An Example of Neurosensory Processing: Audition

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Functional Systems

• An Example of Neurosensory Processing Audition

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Neurosensory Processing of Audition

• As we have seen, sensory systems consist of
  serial chains of neurons that link the periphery
  with the spinal cord or brainstem, thalamus, and
  cerebral cortex.
• The most striking aspect of organization of the
  sensory system is that the peripheral receptor
  sheet, such as the surface of the body, the
  retina, or the cochlea, is systematically mapped
  onto structures of the brain.
• These maps are not strictly isomorphic with the
  size and shape of the periphery, but reflect the
  relative importance of the various regions of the
  receptive sheet.

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Neurosensory Processing of Audition

• Each sensory system first decomposes the sensory
  information and then reconstructs the perception
  by using the different components that
  selectively process one or another aspect of the
  sensory experience.
• Lets lean how this works using the auditory
  sensory system.
• Before a sound signal, such as a speech signal,
  gets to the auditory cortex, it passes through
  levels of peripheral and central processing.
• From the tympanic membrane to the hair cells of
  the organ of Corti in the cochlea, the process of
  detection and replication of the acoustic
  patterns is either mechanical or hydraulic.

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Neurosensory Processing of Audition

• At the level of the cochlea, reception of sound
  energy is analyzed into frequency and amplitude
  representations and is retransmitted in neural
  form via the cochlear branch of the
  vestibulocochlear nerve (CN VIII).
• The central processes of the cochlear nerve
  constitute the first-order neurons.
• The cochlear nerve is accompanied by CN VII, the
  facial nerve, in the auditory canal.
• The two nerves enter the brainstem at the sulcus
  between the pons and the medulla

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Neurosensory Processing of Audition

• The fibers of the cochlear division of CN VIII
  end in the dorsal and ventral cochlear nuclei.
• The cochlear nuclei contain the second-order
  neurons of the auditory pathway.
• From the cochlear nuclei, most fibers of the
  auditory pathway cross the midline and proceed to
  the upper medulla and pons.

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Neurosensory Processing of Audition

• Other fibers ascend in the brainstem
  ipsilaterally.
• Fibers course upward in the ascending central
  auditory pathway of the brainstem called the
  lateral lemniscus.
• The fibers take one of several routes, and
  synapses in the auditory system may occur at one
  or more of the following structures the superior
  olives, the inferior colliculus, and the nuclei
  of the lateral lemniscus.

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Neurosensory Processing of Audition

• All ascending auditory fibers terminate in the
  medial geniculate body of the thalamus.
• In all brainstem nuclei, from the cochlear
  nucleus to the medical geniculate body of the
  thalamus, frequency representations are
  maintained positionally.
• High frequency stimuli are encoded on one side
  of the network, and low frequency stimuli on the
  other.

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Neurosensory Processing of Audition

• With respect to processing, the cells making up
  the cochlear nuclei are hypothesized to process
  the periodicity of monaural signals.
• This is the first level of real information
  processing.
• At the level of the superior olivary nucleus,
  input from both ears is received for the first
  time, from the contralateral and ipsilateral
  pathways.
• Differences in interaural temporal and intensity
  information are compared and matched for cues to
  sound localization and directional hearing.
• These interaural temporal and intensity
  differences are then relayed to the inferior
  colliculus.

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Neurosensory Processing of Audition

• These interaural temporal and intensity
  differences are then relayed to the inferior
  colliculus which also receives the frequency and
  periodicity information from the cochlear
  nucleus.
• As a site of convergence, the inferior colliculus
  is hypothesized to simultaneously code the
  complexity of sounds and their direction in
  space.
• Not until the inferior colliculus has
  reconstructed the changes in sound velocity,
  direction, stimulus frequency, and amplitude, can
  the medical geniculate recognize the complex
  frequency ranges and transitions of both vowels
  and consonants.
• Below this level, categorical feature detection
  is not thought to occur.

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Neurosensory Processing of Audition

• The medical geniculate body is viewed as a
  multi-channel mechanism of the central auditory
  pathway.
• It is thought to be able to store two or more
  items (echoes) simultaneously, in
  uninterrupted pre-categorical form for
  ultra-short periods of time, until a higher order
  analysis assigns them to a more persistent and
  established categorized form.
• The fibers arising from the medial geniculate
  body that course through the temporal lobe are
  called auditory radiations.

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Neurosensory Processing of Audition

• They pass through the internal capsule on route
  to the the bilateral primary auditory areas of
  the brain in the superior temporal gyrus, also
  known as Heschls gyrus (A1).

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Neurosensory Processing of Audition

• The primary auditory cortex (areas 41 and 42) is
  believed to be the area in which initial
  recognition of an auditory stimulus occurs at the
  cortical level.
• Cells in A1 have been found to be sensitive to
  the direction and velocity of frequency changes
  required to identify the location of a sound
  source, especially if the sounds originate in the
  contralateral extrapersonal space.

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Neurosensory Processing of Audition

• Additionally, many neuronal cells show particular
  sensitivity to the features of complex sounds.
• Although they should not be regarded as feature
  detectors per se, they are thought to recognize
  the constancies of size, shape, and position in a
  gestalt sense (e.g., the whole is greater than
  the sum of its parts).
• Many neurons in A1 also show complex temporal
  response patterns, and may be the site of
  auditory short-term memory.
• At this level, however, auditory memory must be
  viewed as a single-channel mechanism, responsive
  to the temporal, or serial order of the
  pre-categorical acoustic input received from the
  medial geniculate body.

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Neurosensory Processing of Audition

• Since A1 has also been hypothesized to recognize
  consistencies of position, size, and shape, it
  seems highly possible that the relevant
  attributes of the pre-categorical acoustic forms
  are matched at this level and categorized into
  some more stable or permanent auditory template
  representations.
• Wernickes area (area 22) is most frequently
  cited as being the auditory association cortex.
• But, as we have seen, an important distinction
  must be made between modality specific or
  unimodal association cortex, and multimodal or
  heteromodal association cortex.

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Neurosensory Processing of Audition

• The unimodal portion of Wernickes area (area 22)
  receives input from A1 and the medical geniculate
  bodies of the thalamus.
• It then relays thalamic pre-categorical and
  primary categorical template matches in
  multiple steps to the progressively more anterior
  parts of the gyrus.

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Neurosensory Processing of Audition

• Here more abstract features of auditory
  information are postulated to be extracted by
  neurons more responsive to individual patterns
  rather than to isolated stimulus features.
• Outputs from these anterior areas are then
  believed to be directed towards the paralimbic
  and limbic structures of the temporal lobe and
  also to the prefrontal and temporoparietal
  heteromodal fields.

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Neurosensory Processing of Audition

• Although cortical auditory recognition of the
  acoustic stimulus occurs in the A1, it is not
  until these auditory templates are relayed to the
  unimodal association cortex that the actual
  experience or perception of the stimulus
  occurs.
• Within the heteromodal portion of Wernickes
  area, modality specific information is lost in
  favor of intermodal associations.

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Neurosensory Processing of Audition

• Output from unimodal AA is projected to several
  heteromodal areas within the temporoparietal
  cortex, and to the prefrontal heteromodal
  association areas as well.

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Neurosensory Processing of Audition

• The importance of heteromodal regions is to
  provide a neural template for intermodal
  associations necessary for many cognitive
  processes, such as language.
• They also provide the initial interaction between
  extensively processed sensory information and
  mood and drive.
• They also provide the initial interaction between
  extensively processed sensory information and
  mood and drive.
• Thus, the heteromodal association areas are
  likely to emphasize associative elaboration of
  perceptual and cognitive processes, and provide
  the firs real linguistic input.

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Neurosensory Processing of Audition

• In summary, the role of the cortex in auditory
  perception can be delineated by three distinct
  regions.
• In the first region, A1 receives pre-categorical
  acoustic forms from the medial geniculate body
  and matches them into cortical template
  representations (gestalts).
• In the second region, AA experiences the
  projected categorical templates and completes the
  process of auditory perception. Finally, the
  heteromodal association regions, receiving the
  perceived auditory information from the unimodal
  association area, integrates this information
  with that from other unimodal association areas,
  some of which my be linguistic or emotional,
  for further cognitive and motor processing.

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Behavioral Manifestations for Auditory Cortical
Lesions

• Unilateral lesions of A1 do not lead to
  contralateral deafness because A1 has access to
  information from both ears, even though the
  influence of the contralateral ear appears
  stronger.

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Behavioral Manifestations for Auditory Cortical
Lesions

• Moreover, because the medial geniculate body has
  major projections not only to A1, but also to AA
  in the adjacent superior temporal gyrus, complete
  cortical word deafness (auditory verbal agnosia)
  is not likely unless there is bilateral damage to
  both A1 and AA.
• Lesions that destroy AA lead to impairments in
  retention and discrimination of auditory
  frequency and sequence.
• The retention impairment reflects auditory-limbic
  disconnections
• The frequency and sequence discrimination
  impairment reflects disturbances of auditory
  template formation.

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Behavioral Manifestations for Auditory Cortical
Lesions

• Lesions in AA of the right hemisphere may lead to
  nonverbal auditory agnosia.
• Such individuals have difficulty identifying
  environmental sounds, familiar melodies, and
  variations in timbre.
• Bilateral lesions in heteromodal association
  areas dependent upon the unimodal AA for input,
  or a strategically situated unilateral left-sided
  lesion, interrupting the transcallosal input from
  the contralateral AA has been found to contribute
  to an inability to understand or repeat spoken
  language.

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Behavioral Manifestations for Auditory Cortical
Lesions

• Damage to the heteromodal portion of Wernickes
  area can contribute to the emergence of what has
  been termed Wernickes aphasia.
• Nonetheless, the language deficit seen in
  Wernickes aphasia will not be confined solely to
  the auditory modality.
• Comprehension deficits are also seen in the
  graphic modality.

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Centrifugal Auditory Input

• As we have seen so far, auditory perception seems
  to involve hierarchically ordered levels or
  stages of processing in which the raw sensory
  data are successively transformed by the
  different nuclei of the central auditory nervous
  system into some organized patterns or gestalts
  of acoustic properties for cortical recognition
  and interpretation.
• Indeed, this type of sequential organization
  seems to imply that all auditory information is
  processed serially from the point it enters our
  ears until the point at which it is perceived
  cortically.
• This is not the case, however, because decisions
  at higher levels can influence processing at
  lower levels by way of the centrifugal auditory
  pathways.

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Centrifugal Auditory Input

• Within the central auditory NS, centrifugal
  connections arise from each of the areas involved
  in the auditory system.
• From A1, centrifugal connections project to
  nuclei one or two levels below their point of
  origin.
• From a group of neurons on the medial side of the
  contralateral superior olive, cochlear efferent
  fibers pass to the cochlea via the crossed
  olivocochlear bundle, and the cochlear division
  of the vestibulocochlear nerve (CN VIII).

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Centrifugal Auditory Input

• Individual connections may be either excitatory
  or inhibitory, but the centrifugal auditory
  pathways appear to be activated by the inhibition
  of transmission of auditory signals through the
  ascending auditory pathways.
• With the addition of centrifugal input, the
  perception of an auditory stimulus is affected by
  bi-directional influencessimultaneous bottom-up
  and top-down processing.
• I have proposed a simple model of auditory speech
  perception based on the notion of parallel
  information processing to help you understand
  these bi-directional influences.

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Theoretical Model of Auditory Speech Perception

• For bottom-up processes, specialized receptors,
  highly sensitive to certain patterns of complex
  auditory stimuli, simultaneously respond to
  multiple-stimulus characteristics arising from
  spatial-temporal changes in the acoustic waves.
• At the first level, the organ of Corti acts as a
  transducer, receiving frequency and amplitude
  properties of the sound energy and retransmitting
  it in neural form.
• The cochlear nucleus is the analyzer.
• It breaks up the neural code into frequency,
  periodicity, and intensity parameters.
• It relays the temporal and intensity patterns to
  the localizer and the frequency pattern to the
  synthesizer.

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Theoretical Model of Auditory Speech Perception

• The localizer, the superior olivary complex,
  receives this relay of temporal and intensity
  parameters from both sides of the auditory
  network.
• It detects interaural differences and matches
  them for stimulus source determination and
  directional hearing.
• At the level of the synthesizer, the inferior
  colliculus, intensity discriminations from the
  localizer and frequency discriminations form the
  analyzer converge simultaneously for relay to the
  buffer.
• The medial geniculate body acts as the buffer.

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Theoretical Model of Auditory Speech Perception

• Pre-categorical echoes of the complex frequency
  ranges and transitions of both vowels and
  consonants are held briefly for higher level
  assignment.
• A1 is the categorizer.
• It receives pre-categorical acoustic input in
  serial order from the buffer.
• This input is matched, not to some internal
  standard pattern, but for constancy of frequency,
  intensity, and periodicity, into a more stable
  auditory template representation.
• Unimportant variations in the acoustic signal are
  smoothed out.

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Theoretical Model of Auditory Speech Perception

• When AA, the perceiver, receives the stable
  templates as well as thalamic excitation about
  semantic content, the actual experience of the
  stimulus occurs.
• This experience is projected to several
  elaborators in the heteromodal areas to be
  utilized in other cognitive, linguistic, and
  motor processes.
• In the top-down, active processes, the status of
  the bottom-up or afferent pathway is tuned by the
  efferent system, thus enhancing or decreasing
  responses of the neuronal brainstem nuclei.
• The categorizer, A1, exerts an inhibitory
  influence on the buffer, the medical geniculate
  body, and the synthesizer, the inferior
  colliculus.

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Theoretical Model of Auditory Speech Perception

• This inhibitory influence decreases the
  responsiveness of these two nuclei to eliminate
  steady acoustic signals and restrict the range of
  frequency detection.
• The synthesizer, in turn, attenuates the
  intensity differences at the level of the
  localizer (the superior olive) to improve
  auditory selective attention and to dampen the
  sensitivity of the transducer to prevent
  saturation or damage to the system.
• Finally, the localizer, the superior olive,
  influences habituation to repeated signal
  presentation to enhance auditory discrimination
  at the level of the analyzer.

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Theoretical Model of Auditory Speech Perception

• This simplified model of speech perception
  supports the notion that the auditory central
  nervous system is a dynamic, self-regulating
  system actively searching for informative
  patterns within the complex of information
  potentially available in the media surrounding
  it.