Feb 07 2007

Hear This.

A member of the SGU forums who goes by the name of Queequeg asked me the following question:

“My question is: is it true that what you hear with only one ear is transmitted to only one hemisphere of the brain? If true what effects on perception and cognition are there compared to when you hear things with both ears/hemispheres? Does the communication allowed across the hemispheres by the corpus callosum completely offset the effects of the information initially being received only by one ear/hemisphere?”

Hearing, actually, is the most bilateral of the special senses. Each ear registers sound in the inner ear. The eardrum translates air vibrations into movement, vibrating the basilar membrane of the cochlea, which in turn triggers so-called hair cells by causing the hair-like appendages of the cell to bend from shearing forces. The bending creates an electrical potential which conducts through the auditory nerve (the VIIIth cranial nerve) to the brainstem. This is the mechanism by which the vibrations of sound are translated into neuro-electrical signals.

The auditory nerve connects to the cochlear nucleus. From there two streams of information are sent to paired nuclei on each side of the brainstem (the superior olivary nuclei). It is at this level – the brainstem level – that the precise timing of auditory signals are preserved and compared left to right. This bilateral processing gives us the ability to judge the direction from which a sound is coming.

So basically, as soon as sound signals come into the brain stem they are mixed from both sides. From that point upward to the cortex where we perceive sound, the signals are mixed from both sides (bilateral). Incidentally, for this reason a stroke or other similar lesion in the brain does not cause hearing loss. There would need to be symmetrical lesions on both sides, and that is unlikely. So hearing loss almost always comes from the ear itself (conductive hearing loss) or from the auditory nerve (neuronal hearing loss).

The second question relates to the corpus callosum and the degree of communication between the two hemispheres. The corpus callosum is a large bundle of axons carrying connections between the two hemispheres of our brain. Each hemisphere is actually capable of consciousness, memory, and thought by itself. For this reason it is sometimes noted that we have two brains, and to an extent this is true. But the corpus collosum and other connections (such as the anterior and posterior commissures) provide robust cross communication between the hemispheres.

For hearing, this does not matter very much since sound processing is bilateral soon after entering the brainstem, and the same information is being sent to both hemispheres. For other functions, however, this connection is vital. For example, vision is one sided – technically it is contralateral. Although information from each eye is mixed, the part of both eyes that sees the right side of the world goes only to the left hemisphere and the part of both eyes that sees the left side of the worlds goes only to the right hemisphere. So the left hemisphere only knows what the right hemisphere is seeing because of cross-communication across the corpus callosum and posterior commissure.

Only the dominant (usually left) hemisphere can speak. So if our right hemisphere sees an object in our left visual field it must send this information to the left hemisphere in order for us to name the object. This does provide some neat phenomena when studying people who have had their corpus callosum severed – which is sometimes done to prevent seizures from propagating from one side to the other. But this is a topic for another blog entry.

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