Hearing

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:

Figure 16.

The middle ear involves the middle ear cavity, which contains the ossicles, which are referred to as the malleus, the incus, and the stapes. These move to focus the sound wave onto the inner ear. There are two parts to the inner ear. These are the semicircular canals and the cochlea. The semicircular canals are involved in balance and the cochlea contains hair cells, which are the sensory receptor cells of the ear.

Sound comes in waves. They first travel through the auditory canal, reaching the tympanic membrane. The membrane vibrates, with the sound waves causing movement of the ossicles. The stapes is the closest to the inner ear. It pushes on a membrane called the oval window, which moves fluid in the cochlea, which triggers activity in the hair cells. The hair cells are attached to the basilar membrane and sound is transmitted into the brain.

The different sound frequencies heard by the human ear are referred to as the pitch of the sound. It is not completely clear how pitch is detected. Different hair cells may fire uniquely depending on the pitch of the sound. Alternatively, different parts of the basilar membrane might be sensitive to different frequencies of sound. It is likely that both ideas are in play with regard to sound detection.