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Getting an Earful
Winter 2017
Peter Woodruff
How can one sense movement of a fluid?
How to sense movement of a fluid?
Hair versus hair cells
How to sense movement of a fluid?
How to sense movement of a fluid?
Sensitive Sharks
Sensitive Sharks
Linear Acceleration Coding by Maculae
Tilt causes shearing
forces on some hair cells
which depolarize as
others hyperpolarize.
Rotational Coding by Semicircular Canals
As one of the canals moves
in an arc with the head, the
internal fluid moves in the
opposite direction, causing
the cupula and stereocilia
to bend. Brain interprets
relative activation of all six
canals to give precise
indication of head
movement.
Sound attributes
• Pitch determined by frequency: vibrations per second, in hertz, Hz
• Loudness determined by amplitude: height of sound wave a sound
intensity, as in decibels, dB
• Timbre: determined by
“complexity and shape”
of sound wave
• Tone: simple sound
Sound attributes
• Pitch determined by frequency: vibrations per second, in hertz, Hz
• Loudness determined by amplitude: height of sound wave a sound
intensity, as in decibels, dB
• Timbre: determined by
“complexity and shape”
of sound wave
• Tone: simple sound
Fun Facts about Hair Cells
• Hair cells can detect deflections of 0.3 nanometers (< size of atom!)
Threshold of hearing: 1 billionth of atmospheric pressure
• Can convert stimulation into nerve impulse in 10 microseconds
• Threshold of pain at 130 dB is 1013 times more intense than
threshold of hearing, at 0 dB!
Tasks of the auditory system
• Resolve intensity (loudness) and frequency (pitch, timbre)
components of sound stimuli
• Localize sound sources in space
Human Hearing
How would YOU code?
1. Frequency?
2. Loudness (Amplitude)?
Cochlear animation
When the stereocilia
are bent in response
to a sound wave, an
electromotile
response occurs.
With every sound
wave, the cell
shortens and then
elongates.
This pushes against
the tectoral
membrane,
selectively amplifying
the vibration of the
basilar membrane.
Frequency segregation in the cochlea – an initial sort of
sound frequencies
Sound Frequency -> Position -> specific neurons
• Pitch to Position: tones of 20,000 Hz and 20 Hz activate different
neurons, leading directly into the auditory nerve.
The spatial layout of frequencies in the cochlea is repeated in other
auditory areas in the brain. This is called tonotopic organization.
Tonotopic Mapping through Auditory Pathway
Minimum sound pressure
Perceived for each frequency
audible
inaudible
Human Hearing Ranges
Sound Spectrua of Various Instruments
Sound Spectra of Various Animals
Coding of Auditory Information
1. In Cochlear Nerve:
a)
b)
Firing rates of neurons
Number of active neurons
Together correlate with perceived
loudness
2. Relative location (tonotopic)
Preserved throughout auditory pathway
Converts frequency (tone) to position
Not whole story
The main pathways and nuclei
are shown for both cochleae.
Max response rate of
neuron:
~1000 firings/second
For sounds of higher
frequency, coding
carried by many
neurons together,
here neurons a - e
Harmony
• Why do tones whose frequencies are in ratios of small numbers
sound good together? See Pythagoras
Harmony
• Why do tones whose frequencies are in ratios of small numbers
sound good together? See Pythagoras
Consonance
A Feeling for Harmony:
Processing Auditory Information
Arrival of auditory message in brain:
Processed
•
•
•
as reflex: jump if loud sound
In auditory cortex
In other brain regions
Auditory Cortex
Humans use at least two strategies for sound
localization
• Strategy 1. for frequencies below 3 kHz: time of arrival differences
can be detected. The threshold of detection is as small as 10
microsec. This translates to a sensitivity of about 1o of arc.
Spatial Localization
also Distance
Processing in the brainstem: sound localization by coincidence cells in
the olivary nuclei
• Q. How can time delays as small as 10 microsec be measured by
neurons that have to operate in the msec time domain?
• A. The medial superior olive (MSO) receives bilateral inputs from
the anteroventral cochlear nuclei. These inputs enter a chain of
coincidence cells.
Time is measured by conduction time in the network
Strategy for higher frequencies
• Strategy 2. for higher frequencies, intensity differences between the
two ears must be used. At these frequencies, the sound wavelength
is so short that the waves cannot bend around the head, so the
head creates a sound shadow that enhances the effect.
Sound localization
• The sound arriving at the
ear that is furthest from the
sound source is delayed
(time difference) and lower
in amplitude (intensity
difference). Both cues used.
• “Wiring” in brain consistent
with this function.
The timing difference
never exceeds .8 msec
Detection of intensity differences in the brainstem
The players
here are the
lateral
superior
olive (LSO)
and the
medial
Whichever ear
receives the
loudest stimulus
can also shut off
activity in the
ascending
pathway from
the less
Hearing Loss
Causes:
ear malformation,
abnormal bone growth,
fluid accumulation due to infection,
poor drainage
hole in eardrum
Treatments: varies with circumstances
surgery,
antibiotics,
hearing aid
Causes:
usually cumulative, slowly
often loud noise exposure
some medications & health conditions
Treatments:
hearing aids
possibly cochlear implant
Mixed Hearing Loss
Electrodes placed on the surface of the cortex can be used to
stimulate the brain of a conscious patient or record its
activity.
Corrected Somatosensory Homuculus