Another well understood example of neural words is birdsongs. Birdsongs are induced internally rather than triggered by external stimuli. The song consists of distinct bursts of sounds (syllables), separated by silent intervals (Fig. 3D).9 In the zebra finch, the syllable sequences are stereotypical and last for several seconds. The song is controlled by a set of nuclei, which form a mostly feed-forward excitatory pathway (Nottebohm et al., 1976). The critical brain area in song production is the high vocal center (HVC), which projects to the robust nucleus of the arcopallium (RA), which, in turn, drives the hypoglossal motor neurons innervating the vocal organ (syrinx). Experiments have demonstrated that the temporal structure of the song is generated by sparse sequential bursts of RA-projecting HVC neurons (Fee et al., 2004Hahnloser et al., 2002; Long and Fee, 2008). Each neuron typically emits a single brief burst of spikes only at one time in the song (Figure 3D and E). It is assumed that each of the sequentially activated neurons is a part of an assembly of approximately 200 neurons, whose other members remain
