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THE SENSES - A PRIMER

By Carl Sherman


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 VISION

The brain devotes more space to processing and storing visual information than all other senses combined.

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While all the senses are important, we tend to rely most on sight. The cornea and lens, transparent structures at the front of the eye, focus light onto the retina, the photosensitive area that covers three-fourths of the back. The shape of the lens changes to adjust for near and distant vision.

How the eye works

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Light striking the surface of a photoreceptor activates a protein, rhodopsin, that initiates a chemical cascade of signals through intricate cellular wiring that ultimately converge in ganglion cells, neurons whose axons form the optic nerve. Processing, sorting, and condensing of signals actually starts here in the retina.

In the brain, visual input passes through intermediate areas to the primary, or striate, visual cortex in the occipital lobe, and then to a secondary visual cortex where the information separates into streams: dorsal pathways, which analyze image size and position; and ventral pathways that discriminate color and shape, allowing us to recognize what we see.

Scientists have identified brain structures specializing in faces, places, and words, among other things. All told, some 30 cortical areas participate in visual processing.



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