If you touch the computer screen with your right index finger because I ask you nicely with sugar on top, without thinking, you know where your index finger is. Not only that but when you touch the screen you know you are touching it with your right index finger, even if you aren't looking. How does this occur? When you touch the screen, an electrical impulse is sent down the length of your finger, down your hand and arm, up your spinal cord to your brain. This electrical impulse stimulates a certain part of your brain called the somatosensory cortex. Also known as the primary sensory cortex, it is found immediately behind the primary motor cortex. The central sulcus, a deep groove running across the cerebrum, separates the two.
Within the somatosensory cortex is a representation of the human body called the homunculus or " little man". Neurons in this location can identify the area of the body being stimulated by the information they receive from the somatic receptors in the skin. So the impulse sent when you touched the screen gets sent to a particular part of the somatosensory cortex that represents the tip of the right index finger. A particular body region is represented on the cortex with an area that is proportional to the density of touch receptors in the body part, not by its actual size. Since your right index finger is very dense with touch receptors, it takes up a lot of cortex compared to say your arm. Therefore, the neurons form a geometrically distorted projection of the body surface.
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