The anatomy of the ITD pathway  

The auditory nerve fibres, known as the  afferent nerve
 fibres, carry information from the 
organ of Corti  to the  brainstem
 and  brain  . Auditory
afferent fibres consist of two types of fibres called type I and type II
fibres. Type I fibres innervate the base of one or two  inner hair cells
 and Type II fibres innervate the
outer hair cells. Both leave the  organ of Corti  through an opening called the habenula perforata. The type I
fibres are thicker than the type II fibres and may also differ in how they
innervate the  inner hair cells 
.  Neurons  with large calyceal endings ensure
preservation of timing information throughout the ITD pathway.

Next in the pathway is the  cochlear nucleus  , which receives mainly ipsilateral (that is, from the
same side) afferent input. The cochlear nucleus has three distinct anatomical
divisions, known as the antero-ventral cochlear nucleus (AVCN), postero-
ventral cochlear nucleus (PVCN) and dorsal cochlear nucleus (DCN) and each
have different neural innervations.

The AVCN contains predominant _bushy cells_ , with one or two profusely
branching  dendrites  ; it is thought that
bushy cells may process the change in the spectral profile of complex stimuli.
The AVCN also contain cells with more complex firing patterns than bushy cells
called _multipolar cells_ , these cells have several profusely branching
dendrites and irregular shaped cell bodies. Multipolar cells are sensitive to
changes in acoustic stimuli and in particular, onset and offset of sounds, as
well as changes in intensity and frequency. The axons of both cell types leave
the AVCN as large tract called the _ventral acoustic stria_ , which forms part
of the  trapezoid body  and travels
to the  superior olivary complex  .

A group of nuclei in  pons  make up the  superior
olivary complex 
(SOC). This is the first stage in auditory pathway to receive input from both
cochleas, which is crucial for our ability to localise the sounds source in
the horizontal plane. The SOC receives input from cochlear nuclei, primarily
the ipsilateral and contralateral AVCN. Four nuclei make up the SOC but only
the medial superior olive (MSO) and the lateral superior olive (LSO) receive
input from both cochlear nuclei.

The MSO is made up of neurons which receive input from the low-frequency
fibers of the left and right AVCN. The result of having input from both
cochleas is an increase in the firing rate of the MSO units. The neurons in
the MSO are sensitive to the difference in the arrival time of sound at each
ear, also known as the interaural time difference (ITD). Research shows that
if stimulation arrives at one ear before the other, many of the MSO units will
have increased discharge rates. The axons from the MSO continue to higher
parts of the pathway via the ipsilateral lateral lemniscus tract.(Yost, 2000)

The  lateral lemniscus  (LL) is
the main auditory tract in the brainstem connecting SOC to the  inferior
colliculus  . The dorsal
nucleus of the lateral lemniscus (DNLL) is a group of neurons separated by
lemniscus fibres, these fibres are predominantly destined for the inferior
colliculus (IC). In studies using an unanesthetized rabbit the DNLL was shown
to alter the sensitivity of the IC neurons and may alter the coding of
interaural timing differences (ITDs) in the IC.(Kuwada et al., 2005) The
ventral nucleus of the lateral lemniscus (VNLL) is a chief source of input to
the inferior colliculus. Research using rabbits shows the discharge patterns,
frequency tuning and dynamic ranges of VNLL neurons supply the inferior
colliculus with a variety of inputs, each enabling a different function in the
analysis of sound.(Batra & Fitzpatrick, 2001) In the inferior colliculus (IC)
all the major ascending pathways from the olivary complex and the central
nucleus converge. The IC is situated in the midbrain and consists of a group
of nuclei the largest of these is the central nucleus of inferior colliculus
(CNIC). The greater part of the ascending axons forming the lateral lemniscus
will terminate in the ipsilateral CNIC however a few follow the commissure of
Probst and terminate on the contralateral CNIC. The axons of most of the CNIC
cells form the brachium of IC and leave the  brainstem  to travel to the ipsilateral  thalamus  . Cells in different parts of the IC tend to be monaural,
responding to input from one ear, or binaural and therefore respond to
bilateral stimulation.

The spectral processing that occurs in the AVCN and the ability to process
binaural stimuli, as seen in the SOC, are replicated in the IC. Lower centres
of the IC extract different features of the acoustic signal such as
frequencies, frequency bands, onsets, offsets, changes in intensity and
localisation. The integration or synthesis of acoustic information is thought
to start in the CNIC.(Yost, 2000)