Somatotopically inappropriate projections from thalamocortical neurons to the SI cortex of the cat demonstrated by the use of intracortical microstimulation.

Single thalamocortical neurons with receptive fields on the toes were antidromically activated by the passage of 300-microseconds, 0.5- to 10-microA pulses through glass micropipette electrodes placed within somatotopically identified regions of the digit representation of the cat first somatosensory (SI) cortex. The somatotopy of the cortex was determined using recordings from single cortical neurons (see "Methods"), and the positions of the all tracks were marked on an enlarged photograph of the postcruciate cortex. In two of the three protocols, a very precise map of the boundary between two adjacent toes was produced prior to attempting intracortical microstimulation. Slopes of the threshold-distance curves at the sites of the lowest recorded thresholds were on the order of 0.8 microA/10 micron. This value, together with information on the anatomy of the cortical arborizations of thalamocortical neurons (Landry and Deschenes, 1981), suggested that currents of 2 and 5 microA would not activate the cortical processes of thalamocortical neurons at distances greater than 50 and 90 microns, respectively. With currents below 5 microA, thalamocortical neurons could be antidromically activated at a number of sites at depths between 340 and 930 microns (layer IV and upper layer III) and between 1,050 and 1,460 microns (layer VI). A total of 13 thalamocortical neurons could be antidromically activated using current pulses of between 0.8 and 5.0 microA, from within tracks at tangential distances of 250-830 microns from the nearest track through the somatotopically appropriate region. Within somatotopically inappropriate regions, cortical neurons frequently had receptive fields on a toe adjacent to that bearing the receptive field of the thalamic neuron(s) under study. The possible relationship of somatotopically inappropriate projections to the reorganization of cortical somatotopy following digit amputation, paw amputation, and nerve section is discussed.