Ultrastructural and electrophysiological analysis of Area X in the untutored and deafened Bengalese finch in relation to normally reared birds.

Birdsong learning bears many similarities to human speech acquisition. Although the anterior forebrain pathway (AFP) is believed to be involved in birdsong learning, the underlying neural mechanisms are unclear. We produced two types of abnormal song learning: young birds untutored from adult “song tutors”, or birds deafened by bilateral cochlear removal before the onset of sensory learning. We then studied how ultrastructure and electrophysiological activity changed in an AFP nucleus, Area X, among these birds at adulthood. Our results showed that, although the size of Area X did not change significantly, the numbers of synapses per unit area and compound synapses and the percent of concave synapses increased significantly in the untutored or deafened birds. The percent of perforated synapses or axo-spinous synapses decreased compared to the normally reared birds, suggesting a decreased efficiency of synaptic transmission in the untutored or deafened birds. We then identified several types of spontaneously firing cells in Area X. Cells with fast and slow firing rates did not show significant electrophysiological differences among the groups, but cells with moderate firing rates, most likely DLM-projecting neurons, fired at significantly lower rates in the untutored and deafened birds. In addition, cells firing irregularly were only found in the deafened birds. Thus, the decreased or irregular electrophysiological activity in the untutored or deafened birds, together with the corresponding ultrastructural findings, could be implicated in the abnormal song production in these two types of birds.