WHAT IS BRAIN LATERALIZATION?

The human brain is a paired organ; it is composed of two halves (called cerebral hemispheres) that look pretty much alike.

The term brain lateralization refers to the fact that the two halves of the human brain are not exactly alike. Each hemisphere has functional specializations: some function whose neural mechanisms are localized primarily in one half of the brain.



Most humans (but not all) have left hemisphere specialization for language abilities. The only direct tests for speech lateralization are too invasive to use on healthy people, so most of what we know in this area comes from clinical reports of people with brain injuries or diseases. Based on these data, and on indirect measures, we estimate that between 70% to 95% of humans have a left-hemisphere language specialization. That means that some unknown percentage of humans (maybe 5% to 30%) have anomalous patterns of specialization. These might include: (a) having a right-hemisphere language specialization or (b) having little lateralized specialization. The more one knows about the neurological mechanisms underlying language abilities, the more complicated these issues become. For instance, some language functions (like prosody-- the emotive content of speech) is specialized in the right hemisphere of people with left-hemisphere language specializations. The bottom line is that, despite overly-simplistic descriptions of left-brain / right-brain stuff one finds in introductory textbooks and the public press, there is still a great deal about brain lateralization that we simply do not yet understand.



WHAT IS HANDEDNESS?



WHAT DOES HANDEDNESS HAVE TO DO

WITH BRAIN LATERALIZATION?



WHO CARES?



This association between hand and brain captured the imaginations of researchers because it would be so useful (so easy, so non-invasive, so cheap) to study patterns of brain asymmetries by using a person's handedness as a marker for brain lateralization (direct methods involve neurosurgery, invasive drug testing, or expensive imaging techniques). I have argued, however, that many fundamental problems exist with this methodology, and advocate going back to the drawing board to work out some of these basic problems, rather than continue to embrace 19th-century methodology.



Better understanding how handedness relates to brain function is relevant to many people, among them: academic researchers, medical clinicians, neurological patients, educators and left-handers. Clarifying the relationship between handedness and functional brain specializations, and learning more about the developmental and neurobiological mechanisms that underlie these relationships, may help us better understand a wide range of seemingly unrelated issues such as dyslexia, stuttering, human variation, comparative brain research, developmental neurobiology of the brain, and the origins of human language.

