[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/areas/neuro/) › The somatosensory cortex, association areas and judging stimulus intensity # 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. Updated Nov 13, 2026 Also called: Sensory homunculus, Weber–Fechner law, Parietal association cortex **See it in 3D:** [Sensory cortex](https://durgical.com/en/anatomy/postcentral-gyrus/) · [Thalamus](https://durgical.com/en/anatomy/thalamus/) · [Middle cerebral artery](https://durgical.com/en/anatomy/middle-cerebral-artery/) · [Anterior cerebral artery](https://durgical.com/en/anatomy/anterior-cerebral-artery/) ## Key points - SI (Brodmann 3, 1, 2) in the postcentral gyrus maps the contralateral body: leg and foot on the medial surface (anterior cerebral artery), face and hand on the lateral surface (middle cerebral artery). - Map size follows receptor density, not body size: lips, tongue and fingers have huge areas. - Thalamic input arrives in layer IV; each vertical column handles one modality from one small skin area. - Parietal association lesions: astereognosis, agraphesthesia; right (non-dominant) parietal lesions cause contralateral neglect. - Weber's law: the just-noticeable change is a constant fraction of the starting intensity, so the scale is compressive. ## 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 **Weber's law**ΔI / I = k **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. **Weber–Fechner law**S = k × log(I / I0) **S** perceived intensity **I 0** threshold intensity Perception grows with the logarithm of the stimulus. **Stevens' power law**S = k × (I − I0)n **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. ## Memory aid Legs hang over the edge into the middle (medial, ACA); the face looks out the side window (lateral, MCA). ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/the-somatosensory-cortex-association-areas-and-judging-stimulus-intensity/) [More Physiology questions →](https://durgical.com/en/practice/?f=physiology) ## Connections ## See it in 3D 4 The structures of Durgical's 3D anatomy where it happens. - [Sensory cortex](https://durgical.com/en/anatomy/postcentral-gyrus/) - [Thalamus](https://durgical.com/en/anatomy/thalamus/) - [Middle cerebral artery](https://durgical.com/en/anatomy/middle-cerebral-artery/) - [Anterior cerebral artery](https://durgical.com/en/anatomy/anterior-cerebral-artery/) ## Related topics 4 - [Somatosensory pathways: dorsal column and anterolateral](https://durgical.com/en/physiology/somatosensory-pathways-dorsal-column-and-anterolateral/) - [Sensory receptors, coding and neuronal circuits](https://durgical.com/en/physiology/sensory-receptors-coding-and-neuronal-circuits/) - [The cerebral cortex and language](https://durgical.com/en/physiology/the-cerebral-cortex-and-language/) - [Position sense, itch and the kinds of somatic sensation](https://durgical.com/en/physiology/position-sense-itch-and-the-kinds-of-somatic-sensation/) ## Related conditions 1 - [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://www.ncbi.nlm.nih.gov/books/NBK549825/) (StatPearls (NCBI Bookshelf), 2024); [Neuroanatomy, Thalamus](https://www.ncbi.nlm.nih.gov/books/NBK542184/) (StatPearls (NCBI Bookshelf), 2024); [Neuroanatomy, Middle Cerebral Artery](https://www.ncbi.nlm.nih.gov/books/NBK526002/) (StatPearls (NCBI Bookshelf), 2024). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/the-somatosensory-cortex-association-areas-and-judging-stimulus-intensity/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/the-somatosensory-cortex-association-areas-and-judging-stimulus-intensity/