Psych202 - Introduction to Cognitive Neuroscience
Hearing 1: The Auditory System and Speech Processing
Overview:
1. What Is Sound?
2. The Auditory Pathway: From Ear to Cortex
3. The Auditory Cortex: Structure and Function
4. Speech Processing in the Brain
Section 1 - What is sound?
Sound =Pressure Waves in Air
● Air is "elastic“: if you try to compress it, it will push back
● Because air has both elasticity and mass, one can imagine air as being
made up of little "lumps of air", where each lump is connected to the
neighbouring lumps by elastic springs
● Sound = sequence of pressure waves which propagate through this elastic
net
Perceptual Qualities of Sound
Section 2 - The Auditory Pathway: From Ear to Cortex
The External Ears (Pinnae)
● Sound is transformed from free-field sound (with head not
present) to the sound at the ear drum.
● This transformation is called the head-related transfer function
(HRTF).
● HRTF adds localization information: this is why ears are shaped
as they are.
The Outer Ear
● Ear canal leads to tympanic membrane (ear drum)
● The ear drum vibrates with incoming sound
● This is the first, and faithful, representation of the sound
The Middle Ear
● The middle ear contains a set of intricate, interconnected bones: stapes, incus, and malleus
● These amplify the motion of the ear drum into pressure waves transmitted via the oval window into
the fluid-filled cochlea (in the inner ear).
, The Inner Ear (Cochlea)
● Basilar membrane inside the cochlea vibrates according to the frequency structure of the sound.
● Hair cells attached to basilar membrane then transduce this mechanical signal into action potentials
in auditory nerve.
The Frequency Spectrum
● A complex sound wave can be produced by adding together
sinusoidal sound waves (pure tones)
● The amplitudes and frequencies of these components determine the
frequency spectrum of the sound.
● Any sound can be fully described by its frequency spectrum.
● It is a mathematical truth that any complicated oscillation can be
broken down into pure sinusoidal oscillations (Fourier analysis).
Tonotopic Map in Cochlea
● The basilar membrane in the cochlea breaks down the frequency
structure of any complex sound
● Higher frequencies make the base vibrate, lower ones the apex.
● Each BM location responds best to a certain frequency, the best
frequency (BF) = characteristic frequency (CF)
● Through its vibrations, the basilar membrane (BM) creates an
orderly map of sound frequency – the tonotopic map.
Coding of Frequency
● Auditory nerve fibres carry spikes synchronized to the basilar membrace
vibrations
● Thus, frequency components of sound are represented in two ways in the
auditory nerve:
○ Temporal coding: sounds with higher frequencies produce higer
rates of synchronized firing
○ Place coding: fibres form a tonotopic map
Coding of Frequency
Tonotopic organisation of auditory nerve and cochlear nucleus
2. Place coding: fibres form a tonotopic map
Hearing 1: The Auditory System and Speech Processing
Overview:
1. What Is Sound?
2. The Auditory Pathway: From Ear to Cortex
3. The Auditory Cortex: Structure and Function
4. Speech Processing in the Brain
Section 1 - What is sound?
Sound =Pressure Waves in Air
● Air is "elastic“: if you try to compress it, it will push back
● Because air has both elasticity and mass, one can imagine air as being
made up of little "lumps of air", where each lump is connected to the
neighbouring lumps by elastic springs
● Sound = sequence of pressure waves which propagate through this elastic
net
Perceptual Qualities of Sound
Section 2 - The Auditory Pathway: From Ear to Cortex
The External Ears (Pinnae)
● Sound is transformed from free-field sound (with head not
present) to the sound at the ear drum.
● This transformation is called the head-related transfer function
(HRTF).
● HRTF adds localization information: this is why ears are shaped
as they are.
The Outer Ear
● Ear canal leads to tympanic membrane (ear drum)
● The ear drum vibrates with incoming sound
● This is the first, and faithful, representation of the sound
The Middle Ear
● The middle ear contains a set of intricate, interconnected bones: stapes, incus, and malleus
● These amplify the motion of the ear drum into pressure waves transmitted via the oval window into
the fluid-filled cochlea (in the inner ear).
, The Inner Ear (Cochlea)
● Basilar membrane inside the cochlea vibrates according to the frequency structure of the sound.
● Hair cells attached to basilar membrane then transduce this mechanical signal into action potentials
in auditory nerve.
The Frequency Spectrum
● A complex sound wave can be produced by adding together
sinusoidal sound waves (pure tones)
● The amplitudes and frequencies of these components determine the
frequency spectrum of the sound.
● Any sound can be fully described by its frequency spectrum.
● It is a mathematical truth that any complicated oscillation can be
broken down into pure sinusoidal oscillations (Fourier analysis).
Tonotopic Map in Cochlea
● The basilar membrane in the cochlea breaks down the frequency
structure of any complex sound
● Higher frequencies make the base vibrate, lower ones the apex.
● Each BM location responds best to a certain frequency, the best
frequency (BF) = characteristic frequency (CF)
● Through its vibrations, the basilar membrane (BM) creates an
orderly map of sound frequency – the tonotopic map.
Coding of Frequency
● Auditory nerve fibres carry spikes synchronized to the basilar membrace
vibrations
● Thus, frequency components of sound are represented in two ways in the
auditory nerve:
○ Temporal coding: sounds with higher frequencies produce higer
rates of synchronized firing
○ Place coding: fibres form a tonotopic map
Coding of Frequency
Tonotopic organisation of auditory nerve and cochlear nucleus
2. Place coding: fibres form a tonotopic map
