Transcript Central auditory pathways

Lecture 12
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Neural discharges
Neural dynamic range
Spontaneous activity
The synchrony of the discharge
Phase locking
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This is the primary factor which determines our
perception of the loudness of the sound.
The first stage of intensity coding is the
frequency rate of the neural discharges varies in
proportion to the stimulus intensity.
It is the spike rate which fundamentally bears
intensity information.
The spike-rate versus the sound level observed
in recordings from single auditory neuron
(primary) at the characteristic frequency shows
that the dynamic range is limited.
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The maximal spike
rate is reached at
20-40 dB above the
level at which the
spike rate is just
noticeably greater
than the
spontaneous rate.
Yet the dynamic
range of the auditory
system is on the
order of 140 dB
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The coding of intensity must involve more
than the spike rate of any individual neuron.
The most likely scheme seems to be that the
brain is most concerned with the total spike
rate or density of neural discharges from all
active units, regardless of their characteristic
frequency.
A fiber exhibiting a particular characteristic
frequency can be stimulated by a higher and
lower tones if they are sufficiently intense.
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If one fiber can be stimulated by tones of
frequencies other than the characteristic
frequency, it follows that a given tone will excite
an array of neural fibers which more or less
reflect the travelling wave pattern in the cochlea.
Then, even at levels of the stimulus at which a
given units response saturates (no longer
increase with the increasing stimulus), the total
discharges per unit time i.e density of discharges
will increase as more fibers are recruited and
those which are already activated respond more
vigorously.
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The first neural responses at barely threshold
intensities appear to be a decrease in the
spontaneous activity, and phase locking of spike
discharges to the stimulus cycle.
Even though the discharge rate may not be
significantly greater than the spontaneous level,
those spikes that occur will tend to be locked in
phase with the stimulus cycle.
Effects such as this might provide some degree
of intensity coding because fibers with higher
spontaneous rates have lower thresholds.
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The neurons dynamic range is the intensity
range over which the auditory nerve
continues to respond with increasing
magnitude.
Saturation is said to have occurred when the
neurons response no longer increases as the
stimulus level is raised.
The following figure shows an example of the
growth and saturation of an auditory neurons
firing pattern with increasing stimulus
intensity.
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The dynamic range of the auditory nerve fiber is
only 20 -40 dB.
In other words, the discharge rate increases
with the stimulus intensity from the threshold
to a level 20-40 dB above it.
At higher intensities the spike rate either levels
off or decreases.
Obviously, a signal fiber cannot accommodate
the 120+ dB range from minimal audibility to
the upper usable limits of hearing.
However, if there were a set of units with
graded thresholds, they could cooperate to
produce the dynamic range of the ear
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For example, if there were four fibers with
similar CF having dynamic ranges of 0-40,
30-70, 60-100, and 90-130 dB, respectively,
then they could conceivably accommodate
the ears dynamic range.
the identification of three relatively distinct
groups of auditory nerve with respect to their
spontaneous rates and thresholds provided
insight to the intensity coding.
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The three fiber groups include:
1. Fibers with high spontaneous rates (over 18
spikes) had the lowest thresholds of the
three groups.
2. Fibers with medium spontaneous rates
(between 0.5-18 spikes).
3. Fibers with low spontaneous rates have the
highest thresholds of the three groups.
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Each inner hair cell receives all three types
of fibers.
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The fibers are also distinguished on the basis
of size, morphology and where they attach to
the inner hair cells.
The high SR fibers have the largest diameters
and the greatest number of mitochondria.
Low SR have the smallest diameter and
relatively fewer mitochondria.
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To sum up, it appears that type I auditory fibers
with similar best frequencies have a considerable
range of thresholds and dynamic ranges, and
both of these parameters are corrolated with the
SRs of the fibers.
In turn, these SR characteristics tend to
categorise themselves in three groups, which are
also distinguishable on the bases of their size,
morphology, and locations of their synapses.
Each inner hair cell synapses with all three types
of fibers.
There is reasonable basis for the coding of
intensity on the basis of the auditory nerve fibers
and their responses.
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An alternative explanation is based on activity
of the auditory nerve fiber reflecting the
pattern of the cochlear excitation. As the
intensity is increased the number of spikes
per second also increases, as does the
frequency range to which the fiber responds.
The frequency range to which the fiber
responds tends to increase more below than
above the characteristic frequency.
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Effect of stimulus
intensity on the
response area of
a single auditory
neuron (CF 2100
Hz)
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Recall that the basilar membrane may be
conceived of as an array of elements that are
selectively responsive to successively lower
frequencies going from base to apex.
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The frequency is
represented along the
horizontal axis from left
to right.
 The vertical axis
represent the spike rate.
(a) Represents the
response area resulting
from stimulation at a
given frequency and
intensity.
(b) Shows the discharge
pattern for an
equivalent amount of
stimulation at a
different frequency
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The frequency change is represented by a simple
displacement of the hypothetical response area
(or excitation pattern) along the frequency axis
and is analogous to the movement of the
travelling wave envelop along the basilar
membrane.
(c) Represents the effect of increasing the stimulus
level at the same frequency as in (a).
 As the stimulus level increases the fibers increase
their spikes rates (until saturation is reached)
 Although some fibers saturate, other fibers with
similar CFs but with different thresholds continue
to increase their discharge rates as the levels
increases.
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As the intensity continues to increase, the
stimulus enters excitatory areas of other
fibers, which responds to frequency at higher
intensities.
The intensity increment is to be coded by
increased overall firing rates among more
fibers and over a wider frequency range.
Increasing the stimulus level also results in
greater synchrony of the individual neural
discharges, so the whole nerve action
potential has a shorter latency and greater
magnitude.
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The implication is that phase locking to the
stimulus cycle would be particularly
important in maintaining frequency coding
when the intensity increment is encoded by
increase in density of discharges per unit
time and by widening the array of active
fibers.
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The auditory nerve will tend to fire at a
particular phase of a stimulating lowfrequency tone. So the inter-spike intervals
tend to occur at integer multiples of the
period of the tone. With high frequency tones
(> 3kHz) phase locking gets weaker, because
the capacitance of inner hair cells prevents
them from changing in voltage sufficiently
rapidly.
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The superior olivary complex (SOC) is the
lowest level at which binaural information is
available.
Cells in the medial superior olive receiving
input from both ipsilateral and contralateral
cochlear nuclei.
Neurons in the SOC code binaural information
through the interaction of excitatory and
inhibitory inputs, which are the result of
intensity and time (phase) differences in the
stimuli at the two ears.
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The lateral SO is principally receptive of to
higher frequencies and interaural level
differences .
LSO neurons sensitive to ILD tend to receive
ipsilateral inputs that are excitatory and
contralateral inputs that are inhibitory
Various cells respond to different ILDs.
Several investigators have identified neurons
in the LSO that respond to ITDs in the
envelops of high frequency signals.
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The medial SO is responsive to lower
frequencies and interaural time differences
Cells sensitive to ITD in the MSO receive
excitatory inputs from the cochlear nuclei on
both sides.
MSO neurons fire maximally in response to a
certain ITD, or a characteristic delay.
Neurons in the MSO appear to be arranged
according to their characteristic delay along
the anteroposterior axis of the nucleus .
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