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
