SLHS 409 Lecture 9 Study Notes

SLHS 409 Lecture 9: Auditory Processing

Place Coding

  • In auditory processing, place coding refers to how different parts of the cochlea respond to different frequencies of sound.

Timing Code

  • The timing code involves the phase-locked firing of auditory nerve fibers, which provides information about the frequency of a stimulus for frequencies below 3 kHz.

Detailed Mechanism of Auditory Coding

  • When a periodic stimulus, such as a pure tone, is presented:

    • The firing pattern of auditory nerve fibers encodes information about the periodicity of the stimulus.

    • The place of firing for action potentials is stereotyped, meaning that each action potential occurs during the compressive phase of the stimulus.

  • As the stimulation frequency increases, if the stimuli become too rapid, the nerve fiber may be unable to produce action potentials on a cycle-by-cycle basis.

Sensorineural Hearing Loss (HL)

  • Sensorineural Hearing Loss involves damage to the inner ear or the cochlea.

    • Hearing aids can help amplify sound to activate remaining viable hair cells.

    • Hair cell regeneration can occur in adult chicken basilar papillae after noise-induced hair cell loss, indicated by new stereociliary bundles.

Cochlear Nerve Pathways

  • The cochlear nerve delivers acoustic information through parallel pathways to tonotopically organized cochlear nuclei in the medulla:

    • Fibers carrying information from the basal end of the cochlea (which detects high frequencies) terminate dorsally.

    • Fibers from the apical end of the cochlea (detecting low frequencies) terminate ventrally in both the ventral and dorsal cochlear nuclei.

  • Each cochlear nerve fiber innervates different areas in the cochlear nuclei, connecting with various types of neurons that project to higher auditory centers.

  • The auditory pathway includes at least four parallel ascending pathways that extract different acoustic information simultaneously from signals sent by cochlear nerve fibers.

Cochlear Nucleus Functions

  • The cochlear nucleus is not simply a relay but contains local circuits for:

    • Extracting and maintaining stimulus representations.

Ventral Cochlear Nucleus (VCN)

  • The VCN extracts both temporal and spectral information:

    • Comprises three types of neurons that intermingle and form pathways through the brainstem:

    • Bushy cells:

      • Project bilaterally to the Superior Olivary Complex (SOC) in the pons.

      • Have two main components:

      • One part travels through the medial nucleus of the trapezoid body (MNTB).

        • Enables comparison of interaural intensity between ears.

      • The other projection courses through the medial superior olive (MSO), allowing for comparison of the time of arrival of sound at both ears.

    • Small spherical bushy cells and globular cells are sensitive to higher frequencies, while large spherical bushy cells respond to lower frequencies and project bilaterally to the MSO.

    • This cellular arrangement helps detect interaural time delays, crucial for localizing low-frequency sounds in the horizontal plane.

Lateral Superior Olive (LSO)

  • The lateral superior olive detects differences in interaural intensity:

    • Small spherical bushy cells excite the LSO ipsilaterally.

    • The LSO receives excitatory input from one ear and inhibitory input from the other, making LSO neurons sensitive to the difference in intensity between ears.

    • The term L (for LSO) refers to intensity level differences.

Inhibition and Sound Localization

  • The globular bushy cells excite neurons in the contralateral medial nucleus of the trapezoid body, which in turn inhibits the principal cells of the LSO.

    • For example, when sound originates from the right, the right ear detects it earlier than the left ear, creating a time delay known as interaural time delay.

  • Individual bushy cells may sometimes fail to fire during certain cycles, leading to a system where a set of cells encodes the timing of low-frequency sounds and the respective frequency at every cycle.

Types of Neurons in Brainstem Pathways

  • The three types of neurons in the auditory pathways are:

    • Bushy cells

    • Stellate cells:

    • Terminate across a wide area.

    • Excite neurons in various regions.
      -Tonotopic arrangement enables the encoding of different sound spectra.

    • Octopus cells:

    • Excite specific neurons that provide sharply timed glycinergic inhibition to the inferior colliculus in the midbrain.

    • Vital for detecting onsets of sounds, allowing animals to detect brief gaps and mark spectral components originating from a single source that start together.

Dorsal Cochlear Nucleus (DCN)

  • The lecture mentions the dorsal cochlear nucleus but does not elaborate on it in the provided text, indicating that further details may be discussed later.