Somatotopic Cortical Maps
 
 
 
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Chapter 12. Somatotopic Cortical Maps

Demonstration 12.4
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Somatotopic Cortical Maps

Apply tactile stimuli to a human figure and see where signals are received in the somatosensory cortex.

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Stimulation of the somatosensory receptors in a body part sends signals to a specific part of the somatosensory cortex. Proportionally larger areas of the somatosensory cortex are devoted to receiving signals from body parts with higher spatial resolution of touch. The shoulders have relatively low spatial resolution of touch, so the person being massaged in this photo will probably be able to discriminate the touch of the thumb but not of the other individual fingers.
Photo: GIPhotoStock / Science Source.

What Are Somatotopic Cortical Maps?

The somatosensory cortex is located in the postcentral gyrus, a strip at the anterior (front) of the parietal lobe, just behind the central sulcus. It consists of two main subregions—the primary somatosensory cortex (or S1) and the secondary somatosensory cortex (or S2). S1 is itself divided into four side-by-side strips—known as areas 3a, 3b, 1, and 2.

Studies have shown that, within any small region of any of the areas and subareas of the somatosensory cortex, adjacent locations receive tactile signals from adjacent locations on the body surface, forming a somatotopic map on the cortex, but a map that is somewhat disarranged and distorted. It’s disarranged in that larger adjacent regions of the cortex don't necessarily receive signals from adjacent parts of the body—for example, the cortical region that receives signals from the face is adjacent to the region that receives signals from the hand and fingers. And it’s distorted in that relatively more cortical space is devoted to receiving signals from body parts where sensory receptors are densely distributed and have small receptive fields, such as the fingertips and lips, while relatively less space is devoted to body parts with fewer receptors that have larger receptive fields, such as the torso, arms, and legs. The size of the cortical representation of a body part corresponds with the spatial resolution of touch on that part.
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Drag and drop each label into the correct blank box on this illustration of the somatosensory cortex.

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Lateral sulcus
Central sulcus
Postcentral sulcus
Posterior parietal cortex
Area 3b
Area 3a
Area 1
Area 2
Area S1
Area S2
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Signals from somatosensory receptors in each part of the body travel to a specific portion of the somatosensory cortex (area S1). Stimulating the body part results in neural activity in that portion of the cortex and in the perception of the stimulation. In fact, stimulating that portion of the cortex with an electrode would produce the same perception of a sensation in that part of the body, without any actual stimulation of the body. This mapping of body parts onto portions of the cortex defines a somatotopic cortical map. Each subarea of S1 contains essentially the same map.
Click on any part of the body or any portion of the somatosensory cortex to reveal the somatotopic map. Only the left somatosensory cortex is depicted here, which would represent only the right side of the body. But for illustrative purposes, both sides of the body are included.

Show somatotopic map
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What Are Somatotopic Cortical Maps?

The somatosensory cortex is located in the postcentral gyrus, a strip at the anterior (front) of the parietal lobe, just behind the central sulcus. It consists of two main subregions—the primary somatosensory cortex (or S1) and the secondary somatosensory cortex (or S2). S1 is itself divided into four side-by-side strips—known as areas 3a, 3b, 1, and 2.

Studies have shown that, within any small region of any of the areas and subareas of the somatosensory cortex, adjacent locations receive tactile signals from adjacent locations on the body surface, forming a somatotopic map on the cortex, but a map that is somewhat disarranged and distorted. It’s disarranged in that larger adjacent regions of the cortex don't necessarily receive signals from adjacent parts of the body—for example, the cortical region that receives signals from the face is adjacent to the region that receives signals from the hand and fingers. And it’s distorted in that relatively more cortical space is devoted to receiving signals from body parts where sensory receptors are densely distributed and have small receptive fields, such as the fingertips and lips, while relatively less space is devoted to body parts with fewer receptors that have larger receptive fields, such as the torso, arms, and legs. The size of the cortical representation of a body part corresponds with the spatial resolution of touch on that part.
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Which is a correct description of the somatosensory cortex?

A.
It consists of two main subregions, S1 and S2; S1 is divided into three side-by-side strips—areas 1, 2 and 3.
B.
It consists of S1, S2, and a third region divided into four side-by-side strips—areas 3a, 3b, 1, and 2.
C.
It consists of two main subregions, S1 and S2; S1 is divided into four side-by-side strips—areas 3a, 3b, 1, and 2.
D.
It consists of two main subregions, S1 and S2; S2 is divided into four side-by-side strips—areas 3a, 3b, 1, and 2.
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Incorrect. The correct answer is С. Click EXPLAIN if you want to review this topic.
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How is the somatotopic map somewhat disarranged and distorted?

A.
Within any small region of the somatosensory cortex, adjacent locations receive tactile signals from adjacent locations on the body surface.
B.
Large adjacent regions of the cortex don't receive signals from adjacent parts of the body, and the cortical space devoted to a body part increases as the size of the body part decreases.
C.
The spatial resolution of touch in a body part determines the size and location of the corresponding part of the somatosensory cortex.
D.
Larger adjacent regions of the cortex don't necessarily receive signals from adjacent parts of the body, and the cortical space devoted to a body part doesn't necessarily correspond to the size of the body part.
Correct! Click EXPLAIN if you want to review this topic.
Incorrect. The correct answer is D. Click EXPLAIN if you want to review this topic.
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The spatial resolution of touch is about the same on the face and on the nose, and the face has a greater surface area than the nose. What do these facts imply about the area of the somatosensory cortex devoted to the face and the nose?

A.
The area devoted to the face is smaller than the area devoted to the nose.
B.
The two areas are about equal.
C.
Not enough information is given to choose an answer.
D.
The area devoted to the face is larger than the area devoted to the nose.
Correct! Click EXPLAIN if you want to review this topic.
Incorrect. The correct answer is D. Click EXPLAIN if you want to review this topic.
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Somatotopic Cortical Maps.
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