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.
• 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 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.