EXPERIENCE-INDUCED PLASTICITY OF CUTANEOUS MAPS IN THE PRIMARY SOMATOSENSORY CORTEX OF ADULT MONKEYS AND RATS

In a first study, the representations of skin surfaces of the hand in the primary somatosensory cortex, area 3b, were reconstructed in owl m onkeys and squirrel monkeys trained to pick up food pellets from small , shallow wells, a task which required skilled use of the digits. Trai ning sessions included limited manual exercise over a total period of a few hours of practice. From an early clumsy performance in which man y retrieval attempts were required for each successful pellet retrieva l, the monkeys exhibited a gradual improvement. Typically, the animals used various combinations of digits before developing a successful re trieval strategy. As the behavior came to be stereotyped, monkeys cons istently engaged surfaces of the distal phalanges of one or two digits in the palpation and capture of food pellets from the smallest wells. Microelectrode mapping of the hand surfaces revealed that the glabrou s skin of the fingertips predominantly involved in the dexterity task was represented over topographically expanded cortical sectors. Furthe rmore, cutaneous receptive fields which covered the most frequently st imulated digital tip surfaces were less than half as large as were tho se representing the corresponding surfaces of control digits. In a sec ond series of experiments, Long-Evans rats were assigned to environmen ts promoting differential tactile experience (standard, enriched, and impoverished) for 80 to 115 days from the time of weaning. A fourth gr oup of young adult rat experienced a severe restriction of forepaw exp loratory movement for either 7 or 15 days. Cortical maps derived in th e primary somatosensory cortex showed that environmental enrichment in duced a substantial enlargement of the cutaneous forepaw representatio n, and improved its spatial resolution (smaller glabrous receptive fie lds). In contrast, tactile impoverishment resulted in a degradation of the forepaw representation that was characterized by larger cutaneous receptive fields and the emergence of non-cutaneous responses. Cortic al maps derived in the hemispheres contralateral to the immobilized fo relimb exhibited a severe decrease of about 50% in the overall areal e xtent of the cutaneous representation of the forepaw, which resulted f rom the invasion of topographically organized cortical zones of non-cu taneous responses, and numerous discontinuities in the representation of contiguous skin territories. The size and the spatial arrangement o f the cutaneous receptive fields were nor significantly modified by th e immobilization of the contralateral forelimb. Similar results were o btained regardless of whether the forelimb restriction lasted 7 or 15 days. These two studies corroborate the view that representational con structs are permanently reshaped by novel experiences through dynamic competitive processes. These studies also support the notion that subj ect-environment interactions play a crucial role in the maintenance of basic organizational features of somatosensory representations.