Foot Fetishes and other fun neurological curiosities

The moment has come to finally talk about the wonderful world of foot fetishes. OK, not just that, but about some interesting cases of neurology that I know about from VS Ramachandran's books, "The Tell-Tale Brain" and "Phantoms in the Brain".

What you see in the image is what is called the "sensory homunculus" - a strip of neurons in the brain whose role is to represent the sensory information from the different parts of the body - the so called "Penfield map". You can see the different body parts represented in the different areas of this strip, some body parts being represented by more neurons than others, hence they appear bigger in the drawing to reflect that.

So what's up with the foot fetishes, so widely spread in the population (from what I'm hearing from the streets)? Well, take a look at the feet and toes representation in the homunculus - they are represented at the "top of the head", in the strip. But right underneath them, the genitals are represented. Now you have to wonder - what would happen if sensory information from the feet or toes would somehow also innervate the genital area of the somatosensory cortex? Of course, it would feel like directly stimulating the genitals. In some people, this is exactly what happens (a similar thing happens with the nipples and the earlobes - they are represented next to each other in the Penfield map).

What's an important piece of evidence that this is the case? Ramachandran once received a phone call. It was a former soldier who stepped on a mine in Irak and got his left leg blown off. He got his leg amputated but the leg area in the brain (which of course remained intact) was now getting inputs from the neighboring areas, like the genitals. So this guy calls Ramachandran and tells him that something weird is happening.

"Doctor, there's something weird happening to me and it's embarassing"

"OK, what's going on?"

"Well... it's weird but... ever since my leg got amputated, my sex life is 10 times better than in the past"

"Really? How come"

"Whenever I have an orgasm, I don't feel it in my penis, I also feel it in my phantom leg", said the patient.

In other words, the sensations from the genitals now were invading the brain areas that didn't get any stimulation from the leg (since the leg got amputated) and they were creating this 10-fold increase in sexual pleasure that the patient was experiencing. This is a completely neurological, non-psychological explanation for foot fetishes.

Something similar but the other way around happens in what's called "apotemnophilia" - the secret desire to amputate one's left hand. Some people have this ever since they were kids and a significant number do eventually amputate their left arm, from a certain specific spot down (they draw with a marker around their arm exactly from where the arm should be amputated). Since this is not legal in the US, these people usually do it in illegal places in Mexico. They then report being much happier after that.

So, are they crazy? Turns out they're not. It's yet again a neurological explanation at hand. If you ask someone like this why they want their arm amputated, they don't say "because I don't feel like it belongs to me". They say "it belongs too much to me". You can then ask them to draw with a marker from which point downwards they want the arm to be amputated. They draw it and then you perform an experiment - connect them to a galvanic skin response device - something that measures one's emotional arousal, similar to a lie detection test. The touch them above the drawn line - nothing happens, a response like in anybody else. But then touch them below the line - a huge galvanic skin response is observed - for them it feels like something super emotional, super significant. It's an explosion of emotion. Therefore, they report as if the arm is "too much" for them.

How is that possible? Turns out that the arm is properly represented on the Penfield map - hence they feel the arm just fine. However, in the next layer of neurons that map the sensation with its associated body part, the hand/arm area is missing. There is no arm representation to be associated to that sensation - there is a "dissonance" going on between experincing the sensation from the arm and representing it as "the sensation of the arm". In other words, their body image doesn't include a left arm from that specific point drawn with the marker, down.

When they eventually amputate their arm, that dissonant sensation goes away (although it can come back as a "phantom limb", later). Here comes the second crazy thing in this already weird story - they prefer sexual partners who also don't have their left hand! Why? Because for them, that's the optimal body map! For them, that second hand is dissonant, like an ugly appendage that doesn't belong there. It's like seeing someone with three hands for us - it looks ugly.

Ask 99%+ of the people about these cases and they will simply call these patients "crazy". But all these cases have absolutely rational, scientific explanations about them - they are not crazy at all. They are rational and reasonable given their conditions.

Here's another one - Charles-Bonnet syndrome. Remember when old people sometimes behave as if they went crazy and are hallucinating people visiting them? They say stuff like "my brother just came in" when their brother has been dead for years or decades. Have they gone nuts? No! Due to their loss of vision, some parts of the visual areas don't send inputs to the visual cortex anymore. But the visual cortex can get inputs from the higher visual areas in the brain - for example from the ventral pathway. The ventral pathway (or the "what" pathway in the brain) can send information to the visual cortex and "fill in" the missing areas that are not stimulated due to vision loss. In those areas, the ventral pathway can simply put the person's brother as a visual data in his or her field of vision.

Yet again the person is not crazy - they are simply tricked by the ventral pathway with a so-called "hallucination" (some people see caricatures or cartoons).

I can recommend VS Ramachandran's books if you want to know more and I might write another of these things in the future if enough people are interested.