There have been numerous surveys of the morphology of the avian telencephalon and attempts to compare it to the various divisions of the telencephalon of other vertebrates. These comparisons were often tenuous, based upon apparent resemblance to structures in mammals. Time does not permit a complete survey of all structures discernible within the forebrain, and the present report is a resume of only some of the recent findings on the avian telencephalon. The major portion of the avian telencephalon consists of a mass of grey matter in the lateral wall of the hemisphere subdivided into several regions, all ending with the suffix “-striaturn,” and with a small region of clearly laminated pallium. As il consequence of the relatively small amount of pallium in birds, most suggestions concerning the nature of the avian telencephalon have derived from the striatal appearance of the cell masses in the forebrain. FIGURE 1 is a Nissl-stained section of a transverse section through the pigeon telencephalon showing the location of the paleo-, neo-, and various subdivisions of the hyperstriatum. It is worth recalling, however, the original reason for designating these areas as striatum. Striatum was simply a descriptive term indicating that these regions consisted of large masses of grey matter with bundles of myelinated axons passing through them, giving them a striated appearance. However, this simple descriptive feature provided no information about the differences between different masses of grey matter within the telencephalon, their connections, or participation in any functional system, and in no way provided any defining characteristics of the cells that would permit comparing them to cell masses in other vertebrates. There was no logical reason to expect that all striatal structures were necessarily comparable, except that they consisted of masses of neurons, glia and axons. Even more puzzling was the striking discrepancy between the relatively small amount of pallium and large volume of striatum in sauropsids, and the apparently inverse ratios of large areas of pallium and only moderate amounts of striatum in mamma1s.t

The organization of the avian telencephalon and some speculations on the phylogeny of the amniote telencephalon

I Fishes.- 1 Structure and Function in Teleost Auditory Systems.- 2 Underwater Localization-A Major Problem in Fish Acoustics.- 3 Central Auditory Pathways in Anamniotic Vertebrates.- II Amphibians.- 4 The Structure of the Amphibian Auditory Periphery: A Unique Experiment in Terrestrial Hearing.- 5 Nonlinear Properties of the Peripheral Auditory System of Anurans.- III Reptiles.- 6 The Reptilian Cochlear Duct.- 7 Physiology and Bioacoustics in Reptiles.- IV Birds.- 8 Structure and Function of the Avian Ear.- 9 Behavior and Psychophysics of Hearing in Birds.- 10 Sound Localization in Birds.- 11 Response Properties of Neurons in the Avian Auditory System: Comparisons with Mammalian Homologues and Consideration of the Neural Encoding of Complex Stimuli.- PartV Mammals.- 12 Directional Hearing in Terrestrial Mammals.- 13 Comparative Organization of Mammalian Auditory Cortex.- 14 Man as Mammal: Psychoacoustics.- 15 The Evolution of Hearing in the Mammals.- VI Future View.- 16 Comparative Audition: Where Do We Go from Here?.

Comparative studies of hearing in vertebrates

Interaural time difference is an important cue for sound localization. In the barn owl (Tyto alba) neuronal sensitivity to this disparity originates in the brainstem nucleus laminaris. Afferents from the ipsilateral and contralateral magnocellular cochlear nuclei enter the nucleus laminaris through its dorsal and ventral surfaces, respectively, and interdigitate in the nucleus. Intracellular recordings from these afferents show orderly changes in conduction delay with depth in the nucleus. These changes are comparable to the range of interaural time differences available to the owl. Thus, these afferent axons act as delay lines and provide anatomical and physiological bases for a neuronal map of interaural time differences in the nucleus laminaris.

https://www.pnas.org/content/pnas/85/21/8311.full.pdf

Axonal delay lines for time measurement in the owl's brainstem.

The ascending auditory pathway in the pigeon (Columba livia) II. Telencephalic projections of the nucleus ovoidalis thalami

1 Introduction.- 2 The Middle Ear of Reptiles and Birds.- 3 The Hearing Organ of Birds and Crocodilia.- 4 The Hearing Organs of Lizards.- 5 The Central Auditory System of Reptiles and Birds.- 6 Sound Localization in Birds.- 7 Hearing in Birds and Reptiles.

Comparative hearing : birds and reptiles

The purpose of this investigation is to determine, by experimental degeneration methods, point to point afferent neuronal connections between the organ of Corti and the primary cochlear nuclei of the pigeon, and to clarify the central termination of the closely associated nerve from the macula lagenae. At the present time, fundamental problems concerning audition in man and higher mammals are being attacked by electrophysiological experimentation on lower forms. This approach should consider comparative anatomical data, some of which this study is intended to provide. Furthermore, a detailed anatomical knowledge of the more simply constructed avian auditory mechanism is necessary to provide a better understanding of the evolution of the auditory system and to serve as a basis for future morphological and physiological investigation.

Projection of the cochlear and lagenar nerves on the cochlear nuclei of the pigeon.

The efferent cochlear bundle in the caiman and pigeon

The effects of experimental removal of the pigeon cupula ampullaris were studied. It could be shown that the hairs of the sensory cells generally remain intact after this operation, suggesting that they are not fixed to the structures of the cupular substance. The cells of the transitional epithelium on the slopes of the crista showed an intense secretory activity as a result of the operation. The secretion appeared in the sections as eosinophilic globules. These globules were found in all parts of the endolymphatic space shortly after the operation, but were removed by the stream of secreted endolymph, except in the region of the sensory area where they were trapped by the hairs.At a later postoperative period the globules in between the hairs are believed to be lifted beyond the tips of the hairs, thereby forming a new “subcupular space”. It is suggested that this could be the result of a histologically invisible secretion from the supporting cells of the sensory areas.

The Effects of Cupular Removal on the Activity of Ampullary Structures in the Pigeon

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Projection of the cochlear and lagenar nerves on the cochlear nuclei of the pigeon.

The efferent cochlear bundle in the caiman and pigeon

The Effects of Cupular Removal on the Activity of Ampullary Structures in the Pigeon