Free Mind Hacks™: Tips & Tools for Using Your Brain by Tom Stafford, Matt Webb Page A

left, sensory on the right
    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 objects — two pencils will do — and 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 pencil — you’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 back — your 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.
    ----
    Note
    My back’s about 35 centimeters across, so my homunculus should have a back that’s 9
     units wide (35 divided by 4 centimeters, approximately). Then the palms should be 45
     units across (my palm is 9 centimeters across; divide that by 2 millimeters to get 45
     units). Calculating in units like this will give you the correct scales — the hand in my
     drawing will be five times as wide as the back.
    ----
    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://en.wikipedia.org/wiki/File :
     Sensory_and_motor_homunculi.jpg ), 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.
    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
     processed — different 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