Click on a body region to see the two-point discrimination threshold and receptive field size for that area.
Fingertip: ~2mm (highly magnified in S1)
Lip: ~5mm
Palm: ~12mm
Forearm: ~40mm
Back: ~65mm
Threshold reflects cortical representation area in S1 (somatosensory cortex) — not finger/back surface area.
Cortical magnification factor:
The somatosensory homunculus (Penfield 1950) shows that body areas with high spatial acuity (fingers, lips, tongue) occupy disproportionately large areas of cortex.
M = cortical area / body surface area (mm²/mm²)
Fingertip: M ≈ 8–10 mm²/mm²
Back: M ≈ 0.05 mm²/mm²
Ratio: ~200:1
This cortical magnification predicts (and explains) the two-point threshold values — smaller receptive fields → lower threshold → higher M.