Your Brain 2

Split Brain

People suffering from intractable epilepsy may be treated by severing communication between their two hemispheres. Such split-brain patients also provide evidence for language lateralization and for understanding contra-lateral brain function.

When the brain is surgically split, certain information from the left side of the body is received only by the right side of the brain, and vice versa. In human who have undergone split-brain operation, the two hemispheres appear to be independent, and messages sent to the brain result in different response, depending on which side receives the message.

For example if a pencil is placed in the left hand of a split-brain whose eyes are closed, the person can use the pencil appropriately but cannot name it because only the left hemisphere can speak. The right brain senses the pencil but the information cannot be relayed to the left brain for linguistic naming because the connections between the two halves have been severed.

Studies of human split-brain patients have also shown that when the inter-hemispheric visual connections are severed, visual information from the right and the left visual fields becomes confined to the left and right hemisphere, respectively.  

Other Experimental Evidence Of Brain Organization

Dichotic listening is an experimental technique that uses auditory signals to observe the behavior of the individual hemispheres of the human brain. Such experiments provide strong evidence of lateralization. Subject who hear two different sound signals simultaneously through earphones, for example they hear curl in one ear and girl in the other, or cough in one ear and laugh in the other. They are more frequently correct in reporting linguistic stimuli (words, nonsense syllables) delivered to the right ear, but are more frequently correct in reporting nonverbal stimuli (musical chords, environmental sounds) delivered to the left ear.

Both hemispheres receive signals from both ears, but the contra-lateral stimuli prevail over the ipsilateral (same side) stimuli because they are processed more robustly. The accuracy with which subjects report what they hear is evidence that the left hemisphere is superior for linguistic processing, and the right hemisphere is superior for nonverbal information.

These experiments are important because they show not only that language is lateralized, but also that the left hemisphere is not superior for processing all sounds; it is only better for those sounds that are linguistics. The left side of the brain is specialized for language, not sound.

Other experimental techniques are using to map the brain and to investigate the independence of different aspects of language and the extent of the independence of language from other cognitive systems, for example ERP.

The electrical signals emitted from the brain to different stimuli. ERP differences result when the subject hears speech sounds versus non speech sounds, with a greater response from the left hemisphere to speech. The ERP experiments show variations in timing, pattern, amplitude, and hemisphere of response. ERPs also show the timing of neuronal activity as the brain processes language, they can provide insight into the mechanisms that allow the brain to processes language quickly and efficiently.

ERP and imaging studies of newborns show that from birth onward, the left hemisphere differentiates between nonlinguistic acoustic processing and linguistic processing of sound. This result indicates that at birth the left hemisphere is primed to process language. The specific localization of language functions is found in the adult brain.