Anatomical demonstration of multiple representation in the forelimb region of the cat motor cortex.

Retrograde transport of HRP was employed to examine the pattern of callosal connections in the forelimb region of area 4 gamma in the cat. According to the conventional view, areas of the motor cortex which contain the representation of distal body parts neither send nor receive callosal fibers. If this is true, then an absence of callosal connections would define the sites of distal forelimb representation. Following multiple injections of HRP into the contralateral motor cortex, many labeled neurons were found in the forelimb region of area 4 gamma. However, within this region, two spatially separate areas were found where labeled neurons were either absent or present in very low density ("callosal holes"). The anatomically defined callosal holes corresponded in size, shape, and location to the physiologically defined digit zones. To provide direct evidence for this correspondence, retrograde HRP transport was combined with intracortical stimulation in the same animal. Small lesions placed in physiologically identified digit zones were located within the anatomically defined callosal holes. Thus, a double representation of the distal forelimb can be defined in area 4 gamma of the cat motor cortex using both anatomical and physiological methods.