Rods and cones are connected to retinal neurons that together form the optic nerve, which sends sight information to the brain for processing. The fovea is the blind spot that is where the optic nerve exits the eye. There are no rods or cones there. Inside the brain, the optic nerves from each eye cross over at the optic chiasm. Much of the visual signal from the left eye goes to the right visual cortex in the occipital lobe, while the reverse is true of the information from the right eye. Vision is not black and white; it is also not two-dimensional. There are three types of cones that get activated to see specific colors. The three colors perceived are green, red, and blue. Cones that are activated by one color get inhibited by its opposite color. This explains why there is an afterimage after staring at a bright light of a certain object. We see things in our environment in three-dimensions because of depth perception. There are different cues used to perceive three dimensions. Part of this comes from the fact that we have two eyes that together provide binocular vision. There are also monocular cues to depth perception, which are possible with just one eye. Things like linear perspective, in which parallel lines converge in the distance, help us see depth, even in a two-dimensional photo.
HEARING Hearing involves the perception of pressure waves by the ears. There are several sections to the anatomy of the ear. The outer ear consists of the pinna or auricle, which is the part that can be seen, the auditory canal, and the tympanic membrane, or ear drum. The anatomy of the ear is described in Figure 16:
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