Inferior

colliculus

Inferior

colliculus

Medial geniculate

Medial geniculate

Superior olive

Superior olive

Cochlear nucleus

Cochlear nucleus

Auditory cortex

Cochlea

Cochl

ea

Auditory nerve

Auditory nerve

cochlear nucleuscochlear nucleus superior olive

Inferior colliculus Inferior colliculus

medial geniculate medial geniculate

auditory cortex

The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory System

is Tonotopically Organized

to nucleus 1

to nucleus 2

to nucleus 3

to nucleus 4

to nucleus 5

Projections form the parallel pathways in ascending auditory system

cells in cochlear nucleus

Each cell type in the cochlear nucleus uniquely transforms an incoming spike train into an output that is different from the input

to nucleus 1

to nucleus 2

to nucleus 3

to nucleus 4

to nucleus 5

cells in cochlear nucleus

to nucleus 1

to nucleus 2

to nucleus 3

to nucleus 4

to nucleus 5

Projections from each cell group in the cochlear nucleus are tontopically organized

dorsalintermediate

ventral

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

excitatory

GABAergic

glycinergic

auditory nerve

LSOMSO

superior olivary complexLSO

MSO

Lateral Superior Olive (LSO) and Medial Superior Olive (MSO)

are both binaural nuclei that process the cues for sound localization

bushy cells

Processing of interaural intensity disparitiesfor localizing high frequencies

Processing of interaural time disparitiesfor localizing low frequencies

BaseBase

With high frequencies, the ears and head block some of the sound, making the sound

louder in one ear than the other, which creates

interaural intensity differences (IIDs)

Right Left

BaseBase

left ear louder

Right Left

BaseBase

Right Left

Equally intense at both ears

BaseBase

Right Left

Right ear louder

BaseBase

With low frequencies, sound waves just bend around the head and ears so there is no difference in sound intensity at the two

ears

Right Left

BaseBase

With low frequencies, however, the sound arrives

at one ear earlier than it does at the other ear,

which creates interaural time differences (ITDs)

Right Left

BaseBase

Right Left

Sound arrives at left ear first- left leads

BaseBase

Right Left

Arrives at both ears at the same time

BaseBase

Right Left

Sound arrives at right ear first- right leads

Processing of interaural time disparitiesfor localizing low frequencies

Processing of interaural intensity disparitiesfor localizing high frequencies

ITD

IID

Spi

kes

Spi

kes

10-20 microsec

Medial Superior OliveMSO

Lateral Superior OliveLSO

bushy cell

bushy cell

LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+Interaural intensity disparity

IID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike count

Formation of EI Property in LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+

+

LSOIID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike countX

Formation of EI Property in LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+

+

LSOIID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike countX

X

Formation of EI Property in LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+LSOIID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike countX

X

X

+

Formation of EI Property in LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+LSOIID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike countX

X

X

X

+

Formation of EI Property in LSO

DNLL

InferiorColliculus

Cochlearnucleus

Cochlea

MNTB

+

+

LSOIID

Excit ear louder Inhib ear louder0

Norm

aliz

ed

Spik

e

Count

1.0

0.5

0

monaural spike countX

X

X

X X

IID Function

Formation of EI Property in LSO

5 04 03 02 01 00-1 0-2 0-3 0-4 0-5 0

.2

.4

.6

.8

1 .0

I ID ( d B )

E x c i t e a rm o r e

in te n s e

In h ib e a rm o r e

in te n s e

IID Distribution of LSO

No

rmal

ized

sp

ike

cou

nt

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0Spik

es

Spik

es

Spik

es

Spik

es

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0

all cells fire

Spik

es

Spik

es

Spik

es

Spik

es

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0Spik

es

Spik

es

Spik

es

Spik

es

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0Spik

es

Spik

es

Spik

es

Spik

es

LSO

MNTB

++

CochlearNucleus

IIDlouder in

excitatory earlouder in

inhibitory ear

0

Low frequencies

High frequencies- above about 3000 Hz

Discharges are phase locked but not to every cycle

Discharges are not phase locked

Discharges are phase locked to every cycle of the sinusoidal signal

Frequencies below 1000 Hz

Frequencies from 1000-3000 Hz

time (ms)

ton

e p

rese

nta

tio

n

Raster display of phase-locked discharges evoked by 5

presentations of a tone burst

tone burst

spik

e co

un

t

time (ms)

Post-stimulus time(PST) histogram

right ear

left ear

Due to phase-locking, the timing of spikes in the auditory nerve fibers from the two ears accurately represents, and

thereby preserves, the ITD in the auditory pathway

spikes at right ear

spikes at left ear

ITD

phase-locked discharges

Interaural time disparities have to be processed ona frequency-by-frequency basis

right ear

left ear

ITD

right ear

left ear

constant phase difference between two ears

ITD

right ear

left ear

phase difference between two ears would continuously change

..

In 1948 Lloyd Jeffress, a professor in the Psychology Department at The University of Texas at Austin, proposed a

model could explain how low frequency sounds are localized.

The model includes: 1) structural features, i.e., delay lines resulting from differences in axonal lenghts;2) The neuronal process of coincidence detection. Specifically, the requirement that action potentials arrive at a target neuron simultaneously to activate the binaural neuron.3) Topographically organized selective features that allows sound location to be repesented as a place of maximal activity.4) How all of those features are activated by interaural time disparities, the cues animals use to localize low frequency sounds.

MSO

Medial Superior Olive

Lateral Superior Olive

MSO

Medial Superior Olive

Interaual time disparity (µsec)

Spi

kes

0 +40-40

MSO neurons are sensitive to Interaural time disparities of 10-20 µs

MSO

Medial Superior Olive

Interaual time disparity (µsec)

Spi

kes

0 +40-40

MSO neurons are sensitive to Interaural time disparities of 10-20 µs

MSO

Medial Superior Olive

Interaual time disparity (µsec)

Spi

kes

0 +40-40

MSO neurons are sensitive to Interaural time disparities of 10-20 µs

Freq 1

Freq 2

Freq 3

Freq 4

Spi

kes

ITD (µsec)

Spik

es

0

ITD (µsec)

Spik

es

0

ITD (µsec)

Spik

es

0

ITD (µsec)

Spik

es

0

MSO

Jeffress Model