Sound Localization

Superior Olivary Complex

Localization: Limits of Performance

Absolute localization: localization of sound without a reference. Humans: 1º - 2º error (near midline); declines

with eccentricity. Relative localization (minimal audible

angle): a measure of acuity (two-point discrimination. About 1º in horizontal plane near midline, 8º

at 70 lateral. 3.5º vertical midline.

Cues for Sound Localization

Horizontal (aural plane): Monaural cues Interaural time disparity (ITD) Interaural level (or intensity) disparity (ILD, IID)

Vertical (interaural or sagittal plane): Pinna cues (directional filtering) No interaural disparities.

Distance: Experience dependent spectral filtering

Localization Cues: Head Shadow

Head generates acoustic shadow creating interaural intensity differences (IIDs).

Localization Cues: Path Length Differences

Head also introduces a path length difference for sounds off median plane, which generates ITDs at the two ears.

ITD maximal at 90º, and depends upon head size: 700 – 800 µs in humans. 350 – 400 µs in cats.

Monaural Cues: Pinna Directionality

Pinna and external meatus are directional filters.

Directionality is frequency dependent.

Cat

Monaural Cues: Head-Related Transfer Functions (HRTF)

Shift in horizontal location generates maximum gain at particular azimuth that is frequency dependent.

Systematic shift in spectral notch is seen for changes in vertical location, but not horizontal.

ITDs

Transient ITDs: difference in time of arrival of first wavefront at two ears.

Sustained sounds generate ongoing time differences: Pure tones: O-ITD evident in fine structure as interaural

phase difference (IPD). Complex sounds: O-ITD evident in envelope.

IPDs generate reliable azimuthal position for pure tones only when period is >2x max. interaural delay.

Therefore, ITDs only useful for low frequencies.

Perception of ITDs and IPDs

ITDs >40 ms perceived as two independent sound sources.

ITDs between 3- and 40 ms are perceived as sound source motion.

ITDs < 1 ms perceived as one sound emanating from the location of the first signal, even if that signal is weaker than the second (Precedence effect).

Perception of ITDs and IPDs

ITDs <0.5 ms perceived as single sound source lateral to midline. Location associated with leading ear. Location varies with ITD or IPD.

ITD effective only for pure tones below about 1 kHz, and for complex high frequency sounds with low frequency amplitude envelopes (AM).

Minimum detectable ITD: 6 – 10 µs (~1 – 2º)

Interaural Intensity (level) differences

Difference in loudness at two ears perceived as lateralized sound source.

Source lateralized to ear receiving louder signal. IIDs produced by head shadow, enhanced by

pinna directionality. IIDs are significant when wavelength is less than

head diameter; in humans, frequencies >2 kHz. Animals with small heads rely exclusively on IID

cues. (Head diameter correlates well with highest frequency of audible sound)

Binaural Interactions

Monaural, E0 (I0): excitatory (rarely, inhibitory) input from contralateral ear, no response from ipsi ear. (vice versa 0E)

Binaural summation, EE: Excited by either ear alone, sums inputs binaurally.

Binaural suppression, EI: Excited by contra ear, inhibited by ipsi (vice versa IE).

BinauralContra

Ipsi

Binaural

Binaural

SPL

FR

Superior Olivary Complex

SOC receives input mainly from AVCN (bushy cells).

SOC principal nuclei include: Medial SO (MSO) Lateral SO (LSO). Medial nuc. of the

trapezoid body (MNTB) Lateral nuc. Of the

trapezoid bodty (LNTB)

Superior Olivary Complex

LSO and MSO comprised mainly of bipolar/multipolar cells.

Dendrites oriented perpendicular to long axis of nuclei.

Ipsilateral afferents end on one primary dendrite, contralateral side the other.

Ramon y Cajal (1909)

MSO

Excited by inputs from ipsilateral and contralateral AVCN (spherical bushy cells).

Inhibited by MNTB and LNTB (not shown)

Two types of binaural interaction: Binaural summation (EE

responses) Monaural (0E or E0). Both

Both0E

EE

IC

CI

MSO

Tonotopically organized, emphasizing low-frequencies.

Dorso-ventral tonotopic gradient (High frequencies ventral).

LSOMSO

MNTB

MSO

Monaural stimulation: MSO cells phase-lock to stimulation of either ear with low-f. tones.

Dichotic stimuli: delay-dependent (ITD/IPD) facilitation and inhibition.

Peak spacing of ITD functions related to the period of stimulus.

ITD functions can be generated with any tone stimulating response area.

MSO

Single peak occurs within the physiologically relevant range (cat: 400 µs).

Characteristic Delay: Common peak across stimulus frequencies.

Model of Azimuthal Localization using ITD Cues (Jeffress 1948)

Each 3rd order neuron receives two excitatory inputs, one from each ear.

ITD’s are decoded by 3rd order neurons acting as “coincidence detectors.

Jeffress Model

Axonal path length differences determine a specific neural delay when coincidence will occur (= characteristic delay).

When the neural delay is offset by an acoustic delay (i.e., ITD) of the same magnitude, then coincidence occurs and the cell fires.

Model vs. Reality

Jeffress model predicts delay lines from both secondary fibers.

MSO only shows delay lines for contralateral AVCN fibers.

Evidence for MSO ITD Map

Each MSO neuron is tuned to characteristic ITD.

MSO neurons with different ITDs organized into gradient (map).

ITD map runs in rostrocaudal direction, perpendicular to tonotopic map.

Auditory midline represented rostrally, contralateral 90º caudally

MNTB

Excited by individual globular bushy cell inputs from contralateral VCN

Calyx of Held is largest synapse in brain.

MNTB cells inhibit LSO and MSO (Gly).

LSOMSO

MNTB

LSO

Excited by ipsilateral ear (globular bushy cells)

Inhibited by contralateral ear (globular bc) via MNTB

Binaural suppression (IE responses)

BinauralContra

IpsiSPL

FR

Lateral Superior Olive

LSO tonotopic representation favors higher frequencies (appropriate for conveying IIDs).

LSOMSO

MNTB

Lateral Superior Olive

Majority of cells receives excitatory input from ipsilateral VCN, inhibitory input from contra VCN via ipsi MNTB. (IE response)

E and I inputs have nearly identical spectral receptive fields.

Processing of IIDs in LSO

LSO neurons are excited by ipsilateral stimulation, inhibited by contralateral stimulation.

IID functions show different sensitivity to contralateral inhibition.

Processing ITDs in Lateral Superior Olive

LSO cells are sensitive to transient ITDs…

…and ongoing ITDs of high-frequency carriers modulated by low-frequency AM.

Binuaral suppression creates characteristic inhibitory delays (troughs in ITD functions)

Outputs of MSO, LSO

MSO projects ipsilaterally: DNLL ICC (dorsal low frequency)

LSO projects bilaterally: Excitatory projections to

DNLL and ventral ICC on contralateral side.

Inhibitory projections to DNLL and ICC on ipsilateral side.

Each ICC responds best to contralateral and frontal auditory space