A New Penile Homunculus?

Homunculus image from Reinhard Blutner.

Leave it to those wacky writers at LiveScience.com to come up with a sure-to-click headline:

Study: Circumcision Removes Most Sensitive Parts

By Ker Than, LiveScience Staff Writer
posted: 15 June 2007 12:53 pm ET

How much does circumcision alter what a man ultimately feels? Scientific studies aiming to answer this question have been inconclusive.

Now researchers prodding dozens of male penises with a fine-tipped tool have found that the five areas most receptive to fine-touch are routinely removed by the surgery.

The finding, announced today, was detailed in the April issue of the British Journal of Urology (BJU) International.

. . .

Morris Sorrells of National Organization of Circumcision Information Resources Center and colleagues created a “penile sensitivity map” by measuring the sensitivity of 19 locations on the penises of 159 male volunteers. Of the participants, 91 were circumcised as infants and none had histories of penile or sexual dysfunction.

For circumcised penises, the most sensitive region was the circumcision scar on the underside of the penis, the researchers found. For uncircumcised penises, the areas most receptive to pressure were five regions normally removed during circumcision—all of which were more sensitive than the most sensitive part of the circumcised penis.

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).

See the G-Rated [i.e., genital-less] flash explanation of homunculus.

Sorrells and colleagues didn't really map a new penile homuculus onto somatosensory cortex, but they did create a "penile sensitivity map" by determining, at 19 different locations,

the fine-touch pressure thresholds of the penis and quantify[ing] the differences in penile sensitivity between men with and without foreskins.

Here's what they found, as measured "using Semmes-Weinstein monofilament touch-test sensory evaluators (North Coast Medical Supply, Morgan Hill, CA, USA)" [ooh, doesn't that sound sexy]:


Fig 3 (Sorrells et al.): Fine-touch pressure thresholds (g) by location on the adult penis, comparing uncircumcised men (white bars) and circumcised men (gray bars), with a range of one sd shown with the error bars.

The authors' conclusion:

The glans in the circumcised male is less sensitive to fine-touch pressure than the glans of the uncircumcised male. The most sensitive location on the circumcised penis is the circumcision scar on the ventral surface. Five locations on the uncircumcised penis that are routinely removed at circumcision were more sensitive than the most sensitive location on the circumcised penis.

However,

Additional study with vibratory, hot and cold thresholds on a wider variety of positions on the penis is needed.

The paper isn't without its critics (Waskett & Morris, 2007):

Poor methods and erroneous statistical analysis mar this paper; e.g. in their Table 2 they fail to compare the same points on the circumcised and uncircumcised penis. Using their data we find no significant differences (Table 1), consistent with previous findings. Only in their multivariate analysis were P values of apparent significance. They claim that several locations on the uncircumcised penis are significantly more sensitive than the most sensitive location on the circumcised penis (the ventral scar), yet their Table 2 shows this applies only to their position 3, the orifice rim of the prepuce. However, after we used the Bonferroni method to correct for multiple comparisons, this significance disappeared. Statistical naiveté is also apparent in their expression of values to up to four significant figures! [gasp!]

And even better! (ibid):

The authors conclude that ‘circumcision ablates the most sensitive parts of the penis’, although they only tested the ability of subjects to detect the lightest touch. Meissner’s corpuscles, being light-touch receptors, would be expected to cause such a measurement to exaggerate the sensitivity of the prepuce. However, sensitivity, particularly when discussing erogenous sensation, depends on several different modes of stimulation and their interaction. In addition, sexual sensation depends upon the types of mechanical stimulation generated during intercourse, which might in turn be influenced by circumcision status. Thus circumcision has the potential to either increase or decrease sexual sensation.

Surprisingly, the study omitted to address sexual pleasure. The existence of a market for lidocaine-based products to reduce penile sensitivity attests to the desire by some men for a penis with reduced, not heightened, sensitivity...

I could perhaps reproduce one of the explicit drawings that depict the stimulated regions (including the disputed position 3), but... er..., well, hmm, maybe not. This is a family blog.

References

Sorrells ML, Snyder JL, Reiss MD, Eden C, Milos MF, Wilcox N, Van Howe RS. (2007). Fine-touch pressure thresholds in the adult penis. BJU Int. 99:864-9.

OBJECTIVE: To map the fine-touch pressure thresholds of the adult penis in circumcised and uncircumcised men, and to compare the two populations. SUBJECTS AND METHODS: Adult male volunteers with no history of penile pathology or diabetes were evaluated with a Semmes-Weinstein monofilament touch-test to map the fine-touch pressure thresholds of the penis. Circumcised and uncircumcised men were compared using mixed models for repeated data, controlling for age, type of underwear worn, time since last ejaculation, ethnicity, country of birth, and level of education. RESULTS: The glans of the uncircumcised men had significantly lower mean (sem) pressure thresholds than that of the circumcised men, at 0.161 (0.078) g (P = 0.040) when controlled for age, location of measurement, type of underwear worn, and ethnicity. There were significant differences in pressure thresholds by location on the penis (P less than 0.001). The most sensitive location on the circumcised penis was the circumcision scar on the ventral surface. Five locations on the uncircumcised penis that are routinely removed at circumcision had lower pressure thresholds than the ventral scar of the circumcised penis. CONCLUSIONS: The glans of the circumcised penis is less sensitive to fine touch than the glans of the uncircumcised penis. The transitional region from the external to the internal prepuce is the most sensitive region of the uncircumcised penis and more sensitive than the most sensitive region of the circumcised penis. Circumcision ablates the most sensitive parts of the penis.

Waskett JH, Morris BJ. (2007). Fine-touch pressure thresholds in the adult penis. BJU Int. 99:1551-2.

Uncensored image here.
(NSFW)