Lateralisation · Grey Matter

Lateralisation is the tendency of some functions to depend more on one hemisphere than the other, most clearly language on the left and the spatial attention to both sides on the right.

Lateralisation. Lateralisation is the tendency of some functions to depend more on one hemisphere than the other, most clearly language on the left and the spatial attention to both sides on the right.

The two hemispheres look alike and share most of their work; the specialisation is a matter of degree. Language production and grammar are left-dominant in about 96 % of strong right-handers and in most left-handers (the share with right-hemisphere dominance rises to about 27 % in strong left-handers). The right hemisphere contributes more to the melody and emotional tone of speech (prosody), to global spatial attention, to recognising faces and to understanding context and implied meaning. The corpus callosum keeps the two sides in constant exchange, so in an intact brain both contribute to almost everything.

Lesions reveal it best. A left-hemisphere stroke often causes aphasia, a right one more often hemispatial neglect and difficulties with prosody and context.

The popular split into a logical left brain and a creative right brain goes far beyond the evidence: an imaging study of more than a thousand people found lateralised hubs, but no sign that some people use one whole hemisphere more than the other.

A rule of thumb that holds reasonably well: left for language and sequence, right for prosody, global space and context, with every task drawing on both.

Questions: What does the right hemisphere add to language if the left does the words? The right temporal and frontal cortex handle much of the melody of speech (prosody): the rise of a question, the emphasis that changes meaning, the tone that signals irony or anger. They also help with what is meant beyond the literal words, such as jokes, metaphors and the thread of a story. After a right-hemisphere stroke a person may speak grammatically in a flat voice, or miss that a remark was sarcastic, a deficit called aprosodia. What happens when the corpus callosum is cut? The operation was done to keep seizures from spreading between the hemispheres, and in daily life its effects are hard to see. In the laboratory, Sperry and Gazzaniga showed each hemisphere words or pictures separately by flashing them in one half of the visual field: an object shown to the left hemisphere could be named, the same object shown to the right hemisphere could not be named but could be picked out by touch with the left hand. Each hemisphere was perceiving and acting on its own, and the speaking left hemisphere would sometimes invent a reason for what the left hand had done. Is language on the left side of the brain in everyone? In most people it is, and the share depends on handedness. Measuring blood flow in both hemispheres during word generation, Knecht and colleagues found right-hemisphere dominance in about 4 % of strong right-handers, 15 % of ambidextrous people and 27 % of strong left-handers, with the rest left-dominant or bilateral. So most left-handers are left-dominant for language too, and a right-hemisphere stroke causes aphasia only in the minority whose language sits on that side.