Hack 12. Build Your Own Sensory Homunculus
All abilities are skills; practice something and your brain will devote more resources to it.
The sensory homunculus looks like a person, but swollen and out of all proportion. It has hands as big as its head; huge eyes, lips, ears, and nose; and skinny arms and legs. What kind of person is it? It's you, the person in your head. Have a look at the sensory homunculus first, then make your own.
1.13.1. In Action
You can play around with Jaakko Hakulinen's homunculus applet (http://www.cs.uta.fi/~jh/homunculus.html; Java) to see where different bits of the body are represented in the sensory and motor cortex. There's a screenshot of it in Figure 1-3.
Figure 1-3. The figure shown is scaled according to the relative sizes of the body parts in the motor and sensory cortex areas; motor is shown on the left, sensory on the right
This is the person inside your head. Each part of the body has been scaled according to how much of your sensory cortex is devoted to it. The area of cortex responsible for processing touch sensations is the somatosensory cortex. It lives in the parietal lobe, further toward the back of the head than the motor cortex, running alongside it from the top of the head down each side of the brain. Areas for processing neighboring body parts are generally next to each other in the cortex, although this isn't always possible because of the constraints of mapping the 3D surface of your skin to a 2D map. The area representing your feet is next to the area representing your genitals, for example (the genital representation is at the very top of the somatosensory cortex, inside the groove between the two hemispheres).
The applet lets you compare the motor and sensory maps. The motor map is how body parts are represented for movement, rather than sensation. Although there are some differences, they're pretty similar. Using the applet, when you click on a part of the little man, the corresponding part of the brain above lights up. The half of the man on the left is scaled according to the representation of the body in the primary motor cortex, and the half on the right is scaled to represent the somatosensory cortex. If you click on a brain section or body part, you can toggle shading and the display of the percentage of sensory or motor representation commanded by that body part. The picture of the man is scaled, too, according to how much cortex each part corresponds to. That's why the hands are so much larger than the torso.
Having seen this figure, you can see the relative amount of your own somatosensory cortex devoted to each body part by measuring your touch resolution. To do this, you'll need a willing friend to help you perform the two-point discrimination test.
Ask your friend to get two pointy objectstwo pencils will doand touch one of your palms with both of the points, a couple of inches apart. Look away so you can't see him doing it. You'll be able to tell there are two points there. Now get your friend to touch with only one pencilyou'll be able to tell you're being touched with just one. The trick now is for him to continue touching your palm with the pencils, sometimes with both and sometimes with just one, moving the tips ever closer together each time. At a certain point, you won't be able to tell how many pencils he's using. In the center of your palm, you should be able to discriminate between two points a millimeter or so apart. At the base of your thumb, you've a few millimeters of resolution.
Now try the same on your backyour two-point discrimination will be about 4 or 5 centimeters.
To draw a homunculus from these measurements, divide the actual width of your body part by the two-point discrimination to get the size of each part of the figure.
That's only two parts of your body. To make a homunculus like the one in Hakulinen's applet (or, better, the London Natural History Museum's sensory homunculus model: http://owen.nhm.ac.uk/piclib/www/image.php?img=87494&cat=6), you'll also need measurements all over your face, your limbs, your feet, fingers, belly, and the rest. You'll need to find a fairly close friend for this experiment, I'd imagine.
1.13.2. How It Works
The way the brain deals with different tactile sensations is the way it deals with many different kinds of input. Within the region of the brain that deals with that kind of input is a surface over which different values of that input are processeddifferent values correspond to different actual locations in physical space. In the case of sensations, the body parts are represented in different parts of the somatosensory cortex: the brain has a somatotopic (body-oriented) map. In hearing, different tones activate different parts of the auditory cortex: it has a tonotopic map. The same thing happens in the visual system, with much of the visual cortex being organized in terms of feature maps comprised of neurons responsible for representing those features, ordered by where the features are in visual space.
Maps mean that qualities of stimuli can be represented continuously. This becomes important when you consider that the evidence for each qualityin other words, the rate at which the neurons in that part of the map are firingis noisy, and it isn't the absolute value of neural firing that is used to calculate which is the correct value but the relative value. (See [Hack #25] on the motion aftereffect for an example of this in action.)
The more cells the brain dedicates to building the map representing a sense or motor skill, the more sensitive we are in discriminating differences in that type of input or in controlling output. With practice, changes in our representational maps can become permanent.
Brain scanning of musicians has shown that they have larger cortical representations of the body parts they use to play their instruments in their sensory areasmore neurons devoted to finger movements among guitarists, more neurons devoted to lips among trombonists. Musicians' auditory maps of "tone-space" are larger, with neurons more finely tuned to detecting differences in sounds,1 and orchestra conductors are better at detecting where a sound among a stream of other sounds is coming from.
It's not surprising that musicians are good at these things, but the neuroimaging evidence shows that practice alters the very maps our brains use to understand the world. This explains why small differences are invisible to beginners, but stark to experts. It also offers a hopeful message to the rest of us: all abilities are skills, if you practice them, your brain will get the message and devote more resources to them.
1.13.3. End Note
1.13.4. See Also