Bumps on the Brain: Right–Left Anatomic Asymmetry as a Key to Functional Lateralization**Preparation of this paper and some of the research reported within were supported in part by U.S. NINCDS Contract #N01-NS-6-2344 and Ontario Mental Health Foundation Grant #803. Some of the results were presented at the 13th Annual Winter Conference on Brain Research, Keystone, Colorado, Jan. 1980 and at the 19th Annual meeting of the Academy of Aphasia, London, Ont., Oct. 1981.

We have found marked anatomical asymmetries between tile upper surfaces of the human right and left temporal lobes. The planum temporale (the area behind Hesch's gyrus) is larger on the left in 65 percent of brains; on the right it is larger in only 11 percent. The left planum is on the average one-third longer than the planum. This area makes up part of the temporal speech cortex, whose importance is well established on the basis of both anatomical findings in aphasic patients ans cortical stimulation at operation.