The parietal lobes are on the back half of the top of the brain, and extend down each side (see Figure 1 in Section 4-3). When surgeons electrically stimulate a strip of tissue running along the anterior (front) part of the parietal lobes, people typically experience tingling or burning sensations of the skin. This area is referred to as the primary somatosensory cortex, and is associated with the perception of tactile (touch) stimuli. When sensory receptors in the skin are activated, people perceive touching, tingling, tickling, burning, or stinging sensations. As can be seen in Figure 1, the tactile receptors from each part of the body map onto specific areas of the cerebral cortex. For example, tactile receptors from the eyes and index finger map onto a part of the parietal lobes immediately adjacent to touch receptors from the thumb. The pathways for tactile information from each side of the body cross over in the medulla, which you learned about in Section 4-1: the somatosensory cortex in the left hemisphere receives tactile sensations from the right side of the body and the somatosensory cortex in the right hemisphere receives tactile sensations from the left side of the body.

Figure 1. The primary somatosensory cortex and the parts of the body from which each cortical section receives sensory input (the picture appears at this link)

The degree to which a part of the body is sensitive to touch depends upon:

(a) the number of tactile receptors in that part of the body;
(b) the amount of somatosensory cortex receiving information from that part of the body.

Figure 1 shows that the most sensitive parts of our bodies, such as our fingers and lips, take up a large amount of the somatosensory cortex, whereas the least sensitive parts of our bodies, such as the back of our necks, take up a small amount of the somatosensory cortex.

People who lose a limb through accidents, disease, or surgery often continue to perceive tactile sensations that seem to arise from the missing body part; and sometimes they even experience severe pain in the missing body part. This phenomenon is referred to as phantom-limb sensations (James, 1887; Ramachandran, 1999). An unusual example of phantom-limb sensations involves a case in which a man, whose foot had been amputated, experienced orgasms in his missing foot (Ramachandran & Blakeslee, 1998)! If you examine Figure 1, you can see why this occurred: the area of the somatosensory cortex for the foot is next to the area for the genitals. It seems that, when this man lost his foot, the brain cells that make up the area for the penis developed connections to those that make up the area for the foot. Now, when his penis is stimulated, especially when experiencing an orgasm, the cells in the foot area of his somatosensory cortex are activated, which causes him to feel as if the orgasm is occurring in his (phantom) foot. This illustrates a phenomenon referred to as brain plasticity — the ability of the brain to respond to environmental events and physical damage by forming new connections among its cells. In general, phantom-limb sensations seem to be due to the development of connections between nearby areas of the somatosensory cortex and the area associated with the missing body part. When the nearby areas are activated by tactile stimuli, the person may feel sensations in the missing body part.

If the somatosensory cortex is damaged, there typically is only a temporary loss of the sense of touch. The more permanent problem involves difficulties with properly interpreting tactile information. For example, people with damage to the somatosensory cortex often have problems identifying objects by touch and also may become clumsy because of their difficulties with perceiving the objects they handle.

Areas around the somatosensory cortex are involved in perceiving several other kinds of bodily information. In general, they seem to be important for sensing and orienting our bodies in space. For example, when the right parietal lobe is damaged, people sometimes do not perceive the left side of their bodies: they act as if the left side does not exist, a phenomenon referred to as left-side neglect. Left-side neglect also often includes a lack of awareness of their surroundings on the left side. Thus, the symptoms of severe left-side neglect generally include the following: people fail to dress the left side of the body, ignore anything in the environment on that side of the body, and/or feel as if parts of their bodies on the left side do not really belong to them. Oliver Sacks (1987) described one such case in a man who showed up at a neurology clinic because of a “lazy” left leg:

He had felt fine all day, and fallen asleep towards evening. When he woke up he felt fine too, until he moved in the bed. Then he found, as he put it, ‘someone’s leg’ in the bed — a severed human leg, a horrible thing! He was stunned, at first, with amazement and disgust — he had never experienced, never imagined, such an incredible thing. He felt the leg gingerly. It seemed perfectly formed, but ‘peculiar’ and cold. ... [H]e threw the damn thing out of the bed. But ... when he threw it out of bed, he somehow came after it — and now it was attached to him. ... He seized it with both hands, with extraordinary violence, and tried to tear it off his body, and, failing, punched it in an access of rage. (pp. 55-56)

Even at this point, the man did not realize that the leg was his own. When asked where his own left leg was, he answered: “I don’t know. ... I have no idea. It’s disappeared. It’s gone. It’s nowhere to be found” (p. 57). When the somatosensory cortex in the left hemisphere is damaged, on the other hand, the resulting problems seem to be much more specific to the sense of touch.

Anosognosia is a word derived from the following: the Greek word, nosos, which means disease; the Latin prefix, a-, which means not or lack of; and the Greek word, gnosis, which means knowledge. Putting all of this together, we can define anosognosia as a disorder in which a person is unaware (does not know) that he or she has a (neurological) disease. People who receive damage to the parietal lobes in the right hemisphere (in addition to other areas of the cerebral cortex) sometimes show paralysis of the left side of the body but seemingly have little or no direct awareness that they are paralyzed. The neurologist, Antonio Damasio (1994), described such an anosognosic woman under his care (referred to by her initials, DJ):

Whenever I asked my patient DJ about her left-side paralysis, which was complete, she would always begin by saying that her movements were entirely normal, perhaps that they had once been impaired but they no longer were. When I would ask her to move her left arm, she would search around for it and, after looking at the inert limb, ask whether I really wanted "it" to move "by itself." When I would say yes, please, she would then take visual notice of the lack of any motion in the arm, and tell me that "it doesn't seem to do much by itself." As a sign of cooperation, she would offer to have the good hand move the bad arm: "I can move it with my right hand." (p. 63)

This passage shows another troubling feature of her anosognosia. In addition to DJ's lack of awareness of her physical condition, she also expressed no concern when she noticed that she couldn't move her left arm: she experienced no distress over what should be a distressing discovery. According to Damasio (1994), anosognosics in general, show little or no emotion when their impairments are pointed out to them. [NOTE: I NEED TO FINISH THIS PART. STUDENTS, DO NOT WORRY ABOUT ANYTHING AFTER THIS POINT.]

xxxxxx news that there was a major stroke, that the risk of further trouble in ... (p. 64)

Anosognosia is associated with damage in .... [SEE PAGES 65-66]

Study Questions for Section 4-5

1. Where is the primary somatosensory cortex located?
2. What kinds of information are processed by the primary somatosensory cortex?
3. Which hemisphere of the primary somatosensory cortex is activated by tactile stimuli on the left side of the body?
4. Which hemisphere of the primary somatosensory cortex is activated by tactile stimuli on the right side of the body?
5. What determines how sensitive a part of the body will be to touch?
6. What are phantom-limb sensations?
7. What is plasticity and how is it important for the development of phantom-limb sensations?
8. How would you describe the main functions of the somatosensory cortex and related areas in the parietal lobes?
9. What is "left-side neglect" and what is its primary cause?
10. How would you define "anosognosia" in your own words?

Go to Quiz 4-5 questions

Go to Readings Section 4-6