6.8.2. How It Works (2)
Nine out of 10 people use their right hand predominantly, and at least 9 out of 10 people have their major functions on their left side.5 This includes around two-thirds of left-handers. Everyone else, a significant minority, either uses the right hemisphere for speech or uses both hemispheres.6
One test of which half of the brain is dominant for language is the Wada test. This involves a short-acting anesthetic (e.g., sodium amytal) being injected into the carotid artery. This transiently anesthetizes the left hemisphere, thus testing the functional capabilities of the affected half of the brain. People for whom the left hemisphere is indeed dominant for language (i.e., most of us) will temporarily become aphasic, losing the ability to comprehend or produce language. If counting at the time, you’ll stop being able to do so for a few beats when injected with the anesthetic.
The reason most people are still left-brainers for language may be due to how our brain functions became lateralized [Hack #69] before the evolution of language, the brain lateralizing separately from the use of our hands.
It has been suggested that the speech areas of the brain developed near the motor cortex because hand gestures were the principal form of communication before speech.7 Studies show that, when a participant observes hand and mouth gestures, parts of the motor cortex (F5) and Broca’s area (found in the left frontal lobe, specifically involved in the production of language) are stimulated. It is argued that before speech our ancestors used gestures to communicate, much as monkeys and apes do now (i.e., lip smacks). And so the human speech circuit is a consequence of the precursor of Broca’s area, which was endowed (before speech) with mechanisms to recognize action made by others, from which speech developed.
Taken from : Mind Hacks
