Organization of the visual system in larval lampreys: an HRP study.

The organization of the visual system of larval lampreys was studied by anterograde and retrograde transport of HRP injected into the eye. The retinofugal system has two different patterns of organization during the larval period. In small larvae (less than 60-70 mm in length) only a single contralateral tract, the axial optic tract, is differentiated. This tract projects to regions in the diencephalon, pretectum, and mesencephalic tegmentum. In larvae longer than 70-80 mm, there is an additional contralateral tract, the lateral optic tract, which extends to the whole tectal surface. In addition, ipsilateral retinal fibers are found in both small and large larvae. Initially, the ipsilateral projection is restricted to the thalamus-pretectum, but it reaches the optic tectum in late larvae. Changes in the organization of the optic tracts coincide with the formation of the late-developing retina and consequently, the origin of the optic tracts can be related to specific retinal regions. The retinopetal system is well developed in all larvae. Most retinopetal neurons are labeled contralaterally and are located in the M2-M5 nucleus of the mesencephalic tegmentum, in the caudolateral mesencephalic reticular area and adjacent ventrolateral portions of the optic tectum. Dendrites of these cells are apparent, especially those directed dorsally, which in large larvae extend to the optic tectum overlapping with the retino-tectal projection. These results indicate that in lampreys, visual projections organize mainly during the blind larval period before the metamorphosis, their development being largely independent of visual function.     

Visual projections in larval Ichthyophis kohtaoensis (Amphibia: gymnophiona)

The visual projection patterns of retinal efferents were studied in larval Ichthyophis kohtaoensis by means of anterogradely transported HRP. Our results show in all larvae a projection contralateral to a thalamic terminal field, a pretectal terminal field, and a basal optic neuropil, but only a sparse innervation of the contralateral tectum. In addition, all larvae possess an uncrossed projection to a thalamic and a pretectal terminal field. The fibers are bilaterally almost confined to the medial optic tract with only a few fibers running in the marginal and basal optic tract. The ipsilateral and contralateral tracts and terminal fields seem to enlarge during larval life. Comparison with other amphibian orders reveals that larval Ichthyophis are unique in that they develop the medial optic tract and the related thalamic and pretectal terminal fields very early in larval life. In addition they possess only a very sparse tectal projection, though it is the largest projection in larval urodeles and anurans. This suggests a selective phylogenetic loss of those ganglion cells or collaterals which project mainly to the tectum in other amphibian orders and a change in the ontogenetic program leading to an earlier development of the medial optic tract in Ichthyophis as compared to urodeles and anurans.     

Retinal projections in the cane toad, Bufo marinus.

The location and extent of retinorecipient areas in the cane toad, Bufo marinus, were established by anterograde transport of cobaltic-lysine complex from the cut optic nerve. Most of the labeled optic axons travelled in the marginal optic tract, while others were in the axial optic tract, and/or the basal optic tract. Retinal projections terminated in both contralateral and ipsilateral targets. In addition to the optic tectum, the main visual center, retinorecipient areas included the suprachiasmatic nucleus, rostral visual nucleus, neuropil of Bellonci, corpus geniculatum thalamicum, ventrolateral thalamic nucleus (dorsal part), posterior thalamic neuropil, uncinate neuropil, pretectal nucleus lentiformis mesencephali and basal optic nucleus. While all of these retinorecipient areas receive optic fibers from both eyes, the ipsilateral retinal projections were observed to be generally sparser than those from the contralateral retina. A sparse optic fiber projection covers the surface of the ipsilateral optic tectum and is most prominent rostromedially and caudolaterally. The position and the extent of each of the retinorecipient areas were determined in relation to a three-dimensional coordinate system. Morphometric analysis showed that 85.3% of the retinorecipient area is in the contralateral optic tectum, 10.4% in contralateral non-tectal areas, 1.6% in the ipsilateral optic tectum and 2.7% in ipsilateral non-tectal areas. The presence of an ipsilateral tectal projection and the well defined pretectal visual neuropil complex may be related to the highly developed visual behavior and visual acuity of Bufo marinus.     

Topographic projections between the nucleus isthmi and the tectum of the frog Rana pipiens.

The connections between the nucleus isthmi and the tectum in the frog have been determined by several anatomical techniques: iontophoresis of horseradish peroxidase into the tectum, iontophoresis of 3H-porline into the nucleus isthmi and the tectum, and Fink-Heimer degeneration staining after lesions of the nucleus isthmi. The results show that the nucleus isthmi projects bilaterally to the tectal lobes. The ipsilateral isthmio-tectal fibers are distributed in the superficial layers of the tectum, coincident with the retionotectal terminals. The contralateral isthmio-tectal fibers travel anteriorly adjacent to the lateral optic tract and cross the midline in the supraoptic ventral decussation, where they turn dorsally and caudally; upon reaching the tectum, the fibers end in two discrete layers, layers 8 and A of Potter. The tectum projects to the ipsilateral nucleus isthmi and there is a reciprocal topographic relationship between the two structures. Thus, a retino-tecto-isthmio-tectal route exists which may contribute to the indirect ipsilateral retinotectal projection which is observed electrophysiologically. The connections between the nucleus isthmi and the tectum in the frog are strinkingly similar to the connections between the parabigeminal nucleus and the superior colliculus of mammals.     

Development of the transient ipsilateral retinotectal projection in the chick embryo: A numerical flourescence-microscopic analysis

The ipsilateral retinotectal projection in the developing chick was examined by using rhodamine-B-isothiocyanate (RITC)as an anterograde and retrograde vital marker for the retinal ganglion cells and their axons. Staining of the entire retina following intravitreal RITC injection between incubation days 3 and 16 revealed a small number of anterogradely labeled fibers in the optic tract and the anterior half of the optic tectum ipsilateral to the injection site. The total number of ipsilaterally projecting fibers was estimated to be about 2,000 on developmental day 9. The ipsilateral projection totally disappeared after day 15. The arrangement of fibers within the ipsilateral projection was examined by local anterograde RITC staining of localized retinal regions between days 9 and 10. The projection was retinotopically organized along the dorsoventral axis such that fibers of dorsal retinal origin projected on the ventral tectal half, whereas fibers of ventral retinal orgin projected on the dorsal tectal half. The localization of ipsilaterally projecting ganglion cell bodies was examined by retrograde RITC staining during days 9 and 15. Ganglion cells of all four quadrants of the central retina contributed to the production of the ipsilateral projection. The ipsilaterally growing retinotectal fibers did not represent collaterals of contralaterally projecting retinotectal axons. We assume that the tendency of early growing retinotectal axons to grow straight, as well as the ability of axonal growth cones to “sample” the environment, lead to a crossing of axons to the contralateral side. Ipsilateral projections would therefore represent “pathfinding errors.” Explanations for the elimination of the ipsilateral retinotectal projection are discussed.     

Development of the optic tract in the cichlid fish Haplochromis burtoni

In the teleost fish, Haplochromis burtoni, the optic tract is composed of 3 distinct components: the marginal tract, which projects to the optic tectum and is by far the largest, and the axial and medial tracts which project to diencephalic targets. In this paper we report on the normal development of these pathways in larval H. burtoni, an African cichlid fish. The earliest optic tract fibers are found in what will become the marginal optic tract. These fibers hug the wall of the diencephalon in a cohesive bundle. The first fibers in the axial tract location appear on day 5, increasing in number between days 6 and 18. Like marginal tract fibers, axial tract fibers form a cohesive bundle. It is not clear from these experiments whether the first axial tract fibers actually arrive at this location at day 5, or whether they are fibers arriving earlier that were physically displaced from the marginal tract at day 5. Medial tract fibers are not evident until day 6 of development and the number of medial tract fibers also increases as the animal gets older. Unlike fibers in the other two pathways, medial tract fibers do not travel together in a bundle. Rather, each one follows an independent trajectory to its target site. Comparison of this larval development with the adult optic tract organization which we have studied earlier suggests constraints on the mechanisms of axon guidance.     

Anomalous ipsilateral retinotectal projections in syrian hamsters with early lesions: Topography and functional capacity

Retinotectal topography, response properties of neurons in superior colliculus, and visual orienting behavior were studied in hamsters whose superior colliculi were innervated by one or the other of two types of anomalous ipsilateral projections. For the first type, an abnormally large uncrossed projection was created by monocular enucleation on the day of birth. This projection extended over the superficial part of the rostral half of the colliculus. The upper visual field was represented medially, and the lower visual field laterally, which corresponds to a normal projection. The rostrocaudal axis was disordered, but showed a slight tendency for nasal visual field to be represented rostrally and temporal field caudally; this tendency corresponds to an inversion of the normal ipsilateral projection, fitting instead the pattern of a contralateral projection. For the second type of anomalous ipsilateral projection, an abnormal intertectal decussation of optic tract fibers was created by neonatal ablation of the superficial layers of one superior colliculus and removal of the ipsilateral eye (Schneider, '73). Retinotectal topography observed in this recrossing projection was predominantly mirror-symmetric to the normal contralateral projection; however, some distortions in retinotopic order were observed, including misplaced fields and local inversions of the mirror-symmetric topography, and distortions of local magnification factor. Response properties of single units found medially in the left colliculus were similar to those found in normal colliculus. Units found more laterally were underresponsive, showing response decrements with repeated stimulation which is abnormal for units in the superficial gray, and many had abnormally large receptive fields. This physiological pattern was reflected in the pattern of errors made in visual orienting to small targets. It was concluded that polarity cues exist in the tectum sufficient to order the terminals of the retinotectal projection independent of the direction of fiber arrival or order in the optic tract as it enters the tectum. In addition, the functional competence of the abnormal recrossing retinotectal projection has been demonstrated by both electrophysiological and behavioral methods.     

Restoration of vision in genetically eyeless axolotls (Ambystoma mexicanum).

Eyes grafted into genetically eyeless axolotls at embryonic stages 26 or 27 (early tailbud stage) are capable of establishing retinotectal connections and restoring near normal vision. Normal vestibulo-ocular reflexes are also present in most of the eyeless mutants having grafted eyes. The animals are capable of accurately localizing objects in visual space and demonstrate following movements in an optokinetic drum. Evoked potentials can be recorded from the surfaces of the tectal lobes of eyeless mutants having a right eye graft which do not differ significantly from those recorded from a normal animal, except that recordings can still be obtained from the ipsilateral tectal lobe in the former following section of the intertectal fibers. This indication of direct retinotectal connections to the ipsilateral tectum was confirmed by histological examination which also showed that the optic fibers entering the diencephalon high on the lateral wall are initially directed toward the normal optic tract position before proceeding to be tectum.     

A retinotopic analysis of the central connections of the optic nerve in the frog

The possibility of retinotopic organization in the optic nerve projections to the contralateral and ipsilateral diencephalon was studied by means of partial retinal lesions and staining for terminal degeneration by the Fink-Heimer technique. A retinotopic pattern of projection was observed in the nucleus of Bellonci, the corpus geniculatum thalamicum and the posterior thalamic nucleus. The temporal quadrant of the retina, and, to a lesser extent, the ventral quadrant projected to the ipsilateral side as well as to the contralateral side. In each diencephalic region noted above, the temporal and dorsal quadrants of the retina were represented more posteriorly (posteroventrally), and ventral and nasal quadrants projected more anteriorly (anterodorsally). The areas of representation for the temporal and ventral quadrants were located superior (superoposterior) to those for the dorsal and nasal quadrants. In their overall configuration and orientation, the retino-diencephalic maps show mirror-image reversal with respect to the retino-tectal projection. Since, in their areal extent, both the retino-diencephalic maps and the retino-tectal map are approximately parallel to the ventricular surface, their mirror-image reversal appears to indicate a reversal in the polarity of developmental processes across the di-mesencephalic junction. The retinotopic organization within the optic tract in the diencephalon and tectum was also analyzed. In the optic tract, the quadrants of the retina are reassembled such that the dorsal and nasal quadrants are widely separated in, respectively, the ventral and dorsal edges of the tract; the temporal and ventral quadrants are systematically represented in intermediate levels in the tract, the temporal quadrant above the dorsal, and the ventral quadrant below the nasal. When the optic tract bifurcates to encircle the tectum, the fibers from the ventral and nasal quadrants enter the dorsomedial arm and the fibers from the temporal and dorsal quadrants enter the ventrolateral arm of the optic tract. The paths taken by optic fibers in traversing the tectum to reach their areas of termination were reconstructed. Many optic fibers show an alignment parallel to an anteroventral posterodorsal axis as they cross the surface of the tectum, but the OS vs IS characterization of the fibroarchitecture of the tectum appears to be an oversimplification.     

Ipsilateral retinofugal projections in a percomorph bony fish: their experimental induction, specificity and maintenance

Adult bony fish possess only a small ipsilateral retinofugal projection, if any. Experimental manipulation, such as unilateral enucleation, can lead to an enhancement of this projection. We examined the patterns of, as well as the conditions for the development and maintenance of an enhanced ipsilateral retinofugal projection (EIRP) after nerve crush, after enucleation, and after various combinations of both types of surgery in juvenile and adult Haplochromis burtoni (Cichlidae). Retinal projections were labeled either unilaterally with horseradish perixodase, or with the lipophilic fluorescent dye DiI in aldehyde-fixed animals, or bilaterally with differently colored fluorescent dextran amines. Unilateral nerve crush always leads to the regeneration of retinofugal fibers to the contralateral tectum but spares some contralateral diencephalic nuclei. In addition, unilateral or bilateral nerve crush in many cases, and unilateral enucleation in some cases, leads to the development of an EIRP to the ipsilateral diencephalon and tectum. This EIRP persists (4 months and longer postoperatively) in only 10% of the unilaterally enucleated animals, in none of the animals subjected to unilateral nerve crush and in 79% of the animals subjected to bilateral nerve crush. All unilaterally enucleated animals in which the remaining, contralateral optic nerve was crushed develop and maintain an EIRP. These data suggest that nerve crush alone is sufficient to cause regenerating fibers to project, at least transiently, to the ipsilateral side of the brain. When the normal contralateral projection is either absent or in the process of regeneration, an EIRP can be maintained. In the latter case, alternate bands or patches of ipsi- and contralateral fibers in the tectum may result. Ipsilateral fibers follow unusual pathways by recrossing at the rostral diencephalon. Likewise, regenerating contralateral retinal fibers grow differently in this area; here, where the optic-nerve projection is reorganized into the optic tract, many regenerating fibers are deflected to the ipsilateral side of the brain. Despite atypical routes taken by some fibers, the EIRP nevertheless ends only in specific retinorecipient areas. An EIRP develops independently of the age of the animal, independently of the time lapse between enucleation and nerve lesion, and independently of persisting debris. However, in animals receiving an optic nerve lesion a long time after unilateral enucleation, the size of the EIRP and its tectal extent are reduced compared to that in animals enucleated around the same time as receiving the crush of the contralateral optic nerve.     

Telencephalic efferents of the tiger salamander Ambystoma tigrinurn tigrinum (Green)

The efferent projections of the telencephalon in the tiger salamander were examined by the Nauta and Fink-Heimer methods following unilateral hemispherectomies, rostral hemispheric ablations and pallial lesions. The cerebral hemisphere connects with most areas of the contralateral hemisphere via the pallial, anterior and habenular commissures. The descending fibers travel in the medial and lateral forebrain bundles and in the tracts comprising the stria medullaris. Degenerating fibers and terminals were present throughout the diencephalon but were more abundant ipsilaterally. Fibers reach the pretectum and optic tectum via dorsal and ventral pathways. There is a heavy projection to the midbrain tegmentum and a sparse projection to the tectum via the ipsilateral lateral forebrain bundle. This tract continues into the medulla oblongata and the cervical spinal cord. Rostral and dorsal hemispheric ablations revealed that the majority of fibers forming the olfacto-peduncular tract originate in the ventral, rostral one-third of the hemisphere. It was also determined that the majority of the descending efferent fibers located in the lateral forebrain bundle originate from the caudal lateral hemispheric wall, and that these fibers form connections characteristic of mammalian corticofugal and striatofugal systems. The cytoarchitecture and connections of the caudal lateral hemispheric wall suggest that it is homologous to parts of motor isocortex and amygdala of amniotes.     

Retinofugal and retinopetal projections in the green sunfish, Lepomis cyanellus

The retinofugal and retinopetal connections in the green sunfish were studied by autoradiographic and horseradish peroxidase methods. All retinofugal fibers decussate in the optic chiasm. Some fibers project to contralateral preoptic and hypothalamic nuclei while others recross to project to the comparable ipsilateral nuclei. Contralaterally, the medial optic tract projects to the periventricular thalamic and pretectal nuclei and, sparsely, to the rostral optic tectum. The dorsal optic tract projects to the parvocellular portion of the superficial pretectal nucleus, the central pretectal nucleus, nucleus corticalis, and the rostral portion of the optic tectum. The ventral optic tract primarily projects to the caudal portion of the optic tectum, giving off fibers in route to innervate various nuclei, including the parvocellular superficial pretectal nucleus and the dorsal and ventral accessory optic nuclei. The axial optic tract projects to the dorsal accessory optic nucleus, the central pretectal nucleus, and the caudal optic tectum. Retinal fibers reach the ipsilateral thalamus, pretectum and other sites via a redecussation through the posterior commissure. From outgroup analysis it is concluded that such redecussating fibers are an independently derived character within actinopterygians and are homoplasous to nondecussating ipsilateral retinal projections in other vertebrates. Neurons retrogradely labeled with horseradish peroxidase were found to form a rostrocaudal column from the olfactory bulb and nerve through the ventral telencephalon to caudal diencephalic levels along the medial aspect of the optic tract. It is possible that all these neurons consist of one population of migrated ganglion cells of the nervus terminalis.     

Long term regeneration of contralateral and induced ipsilateral retinal projections to the remaining optic tectum ofRutilus rutilus

The regeneration of optic tract fibers hs been investigated in Rutilus kept at 18-20 degrees C, 6-7 months after ablation of one optic tectum and simultaneous section of the optic nerve from the contralateral eye. The labeling of the optic fibers obtained following injection of either tritiated proline or HRP in either of the eyes showed the existence of a normal contralateral retino-tectal projection to strata opticum, fibrosum et griseum superficial (SFGS), griseum centrale, and album centrale. Furthermore, it demonstrated the presence of a conspicuous newly-formed ipsilateral retino-tectal projection to both superficial and deep layers of SFGS in the form of horizontal bands. The partial overlapping of ipsi- and contralateral projections in SFGS was confirmed by a double-labeling technique (HRP and tritiated proline). The results suggest a retinal hyperinnervation of the remaining optic tectum.     

Retinal projections in the freshwater butterfly fish, Pantodon buchholzi (Osteoglossoidei). I. Cytoarchitectonic analysis and primary visual pathways.

The freshwater butterfly fish, Pantodon buchholzi, is a member of the most primitive radiation of teleosts. The retinofugal projections were studied in this fish with autoradiographic and horseradish peroxidase (HRP) methods, and the cytoarchitecture of the retinorecipient regions in the diencephalon and pretectum was analyzed with Bodian-, cresylecht-violet- and acetylcholinesterase-reacted sections. The rostral diencephalon of Pantodon contains a large retinorecipient nucleus, not previously identified in any other fish, i.e. nucleus rostrolateralis. Other nuclei that are described correspond to those previously recognized in other species. The majority of retinorecipient nuclei are positive for acetylcholinesterase, particularly those in the pretectum, as has been found in other species of teleosts. Most of the retinofugal fibers decussate in the optic chiasm. Some fibers project via the axial optic tract to preoptic nuclei and a region in the rostral hypothalamus. Fibers leave the medial optic tract to terminate in nucleus rostrolateralis and in dorsal and ventral thalamic nuclei, accessory optic and tubercular nuclei, periventricular and central pretectal nuclei, and sparsely in the deep tectal fascicle and terminal field. Dorsal optic tract fibers project to the dorsal accessory optic nucleus, superficial and central pretectal nuclei, and superficial and deep tectal layers. Ventral optic tract fibers project to the superficial pretectum, accessory optic nuclei, posterior tuberculum, nucleus corticalis in the central pretectum, and superficial tectal layer. Fibers that remain in the ipsilateral optic tract project to most of the targets reached by contralaterally projecting fibers. A few fibers in the contralateral medial optic tract redecussate via the posterior commissure to reach the ipsilateral periventricular pretectum. No labeled retinopetal cells caudal to the olfactory bulb were identified in any of the HRP cases.     

Retinal fibers alter tectal positional markers during the expansion of the retinal projection in goldfish

Although widely accepted, the theory, that neurones carry immutable cytochemical markers which specify their synaptic connections, is not consistent with plastic reorganizations. Half retinal fish were therefore tested for changed markers following expansion. Optic nerve crush at the time of the half retinal ablation resulted in regeneration of a normal, restricted projection; but nerve crush following expansion (many months later) resulted in reestablishment of the expanded projection, assessed both by electrophysiological mapping and by radioautography. Since this implied changed markers, the half retina and tectum were tested independently using the ipsilateral tectum and eye as controls. In normal fish, removal of one tectum and deflection of the corresponding optic tract toward the remaining tectum resulted in regeneration of a positionally normal but ipsilateral map. In experimental fish, after the half retina had expanded its projection to the contralateral tectum, its optic tract was deflected to the control tectum. After 40 days it had regenerated a normal, restricted map indicating that the retinal markers had not changed. Such restricted projections did not expand in the presence of the normal projection even after a year or more. Similarly, the optic tract from the normal eye was deflected to cause innervation of the tectum containing the expanded half retinal projection. After 40 days, the projection regenerated from the normal eye was similar to the expanded half retinal projection. Areas of the normal retina corresponding to the missing areas of the half retina were not represented. Tectal markers had been altered by the half retinal fibers. In a final group, tecta were denervated and tested at various intervals by innervation from ipsilateral half retinal eyes. After five months of denervation, the regenerating fibers were no longer restricted to the rostral tectum but formed an expanded projection initially. Apparently tectal markers are induced by the retinal fibers, changed during expansion, and disappear during long-term denervation.     

Comparative neurology of the optic tectum in ray-finned fishes: patterns of lamination formed by retinotectal projections

Retinotectal projections were studied in 33 different species of Actinopterygii, the ray-finned fishes, with horseradish peroxidase and cobalt tracing techniques. The distribution of retinorecipient layers in the contralateral optic tectum was analyzed. In addition, the degree of differentiation of the stratum periventriculate, and the presence of ipsilateral retinotectal projections was examined. Retinofugal fibers are labeled in the stratum opticum (SO), stratum fibrosum et griseum superficiale (SFGS), stratum griseum centrale (SGC), stratum album centrale (SAC) and stratum periventriculare (SPV). Some species lack the projection to the SO, others lack the projection to the SGC, and a third group of fishes lack both projections. Five different patterns of retinorecipient tectal strata are distinguished. These patterns correlate with the species' taxonomic position. Evolutionary trends of tectal lamination and retinotectal innervation are described. The retinotectal projection patterns provide a useful indicator of phylogenetic relationships. Some of our data suggest different relationships between actinopterygian species than hitherto believed.     

The development and restriction of the ipsilateral retinofugal projection in the chick

Although it is generally believed that the central projections of the retina in birds are entirely crossed, using wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) as an anterograde tracer, we have found that in normal posthatched chicks there is a small ipsilateral retinofugal projection to the diencephalon and midbrain. Most of the ipsilateral fibers appear to be directed to the lateral anterior and dorsolateral anterior nuclei of the thalamus, to the pretectal region, and to the ectomammillary nucleus and the adjoining nucleus externus. Even in the best preparations the numbers of ipsilateral fibers are so small that it is hardly surprising that they have been overlooked in previous axonal degeneration and autoradiographic experiments. A significantly larger ipsilateral retinal projection develops during the second week of incubation. The ipsilaterally directed fibers can be first seen on the fifth day of incubation and their numbers appear to increase until about embryonic day 12. At this stage the projection involves substantially more fibers than at hatching and is also more extensive in its distribution; in fact, in its general organization (but not its size) it closely parallels the normal crossed retinofugal system, contributing fibers to essentially all the primary visual relay nuclei in the diencephalon and midbrain and to much of the optic tectum, where the densest projection is to its caudomedial aspect. During the second week of incubation there is also a small number of retinal fibers, which after crossing in the optic chiasm, recross the midline in the posterior and tectal commissures (and also in the tectal roof plate), before ending in the pretectal region of the ipsilateral side. In addition, there is a markedly aberrant projection from the retina into the contralateral optic nerve. Most of the ipsilateral retinal fibers are eliminated between the twelfth and sixteenth days of incubation, and by day 17 the ipsilateral projection is reduced to its mature form. The progressive reduction in the ipsilateral projection occurs at a time when it is known (from other studies) that there is an appreciable loss of retinal ganglion cells; but whether the reduction is due to neuronal death or to the selective elimination of ipsilateral axon collaterals remains to be determined. The existence of a significant ipsilateral retinofugal component early in development, probably accounts, in part, for the distinctive and persistent ipsilateral projection that occurs if one eye is removed during the first few days of incubation.     

Bilateral tectal innervation by regenerating optic nerve fibers in goldfish: a radioautographic, electrophysiological and behavioral study.

Following unilateral enucleation and optic nerve crush in goldfish, the remaining nerve regenerates and innervates both optic tecta. Approximately 5% of the nerve fibers reach the ipsilateral optic tectum (IOT) via the ipsilateral tract at the chiasma. Comparable debris in both tracts was not sufficient to result in an IOT projection since when both nerves were crushed simultaneously the usual pattern was seen, i.e., each nerve innervated a contralateral optic tectum (COT). When the arrival of one nerve at the chiasma was delayed by staggering the nerve crushes, the nerve that first arrived at the chiasma partially innervated the Iot. In most instances the entire IOT was innervated, however, the stratigraphic distribution of fibers in the various tectal lamina was atypical. Electrophysiological analysis indicated that fibers from each area of the retina innervated the IOT visuotopically. The COT was ablated in order to determine whether the IOT projection could mediate behavior. All fish failed to respond to changes in illumination as measured by respiration and failed to swim with or against the stripes in an optomotor drum. Thus, the IOT input, possibly because of its sparseness, could not be shown to be behaviorally functional.     

A radioautographic study of retinal projections in Type I and Type II lizards

The retinofugal projections of 5 species (Acanthodactylus boskianus, Scincus scincus, Tarentola mauritanica, Uromastix acanthinurus and Zonosaurus ornatus) belonging to 5 different families of Type I and Type II lizards have been examined by means of the radioautographic method. In the 5 species the retinal ganglion cells project to the contralateral hypothalamus (nucleus suprachiasmaticus), thalamus (nucleus geniculatus lateralis pars ventralis, nucleus geniculatus lateralis pars dorsalis), pretectum (nuclei lentiformis mesencephali, geniculatus pretectalis, postero-dorsalis griseus tectalis), tectum opticum (layer 2 to layer 6 of the stratum griseum et fibrosum superficiale) and tegmentum mesencephali (nucleus opticus tegmenti). Ipsilateral optic fibers were never observed in Uromastix acanthinurus, whereas an uncrossed quota was visible in both nucleus geniculatus lateralis pars dorsalis and nucleus postero-dorsalis in the other species. An ipsilateral retinotectal projection was observed only in Tarentola mauritanica. With the exception of the nucleus griseus tectalis the contralateral optic centers identified in this material have to a large extent been observed in other reptiles belonging to the different orders. The presence in reptiles of a general pattern of contralateral visual projections indicates that these were established very clearly in the course of evolution. Similarities become apparent when this plan is compared with that observed in birds. In marked contrast the ipsilateral component in reptiles is unstable and mutable in nature. This ipsilateral retinotectal projections do not appear to be a feature restricted to Type I lizards. On the other hand, the presence of this optic component cannot be linked solely to nocturnal habits.