Download Neural Encoding Talk

Sound perception and
it’s neural coding
03/29/2012 Dr. Sorinel A. Oprisan
PHYS 150
Sensations and perception
“In psychology, sensation and perception are stages of
processing of the senses in human and animal systems, such as
vision, auditory, vestibular, and pain senses.”
(http://en.wikipedia.org/wiki/Sensation_and_perception)
Sensations are the first stages in the functioning of senses to
represent stimuli from the environment, and perception is a
higher brain function about interpreting events and objects in the
world.
(David G. Myers (2004). Exploring Psychology, Macmillan.
pp. 140–141. ISBN 9780716786221)
Review of concepts (6 min)
http://www.sumanasinc.com/webcontent/animations/content/soundtransduction.html
Sensations and perception
Sensations and perception
Sound introduced into the cochlea via the oval window flexes the
basilar membrane and sets up traveling waves along its length.
The taper of the membrane is such that these traveling waves are
not of even amplitude the entire distance, but grow in amplitude to
a certain point and then quickly fade out. The point of maximum
amplitude depends on the frequency of the sound wave.
Each point on the basilar membrane oscillates up and down at the
same frequency as the sound. What differs from point to point is
the size of the oscillation.
Displacement of the basilar membrane in response to a pure tone
Zwislocki and Feldman, 1963
Sensations and perception
The pinna and middle ear act as mechanical transformers and amplifiers, so that by the
time sound waves reach the organ of Corti, their pressure amplitude is 22 times that of
the air impinging on the pinna.
Hair cells are the sensory receptors of both the auditory system and the vestibular
system in all vertebrates.
In mammals, the auditory hair cells are located within the organ of Corti on a thin
basilar membrane in the cochlea of the inner ear. Mammalian cochlear hair cells come in
two anatomically and functionally distinct types: the outer and inner hair cells. Damage
to these hair cells results in decreased hearing sensitivity, i.e. sensorineural hearing
loss.
The organ of Corti contains 15,000-20,000 auditory nerve receptors. Each receptor has
its own hair cell. The shear on the hairs opens non-selective transduction ion channels
that are permeable to potassium and calcium, leading to hair cell plasma membrane
depolarization, activation of voltage-dependent calcium channels at the synaptic
basolateral pole of the cells which triggers vesicle exocytosis and liberation of glutamate
neurotransmitter to the synaptic cleft and electrical signaling to the auditory cortex via
spiral ganglion neurons.
Confocal and scanning electron micrographs of inner and outer hair bundles (scale bar
1 mm)
Eric A. Stauffer & Jeffrey R. Holt, J Neurophysiol, 98, 2007
Auditory nerve
Each auditory nerve fiber responds to a narrow band of
frequencies, with a phase locked response that increases in
rate with sound intensity
A tuning curve shows the amplitude of the input at each
frequency required to produced an equal response in a
device
Frequency threshold curves (FTCs), or tuning curves, plot
the minimum intensity of sound needed at a particular
frequency to just stimulate an auditory nerve fibre above
spontaneous activity.
Damage to the cochlea easily abolishes the tip, and
explains some features of Sensori-Neural Hearing Loss:
raised thresholds and reduced frequency selectivity.
Characteristics of auditory nerve tuning
curves
• Band-pass in shape, about 1/3 octave
wide.
• Best or “characteristic” frequency.
• Steep high frequency slope
• Extended low frequency tail
Even when you just present a tone at a moderate intensity, more than one nerve fiber
will respond. The brain has to look at the pattern of response across nerve fibers
(sometimes called the “excitation pattern”).
Auditory nerve fibers respond at some
rate even when no sound is presented =
spontaneous activity.
A hair bundle = 20 to > 300 stereocilia (1mm – 100mm in length, a tip link (pink) is 150
nm long, a channel (yellow) is < 10 nm in diameter, and its gate (orange) moves by 4
nm)
A robust stimulus (60 dB sound-pressure level) deflects the bundle only 10 nm, whereas
a threshold stimulus moves the bundle less than 1 nm.
Applied force (green arrow) extends the tip link. When a channel opens (curved orange
arrow), relax the tip link, causing the bundle to move still further (red arrow).
A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772
A stimulus force (green arrow) initially deflects the hair bundle, opening a transduction
channel. A Ca2+ ion (red) that enters through the channel interacts with a molecular
motor, probably myosin, and causes it to slip down the stereocilium’s actin cytoskeleton
(orange arrow). Slackening of the tip link fosters a slow movement of the bundle in the
positive direction (dashed red arrow). The reduced tension in the tip link then permits the
channel to reclose.
A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772
If the internal-energy content of a two-state channel is EO in the open state and EC in the
closed state, the equilibrium probabilities of these two configurations, respectively pO
and pC, are related by the Boltzmann equation
A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772
(Fettiplace& Mackney, 2006
http://www.leica-microsystems.com/science-lab/the-patch-clamp-technique-an-introduction/
A virtuous loop.
Sound evoked perturbation of the organ of Corti elicits a motile
response from outer hair cells, which feeds back onto the organ of
Corti amplifying the basilar membrane motion.
Two-tone suppression
If a tone at a fiber's CF is
played just above threshold
for that fiber, the fiber will
fire. But if a second tone is
also played, at a frequency
and level in the shaded
area of the next diagram,
then the firing rate will be
reduced. This two-tone
suppression demonstrates
that the normal auditory
system is non-linear.
Two-tone inhibition is the
result of mechanical
processes inthe cochlea.
Place Code Theory: Helmholtz's theory
Sensation of a low frequency pitch derives exclusively from the motion of a particular
group of hair cells, while the sensation of a high pitch derives from the motion of a
different group of hair cells
Temporal Code Theory: According to temporal code theory, the location of activity
along the basilar membrane is irrelevant. Rather, pitch is coded by the firing rates of
nerve cells in the audotry nerve. But a single nerve cell can not signal at a rate of 20,000
Hz
Cochlear Microphonic: The cochlear microphonic is a discovery that cast doubt on
Helmholtz's place code and supports the temporal code theory. It was discovered by
Wever.
Phase Locking is an empirical observation that supports the volley principle.
Sensations and perception
Tonotopic organization: The spatial layout of frequencies in the cochlea along the basilar
membrane is repeated in other auditory areas in the brain.
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.
html
Auditory cortex
Located in the temporal lobe, the auditory cortex is the primary receptive area
for sound information. The auditory cortex is composed of Brodmann areas 41
and 42, also known as the anterior transverse temporal area 41 and the
posterior transverse temporal area 42, respectively. Both areas act similarly and
are integral in receiving and processing the signals transmitted from auditory
receptors. (http://en.wikipedia.org/wiki/Sensation_and_perception)
There are between 30,000 and 40,000 nerve fibres in the cochlea of a normal hearing
adult (Tylstedt, 2003).
The number of myelinated axons per millimetre length of the cochlear duct:
-300 fibres per millimetre at the basal end of the cochlea
-reaches a maximum in the lower second turn with 1400 fibres per millimetre-decreases again towards the cochlear apex to about 400 fibres per millimetre.
This corresponds to an average of 15 nerve fibres per inner hair cell in the lower second
turn and 3-4 nerve fibres per inner hair cell at the base and apex (Spoendlin and Schrott,
1989).
Average interneural distance of 700 nm in the lower second turn of the cochlea and
3300 nm at the basal end.
Temporal response patterns of a low-frequency axon in the auditory nerve. The stimulus
waveform is indicated beneath the histograms, which show the phase-locked responses
to a 50-ms tone pulse of 260 Hz. Note that the spikes are all timed to the same phase of
the sinusoidal stimulus. (After Kiang, 1984.)
Each hair cell has about 10-20 auditory nerve fibers connected to it. These fibers have
different thresholds.
Inner hair cells stimulate the afferent auditory nerve, outer hair cells generally do not, but
are innervated by the efferent auditory nerve. Efferent activity may influence the
mechanical response of the basilar membrane via the outer hair cells.