Lecture 8 - Auditory Sensation and Perception 1 Flashcards
How does the human auditory system work?
Outer, middle and inner work
Transduction
What is sound caused by?
By the movement/vibration of an object.
The movement alternately squeezes air molecules together and pulls them apart.
Creates a longitudinal pressure wave in the air.
What is the decibel scale?
A logarithmic scale of relative intensities.
Reduces a wide range of values to a smaller scale.
Calculated with reference to our hearing threshold.
What is a period?
Time taken to complete one wavelength.
What is frequency?
Number of periods per second.
What is timbre?
Refers to that quality which can make two sounds with the same pitch and loudness seem dissimilar.
Related to complexity
What is the difference between pure and complex tones?
Pure tone - single frequency
Complex tones - more than one frequency
What are harmonics?
For a sound to have a clear pitch, the partials must be integer multiples of the fundamental frequency.
If a sound has inharmonic partials, it will be unpitched.
What fundamental frequency does a harmonic complex tone have?
150Hz
Explain the properties of the outer ear
Visible part of the ear (auricle/pinna)
Not vital for perception, but does have a perceptual effect.
Explain the properties of the ear canal
3cm long
Extends down to the eardrum
Resonant frequency of 1-5kHz
Explain the properties of the middle ear
Eardrum –> ossicles –> oval window.
Two membranes joined by bones.
Why do we need the middle ear?
Vibrations must travel from air to fluid
The eardrum is much bigger than the oval window.
What is the main structure of the inner ear?
The cochlea
How does the inner ear work?
It works as a frequency analyser by breaking incoming complex sounds into pure tone components.
It also works as a transducer by converting mechanical energy at these different frequencies into electrical activity to travel to the brain.
What are the elements of the cochlea?
- Two chambers separated by the cochlear partition.
Filled with perilymphic fluid
Vibrations flow from oval –> round window and back.
What does the cochlear partition contain?
The basilar membrane
What sits on top of the basilar membrane? (BM)
The Organ of Corti
What does the organ of corti contain?
Tectorial membrane
Hair cells (inner and outer)
Explain the transduction process (4)
- BM moves in response to vibrations in the perilymph
- BM vibrates at the same frequencies as the incoming sound
- These vibrations bend the stereocilia of the inner hair cells against the TM.
- Triggers the release of glutamate (neurotransmitter) and an electrical signal is sent to the brain.
How do we know what pitch we’re hearing?
Place coding - WHERE on the BM the firing is coming from.
Temporal (rate) coding - WHEN the firing is occuring.
What is place coding?
At the base, the BM is anchored, narrow and stiff.
At the apex, it is free, wide and loose.
Different resonant frequencies at different points!
Explain temporal coding (4)
- BM moves in response to vibrations in the perilymph.
- BM vibrates at the same frequencies as the incoming sound.
- Stereocilia stimulated by peaks in BM vibration.
- Firing occurs at the period of the incoming waveform.
What is the problem with place coding?
Place coding is less reliable at low frequencies.
Broader regions of activity on the BM at lower frequencies.
Less specific encoding of pitch
What is the problem with temporal coding?
Temporal coding breaks down at high frequencies.
Not enough time for cells to “recharge”.
This leads to temporal “smearing”.
What do inner hair cells detect?
Motion on the BM
What is cochlear turning?
Outer hair cells amplify the motion of the BM
What does intensity coding rely on?
The fact that there are low-and-high threshold auditory nerve fibres.
Low-threshold fibres discriminate quiet and moderate (low-vs. medium-amplitude) sounds.
High-threshold fibres kick in to discriminate moderate and loud (high-amplitude) sounds.
What is the cochlear nucleus?
A relay station.
Sends neural activity to other nuclei in the brainstem.
What is the primary auditory area?
Analyses higher-order features.
Less reliant on precise timing.
What is the acronym to remember the elements of the auditory pathway?
- Carrying: Cochlear nucleus
- Sounds: Superior olive
- Into: Inferior colliculus
- My: Medial geniculate
- Primary auditory cortex!
What is the superior olive?
It analyses location.
Relies on precise timing so happens early.
