possess unique motif repertoires, disjoint sets of motifs will generally
correspond to individual identity.Therefore, attending to the motif structure
captures a significant portion of the individual variation in the signal.
The behavioral data inform a number of hypotheses regarding the neural
mechanisms of vocal recognition in starlings. First, the representational
architecture of forebrain auditory regions should reflect the segmentation
of song at the level of the motif. When similarity/difference computations
are invoked, as they must be for recognition to proceed, the input to such
computations should be some representation of the motif. Determination
of the precise acoustic information corresponding to such a representation
has yet to be accomplished but is amenable to behavioral study in the laboratory. Investigations of these representations at the neural level are likely
to tell a great deal about the manner in which complex auditory objects are
represented and processed. Second, the behavioral strategy employed by
starlings for vocal recognition suggests that the upper bound on the capacity for accurate recognition is constrained by the memory capacity of the
system for specific motifs. If the representation of a motif is coded by
dynamic temporal and spatial patterns of connectivity among local feature
detectors, then these memory constraints may derive directly from the perceptual mechanisms for coding complex stimuli.
3.3.4. Songbird Auditory System: Anatomy
The basic plan of the passerine auditory system follows a general reptilebird pattern of connections (Ulinski and Margoliash 1990; Carr 1992; Carr
and Code 2000). In birds, the auditory nerve projects to two cochlear nuclei,
the nucleus magnocellularis and the nucleus angularis. These nuclei project
in turn to second-order olivary nuclei, to the lemniscal nuclei, and contralaterally to the central nucleus of the nucleus mesencephalicus lateralis
dorsalis (MLd), the avian analog of the inferior colliculus. The central
nucleus of the MLd projects to a major target in the medial portion of the
dorsal thalamus, the nucleus ovoidalis (Ov), and to a lesser extent to the
subjacent nucleus semiluminaris parovoidalis (SPO) (Karten 1968) and a
region surrounding the Ov-SPO complex referred to as the “shell” (Durand
et al. 1992). There are a wealth of connections and patterns of connections
in the more peripheral auditory structures not reviewed here. In addition
to the primary projections to the auditory telencephalon, auditory fibers
also project from the thalamus to the hypothalamus, providing one possible
route for interaction between auditory, neuromodulatory (Li and Sakaguchi
1997), and endocrine systems (Durand et al. 1992).
Nissl and Golgi preparations of male zebra finch brains (Bonke et al.
1979b) confirm the general pattern of organization observed in other
species (Karten 1968) and demonstrate that the caudal medial portion of
the avian telencephalon is composed of five cytoarchitectonic subregions—
L1, L2a, L2b, L3 and L—called the field L complex (Fortune and
7. Neuroethology of Vocal Communication
361
correspond to individual identity.Therefore, attending to the motif structure
captures a significant portion of the individual variation in the signal.
The behavioral data inform a number of hypotheses regarding the neural
mechanisms of vocal recognition in starlings. First, the representational
architecture of forebrain auditory regions should reflect the segmentation
of song at the level of the motif. When similarity/difference computations
are invoked, as they must be for recognition to proceed, the input to such
computations should be some representation of the motif. Determination
of the precise acoustic information corresponding to such a representation
has yet to be accomplished but is amenable to behavioral study in the laboratory. Investigations of these representations at the neural level are likely
to tell a great deal about the manner in which complex auditory objects are
represented and processed. Second, the behavioral strategy employed by
starlings for vocal recognition suggests that the upper bound on the capacity for accurate recognition is constrained by the memory capacity of the
system for specific motifs. If the representation of a motif is coded by
dynamic temporal and spatial patterns of connectivity among local feature
detectors, then these memory constraints may derive directly from the perceptual mechanisms for coding complex stimuli.
3.3.4. Songbird Auditory System: Anatomy
The basic plan of the passerine auditory system follows a general reptilebird pattern of connections (Ulinski and Margoliash 1990; Carr 1992; Carr
and Code 2000). In birds, the auditory nerve projects to two cochlear nuclei,
the nucleus magnocellularis and the nucleus angularis. These nuclei project
in turn to second-order olivary nuclei, to the lemniscal nuclei, and contralaterally to the central nucleus of the nucleus mesencephalicus lateralis
dorsalis (MLd), the avian analog of the inferior colliculus. The central
nucleus of the MLd projects to a major target in the medial portion of the
dorsal thalamus, the nucleus ovoidalis (Ov), and to a lesser extent to the
subjacent nucleus semiluminaris parovoidalis (SPO) (Karten 1968) and a
region surrounding the Ov-SPO complex referred to as the “shell” (Durand
et al. 1992). There are a wealth of connections and patterns of connections
in the more peripheral auditory structures not reviewed here. In addition
to the primary projections to the auditory telencephalon, auditory fibers
also project from the thalamus to the hypothalamus, providing one possible
route for interaction between auditory, neuromodulatory (Li and Sakaguchi
1997), and endocrine systems (Durand et al. 1992).
Nissl and Golgi preparations of male zebra finch brains (Bonke et al.
1979b) confirm the general pattern of organization observed in other
species (Karten 1968) and demonstrate that the caudal medial portion of
the avian telencephalon is composed of five cytoarchitectonic subregions—
L1, L2a, L2b, L3 and L—called the field L complex (Fortune and
7. Neuroethology of Vocal Communication
361