Self-organisation can generate the discontinuities in the somatosensory map

The primary somatosensory cortex contains a topographic map of the body surface, with two notable discontinuities-the representation of the face is next to that of the hands, and that of the feet is next to the genitals. Farah [Why does the somatosensory homunculus have hands next to face and feet next to genitals? a hypothesis. Neural Computation 10(8) (1998) 1983-1985] has suggested that these discontinuities are due to the mechanisms of self-organisation which underlie cortical map development. The typical position of the foetus in the womb means that these two pairs of body parts will often touch and hence their representations will be simultaneously co-active, even though they are distal in terms of the body surface. We use the Kohonen self-organising map algorithm to provide an existence proof of the plausibility of Farah's hypotheses. We then use the model to test the viability of other possible causes of the known map structure and to explore the limitations of self-organisation for explaining the features of the somatosensory map. The model shows that (a) the Kohonen algorithm requires high frequencies of co-activation to introduce a selective discontinuity into the map, and (b) that higher frequency of separate activation of the critical patterns alone is not sufficient to generate the selective discontinuity and (c) the consistency of near-optimal map formation, and in particular the medial-lateral ordering, cannot be reliably generated by a simple Kohonen algorithm.