Functional organization of the projection from area 2 to area 4 gamma in the cat

It has been shown that tetanic stimulation of area 2 of the somatosensory cortex produces long-term potentiation in neurons in area 4 gamma, and this has been suggested as the basis of learning new motor skills. The purpose of this study was to further elucidate the characteristics of this projection by the use of evoked potentials in area 4 gamma elicited by intracortical microstimulation of area 2. The experiments were carried out in cats and the following results were obtained. 1) In six animals, stimulation of a given site in area 2 elicited evoked potentials in a restricted region of area 4 gamma, the size of which ranged from 1 to 1.5 mm(2). These responses were labeled ''localized responses.'' The evoked potentials were recorded from the superficial layers of the cortex, and were positive monophasic in shape. 2) In 16 animals, stimulation of a given site in area 2 elicited a focal response that was surrounded by smaller positive and monophasic potentials of <50% amplitude that spread broadly over area 4 gamma. These responses were labeled ''graded responses.'' 3) The sites that produced focal evoked potentials in area 4 gamma extended along the direction of the radial fibers in area 2. These sites were defined as most effective segments (MESs). 4) The receptive fields of neurons along the MES in area 2 were similar to those of neurons recorded at the foci of the evoked potentials in area 4 gamma. The results demonstrate that some of the projections from area 2 to area 4 gamma are highly specific and that the somatosensory and motor areas that are connected by these specific projections receive functionally related peripheral input. These results are discussed in relation to possible mechanisms underlying motor learning.