Unit 3 The Ear-Auditory

Hearing: The Auditory System Stimulus=sound Sound waves are pulses of moving molecules. A sound wave’s AMPLITUDE is its LOUDNESS. Loudness is measured in decibels. Absolute threshold = 0 decibels 10 db = 10 x louder 60 db= normal conversation 100 db = 10,000,000,000 x louder! (subway passing) 76 A sound wave’s FREQUENCY is its PITCH: low frequency = low pitch high frequency = high pitch Frequency is number of wavelengths per second, measured in Hz (hertz). A low pitch sound of 35 Hz is traveling at a frequency of 35 wavelengths per second. Hz = wavelength/second 78 You do not have to know all the parts in this video! 79 The ear is divided into three parts: outer, middle, and inner. 80 Outer Ear What we think of as the “ear” is really just a portion of the outer ear. The outer ear consists of the auricle/pinna and auditory canal. The pinnae channel sound waves into the auditory canal, towards the eardrum (which is part of the middle ear). 81 Middle Ear The middle ear transforms the sound wave vibrations into “middle ear” vibrations: First the eardrum / tympanic membrane vibrates, which in turn moves the ossicles, which send the vibrations to the inner ear. The ossicles are the smallest bones in the body. These bones are called the hammer, the anvil, and the stirrup. 82 Inner Ear The inner ear contains the cochlea (and the semicircular canals, which house the vestibular system / balance). The cochlea is filled with fluid. The stirrup footplate (of the middle ear) pushes against the cochlear membrane at the oval window, sending vibrations through the fluid. This vibrating fluid causes the basilar membrane to vibrate. When the basilar membrane vibrates, hair cells attached to the basilar membrane move back and forth. When the tips of the moving hair cells get pulled by the movement, electrical signals are sent to the thalamus via the cochlear nerve. 83 From the ear, the cochlear nerve travels (indirectly) to the THALAMUS, which projects to the AUDITORY CORTEX. 84 Theories of Hearing • Place theory (Helmholtz, 1863) (TONOTOPIC) – A pitch has a corresponding place on the basilar membrane that vibrates when that specific sound comes into the ear – High pitches vibrate near the base of the cochlea, closer to the oval window – Low pitches vibrate near the tip / apex of the basilar membrane, curled in the middle of the cochlea – Brain determines pitch by recognizing where on the basilar membrane the neural signal is coming from 85 86 87 Theories of Hearing • Frequency theory (Rutherford, 1886) – The problem with Place Theory is that low pitches/frequencies don’t place as specifically as high pitches do – How does the ear transduce low frequencies, so that we can get clear signals to the brain? – Frequency Theory says that the brain reads frequency (pitch) by monitoring the frequency of neural impulses traveling up the auditory nerve • For example, a 100 Hz sound would stimulate neurons to fire 100 times per second 88 Theories of Hearing • The problem with Frequency Theory is that neurons can only fire at a maximum rate of 1,000 times per second • How does ear send neural signals at a frequency higher than that? • Several neurons coordinate their firing, so that the sum of their signals together is the correct frequency • This is called the VOLLEY PRINCIPLE 89 Theories of Hearing Place Theory best explains how we sense high pitches Frequency Theory best explains how we sense low pitches Some combination is possible as an explanation for medium pitches Localizing Sound If a sound comes from the left, it will reach the left ear sooner and will be louder in the left ear. Same for the right: A sound from the right side will reach the right ear sooner and will be louder in the right ear. Your brain used these minute differences to calculate where a sound is coming from. If a sound reaches the two ears at the same time, the clues are missing, and a person will often localize a sound source incorrectly. This can happen if the sound is directly in front of, behind, above, or below a person’s head. 91 Hearing Loss Conductive Hearing Loss – deafness caused by a problem in the outer or middle ear, preventing vibrations from reaching the inner ear Sensorineural hearing loss – deafness caused by damage to the inner ear/cochlea hair cells or the cochlear nerve Sensorineural is the more common type of deafness. Can be caused by genetics, aging, or prolonged exposure to loud noises. Cochlear implants provide electrical stimulation directly to the cochlear nerve in the inner ear, to allow sound signal to be sent to the brain. 92