Interaural time difference
Experiments conducted by Woodworth (1938) tested the duplex theory by using a solid sphere to model the shape of the head and measuring the ITDs as a function of azimuth for different frequencies.
Initial measurements found that there was a maximum time delay of approximately 660 μs when the sound source was placed at directly 90° azimuth to one ear.
The results concluded that when a sound played had a frequency less than 1500 Hz, the wavelength is greater than this maximum time delay between the ears.
Therefore, due to the size of the head and the distance between the ears there is a reduced phase difference, so localisations errors start to be made.
Feddersen et al. (1957) also conducted experiments taking measurements on how ITDs alter with changing the azimuth of the loudspeaker around the head at different frequencies.
However, after blocking EPSPs with a glutamate receptor blocker, they determine that the size of inhibitory inputs was too marginal to appear to play a significant role in phase locking.
Franken et al. also examined anatomical and functional patterns within the superior olive to clarify previous theories about the rostrocaudal axis serving as a source of tonotopy.
Evidence from Franken et al. shows that the processing is affected by inputs that precede the binaural signal, which would alter the functioning of voltage-gated sodium and potassium channels to shift the membrane potential of the neuron.
Furthermore, the shift is dependent on the frequency tuning of each neuron, ultimately creating a more complex confluence and analysis of sound.
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.
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.
(Yost, 2000) The lateral lemniscus (LL) is the main auditory tract in the brainstem connecting SOC to the inferior colliculus.
(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).
In their review of localisation and lateralisation studies, Durlach, Thompson, and Colburn (1981), cited in Moore (1996) found a "clear trend for poor localization and lateralization in people with unilateral or asymmetrical cochlear damage".
