Brain and Behavior

Sensation and Perception

How does the brain process vision?

One-third of the brain is used to process visual information, which tells us just how important vision is to human beings. The processing of visual information starts in the eyes, the visual sense organs. On the retina, the inner back surface of the eye, there are a number of neurons that are specialized to fire in response to light. These specific cells are called rods and cones. Rods respond to night vision and process information in shades of black, white and gray. Cones fire in bright light and respond to color. The axons of rods and cones connect to other cells in the retina, where some preliminary visual processing is done. These neurons then connect to ganglion cells, whose axons bundle together, like wires in a cable, to form the optic nerve.

The optic nerve exits through the back of the retina and travels to the brain. All the ganglion cells that respond to the right side of the visual field will go to the left side of the brain. Likewise the ganglion cells that respond to the left side of the visual field will go to the right side of the brain. Any ganglion axon that needs to cross over to the opposite side of the brain is able to do so at the optic chiasm, in the middle of the brain. The two branches of the optic nerve next connect to the corresponding lobes of the thalamus. The thalamus is the gate keeper for sensory information. Based on feedback from the cortex, the thalamus lets some information through to activate neurons connected to the cortex but stops other messages in their tracks. Neurons in the thalamus send visual information up to the primary visual cortex in the occipital lobe, known as VI or Brodman area 17. From there, the fundamental features of the visual stimulus are processed. As discussed above, these neurons connect to association cortices where the basic visual features are then synthesized into larger patterns.


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