chapter 11 pt 2 Flashcards
Békésy’s place theory of hearing
frequency of sound is indicated by the placed on the organ of corti that has the highest firing rate
different parts of the membrane vibrate the most depending on the frequency
tonotopic map
the cochlea shows an orderly map of frequencies along its length
- apex responds best to low frequencies
- base responds best to high frequencies
frequency tuning curves
we can determine the threshold for specific frequencies for single neurons
done by presenting pure tones of different frequencies and measuring the sound level necessary to cause the neuron to increase its firing above the baseline or “spontaneous” rate in the absence of sounds
the level = threshold for that frequency
plotting each threshold for each frequency = frequency tuning curve
characteristic frequency
frequency to which the neuron is most sensitive
place theory
based on the relation between a sound’s frequency and the place along the basilar membrane that is activated
phase locking
because nerve fibers fire at the same time in the sound stimulus, the sound produces a pattern of nerve firing in groups of neurons that matches the frequency of the sound stimulus
= bursts of firing and not firing
the timing of firing of groups of neurons provides information about the fundamental frequency of a complex tone, and this information exists even if the fundamental frequency or other harmonics are absent
which area of the basilar membrane responds best ot low frequencies
the apex
which area do the basilar membrane responds best to high frequencies
the base
how does the cochlear act like a filter
the different places of maximum vibration along the length of the basilar membrane separate sound stimuli by frequency
- high freq = more vibration near the base
- low freq = more vibration near the apex
means that vibration of the membrane sorts or filters by frequency so hair cells are activated at different places along the cochlea for different frequencies
what are two things that reflects the cochlea’s filtering action
- the neurons respond best to one frequency
- each frequency is associated with nerve fibres located at a specific place along the basilar membrane, with fibers oringinating near the base of the cochlea having high characteristic frequencies and those originating near the apex having low characteristic frequencies
what is the main purpose of outer hair cells
to influence the way the basilar membrane vibrates by changing length
the mechanical response of their longening and shortening pushes and pulls on the basilar membrane, which increases the motion of the basilar membrane and sharpens its response to specific frequencies
how do outer hair cells change length
ion flow in outer hair cells causes mechanical changes inside the cell that causes the cell to expand and contract
the outer hair cells become elongated when the stereocilia bend in one direction and contract when they bend in the other direction
cochlear amplifier
outer hair cells make the basilar membrane more sensitive to different frequencies and play and important role in the frequency selectivity of auditory nerve fibers
without outer hair cells, it would take much higher intensities of sound to get auditory nerve fibers to respond
missing fundamental
critique of place theory
- effect where removing the fundamental frequency of a complex tone does not change the tone’s pitch - means there isn’t a peak vibration at the place associated with the pitch because there isn’t a fundamental frequency entering the ear
how does place theory address the missing fundamental
complex tones cause peaks in vibration for both the fundamental and the harmonics - we can still determine the pitch without the fundamental by recognizing the pattern of peaks in the harmonics
this only works for low harmonics, however! these are called resolved harmonics
can’t fully explain missing fundamental
resolved harmonics
harmonics in a complex tone that create separated peaks in basilar membrane vibration and so can be distinguished from one another - usually lower harmonics of a complex tone
unresolved harmonics
higher harmonics of a complex tone that don’t create separate peaks in basilar membrane vibration and can’t be distinguished from each other - result in a weak perception of pitch without the fundamental frequency
amplitude modulated noise
critique of place theory
a stimulus that doesn’t create vibration pattern on the basilar membrane that corresponds to a specific frequency since it is made up of so many random frequencies and that fluctuates between different levels of loudness
we can still determine pitch from this type of noise
how is phase locking linked to pitch perception
they both only occur for frequencies up to about 5000Hz
when tones are strung together to create a melody, we only perceive a melody if the tones are below 5000Hz
our sense of musical pitch may be limited to those frequencies that create phase locking
temporal coding
the connection between the frequency of a sound stimulus and the timing of the auditory nerve fiber firing
where do signals generated in the cochlea go?
transmitted out of the cochlea in nerve fibers of the auditory nerve
they get carried along the auditory pathway, eventually reaching the auditory cortex
what is the sequence of subcortical structures that auditory nerve fibers synapse in
cochlear nucleus > superior olivary nucleus in the brain stem > inferior colliculus in the midbrain > medial geniculate nucleus in the thalamus
where do auditory nerve fibers go after the medial geniculate nucleus
primary auditory cortex in the temporal lobe of the cortex
what is the function of processing in the superior olivary nucleus
this is the first place that signals from the left and right ears first meet so it is important for locating sounds
what happens as nerve impulses travel up the SONIC MG pathway to the auditory cortex?
temporal information decreases
phase locking, which occured up to about 5000Hz in auditory nerve fibers, occurs only up to 100-200Hz in the auditory cortex
however, there are areas of the auditory cortex that respond to pitch
pitch neurons
cortical neurons in marmoset (monkeys) that respond to stimuli associated with a specific pitch - they fire at the pitch of a complex tone even if the harmonic or other harmonics of the tone are not present
anterior auditory cortex
part of the cortex that is close to the front of the brain and is most responsive to pitch
respond to resolved harmonics, but not unresolved harmonics - suggests these areas are involved in pitch perception (bc resolved harmonics are associated with pitch perception)
what happens when the outer hair cells are damaged?
loss of hearing sensitivity and loss of sharp frequency tuning - makes it harder to separate out sounds
inner hair cell damage
can also cause a loss of sensitivity - inner hair cells can sometimes be lost over an entire region of the cochlea
for both inner and outer - hearing loss occurs for the frequencies corresponding to the frequencies detected by the damaged hair cells
presbycusis
a form of sensorineural hearing loss that occurs as a function of age and is usually associated with a decrease in the ability to hear high frequencies
results from the cumulative effects over time of noise exposure, ingestion of drugs that damage the hair cells, and age related degeneration
noise induced hearing loss
occurs when loud noises cause degeneration of the hair cells
leisure noise
noise associated with leisure activities such as listening to music, hunting, and woodworking - exposure to high levels of leisure for extended periods can cause hearing loss
hidden hearing loss
hearing loss that occurs at high sound levels, even though the person’s thresholds, as indicated by the audiogram, are normal
audiogram
a plot of hearing loss versus frequency
normal hearing is indicated by a horizontal function at 0dB on the audiogram
