Intracortical stimulation in pyramidotomized monkeys

In order to examine, separately, the organizations of pyramidal and extrapyramidal projections from the primary motor cortex, efforts were made to map the forelimb area of two rhesus monkeys with microstimulation before and after unilateral pyramidotomy. However, microstimulation was not effective in evoking motor responses following complete pyramidal tract section. Movements were evoked using a modified intracortical electrode with a large exposed tip and using stimulation parameters similar to those used for surface stimulation. The results from this modified intracortical stimulation generally agree with those from surface stimulation studies in that: (1) the extrapyramidal topography is similar to the normal motor cortex topography and (2) while peripheral responses can be evoked from the cortex following pyramidotomy, greater spatial and temporal summation are necessary to evoke these responses. In addition, the modified intracortical technique revealed a more widespread post-pyramidotomy digit representation than observed previously with surface stimulation. Results from an incomplete pyramidal tract lesion suggest that recovery of motor function may include plastic changes in surviving corticospinal axons.