Auditory System II: Auditory Pathways And
Language
The vestibulocochlear or eighth
cranial nerve transmits information from both the cochlea and vestibular apparatus. Each fibre of the
cochlear nerve is selectively tuned to a characteristic frequency, which is
determined by its site of origin within the cochlea (see Chapter 27). These
fibres are then arranged according to the location of their innervating hair
cells along the basilar membrane (BM), and this tonotopic organization is
maintained throughout the auditory pathway.
On entering the brainstem the
cochlear nerve synapses in the cochlear nuclear complex of the medulla.
Auditory pathways
•
The cochlear
nucleus is divided into a ventral (VCN) and dorsal (DCN)
part. The VCN projects to the superior olivary complex (SOC)
bilaterally. The DCN projects via the dorsal acoustic striae to the
contralateral nucleus of the lateral lemniscus and inferior
colliculus.
•
The SOC
contains spindle-shaped neurones with a lateral and medial dendrite, which
receive an input from each ear. It is the first site of binaural interactions
and so is important in sound localization. In the medial part of the SOC this
input is excitatory from each ear (EE cells) whereas in the lateral
SOC the neurones have an excitatory input from one ear and an inhibitory
input from the other (EI cells).
•
The EE
cells by virtue of their input are important in the localization of sounds of
low frequency (<1.4 kHz) where the critical factor is the delay (Δt) in the
sound reaching one and then the other ear. One possible arrangement relies on
the differential localization of the synaptic inputs to a single SOC neurone
from the two ears.
•
The EI
cells are important in the localization of higher frequency sounds where the
difference in intensity (ΔI) of sound between the two ears is important (ΔI
being generated as a result of the head acting as a shield). Sounds of
frequencies greater than 1.4 kHz (in the case of humans) rely on ΔI for
localization. In the case of sounds originating in the midline, there will be
no Δt and no ΔI, and there is some confusion in localization which can be
overcome to some extent by moving the head or using other sensory cues.
•
The
localization of sound within the vertical plane is dependent in some way on the pinna.
•
The SOC
not only projects rostrally to the inferior colliculus (IC), but
also has an important input to the cochlea where it primarily controls the OHCs
and by so doing the response properties of the organ of Corti (see Chapter 27).
The projection to the IC is tonotopic, and this structure also receives an
input from the primary auditory cortex (A1) and other sensory
modalities. In this respect it interacts with the superior colliculus and is involved
in the orienting response to novel audiovisual stimuli (see Chapters 25 and
56).
•
The IC
projects to the medial geniculate nucleus of the thalamus (MGN),
which projects to the A1 in the superior temporal gyrus. This area
corresponds to Brodmann’s areas 41 and 42, with the thalamic afferent input
synapsing in layers III and IV of the cortex. The columnar organization of A1
is poorly defined, but the tonotopic map is maintained so that low-frequency
sounds are located posteriorly and high-frequency sounds anteriorly.
Language
Language is organized in the
dominant, typically left hemisphere and is best developed and most studied in
the human brain.
• The
localization and network subserving language is controversial as much of the
early work used lesion studies, which of late has been refined using functional
imaging studies.
• Language
dysfunction typically occurs in the context of stroke but can be affected in
isolation in some neurodegenerative conditions – such as primary progressive
aphasia.
• Developmentally
abnormalities in language can occur in isolation or be part of a more
widespread problem such as autism, learning disabilities, and importantly can
also be seen with hearing problems.
