Phantoms in the Brain: Probing the Mysteries of the Human Mind by V. S. Ramachandran, Sandra Blakeslee Page B
Authors: V. S. Ramachandran, Sandra Blakeslee
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genitals. Nearby stimuli evoked sensa−
Figure 2.1 (a) The representation of the body surface on the surface of the human brain (as discovered by Wilder Penfield) behind the central sulcus. There are many such maps, but for clarity only one is shown here.
The homunculus ("little man") is upside down for the most part, and his feet are tucked onto the medial surface (inner surface) of the parietal lobe near the very top, whereas the face is down near the bottom of the outer surface. The face and hand occupy a disproportionately large share of the map. Notice, also that the face area is below the hand area instead of being where it should — near the neck — and that the genitals are represented below the foot. Could this provide an anatomical explanation of foot fetishes'? (b) A whimsical 24
three−dimensional model of the Penfield homunculus — the little man in the brain — depicting the representation of body parts. Notice the gross overrepresentation of mouth and hands. Reprinted with permission from the British Museum, London.
tions in the feet. As Penfield followed this strip down from the top of the brain, he discovered areas that receive sensations from the legs and trunk, from the hand (a large region with a very prominent representation of the thumb), the face, the lips and finally the thorax and voicebox. This "sensory homunculus," as it is now called, forms a greatly distorted representation of the body on the surface of the brain, with the parts that are particularly important taking up disproportionately large areas. For example, the area involved with the lips or with the fingers takes up as much space as the area involved with the entire trunk of the body. This is presumably because your lips and fingers are highly sensitive to touch and are capable of very fine discrimination, whereas your trunk is considerably less sensitive, requiring less cortical space. For the most part, the map is orderly though upside down: The foot is represented at the top and the outstretched arms are at the bottom. However, upon close
examination, you will see that the map is not entirely continuous. The face is not near the neck, where it should be, but is below the hand. The genitals, instead of being between the thighs, are located below the foot.4
These areas can be mapped out with even greater precision in other animals, particularly in monkeys. The researcher inserts a long thin needle made of steel or tungsten into the monkey's somatosensory cortex—the strip of brain tissue described earlier. If the needle tip comes to lie right next to the cell body of a neuron and if that neuron is active, it will generate tiny electrical currents that are picked up by the needle electrode and amplified. The signal can be displayed on an oscilloscope, making it possible to monitor the activity of that neuron.
For example, if you put an electrode into the monkey's somatosensory cortex and touch the monkey on a specific part of its body, the cell will fire. Each cell has its territory on the body surface—its own small patch of skin, so to speak—to which it responds. We call this the cell's receptive field. A map of the entire body surface exists in the brain, with each half of the body mapped onto the opposite side of the brain.
While animals are logical experimental subjects in which to examine the detailed structure and function of the brain's sensory regions, they have one obvious problem: Monkeys can't talk. Therefore, they cannot tell the experimenter, as Penfield's patients could, what they are feeling. Thus a large and important dimension is lost when animals are used in such experiments.
But despite this obvious limitation, a great deal can be learned by doing the right kinds of experiments. For instance, as we've noted, one important question concerns nature versus nurture: Are these body maps on the surface of the brain fixed, or can they change with experience as we grow from newborns to infancy, through
Figure 2.1 (a) The representation of the body surface on the surface of the human brain (as discovered by Wilder Penfield) behind the central sulcus. There are many such maps, but for clarity only one is shown here.
The homunculus ("little man") is upside down for the most part, and his feet are tucked onto the medial surface (inner surface) of the parietal lobe near the very top, whereas the face is down near the bottom of the outer surface. The face and hand occupy a disproportionately large share of the map. Notice, also that the face area is below the hand area instead of being where it should — near the neck — and that the genitals are represented below the foot. Could this provide an anatomical explanation of foot fetishes'? (b) A whimsical 24
three−dimensional model of the Penfield homunculus — the little man in the brain — depicting the representation of body parts. Notice the gross overrepresentation of mouth and hands. Reprinted with permission from the British Museum, London.
tions in the feet. As Penfield followed this strip down from the top of the brain, he discovered areas that receive sensations from the legs and trunk, from the hand (a large region with a very prominent representation of the thumb), the face, the lips and finally the thorax and voicebox. This "sensory homunculus," as it is now called, forms a greatly distorted representation of the body on the surface of the brain, with the parts that are particularly important taking up disproportionately large areas. For example, the area involved with the lips or with the fingers takes up as much space as the area involved with the entire trunk of the body. This is presumably because your lips and fingers are highly sensitive to touch and are capable of very fine discrimination, whereas your trunk is considerably less sensitive, requiring less cortical space. For the most part, the map is orderly though upside down: The foot is represented at the top and the outstretched arms are at the bottom. However, upon close
examination, you will see that the map is not entirely continuous. The face is not near the neck, where it should be, but is below the hand. The genitals, instead of being between the thighs, are located below the foot.4
These areas can be mapped out with even greater precision in other animals, particularly in monkeys. The researcher inserts a long thin needle made of steel or tungsten into the monkey's somatosensory cortex—the strip of brain tissue described earlier. If the needle tip comes to lie right next to the cell body of a neuron and if that neuron is active, it will generate tiny electrical currents that are picked up by the needle electrode and amplified. The signal can be displayed on an oscilloscope, making it possible to monitor the activity of that neuron.
For example, if you put an electrode into the monkey's somatosensory cortex and touch the monkey on a specific part of its body, the cell will fire. Each cell has its territory on the body surface—its own small patch of skin, so to speak—to which it responds. We call this the cell's receptive field. A map of the entire body surface exists in the brain, with each half of the body mapped onto the opposite side of the brain.
While animals are logical experimental subjects in which to examine the detailed structure and function of the brain's sensory regions, they have one obvious problem: Monkeys can't talk. Therefore, they cannot tell the experimenter, as Penfield's patients could, what they are feeling. Thus a large and important dimension is lost when animals are used in such experiments.
But despite this obvious limitation, a great deal can be learned by doing the right kinds of experiments. For instance, as we've noted, one important question concerns nature versus nurture: Are these body maps on the surface of the brain fixed, or can they change with experience as we grow from newborns to infancy, through
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