Before we talk about the auditory system, we need to understand a few things about the nature of sound. Sound is simply a vibration that travels through some medium. For humans, that medium is usually air.
Measurable attributes of sound waves include length (measured in Hertz [Hz]) and amplitude (measured in decibels [dB]).
How can we tell from where sounds are coming?
When an object makes a sound, the sound waves reach our ears at slightly different times and at slightly different intensities. These interaural time and intensity differences allow us to determine the location of the sound. If there are little or no time and intensity differences, it is difficult to determine the location. For example, if a sound is coming from directly behind, above, or in front of us, it will be difficult to determine where it is coming from, because there are no time and intensity differences between the ears. What can you do to create differences in such situations?
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Page 1 of 1 SU_PSY3001_Cognitive © 2009 South University
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!