Neural centres for hearing and singing

Three different groups of nuclei are known to play roles in the production of song. One group of brain nuclei is involved in processing auditory information, a second in producing the motor program for song (Fig. 9.7a), and a third in learning and developing the song pattern (Fig. 9.7b). One nucleus, HVc, is a member of all three groups. Sometimes, HVc is referred to as the 'higher vocal centre', but attaching functional names to brain regions can be misleading, and this and other nuclei are referred to here by accepted abbreviations. In an adult male zebra finch, HVc contains about 35000 neurons. There is a good correlation between the size of HVc and the complexity of song in different species.

HVc exerts direct control over the production of song through connections it makes with nucleus RA. From RA, axons project to groups of motor neurons that innervate the syrinx and some respiratory muscles. Birds in which HVc or RA has been destroyed cannot sing, although they still court females by adopting the same postures as during singing (Nottebohm et al., 1976), and they produce alarm cries and some other calls. Anthony Yu and Daniel Margoliash (1996) implanted fine wire electrodes into the brains of zebra finches and recorded spikes from single neurons while the birds were moving around their cages and singing normally. Many of the neurons in HVc started to spike before a bird began to sing, and remained excited until just before the end of the song. During a song, spike rate increased and decreased in a characteristic manner whenever the bird sang a particular syllable. A particular neuron was most excited when a particular syllable was sung, and each syllable would be associated with excitation of a unique population of HVc neurons. In contrast, individual neurons in RA produced discrete bursts of spikes coinciding with a specific series of notes that occurred in a number of different syllables throughout the song. In other experiments, Eric Vu and colleagues (Vu, Mazurek & Kuo, 1994) used similar electrodes to stimulate neurons in singing birds with a short train of pulses that lasted less than the duration of a syllable. Stimuli to small regions of

Figure9.7 Diagrams to show the principal brain nuclei involved in the generation and development of bird song. (a) The control of singing in a mature adult. Motor neurons of syringeal muscles are activated by neurons from RA which, in turn, is activated by neurons from HVc. HVc receives inputs from Field L, which is a major sensory area involved in auditory processing, and from Uva and NIf. (b) Nuclei involved in the development of song. Notice that nucleus lMAN receives feedback from HVc via Area X

Figure9.7 Diagrams to show the principal brain nuclei involved in the generation and development of bird song. (a) The control of singing in a mature adult. Motor neurons of syringeal muscles are activated by neurons from RA which, in turn, is activated by neurons from HVc. HVc receives inputs from Field L, which is a major sensory area involved in auditory processing, and from Uva and NIf. (b) Nuclei involved in the development of song. Notice that nucleus lMAN receives feedback from HVc via Area X

HVc disrupted the syllable the bird was singing and interfered with the order of subsequent syllables within a motif. For example, if a bird's motif consisted of syllables A B C D, a stimulus delivered just after syllable B might alter the motif's structure to A B G, combining syllables C and D to an unusual, abbreviated form, G. The electrical stimuli neither stopped the song nor interfered with the pattern of syllables during following motifs. Stimuli delivered to RA had a more restricted effect, disrupting the current syllable without affecting later syllables within the motif. A stimulus delivered to RA just after B in the sequence A B C D would cause the bird to sing A B C' D, where C' is an altered form of syllable C.

These two kinds of experiment suggest that individual neurons in HVc control particular sequences of syllables within a motif by recruiting groups of neurons in RA in a specific order. The RA neurons each excite a pool of motor neurons that control muscles of the syrinx to produce a particular combination of notes, and this correlates well with the anatomy of RA, which is arranged topographically so that a particular region in it corresponds with a particular group of motor neurons and muscles (Vicario, 1991). HVc is not organised topographically.

HVc has sensory as well as motor properties. In an anaesthetised bird, many HVc neurons respond well to recordings of the bird's own song, but not to the songs of other birds. The neurons recognise both the order of notes within a syllable and the order of syllables within a motif. Specificity for the order of syllables in a motif is particularly interesting because it means the HVc neurons need to collect information over time, for example to distinguish order A B C D from A C B D. It is not clear whether these auditory responses play a role in the control of singing in the adult because they are probably suppressed when the bird sings (McCasland & Konishi, 1981), but they are important for the normal development of song.

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