Audio-vocal learning in budgerigars

Vocal learning in Budgerigars (Melopsittacus undulatus): effects of an acoustic reference on vocal matching

Budgerigars were trained to produce specific vocalizations (calls) using operant conditioning and food reinforcement. The bird's call was compared to a digital representation of the call stored in a computer to determine a match. Once birds were responding at a high level of precision, we measured the effect of several manipulations upon the accuracy and the intensity of call production. Also, by differentially reinforcing other aspects of vocal behavior, budgerigars were trained to produce a call that matched another bird's contact call and to alter the latency of their vocal response. Both the accuracy of vocal matching and the intensity level of vocal production increased significantly when the bird could hear the template immediately before each trial. Moreover, manipulating the delay between the presentation of an acoustic reference and the onset of vocal production did not significantly affect either vocal intensity or matching accuracy. Interestingly, the vocalizations learned and reinforced in these operant experiments were only occasionally used in more natural communicative situations, such as when birds called back and forth to one another in their home cages.

Genetic components of vocal learning

Vocal learning is a rare trait. Humans depend on vocal learning to acquire spoken language, but most species that communicate acoustically have an innate repertoire of sounds that they use for information exchange. Among the few non-human species that also rely on vocal learning, songbirds have provided by far the most information for understanding this process. This article concentrates on the genetic components of vocal learning in humans and birds. We summarize the existing evidence for a genetic predisposition towards acquiring the species-specific human and avian vocal repertoires. We describe the approaches used for finding genes involved in shaping the neural circuitry required for vocal learning or in mediating the learning process itself. Special attention is given to a particular gene, FOXP2, which has been implicated in a human speech and language disorder. We have studied FoxP2 in avian vocal learners and non-learners and review evidence that links both the molecule and its close homologue FoxP1 to the development of brain regions implicated in vocal learning and to their function. FoxP2 has a characteristic expression pattern in a brain structure uniquely associated with learned vocal communication, Area X in songbirds, or its analogue in parrots and hummingbirds. In both avian song learners and non-learners FoxP2 expression predominates in sensory and sensory-motor circuits. These latter regions also express FoxP2 in mammals and reptiles. We conclude that FoxP2 is important for the building and function of brain pathways including, but not limited to, those essential for learned vocal communication.

Development of auditory sensitivity in budgerigars (Melopsittacus undulatus)

Auditory feedback influences the development of vocalizations in songbirds and parrots; however, little is known about the development of hearing in these birds. The auditory brainstem response was used to track the development of auditory sensitivity in budgerigars from hatch to 6 weeks of age. Responses were first obtained from 1-week-old at high stimulation levels at frequencies at or below 2 kHz, showing that budgerigars do not hear well at hatch. Over the next week, thresholds improved markedly, and responses were obtained for almost all test frequencies throughout the range of hearing by 14 days. By 3 weeks posthatch, birds' best sensitivity shifted from 2 to 2.86 kHz, and the shape of the auditory brainstem response (ABR) audiogram became similar to that of adult budgerigars. About a week before leaving the nest, ABR audiograms of young budgerigars are very similar to those of adult birds. These data complement what is known about vocal development in budgerigars and show that hearing is fully developed by the time that vocal learning begins.

Organization of the avian basal forebrain: chemical anatomy in the parrot (Melopsittacus undulatus)

Hodological, electrophysiological, and ablation studies indicate a role for the basal forebrain in telencephalic vocal control; however, to date the organization of the basal forebrain has not been extensively studied in any nonmammal or nonhuman vocal learning species. To this end the chemical anatomy of the avian basal forebrain was investigated in a vocal learning parrot, the budgerigar (Melopsittacus undulatus). Immunological and histological stains, including choline acetyltransferase, acetylcholinesterase, tyrosine hydroxylase, dopamine and cAMP-regulated phosphoprotein (DARPP)-32, the calcium binding proteins calbindin D-28k and parvalbumin, calcitonin gene-related peptide, iron, substance P, methionine enkephalin, nicotinamide adenine dinucleotide phosphotase diaphorase, and arginine vasotocin were used in the present study. We conclude that the ventral paleostriatum (cf. Kitt and Brauth [1981] Neuroscience 6:1551-1566) and adjacent archistriatal regions can be subdivided into several distinct subareas that are chemically comparable to mammalian basal forebrain structures. The nucleus accumbens is histochemically separable into core and shell regions. The nucleus taeniae (TN) is theorized to be homologous to the medial amygdaloid nucleus. The archistriatum pars ventrolateralis (Avl; comparable to the pigeon archistriatum pars dorsalis) is theorized to be a possible homologue of the central amygdaloid nucleus. The TN and Avl are histochemically continuous with the medial aspects of the bed nucleus of the stria terminalis and the ventromedial striatum, forming an avian analogue of the extended amygdala. The apparent counterpart in budgerigars of the mammalian nucleus basalis of Meynert consists of a field of cholinergic neurons spanning the basal forebrain. The budgerigar septal region is theorized to be homologous as a field to the mammalian septum. Our results are discussed with regard to both the evolution of the basal forebrain and its role in vocal learning processes.

Telencephalic nuclei control late but not early nestling calls in the budgerigar

Bilateral lesions targeting the central nucleus of the anterior archistriatum (AAc) were placed in nestling budgerigars (Melopsittacus undulatus) aged 5, 9, 13, 22, 26, and 33 days post-hatch in order to evaluate the role of the telencephalon in producing nestling vocalizations in this species. In budgerigars, AAc is the final common pathway from telencephalic vocal control nuclei to brainstem respiratory and syringeal motorneuron pools. The results show that lesions destroying AAc bilaterally in addition to surrounding archistriatum and neostriatum do not alter the production of early simple patterned foodbegging calls but do prevent both the normal transition at 3-4 weeks post-hatch to more complex begging calls as well as the emergence of individually-distinctive contact calls around the time of fledging. These vocal results are strikingly similar to those obtained in previous studies in which early deafening of nestlings (Heaton and Brauth, 1999) and early lesioning of auditory areas in the anterior telencephalon (Hall WS, Brauth SE, Heaton JT. Comparison of the effects of lesions in nucleus basalis and field 'L' on vocal control learning and performance in the budgerigar (M. undulatus), Brain Behav. Evol., 1994;44:133-148) did not affect call production until 3-4 weeks post-hatch. These data combined support the idea that neither auditory feedback nor telencephalic sensorimotor circuits are necessary for the production of nestling calls before 3 weeks post-hatch.

Molecular mapping of brain areas involved in parrot vocal communication

Auditory and vocal regulation of gene expression occurs in separate discrete regions of the songbird brain. Here we demonstrate that regulated gene expression also occurs during vocal communication in a parrot, belonging to an order whose ability to learn vocalizations is thought to have evolved independently of songbirds. Adult male budgerigars (Melopsittacus undulatus) were stimulated to vocalize with playbacks of conspecific vocalizations (warbles), and their brains were analyzed for expression of the transcriptional regulator ZENK. The results showed that there was distinct separation of brain areas that had hearing- or vocalizing-induced ZENK expression. Hearing warbles resulted in ZENK induction in large parts of the caudal medial forebrain and in 1 midbrain region, with a pattern highly reminiscent of that observed in songbirds. Vocalizing resulted in ZENK induction in nine brain structures, seven restricted to the lateral and anterior telencephalon, one in the thalamus, and one in the midbrain, with a pattern partially reminiscent of that observed in songbirds. Five of the telencephalic structures had been previously described as part of the budgerigar vocal control pathway. However, functional boundaries defined by the gene expression patterns for some of these structures were much larger and different in shape than previously reported anatomical boundaries. Our results provide the first functional demonstration of brain areas involved in vocalizing and auditory processing of conspecific sounds in budgerigars. They also indicate that, whether or not vocal learning evolved independently, some of the gene regulatory mechanisms that accompany learned vocal communication are similar in songbirds and parrots.

Vocal production mechanisms in the budgerigar (Melopsittacus undulatus): the presence and implications of amplitude modulation

In this paper acoustic evidence is presented for the presence of amplitude modulation in budgerigar (Melopsittacus undulatus) contact calls and learned English vocalizations. Previously, acoustic analyses of budgerigar vocalizations have consisted solely of visual inspection of spectrograms or power spectra (derived from Fourier transformation). Such analyses have led researchers to conclude that budgerigar vocalizations are primarily frequency-modulated, harmonic vocalizations. Although budgerigar calls have been shown to contain regions that are modulated in amplitude, the implications of this fact have been largely ignored. Amplitude modulation, the nonlinear interaction between two separate signals that results in the creation of new, heterodyne (sum and difference) frequencies, can produce a very complex Fourier spectrum that may resemble that produced by a harmonic vocalization. In this paper, the acoustic principles necessary for identifying amplitude modulation present in signals are outlined, and followed by data demonstrating that amplitude modulation is a prominent feature not only of natural budgerigar contact calls, but also of their learned English vocalizations. It is illustrated how analyzing a vocalization that contains amplitude modulation as if it were harmonic can result in misinterpretations of the acoustic and physical properties of the sound and sound source. The implications of amplitude modulation for studies of the ontogenetic, physical, and neural basis of budgerigar vocalizations are discussed, and a potential model for how the budgerigar syrinx may function to produce amplitude modulation is proposed.

Methionine enkephalin immunoreactivity in the brain of the budgerigar (Melopsittacus undulatus): similarities and differences with respect to oscine songbirds

The brain of the budgerigar (Melopsittacus undulatus), a small parrot that acquires new vocalizations throughout life, was examined for immunoreactivity to the opioid peptide methionine enkephalin (mENK). mENK is a highly prominent feature of the chemical architecture of the forebrain vocal system of oscine songbirds. Forebrain vocal control nuclei are believed to have evolved independently in parrots and songbirds (Streidter [1994] J. Comp. Neurol. 343:35-56); however, recent studies have found similarities in the neural organization of vocal control pathways in budgerigars and songbirds (Durand et al. [1997] J. Comp. Neurol. 377:179-206). Among the similarities are the existence of recursive pathways interconnecting vocal control neurons in the archistriatum, basal ganglia (i.e., lobus parolfactorius), and dorsal thalamus. In the present study, we found that all vocal control nuclei within the budgerigar forebrain exhibit prominent mENK-like immunoreactivity (ELI) in fibers and somata. We also found striking similarities between the morphology of ELI elements in budgerigar vocal control nuclei and that described previously in songbird vocal nuclei. Despite these similarities, the budgerigar dorsal striatopallidum (lobus parolfactorius, paleostriatum augmentatum, and paleostriatum primitivum) and somatomotor (anterior) archistriatum exhibit unique patterns of ELI. The dorsal striatopallidum contained far less ELI, whereas the archistriatum contained far more than would be expected on the basis of previous studies of opioid peptides in other avian species, including pigeons, chickens, and songbirds. These differences may reflect neural specializations unique to the budgerigar that contribute to the extraordinary flexibility of the vocal motor system of this species to acquire socially significant stimuli throughout life.