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.

In humans, the most obvious functional specialization is speech and language abilities. In the mid-1800s, Paul Broca (a French neurosurgeon) identified a particular area of the left hemisphere that plays a primary role in speech production. Shortly afterwards, a German neurologist, Carl Wernicke, identified another part of the left hemisphere primarily concerned with language comprehension.

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.