Differential sensitivities of the two visual pathways of the chick to labelling by fluorescent retrograde tracers.

This study investigates the neurone structure-specific differences of sensitivities of fluorescent tracers. The tracers were used for retrograde labelling of contralateral projections in the two visual pathways of the chick. Rhodamine B Isothiocyanate (RITC), Fluorogold (FG) and True blue (TB) were injected into either the visual Wulst (thalamofugal pathway) or the nucleus rotundus (Rt; tectofugal pathway) and the retrogradely labelled neurones in the nucleus geniculatus lateralis pars dorsalis (GLd) or the optic tectum, respectively, were counted. Differential retrograde labelling in the two pathways was observed. In the thalamofugal pathway, both the contralateral and ipsilateral GLd cells were labelled by all three tracers (RITC, FG and TB). However, in the tectofugal pathway, whereas RITC labelled both the ipsilateral and contralateral tectal neurones, FG or TB labelled effectively only the ipsilateral tectal neurones. It was clear that FG and TB were taken up by the nerve endings and transported part-way along the axon but failed to be transported to the cell bodies of the contralateral tectal neurones. In addition, red beads and green beads were also injected into Rt and the differential labelling was also observed. Red beads labelled both ipsilateral and contralateral tectal neurones but green beads labelled only the ipsilateral tectal neurones. Since the contralateral tectal projections consist of divergent axon collaterals, the present study suggests that various retrograde tracers are not transported in these axon collaterals to label cell bodies. The contralaterally projecting neurones in the thalamofugal pathway are not axon collaterals and they were labelled by all of the tracers used.     

Structural asymmetry in the thalamofugal visual projections in 2-day-old chick is correlated with a hemispheric difference in synaptic density in the hyperstriatum accessorium.

Differences in visual discrimination ability between the left and right eyes of chicks, which are most prominent in young males, may result from a structural asymmetry in the organization of the visual projections from the thalamus to the visual Wulst. This asymmetry in projections is no longer present by 21 days in males when the contralateral projections from the right thalamus to the left hyperstriatum have developed. Since the asymmetry of the thalamo-hyperstriatal system results in a differential input of fibres to regions of the hyperstriatum which in turn project to the hyperstriatum accessorium (HA), one of the major differences expected within this region would be an asymmetry in the numerical density of synapses (Nv.syn/microns3). When this was examined in the hyperstriatum accessorium of 2-day-old male chicks, the density of synapses in the right HA was found to be significantly higher (22%, P less than 0.05) than in the left HA. The consequences of this asymmetry in synaptic density in the HA could be widespread and influential within the chick visual system.     

Integration of information from both eyes by single neurons of nucleus rotundus, ectostriatum and lateral neostriatum in the zebra finch (Taeniopygia guttata castanotis Gould)

Although the optic nerve in birds crosses completely, visual information from the ipsilateral eye also reaches the ectostriatum, the telencephalic statibon of the tectofugal pathway, by recrossing fibers. These recrossing projections connect the contralateral tectum opticum with the ipsilateral nucleus rotundus, which in turn projects to the ectostriatum. The ectostriatum itself projects to the overlying lateral neostriatum, an area which serves an important role in sexual imprinting. This study shows that contralateral and ipsilateral information converges on single neurons within the nucleus rotundus, the ectostriatal region and the lateral neostriatum. In the three brain areas almost all isolated neurons exhibited responses to contralateral as well as bilateral visual stimuli. The number of neurons responding to ipsilateral stimuli increases from nucleus rotundus to the lateral neostriatum. We did not find any neurons driven exclusively by ipsilateral stimuli. The strength of ipsilateral responses is rather weak within the nucleus rotundus and ectostriatum, but shows a sharp increase in the lateral neostriatum. For most neurons of nucleus rotundus and ectostriatum, an additional ipsilateral stimulus did not significantly affect the response to a contralateral one. In contrast, a strong excitatory effect can be found in the neostriatum. The results are in agreement with previous evoked-potential studies and give new insights on the integration of ipsilateral and contralateral stimuli in zebra finch tectofugal visual pathway.     

Sex difference in the visual projections of young chicks: a quantitative study of the thalamofugal pathway

We investigated quantitatively the thalamo-hyperstriatal visual projections of chickens by injecting the fluorescent dye True blue (TB) in either side of the Wulst on day 2 post hatching. The ratios resulting from the comparison of the number of cells retrogradely labelled in the nuclei dorsolateralis anterior thalami (DLA) contralateral and ipsilateral to the site of injection (C/I ratios) revealed a structural asymmetry in the thalamofugal projections of male chicks (n = 17) but not in those of females (n = 12). In the males, TB injections in the right hyperstriatum resulted in higher C/I ratios (37 +/- 14%) compared to injections in the left hyperstriatum (22 +/- 7%). Thus, the left DLA (fed by the right eye) was found to give rise to a more developed visual projection compared to the right DLA (fed by the left eye). No such difference was found in female chicks (41 +/- 13% and 38 +/- 12%, respectively): each DLA gives rise to similarly developed visual projections. We also report evidence of higher densities of cell bodies labelled in the DLA of females compared to males. These results are discussed in the light of experiments demonstrating sex differences in functional asymmetry in chicks tested with the left or right eye occluded.     

'Natural' and artificial monocular deprivation effects on thalamic soma sizes in pigeons

The dominance for visual pattern analysis of the left hemisphere in normal pigeons and the concomitant morphological asymmetries in the optic tectum can be attributed to a 'natural' prehatch monocular deprivation of the left eye resulting from an asymmetrical embryonic position within the egg. Using control animals and pigeons which were monocularly deprived for 10 days after hatching, the present study could show that the cellular soma sizes of the nucleus rotundus within the tectofugal visual pathway are modified by light experience depending on the timepoint and direction of lateralized stimulation. Although rotundal cell size is thus ontogenetically modified in an activity-dependent manner, a detailed comparison makes it likely that the mechanisms which govern developmental plasticity of visual pathways differ between birds and mammals.     

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.     

A tecto-rotundo-telencephalic pathway in the rattlesnake: evidence for a forebrain representation of the infrared sense

Rattlesnakes possess a sensory system specialized for the detection of infrared (IR) radiation. IR signals ascend as far as the optic tectum, where they generate a spatiotopic map. It is unknown if such signals reach the forebrain, but the existence of prominent tectothalamic pathways in other vertebrates makes this a distinct possibility. In nonmammalian forms, the major target of ascending tectal visual signals is nucleus rotundus, a thalamic nucleus that projects in turn to the subpallial telencephalon. We sought to determine whether a tecto-rotundo-telencephalic system exists in rattlesnakes and, if so, whether it carries IR as well as visual information. We have identified a thalamic nucleus in the rattlesnake Crotalus viridis that matches the n. rotundus of other reptiles in its topographic location, cytoarchitecture, and connections. Using anterograde and retrograde transport of HRP, we have demonstrated a strong ipsilateral and weaker contralateral tectorotundal projection. Tectorotundal cells lay primarily in the deeper tectal layers, which receive input from the IR system, but also in the superficial, visual layers. In n. rotundus, single units recorded extracellularly invariably responded to visual stimuli, but many were also sensitive to unimodal IR stimuli. IR and visual receptive fields were very large and often bilateral. Some rotundal units appeared sensitive to substrate vibration. Most habituated rapidly. Nucleus rotundus was found to project to a sector of the ipsilateral anterior dorsal ventricular ridge (ADVR) of the telencephalon. Single units in this region of the ADVR resembled those in rotundus, responding to visual, IR, and/or vibrational stimuli and possessing large, often bilateral receptive fields. These findings demonstrate the existence of a tecto-rotundo-telencephalic pathway in rattlesnakes and suggest that this system conveys IR as well as visual information to the forebrain. Ascending tectofugal pathways have been implicated in the discrimination of form. Thus, pattern recognition may have to be added to orientation as a proper function of the IR system of pit vipers.     

The architecture of an inhibitory sidepath within the avian tectofugal system

The tectofugal system dominates vision in most avian species. A key component of this pathway is the projection from the optic tectum onto the nucleus rotundus and the nucleus subpretectalis. Since subpretectalis has inhibitory projections onto rotundus, it constitutes a modulatory tectofugal sidepath to the tectorotundal system. We analyzed the connections and the immunocytochemical pattern of the subpretectalis in pigeons and show that it receives afferents from some tectal celltypes and from the nucleus pretectalis. Subpretectalis-neurons project non-topographically onto pretectalis and the rostrolateral rotundus. In addition, our immunocytochemical data make it likely that the cells of the subpretectalis receive glutamatergic and GABAergic input. These data provide evidence that the tectofugal sidepath over the subpretectalis could be involved in two major functions: The first is a modulation of attentional shifts from one eye to the other, while the second is a temporal fine-tuning of rotundal units.     

Tectal efferents in the banded water snake,Natrix sipedon

Visual information reaches the dorsal thalamus by two distinct routes in most reptiles. Retinal efferents terminate directly in the dorsal lateral geniculate nucleus (DLGN). Retinal information is also channeled indirectly through the tectum to nucleus rotundus. Retinal projections to DLGN and tectum are also well esablished in snakes, but the status of the tecto-rotundal link of the indirect visual pathway is uncertain. Thus, tectal efferents were studied with Fink-Heimer methods in banded water snakes (Natrix sipedon). The tectum gives rise to crossed and uncrossed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and osterior commissures. tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mesencephali, lateral habenular nuclei, and posterodorsal nuclei. Ascending projections reach the ventral lateral geniculate and suprapeduncular nuclei. there is a diffuse projection to the central part of the caudal thalamus and a dense, bilaternal projection to the DLGN. These results indicate that the relation of the tectum to the dorsal thalamus is different in snakes than in other reptiles. Nucleus rotundus is either absent or poorly differentiated and there is a strong convergence of the direct and indirect visual pathways at DLGN.     

Mapping the avian visual tectofugal pathway using 3D reconstruction

Image processing in amniotes is usually accomplished by the thalamofugal and/or tectofugal visual systems. In laterally eyed birds, the tectofugal system dominates with functions such as color and motion processing, spatial orientation, stimulus identification, and localization. This makes it a critical system for complex avian behavior. Here, the brains of chicks, Gallus gallus, were used to produce serial brain sections in either coronal, sagittal, or horizontal planes and stained with either Nissl and Gallyas silver myelin or Luxol fast blue stain and cresyl echt violet (CEV). The emerging techniques of diffusible iodine-based contrast-enhanced computed tomography (diceCT) coupled with serial histochemistry in three planes were used to generate a comprehensive three-dimensional (3D) model of the avian tectofugal visual system. This enabled the 3D reconstruction of tectofugal circuits, including the three primary neuronal projections. Specifically, major components of the system included four regions of the retina, layers of the optic tectum, subdivisions of the nucleus rotundus in the thalamus, the entopallium in the forebrain, and supplementary components connecting into or out of this major avian visual sensory system. The resulting 3D model enabled a better understanding of the structural components and connectivity of this complex system by providing a complete spatial organization that occupied several distinct brain regions. We demonstrate how pairing diceCT with traditional histochemistry is an effective means to improve the understanding of, and thereby should generate insights into, anatomical and functional properties of complicated neural pathways, and we recommend this approach to clarify enigmatic properties of these pathways.     

Extratelencephalic projections of the avian visual Wulst. A quantitative autoradiographic study in the pigeon Columbia livia

The efferent projections of the pigeon visual Wulst upon the diencephalon and mesencephalon were investigated using the autoradiographic technique following the combined injection of [3H] proline and [3H] leucine into the rostral hyperstriatum accessorium. Repeated measures of silver grain densities were performed bilaterally in different brain structures using a computer-assisted system of image analysis. The density values were compared (Mann-Whitney U-Test) with those recorded in three homolateral control structures (tractus opticus, n. rotundus, n. pretectalis principalis) and in corresponding contralateral areas and nuclei. The data showed ipsilateral projections from the visual Wulst and via the tractus septomesencephalicus upon the dorsal thalamus (n.: dorsolateralis anterior superficialis parvocellularis), ventral thalamus (n.: intercalatus, ventrolateralis, geniculatus lateralis pars ventralis--GLv), pretectum (n.: superficialis synencephali, geniculatus pretectalis, griseus tectalis, pretectalis: diffusus, pars lateralis and pars medialis, area pretectalis) as well as to the nucleus of the basal optic root, n. spiriformis medialis and optic tectum (layer 2-4, 6, 7, 12 and 13). Crossed projections were observed to pass through the supraoptic decussation and the posterior commissure, however only the contralateral n. GLv was found to be significantly labeled. Interspecies variations in the organization of descending visual Wulst projections, related to the terminal distribution and relative size of the crossed components may be linked to differences in the degree of overlap of the binocular fields. Correspondingly, this may reflect the degree of bilateralization upon the Wulst of direct input from the visual thalamus.     

Topographic arrangement of the rotundo‐entopallial projection in the pigeon (Columba livia)

The tectofugal pathway (retina – optic tectum – nucleus rotundus – entopallium) is a prominent route mediating visual discrimination in diurnal birds. Several lines of evidence have shown that at the tecto‐rotundal stage this pathway is composed of multiple parallel channels. Anatomical studies show that the nucleus rotundus is composed of at least four subdivisions, according to differences in cytoarchitectonic, histochemical, and hodological properties. Each of these subdivisions is in receipt of a highly convergent, nontopographic tectal projection, originating from a distinct subset of tecto‐rotundal neurons. Physiological studies show that neurons of different subdivisions respond specifically to different visual dimensions, such as color, luminance, two‐dimensional motion, and in‐depth motion. At present it is less clear whether or to what extent this channel segregation is preserved at the telencephalic stage of the tectofugal pathway. The entopallium shows no obvious subdivisions or laminations. Nevertheless, tract‐tracing experiments show that separate portions of the entopallium receive efferent projections from different rotundal subdivisions, in a way that maintains the rostrocaudal order of these subdivisions. In the present study we investigate in detail the topography of the rotundo‐entopallial projection by means of anterograde and retrograde neuronal tracers. Our results confirm the zonal topography proposed by previous studies and indicate that each zone in the entopallium receives a direct and topographically organized projection from its corresponding rotundal subdivision. These results suggest that the spatial arrangement of the different rotundal functional modules is preserved at the entopallial level. J. Comp. Neurol. 518:4342–4361, 2010. © 2010 Wiley‐Liss, Inc.     

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. III. The tectorotundal projection

Nucleus rotundus is the primary thalamic recipient of projections from the optic tectum in pond turtles. Although the projection of the retina to the optic tectum is known to be topographically organized, earlier studies suggest that the tectorotundal projection is not topographically organized. Three types of analyses are used in this paper to characterize the organization of the projection of the optic tectum to nucleus rotundus. First, large iontophoretic injections of horseradish peroxidase into the optic tectum anterogradely fill axons with reaction product after the use of a cobalt-enhanced diaminobenzidine procedure. These preparations show that shafts of axons in the tectothalamic tract give rise to thinner, primary collaterals that enter nucleus rotundus from its caudolateral aspect and form sparsely branching arbors within the nucleus. Very thin secondary collaterals branch from these collateral bear terminal collaterals with frequent varicosities. Although the total size of such arbors is unknown, the evidence suggests that each arbor is large in relation to the size of nucleus rotundus. Thus, injection sites restricted to central tectum label axons throughout nucleus rotundus. Second, subtotal lesions of the tectum produce degeneration throughout nucleus rotundus in silver degeneration preparations. Finally, analysis of electron microscopic degeneration material indicates that tectal boutons are distributed along the full lengths of the dendrites of rotundal neurons, but not on their somata. These boutons form asymmetric synaptic junctions and contain round synaptic vesicles. In view of the relatively large size of the dendritic fields of rotundal neurons, these data suggest that the tectorotundal projection is both strongly convergent on individual neurons and strongly divergent from single tectorotundal axons. This type of organization is consistent with physiological evidence that rotundal neurons have receptive fields that cover at least one-half of the contralateral visual field and often include the entire hemifield. It seems unlikely that nucleus rotundus can be involved in neuronal transactions that preserve detailed spatial information, but it may be involved in processing information on other visual parameters such as stimulus velocity or color.     

Changes in metabolic activity in the hyperstriatum of the chick before and after hatching

Changes in metabolic activity in the hyperstriatal regions of the chick forebrain have been assessed just prior to and after hatching using ^14C 2-deoxyglucose (2-DG) autoradiography. Embryos were injected on day E19, followed by either exposure to light for 30 min or being held in darkness. Other embryos were injected on day E20, after pipping of the egg shell had occurred, and chicks were injected on day 1 (D1) after hatching, followed by light exposure. In the E19 groups metabolic activity in visual regions of the hyperstriatum accessorium (HA) was significantly higher than that in the hyperstriatum dorsale (HD), the region which receives the thalamofugal visual projections. The result was the same in both the light and dark exposed embryos, indicating that the high level of activity in HA on day E19 is not visually driven and that HA may be processing inputs from other sensory modalities. At stage E20 the activities of HA and HD did not differ and by day 1 post-hatching HD activity exceeded that of HA. Activity in HA fell between E19 and E20, while in HD activity rose between E20 and D1. The developmental sequence of metabolic activity levels in the intermediate medial hyperstriatum ventrale (IMHV), a region involved with imprinting memory formation, was higher on E19 and D1 than on E20. E20 is thus a quiescent period of neural activity in the hyperstriatum prior to hatching. Although a small number of the embryos showed distinct hemispheric asymmetries in metabolic activity, overall there was no significant asymmetry in the embryo groups. The implications of these results for imprinting and early perceptual processing are discussed: it appears that HA activity may be inhibited or limited during the sensitive period for visual imprinting, thereby temporarily diminishing the importance of the thalamofugal visual pathway relative to the tectofugal pathway in the imprinting process.     

A second ascending visual pathway from the optic tectum to the telencephalon in the pigeon (Columba livia)

Previous studies in the pigeon (Karten and Revzin: Brain Res. 2:368-377, '66; Karten and Hodos: J. Comp. Neurol. 140:35-52, '70) have described an ascending tectofugal visual pathway from the optic tectum to the ectostriatum by way of the nucleus rotundus of the thalamus. This present study used anterograde autoradiographic and retrograde horseradish peroxidase pathway-tracing techniques to investigate another ascending tectofugal pathway in the pigeon. Injections of 3H-proline/leucine confirmed a previous report that the optic tectum projects to the nucleus dorsolateralis posterior of the thalamus (DLP). This projection is predominantly ipsilateral and is confined to a large-celled caudal region of the nucleus (DLPc); the rostral region of the nucleus (DLPr) is not tectorecipient. Injections of horseradish peroxidase in DLPc labeled cells predominantly ipsilaterally in layers 8-15 of the optic tectum. Injections of 3H-proline/leucine placed in the DLPc labeled a discrete region of the ipsilateral telencephalon. Similar injections of DLPr labeled a contiguous, but more rostral, region of the neostriatum intermedium. Nissl- and silver-stained material indicated that the region in which DLP terminates is cytoarchitecturally distinct from ventromedial ectostriatal core and belt. Injections of horseradish peroxidase at various locations in the neostriatal DLP terminal field demonstrated a rostrocaudal ordering of the DLP projection upon the neostriatum intermedium. Single-unit recording demonstrated that cells in DLPc respond to whole-field illumination at the same latency as cells in the nucleus rotundus, indicating that the tecto-DLPc-neostriatal pathway transmits visual information to the telencephalon. We suggest that comparable pathways may exist in both reptiles and mammals.     

Telencephalic projections of the nucleus rotundus in the pigeon (Columba livia)

The nucleus rotundus receives a massive ascending projection from the optic tectum. Stereotaxic lesions were placed in the nucleus rotundus and its efferent projections studied by means of the Nauta-Gygax and Fink-Heimer methods for degenerating axons and terminals. Efferent rotundal axons form the lateral part of the fasciculus prosencephali lateralis (FPL), pass through the lateral portion of the paleostriatum primitivum (PP) and augmentatum (PA) and enter the overlying ectostriatum. The ectostriatum was found to consist of at least two distinct cytologic regions: a central core (E) containing larger and more scattered cells and a peripheral belt with smaller, more densely packed neurons (Ep). Rotundal efferents terminate on the full extent of the central core of larger neurons. No evidence was found of any significant projection to the overlying periectostriatal belt or to the surrounding neostriatum. All portions of the central core of the ectostriatum receive projections from the nucleus rotundus. This projection is topologically organized on a rostro-caudal axis relative to the nucleus rotundus. There was no indication of the existence of inter-rotundal, rotundo-tectal or rotundus to contralateral ectostriatal projections. These findings demonstrate the existence of a massive ascending visual pathway to the telencephalon, arising from the optic tectum and terminating in a cytologically distinct projection field within the telencephalon. The relationship of this pathway to known visual pathways in mammals and reptiles is discussed.     

Modulatory effects of the nucleus of the basal optic root on rotundal neurons in pigeons

The present paper reports for the first time in birds the modulatory effects of the nucleus of the basal optic root (nBOR) on visual neurons in the nucleus rotundus in particular and those of the accessory optic system on the tectofugal pathway in general. Pharmacological blockade of the nBOR by lidocaine led to a decrease or increase in visual responsiveness of rotundal cells, suggesting excitatory or inhibitory actions of the nBOR on rotundal cells. These results were confirmed by changes in the excitability of rotundal cells following electrical stimulation of the nBOR. Response latency measurements implied that there might be at least two pathways from the nBOR to the nucleus rotundus, one being a direct excitatory pathway and the other an indirect inhibitory pathway possibly mediated by the subpretectal nucleus and the interstitio-pretecto-subpretectal nucleus, which have been thought to send inhibitory afferents to the nucleus rotundus. Taken together with previous neuroanatomical and immunocytochemical studies, it is suggested that modulatory interactions might exist between the nBOR and the nRt in particular and between the accessory optic system and the tectofugal pathway in general in birds.     

Tectal efferents in the blind cave fish Astyanax hubbsi.

Suction lesions were placed in the optic tecta of 36 blind cave fish. Three main bundles of tectal efferents were observed. A large, caudally directed fascicle distributes to the ipsilateral torus semicircularis, nucleus isthmi, and lateral tegmental areas of the mesencephalon and pons via the ipsilateral tectobulbar tract. Contralaterally, this fascicle descends to pontine levels as the contralateral tectobulbar tract. A second, rostrally directed bundle exits from the tectum at two levels. A small fascicle leaves from the caudal tectum and ascends rostrally as the commissura transversa. This bundle then joins with more rostrally exiting fibers and the combined fascicles collect in the area of the medial optic tract. They remain in this position until the level of the postoptic commissure where they decussate. Subsequently, this bundle moves caudally and enters the contralateral tectum at its most rostral extreme. The third bundle of tectofugal efferents leaves the tectum medially, at the level of the lesion, and enters the tectal commissure, through which it is distributed to the ipsilateral torus longitudinalis and contralateral optic tectum.     

A morphological study of the nucleus subpretectalis of the pigeon

In pigeons, the tectofugal system is functionally as well as structurally lateralized. So for example the right nucleus rotundus is less modulated by right forebrain influences than the left nucleus rotundus by the left ones. This functional lateralization pattern may depend on a dynamic balance between left and right tectal processing. Apart from inhibitory interactions at tectal level, suppressive influences might directly affect rotundal neurons by GABAergic input from a cluster of nuclei, the bed nuclei of the tecto-thalamic tract. A major afferent of these nuclei is the side branch of the tectorotundal projection which is of bilateral origin and which is involved in the regulation of ipsilateral as well as bilateral visual processing. Hence, an important role of the bed nuclei could be the interhemispheric communication and in turn the mediation of functional asymmetries. In a first step to unravel asymmetric influences of these nuclei, the present study investigated if the largest of the bed nuclei, the nucleus subpretectalis displays morphological asymmetries in the pigeon. We found that the nucleus subpretectalis in fact exhibits asymmetric cell sizes with larger cell bodies on the left side. This asymmetrical pattern was not present in dark-incubated animals indicating that cell size asymmetries within nucleus subpretectalis are induced by asymmetric photic stimulation during embryonic development.