7 The central auditory nervous system
7.2 Coding of information in the higher auditory centres
We have seen that in the cochlear nerve, information about sound intensity is coded for in two ways: the firing rates of neurons and the number of neurons active. These two mechanisms of coding signal intensity are found throughout the auditory pathway and are believed to be the neural correlates of perceived loudness. The tonotopic organisation of the auditory nerve is also preserved throughout the auditory pathway; there are tonotopic maps within each of the auditory nerve relay nuclei, the medial geniculate nucleus (MGN, labelled meidal geniculate body in Figure 27) and the auditory cortex. Conversion from frequency to position that originates on the basilar membrane is maintained all the way up to the auditory cortex. One source of information about sound frequency is therefore derived from tonotopic maps; the location of active neurons in the auditory nuclei and in the cortex is an indication of the frequency of a sound. Phase locking as a means of frequency coding is also present in centres further along the pathway.
There are, in fact, two distinct pathways that occur in the CANS:
- The ‘what’ pathway which is monaural and receives information from only one ear. This pathway is concerned with the spectral (frequency) and temporal (time) features of a sound and is hardly concerned with the spatial aspects. It focuses mainly on identifying and classifying different types of sound.
- The ‘where’ pathway which is binaural and receives information from both ears. It is involved in the localisation of a sound stimulus.
Despite the apparent dichotomy of these two processing pathways, the same types of acoustic cues may be important for the analysis that occurs in each. For example, spectral information is used in the ‘where’ pathway for determining a sound's elevation; and temporal information, used for our perception of frequency in the ‘what’ pathway, is also used in the ‘where’ pathway for determining a sound's horizontal location.
7.2.1 The ‘what’ pathway
The main nucleus involved in the ‘what’ pathway is the cochlear nucleus which has three main components, each of which is tonotopically organised; cells with progressively higher characteristic frequencies are arrayed in an orderly progression along one axis (Figure 29). The cochlear nuclei contain neurons of several types, each of which encodes a specific parameter of a stimulus (frequency, intensity, time): stellate cells encode stimulus frequency and intensity, bushy cells provide information about the timing of acoustical stimuli, and are involved in locating sound sources along the horizontal axis, and fusiform cells are thought to participate in the localization of sound sources along a vertical axis.
Figure 29 The representation of stimulus frequency in the cochlear nucleus. Stimulation with two sounds of different frequency causes vibration of the basilar membrane at two different positions (top). This in turn excites two distinct populations of afferent fibres, which project onto the cochlear nucleus in an orderly fashion
7.2.2 The ‘where’ pathway
The ‘where’ pathway involves the ventral cochlear nuclei, the superior olivary complex and the inferior colliculus. The superior olivary complex is composed of the lateral superior olive (LSO) and the medial superior olive (MSO).
The neurons in the superior olivary complex are the first brainstem neurons to receive strong inputs from both cochleae and are involved in sound localisation.
The MSO receives excitatory inputs from the cochlear nuclei on both sides and is tonotopically organised. It is involved in the localisation of sound in the horizontal plane by processing information about auditory delays. Units in the MSO increase their firing rate in response to sounds from both ears as opposed to one ear, and these excitatory–excitatory (EE) units will increase their discharge rate further in response to sounds that reach both ears with a certain delay. In other words, a unit will discharge at the greatest rate when there is a particular interaural delay. This aids in localising sound in the horizontal plane.
The LSO is also involved with sound localisation but instead of using interaural time delays, it employs intensity differences to calculate where a sound originated. Information from the ipsilateral (same side) inputs to the LSO is usually excitatory and results in an increase in discharge rate of the neuron. Contralateral stimulation of the LSO is usually inhibitory. Thus, stimulation from both ears may decrease the firing rate of the neuron relative to the firing rate when only the ipsilateral ear receives sound. These excitatory–inhibitory (EI) units discharge with a few spikes when there is approximately equal stimulation of both ears and discharge rate increases as a function of changing the interaural level difference. The LSO therefore appears to form a network for processing interaural level differences, which are used to determine the location of sound sources.
The use of interaural time and intensity differences in sound localisation will be dealt with in more detail in Section 12.
The inferior colliculus is part of the tectum and is the most prominent nucleus in the brainstem. It receives inputs from the olivary complex and the cochlear nucleus. Units in the inferior colliculus appear to be mainly EI units although there are EE units as well. They are tonotopically organised in sheets of cells (as in the cochlear nucleus). Cells in different parts of the inferior colliculus are either monaural, in that they respond to input from one ear only, or binaural, responding to bilateral stimulation. Both the spectral processing that takes place in the cochlear nucleus and the binaural processing that occurs in the olivary complex are seen in the inferior colliculus. In fact, the inferior colliculus is the termination of nearly all projections from brainstem auditory nuclei. It is therefore a ‘watershed’ for information processing where the ‘what’ and ‘where’ pathways converge on a single tonotopic map. Outputs of the inferior colliculus project mainly to the medial geniculate nucleus.
The medial geniculate nucleus is also tonotopically organised. Neurons with the same characteristic frequency are arrayed in one layer, so that the nucleus consists of a stack of neural laminae that represent successive stimulus frequencies. Sensitivity to interaural time or intensity differences is maintained. Axons leaving the MGN project to the auditory cortex. The neural responses of cortical cells in response to sound have been studied extensively in primates. In general, neurons are relatively sharply tuned for sound frequency and possess characteristic frequencies covering the audible spectrum of frequencies. In electrode penetrations made perpendicular to the cortical surface, the cells encountered tend to have similar characteristic frequencies, suggesting columnar organisation on the basis of frequency, the so-called ice-cube model of the auditory cortex (Figure 30). Although most of the neurons in the primary auditory cortex are sensitive to stimulation through either ear, their sensitivities are not identical. Instead the cortex is divided into alternating strips of two types. Half of these strips contain EE neurons and respond more to stimulation from both ears than to either ear separately, and the other half consist of EI neurons which are stimulated by unilateral input but inhibited by stimulation from the opposite ear. The strips of EE and EI cells run at right angles to the axis of tonotopic mapping so that the primary auditory cortex is partitioned into columns responsive to every audible frequency and to each type of interaural interaction.