Bird song
and its neurobiological substrates are central topics
in ethology and neuroethology. Birds learn their songs
from external tutors, and have an elaborate system of
forebrain, mid brain, and brainstem nuclei involved
in aspects of song memorization, learning, perception,
and production. Behavioral studies have identified critical
elements of the theory of song learning, but the neural
implementation of this theory has yet to be described.
A connectionist modeling study has demonstrated that
neurons in the nucleus ovoidalis, part of the ascending
auditory system, exhibit responses to complex stimuli
such as song that can be predicted from the neurons’
responses to simple stimuli such as tone and noise bursts.
Thus, there is no evidence that the auditory response
properties of these neurons are modified by the song
learning process.
Indeed, to
date the modification of neural sensory structures that
must accompany song memorization early in life has yet
to be described. A recent 2DG study suggests field L
(which receives from ovoidalis and projects to HVc)
may be an attractive candidate structure. In contrast,
there is strong evidence that the forebrain nucleus
HVc is a site of sensorimotor integration in the birdsong
system. Auditory neurons in the HVc of all species tested
respond selectively to the individual bird’s own
(autogenous) song, including neurons that exhibit strict
temporal combination sensitivity. The parameters of
autogenous song are specified by environmental influences
during vocal learning, hence it is hypothesized that
the HVc neurons shape, and are shaped by, the sensorimotor
phase of learning. The representation of autogenous
song has unusual features, including a global synchrony
related to syllabic features of the song. These features
vary between individuals, emphasizing the idiosyncratic
nature of neural representations of such forms of complex
learning.
The syllabic
representation at the level of HVc is also observed
in single neuron activity recorded during singing in
zebra finch. HVc neurons have a motor recruitment activity
pattern that is specific for each syllable type, independent
of syllable position, and varies from neuron to neuron.
Thus, during production HVc neurons encode for syllable
identity.
The syllable
is a motor program, generally involving all syringeal
muscles, and co-articulation with the vocal tract, respiratory,
and postural systems. Hence, the code at the level of
HVc is not muscle based. Elucidation of the HVc code
in the adult, and its establishment during sensorimotor
development, will be of major importance for determining
the neural mechanisms of idiosyncratic learning. Understanding
the mapping between the auditory response and motor
recruitment properties of the same HVc neurons remains
a major theoretical challenge - how is auditory feedback
used to guide vocal learning if it has no apparent relation
to the ongoing motor program at the physiological level.
To date, however, these same properties have yet to
be assessed in juvenile birds. The motor recruitment
properties of neurons in RA, which is the major forebrain
output structure and receives from HVc, differ dramatically
from those of HVc neurons. RA neurons exhibit highly
synchronized bursts of activity associated with sub-syllabic
acoustic features such as notes. Thus, there is a hierarchical
organization of the motor system that is related to
the hierarchical organization of the segmental temporal
structure of the vocalizations. The analysis of the
mechanisms of syringeal action will be essential for
a deep understanding of coding in this system.
