| Abstract: Recordings were made from neurons in layers II, III, and IV of rat barrel cortex. The animals were raised either from the day of birth (P0) or from P2, P4, or P7 with just the D1 vibrissa protruding on one side of the face and the contralateral side intact. Follicles were not ablated, but vibrissae were carefully removed by applying steady tension to the base of each vibrissa. Deprivation was continued until the day of recording (P30-P90), though in most cases vibrissae were allowed to regrow for 4-7 d prior to recording. The area of cortex driven by stimulating the spared D1 vibrissa was found to be enlarged in uni-vibrissae animals, but the characteristic anatomical map of the barrel field, defined by cytochrome oxidase staining, retained its normal form. In animals deprived from P0, layer IV cells outside the D1 barrel responded with short latencies (5-10 msec) to D1 stimulation, a condition never observed in normally reared animals. Short-latency responses to stimulation of regrown, deprived vibrissae were still present in layer IV despite the deprivation.
Plasticity decreased rapidly in layer IV between P0 and P4 as judged by two measures: first, the percentage of cells in neighboring barrels that showed short-latency responses to D1 fell from 30% in P0 deprived animals to 18% in P2 and 13% in P4 deprived animals. Second, the percentage of cells in barrels surrounding Dl with larger responses to D1 stimulation than to stimulation of their anatomically related vibrissa also fell from 37% in P0 to 23% in P2 and 12% in P4 deprived animals. The percentage of "shifted cells" showed no further reduction in P7 deprived animals (14%).
Plasticity in layers II and III showed little sign of decreasing between P2 and P7 after an initial drop between P0 and P2. Therefore, deprivation started at P4 and P7 had a far greater effect on layers II and III than on layer IV. In animals deprived from P4 onward, not only were responses to D1 stimulation greater in barrels neighboring D1 (in layers II/III), but responses were smaller to principal vibrissa stimulation. This suggests increased lateral transmission from the "experienced" barrel and a failure of vertical transmission within the "deprived" barrels.
These results show that changes in the balance of experience acquired through vibrissae can affect development of connectivity in the barrel cortex. The main locus of plasticity is cortical when deprivations are started at P4 and beyond. Cells in layers II and III showed greater plasticity at all ages and appeared to have a longer critical period for experience-dependent plasticity than did cells in layer IV.
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