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Humans and other animals generally perceive motion independently of the cues that define the moving object. To understand the underlying mechanisms of this generalization of stimulus attributes, we have examined the cellular properties of avian wide-field tectal neurons that are sensitive to a variety of moving stimuli but not to static stationary stimuli. This in vitro study showed phasic signal transfer at the retinotectal synapse and binary dendritic responses to synaptic inputs that interact in a mutually exclusive manner in the postsynaptic tectal neuron. A model of the tectal circuitry predicts that these two cellular properties mediate sensitivity to a wide range of dynamic spatiotemporal stimuli, including moving stimuli, but not to static stationary stimuli in a tectal neuron. The computation that is independent of stimulus detail is initiated by tectal neurons and is completed by rotundal neurons that integrate outputs from multiple tectal neurons in a directionally selective manner.

The stable and specific locking-in of pre- and postsynaptic membranes in synaptogenesis may be mediated by integral membrane proteins, such as members of the cadherin family. Cadherins are ideal candidate molecules for mediating synaptic specificity because they are differentially expressed in functionally connected brain structures. We studied the expression of four classic cadherins (R-cadherin, N-cadherin, cadherin-6B and cadherin-7) at the synaptic level on the somata and the proximal neurites of identified neuron populations that were traced selectively in the developing chicken visual system. Three major findings were observed. (1) Synapses on somata of shepherd's crook cells of the optic tectum are associated preferentially with one cadherin subtype. (2) In an isthmic nucleus that contains a mixed population of cells expressing different cadherins, somatic synapses tend to express the same cadherin subtype as the rest of the cell. (3) In the oculomotor complex, two cadherin subtypes are expressed only by synapses on the axon hillock. However, another neuron type that projects from the tectum to the isthmic nucleus does not show such selective synaptic cadherin staining. Our findings support the idea that a cadherin-based adhesive mechanism can mediate synaptic specificity.

Zusammenfassung Bei allen Wirbeltieren ist das visuelle Mittelhirn eingebunden in die Raumorientierung und speziell in die Ausrichtung der sensorischen Systeme. Wie autonom aber ist diese Struktur? Bei Fischen, Amphibien, Reptilien und Vögeln wird das optische Tectum als wichtigstes visuelles Zentrum betrachtet, das eine zentrale Funktion bei der gesamten Verhaltenssteuerung hat. Dem Colliculus superior der Säuger wird dagegen nur eine untergeordnete Rolle als ausführende Station unter Kontrolle des Kortex zugewiesen. Diese klassische Sicht auf die Gehirnorganisation der Wirbeltiere ist nach neueren Befunden nicht mehr aufrechtzuerhalten. Wir haben in diesem Artikel verschiedene Befunde zusammengestellt, die bei allen Wirbeltieren auf eine vergleichbare Integration des visuellen Mittelhirns in die Verarbeitung sensorischer Stimuli hinweisen. In vergleichend anatomischen Arbeiten sind identische Projektionen und Zelltypen beschrieben worden. Läsionsexperimente führen zu Ausfallserscheinungen, die nicht mit der klassischen Vorstellung erklärt werden können. Bei Vögeln und bei Säugern werden zunehmend sensorische Verarbeitungsleistungen im Mittelhirn beschrieben, deren Ergebnisse nicht nur zur Auslösung adäquater Orientierungsreaktionen genutzt werden. Funktionelle Untersuchungen deuten vielmehr an, dass die Aktivität an das Vorderhirn weitergegeben wird und dort zu weiteren Wahrnehmungsleistungen oder zu kognitiver Kontrolle beiträgt. Dies deutet eine eigenständige Bedeutung des Mittelhirns in der Verarbeitung an, die weit über die klassische Sicht als Reflexzentrum hinausgeht.

In everyday life we constantly perceive and discriminate between a large variety of sensory inputs, the vast majority of which consist of more than one modality. We performed two experiments to investigate whether chickens use the information present in multimodal signals. To test whether audiovisual stimuli are better detected than visual or acoustic stimuli alone, we first measured the detection threshold with a staircase paradigm. We found that chickens were able to detect weaker stimuli using audiovisual stimuli. Next, we tested whether the multimodal nature of a stimulus also increases the discrimination between two stimuli by measuring the smallest difference that the animals could still distinguish from each other. We found that chickens can discriminate smaller differences using audiovisual stimuli in comparison to visual stimuli alone, but not in comparison to acoustic stimuli alone. Thus, even in a relatively unspecialized species such as the chicken, the benefits of multimodal integration are exploited for sensory processing.

During development of the nervous system, neurotrophic interactions are essential for the synchronization of substructures such as retina and visual midbrain. However, morphological studies of postsynaptic elements after ablation of afferents are sparse. We investigated the effect of uni- and bilateral eye anlagen removal on identified projection neurons in the chicken optic tectum. Without retinal input, neurons in the stratum griseum centrale express their specific cell adhesions molecules, retain large dendritic fields and form specialized dendritic endings; however, the latter are deformed and extend over a much larger area. Our results show that even monosynaptically innervated tectal neurons develop largely independently from trophic retinal inputs and only become dependent on these after synaptic contact with retinal afferents.

Most systematic studies of the avian visual system have focused on Neognathous species, leaving virtually unexplored the Palaeognathae, comprised of the flightless ratites and the South American tinamous. We investigated the visual field, the retinal topography, and the pattern of retinal and centrifugal projections in the Chilean tinamou, a small Palaeognath of the family Tinamidae. The tinamou has a panoramic visual field with a small frontal binocular overlap of 20°. The retina possesses three distinct topographic specializations: a horizontal visual streak, a dorsotemporal area, and an area centralis with a shallow fovea. The maximum ganglion cell density is 61,900/ mm(2) , comparable to Falconiformes. This would provide a maximal visual acuity of 14.0 cycles/degree, in spite of relatively small eyes. The central retinal projections generally conform to the characteristic arrangement observed in Neognathae, with well-differentiated contralateral targets and very few ipsilateral fibers. The centrifugal visual system is composed of a considerable number of multipolar centrifugal neurons, resembling the "ectopic" neurons described in Neognathae. They form a diffuse nuclear structure, which may correspond to the ancestral condition shared with other sauropsids. A notable feature is the presence of terminals in deep tectal layers 11-13. These fibers may represent either a novel retinotectal pathway or collateral branches from centrifugal neurons projecting to the retina. Both types of connections have been described in chicken embryos. Our results widen the basis for comparative studies of the vertebrate visual system, stressing the conserved character of the visual projections' pattern within the avian clade.

Dendritic computation is essential for understanding information processing in single neurons and brain circuits. Optical methods are suited best to investigate function and biophysical properties of cellular compartments at high spatial and temporal resolution. Promising approaches include the use of voltage sensitive dyes, genetically encoded voltage sensors, or hybrid voltage sensors (hVoS) consisting of fluorescent proteins and voltage-dependent quenchers that, so far, are not available in avian neuroscience.We have adapted a hVoS system for a chicken midbrain slice preparation by combining genetically expressed farnesylated eGFP with dipicrylamine (DPA). Depending on the cellular potential, DPA is shifted in the membrane, resulting in quenching of eGFP fluorescence linearly to the membrane potential by Förster resonance electron transfer.In ovo electroporation resulted in labelled neurons throughout the midbrain with a high level of fine structural detail. After application of DPA, we were able to optically record electrically evoked action potentials with high signal-to-noise ratio and high spatio-temporal resolution.Standard methods available for avian neuroscience such as whole-cell patch clamp yield insufficient data for the analysis of dendritic computation in single neurons. The high spatial and temporal resolution of hVoS data overcomes this limitation. The results obtained by our method are comparable to hVoS data published for mammals.With the protocol presented here, it is possible to optically record information processing in single avian neurons at such high spatial and temporal resolution, that cellular and subcellular events can be analysed.

Pit vipers (Crotalinae) have a specific sensory system that detects infrared radiation with bilateral pit organs in the upper jaw. Each pit organ consists of a thin membrane, innervated by three trigeminal nerve branches that project to a specific nucleus in the dorsal hindbrain. The known topographic organization of infrared signals in the optic tectum prompted us to test the implementation of spatiotopically aligned sensory maps through hierarchical neuronal levels from the peripheral epithelium to the first central site in the hindbrain, the nucleus of the lateral descending trigeminal tract (LTTD). The spatial organization of the anatomical connections was revealed in a novel in vitro whole-brain preparation of the western diamondback rattlesnake (Crotalus atrox) that allowed specific application of multiple neuronal tracers to identified pit-organ-supplying trigeminal nerve branches. After adequate survival times, the entire peripheral and central projections of fibers within the pit membrane and the LTTD became visible. This approach revealed a morphological partition of the pit membrane into three well-defined sensory areas with largely separated innervations by the three main branches. The peripheral segregation of infrared afferents in the sensory epithelium was matched by a differential termination of the afferents within different areas of the LTTD, with little overlap. This result demonstrates a topographic organizational principle of the snake infrared system that is implemented by maintaining spatially aligned representations of environmental infrared cues on the sensory epithelium through specific neuronal projections at the level of the first central processing stage, comparable to the visual system.

The anuran torus semicircularis consists of several subnuclei that are part of the ascending auditory pathway as well as audiomotor interface structures. Additionally, recent anatomical studies suggest that the midbrain tegmentum is an integral part of the audiomotor network. To describe the chemoarchitecture of these nuclei, taking into account the toral subdivisions, we investigated the distribution of serotonin, leucine-enkephalin, substance P, tyrosine-hydroxylase, dopamine D2-receptor, parvalbumin, aspartate, GABA, and estrogen-binding protein-immunoreactivity in the midbrain of Bombina orientalis, Discoglossus pictus and Xenopus laevis. In the torus semicircularis, the highest density of immunoreactive fibers and terminals for all transmitters was found in the laminar nucleus. Parvalbumin-like immunoreactivity was highest in the principal nucleus, and D2-receptor-like immunoreactivity was uniformly distributed throughout the torus. In the tegmentum, axons and/or dendrites were stained with all antibodies except estrogen-binding protein. Additionally, heavily stained enkephalin and substance P-immunopositive fiber plexus were found in the lateral and dorsal tegmentum. The immunostainings revealed no qualitative differences between the three species. Immunopositive cell bodies were labeled in several brain areas, the connectivity of which with torus and tegmentum is discussed on the background of functional questions. The putative neuromodulatory innervation of both the laminar nucleus of the torus semicircularis and the tegmentum may be the anatomical basis for the influence of the animal's endogenous state on the behavioral reaction to sensory stimuli. These data corroborate earlier anatomical and physiological findings that the neurons of these nuclei are key elements in the audio-motor interface.

The widefield vertical neurons of the lower stratum griseum superficiale (SGS3) and upper stratum opticum (SO) of the superior colliculus provide an extrageniculate route for visual information to reach the pulvinar. Previous physiological studies indicate that SGS3/SO neurons have large receptive fields and respond to small moving stimuli. We sought to better characterize the dendritic morphology of SGS3/SO neurons with intracellular filling in slice preparations of the ground squirrel superior colliculus. We found that dendrites of widefield vertical cells end in monostratified arrays of spiny terminal specializations called "bottlebrush" dendritic endings. Two major subtypes of neurons are described. Type I neurons have somata restricted to the SGS3 and bottlebrush endings in the most superficial sublayer of the SGS. Type II neurons are found at the base of the SGS and in the upper SO, and have bottlebrush endings arrayed within the middle sublayers of the SGS. Bottlebrush endings may sample and integrate laminated afferents to the superior colliculus, and cellular subtypes may underlie multiple information streams within the tectopulvinar pathway. A similar dendritic morphology and projection pattern can be found in cells of the avian optic tectum that project upon the nucleus rotundus, a thalamic nucleus homologous to the mammalian caudal/inferior pulvinar. Because motion processing is a dominant feature of the avian tectorotundal pathway, the current results suggest that both dendritic morphology and motion processing are conserved features of widefield vertical cells in the tectopulvinar pathway of vertebrates.

In this contribution, it is argued that the prevailing view of the sensory system as an intricate, but passive processor of external information does not capture the full complexity of brain performance. Instead, we try to reinforce a notion that sees the brain as a system embedded within the environment and actively exploring it. We will attempt to emphasize the bidirectional interaction between brain and environment at all levels of processing and at different scales of the system description. This modified approach to the understanding of the brain has profound consequences for experimental investigations, starting with the experimental design and extending into data analysis and interpretation.

Acoustic signals are extensively used for guiding various behaviors in frogs such as vocalization and phonotaxis. While numerous studies have investigated the anatomy and physiology of the auditory system, our knowledge of intrinsic properties and connectivity of individual auditory neurons remains poor. Moreover, the neural basis of audiomotor integration still has to be elucidated. We determined basic response patterns, dendritic arborization and axonal projection patterns of auditory midbrain units with intracellular recording and staining techniques in an isolated brain preparation. The subnuclei of the torus semicircularis subserve different tasks. The principal nucleus, the main target of the ascending auditory input, has mostly intrinsic neurons, i.e., their dendrites and axons are restricted to the torus itself. In contrast, neurons of the magnocellular and the laminar nucleus project to various auditory and non-auditory processing centers. The projection targets include thalamus, tegmentum, periaqueductal gray, medulla oblongata, and – in the case of laminar neurons – the spinal cord. Additionally, tegmental cells receive direct auditory input and project to various targets, including the spinal cord. Our data imply that both auditory and premotor functions are implemented in individual toral and tegmental neurons. Their axons constitute parallel descending pathways to several effector systems and might be part of the neural substrate for differential audiomotor integration.

The chicken optic tectum (TeO) and its mammalian counterpart, the superior colliculus, are important sensory integration centers. Multimodal information is represented in a topographic map, which plays a role in spatial attention and orientation movements. The TeO is organised in 15 layers with clear input and output regions, and further interconnected with the isthmic nuclei (NI), which modulate the response in a winner-takes-all fashion. While many studies have analysed tectal cell types and their modulation from the isthmic system physiologically, little is known about local network activity and its modulation in the tectum. We have recently shown with voltage-sensitive dye imaging that electrical stimulation of the retinorecipient layers results in a stereotypic response, which is under inhibitory control [S. Weigel & H. Luksch (2012) J. Neurophysiol., 107, 640–648]. Here, we analysed the contribution of acetylcholine (ACh) and the NI to evoked tectal responses using a pharmacological approach in a midbrain slice preparation. Application of the nicotinic ACh receptor (AChR) antagonist curarine increased the tectal response in amplitude, duration and lateral extent. This effect was similar but less pronounced when γ-aminobutyric acidA receptors were blocked, indicating interaction of inhibitory and cholinergic neurons. The muscarinic AChR antagonist atropine did not change the response pattern. Removal of the NI, which are thought to be the major source of cholinergic input to the TeO, reduced the response only slightly and did not result in a disinhibition. Based on the data presented here and the neuroanatomical literature of the avian TeO, we propose a model of the underlying local circuitry.

In the present study an isolated preparation of the complete anuran central nervous system (CNS) is described which can be kept alive for several days and allows tracing, immunohistochemical and electrophysiological studies. A simple perfusion chamber is being used in which the isolated CNS preparation is superfused with oxygenated Ringer. The use of an isolated CNS has many advantages including: (1) virtually all areas are easily accessible at the same time without having the problem of blood vessels that hinder access; (2) large lesions and massive tracer applications are possible without survival problems of the animal, and tracers will not be translocated by blood circulation; (3) since pulsations caused by the pressure changes of blood circulation do not occur, intracellular recordings are comparatively easy and stable; and (4) this approach offers the possibility of working on the same brain for several days by storing the preparation in a refrigerator overnight at low temperatures, thus allowing extensive utilization of a single preparation and reduction in the number of experimental animals required. Some applications to the anuran auditory system illustrate that the isolated anuran CNS is well-suited for a variety of neuroanatomical and physiological techniques.

Microwire arrays are a powerful tool for the exertion of localized thermal stress on cellular networks. Combining microwire arrays with a set of orthogonal axon-guiding microchannels on-chip allows for the positioning of neurites, as well as control over their polarity. In this paper, we present a new fabrication approach, based on standard clean room fabrication and sacrificial layer etching, for the integration of microwire arrays into neurite guiding structures. The system permits the application of strong temperature gradients, enabling localized thermal stimulation inside microchannels.

SYNOPSIS. Anurans respond to different acoustic signals in a distinct manner. Moreover, acoustic behavior strongly depends on the inner state of the animal and the social context. Neuroanatomical studies as well as extra- and intracellular recordings were carried out to examine the problem of audio-motor interfacing. Acoustic signals are processed in a partly hierarchical and partly distributed manner in the anuran central nervous system. Features are represented in a topographical manner. Auditory maps in the midbrain torus semicircularis are read by a subset of neurons of the laminar and magnocellular subnuclei, which interconnect the auditory pathway and premotor centres. These, in turn, feed into neural networks controlling vocalization, locomotion, or autonomic functions. Our data on the cytoarchitectonic organization and connectivity of the interfacing neurons give rise to a population coding hypothesis which may explain the differential evaluation of acoustic signals. Additionally, immunohistochemical findings reveal strong neuromodulatory and hormonal input, especially onto interfacing neurons, suggesting intense influence on the physiological properties of these cells.

The midbrain is an important processing area for sensory information in vertebrates. The optic tectum and its mammalian counterpart, the superior colliculus, receive multimodal, topographic information and contain a sensory map that plays a role in spatial attention and orientation movements. Many studies have investigated the tectal circuitry by cytochemistry and by characterization of particular cell types. However, only a few studies have investigated network activation throughout the depth of the tectum. Our study provides the first data on spatiotemporal activity profiles in the depth and width of the avian optic tectum. We used an optical imaging approach with voltage-sensitive dyes to investigate population responses at a high temporal and spatial resolution. With the necessary caution due to cell extension across several layers, we can thus link our findings tentatively with the general layout of the avian optic tectum. Single electrical stimuli in the retinorecipient layers 1–4 evoked a complex optical response pattern with two components: a short, strong transient response and a weaker persistent response that lasted several hundred milliseconds. The response started in layer 5 and spread within this layer before it propagated into deeper layers. This is in line with neuroanatomical and earlier physiological data. Analysis of temporal sequence and pharmacological manipulations revealed that these responses were mainly driven by postsynaptic activation. Thus tectal network responses to patterned input can be studied by voltage-sensitive dye imaging.

Topographically organized neurons represent multiple stimuli within complex visual scenes and compete for subsequent processing in higher visual centers. The underlying neural mechanisms of this process have long been elusive. We investigate an experimentally constrained model of a midbrain structure: the optic tectum and the reciprocally connected nucleus isthmi. We show that a recurrent antitopographic inhibition mediates the competitive stimulus selection between distant sensory inputs in this visual pathway. This recurrent antitopographic inhibition is fundamentally different from surround inhibition in that it projects on all locations of its input layer, except to the locus from which it receives input. At a larger scale, the model shows how a focal top-down input from a forebrain region, the arcopallial gaze field, biases the competitive stimulus selection via the combined activation of a local excitation and the recurrent antitopographic inhibition. Our findings reveal circuit mechanisms of competitive stimulus selection and should motivate a search for anatomical implementations of these mechanisms in a range of vertebrate attentional systems.

One strategy in localizing a sound source in the azimuthal plane is the comparison of arrival times of sound stimuli at the two ears. The processing of interaural time differences (ITDs) in the auditory brainstem was suggested by the Jeffress model in 1948. In chicks, binaural neurons in the nucleus laminaris (NL) receive input from both ipsilateral and contralateral nucleus magnocellularis (NM) neurons, with the axons of the latter acting as delay lines. A given neuron in the NL responds maximally to coinciding input from both NM neurons. To achieve maximum resolution of sound localization in the NL, the conduction velocity along these delay lines must be precisely tuned. Here, we examined the development of this velocity between embryonic days (E)12 and E18. Our optical imaging approach visualizes the contralateral delay lines along almost the complete NL of the chicken embryo. Optical imaging with the voltage-sensitive dye RH 795 showed no significant differences in the velocity between E12 and E15, but a significant increase from E15 to E18, at both 21 degrees C and 35 degrees C. Surprisingly, at 21 degrees C the conduction velocity in the dorso-lateral part of the NL was significantly higher compared to the situation in the ventro-medial part. The observed development in contralateral conduction velocity may be due to a developmental increase in myelination of the NM axons. Indeed, antibody staining against myelin-associated glycoprotein (alpha-MAG) showed no myelination of the NM axon branches within the NL at E12 and E15. On the other hand, a clear alpha-MAG immunoreactivity occurred at E18. Our results therefore describe the developmental physiological properties of the delay line in the chicken embryo.

In the auditory system voltage-activated currents mediated by potassium channels Kv1.1 and Kv3.1b and their interaction with sodium inward currents play a crucial role for computational function. However, it is unresolved how these potassium channels are developmentally regulated. We have therefore combined a biochemical investigation of Kv1.1 and Kv3.1b protein expression with electrophysiological recordings of membrane currents to characterize neuronal differentiation in the auditory brain stem of the chick. Differentiation in vitro was compared with cells prepared from corresponding embryonic stages in vivo. Using a computer model based on the empirical data we were then able to predict physiological properties of developing auditory brain stem neurons. In vivo Kv3.1b expression increased strongly between E10 and E14, a time of functional synaptogenesis in the auditory brainstem. We also found this increase of expression in vitro, again coinciding with synaptogenesis in the cultures. Whole-cell patch recordings revealed a corresponding increase of the (Kv3.1-like) high threshold potassium current. In contrast, Kv1.1 protein expression failed to increase in vitro, and changes in (Kv1.1-like) low threshold potassium current with time in culture were not significant. Electrophysiological recordings revealed that sodium inward currents increased with cultivation time. Thus, our data suggest that Kv3.1b expression occurs with the onset of functional synaptogenesis, while a different signal, absent from cultures of dissociated auditory brain stem, is needed for Kv1.1 expression. A biophysical model constructed with parameters from our recordings was used to investigate the functional impact of the currents mediated by these channels. We found that during development both high and low threshold potassium currents need to be increased in a concerted manner with the sodium conductance for the neurons to exhibit fast and phasic action potential firing and a narrow time window of coincidence detection.

Avian nucleus isthmi pars parvocellularis (Ipc) neurons are reciprocally connected with the layer 10 (L10) neurons in the optic tectum and respond with oscillatory bursts to visual stimulation. Our in vitro experiments show that both neuron types respond with regular spiking to somatic current injection and that the feedforward and feedback synaptic connections are excitatory, but of different strength and time course. To elucidate mechanisms of oscillatory bursting in this network of regularly spiking neurons, we investigated an experimentally constrained model of coupled leaky integrate-and-fire neurons with spike-rate adaptation. The model reproduces the observed Ipc oscillatory bursting in response to simulated visual stimulation. A scan through the model parameter volume reveals that Ipc oscillatory burst generation can be caused by strong and brief feedforward synaptic conductance changes. The mechanism is sensitive to the parameter values of spike-rate adaptation. In conclusion, we show that a network of regular-spiking neurons with feedforward excitation and spike-rate adaptation can generate oscillatory bursting in response to a constant input.

The optic tectum plays a central role in the network that interfaces between sensory stimuli and behavioral motor patterns. Situated dorsally in the midbrain, the tectum consists of multiple laminae with specific cell types and connectivity. The tectum receives a strong retinal projection that forms a map of visual space which acts as a master coordinate system for other sensory afferents (auditory, somatosensory, etc.), leading to a multimodal representation of the sensory environment. Although the size of the optic tectum is variable among vertebrate species, the sensorimotor functions of the tectum appear to be relevant for all vertebrate classes.

We consider the effect of distributed delays in neural feedback systems. The avian optic tectum is reciprocally connected with the nucleus isthmi. Extracellular stimulation combined with intracellular recordings reveal a range of signal delays from 4 to 9 ms between isthmotectal elements. This observation together with prior mathematical analysis concerning the influence of a delay distribution on system dynamics raises the question whether a broad delay distribution can impact the dynamics of neural feedback loops. For a system of reciprocally connected model neurons, we found that distributed delays enhance system stability in the following sense. With increased distribution of delays, the system converges faster to a fixed point and converges slower toward a limit cycle. Further, the introduction of distributed delays leads to an increased range of the average delay value for which the system's equilibrium point is stable. The enhancement of stability with increasing delay distribution is caused by the introduction of smaller delays rather than the distribution per se., 7 pages, 4 figures

The magnetic compass of migratory birds has been suggested to be light-dependent. Retinal cryptochrome-expressing neurons and a forebrain region, "Cluster N", show high neuronal activity when night-migratory songbirds perform magnetic compass orientation. By combining neuronal tracing with behavioral experiments leading to sensory-driven gene expression of the neuronal activity marker ZENK during magnetic compass orientation, we demonstrate a functional neuronal connection between the retinal neurons and Cluster N via the visual thalamus. Thus, the two areas of the central nervous system being most active during magnetic compass orientation are part of an ascending visual processing stream, the thalamofugal pathway. Furthermore, Cluster N seems to be a specialized part of the visual wulst. These findings strongly support the hypothesis that migratory birds use their visual system to perceive the reference compass direction of the geomagnetic field and that migratory birds "see" the reference compass direction provided by the geomagnetic field.

The vertebrate nervous system has been shown to contain high concentrations of intracellular calcium-binding proteins, each of them with a restricted expression pattern in specific brain regions and specific neuronal subpopulations. Using immunohistochemical staining techniques, we analyzed the expression pattern of calbindin, calretinin and parvalbumin in visual brain areas of a songbird species, the zebra finch (Taeniopyga guttata). Here we show that the analyzed proteins are expressed in a complementary fashion within different brain substructures generally corresponding to functional subpathways of the avian visual system. In detail, calbindin is expressed in the brain structures that belong to the thalamofugal pathway, whereas parvalbumin-positive neurons are found in the brain structures that are part of the tectofugal visual pathway. Originally, the expression of calcium-binding proteins has been associated with specific morphological or neurochemical criteria of neurons. Our results suggest that their expression pattern also indicates a functional segregation of brain substructures linked to vision in the zebra finch brain. As the selective labeling of functional streams has also been shown for the visual system in mammalian species, function-selective expression of calcium-binding proteins might be a general feature of vertebrates.

Vitamin A is a major morphogen for the visual system. Most of its effects are mediated by retinoic acid (RA), whose developmental functions include pattern formation, neuronal differentiation and possibly axonal guidance. Although RA has been suggested to regulate development of the retina and its central projection, little is known about the distribution of retinoid receptors and binding proteins in the optic tectum, which in birds is the direct target of most retinofugal axons. We investigated the spatial and temporal distribution of the cellular retinoic acid binding protein-I (CRABP-I) in the chick midbrain. While the precise role of CRABP-I is still unknown, this is an intracellular transport protein for RA, which tends to be expressed in cells that are responsive to retinoic acid. Our data show immunoreactivity of CRABP-I in the tectal anlage at E2.5 and during the entire period of embryonic development. It was found in differentiating neurons of the generative zone, in migrating cells of the prospective stratum griseum et fibrosum superficiale and in mature neurons in this layer. In addition, we detected retinoid receptors RARalpha, RARbeta, RXRalpha, RXRbeta and RXRgamma in the developing tectum. Cell culture experiments demonstrate CRABP-I expression in a subpopulation of tectal neurons as they differentiate in vitro. These results are consistent with a regulatory role of RA in tectal neurogenesis and physiology.

The cholinergic division of the avian nucleus isthmi, the homolog of the mammalian nucleus parabigeminalis, is composed of the pars parvocellularis (Ipc) and pars semilunaris (SLu). Ipc and SLu were studied with in vivo and in vitro tracing and intracellular filling methods. 1) Both nuclei have reciprocal homotopic connections with the ipsilateral optic tectum. The SLu connection is more diffuse than that of Ipc. 2) Tectal inputs to Ipc and SLu are Brn3a-immunoreactive neurons in the inner sublayer of layer 10. Tectal neurons projecting on Ipc possess "shepherd's crook" axons and radial dendritic fields in layers 2-13. 3) Neurons in the mid-portion of Ipc possess a columnar spiny dendritic field. SLu neurons have a large, nonoriented spiny dendritic field. 4) Ipc terminals form a cylindrical brush-like arborization (35-50 microm wide) in layers 2-10, with extremely dense boutons in layers 3-6, and a diffuse arborization in layers 11-13. SLu neurons terminate in a wider column (120-180 microm wide) lacking the dust-like boutonal features of Ipc and extend in layers 4c-13 with dense arborizations in layers 4c, 6, and 9-13. 5) Ipc and SLu contain specialized fast potassium ion channels. We propose that dense arborizations of Ipc axons may be directed to the distal dendritic bottlebrushes of motion detecting tectal ganglion cells (TGCs). They may provide synchronous activation of a group of adjacent bottlebrushes of different TGCs of the same type via their intralaminar processes, and cross channel activation of different types of TGCs within the same column of visual space.

Der Bereich der Neurobionik kann eingeteilt werden in die Neuroprothetik, die Biohybridelemente und den Rechenvorschriften, mit denen die Natur Informationsverarbeitung betreibt. In allen diesen Bereichen hat es in den letzten Jahren einen enormer Fortschritt gegeben, der hier anhand von verschiedenen Beispielen skizziert wird. Durch das zunehmende Verstandnis der biologischen Elemente sowie den technische Fortschritt im Bereich Miniaturisierung und Materialien ist zu erwarten, dass die Neurobionik in den kommenden Jahren eine Fuhrungsrolle in der bionischen Forschung beanspruchen wird.

Dissociated primary cell cultures of the nervous system are usually composed of many different cell types, which makes it difficult to investigate a specific cell type and to describe its development in vitro without direct or indirect influence of other cell types. Although various methods have been published to specifically separate either neurons or glial cells, there is still a need for simple protocols to isolate distinct neuronal subpopulations. Here we describe a method to purify specific neuronal subtypes from the chick embryonic midbrain. Embryonic (E10) optic tecta were dissociated and a cell suspension was produced. Cells were separated by magnetic cell sorting (MACS) based on their specific expression of somatic N-cadherin. After cultivation on poly-D-lysine coated dishes in serum-free culture medium supplemented with B27, cells were fixed and analyzed with immuncytochemistry. Enriched primary cultures contained about 70% of N-cadherin positive cells compared to 46% before sorting. 7 days after cultivation, N-cadherin expression and its co-localization with synapses was demonstrated.

The dorsal striatopallidal system of tetrapods consists of the dorsal striatum (caudate-putamen in mammals) and the dorsal pallidum. Although the existence of striatal and pallidal structures has been well documented in anuran amphibians, the exact boundaries of these structures have so far been a matter of debate. To delineate precisely the dorsal striatopallidal system of anurans, we used quantitative analysis of leucine-enkephalin immunohistochemistry (in Bombina orientalis, Discoglossus pictus, Xenopus laevis, and Hyla versicolor), retrograde neurobiotin tracing studies (injections in the central and ventromedial thalamic nuclei in H. versicolor), and double-labeling tracing studies (injections in the lateral forebrain bundle and the caudal striatum in B. orientalis). Immunohistochemistry revealed that enkephalin-positive neurons are located mainly in the rostral and intermediate striatum. Neurobiotin tracing studies demonstrated that neurons projecting to the central and ventromedial thalamic nuclei are found in the intermediate and caudal striatum. Double-labeling studies revealed that the population of neurons in the rostral and intermediate striatum innervating the caudal striatum is separated from neurons projecting into the lateral forebrain bundle. Neurons that project to both the caudal striatum and the lateral forebrain bundle are found only in the dorsal part of the intermediate striatum. Taken together, our results suggest that the rostral striatum of anurans is homologous to the striatum proper of mammals, whereas the caudal striatum is comparable to the dorsal pallidum. The intermediate striatum represents a transition area between the two structures. J. Comp. Neurol. 468:299–310, 2004. © 2003 Wiley-Liss, Inc.

We studied the development of the projection from the external nucleus of the inferior colliculus (ICX) to the optic tectum (OT) in the barn owl. The projection was labeled by tracer application in vitro to either the OT or the ICX, or by staining ICX cells intracellularly with biocytin. The axons of ICX neurons bifurcated into an ascending branch that projected toward the OT and a descending branch that coursed caudally to an unknown target in the brainstem. Axons of the ICX were observed to grow into the OT from embryonic day 16 (E16) on. From E22 on, side branches of the axonal projections could be found within the OT. At the day of hatching (E32), the projection displayed a dorsoventral topography comparable to the adult owl; however, atopically projecting cells remained. The complexity of the axonal arborization in the adult barn owl was found to be slightly increased compared with the hatchling. The terminal area of individual ICX cells in the OT of the adult barn owl was still broad, a finding that had not been expected from the sharply defined physiological response properties of the bimodal neurons in the space map of the OT. However, the width of the termination zone was in accordance with the large dendritic tree of the adult ICX cells, because both spanned comparable angles in their respective maps. Our data suggest that a coarse projection from the ICX to the OT can develop without coherent sensory input and may, therefore, be innately determined.

Pit vipers (Crotalinae) have a specific sensory system that detects infrared radiation with bilateral pit organs in the upper jaw. Each pit organ consists of a thin membrane, innervated by three trigeminal nerve branches that project to a specific nucleus in the dorsal hindbrain. The known topographic organization of infrared signals in the optic tectum prompted us to test the implementation of spatiotopically aligned sensory maps through hierarchical neuronal levels from the peripheral epithelium to the first central site in the hindbrain, the nucleus of the lateral descending trigeminal tract (LTTD). The spatial organization of the anatomical connections was revealed in a novel in vitro whole-brain preparation of the western diamondback rattlesnake (Crotalus atrox) that allowed specific application of multiple neuronal tracers to identified pit-organ-supplying trigeminal nerve branches. After adequate survival times, the entire peripheral and central projections of fibers within the pit membrane and the LTTD became visible. This approach revealed a morphological partition of the pit membrane into three well-defined sensory areas with largely separated innervations by the three main branches. The peripheral segregation of infrared afferents in the sensory epithelium was matched by a differential termination of the afferents within different areas of the LTTD, with little overlap. This result demonstrates a topographic organizational principle of the snake infrared system that is implemented by maintaining spatially aligned representations of environmental infrared cues on the sensory epithelium through specific neuronal projections at the level of the first central processing stage, comparable to the visual system.

At least three identified cell types in the stratum griseum centrale (SGC) of the chick optic tectum mediate separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus. Two of these, SGC type I and type II, constitute the major direct inputs to rotundal subdivisions that process various aspects of visual information, e.g., motion and luminance changes. Here, we examined the responses of these cell types to somatic current injection and synaptic input. We used a brain slice preparation of the chick tectum and applied whole-cell patch recordings, restricted electrical stimulation of dendritic endings, and subsequent labeling with biocytin. Type I neurons responded with regular sequences of bursts (“chattering”) to depolarizing current injection. Electrical stimulation of retinal afferents evoked a sharp-onset EPSP/burst response that was blocked with CNQX. The sharp-onset EPSP/burst response to synaptic stimulation persisted when the soma was hyperpolarized, thus suggesting the presence of dendritic spike generation. In contrast, the type II neurons responded to depolarizing current injection solely with an irregular sequence of individual spikes. Electrical stimulation of retinal afferents led to slow and long-lasting EPSPs that gave rise to one or several action potentials. In conclusion, the morphological distinct SGC type I and II neurons also have different response properties to retinal inputs. This difference is likely to have functional significance for the differential processing of visual information in the separate pathways from the retina to different subdivisions of the thalamic nucleus rotundus.

The dendritic development of a well-characterized retino-recipient neuronal type in the chicken optic tectum has been traced with intracellular labeling. Normal dendritic development can be divided into three phases: extension, differentiation and pruning. During the first phase, cells extend their dendrites, generate large dendritic fields and position their distal endings in a certain retino-recipient tectal layer. In the second phase, these dendritic endings arborize into characteristic bottlebrush-like structures, while the overall morphology of the neurons remains unaltered. After hatching, the number and width of the bottlebrush endings are reduced. The findings are discussed with respect to the innervation of the optic tectum by retinal afferents and possible guidance mechanisms for synapse formation in this system.

Many organisms use multimodal maps to generate coherent neuronal representations that allow adequate responses to stimuli that excite several sensory modalities. During ontogeny of these maps, one modality typically acts as the dominant system the other modalities are aligned to. A well studied model for the alignment of sensory maps is the calibration of the auditory space map by the visual system in the optic tectum of the barn owl. However, a projection from the optic tectum to the site of plasticity in the auditory pathway that could deliver an instructive signal has not been found so far. We have analyzed the development of the connectivity between the bimodal (visual and auditory) map of space in the barn owl’s optic tectum and the auditory space map in the inferior colliculus with tracing methods and intracellular fills. Neurons in the tectal stratum griseum centrale were found to be suited to deliver an alignment signal from the visual midbrain to the auditory pathway. These neurons are presumably part of the efferent tectal projection pathway that mediates head saccades. The implications of a sensory alignment signal possibly being delivered by a (pre)motor command pathway are discussed.

In avian and mammalian brains, visual information from the retina is conveyed to the telencephalon via two separate pathways: the thalamofugal and the tectofugal pathways. Recently, Karten et al. ([1997] J. Comp. Neurol. 387:449–465) examined a portion of the tectofugal pathway, the projection from the optic tectum to the nucleus rotundus thalami, in pigeons. They defined two distinct subpopulations of tectal neurons projecting from the stratum griseum centrale (SGC; tectal layer 13) to specific divisions of the rotundus. The goal of this study in chick was to verify the existence of the type I and type II SGC neurons, as defined by Karten et al., and then examine in greater detail the connectivity and morphology of these SGC neurons. Furthermore, our results suggest how the unique morphological characteristics of SGC neurons contribute to the large receptive fields (20–50°) found in physiological recordings and the SGC neuronal response to extremely small (ca. 0.05°), fast-moving (100°/second) stimuli. By injecting retrograde tracer into various divisions of the chick rotundus, we verified that, indeed, the chick did possess type I and type II SGC neurons, as well as a “new” type of SGC neuron, type III, that is not found in the pigeon. We then used intracellular cell-filling techniques to define further these three types of SGC neurons. Our examination revealed the following: Type I SGC neurons had large, circular dendritic fields (average diameter, 1,725 μm) composed of smooth dendrites and ending in spine-rich, bottlebrush endings located in retinorecipient tectal layer 5b; type II SGC neurons had elliptical dendritic fields (average 1,447 μm) and dendritic endings located never more superficially than tectal layer 8; and type III SGC neurons had large dendritic fields (average 1,800 μm) of unknown shape and bottlebrush dendritic endings located in retinorecipient tectal layer 4. We suggest that the neuronal features of the SGC neurons (i.e., bottlebrush dendritic endings and large dendritic fields) are key morphological characteristics for the detection of motion within the tectofugal pathway. Furthermore, because neurons with similar morphology have also been found in the tecta of both mammals and reptiles, we suggest that these neuronal features are fundamental components of a phylogenetically conserved system used for the “extrastriate” detection of motion in vertebrates. J. Comp. Neurol. 396:399–414, 1998. © 1998 Wiley-Liss, Inc.

The amphibian optic tectum and pretectum have been analyzed in detail anatomically and physiologically, and a specific model for tecto-pretectal interaction in the context of the visual guidance of behavior has been proposed. However, anatomical evidence for this model, particularly the precise pattern of pretecto-tectal connectivity, is lacking. Therefore, we stained pretectal neurons intracellularly in an in-vitro preparation of the salamanders Plethodon jordani and Hydromantes genei. Our results demonstrate that the projections of neurons of the nucleus praetectalis profundus are divergent and widespread. Individual neurons may project divergently to telencephalic (ipsilateral amygdala and striatum), diencephalic (ipsi-and contralateral thalamus, contralateral pretectum), and mesencephalic (ipsi- and contralateral tectum and tegmentum) centers, and to the ipsi- and contralateral medulla oblongata and rostral spinal cord. The projection of pretectal cells to the optic tectum is bilateral; axonal structures do not show discernible patterns and are present in all layers of the superficial white matter. A classification of pretectal neurons on the basis of axonal termination pattern or dendritic arborization has not been possible. Our results do not support the hypothesis that a distinct class of pretectal neurons projects to a particular subset of tectal cells. Rather, the pretectum appears to influence the tectum indirectly, acting either on retinal afferents or modulating inhibitory interneurons.

Interactions between pretectum and optic tectum of salamanders were analyzed by recording evoked potentials (EPs) in the optic tectum in vitro in response to stimulation of the contralateral optic nerve. Neither lesioning of the pretectum nor ablation of the medulla oblongata including the nucleus isthmi altered the shape of tectal EPs, suggesting that the tectal EP in this preparation reflects activation of tectal circuitry by retinal afferents without a major contribution from non-retinal afferents. To analyze the effect of stimulation of the pretectum on the tectal EP, we stimulated the pretectal area pharmacologically. Amplitudes of tectal EPs decreased rapidly after stimulation of the pretectum and recovered within minutes (glutamate) or hours (kainic acid). The pretectal influence on the tectal EP might act presynaptically on retinal afferents or by modulating the response of inhibitory interneurons.

To analyse cellular computation in the vertebrate brain, a thorough knowledge of the underlying anatomy, physiology and connectivity of the neuronal substrate is essential. This review compiles data on one of the best known structures of the vertebrate brain, the optic tectum of birds. The functions of this structure are multifold, but can be attributed largely to orientation and the basic analysis of sensory data in a spatial context. In the tectum, a wealth of data on physiology and anatomy has been gathered over more than a century and provides an excellent background for computational studies. The analysis of the optic tectum is facilitated by several principles of organisation, including the retinotopic input and the highly laminated layout with separated input and output layers. Moreover, the molecular mechanisms guiding the development and connectivity have been analysed in detail. As the avian tectum and the mammalian superior colliculus are partly homologous, the cellular mechanisms unraveled in the tectum can also be transferred to the colliculus and thus contribute to the understanding of the vertebrate visual system in general.