The parietal lobes are on the back half of the top of the brain, and extend down each side (see Figure 1). Areas within the parietal lobes serve a wide variety of functions associated primarily with combining sensory information important for perceiving the space around one’s body, as well as for perceiving the body itself.
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 called the primary somatosensory cortex. Activity in this part of the parietal lobes is associated with tactile (touch) perception. When sensory receptors in the skin are activated, people perceive touching, tingling, tickling, burning, or stinging sensations because the information is transmitted to corresponding areas within the somatosensory cortex. Figure 2 illustrates how the tactile receptors from each part of the body map onto specific areas of the somatosensory cortex (see the picture on the left-hand side of Figure 2). 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. (Note: you will learn about the primary motor cortex — pictured on the right in Figure 2 — in Section 3-10.)
The pathways for tactile information from each side of the body cross over in medulla oblongata, which as you learned in Section 3-4 is in the brain stem. Because of this, 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.
The degree to which a part of the body is sensitive to touch depends upon:
- the number of tactile receptors in that part of the body;
- the amount of the somatosensory cortex receiving input from that part of the body.
Figure 2 shows that the most sensitive parts of our bodies, such as our fingers and lips, make up a large amount of the somatosensory cortex, whereas the least sensitive parts of our bodies, such as the back of our necks, make up a small amount of the somatosensory cortex.
People who lose a limb or other body parts through accidents, disease, or surgery often continue to perceive tactile sensations that seem to arise from the missing body part. This phenomenon is referred to as phantom-limb sensations (James, 1887; Ramachandran & Hirstein, 1998). 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). This bizarre case can be understood by examining the map of the somatosensory cortex in Figure 2: the area for the foot is next to the area for the genitals. So what does this mean? It seems that, when this man lost his foot, the brain cells that receive sensory input from the penis developed connections to those that reccieved input from the now-missing foot. After these connections formed, sensations involving the penis, such as those from an orgasm, activated cells in the foot area of his somatosensory cortex, which caused him to experience the orgasm as occurring in his missing foot.
This example illustrates a phenomenon referred to as brain plasticity — the ability of the brain to respond to environmental events or physical damage by forming new connections among its cells. In the case of phantom limbs, these new connections are most likely to form between adjacent areas of the somatosensory cortex. For example, look at the map of the somatosensory cortex in Figure 2, and try to figure out which part of the body is most likely to form new connections to the thumb and index finger of the missing hand. New connections are most likely to form between the areas for these two fingers and areas for parts of the face.
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 (1985) 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.
Body Integrity Identity Disorder
The mental image that people suffering from Body Integrity Identity Disorder (BIID) have of their bodies do not match the way their bodies actually look. To be specific, the mental image is that of an amputee: their view of themselves is that they are missing a limb that actually is there. It does not seem to be a psychotic delusions (Blom, Hennekam, & Denys, 2012), and other than the distorted view of their bodies, they generally do not suffer from severe mental disorders (although depression may develop because of the mismatch between their body image and the actual appearance of their bodies). People with BIID have a strong desire either to amputate the limb or to cause it to become paralyzed. The following video shows a man with BIID.
Study Questions for Section 3-8
- Where is the primary somatosensory cortex located?
- What kinds of information are processed by the primary somatosensory cortex?
- Which hemisphere of the primary somatosensory cortex is activated by tactile stimuli on the left side of the body?
- Which hemisphere of the primary somatosensory cortex is activated by tactile stimuli on the right side of the body?
- What determines how sensitive a part of the body will be to touch?
- What are phantom-limb sensations?
- What is plasticity and how is it important for the development of phantom-limb sensations?
- How would you describe the main functions of the somatosensory cortex and related areas in the parietal lobes?
- What is “left-side neglect” and what is its primary cause?
Damasio, A. (1994). Descartes’ error: Emotion, reason, and the human brain. New York: Avon Books.
James, W. (1887). Consciousness of lost limbs. Proceedings of the American Society for Psychical Research, 1, 249-258. Retrieved September 18, 2011, from http://psychclassics.yorku.ca/James/lostlimbs.htm
Ramachandran, V. S., & Blakeslee, S. (1998). Phantoms in the brain: Probing the mysteries of the mind. New York: William Morrow.
Ramachandran, V. S., & Hirstein, W. (1998). The perception of phantom limbs: The D.O. Hebb lecture. Brain, 121, 1603–1630.
Sacks, O. (1985). The man who mistook his wife for a hat and other clinical tales. New York: Harper & Row.