Dr Robert H. Mannell
Department of Linguistics
Click here for an explanation of some of the terminology used in this lecture.
The readings for this lecture are as follows:-
|Figure 1: An overview of afferent ipsilateral and contralateral interactions in the auditory brainstem.|
Each auditory fibre branches: one branch projects rostrally, one branch projects caudally.
Rostral branch:- antereoventral cochlear nucleus (AVCN)
All three cochlear nuclei exhibit tonotopic mapping of frequency, but:-
A number of cell types have been identified and characterised by their shape and size. These cell types include:-
Cells in the cochlear nuclei can be classified by their temporal response. Post-stimulus time histograms (PSTH) examine the response of a neuron to a stimulus (often a tone or a band of noise). In the cochlear nuclei, a number of classes of neuron, based on their temporal response, have been identified. They are:-
Inhibition of the primary-like cells of the AVCN shows similar patterns to the inhibition of the primary auditory nerve fibres. Inhibitory patterns show an excitatory response area and inhibitory sidebands.
Some frequency/intensity values excite certain types of neuron.
Other frequency/intensity values inhibit those neurons (sidebands).
1) ventral cochlear nucleus - simple tonotopic response - feeds to superior olivary complex where temporal analysis occurs
2) dorsal cochlear nucleus - complex frequency analysis bypasses superior olivary complex and goes directly to lateral lemniscus and inferior colliculus
The Superior Olivary Complex consists of the Medial Nucleus of the Superior Olive (MSO), the Lateral Nucleus of the Superior Olive (LSO) and the Medial Nucleus of the Trapezoid Body (MTB). It also consists of a number of pre-olivary and peri-olivary nuclei which receive mostly efferent innervation.
Fibres from the DCN and some of the fibres from the PVCN bypass the Superior Olive Complex. Fibres from the AVCN (and some fibres from the PVCN) project to the ipsilateral LSO and MSO and to the contralateral MSO and (via the contralateral MTB) to the contralateral LSO. (see Pickles, 1998, figs 6.8 and 6.9, pp 180-181).
In the MSO and the LSO there are two major types of neuron:-
The MSO mainly receives innervation from both the ipsilateral and the contralateral AVCN.
The MSO has a tonotopic response pattern that favours low frequencies. This particularly enhances the MSO's analysis of interaural temporal disparities in a waveform as low frequencies better preserve phase information.
The majority of MSO cells are EE cells, but there is also a significant minority of EI cells.
MSO EE cells determine the direction that a signal comes from on the basis of interaural timing (phase) differences.
EE cells display:-
Different EE cells appear to have different "characteristic delay" and respond most strongly when the delay between the two ears matches this delay time. That is, when the phase difference between the two ears matches the characteristic delay time of the cell the signals from each ear appear to that cell to be aligned and thus the signals from the two ears reinforce each other to the greatest degree.
The LSO mainly receives innervation from both the ipsilateral the contralateral AVCN. Innervation from the contralateral AVCN is via the ipsilateral MTB.
In the LSO, most cells are EI cells.
The LSO analyses interaural intensity differences. LSO EI neurons respond to these interaural intensity differences. When the interaural intensity difference is least (for signals on the medial plane) the signal from the contralateral ear most effectively cancels out signal from the ipsilateral ear.
The LSO cells are also sensitive to interaural timing differences although this appears to be a secondary function of the LSO.
The LSO has a tonotopic response pattern that favours high frequencies. This particularly enhances the LSO's analysis of interaural intensity disparities as intensity differences are greatest at high frequencies because higher frequencies experience less diffraction around the head.
The MTB is a passive relay which conveys information from the cochlear nucleus of contralateral ear.
The Lateral Lemniscus has two main nuclei:-
Ventral Nucleus - Input from the contralateral Cochlear Nucleus and output to the ipsilateral Inferior Colliculus.
Dorsal Nucleus - Bilateral input from the Cochlear Nucleus and bilateral output to the Inferior Colliculus.
The Inferior Colliculus (IC) is an auditory relay and auditory reflex centre.
The Central nucleus is characterised by iso-frequency sheets (see Pickles, fig. 6.16c, p190). Each of these sheets represent a single characteristic frequency (cf), with low CF sheets rostrally and high CF sheets caudally. This suggests the possibility of multiple, parallel, representations of each frequency represented in a three dimensional matrix where one dimension (caudal to rostral) represent frequency whilst the other two dimensions represent some other parameters (eg. signal direction). It may be that this structure is a map of frequency in space.
The Dorsal Cortex and the Paracentral Nuclei are the most peripheral examples of a diffuse or non-specific auditory system surrounding the specific or core auditory system. The diffuse or nonspecific parts of the auditory system have auditory as well as other input. Little is known about the function of these diffuse auditory centres.
Pickles (1988) speculates on the function of the Inferior Colliculus (and specifically the Central Nucleus) as follows:-
"Localizing sound by interaural time disparities requires the preservation of accurate time relations. These are lost in the dorsal cochlear nuclear nucleus by the circuitry needed for complex amplitude and frequency analysis. It is therefore appropriate that the direction of the sound should be extracted separately. The inferior colliculus, by combining information from both sources, might therefore be able to code simultaneously the complexity of sounds and their direction in space." (p194)
The Medial Geniculate Body accepts afferents from Inferior Colliculus and projects to cerebral cortex.
The Medial Geniculate Body can be divided into three regions:-
Ventral Division (v) - The Ventral Division is a specific auditory relay with input from the Inferior Colliculus and it projects principally to AI area of auditory cortex.
Dorsal Division (d) - Diffuse auditory division with both auditory (Inferior Colliculus) and non-auditory (eg. somatosensory) input. Has projections to the AII area of the auditory cortex.
Medial Division (m) - Diffuse auditory division with both auditory (pericentral nucleus of the Inferior Colliculus) and non-auditory (eg. somatosensory) input. The medial division has nonspecific projections to auditory cortex.
The Ventral Division is tonotopically organised.
Some neurons have complex temporal properties.
Some neurons are binaurally sensitive.
Some neurons are sensitive to interaural intensity differences (high frequency).
Some neurons are sensitive to interaural time differences (low frequency).
The auditory centrifugal pathways are efferent or descending pathways that carry information from more central to more peripheral levels of the auditory system.
The auditory centrifugal pathways run close to, but usually not within, the tracts containing the auditory afferent pathways.
Efferent pathways enable more central processes to influence more peripheral processes. Such affects might include:-
This centre is found in the Superior Olivary Complex of the brainstem and the efferent neurons that arise in this bundle have their point of origin around, but not within, the afferent nuclei (MSO, LSO, MTB) as well as in the pre-olivary and peri-olivary nuclei.
The cells of the olivocochlear bundle are differentiated into different types:-
|Figure 2: An overview of the Efferent connections of the Olivocochlear Bundles from the left and right Olivary regions to the left and right cochlea. Two bundles of fibres descend from each side. The Crossed Olivocochlear Bundles (COCB) represent about 1/3 of the fibres and the Uncrossed Olivocochlear Bundles (UCOCB) represent about 2/3 of the fibres originating on each side. Other fibres not represented in this diagram make up the remaining 6%.|
The olivocochlear bundles:-
The Cochlear Nuclei receive efferent fibres from:-
Efferent innervation from the Olivary Complex to the cochlear nucleus is both excitatory and inhibitory. There is also evidence that such innervation is both ipsilateral and contralateral.
The main effect of this innervation is enhancement of the detection of a signal in continuous background noise.
A number of descending pathways have been detected:-
These descending pathways are both inhibitory and excitatory.
Pickles (1988) speculates:-
"We therefore have a system in which reflexes can be established at many levels, and in which the cortex controls the reflexes through descending influences..." (p253)
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