The same is true for about 3 percent of right-handers. While the two sides of the brain are roughly equal in size, in most people these parts of the left hemisphere are slightly larger.īut like handedness, size discrepancy and language dominance are reversed in a minority for about 30 percent of lefties, the right hemisphere rules in these regards. Roughly a decade after his discovery of “Broca’s area,” neurologist Carl Wernicke localized the ability to understand words in another part of the left hemisphere, the superior temporal lobe: “Wernicke’s area.” Following the observation that stroke or injury to the left side of the brain often disrupts the ability to speak, some 150 years ago the physician Paul Broca identified a region in the left frontal lobe as crucial in speech production. In terms of mental function, the first studied and best documented consequence of brain asymmetry involves language. But it has become clear that equivalents are actually quite widespread: about 65 percent of chimpanzees use the right hand for demanding tasks, for example, and 90 percent of parrots pick up objects with the left foot. Until fairly recently, scientists assumed handedness (and presumably the cerebral asymmetry that underlies it) to be uniquely human. The remaining ten percent are left-handed or ambidextrous. About 90 percent of people are right-handed: they have much better control over fine movements with their right than left hand, and preferentially use their right hand for most activities. Whatever the reason, the most obvious manifestation of brain lateralization is handedness.
The origins of cerebral asymmetry are unclear, but may come down to an evolutionary advantage in efficiency: locating a specific cognitive function in a compact area on one side of the brain frees neural space for other functions on the other, and reduces the frequency of transmission errors. And like so much in neuroscience, far from fully understood. The reality is not so simple-and a good deal more interesting. A host of popular books, educational strategies, and even therapeutic interventions have ensued, promising to enhance abilities and relieve mental maladies by optimizing function on one or the other side of the cerebral cortex. Yet in recent years, the two sides of the brain have come to symbolize two sides of human nature the left brain hailed (or disparaged) as “logical,”“analytical, and “intellectual,” and the “intuitive” right brain as the avatar of emotion and creativity. Symmetry only goes so far, however-there are differences, too.
The human brain is basically symmetrical, split down the middle: the right cerebral hemisphere receives sensory input from and directs movement on the left side of the body, while the left hemisphere governs corresponding functions for the right side.
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