Non-overlapping thalamocortical projections for separate forepaw digits before and after cortical reorganization in the raccoon

The fluorescent dye retrograde tracing technique was used to examine the projection from ventroposterior lateral thalamus to primary somatosensory cortex in the raccoon. Results are presented from fifteen experiments using Fast Blue in combination with either Nuclear Yellow or Diamidino Yellow Dihydrochloride. Injections of two dyes into adjacent but separate digit regions of cortex produced single-labeling of thalamic neurons, suggesting that projections for each digit are separate and independent with no branching to adjacent parts of the somatotopic map. Double-labeling was only seen when the cortical injection sites were overlapping. Similar results were seen in six cases in which amputation of the 5th digit of the contralateral forepaw was carried out four months earlier. These results suggest that neither the strengthening of existing collaterals nor growth of new collaterals in the thalamocortical projection are likely to be responsible for the physiological reorganization that is seen in raccoon cortex after peripheral deaffereptation.