Transcript Slides from Lecture 11/24/2004 (Pascal Wallisch)

Sensation
&
Perception
16 – The Auditory System
Administrative stuff
The final
• Friday, December 10th, 10.30 am -12.30 pm
• Comprehensive, with an emphasis on things
that haven’t been covered in previous exams.
• Like the midterms. Interpreting graphs,
drawing diagrams, multiple choice and short
written answers.
How to do well in class?
1.: Attend class!
• Class attendance
correlates with GPA
typically around 0.3
(Schuman et al., 1985)
but can reach
correlations of up to
0.4! (Larrieu, 2004)
2.: Study! (a lot)
• The effect of studying
seems to be highly
nonlinear. It only
makes a significant
difference if one
studies more than 5
hours per day!
(Schuman et al., 1985).
3. Don’t drink!
• Effects of alcohol consumption on GPA
have been consistently found to be negative
and significant (Finnell & Jones, 1975). The
magnitude of the correlations is in the
ballpark of -0.2 (Larrieu, 2004).
4. Don’t do drugs!
• Effects of Marihuana consumption on GPA
have been found to be significantly
negative. The magnitude of the correlation
is in the ballpark of -0.25 (Larrieu, 2004).
Summary
•
•
•
•
Attend as many classes as possible
Study at least 5 hours a day (outside of class)
Don’t drink
Don’t do drugs
•  Be a complete nerd (or act like one).
• Caveat: The effects outlined above might be
non-independent and might not be causal.
The Auditory System
If a tree falls but there is no one
there to listen, does it make a sound?
Profound question?
Silly question?
Why focusing on the visual system
for most lectures?
• This approach has a good rationale:
• Rather studying one system very thoroughly than all
systems superficially.
• Primary Goal: Achieving understanding of one
system rather than amassing easily forgotten
factoids about many system.
• Secondary Goal: Extend the concepts introduced in
the visual system to other perceptual systems.
Why focusing on the visual system?
•More than 50 %
of the macaque
brain is visually
responsive.
•It is certainly the
most studied
perceptual system
•Possibly also the
best understood.
Now we will use this approach to
• Outline similarities and differences between
the auditory and visual system.
• Essentially understanding the auditory system
in terms of the concepts we introduced in the
context of the visual system.
Functional classification of external
perceptual systems
• Long range systems
• Vision (Seeing)
• Audition (Hearing)
•
•
•
•
Short range systems
Gustation (Taste)
Olfaction (Smell)
Somatosensation
(Touch, Pain, etc.)
Vision and Audition are both long
range, external perceptual systems.
The physical stimulus
• Vision:
• Electromagnetic waves (in humans with a
wavelength of around 400-700 nm)
• Audition:
• Pressure waves in air (in humans with a
frequency of around 20-20000 Hz)
Refresher on wave-physics
•
•
•
•
•
•
•
•
•
•
1 Hz = 1 cycle per second (Unit of frequency)
Conversion: Frequency (f) = Speed (c)/Wavelength(λ).
Example 1: Speed of light in vacuum/air ≈ 3*108 m/s
f = c/λ  3*108 / 400 * 10-9 = 7.5 * 1014 = 750 TeraHz
f = c/λ  3*108 / 700 * 10-9 = 4.29 * 1014 = 429 TeraHz
We see in a range of 400-700 nm = 429-750THz
Example 2: Speed of sound in air (at 20º C) ≈ 343 m/s
λ = c/f  343 / 20 = 17.2 m
λ = c/f  343 / 20000 = 1.72 cm
We hear air waves with a wavelength of 1.72 cm to 17.2 m.
Stimulus comparison
• The physical stimulus of both the auditory
system and the visual system can be
described as a wave-phenomenon with a
certain wavelength and frequency.
• However, the physical quality of the waves
(electromagnetic vs. air-pressure) as well as
the frequency-ranges (or wavelengths) are
quite different.
Psychological qualities of sound
Amplitude
 Loudness
Wavelength = Cyclelength.
Frequency = 1/Wavelength
 Pitch
Some examples
300 Hz,
low.
3000 Hz,
high.
Big
amplitude,
loud
Small
amplitude,
loud
300 Hz,
low.
3000 Hz,
high.
Small
amplitude,
not as loud
Small
amplitude,
not as loud
Intensity/Loudness
• The amplitude corresponds to Sound Pressure
• Sound Pressure levels is measured in decibels (dB SPL)
• Just like with luminances in vision, the auditory system
is capable of hearing sounds over many orders of
magnitude of sound pressure.
• Hence, the decibel scale is logarithmic. Multiplying the
sound pressure by 10 adds 20 dB.
• Loudness estimate are largely a linear function of sound
intensity as measured in dB.
Stimulus Correspondences
• Visual System
• Auditory System
• Frequency = Hue, Color.
• Amplitude = Luminance
• Frequency = Pitch
• Amplitude = Loudness
Example:
Light that contains all frequencies equally appears white:
Sound that contains all frequencies equally is called “white noise”
Anatomy of the auditory system
Equalizer
system
Coarse
amplification
Contains
primary
receptors
(transducers)
The inner ear
• Filled with fluid
• Contains the Organ of Corti, which contains the
transducers that transduce mechanical pressure waves
into action potentials.
• The cochlea is tonotopically organized. Frequencies map
orderly along the length of the cochlea.
• Hair cells near the Apex respond best to low frequencies
• Hair cells near the Base respond best to high frequencies
• Base = oval window, connection to Stapes
The inner ear (cochlea)
The inner ear uses a place code to
decode fundamental frequencies
The organ of corti
• Corresponds to the retina in it’s function as an
array of primary transducers.
• Contains two membranes (Basilar membrane and
tectorial membrane) and Hair cells.
• The hair cells sit on the basilar membrane.
• Their cilia move against the tectorial membrane.
• They bend. This causes action potentials by the
hair cells.
Hair cells
• The primary transducing receptors of the
auditory system
• They have cilia that bend
• Bending of the cilia of the inner hair cells
generates action potentials. They are the
transducers.
• The outer hair cells have modulatory functions,
effectively narrowing the frequency tuning of
the basilar membrane.
Hair cells
Visual:
The auditory pathway
Auditory
Retina
Cochlea
Chiasma
SON
SC/LGN
IC
V1
MGN
V2, etc.
A1
A2, etc.
Big difference between the systems:
• The temporal fidelity in the auditory system is
much higher than that in the visual system.
• This temporal fidelity is for example needed
in sound localization, where interaural time
differences are utilized (on the order of
microseconds)
Temporal order judgements
1
2
Visual fusion threshold
40 ms
Auditory fusion threshold
2 ms (!)
This is not
wrong, it’s
british english
Visual and Auditory illusions
• There are illusions in both systems.
• Most of the auditory ones require a very controlled
stimulus administration that is not possible here (e.g.
headphones)
• Hence, the demonstration of Shepard’s Tones, an
ever-ascending scale of tones should be sufficient (as
a proof of concept)
• Psychophysics of the effect is well established,
underlying cortical mechanisms are not.
Shepard’s Tones
• Locally, each sound will
appear higher than the
previous one.
• This is due to the relative
spacing of tone
components (1 octave
apart) and appropriate
scaling (amplitude).
• The brain makes a
(wrong) perceptual
decision, based on in-built
assumptions
Interactions between the systems
• Neat separation of visual vs. auditory system is a
textbook phenomena.
• Many real life phenomena have corresponding sights
and sounds.
•  There is crosstalk between the systems, the brain
uses multimodal information to disambiguate the
perceptual world.
• Primary examples: The McGurk effect
• And Motion Disambiguation
The McGurk effect
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Auditory: Ba
Visual: Ga
Fused percept: Da
Discovered by McGurk in 1976 by accident
Neural mechanisms remain unknown
Motion disambiguation by sound
Back to the tree
• When it falls, it creates a pressure wave in the air.
• If there is no organism with an auditory system
present that interprets these oscillations in the air as
a sound, it will make no noise.
• Even if organisms with auditory systems are
present, it will sound different to different
organisms (if they have different auditory systems).