4417 Hearing Science
Final Exam study guide, 2013
This document lists the key concepts from the material to be covered on the 4417 final exam.
It is not, however, an exhaustive list of possible exam question topics. The final will concentrate on the
material we have covered in the second half of the course, but familiarity with fundamental concepts and
skills from the first half will be assumed.
If you have a clear understanding of the material, you should be able to answer most of these questions
in one or two sentences. To prepare for the exam, it might be good to concentrate on areas for which you
can’t yet formulate such short answers.
Inner ear anatomy
• Organ of Corti
o Tectorial membrane
o Inner hair cells
o Outer hair cells
o Nerve fiber
• What sort of filter does the mechanical response of a point on the basilar membrane (BM)
o Band pass
• What is the BM travelling wave?
o Doesn’t correspond to movement of any material it’s a consequence of each place on the
BM moving up and down in response to stimulation
• How is characteristic frequency mapped on the BM?
o High to low from base to apex
• Why does characteristic frequency vary along the BM?
o Because the width of the BM and the mass increases as you move from base to apex
o Stiffness decreases (k decreases) – stiffness really drives it
o Causes decrease in resonant frequency
• What is the difference between the passive and active response of the BM?
o Active response increases response for given input
o Amplitude is much bigger, more vibration • More specifically, how do the effects of stimulus frequency and amplitude on the amplitude of
BM vibration differ between active and passive responses?
o Increased amplitude and vibration for the same stimulus in an active response
• What cells are responsible for this?
• How do they do it?
o OHC change in length amplifies movement of the BM at a CF
o The stereocilia bend and change their length with the hair cells this exerts a force on the
rest of the structures
o OHCs respond to vibration by stretching and contracting to amplify the response which
causes sharpen the tuning of BM
• What are otoacoustic emissions?
o Byproduct of activity of OHCs in healthy ears
o Activity of OHCs produce vibration recordable in canal with microphone
o These can travel outwards
• Why are active OHCs necessary to produce them?
o Used for screening for hearing loss in infants, newborns and young children
o Determine if hearing loss is cochlear or more central
o Monitor medications toxic to the ear, abnormal emissions can mean cochlear dysfunction
o Assess effects of noise exposure in occupational settings
• Why is the high electrical potential in scala media important for hearing?
o Allows the hair cells to depolarize for neural transduction
• What are the (multiple) steps in the conversion from sound in the ear canal to action potentials in
the auditory nerve?
o Stimuli Hair cells bend tiplinks open K+ flows in depolarization of IHC
Ca2+ enter cells NT released Na+ channels open on axon depolarization of
neuron = AP
• How do inner and outer hair cells work?
o Tips of stereocilia are embedded in the tectorial membrane
o OHC change mechanical properties of tectorial membrane to sharpen frequency response
o IHCs transduce mechanical vibration into electrical
• How does inner hair cell response differ for low and high-frequency inputs?
o The IHC receptor potential is low pass filtered
o Below 1 kHz stimulus fine structure is well represented
o Above 1 kHz the fine structure is attenuated
o Above 5 kHz only the stimulus envelope is transduced
• How does this change the information transmitted by the auditory nerve?
o The fine structure is transmitted, ie. Phase locking etc.
• How does a hair cell produce action potentials in the auditory nerve?
o Cell membrane permeability changes in the axon during a positive feed back cascade
from an adequate stimulus o Ion cross membrane, depolarize one place on the axon
o This place is now in a refractory period
o Depolarization propagates
• Be familiar with neural anatomy slides in I1 Powerpoint
• What are afferent and efferent nerve fibers? What functions do they serve?
o Afferent goes to the brain efferent comes from the brain
o Efferent connect to the OHC to control amplification and connects to IHC in indirect way
• What are radial and spiral afferent fibers?
o Radial – (I) only synapse with one or two IHCs, thicker faster
Shoot straight into spiral lamina in centre of cochlea
o Spiral – II synapse with about 10 OHCs
Neural encoding of sound
• What is tonotopy? How is it preserved in the auditory nerve?
o The auditory mapping of frequencies
• For an auditory nerve fiber ...
• What is meant by firing rate? Spontaneous rate? Neural threshold? Dynamic range?
o Firing ate = AP/sec
o Spontaneous rate = baseline firing rate
o Neural threshold = minimum stimulus level causing an increase in discharge
o Dynamic range = range between threshold and saturation
• Tuning curve? Rate-level function?
o Measures how large an input is required to elicit a given output level as a function of the
frequency o Rate- level function – firing is observed across many neurons from a given part of the
BM, level can be encoded across a larger dynamic range
• How do those properties depend on the BM response and the OHCs?
o Different fibers have different rates in different areas of the BM
o OHCs affect amplification of response
• What are two ways of encoding stimulus frequency in the auditory nerve?
o Place theory – which fibers fire tell which BM place = what stimulus frequency
o Temporal theory – (timing theory) determining by period of neuron firing patterns
Ie. Phase locking
• Do they both work at high and low frequencies?
o No – temporal relies on phase locking which occurs at < 5 kHz, to estimate frequency,
without it the intervals are random and give no info
o Place theory – both
• Similar: what is phase locking, and does it work at high and low frequencies?
o Spikes occurring at the same phase of a waveform
o Only works at CF < 5 kHz
o Falls apart at 1 kHz
• What is the volley theory? What is entrainment?
o Volley Theory – does not require entrainment of every fiber to convey temporal code, it
o Entrainment = whether fiber fires on every cycle of the waveform
Most fibers max firing rate is < 500 Hz, they cannot fire on every cycle for
frequencies above this
• What are two ways of encoding stimulus intensity in the auditory nerve?
o Firing rate – extent to which low and high spontaneous rate fibers fire
o Spread of excitation – more neurons will be stimulated at higher levels
• Why does that depend on having a diverse population of nerve fibers?
o A diverse population of fibers allows for more diversity in frequency so it gives more
information about the sound
Sensitivity - detection and discrimination
• What is a detection threshold for a sound?
o MAF – minimum audible field (loudspeakers)
o MAP – minimum audible pressure (headphones)
o Across frequency the normal threshold of hearing is 0 db HL
o Hearing is normal for threshold < 20 dB HL
• Why is detection probabilistic?
o Because there’s individual differences
• What is a two-alternative forced choice task (2AFC)? Why bother with that?
o Instead of providing yes/no answers, they have to pick choice 1 or choice 2
o Improves sensitivity because it eliminates answer bias
• What is a psychometric function?
o Relationship between parameter of stimulus and subjective response • What is a discrimination threshold?
o The threshold required to discriminate a certain percent (eg. 75%) between to stimuli
• What is a JND?
o Just noticeable difference
o The difference required between to tones to discriminate level or frequency
o Rule of thumb JND = 1 dB
• What is a Weber fraction?
o JND between two stimuli is proportional to the magnitude of the stimuli
o For limited stimulus ranges, holds for frequency and intensity discrimination
o Delta S / S
• What is Weber’s law? Is it really a “law”?
o Constant for a particular task
o Describing what you see for those experiments – it doesn’t have to be
• Why do frequency discrimination thresholds depart from Weber’s law at high frequencies?
o Doesn’t hold true
• What is the JND for intensity?
• How do detection thresholds vary across frequency?
o There is an optimal range
• What filtering characteristics of the auditory periphery are responsible for that?
o The auditory system acts as a high pass filter, filters out sounds < 4kHz
• What is the dB HL scale?
o Hearing level
o Defined with respect to the normal
• What is A-weighting in sound level measurement and why is it used?
o Standard type of filtering that can be applied to microphone signal
o Used to mea