SI Finger Representation

Despite limitations- brain imaging studies of the SI finger representation examine a number of features- such as overlapping representations from adjacent fingers- spatial extent of different finger representations- and the multiplicity of representations. Penfield's classic work illustrates a single hand representation with the thumb occupying a larger extent than that for any other finger- with all of the latter having nearly equal territories.181 Later studies indicate larger representations for the thumb and index fingers in humans233-236 and monkeys.119-152-172

Studies based on MEG or somatosensory evoked potential (SEP) measurements8-221-239 indicate single and unique centers of mass for each finger. Electrically stimulating the proximal interphalangeal joint of each finger activated spatially separable magnetic or electrical field patterns for the initial 20 to 30 msec latency peak potentials. Source localizations- calculated from the equivalent current dipole (ECD) distributions- place non-overlapping foci for fingers 5 to 1- respectively- to a medial to lateral extent of parietal cortex in several studies.8-10-221-239 All recordings reflect responses in or near the central sulcus. One study demonstrates distortions in the SI map by showing a ranking from largest to smallest for- respectively- thumb to finger 5 in the anterior to posterior lengths of ECD distributions.221 In measurements from published figures- the average tangential distance between digits 5 and 1 is 18 mm in 4 subjects8 and 23 mm between electric dipoles in 3 subjects. 221 This suggests approximately 3 to 4 mm sagittal width per digit- if each finger is assumed to have an equal representation. These data- however- do not show whether overlap exists at the boundaries between digit zones.

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