time. You will find that, when the U is touching the fingertips or
the lips, the ends of the U can be very close together—perhaps as
little as 1 or 2 millimeters apart—and the subject will still be able
to experience two separate points of touch. This represents a high
degree of somatosensory acuity—a result of the densely packed so-
matosensory dendrites connected to many separate neurons each
having small receptive fields and projecting eventually to a region
of the parietal lobe ($1) where large numbers of neurons receive the
signals.
In contrast, if the experimenter touches the U to the subject’s
back, the ends of the U must be moved apart as much as 1 or 2
centimeters (ten times the separation needed for the hand) before
the subject experiences two separate points of touch. If the ends
of the U are close together, say, several millimeters, it will feel like
a single point of touch. This represents a relatively low degree of
somatosensory acuity and reflects many fewer somatosensory re-
ceptor neurons sending signals to the cortex, where fewer cortical
neurons receive and process the information.From the primary somatosensory cortex, information is sent to other
more posterior regions of the parietal lobe. These regions are named
S2,S3,S4,S5, and so on, and are collectively called the secondary
somatosensory cortex. They contain maps of the body, but the things
represented in these maps are far less clear than is the case for S1.
While lesions in S1 produce a simple loss of sensation in a particular
region of the body, lesions in secondary somatosensory cortex are
associated with various kinds of somatosensory weirdness, in which
touch sensations may feel weird and confusing in different ways—
somatosensory agnosias. They may also cause neglect syndromes, in