The Journal of Neuroscience, September 1, 2001, 21(17):6836-6845
An Avian Basal Ganglia Pathway Essential for Vocal Learning Forms
a Closed Topographic Loop
Minmin
Luo,
Long
Ding, and
David J.
Perkel
Department of Neuroscience, University of Pennsylvania,
Philadelphia, Pennsylvania 19104
The mammalian basal ganglia-thalamocortical pathway is important
for motor control, motor learning, and cognitive functions. It contains
parallel, closed loops, at least some of which are organized
topographically and in a modular manner. Songbirds have a circuit
specialized for vocal learning, the anterior forebrain pathway (AFP),
forming a basal ganglia loop with only three stations: the pallial
("cortex-like") lateral magnocellular nucleus of the anterior
neostriatum (lMAN), the basal ganglia structure area X, and the medial
portion of the dorsolateral thalamic nucleus (DLM). Several properties
of this pathway resemble those of its mammalian counterpart, but it is
unknown whether all projections in the loop are topographically
organized, and if so, whether topography is maintained through the
entire loop. After small single- or dual-tracer injections into area X
and/or the lMAN of adult zebra finches, we found that the area X to DLM
projection is topographically organized, and we confirmed the
topography for all other AFP projections. Quantitative analysis
suggests maintained topography throughout the loop. To test this
directly, we injected different tracers into corresponding areas in
lMAN and area X. We found somata retrogradely labeled from lMAN and terminals anterogradely labeled from area X occupying the same region
of DLM. Many labeled somata were tightly surrounded by tracer-labeled
terminals, indicating the microscopically closed nature of the AFP
loop. Thus, like mammals, birds have at least one closed, topographic
loop traversing the basal ganglia, thalamus, and pallium. Each such
loop could serve as a computational unit for motor or cognitive functions.
Key words:
basal ganglia; thalamus; songbird; birdsong; topography; zebra finch; vocal learning
Copyright © 2001 Society for Neuroscience 0270-6474/01/21176836-10$05.00/0