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Brein en Cognitie 1: zelfstudieopgaves van week 4 The Auditory System
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Brein en Cognitie 1: zelfstudieopgaves van week 4 The Auditory System
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Summary and assignment on The Auditory SystemThe auditory system is the basis of hearing and is closely connected to the vestibular system.
This lecture will cover the general structure and functioning of the auditory system as well as
its relationship to language perception and production.
Learning objectives
The student will be able to apply the basic principles of physics to the processing of auditory
information at both the anatomical and physiological level and be able to identify a number of
similarities and differences in relation to the visual system. The student will also be able to
explain the relationship between the auditory system and the production and perception of
language and subsequently use this as a means to explain the different forms of aphasia.
Lastly, the student will be able to identify the neural structures of the vestibular system.
Summary
Sounds in the air consist of propagating differences in air pressure. At the hearing threshold,
these fluctuations are very small. Due to the enormous dynamic range in which the ear
functions, it is useful to indicate the relative strength of sound in a logarithmic scale. For this
purpose, a so-called decibel scale is used; this shows the ratio of a volume in relation to a
reference level. Sound can be represented as pressure variation over time, but it is also often
shown in its decomposed frequency components. The view that is created is then called the
spectrum of the sound.
The auditory system consists of the peripheral auditory system (ear and auditory nerve) and
the central auditory system in the brain. The ear itself is often divided into the outer ear (the
pinna and external auditory canal), the middle ear (the eardrum and ossicles) and the inner
ear (the cochlea). The sound that reaches the eardrum is transmitted to the fluid in the inner
ear (cochlea) via the ossicles known as the hammer, anvil and stirrup. The pressure wave in
the cochlea sets the basilar membrane in motion; high tones (15,000 Hz) produce vibrations
in the front while low tones (250 Hz) produce vibrations in the back. The mechanical
vibrations of the basilar membrane in the cochlea that are detected by hair cells are passed
onto the auditory nerve fibres, which in turn transmit this information to the cochlear nuclei.
Because the tonotopic organisation is preserved in the auditory nerve, the cochlear nucleus, in
turn, also shows a tonotopic map. In other words, each pitch has its own nerve cell in the
cochlear nucleus. Another area in the brain, the medial superior olivary nucleus, helps with
directional hearing. Action potentials enter this olivary nucleus from the two cochlear nuclei,
from the left and right ear. The time difference between these signals indicates the extent to
which the sound came from the left or from the right. This information then moves onto the
higher levels of processing, first to the thalamus (MGN) and then to the primary auditory
cortex.
The neural code for sound is fundamentally different from the coding of visual stimuli.
Locations in the visual field can be encoded directly by activity at corresponding locations on
the retina. In contrast, the spatial characteristics of a sound source need to be reconstructed on
the basis of multiple monaural and binaural cues. The most important cues for determining the
location of a sound source in the horizontal plane are: the time differences in the arrival of
sound at the two ears and the differences in intensity of the sound at the two ears, which is
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