Two to nine months after the median nerve was transected and ligated in adult owl and squirrel monkeys, the cortical sectors representing it within skin surface representations in Areas 3b and 1 were completely occupied by ‘new’ and expanded representations of surrounding skin fields. Some occupying representations were ‘new’ in the sense that

• (1)
  there was no evidence that these skin surfaces were represented in this region prior to median nerve transection; and
• (2)
  these skin surfaces retained their normal representation elsewhere within these two cortical representations of hand surfaces. Large ‘new’ representations of the dorsal surfaces of digits 1 and 2 (innervated by the radial nerve) and large ‘new’ representations of the hypothenar eminence (innervated by the ulnar nerve) were consistently recorded.

Some surrounding skin surface representations expanded into the former median nerve zone, so that bordering skin surfaces (the ulnar insular palmar pad, the third digital palmar pad, glabrous ulnar digit 3. radial hand dorsum) were represented over far larger than normal cortical areas. These expanded representations of always-innervated skin sometimes appeared to move in entirety into the former median nerve representational zone. Significant changes in the representation of skin surfaces away from the median nerve representational zone (e.g. in the zone of representation of glabrous digit 4) were also consistently recorded. Reorganizational changes following median nerve sections were much more variable in Area 1 than in Area 3b.

The topographic order of the reorganized cortical zone was comparable to normal. In at least most cortical sectors, there was a consistent, maintained relationship between receptive field size and magnification i.e. as representations enlarged, receptive fields were correspondingly reduced in size.⁴¹

These studies indicate that topographic representations of the skin surface in adult monkeys are maintained dynamically. They clearly reveal that this projection system retains a self-organizing capacity in adult monkeys. They suggest that processes perhaps identical to a part of the original developmental organizing processes (by which details of field topographics are established) are operational throughout life in this projection system in primates. Some ofthe implications of these studies for the neural origins of tactile perception are discussed.