Wilder Penfield mapped the somatosensory cortex in the 1930s by touching the exposed brains of awake patients during surgery and recording where they felt sensation. The resulting map — the homunculus — is not proportional to body size. It's proportional to sensitivity and complexity of use.
Hands and face dominate. The torso barely registers. This is what makes phantom limbs possible: the map persists after the territory is gone. Click a region to see how much cortical space it holds.
Penfield's maps revealed something counterintuitive: the brain allocates cortical space by precision and use, not by the physical scale of the body part. The hands and face together account for roughly half the somatosensory cortex. The entire torso — the largest part of the body — gets a thin strip.
The homunculus is often rendered as a grotesque figure with enormous hands and lips, tiny torso and legs. That image is accurate. Your brain carries a model of your body, and that model is not a faithful miniature. It's a weighted map drawn by decades of fine manipulation and high-density sensing.
This is why phantom limbs persist. The body map is in the cortex, not in the limb. After amputation, the neurons representing the missing hand don't go quiet — they remain, ready, receiving no signals. The sensation of the hand is not a memory or an illusion. It's what happens when the map has no territory to report on, and it keeps reporting anyway.
This is a simplified representation. Penfield's original maps were derived from intraoperative stimulation during epilepsy and tumor surgeries — awake patients, exposed cortex, electrode probes. The maps vary across individuals and are now understood to be more plastic than Penfield knew. The somatosensory cortex reorganizes with experience: musicians have larger representations for their instrument hand; Braille readers expand their fingertip maps. The values here are approximate averages from modern neuroimaging studies.
The model also conflates the primary somatosensory cortex (S1) with the broader map. There are multiple body maps in the brain — S1, S2, the posterior parietal cortex, and others. Phantom limb phenomena involve interactions across all of them.
Based on Penfield & Rasmussen (1950), The Cerebral Cortex of Man, and subsequent neuroimaging studies (Nakamura et al. 1998, Elbert et al. 1995). Cortical area percentages are approximate estimates from the literature, not precisely measured values.