sound source localization

# Sound Source Localization Flashcards - Quizlet Import Format

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What is the pathway from cochlear nuclei to auditory cortex?

Cochlear nuclei → Superior olives (LSO and MSO) → Inferior colliculus (IC) → Medial geniculate nucleus of thalamus (MGB) → Auditory cortex (AC)

What are the three main cues used to localize sound sources?

1. Monaural cues (spectral notches), 2. Interaural time difference (ITD), 3. Interaural level difference (ILD)

What are monaural cues?

Things that a single ear detects, such as spectral notches

What are binaural cues?

Features that require comparison of sound reaching the two ears, including ITD and ILD

What information do binaural cues (ITD and ILD) provide?

Help figure out the source of sound in the horizontal plane

What cue is used to determine the elevation of a sound source?

Spectral notches in a complex sound

What are spectral notches?

Frequencies in a sound that fail to reach the tympanic membrane because they are absorbed by the pinna and ear canal

How do spectral notches help with localization?

Different frequencies are absorbed at different elevations, and we learn which notches occur with which elevations

Which nucleus processes interaural timing differences (ITD)?

Medial superior olive (MSO)

Which nucleus processes interaural level differences (ILD)?

Lateral superior olive (LSO)

What is the response pattern of spiral ganglion cells in the auditory nerve?

Initial burst of action potentials followed by a sustained response

What types of cochlear nuclei neurons are important for sound localization?

Bushy cells (spherical and globular) - they send axons to brainstem nuclei for sound source localization

What are the two main parts of the superior olive?

Medial superior olive (MSO) and Lateral superior olive (LSO)

What type of input do MSO neurons receive?

Excitatory inputs from BOTH left and right cochlear nuclei (bilateral excitation)

What type of input do LSO neurons receive?

Excited by ipsilateral cochlear nuclei and indirectly inhibited by contralateral cochlear nuclei (via MNTB)

What is the MNTB and what is its function?

Medial nucleus of trapezoid body - provides inhibition to LSO from the contralateral side

How many dendrites do MSO neurons have and what innervates them?

Two dendrites: lateral dendrite innervated by ipsilateral ventral cochlear nuclei, medial dendrite innervated by contralateral ventral cochlear nuclei

Why is inhibitory input from MNTB to MSO important?

Makes it so that only when action potentials from the two cochlear nuclei arrive simultaneously do their EPSPs drive action potentials in MSO neurons

What does inhibition do to MSO neuron responses?

Suppresses the response if excitation from the two sides does not reach the neuron simultaneously

What is the Jeffress model?

A model for sound source localization using delay lines from the two ears that converge on a common target, with individual neurons acting as coincidence detectors for simultaneous arrival of action potentials

What are coincidence detectors in the Jeffress model?

Neurons that detect (respond to) the simultaneous arrival of action potentials from both ears

In the Jeffress model, where are axon lengths equal from right and left cochlear nuclei?

In the rostral MSO

In the Jeffress model, what must happen for a caudal MSO neuron to fire?

Sound must arrive earlier in the contralateral ear to compensate for the difference in axon lengths (ipsilateral axons are shorter to caudal MSO)

What was the prevailing model of MSO organization until recently?

That MSO neurons behave like nucleus laminaris neurons with delay lines creating a topographic map for interaural delay

How were axons thought to enter the MSO in the old delay line model?

Contralateral CN axons enter from the rostral end, ipsilateral CN axons enter from the caudal end (opposite directions)

What is a characteristic delay of an MSO neuron?

The specific interaural time difference at which a neuron spikes only when action potentials from ipsilateral and contralateral cochlear nuclei reach it at the same time

Why is phase locking necessary for MSO function?

For coincidence detection to occur reliably and regularly, neurons in the cochlear nuclei must fire action potentials to a particular phase of the sound wave

At what frequencies does phase locking occur in cochlear nuclei neurons?

Only at sound frequencies below 2 kHz (same as spiral ganglion cells)

What is required for synaptic security in the cochlear nuclei?

The synapse between spiral ganglion cell and cochlear nuclear neuron must always generate an action potential if the presynaptic axon spikes, and the delay must always be the same

What type of cochlear nucleus cells provide extremely precise and quick synaptic transmission?

Spherical bushy cells

Do birds and mammals use the same strategy for ITD processing?

No - nucleus laminaris of birds (and some reptiles) uses delay lines, but mammalian MSO does not use delay lines

How do axons actually enter the mammalian MSO?

Axons from cochlear nuclei on both sides enter the MSO from its rostral extreme and then move backward

Is there a topographic map for interaural timing difference in mammalian MSO?

No - individual MSO neurons have tightly tuned characteristic delays but there is no topographic map (no place code) for ITD

What is the current best model for mammalian MSO function?

Two-channel model - predicts MSO uses a relative rate code to signal ITD

How does the two-channel model predict MSO responses change with sound location?

As a sound source moves away from the midline, the response of contralateral MSO increases

What is the key structure that enables LSO to compute ILD?

Inhibitory neurons in the medial nucleus of trapezoid body (MNTB)

How does MNTB enable ILD computation?

MNTB turns the excitatory input from contralateral AVCN into an inhibitory output to LSO

What does the LSO receive from the ipsilateral side?

Excitatory input from ipsilateral cochlear nuclei

What does the LSO receive from the contralateral side?

Inhibitory input via MNTB

Why is ILD more effective at high frequencies than low frequencies?

High frequencies are reflected, creating a sound shadow on the opposite side with lower amplitude. Low frequencies wrap around the head so similar amplitudes reach both ears

What ILD is produced by a 6 kHz tone when the sound source is at right angles?

Almost 25 dB (SPL at nearer ear is 25 dB higher than at trailing ear)

What ILD is produced by a 500 Hz tone at right angles?

Less than 5 dB

What ILD is produced by a 3 kHz tone at right angles?

No more than 10 dB

What do the ILD data tell us about frequency limitations?

ILD is useful only for high frequencies; for low frequencies we must use interaural timing differences

What is the anatomical division of labor for sound localization?

ITD processed by MSO (coincidence detectors with characteristic delay), ILD processed by LSO (inhibitory circuit for comparison across hemispheres)

Why does MSO only compute ITD for low frequencies?

Because spiral ganglion cells do not phase lock to high frequency sounds (above 1.5-2 kHz)

Why does LSO only compute ILD for high frequencies?

Because there is no interaural level difference for low frequencies (they wrap around the head)

What are the three features of sound stimuli that inform sound source location?

1. Monaural cues (spectral notches), 2. ITD (interaural timing difference), 3. ILD (interaural level difference)

What does raising outer hair cells (OHC) do to sound perception?

Does not change frequency, but intensity has to be higher because basilar membrane has to mechanically move up more

Where do cues from spectral notches become important in the auditory cortex?

In the core areas, particularly the caudal parabelt

What is special about the synapse in the anterior ventral cochlear nucleus (AVCN)?

Has hundreds of synapses, action potential happens every time at exactly the same time with only 0.5 millisecond delay (bushy cells)

What is the calyx of Held?

A large end bulb synapse found in the MNTB (from AVCN to MNTB neurons)

What is the difference between spherical and globular bushy cells?

Only different in shapes - spherical has sharp onset, globular onset is not as sharp. AVCN projects to MNTB

What pathway involves the dorsal cochlear nucleus (DCN)?

DCN → crosses midline → Lateral lemniscus (LL) → Inferior colliculus (IC) → MGB

What is unknown about the posterior ventral cochlear nucleus (PVCN)?

We don't know what the PVCN is doing (function is unclear)

Why is the AVCN important?

It is critical for sound localization circuits, providing precisely timed inputs to MSO and LSO/MNTB

Describe the auditory pathway overview image

Shows bilateral pathway from cochlear nuclei (CN) on both sides projecting to superior olives (MSO and LSO), then ascending through inferior colliculus (IC), to medial geniculate body (MGB) in thalamus, and finally to auditory cortex (AC)

Describe the image showing three localization cues

Shows a head from above with sound source positions. Spectral notches shown as vertical position (elevation), ITD shown as time difference between ears for horizontal localization, ILD shown as intensity difference between ears for horizontal localization

Describe the MSO neuron structure image

Shows bipolar neuron with two dendrites extending laterally and medially. Lateral dendrite receives input from ipsilateral cochlear nucleus, medial dendrite receives input from contralateral cochlear nucleus. Inhibitory inputs from MNTB shown on cell body

Describe the Jeffress model conceptual diagram

Shows sound sources at different positions creating different arrival times at left and right ears. Delay lines of different lengths converge on coincidence detector neurons arranged in a row, each responding to a specific ITD

Describe the Jeffress model with delay lines image

Shows axons from left and right cochlear nuclei entering from opposite ends with progressively different lengths creating delay lines. Coincidence detector neurons arranged along the middle respond when inputs arrive simultaneously based on axon length differences

Describe the MSO delay line mechanism image

Shows axon lengths translated into conduction times. In rostral MSO, lengths are equal from both sides. In caudal regions, ipsilateral axons are shorter, so contralateral sound must arrive earlier to achieve coincidence

Describe the old MSO model anatomical diagram

Shows MSO with contralateral CN axons entering from rostral end and ipsilateral CN axons entering from caudal end (opposite directions), creating systematic delay lines and a topographic ITD map along the MSO length

Describe the phase locking requirement image

Shows sound waves with action potentials locked to specific phases. MSO neurons spike only when inputs from both sides arrive simultaneously. This requires phase-locked firing in cochlear nuclei, which only occurs below 2 kHz

Describe the two-channel model graphs

Two graphs showing MSO response rates vs. ITD. As sound moves from midline toward one side, the contralateral MSO increases firing rate. Uses relative rate code rather than place code to signal sound location

Describe the LSO circuit diagram

Shows ipsilateral cochlear nucleus providing excitatory input directly to LSO. Contralateral cochlear nucleus provides excitatory input to MNTB, which then provides inhibitory input to LSO. LSO computes difference between excitation and inhibition

Describe the LSO computation image

Shows how LSO neurons receive ipsilateral excitation and contralateral inhibition (via MNTB). When sound is louder on ipsilateral side, excitation exceeds inhibition and LSO fires. Balance shifts with sound position

Describe the frequency-dependent ILD effectiveness image

Shows head with high frequency sound waves reflecting off head creating sound shadow (large ILD), while low frequency waves wrap around head reaching both ears equally (minimal ILD). Demonstrates why ILD only works for high frequencies