A comparative study on the cortico-hypoglossal connections in primates, using biotin dextranamine

The anterograde projections of the motorcortical tongue area to the hypoglossal nucleus and neighbouring structures were studied in the rhesus monkey, squirrel monkey, saddle-back tamarin and tree shrew. Biotin dextranamine served as tracer. Direct projections into the hypoglossal nucleus were only found in the rhesus monkey and squirrel monkey. All four species, however, showed a direct projection into the dorsal and parvocellular reticular formation which in turn projects into the hypoglossal nucleus. The findings suggest a phylogenetic trend in the projections of the motorcortical tongue area from non-primate mammals via non-human primates to man in the sense that the cortico-motoneuronal connection is strengthened towards man. This might be one reason for the superior role the tongue plays in human vocal behaviour in contrast to non-human vocalization.