Neural coding of speech sound in the telencephalic auditory area of the mynah bird

Functional organization of the forebrain auditory centres of the European starling: a study based on natural sounds

The field L complex is thought to be the highest auditory centre and the input in the song vocal nuclei. Different anatomical and functional subdivisions have been described in field L. Auditory neurons of field L are well activated by natural sounds and especially by species-specific sounds. A complex sound coding appears to exist in field L. However, until now, the spatial organization of the different functional subdivisions has been described only using artificial sounds. Here, we investigated the spatial distribution of neuronal responses in field L to species-specific songs. Starlings seemed to be a very appropriate species for this investigation, both because of their complex vocal behaviour that implies different levels of categorization and their neuronal responses towards complex song elements. Multi-unit recordings were performed in wild starlings that were awake. The method of backward correlation was used to visualize the functional organization and we represented the neuronal responses as both activity maps and correlation maps. The use of natural sounds allowed us to define several functional sub-areas with different neuronal processing. These results show that field L is involved in a more complex task than simple frequency processing.

Sound processing in the auditory-cortex homologue of songbirds: functional organization and developmental issues

Recent literature on the Field L of songbirds, showing that some neurons present a clear selectivity towards complex sounds, especially conspecific songs, is reviewed. Furthermore, studies on European starlings have revealed a complex functional organization in this central auditory area, with subareas exhibiting different response features. Interestingly, both the functional organization and the neuronal specialization can be drastically affected by early deprivation, clearly showing the existence of a developmental plasticity. Some recovery seems to remain possible at later stages, and social factors may be involved.

Hemispheric specialization in the primary auditory area of awake and anesthetized starlings (Sturnus vulgaris)

Although evidence exists for a lateralization of song production, few studies have focused on the perceptual aspect of lateralization in songbirds. In the present study, the authors recorded neuronal responses to a variety of species-specific and artificial, nonspecific stimuli in both hemispheres of awake and anesthetized male starlings (Sturnus vulgaris). Recordings were made in the primary auditory area of the songbird brain, the Field L complex. The right hemisphere exhibited significantly more responsive units than the left hemisphere in awake birds, and this difference was significantly reduced in anesthetized birds. Furthermore, clear hemispheric specialization toward categories of behaviorally relevant stimuli and precise parameters of these stimuli were found. The main auditory area of the starling's brain thus appears to show some degree of lateralization.

Feature analysis of natural sounds in the songbird auditory forebrain

Although understanding the processing of natural sounds is an important goal in auditory neuroscience, relatively little is known about the neural coding of these sounds. Recently we demonstrated that the spectral temporal receptive field (STRF), a description of the stimulus-response function of auditory neurons, could be derived from responses to arbitrary ensembles of complex sounds including vocalizations. In this study, we use this method to investigate the auditory processing of natural sounds in the birdsong system. We obtain neural responses from several regions of the songbird auditory forebrain to a large ensemble of bird songs and use these data to calculate the STRFs, which are the best linear model of the spectral-temporal features of sound to which auditory neurons respond. We find that these neurons respond to a wide variety of features in songs ranging from simple tonal components to more complex spectral-temporal structures such as frequency sweeps and multi-peaked frequency stacks. We quantify spectral and temporal characteristics of these features by extracting several parameters from the STRFs. Moreover, we assess the linearity versus nonlinearity of encoding by quantifying the quality of the predictions of the neural responses to songs obtained using the STRFs. Our results reveal successively complex functional stages of song analysis by neurons in the auditory forebrain. When we map the properties of auditory forebrain neurons, as characterized by the STRF parameters, onto conventional anatomical subdivisions of the auditory forebrain, we find that although some properties are shared across different subregions, the distribution of several parameters is suggestive of hierarchical processing.

Adjustable frequency selectivity of auditory forebrain neurons recorded in a freely moving songbird via radiotelemetry

One of the hearing system's basic properties that determines the detection of signals is its frequency selectivity. In the natural environment, a songbird may achieve an improved detection ability if the neuronal filters of its auditory system could be sharpened to adapt to the spectrum of the background noise. To address this issue, we studied 35 multi-unit clusters in the input layer of the primary auditory forebrain of nine European starlings (Sturnus vulgaris). Microelectrodes were chronically implanted in this songbird's cortex analogue and the neuronal activity was transmitted from unrestrained birds via a miniature FM transmitter. Frequency tuning curves (FTCs) and inhibitory sidebands were determined by presenting a matrix of frequency-level combinations of pure tones. From each FTC, the characteristic frequency (CF) and several parameters describing the neurons' filter characteristics were derived and compared to the same recording site's filter function while simultaneously stimulating with a continuous CF tone 20 dB above the response threshold. Our results show a significant improvement of frequency selectivity during two-tone stimulation, indicating that spectral filtering in the starling's auditory forebrain depends on the acoustic background in which a signal is presented. Moreover, frequency selectivity was found to be a function of the time over which the stimulus persisted, since FTCs were much sharper and inhibitory sidebands were largely expanded several milliseconds after response onset. Neuronal filter bandwidths during two-tone stimulation in the auditory forebrain are in good agreement with psychoacoustically measured critical bandwidths in the same species. Radiotelemetry proved to be a powerful tool in studying neuronal activity in freely behaving birds.

Vocal control pathways through the anterior forebrain of a parrot (Melopsittacus undulatus)

A feature of the telencephalic vocal control system in the budgerigar (Melopsittacus undulatus) that has been hypothesized to represent a profound difference in organization from the oscine vocal system is its reported lack of an inherent circuit through the anterior forebrain. The present study reports anatomical connections that indicate the existence of an anterior forebrain circuit comparable in important ways to the "recursive" pathway of oscine songbirds. Results from anterograde and retrograde tracing experiments with biocytin and fluorescently labeled dextran amines indicate that the central nucleus of the anterior archistriatum (AAc) is the source of ascending projections upon the oval nuclei of the anterior neostriatum and ventral hyperstriatum (NAo and HVo, respectively). Efferent projections from the latter nuclei terminate in the lateral neostriatum afferent to AAc, thereby forming a short recurrent pathway through the pallium. Previously reported projections from HVo and NAo upon the magnocellular nucleus of the lobus parolfactorius (LPOm), and after LPOm onto the magnocellular nucleus of the dorsal thalamus (DMm; G.F. Striedter [1994] J. Comp. Neurol. 343:35-56), are confirmed. A specific projection from DMm onto NAom is also demonstrated; therefore, a recurrent pathway through the basal forebrain also exists in the budgerigar vocal system that is similar to the anterior forebrain circuit of oscine songbirds. Parallels between these circuits and mammalian basal ganglia-thalamo-cortical circuits are discussed. It is hypothesized that vocal control nuclei of the avian anterior neostriatum may perform a function similar to the primate supplemental motor area.

Functional organization of the auditory cortex: maps and mechanisms

Recent studies have led to a better understanding of several aspects of the organization and physiological mechanisms involved in the processing of information in the auditory cortex. A wide range of approaches have revealed new information regarding the histochemistry, cortico-cortical connections, single-unit physiology, and functional spatial organization, as well as mechanisms and effects of representational and learning-induced cortical plasticity.

High-order auditory filters

The past year has seen some important advances in our understanding of central auditory function. Several central auditory neurons that differ from those in the periphery in their selectivity for various spectral and temporal parameters of complex sound have been described. Although central specializations for the processing of complex sound have been known for some time, recent findings suggest that these high-order filter properties are more widespread than previously thought. Significant progress has been made in our understanding of the neural mechanisms by which some high-order filter properties, such as delay-tuning, amplitude-tuning, and complex frequency tuning are performed by the central auditory system. New evidence has clarified the role of high-order auditory filters in auditory learning and perception, and the regions in which they are found.