoral stimulation produced significant activations predominantly in bilaterally prefrontal areas (BA 6, 8 and 45), the precentral, parietal and postcentral gyri, including S1 (BA 2 and 3; 40–70% probability) and S2 (BA 43 and ventral BA 40, 30–60% probability). In addition, distributed activations were also seen in the anterior and posterior parts of the insula and the putamen.

Fig. 3 (Michels et al., 2009). Illustration of the random-effect group-activation pattern for the contrast ‘electrical clitoral stimulation versus rest’ (orangeyellow color code; p less than 0.02 uncorrected for multiple comparisons) and for the contrast ‘electrical hallux stimulation versus rest’ (greenblue color code; p less than 0.001 uncorrected for multiple comparisons) on a group average brain. A cluster extent threshold of p less than 0.05 is applied for both contrasts. Electrical clitoral stimulation elicited bilateral activations of lateral surface of S1 as indicated by the white circles.

The major result was similar to the penile homuculus findings of Kell et al. (2005): a failure to replicate the original 1937 studies of Penfield and Boldrey. Although the statistical thresholds here for the clitoral stimulation were not stringent enough, the authors use this to their advantage:
We found no evidence of clitoral representation in the mesial wall, even when using unconventionally low statistical thresholds. This finding is further substantiated by other recent cytoarchitectonic studies revealing that BA 2 does not reach the inter-hemispheric fissure and BA 3 and BA 1 reach the postcentral mesial wall with a probability of only 30% . Our results are also in good agreement with [neuroanatomical] studies on nonhuman primates.
In conclusion, it appears that Michels et al. (2009) have indeed mapped out a new clitoral homunculus, to go along with the new penile homunculus. The standard somatosensory images2 should be revised accordingly.


1 Somatotopy, or somatotopic organization refers to
the maintenance of spatial organisation within the central nervous system. For example, sensory information maintains its structure (i.e. sensory information on the hand remains next to sensory information on the arm) throughout the spinal cord and brain.
Foot fetishes aside, the mapping of the genitals next to the toes is in violation of somatotopic organization.

[like the standard homunculi shown above and below]


Kell CA, von Kriegstein K, Rösler A, Kleinschmidt A, Laufs H. (2005). The sensory cortical representation of the human penis: revisiting somatotopy in the male homunculus. J Neurosci. 25:5984-7.

Michels, L., Mehnert, U., Boy, S., Schurch, B., & Kollias, S. (2009). The somatosensory representation of the human clitoris: An fMRI study NeuroImage. DOI: 10.1016/j.neuroimage.2009.07.024

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At August 10, 2009 9:50 PM, Blogger Jonathan Hepburn said...

I think they're still missing something: There is no way that either diagram accurately depicts the nipples (and breasts).

At August 10, 2009 10:32 PM, Blogger The Neurocritic said...

Yes, both of the homunculus figures are off there...

At August 11, 2009 8:04 AM, Blogger Andy said...


"stimulation was neither felt painful nor elicited... any sexual arousal"

I do not possess a clitoris, but I expect that even in the most clinical of situations, stimulation of my penis would, eventually, result in some sexual arousal.

I see they used a scale to check for arousal: -10 being lots, +10 being none. The range: −7.5 to +8. It's a shame they didn't add this to the analysis!

At August 11, 2009 5:59 PM, Blogger The Neurocritic said...

Andy - I agree it would be interesting to consider the arousal ratings and whether they affected the results. But you'd need to recruit more participants for this. There were only 15 originally, and data from 2 women were dropped from the study because of excessive head motion during the scan.

At August 12, 2009 12:03 PM, Blogger Neuroskeptic said...

It would be nice if they'd stimulated a few more random body parts, to verify that the paradigm was revealing a homunculus rather than, I don't know, making people move around in the scanner when their toe was stimulated because it was a bit uncomfortable.

At August 15, 2009 1:27 PM, Anonymous Anonymous said...

Sorry to change the subject, but a fascinating study on love and hate, Oct. 1 08, in this blog. Questions: Is hate the same as anger, intensified? If so, is anger (I think it is) the symptomological display of depression? (Seems to be so in adolescents.) Is the opposite of love not hate, but fear? Sorry to distract. Now I'm going to read the Homunculus blog. Gary Sturni

At August 22, 2009 7:07 PM, Anonymous Still unmapped and anonymous said...

Gosh, neurocritic, I was all ready to lap this up until I remembered that there was a paper published in the mid-90s that had stimulated research in this area. To wit:

Allison, T., McCarthy, G., Luby, M., Puce, A. and Spencer, D. D. (1996). Localization of functional regions of human mesial cortex by somatosensory evoked potential recording and by cortical stimulation. Electroencephalography and clinical neurophysiology, 100(2):126-40.

It is true that there are few decently conducted studies of the the somatosensory mapping of the genitalia, so I think this study is worthy of mention since it is not an fMRI study but did suggest that the homonculitic representation of the genitalia were displaced towards (wait for it) the cingulate sulcus.

At August 24, 2009 7:52 PM, Blogger The Neurocritic said...

Gosh, Still unmapped and anonymous, I wasn't aware of the Allison et al. (1996) paper, thanks for the reference! From the abstract:

"We describe methods of localizing functional regions of the mesial wall, based on 47 patients studied intraoperatively or following chronic implantation of subdural electrodes. Somatosensory evoked potentials were recorded to stimulation of posterior tibial, dorsal pudendal, median, and trigeminal nerves. ... The foot sensory area occupied the posterior paracentral lobule, while the genitalia were represented anterior to the foot sensory area, near the cingulate sulcus."

And I do appreciate your puns.

At January 02, 2012 6:15 PM, Blogger Nicole said...

I stumbled upon this blog as a woman (with a biology background so I don't mind digging around on a neurology site) who finds her orgasms to be significantly heightened by having the bottoms of her feet held by her partner. It's like night and day, the sensation is so much more intense. I looked around online thinking surely there would be some sex sites with this little tip, but found nothing. I certainly don't know this body of literature, but is there any way that the proximity of the genitals to the feet in the homunculus could be responsible for this? It happens without fail, and I've had male sexual partners that report the same thing. Sorry to bother you with this, but I just can't find any information to explain this unexpected upside of my feet. Sincerely and thank you in advance, Nicole

At April 12, 2019 10:19 AM, Blogger Haven said...

Who is the illustrator of final diagram depicting the female anatomy?


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