The approximate size of the receptive fields serving light touch can be measured by the two-point touch threshold test. In this procedure, the two points of a pair of calipers are lightly touched to the skin at the same time. If the distance between the points is sufficiently great, each point will stimulate a different receptive field and a different sensory neuron—two separate points of touch will thus be felt. If the distance is sufficiently small, both points will touch the receptive field of only one sensory neuron, and only one point of touch will be felt (fig. 10.5).
The two-point touch threshold, which is the minimum distance at which two points of touch can be perceived as separate, is a measure of the distance between receptive fields. If the distance between the two points of the calipers is less than this minimum distance, only one "blurred" point of touch can be felt. The two-point touch threshold is thus an indication of tactile acuity (acus = needle), or the sharpness of touch perception.
The tactile acuity of the fingertips is exploited in the reading of braille. Braille symbols are formed by raised dots on the page that are separated from each other by 2.5 mm, which is slightly greater than the two-point touch threshold in the finger-
P S The phenomenon of the phantom limb was first described by a neurologist during the Civil War. In this ^ account, a veteran with amputated legs asked for someone to massage his cramped leg muscle. It is now known that this phenomenon is common in amputees, who may experience complete sensations from the missing limbs. These sensations are sometimes useful; for example, in fitting prostheses into which the phantom has seemingly entered. However, pain in the phantom is experienced by 70% of amputees, and the pain can be severe and persistent.
One explanation for phantom limbs is that the nerves remaining in the stump can grow into nodules called neuromas, and these may generate nerve impulses that are transmitted to the brain and interpreted as arising from the missing limb. However, a phantom limb may occur in cases where the limb has not been amputated, but the nerves that normally enter from the limb have been severed. Or it may occur in individuals with spinal cord injuries above the level of the limb, so that sensations from the limb do not enter the brain. Current theories propose that the phantom may be produced by brain reorganization caused by the absence of the sensations that would normally arise from the missing limb. Such brain reorganization has been demonstrated in the thalamus and in the representational map of the body in the postcentral gyrus of the cerebral cortex.
tips (table 10.3). Experienced braille readers can scan words at about the same speed that a sighted person can read aloud—a rate of about 100 words per minute.
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