The somatosensory cortex, association areas and judging stimulus intensity
- Concept
- Exam priority: medium
The strip of brain behind the central sulcus maps the body; areas behind it turn raw touch into shapes, objects and a sense of your whole body.
Also called: Sensory homunculus, Weber–Fechner law, Parietal association cortex
See it in 3D: Sensory cortex · Thalamus · Middle cerebral artery · Anterior cerebral artery
What it is
Touch and position signals relayed by the thalamus reach the primary somatosensory cortex (SI) in the postcentral gyrus, where the opposite half of the body is mapped in order. Secondary (SII) and parietal association areas combine these signals into texture, shape, object identity and spatial awareness. The cortex also judges how strong a stimulus is, on a compressed (roughly logarithmic) scale, and sends signals back down to adjust the gain of its own relays.
Key facts
- Area 3a receives muscle and joint input, area 3b the core cutaneous input, area 1 texture, area 2 size and shape (touch plus position).
- SII lies in the upper wall of the lateral sulcus, receives input from both sides of the body and helps compare objects by touch.
- Areas 5 and 7 (superior parietal lobule) merge touch, position, vision and hearing to locate the body in space.
- Losing SI impairs fine localization, two-point discrimination, weight and texture judgment; crude pain and temperature are partly kept (thalamus).
- Corticofugal fibres from the cortex to the thalamus, dorsal column nuclei and dorsal horn mostly inhibit relays when input is strong, widening the usable range.
Equations
- ΔI
- smallest detectable change in intensity
- I
- starting intensity
- k
- Weber fraction (constant for each sense)
Holds best in the middle of the intensity range.
- S
- perceived intensity
- I0
- threshold intensity
Perception grows with the logarithm of the stimulus.
- n
- exponent: below 1 for brightness or loudness, above 1 for electric shock
Fits real intensity ratings over a wider range.
Clinical notes
- Anterior cerebral artery stroke: contralateral leg and foot sensory loss and weakness; middle cerebral artery stroke: face and arm.
- Right parietal stroke: the patient ignores the left side of space and body, may deny the deficit, and draws only half a clock.
Test yourself
Think of the answer, then tap the question to see it.
A patient can feel a key placed in the left hand but cannot name it with eyes closed. Where is the lesion most likely?
In the right parietal association cortex (areas 5 and 7 or SII): basic touch is intact, but combining it into object identity (stereognosis) is lost.
Which artery supplies the somatosensory cortex for the contralateral foot?
The anterior cerebral artery, because the leg and foot are mapped on the medial surface of the hemisphere (paracentral lobule).
A person can just tell 100 g from 105 g. By Weber's law, what is the smallest difference they can detect from 1 kg?
About 50 g: the Weber fraction is 5/100 = 0.05, and 0.05 × 1000 g = 50 g.
Why is the cortical area for the fingertips much larger than for the back?
Cortical area follows receptor density and discrimination needs; fingertips have many small receptive fields, the back few large ones.
Connections
See it in 3D4
The structures of Durgical's 3D anatomy where it happens.
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Neuroanatomy, Postcentral Gyrus (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Thalamus (StatPearls (NCBI Bookshelf), 2024); Neuroanatomy, Middle Cerebral Artery (StatPearls (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.