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For centuries, the apparent symmetry of the hemispheres of the brain stimulated speculations about the duality of mind or gradients of function in the planes of the three dimensions: front-to-back (Gall, Broca), left-to-right (Dax) and top-to-bottom (Jackson). Front-to-back has lost its appeal and top-to-bottom is canonical (albeit still contentious), leaving left-to-right as the currently hot topic in the arena of human frontier science.

Legend has it that Gall chose the frontal (supraorbital) part of the brain as the seat of the language instinct because certain verbose students of his acquaintance had protruding eyes. If true, the legend proves that the wildest of hunches occasionally is pregnant with consequences. In this case, the hunch was the existence of a fronto-occipital gradient of faculties subserved by the brain. Gall's insight triggered the French anthropologist's lively formulation of the Dax–Broca doctrine, conjoining Dax's claim that the speech faculty resides in the left hemisphere with Broca's originally limited claim that the language faculty resides in the frontal cortex. Dax and Broca arrived at their claims from consideration of patients with speech deficits after stroke or brain damage; on the basis of observations originally reported in 1836, Marc Dax claimed that aphasia invariably follows a left-hemisphere lesion.

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