Questions: Primary Sensory Cortices and Somatotopic Organization

Question 1

A violinist practices intensively for years. Neuroimaging shows that S1 cortical territory dedicated to the fingers of their fretting hand has expanded. Which principle best explains this finding?

• A. The cortex generates new neurons to accommodate skilled performers (adult neurogenesis in S1)
• B. Experience-dependent plasticity: neurons receiving more frequent, intense input outcompete neighbors and claim more cortical territory
• C. The sensory homunculus is genetically predetermined and cannot change; the imaging result reflects measurement error
• D. The expansion reflects increased fingertip mass and skin surface area from callus formation

Answer: (B)

The cortical maps in S1 are competitive: neurons with more active inputs expand their territory by strengthening connections and taking over cortical space from less active neighbors. This is experience-dependent plasticity operating on the existing neuron population — not neurogenesis. The violinist's active fingers drive more frequent and intense sensory input, winning cortical competition. This same mechanism explains why amputation causes neighboring representations to invade the vacated cortical territory.

Question 2

Why do the lips occupy far more cortical area in S1 than the entire back, even though the back has a much larger surface area?

• A. The back has more total skin receptors, but they are less densely packed so less cortical area is needed for spatial integration
• B. S1 area reflects innervation density and the computational demand for fine spatial discrimination — lips require high-resolution touch and have many closely-spaced receptors, while the back requires only coarse discrimination
• C. This is an artifact of the surgical stimulation studies by Penfield; the actual cortical representation is more proportional to body size
• D. The back is underrepresented because it sits at the edge of the homunculus where cortical space runs out

Answer: (B)

The sensory homunculus is not a scale model of the body — it is a map of *computational demand for discriminative touch*. Lips and fingertips require the ability to distinguish stimuli millimeters apart, which demands many closely-spaced cortical neurons processing fine-grained spatial information. The back can only resolve stimuli centimeters apart and needs far fewer dedicated neurons. Cortical area tracks innervation density (receptors per unit area of skin), which reflects tactile resolution requirements, not physical body size.

Question 3

The sensory homunculus in S1 represents the body in proportion to physical body surface area, producing an approximately life-like body map on the cortical surface.

• T. True
• F. False

Answer: False

The homunculus is famously grotesque precisely because it is NOT proportional to body size. It is proportional to innervation density — the number and density of sensory receptors per body region. High-discrimination body parts (lips, tongue, fingertips, genitals) are massively over-represented relative to their physical size. Low-discrimination parts (back, thigh, upper arm) are massively under-represented. This distortion is a direct readout of where the nervous system invests precision processing resources.

Question 4

Cortical maps in primary sensory areas can be reorganized following amputation of a body part, with neighboring representations expanding into the vacated cortical territory.

• T. True
• F. False

Answer: True

This is one of the most striking demonstrations of adult cortical plasticity. Following amputation, input from the amputated body part ceases, and the cortical neurons formerly driven by that input begin responding to neighboring body parts instead. The 'deprived' cortical territory is progressively taken over by active neighboring representations. This reorganization can be substantial — in some studies, stimulation of the face activates cortical regions formerly representing a missing hand, because facial skin lies adjacent to the hand representation in S1.

Question 5

Why does cortical area in S1 reflect innervation density rather than body part size, and what does this reveal about S1's function?

Model answer: S1's function is spatial discrimination of touch — detecting where on the body a stimulus occurred and distinguishing nearby stimuli. Fine discrimination requires many closely-spaced cortical neurons, one for each closely-spaced receptor. High-discrimination body regions (fingertips, lips) pack many receptors into small skin areas; each receptor needs its own cortical representation to preserve spatial resolution. Low-discrimination regions (back) have widely spaced receptors and need few cortical neurons. Cortical area therefore reflects the resolution requirements of touch perception, revealing that S1 is organized around perceptual precision, not anatomy.

This principle — that cortical real estate tracks computational demand rather than physical size — is a general organizing principle of sensory cortex. It applies in V1 (central visual field is over-represented relative to periphery because central vision requires finer spatial resolution) and in other primary sensory areas, not just S1.