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.