Broca zoomed in on the relevant area of the left hemisphere associated with speech deficits and made it impossible to ignore that anatomical reality of an uneven brain with localized functions. "This opened the way," writes Oliver Sacks, "to a cerebral neurology, which made it possible, over decades, to 'map' the human brain, ascribing specific powers - linguistic, intellectual, perceptual, etc.-- to equally specific 'centers' of the brain."
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It didn't take long for Broca's peers to come up with challenges to the infant theory of cerebral localization. Broca embraced such skeptical questioning and was able to show that damage to one area of the left hemisphere would impair patients differently than damage contained specifically within this speech "center." But the more puzzling exceptions were the small number of patients with either left-hemisphere damage and no difficulties with spoken language, or the tell-tale inability to speak, yet only frontal lobe damage in the right hemisphere.
Broca could only surmise that for a minority of people, localization of the speech center must be reversed between the left and right hemispheres. At one point he suggested that these people might also have their handedness reversed, i.e., that such individuals would be left-handed, but later he said these two exceptions to the left-side dominance for speech weren't necessarily united. That is, they didn't have to be one and the same anomaly. Satisfied with the broader theory of hemispheric localization and the explanation it provided for speechless patients like Leborgne and Lelong, Broca was apparently ready to move on to the next scientific inquiry of the day.
A few years after the sonic boom from his findings had subsidied--"How could two seemingly identical masses of grey matter of the brain be so different?"--other thinkers hurried to tie up what they perceived to be the obvious, and as one current scholar put it, "psychologically seductive" theory that Broca's research had inadvertently led them to conclude.
Broca correctly observed that the faculties for speech production reside in a particular area of the left hemisphere, except for the rare instances when they don't. Because some people are an exception to the language-to-the-left rule, and because a similarly small proportion of people are left-handed, everyone and his cousin in the medical establishment figured the two must go hand in hand; lefties should have language lateralized to the right.
What's interesting about this conclusion is that few people in nineteenth-century Europe would have admitted to being left-handed. Detecting someone's left-handedness would have been difficult, with eating, writing, and other major tasks all usually carried out with the right hand. What's also interesting about this conclusion is that it's wrong. Nearly 99 percent of right-handers have language located in the left hemisphere, and about 70 percent of lefties do. A different proportion, yes, but hardly the opposite; most lefty brains are like righty brains, at least as far as speech function is concerned. The rest either have language in the right hemisphere, or have it distributed more evenly between the two sides of the brain.