Central somatotopic map theory
Explanation
The somatotopic map is the organised representation of the body in the brain, especially in the somatosensory cortex (postcentral gyrus) and the primary motor cortex (precentral gyrus). Wilder Penfield, Canadian neurosurgeon, mapped these representations in the 1930s-50s through direct electrical stimulation of the brains of awake patients during epilepsy operations, giving rise to the famous Penfield homunculus.
Each part of the body has its cortical representation, with sizes proportional to the density of receptors and to functional importance, not to physical size. Hands, lips and tongue occupy disproportionately large areas; the back and trunk, comparatively small ones. This reflects the importance of fine manipulation and oral communication in humans.
The map is not static but plastic. Learning a fine skill (playing the violin, reading Braille) expands the corresponding cortical area. After amputation, the area of the lost limb can be invaded by neighbouring areas, producing the sensation of a phantom limb (sensations of the absent limb, often painful). This demonstrates the continuous plasticity of the body map.
For bodily consciousness, the somatotopic map is central. It is the basis of proprioception (knowing where each part of the body is) and of the body schema (the coherent sense of having a body). Alterations of the map produce fascinating pathologies: neglect (ignoring half of the body), somatoparaphrenia (the arm is not mine), heautoscopy (seeing oneself from outside), phantom limb.
Antonio Damasio and others have proposed that representations of the body in the brain (not only somatosensory but also interoceptive: state of internal organs, visceral regulation) are the basis of the proto-self and of primary consciousness. Without a coherent map of the bodily state, consciousness would lose its subjective anchor.
The study of the somatotopic map has had clinical applications (rehabilitation after stroke, neurally controlled prostheses, treatments for phantom limb such as Ramachandran's mirror box). And it has contributed to broader theories of embodied cognition, according to which much of abstract cognition is metaphorically built upon primitive bodily schemes.
Strengths
- Decisive empirical foundations (cortical mappings).
- Example of philosophical honesty in the face of data.
- Lasting clinical contributions.
- Takes first-person experience seriously.
Main critiques
- Interactionist dualism with the classical problems.
- Philosophical conclusions over-interpreting the data.
- Generalisation not supported by later neuroscience.
- Disputed metaphysics.