Biomedical Engineering Reference
In-Depth Information
This suggests that the HVC activity is sequenced to orchestrate the produc-
tion of syllables and motifs, so that interrupting its progress during such a
sequence causes a reset or reinitiation of the activity. It also suggests that RA
is a “junction box” where HVC signals are combined to produce song, but
does not itself initiate or command the song. The connections in the “junc-
tion box” can be altered by modification of its input connections from HVC
and the anterior forebrain pathway, so song production can be influenced in
RA but not started there.
Work on zebra finches (
Taeniopygia guttata
) has analyzed in detail the
exact time relation between the firing of neurons in HVC and RA during
song [Hahnloser et al. 2002]. This work has shown that during the vocaliza-
tion of a motif, lasting approximately 1 s, a given RA-projecting neuron in
HVC is active only during one window of temporal size 6
.
1
±
2msinthis
motif, and produces 4
.
5
2 spikes in that temporal window. If a motif is
repeated, each HVC neuron will repeat its spiking in the same time window.
On the other hand, RA neurons generate highly stereotyped sequences of ac-
tion potential bursts, typically a few bursts of approximately 10 ms duration
per motif, well time-locked with parts of syllables [Chi and Margoliash 2001].
The picture emerging from these experiments is that during each time win-
dow in the RA sequence, RA neurons are driven by a subpopulation of
RA-projecting HVC neurons which are active only during that window of
time [Hahnloser et al. 2002]. These experiments suggest that the premotor
burst patterns in RA are basically driven by the activities of HVC neurons
[McCasland 1987]. In that case, the architecture of the connectivity between
the HVC nucleus and the RA nucleus will determine the complex patterns of
activity in the RA neurons. Additional nuclei afferent to HVC are likely to be
part of the motor pathway. One of these, the nucleus interfacialis (Nif), seems
to be critical at the moment of generation of the basic oscillations which even-
tually drive HVC into generating the sparse activity that ultimately drives
the syrinx.
±
8.2 The AFP Pathway
A second set of interconnected nuclei is the anterior forebrain pathway
(AFP), also shown in Fig. 8.1. It connects indirectly the nuclei HVC and
RA. In contrast to the motor pathway, the AFP contributes only minimally
to the production of song in adults [Brainard and Doupe 2000]. However,
it has been shown that lesions to these nuclei during learning profoundly
alter the bird's capability for developing normal song [Bottjer et al. 1984,
Scharff and Nottebohm 1991]. Three nuclei are part of the AFP: area X, the
medial nucleus of the dorsolateral thalamus (DLM) and the lateral magno-
cellular nucleus of the anterior neostratum (lMAN). The way in which this
system operates during learning is not known. The motor pathway and the
AFP are not isolated. Some neurons in RA receive signals from both HVC
