Visual-field-specific heterogeneity within the tecto-rotundal projection of the pigeon

Seeing the Forest for the Trees, and the Ground Below My Beak: Global and Local Processing in the Pigeon's Visual System

Non-human animals tend to solve behavioral tasks using local information. Pigeons are particularly biased toward using the local features of stimuli to guide behavior in small-scale environments. When behavioral tasks are performed in large-scale environments, pigeons are much better global processors of information. The local and global strategies are mediated by two different fovea in the pigeon retina that are associated with the tectofugal and thalamofugal pathways. We discuss the neural mechanisms of pigeons' bias for local information within the tectofugal pathway, which terminates at an intermediate stage of extracting shape complexity. We also review the evidence suggesting that the thalamofugal pathway participates in global processing in pigeons and is primarily engaged in constructing a spatial representation of the environment in conjunction with the hippocampus.

Neurons in the pigeon visual network discriminate between faces, scrambled faces, and sine grating images

Discriminating between object categories (e.g., conspecifics, food, potential predators) is a critical function of the primate and bird visual systems. We examined whether a similar hierarchical organization in the ventral stream that operates for processing faces in monkeys also exists in the avian visual system. We performed electrophysiological recordings from the pigeon Wulst of the thalamofugal pathway, in addition to the entopallium (ENTO) and mesopallium ventrolaterale (MVL) of the tectofugal pathway, while pigeons viewed images of faces, scrambled controls, and sine gratings. A greater proportion of MVL neurons fired to the stimuli, and linear discriminant analysis revealed that the population response of MVL neurons distinguished between the stimuli with greater capacity than ENTO and Wulst neurons. While MVL neurons displayed the greatest response selectivity, in contrast to the primate system no neurons were strongly face-selective and some responded best to the scrambled images. These findings suggest that MVL is primarily involved in processing the local features of images, much like the early visual cortex.

The expression of DARPP-32 in adult male zebra finches (Taenopygia guttata)

Although the catecholaminergic circuitry in the zebra finch brain has been well studied, there is little information regarding the postsynaptic targets of dopamine. To answer this question, we looked at overall patterns of immunoreactivity for DARPP-32 (a dopamine and cAMP-regulated phosphoprotein, present mostly in dopaminoceptive neurons) in adult male zebra finches. Our results demonstrated that as in mammals and other avian species, DARPP-32 expression was highest in both medial and lateral striatum. Interestingly, a specific pattern of immunoreactivity was observed in the song control system, with 'core' song control regions, that is, LMANcore (lateral magnocellular nucleus of the anterior nidopallium), RA (nucleus robustus arcopallialis) and HVC being less immunoreactive for DARPP-32 than 'shell' areas such as LMANshell, RAcup, AId (intermediate arcopallium) and HVCshelf. Our results suggest that whereas dopamine may modulate the shell pathways at various levels of the AFP, dopaminergic modulation of the core pathway occurs mainly through Area X, a basal ganglia nucleus. Further, secondary sensory cortices including the perientopallial belt, Fields L1 and L3 had higher DARPP-32-immunoreactivity than primary sensory cortical areas such as the pallial basolateral nucleus, entopallium proper and Field L2, corresponding to somatosensory, visual and auditory systems, respectively. We also found DARPP-32-rich axon terminals surrounding dopaminergic neurons in the ventral tegmental area-substantia nigra complex which in turn project to the striatum, suggesting that there may be a reciprocal modulation between these regions. Overall, DARPP-32 expression appears to be higher in areas involved in integrating sensory information, which further supports the role of this protein as a molecular integrator of different signal processing pathways.

The mesencephalic GCt-ICo complex and tonic immobility in pigeons (Columba livia): a c-Fos study

Tonic immobility (TI) is a response to a predator attack, or other inescapable danger, characterized by immobility, analgesia and unresponsiveness to external stimuli. In mammals, the periaqueductal gray (PAG) and deep tectal regions control the expression of TI as well as other defensive behaviors. In birds, little is known about the mesencephalic circuitry involved in the control of TI. Here, adult pigeons (both sex, n = 4/group), randomly assigned to non-handled, handled or TI groups, were killed 90 min after manipulations and the brains processed for detection of c-Fos immunoreactive cells (c-Fos-ir, marker for neural activity) in the mesencephalic central gray (GCt) and the adjacent nucleus intercollicularis (ICo). The NADPH-diaphorase staining delineated the boundaries of the sub nuclei in the ICo-GCt complex. Compared to non-handled, TI (but not handling) induced c-Fos-ir in NADPH-diaphorase-rich and -poor regions. After TI, the number of c-Fos-ir increased in the caudal and intermediate areas of the ICo (but not in the GCt), throughout the rostrocaudal axis of the dorsal stratum griseum periventriculare (SGPd) of the optic tectum and in the n. mesencephalicus lateralis pars dorsalis (MLd), which is part of the ascending auditory pathway. These data suggest that inescapable threatening stimuli such as TI may recruit neurons in discrete areas of ICo-GCt complex, deep tectal layer and in ascending auditory circuits that may control the expression of defensive behaviors in pigeons. Additionally, data indicate that the contiguous deep tectal SCPd (but not GCt) in birds may be functionally comparable to the mammalian dorsal PAG.

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

Connectivity and neurochemistry of the commissura anterior of the pigeon (Columba livia)

The anterior commissure (AC) and the much smaller hippocampal commissure constitute the only interhemispheric pathways at the telencephalic level in birds. Since the degeneration study from Zeier and Karten (), no detailed description of the topographic organization of the AC has been performed. This information is not only necessary for a better understanding of interhemispheric transfer in birds, but also for a comparative analysis of the evolution of commissural systems in the vertebrate classes. We therefore examined the fiber connections of the AC by using choleratoxin subunit B (CTB) and biotinylated dextran amine (BDA). Injections into subareas of the arcopallium and posterior amygdala (PoA) demonstrated contralateral projection fields within the anterior arcopallium (AA), intermediate arcopallium (AI), PoA, lateral, caudolateral and central nidopallium, dorsal and ventral mesopallium, and medial striatum (MSt). Interestingly, only arcopallial and amygdaloid projections were reciprocally organized, and all AC projections originated within a rather small area of the arcopallium and the PoA. The commissural neurons were not GABA-positive, and thus possibly not of an inhibitory nature. In sum, our neuroanatomical study demonstrates that a small group of arcopallial and amygdaloid neurons constitute a wide range of contralateral projections to sensorimotor and limbic structures. Different from mammals, in birds the neurons that project via the AC constitute mostly heterotopically organized and unidirectional connections. In addition, the great majority of pallial areas do not participate by themselves in interhemispheric exchange in birds. Instead, commissural exchange rests on a rather small arcopallial and amygdaloid cluster of neurons.

Integrating brain, behavior, and phylogeny to understand the evolution of sensory systems in birds

The comparative anatomy of sensory systems has played a major role in developing theories and principles central to evolutionary neuroscience. This includes the central tenet of many comparative studies, the principle of proper mass, which states that the size of a neural structure reflects its processing capacity. The size of structures within the sensory system is not, however, the only salient variable in sensory evolution. Further, the evolution of the brain and behavior are intimately tied to phylogenetic history, requiring studies to integrate neuroanatomy with behavior and phylogeny to gain a more holistic view of brain evolution. Birds have proven to be a useful group for these studies because of widespread interest in their phylogenetic relationships and a wealth of information on the functional organization of most of their sensory pathways. In this review, we examine the principle of proper mass in relation differences in the sensory capabilities among birds. We discuss how neuroanatomy, behavior, and phylogeny can be integrated to understand the evolution of sensory systems in birds providing evidence from visual, auditory, and somatosensory systems. We also consider the concept of a "trade-off," whereby one sensory system (or subpathway within a sensory system), may be expanded in size, at the expense of others, which are reduced in size.

Local cholinergic interneurons modulate GABAergic inhibition in the chicken optic tectum

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 acid(A) 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.

Figure-ground discrimination in the avian brain: the nucleus rotundus and its inhibitory complex

In primates, neurons sensitive to figure-ground status are located in striate cortex (area V1) and extrastriate cortex (area V2). Although much is known about the anatomical structure and connectivity of the avian visual pathway, the functional organization of the avian brain remains largely unexplored. To pinpoint the areas associated with figure-ground segregation in the avian brain, we used a radioactively labeled glucose analog to compare differences in glucose uptake after figure-ground, color, and shape discriminations. We also included a control group that received food on a variable-interval schedule, but was not required to learn a visual discrimination. Although the discrimination task depended on group assignment, the stimulus displays were identical for all three experimental groups, ensuring that all animals were exposed to the same visual input. Our analysis concentrated on the primary thalamic nucleus associated with visual processing, the nucleus rotundus (Rt), and two nuclei providing regulatory feedback, the pretectum (PT) and the nucleus subpretectalis/interstitio-pretecto-subpretectalis complex (SP/IPS). We found that figure-ground discrimination was associated with strong and nonlateralized activity of Rt and SP/IPS, whereas color discrimination produced strong and lateralized activation in Rt alone. Shape discrimination was associated with lower activity of Rt than in the control group. Taken together, our results suggest that figure-ground discrimination is associated with Rt and that SP/IPS may be a main source of inhibitory control. Thus, figure-ground segregation in the avian brain may occur earlier than in the primate brain.

Telencephalic organization of the olfactory system in homing pigeons (Columba livia)

Pigeons use olfactory cues to navigate over unfamiliar areas, and any impairment of the olfactory system generates remarkable reduction of homing performance. Lesion and deprivation studies suggest a critical involvement of the right nostril and thus, the right olfactory bulb (OB) and the left piriform cortex (CPi) for initial orientation. This functional pattern suggests that OB and CPi are asymmetrically connected with a stronger projection from the right OB to the left CPi. However, the structural organization of the olfactory system is not unequivocally clarified yet. Thus, we re-analyzed the system by antero- and retrograde tract tracing with biotinylated dextran amine and choleratoxin subunit B, and we especially evaluated quantitative differences in the number of cells in the OB innervating the left and right CPi. Our anterograde tracing data verified a strong bilateral input to the CPi, and the prepiriform cortex (CPP), as well as small projections to the ipsilateral medial septum and the dorsolateral corticoid area and the nucleus taeniae of the amygdala in both hemispheres. Apart from the bilateral bulbar afferents, CPi in turn receives unequivocal input from the ipsilateral CPP, hyperpallium densocellulare, dorsal arcopallium, and from a cluster of cells located within the frontolateral nidopallium. Thus, an indirect connection between OB and CPi is only mediated by the CPP. For quantitative analysis of bulbar input to the CPi, we counted the number of ipsi- and contralaterally projecting neurons located in the OB after injections into the left or right CPi. Retrogradely labeled cells were found bilaterally in the OB with a higher number of ipsilaterally located cells. The bilaterality index did not differ after left- or right-sided CPi injections indicating that the functional lateralization of the olfactory system is not simply based on differences in the number of projecting axons of the major processing stream.

Ascending and descending mechanisms of visual lateralization in pigeons

Brain asymmetries are a widespread phenomenon among vertebrates and show a common behavioural pattern. The right hemisphere mediates more emotional and instinctive reactions, while the left hemisphere deals with elaborated experience-based behaviours. In order to achieve a lateralized behaviour, each hemisphere needs different information and therefore different representations of the world. However, how these representations are accomplished within the brain is still unknown. Based on the pigeon's visual system, we present experimental evidence that lateralized behaviour is the result of the interaction between the subtelencephalic ascending input directing more bilateral visual information towards the left hemisphere and the asymmetrically organized descending telencephalic influence on the tecto-tectal balance. Both the bilateral representation and the forebrain-modulated information processing might explain the left hemispheric dominance for complex learning and discrimination tasks.

Timing of ascending and descending visual signals predicts the response mode of single cells in the thalamic nucleus rotundus of the pigeon (Columba livia)

Neurons of the pigeon's diencephalic n. rotundus were demonstrated to show visual responses of short and long latency representing ascending signals of the retino-tecto-rotundal system and descending signals from telencephalo-tecto-rotundal fibers. Pigeons thus provide an ideal model to investigate the convergence of ascending and descending visual processing streams at single cell level. Although it is known that rotundal responses of long latency show distinct response characteristics, dependent on the stimulus being presented monocularly or binocularly, the mechanisms underlying these response differences are still unclear. While it is possible that the simultaneity of eye stimulation produces a change of processing, it is also possible that the relative timing and order between ipsilateral and contralateral signals are the decisive variable. To test between both possibilities, we recorded from cells in the pigeon's n. rotundus while providing monocular or binocular visual stimulation and varying the delay and order of eye presentations. We revealed that the precise temporal interaction and order of ascending and descending inputs to the tectum decide about late responses with burst or tonic characteristics. When descending signals reached the tectum before the ascending signals, rotundal cells showed late responses that were characterized by burst activity patterns. When ascending input reached the tectum first, responses with tonic characteristic were observed. These effects might become mediated by intratectal mechanisms, the nucleus ventrolateralis thalami, or the bed nuclei of the tectothalamic tract and might constitute the neural basis of a bihemispheric gating function.

Organization of telencephalotectal projections in pigeons: Impact for lateralized top-down control

Birds display hemispheric specific modes of visual processing with a dominance of the right eye/left hemisphere for detailed visual object analysis. In pigeons, this behavioral lateralization is accompanied by morphological left-right differences in the ascending tectofugal pathway. This system is also asymmetrically modulated by descending telencephalotectal input whereby the left forebrain displays a much more pronounced physiological control over ipsilateral left and contralateral right visual thalamic processes. In the present study we aimed to answer the question if this top-down asymmetry that up to now had been demonstrated in single cell recording studies is due to anatomical asymmetries in the size of the fiber systems descending from the telencephalon to the tectum. We approached this question by means of a quantitative retrograde tracing study. Cholera toxin subunit B (CtB) was injected unilaterally into either the left or right optic tectum of adult pigeons. After immunohistochemical detection of CtB-positive cells, the number of ipsi- and contralaterally projecting neurons was estimated. Retrogradely labeled cells were located within the arcopallium, the hyperpallium apicale (HA) and the temporo-parieto-occipital area (TPO). Descending projections from HA, arcopallium, and TPO were mainly or exclusively ipsilateral with the contralateral projection being extremely small. Moreover, there was no difference between left and right hemispheric projections. These anatomical data sharply contrast with behavioral and electrophysiological ones which reveal an asymmetric and bilateral top down control. Therefore, contralateral and lateralized forebrain influences onto tectofugal processing are possibly not the direct result of asymmetrical descending axon numbers. Those influences emerge by a lateralized intra- and/or interhemispheric integration of ascending and descending input onto the rotundus.

Visual responses and afferent connections of the n. ventrolateralis thalami (VLT) in the pigeon (Columba livia)

The nucleus ventrolateralis thalami (VLT) in pigeons receives direct retinal and forebrain projections and has reciprocal connections with the optic tectum. Although VLT is a component of the avian visual system, no study directly examined its connections or its cellular response characteristics. We, therefore, recorded from single units in the pigeon's VLT while visually stimulating the ipsi- and/or contralateral eye. In addition, tracing experiments were conducted to investigate its afferent connections. Electrophysiologically, we discovered three types of neurons, two of which were probably activated via a top-down telencephalotectal system (latencies > 100 ms). Type I neurons responded to uni- and bilateral and type II neurons exclusively to bilateral stimulation. Type III neurons were probably activated by retinal or retinotectal input (latencies < 27 ms) and responded to contra- and bilateral stimulation. Retrograde tracer injections into the VLT revealed an ipsilateral forebrain input from the visual Wulst, from subregions of the arcopallium, and bilateral afferents from the optic tectum. Most intriguing was the direct connection between the VLTs of both hemispheres. We suggest that the avian VLT is part of a system that integrates visuomotor processes which are controlled by both forebrain hemispheres and that VLT contributes to descending tectomotor mechanisms.

Evidence for a cortical--basal ganglia projection pathway in female zebra finches

The anterior forebrain pathway in songbirds is a specialization of the avian basal ganglia pathway and is prominent in males that sing, but seem to be absent or incomplete in females that do not sing. We studied the connectivity in females in the in vitro slice preparation by applying the tracer Fluoro Ruby, biotinylated dextran amine, and cholera toxin B. We identified (1) retrograde labeled neurons in the lateral magnocellular nucleus of the anterior nidopallium (LMAN) projecting to the medial striatum (MSt), and (2) we identified fibers in the MSt labeled by anterograde transport after tracer injection into LMAN. Our data clearly demonstrate the existence of a cortico-basal ganglia pathway in female birds.

Tectal mosaic: Organization of the descending tectal projections in comparison to the ascending tectofugal pathway in the pigeon

The optic tectum of vertebrates is an essential relay station for visuomotor behavior and is characterized by a set of connections that comprises topographically ordered input from the eyes and an output that reaches premotor hindbrain regions. In the avian tectofugal system, different ascending cell classes have recently been identified based on their dendritic and axonal projection patterns, although comparable information about the descending cells is missing. By means of retrograde tracing, the present study describes the detailed morphology of tectal output neurons that constitute the descending tectobulbar and tectopontine pathways in pigeons. Descending cells were more numerous in the dorsal tectum and differed in terms of 1) their relative amount of ipsi- vs. contralateral projections, 2) the location of the efferent cell bodies within different tectal layers, and 3) their differential access to visual input via dendritic ramifications within the outer retinorecipient laminae. Thus, the descending tectal system is constituted by different cell classes presumably processing diverse aspects of the visual environment in a visual field-dependent manner. We demonstrate, based on a careful morphological analysis and on double-labeling experiments, that the descending pathways are largely separated from the ascending projections even when they arise from the same layers. These data support the concept that the tectum is arranged as a mosaic of multiple cell types with diverse input functions at the same location of the tectal map. Such an arrangement would enable the tectal projections onto diverse areas to be both retinotopically organized and functionally specific.

Afferent and Efferent Connections of the Nucleus Rotundus Demonstrated by WGA-HRP in the Chick

Organization of the fibre connections in the chick nucleus rotundus (Rt) was investigated by an axonal tracing method using wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). After an injection of WGA-HRP into the Rt, labelled neurones were observed in the striatum griseum centrale (SGC) in both sides of the tectum (TO) and in the ipsilateral nucleus subpretectalis/nucleus interstito-pretecto-subpretectalis (SP/IPS). Labelled fibres and terminals were also found in the ipsilateral ectostriatum (Ect). These fibre connections were topographically organized rostrocaudally. In the TO-Rt projection, the rostral and the dorsocaudal parts of the Rt received afferents from the superficial part of the SGC, the middle part of the Rt received afferents from the intermediate part of the SGC, and the ventrocaudal part of the Rt received mainly fibres from the deep part of the SGC. These topographic projections were accompanied by a considerable number of diffuse projections to the thalamic regions surrounding the Rt. In addition, the rostral and middle caudal parts of the Rt received afferents from the lateral and medial parts of the SP/IPS, respectively, and respective parts of the Rt sent efferents to the lateral and medial parts of the Ect.

Internal structure of the nucleus rotundus revealed by mapping cadherin expression in the embryonic chicken visual system

The nucleus rotundus is the largest nucleus of the avian thalamus. It is an important center of visual information processing and conveys information from the optic tectum to the ectostriatum in the telencephalon. The nucleus rotundus is generally believed to contain internal divisions processing information on color, form, motion, and looming of visual objects. The detailed arrangement of these internal divisions is unclear. Here, we map the expression of four classic cadherins (N-cadherin, R-cadherin, cadherin-6B, and cadherin-7), which are markers for specific functional gray matter divisions and their fiber connections in the vertebrate brain. Results show that each cadherin is expressed by one coherent part of the nucleus rotundus that is connected to other brain structures by fiber tracts expressing the same subtype of cadherin. Overall, the expression of the four cadherins encompasses almost the entire nucleus rotundus. The four cadherin-expressing parts show different degrees of overlap. For example, the cadherin-6B part and the cadherin-7 part overlap extensively, whereas the R-cadherin part and the cadherin-6B part show little overlap and are partially complementary. Regions with shallow gradients of cadherin expression alternate with regions that show relatively abrupt changes in cadherin expression. At some points, changes of cadherin expression are also arranged in a pinwheel-like fashion, alternating between clockwise and counterclockwise orientations. In general, these results are reminiscent of the organization of functional modules in the mammalian visual cortex. It is speculated that each domain of cadherin expression corresponds to a functional domain, which processes a specific stimulus feature.

Tracing developing pathways in the brain: a comparison of carbocyanine dyes and cholera toxin b subunit

The present study examined the efficiency of fluorescent carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylinodocarbocyanine perchlorate and cholera toxin B subunit in tracing the crossed tectal projection to the nucleus rotundus of the thalamus (tectorotundal pathways) of paraformaldehyde-fixed and living chick embryos. The tracers were injected into the optic tectum under three experimental conditions (carbocyanine postfix, carbocyanine in vivo, and cholera toxin B subunit in vivo) and the anterograde transport of the nucleus rotundus was monitored and compared. In the carbocyanine postfix method, small crystals of carbocyanine dye were inserted into the tectum of paraformaldehyde-fixed embryos. A 6-month post-insertion period was required to label the crossed tectorotundal pathway. Results showed that tectal neurons did not begin to innervate the ipsilateral nucleus rotundus until embryonic day 9 and the contralateral nucleus rotundus until embryonic day 17. This slow progression of labeling through the crossed tectal projection resulted in significant contrast of the labeling between the ipsilateral and contralateral nuclei rotundus. In the carbocyanine in vivo method, a small volume of carbocyanine dye solution was injected into the tectum of living embryos. A 8- to 12-h survival period was sufficient enough to label the tectorotundal pathway. By embryonic day 8, the labeled axons terminated in the ipsilateral nucleus rotundus and the crossed tectorotundal projection was first detected by embryonic day 10. Similarly, in the cholera toxin B subunit in vivo method, a small volume of cholera toxin B subunit solution was injected into the tectum of living embryos. After a 6- to 10-h survival period, heavily labeled axons were found to innervate bilaterally the nucleus rotundus by embryonic day 8. This appeared to be the earliest schedule for detecting the crossed tectorotundal projection, compared with that of both the postfix and in vivo methods of carbocyanine dye. Based on the differences in the detectability of the crossed tectorotundal projection between the postfix and in vivo methods, the present data suggest that the former method is of limited purpose for labeling tectal collaterals during embryogenesis. Moreover, given the rapid transport rate and absence of photobleaching, which is often seen when using carbocyanine dye, the cholera toxin B subunit in vivo method appears to be the tracer of choice for investigating embryonic pathways.

Spatial organization of the pigeon tectorotundal pathway: an interdigitating topographic arrangement

The retinotectofugal system is the main visual pathway projecting upon the telencephalon in birds and many other nonmammalian vertebrates. The ascending tectal projection arises exclusively from cells located in layer 13 of the optic tectum and is directed bilaterally toward the thalamic nucleus rotundus. Although previous studies provided evidence that different types of tectal layer 13 cells project to different subdivisions in Rt, apparently without maintaining a retinotopic organization, the detailed spatial organization of this projection remains obscure. We reexamined the pigeon tectorotundal projection using conventional tracing techniques plus a new method devised to perform small deep-brain microinjections of crystalline tracers. We found that discrete injections involving restricted zones within one subdivision retrogradely label a small fraction of layer 13 cells that are distributed throughout the layer, covering most of the tectal representation of the contralateral visual field. Double-tracer injections in one subdivision label distinct but intermingled sets of layer 13 neurons. These results, together with the tracing of tectal axonal terminal fields in the rotundus, lead us to propose a novel "interdigitating" topographic arrangement for the tectorotundal projection, in which intermingled sets of layer 13 cells, presumably of the same particular class and distributed in an organized fashion throughout the surface of the tectum, terminate in separate regions within one subdivision. This spatial organization has significant consequences for the understanding of the physiological and functional properties of the tectofugal pathway in birds.

Organization of the ectostriatum based on afferent connections in the zebra finch (Taeniopygia guttata)

In birds with laterally-located eyes, such as zebra finches and pigeons, the tectofugal visual pathway is the most prominent route from the retina to the telencephalon. However, little is known about exactly how the visual information is processed in this pathway, especially at the core region of the ectostriatum (Ec) in the telencephalon. In order to reveal a detailed organization of Ec, we decided to systematically analyze the afferent connections of Ec by injecting small amounts of sensitive tracers (biotinylated dextran amine and cholera toxin subunit B) selectively into different regions of Ec and the thalamic center of the tectofugal pathway (the nucleus rotundus, Rt). The present study revealed a clearer picture of the organization of Ec subdivisions than previously known. The present results showed that the anterior portion of Rt sent a heavy projection to the ventral region of the anterior Ec, whereas the more caudal subdivisions of Rt sent projections to more caudal and dorsal portions in Ec. The results suggest that Ec subdivisions appear to be arranged along an axis 'rotated' in the anterior direction, almost parallel to other major telencephalic laminae. These results may clarify the physiological and chemical heterogeneity of Ec found in the previous studies. The present findings also provide an insight into the possible organization of a visual processing center in a non-mammal.

Electrophysiological and anatomical evidence for a direct projection from the nucleus of the basal optic root to the nucleus rotundus in pigeons

A direct projection of the nucleus of the basal optic root (nBOR) onto the nucleus rotundus (Rt) in the pigeon would link the accessory optic system to the ascending tectofugal pathway and could thus combine self- and object-motion processes. In this study, injections of retrograde tracers into the Rt revealed some cells in central nBOR to project onto the ipsilateral Rt. Contrary, injections into the diencephalic component of the ascending thalamofugal pathway resulted in massive labeling of neurons in dorsal nBOR. Single unit recordings showed that visual nBOR units could be activated by antidromic stimulation through the Rt. Successful collision tests applied to nBOR cells revealed that the connection between nBOR and Rt is direct. These data provide strong evidence for a direct and differential projection of nBOR subcomponents onto the thalamic relays of the two ascending visual pathways.

Structural organization of parallel information processing within the tectofugal visual system of the pigeon

Visual information processing within the ascending tectofugal pathway to the forebrain undergoes essential rearrangements between the mesencephalic tectum opticum and the diencephalic nucleus rotundus of birds. The outer tectal layers constitute a two-dimensional map of the visual surrounding, whereas nucleus rotundus is characterized by functional domains in which different visual features such as movement, color, or luminance are processed in parallel. Morphologic correlates of this reorganization were investigated by means of focal injections of the neuronal tracer choleratoxin subunit B into different regions of the nuclei rotundus and triangularis of the pigeon. Dependent on the thalamic injection site, variations in the retrograde labeling pattern of ascending tectal efferents were observed. All rotundal projecting neurons were located within the deep tectal layer 13. Five different cell populations were distinguished that could be differentiated according to their dendritic ramifications within different retinorecipient laminae and their axons projecting to different subcomponents of the nucleus rotundus. Because retinorecipient tectal layers differ in their input from distinct classes of retinal ganglion cells, each tectorotundal cell type probably processes different aspects of the visual surrounding. Therefore, the differential input/output connections of the five tectorotundal cell groups might constitute the structural basis for spatially segregated parallel information processing of different stimulus aspects within the tectofugal visual system. Because two of five rotundal projecting cell groups additionally exhibited quantitative shifts along the dorsoventral extension of the tectum, data also indicate visual field-dependent alterations in information processing for particular visual features.

Lateralized interhemispheric transfer of color cues: evidence for dynamic coding principles of visual lateralization in pigeons

Visual feature discrimination tasks in pigeons reveal a right eye/left hemisphere dominance at the population level. Anatomical studies and lesion data show this visual lateralization to be related to asymmetries of the tectofugal system, which ascends from the tectum over the n. rotundus to the forebrain. Anatomically, this system is characterized by numerous morphological and connectional asymmetries which result in a bilateral visual representation in the dominant left hemisphere and a mostly contralateral representation in the subdominant right hemisphere. Ontogenetically, visual lateralization starts with an asymmetrical embryonic position within the egg, which leads to asymmetries of light stimulation. Differences in exposure to light stimulation between the eyes result in activity differences between the ascending tectofugal pathways of the left and the right hemisphere, which are transcribed during a critical time span into morphological asymmetries. The asymmetries established after this transient period finally start to determine the lateralized processes of the visual system for the entire life span of the individual. We now can show that these anatomical lateralizations are accompanied by asymmetries of interocular transfer, which enable a faster shift of learned color cues from the dominant right to the left eye than vice versa. In summary, our data provide evidence that cerebral asymmetries are based both on "static" anatomical and on "dynamic" process-dependent principles.

Morphologic fate of diencephalic prosomeres and their subdivisions revealed by mapping cadherin expression

The expression of four cadherins (cadherin-6B, cadherin-7, R-cadherin, and N-cadherin) was mapped in the diencephalon of chicken embryos at 11 days and 15 days of incubation and was compared with Nissl stains and radial glial topology. Results showed that each cadherin is expressed in a restricted manner by a different set of embryonic divisions, brain nuclei, and their subregions. An analysis of the segmental organization based on the prosomeric model indicated that, in the mature diencephalon, each prosomere persists and forms a coherent domain of gray matter extending across the entire transverse dimension of the neural tube, from the ventricular surface to the pial surface. Moreover, the results suggest the presence of a novel set of secondary subdivisions for the dorsal thalamus (dorsal, intermediate, and ventral tiers and anteroventral subregion). They also confirm the presence of secondary subdivisions in the pretectum (commissural, juxtacommissural, and precommissural). At most of the borders between the prosomeres and their secondary subdivisions, changes in radial glial fiber density were observed. The diencephalic brain nuclei that derive from each of the subdivisions were determined. In addition, a number of previously less well-characterized gray matter regions of the diencephalon were defined in more detail based on the mapping of cadherin expression. The results demonstrate in detail how the divisions of the early embryonic diencephalon persist and transform into mature gray matter architecture during brain morphogenesis, and they support the hypothesis that cadherins play a role in this process by providing a framework of potentially adhesive specificities.