Fig. 1.

Contributions of an avian basal ganglia-thalamocortical circuit to vocal motor plasticity. (A) The AFP of songbirds indirectly connects the motor nuclei HVC and RA, and consists of the basal ganglia (striato-pallidal) nucleus Area X (9, 10), the thalamic nucleus DLM, and the cortical nucleus LMAN. Although LMAN is not a layered structure with classic pyramidal cells, it shares many anatomic and physiologic similarities with the mammalian prefrontal cortex: LMAN lies anterior to motor and premotor areas, receives input from a dorsal thalamic nucleus that is not a primary sensory area, projects to the basal ganglia and to the primary song motor nucleus RA, and is more densely innervated by dopaminergic fibers than the region surrounding it (31, 60–62). Moreover, as with the prefrontal cortex, stimulation of LMAN does not evoke motor (song) output (24, 46, 63). Alternatively, or in addition, with respect to the hypothesis that avian nuclei are homologous to mammalian cortical layers (11, 12, 64), LMAN shares some but not all of the features of layer II/III intracortically connecting neurons. Yellow, green, and purple boxes indicate basal ganglia, thalamic, and cortical structures, respectively. Str, striatum; GPi, internal segment of the globus pallidus. (B) Removal of auditory feedback (via deafening) induces gradual degradation of song (20). (C) Lesions of LMAN prevent deafening-induced changes in song (17). (D) Experimental paradigm: To investigate basal ganglia contributions to cortical activity and motor plasticity, lesions of Area X were made before deafening and their effects on LMAN activity and song were examined.