You work for a surgical equipment manufacturing company. You are part of a design team developing a new machine to monitor pa

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How We Hear Sound energy reaching the ear must be changed into neuronal messages before it can be further processed in the brain.

Hair cells in the cochlea are the site of transduction. A sound wave enters the outer ear, travels through the middle ear, and arrives at the entrance of the cochlea, which is filled with fluid.

Hair cells are embedded in the basilar membrane and move back and forth in response to sound waves. The inner hair cells change the sound energy into neuronal messages.

Different areas of the basilar membrane analyze different frequencies of sound. Hearing damage occurs when inner hair cells are damaged and can no longer change sound energy into neuronal messages. Exposure to intense sound is the leading cause of hearing loss, but various drugs and environmental toxins can also destroy hair cells.

Neuronal messages leave the cochlea via the auditory nerve and travel to the auditory cortex located in the temporal lobe.

Neurons in the auditory cortex are specialized in perceiving frequency, a change in frequency, speech sounds (possibly), and binaural disparity (which contributes to the ability to localize sounds). Similar to the retinotopic map in the PVC, there is a map of the basilar membrane in the auditory cortex. Specific parts of the cortex analyze information from specific parts of the basilar membrane. Hearing impairments can occur from damage at any point along the auditory perception pathway, not just in the cochlea.

The maximum volume on MP3 players can exceed 100 dB, which can cause hearing damage over a period of time. People tend to increase the volume when listening in a noisy environment such as subways, and surveys show that young people frequently listen at maximum levels. There has been an increase in permanent hearing damage in young people, and it is even becoming a public health issue. When hair cells are exposed to high levels of sound on a regular basis, they become less resilient and eventually die. Hair cells do not regrow, and when enough hair cells are damaged, speech perception is impaired. Save your hearing and turn down the volume the next time you listen to an MP3 player!

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Types of Color Blindness

Color blindness is genetic and is carried on the X chromosome. As women have two X chromosomes, a normal gene on one X chromosome compensates for a defective gene on the other X chromosome. But because men have a single X chromosome (XY), color blindness is more prevalent in men than in women. Most cases of color blindness involve the absence of red or green cones.

Normal: People with normal vision have red, green, and blue cones.

Protanopia: People with protanopia do not have red cones.

Deuteranopia: People with deuteranopia do not have green cones.

Tritanopia: People with tritanopia cannot distinguish blues and yellows.

Achromatopsia: It is a rare vision disorder in which people do not perceive color at all. Their cones are not functional, which also means they can neither see much detail nor see well in daylight. Oliver Sacks wrote a book titled Island of the Colorblind that investigated people on a small Micronesian island where achromatopsia affects about 5 percent of the population.

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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.