Motor and somatosensory corticostriatal projection magnifications in the squirrel monkey

1. Motor and somatosensory cortex project massively to the primate striatal matrix, terminating in distributed sets of overlapping projection zones (matrisomes) within the putamen. To study this system quantitatively, we have developed a computer-assisted estimation of the changes in magnification that occur as motor and somatosensory cortical body representations are projected onto the putamen. 2. Cortical and striatal body maps were assessed in squirrel monkeys by injecting anterograde tract tracers into electrophysiologically identified body-part representations in cortical areas 4, 3a, 3b, and 1. Relative projection magnification was defined as the ratio of the cortical injection site volume to the striatal projection site volume. 3. Magnification comparisons indicate that the tracers wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) and S-35-methionine have similar sensitivities. 4. The relative proportions of body-part representations in the striatal maps were not significantly different from those in cortical maps. Both had large representations of hand, foot, and mouth, and smaller representations of trunk. 5. The relative magnification of the motor cortex projection to the striatum was roughly twice as large as those of projections from individual somatosensory areas. 6. These findings suggest that, in the sensorimotor striatum, motor and somatosensory inputs may undergo different proportions of local processing at the borders of their distribution zones (striosomes and matrisomes).