The Skin is Also a Map

I was originally going to start posting on the actual senses with vision, but this is a far better way to go about it!

I’ve had a couple comments on patterns of activity defining the senses. I’m going to use our sense of touch to point out that it isn’t really the patterns of activity that define a sense, but where that activity comes from, and where it goes to.

He's not a freak! He's a Homunculus...

Before you decide to define the above figure as a bit of a freak, just remember, that’s how you have always felt inside.

I’m being serious, that is how you actually feel inside.

Here’s a bit of a history lesson: In the 1950’s a neurosurgeon called Wilder Penfield was trying to find the root of epilepsy in the brain. A lot of modern medicine, and surgery was quite new then, so he thought he would get an electrode out and stimulate the brain to see what happened (hoping he could elicit epilepsy). What he found was a massive shock to the neuroscientific community.

Penfield found that when he touched a specific part of the brain (what is known as the post-frontal gyrus in the temporal/parietal lobe, or the primary sensory cortex) the patient would feel something, like a fluttering in one part of their body. More shockingly Penfield found that if he moved the electrode the sensation would move (e.g. if it started in their hand it could move up their arm, or from their foot up their leg). What he had discovered was a topographic map of the somatosensory world (a fancy name for how we feel things physically).

An overview of the sensory map for touch

This meant that we did not randomly feel the world, but that our brain created a special map that represented our skin surface. This map of the skin surface is in the same place in everyone’s brain, and is called the primary sensory cortex (or somatosensory cortex, see below).

Even more importantly, some areas were bigger than others, giving us the homunculus that you see above. The reason for this was that there are more nerves in those areas. There are more nerves there because we rely on those areas more for our sense of touch. If you think about it, when you want to feel something you use your hands. Babies on the other hand always use their mouths.

Now what about those other squiggly lines...

There’s a simple way to test this. You will need:

  • 2 people
  • 2 pins
  • A blindfold

One of you has the pins, whilst the other wears the blindfold. The one with the pins has to poke the person wearing the blindfold with either 1 or 2 of the pins, they then ask how many pins they used.

You have to use the pins closely together. If you poke them really close together then the person wearing the blindfold won’t be able to tell the difference between 1 or 2 pins. But this changes over different parts of the body. On the hand you will probably have to get the pins within a few millimetres of each other, on the fingertips it will have to be even closer. On areas like the back however it might be difficult for the blindfolded person even if they are centimetres apart.

Please, if you try this out, don’t poke the pins too hard. We don’t want anyone to get hurt! You might want to avoid trying it in the mouth as well…

So what was the relevance of this post to the opening paragraphs?

Well as we can see from this map, there is an area of the brain dedicated to feeling certain parts of the body, no matter how it is stimulated. Patterns of activity will define the intensity and length of a stimulus. But it will not define the stimulus itself. The brain has a special programmed map for that. In fact it has a special map for all of our senses, and normally more than one!

The map for hearing (audition) is pretty simple, each frequency (in Hz) represents a different pitch of noise.

This map of the Visual cortex (for sight) is a bit more complicated… All I’m going to say is that it corresponds to both colour and directions!

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  1. Argus

     /  07/11/2012

    The part that I like most was the (to paraphrase a bit) “his investigations shocked the scientific community”. ‘Twas ever thus, a smug and complacent closed-shop of ‘experts’ fiercely defending their turf gets rocked by an outsider.

    And before long, of course, it becomes part of the expertise and “Hah! We knew that all along!”

    • And you sir have just described Science’s main vice over the last few centuries.

      I dare say that we are getting better at accepting new ideas and ‘paradigm shifts’ as they are no called. But there are still a few who find it difficult. Prof. Watson (DNA discoverer, ish) apparently finds it difficult to believe the importance of epigenetics, an advancement on his own field.

  1. The Skin is Also a Map | Science News |
  2. Sight: The Science of Vision | Science Defined

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