Damage along any part of the visual pathway can affect vision even if the eye itself is normal.
When you view an object with both eyes, each retina is exposed to slightly different images. This is known as retinal disparity. You can test this yourself. Hold your right thumb in front of your face and move it to the right. Close your left eye and view your thumb with only your right eye. Now, close your right eye and view your thumb with your left eye. Did you see a different image with each eye?
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Page 1 of 1 SU_PSY3001_Cognitive © 2009 South University
How We See
The human eye is sensitive to wavelengths of light from 400–700 nm. This segment of visible light is part of the electromagnetic spectrum that includes energy such as ultraviolet, infrared, and gamma rays. Different animals can see different parts of the electromagnetic spectrum. For example, many insects can see ultraviolet light, while humans cannot.
For light energy to be processed by the human brain, it must first be changed into neuronal messages. This process is known as transduction.
Each sensory system has specific structures that complete the transduction process. In the eye, the photoreceptors (rods and cones) at the back of the retina accomplish this task.
Light energy is transduced into a neuronal signal in the rods and cones. After leaving the eye through the optic nerve, this neuronal information travels through the optic chiasm, the lateral geniculate nucleus (LGN), and, finally, the primary visual cortex (PVC) (located in the occipital lobe of the brain). Various types of processes occur in the LGN, and several types of specialized cells and neurons reside in the PVC. The neurons in the PVC analyze color, line orientation, and retinal disparity (which contributes to depth perception), and some neurons appear to respond only to faces. In addition, there is a retinotopic map in the PVC, where regions of the retina have corresponding areas of analysis in the PVC. This neuronal information leaves the PVC and travels to various parts of the brain for further analysis and integration with other types of information. Several changes in vision take place due to the structure of the eye. One of these changes takes place every day around dusk. During the day, vision relies predominately on the cones. The cones allow us to see details in objects and help us perceive color. The cones are clustered in the center of the retina in an area known as the macula. In low-light conditions, the visual system shifts to relying on the rods, which allow us to see in our peripheral vision and which are sensitive to movement. However, the rods are not able to discern detail. They also require an adaptation period, which is why we cannot see well if suddenly exposed to darkness (for example, when walking into a movie theater). As the sun sets, we slowly shift to rod vision. During this shift, our visual acuity decreases and remains poor until we shift back to the cone system in high levels of light. A practical application of this shift is in driving. If you are driving at dusk, you may think you can see just as well as in full daylight but you cannot. Your vision is significantly impaired when driving in full darkness, too. Therefore, you should drive more slowly and cautiously at night.