Transcript Auditory information processing at the cortical level

Lecture 30
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The primary auditory cortex appears to be
well organised with respect to frequency and
carries on its surface a “map” of the cochlea,
as is found in the subcortical nuclei.
High frequency excitation, orignating in the
base of the cochlea, is received in neurons
located in the more medial portion of the
primary area, deep in the lateral fissure.
Low frequency information from the base of
the cochlea is handled by neurons located
more laterally.
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Auditory information processing in the auditory
cortex occurs due to a group of neurons
organised in a vertical manner (columnar
organisation).
The most clear-cut parameter along which this
organisation has been observed is the
characteristic frequency of the nerve cells.
Those neurons are sharply selective to one
frequency of stimulation tend to the same
characteristic frequency if they lie within the
same column
The nerve cells of the auditory cortex appear to
reflect increased specialisation.
The nerve cells become increasingly selective in
their response to novel stimuli or certain features
of the stimulus.
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Which functions can still be accomplished (or
relearned) when the auditory cortex has been
removed?
Which ones can be carried out only when the
cortex is intact?
These questions have been investigated in
laboratory animals, particularly cats. The
approach has been to bilaterally ablate the
auditory cortex and then test the animals ability
to perform or relearn various sound
discrimination tasks.
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There are some tasks that can be
accomplished after bilateral ablation:
1. The onset of the sound
2. Changes in the tonal intensity
3. Changes in the frequency of the tonal
stimulus
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*however, the type of the frequency
discrimination is critical especially when the
discrimination is between sequence of tones
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Bilaterally ablated animals cannot
discriminate:
1. Changes in the tonal duration
2. Changes in the temporal pattern of a tonal
sequence
3. Sound localisation in space
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Neff (1967) suggested that the auditory
cortex plays an important role in the
accurate localisation of sounds in space.
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Most studies on humans with central auditory
lesions have employed speech tests.
Bilateral temporal lobe damage in humans
have shown to result in impaired ability to
make temporal pattern discriminations.
Temporal lobe damage in humans has also
been reported to result in impaired sound
localisation in space.
These results confirm the animals studies.
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