IMPLICATIONS FOR LATERALIZATION OF BIRD SONG FROM UNILATERAL GATING OF BILATERAL MOTOR PATTERNS

FUNCTIONAL lateralization of the brain, once considered unique to huma n language, remains poorly understood(1-5). The most convincing exampl e of this phenomenon in animals is singing behaviour in songbirds(1,4) , in which asymmetries range from a clear unilateral dominance(6-8) to approximately equal contributions from each side of the vocal organ, the syrinx(9). Here we report that in brown thrashers (Toxostoma rufum ) only the activity of muscles that gate sound production by regulatin g airflow through each side of the syrinx is lateralized. Other syring eal muscles that primarily control the phonetic structure of vocalizat ions are active on both sides of the syrinx. This explains the puzzlin g absence of laterality in the morphology(10,11) and activity(12,13) o f the higher central song control nuclei and suggests that song latera lization did not evolve as a means of achieving a single 'executive' c ommand centre, or as a way of economizing on motor circuits to free br ain space for other tasks(14,16).