portional representation for various parts of the body.Another notable feature of the cortical map is that it does not have
the same relative scale between various body regions as does the
actual body. For example, the region of the cortical map that receives
sensory information from the fingers and hand is about the same
size as the region of the map receiving information from much of the
rest of the body—from the arm through the neck, torso, hip, and leg.
The lips and tongue also are represented by an area of cortical surface
larger proportionally to the actual sizes of the lips and tongue on the
body.
These features are related to somatosensory sensitivity. Our fingers
and our lips are the two areas of the body where we have the keenest
sense of touch. Large numbers of somatosensory neurons send den-
drites into the skin of our fingertips and our lips. These dendrites are
densely packed, and the neurons have relatively small receptive fields,
producing a high level of acuity in the sense of touch in these regions.
This allows for the detection of subtle aspects of texture in things we
are touching with our fingertips or with our lips. In contrast, regions
of the body such as the arm, back, or leg have relatively poor so-
matosensory acuity.
Somatosensory acuity can be easily measured using what is called
the two-point discrimination test. This test is best done with an-
other person—it is more fun that way, too. Take a wire paperclip
and bend it into a U-shape so that the two ends of the paperclip
wire can be adjusted to be very close together or farther apart. One
person—the experimental subject—closes her or his eyes. The ex-
perimenter touches the U-shaped wire very gently on the subject’s
skin such that the two ends of the U touch the skin at the same