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