Human brain theory

ISBN 978-3-00-068559-0

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

4.5.  The phase module 


The phase module plays an important role in spatial hearing. Its function was already researched some time ago. In spatial hearing, sound waves arriving laterally from the listener arrive in the left and right ear with a phase shift. This phase shift can be detected in the hearing system.

In principle, a divergence module is used for this purpose.

Here, the basilar membrane is practically unrolled into the plane so that a strip of input neurons receives the output of the hair cells. Since the position of the neurons along the basilar membrane encodes the sound frequency, the neurons of the neuron strip whose assigned frequency is contained in the sound heard are now always active.

If the basilar membrane of the other ear is now unwound analogously and with some distance parallel to it, two strips of input neurons are created that are similarly well-ordered according to assigned frequencies. The left strip receives the input from the left ear, the right strip receives the input from the right ear.

Between these input strips are the output neurons of the phase module. Their number is many times higher compared to the input neurons. For example, if there are 100 output neurons for every pair of input neurons, distributed between them with approximately equal spacing, the phase module can determine 100 different sound directions at this frequency.

The direction is determined by the difference in propagation time. Both excitations propagate from the two neurons on the outside towards the centre. The excitation maximum of the hair cell is directly coupled to the crests of the sound wave. There is usually a phase difference between the left and right sound wave, which is directly coupled to the sound direction.

The synapses of the output neurons are specially designed to recognise simultaneously incoming signals from the left and right. Only when an action potential arrives from the left and from the right at the same time does the output neuron generate its own action potential. Its synapses are very large and specially constructed. They are called Held's calyx.

The signal path of the left signal has a path difference to the signal path of the right signal if the output neuron is not in the middle between the two input neurons. The further an output neuron is from the centre, the greater the path difference imposed on the left and right signals.

However, since the left and right input signals already have a transit time difference, it is reversed by exactly one of the many input neurons through the path difference. This output neuron generates an output signal, while the other output neurons remain silent at this frequency.

This is because the signal propagates along the axons at a finite speed. The longer the path, the more time the signal needs for this path. If the left and right paths are different, this creates a difference in propagation time that cancels out the difference in propagation time of the input signals,

Thus, the output neurons generate a maximum-coded signal at a frequency, which encodes the sound direction. The basis for this phase module is the phase shift of the sound waves between the left and right ear.

Monografie von Dr. rer. nat. Andreas Heinrich Malczan