N-methyl-D-aspartate antagonists prevent interaction of binocular maps in Xenopus tectum

Glutamate receptors appear to play a key role in several forms of experience-dependent modification of both the strength of synapses and synaptic connectivity. In developing Xenopus frogs, the connections made by isthmotectal axons relaying visual input from the eye to the ipsilateral tectum are markedly influenced by the visual activity of contralateral retinotectal axons, and normal binocular visual input is necessary in order for the ipsilateral visuotectal map to come into register with the contralateral map. We have tested whether NMDA receptors play a role in establishment of the topographic matching of binocular maps during development. We have examined the effects of chronic treatment of tectum with either the receptor agonist NMDA or the antagonists APV or CPP applied throughout early postmetamorphic life using subpial implants of drug-impregnated elvax. Both antagonists blocked the matching of the ipsilateral map to the contralateral map, while NMDA permitted such matching. Our data therefore indicate that NMDA receptors are involved in the experience-dependent establishment of matching binocular maps during development.     

Ultrastructure of the crossed isthmotectal projection in Xenopus frogs

The nucleus isthmi NI of frogs is a relay for input from the eye to the ipsilateral tectum; each NI receives retinotopic input from one tectum and sends retinotopic output to both tecta. The crossed isthmotectal projection in Xenopus displays tremendous plasticity during development. Physiological and anatomical studies have suggested that the location at which a developing isthmotectal axon will terminate is determined by the correlation of its visually evoked activity with the activity of nearby retinotectal terminals. What structures could mediate such communication? We have examined quantitatively the ultrastructural characteristics of crossed isthmotectal axons and synapses in order to determine whether retinotectal axons communicate directly with isthmotectal axons via axo-axonic synapses or whether the communication is indirect, e.g., via common postsynaptic dendrites. Our results support the conclusion that isthmotectal axons interact with retinotec tal axons indirectly and that tectal cell dendrites are the critical site of interaction.     

The development of the nucleus isthmi in Xenopus laevis. I. Cell genesis and the formation of connections with the tectum

The nucleus isthmi (NI) of the amphibian relays visual input from one tectum to the other tectum and thus brings a visual map from the eye to the ipsilateral tectum. This isthmotectal visual map develops slowly; it is first detected electrophysiologically at stages 60-62, the age at which the eyes begin their dorsalward migration and the region of binocular overlap beings to increase in extent. During this critical period of life, normal binocular visual input is required for establishment of normal topographic isthmotectal projections. In this study, we have used anatomical methods to trace cell birth, cell death, and formation of connections by the nucleus isthmi during the critical period. Tritiated thymidine labelling demonstrates that cells in the nucleus isthmi are generated throughout most of tadpole life (stages 29-62). Most cells conform to an orderly ventrodorsal gradient starting from stage 29 and extending to stages 56; later cells are inserted at apparently random locations in the nucleus. We have re-examined the hypothesis of Tay and Straznicky ('80) that the order of cell genesis in the NI and tectum could help establish proper isthmotectal connections, and we find that a timing mechanisms does not explain the two-dimensional topography of the isthmotectal map but that timing may aid in proper mediolateral positioning of isthmotectal axons at the points where they first enter the tectum. Horseradish peroxidase labelling was used to investigate whether anatomical projections from tectum to NI and from NI to tectum are present prior to the onset of eye migration. The results show that there are tectoisthmotectal projections by stage 52. Moreover, isthmotectal axons grow into as yet monocular tectal regions prior to the onset of eye migration. At stage 60, when binocular overlap begins, isthmotectal axons are visible throughout the tectum but are densely branched only at the rostral tectal margin, the location where they are predicted to occur on the basis of electrophysiological maps.     

Evidence for shifting connections during development of the chick retinotectal projection

The pattern in which optic axons invade the tectum and begin synaptogenesis was studied in the chick. The anterogradely transported marker, horseradish peroxidase, was injected into one eye of embryos between 5 and 16 days of development (E5 to E16). This labeled the optic axons in the brain. The first retinal axons arrived in the most superficial lamina of the tectum on E6. They entered the tectum at the rostroventral margin. During the next 6 days of development the axons grew over the tectal surface. First they filled the rostral tectum, the oldest portion of the tectum, and then they spread to the caudal pole. Shortly after the first axons entered the tectum on E6, labeled retinal axons were found penetrating from the surface into deeper tectal layers. In any given area of the tectum, optic axons were seen penetrating deeper layers shortly after arriving in that area. Electron microscopic examination showed that at least some of the labeled axons in rostral tectum formed synapses with tectal cells by E7. These results show two things which contrast with results from previous studies. First, there is no delay between the time the retinal axons enter the tectum and the time they penetrate into synaptic layers of the tectum. Second, the first retinotectal connections are formed in rostral tectum and not central tectum. Retrograde tracing showed the first optic axons that arrived in the tectum were from ganglion cells in central retina. Previous studies have shown that the ganglion cells of central retina project to the central tectum in the mature chick. This opens the possibility that the optic axons from central retina, which connect to rostral tectum in the young embryo, shift their connections to central tectum during subsequent development. As they enter the tectum the growth cones of retinal axons appear to be associated with the external limiting membrane. During the time that connections would begin to shift in the tectum a second population of axons appears at the bottom of stratum opticum, some with characteristics of growth cones. This late-appearing population may represent axons shifting their connections. These results have implications for theories on how the retinotopic pattern of retinotectal connections develops.     

Isthmotectal axons maintain normal arbor size but fail to support normal branch numbers in dark-reared Xenopus laevis

Developing binocular projections to the Xenopus tectum require visual input in order to establish matching topographic maps. In dark-reared Xenopus, the ipsilateral eye's map, relayed via the retino-tecto-isthmotectal pathway, fails initially to acquire normal rostrocaudal order. Moreover, with extended time in the dark, the ipsilateral map becomes progressively less well organized. This phenomenon showed that without binocular cues, the isthmotectal axons are unable to locate proper sites for their terminal zones but left open the issue of whether the axons are able to establish arbors of normal dimensions and/or to sustain normal numbers of branches. In order to test whether dark-rearing modifies isthmotectal axon branching, we have used horseradish peroxidase to examine axons of Xenopus after dark-rearing for periods from 3 to 298 weeks. The results demonstrate that these axons never acquire more than about half the normal numbers of terminals. Surprisingly, however, the dark-reared axons' terminal zones are normal in mediolateral and rostrocaudal extent despite the lack of binocular cues that normally could constrain arbor size by inducing pruning of branches in regions with mismatched visual inputs. The effects of dark-rearing are reversible. After a return to normal lighting conditions, the recovery process begins quickly, with a significant increase in branch numbers within 4 weeks. The terminal zone remains of normal dimensions. These results support the hypothesis that correlated binocular visual input is essential for the maintenance of normal numbers of isthmotectal branches but that normal termination zone size can be established in the absence of visual cues.     

Visuotopic organization of the superior colliculus in normal and Siamese cats

The visuotopic organization of the superior colliculus of normally pigmented and Siamese cats was investigated with microelectrodes. In normal cats, the representation of the ipsilateral hemifield in the rostral tectum was found to be binocular. In addition, this representation included more of the ipsilateral hemifield than has been previously reported.

Both normally pigmented and Siamese cats were found to have about 40° of the ipsilateral hemifield represented in the rostral tectum and normally pigmented and Siamese cats do not appear to differ in this regard (see ref. 2). However, the superior colliculus of Siamese cats is abnormal in other ways. The representation of the ipsilateral hemifield in the rostral tectum of Siamese cats was activated only by the contralateral eye. Furthermore, in the larger caudal representation of the contralateral hemifield, very few neurons were activated by the ipsilateral eye. The reduction in such activation is not simply the result of misrouting of ipsilateral retinotectal fibers to the contralateral tectum in a manner similar to the misrouting of retinogeniculate fibers^11,12, since there were also very few neurons with abnormally placed receptive fields. Instead, the recordings from the tectum are similar to those obtained from striate cortex of some Siamese cats¹³ where few neurons were activated by input relayed from the normal ipsilateral or the abnormal contralateral retinogeniculate projections. As in the geniculostriate system^11–13, those neurons with receptive fields abnormally placed in ipsilateral hemifield were in part of the tectal representation of the first 20° of the contralateral hemifield; neurons with normally placed receptive fields related to the ipsilateral eye were 20°–40° into the representation of the contralateral hemifield.     

Latency and temporal overlap of visually elicited contralateral and ipsilateral firing in Xenopus tectum during and after the critical period

The mechanisms underlying the development of proper topographic registration of binocular maps in the tectum of Xenopus laevis involve correlation of activity patterns of ipsilateral and contralateral inputs. Recent evidence implicates NMDA-type glutamate receptors in this process. In general, NMDA receptors are considered to function optimally when there are multiple, simultaneous excitatory inputs to a dendrite. In the binocular system of the frog, however, the ipsilateral eye's response to a visual stimulus reaches the tectum later than the contralateral eye's response. The reason for this delay is that the ipsilateral pathway to the tectum is indirect, involving a relay in the opposite tectum and nucleus isthmi. In this paper, we evaluate the duration of the delay between arrival of contralateral and ipsilateral input in response to cessation of light and we also gauge the extent of temporal overlap in responses of the two inputs. We find that the average delay is about 10 ms and that this delay is not significantly different during the critical period vs later in development. The temporal overlap is 40-60 ms in duration. We conclude that the intertectal delay does not prevent a substantial period of simultaneous firing of ipsilateral and contralateral inputs in response to sudden changes in illumination. Therefore, the firing patterns of these afferents are compatible with a mechanism of activity-dependent alignment of binocular maps in the tectum.     

Caudal topographic nucleus isthmi and the rostral nontopographic nucleus isthmi in the turtle, Pseudemys scripta

Isthmotectal projections in turtles were examined by making serial section reconstructions of axonal and dendritic arborizations that were anterogradely or retrogradely filled with HRP. Two prominent tectal-recipient isthmic nuclei--the caudal magnocellular nucleus isthmi (Imc) and the rostral magnocellular nucleus isthmi (Imr)--exhibited strikingly different patterns of organization. Imc cells have flattened, bipolar dendritic fields that cover a few percent of the area of the cell plate constituting the nucleus and they project topographically to the ipsilateral tectum without local axon branches. The topography was examined explicitly at the single-cell level by using cases with two injections at widely separated tectal loci. Each Imc axon terminates as a compact swarm of several thousand boutons placed mainly in the upper central gray and superficial gray layers. One Imc terminal spans less that 1% of the tectal surface. Imr cells, by contrast, have large, sparsely branched dendritic fields overlapped by local axon collaterals while distally, their axons nontopographically innervate not only the deeper layers of the ipsilateral tectum but also ipsilateral Imc. Imr receives a nontopographic tectal input that contrasts with the topographic tectal input to Imc. Previous work on nucleus isthmi emphasized the role of the contralateral isthmotectal projection (which originates from a third isthmic nucleus in turtles) in mediating binocular interactions in the tectum. The present results on the two different but overlapping ipsilateral tecto-isthmo-tectal circuits set up by Imc and Imr are discussed in the light of physiological evidence for selective attention effects and local-global interactions in the tectum.     

Synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi in Rana pipiens

The nucleus isthmi is reciprocally connected to the ipsilateral optic tectum. Ablation of the nucleus isthmi compromises visually guided behavior that is mediated by the tectum. In this paper, horseradish peroxidase (HRP) histochemistry and electron microscopy were used to explore the synaptic interrelationships between the optic tectum and the ipsilateral nucleus isthmi. After localized injections of HRP into the optic tectum, there are retrogradely labeled isthmotectal neurons and orthogradely labeled fibers and terminals in the ipsilateral nucleus isthmi. These terminals contain round. Clear vesicles of medium diameter (40–52 nm). These terminals make synaptic contact with dendrites of nucleus isthmi cells. Almost half of these postsynaptic dendrites are retrogradely labeled, indicating that there are monosynaptic tectoisthmotectal connections. Localized HRP injection into the nucleus isthmi labels terminals primarily in tectal layers B, E, F, and 8. The terminals contain medium-sized clear vesicles and they form synaptic contacts with tectal dendrites. There are no instances of labeled isthmotectal terminals contacting labeled dendrites. Retrogradely labeled tectoisthmal neurons are contacted by unlabeled terminals containing medium-sized and small clear vesicles. Fifty-four percent of the labeled fibers connecting the nucleus isthmi and ipsilateral tectum are myelinated fibers (average diameter approximately 0.6 μm). The remainder are unmyelinated fibers (average diameter approximately 0.4 μm). © 1994 Wiley-Liss, Inc.     

The central projections in the retina in Necturus maculosus

The projections of the retina in Necturus maculosus were studied by injecting radioactive proline into one eye. Labeling was seen in both the contralateral and ipsilateral diencephalon and tectum. The contralateral fibers are divided into three major tracts: the marginal, axial, and basal. The ipsilateral fibers separate into a marginal and an axial optic tract. The contralateral and ipsilateral axial optic tracts have a similar distribution. The contralateral and ipsilateral marginal optic tracts projecting to the diencephalon also have a similar distribution. However, in the tectum the ipsilateral marginal optic tract ends in the anterior third while the contralateral extends almost the entire length of the tectum. The retinotectal ipsilateral projection ends in clumps as has been described in other vertebrates. A direct ipsilateral retinotectal projection has not been described in any other amphibian.     

The effect of cortical and tectal lesions on the visual fields of binocularly deprived cats

The visual fields of seven cats raised with binocular lid suture were measured before and after various neural lesions. Each of the cats preoperatively responded with each eye to stimuli from 90° ipsilateral through to the midline. A transection of the optic chiasm rendered one cat bline on the visual field tests. Large bilateral occipito-temporal cortical ablations (4 cats) did not measurably affect orienting responses or the extent of visual field. Unilateral occipito-temporal cortical ablations (2 cats) also had no affect on the visual fields, but subsequent ablations of the contralateral superior colliculus produced permanent blindness in the hemifield contralateral to the ablated tectum. These two cats also were apparently blind with the eye contralateral to the ablated tectum; but with the other eye, the cats retained their preoperative orienting responses. These data are consistent with the hypothesis that, with early binocular deprivation, cats develop dependence upon retinotectal and not thalamocortical pathways for visually guided orienting behavior.     

Ultrastructure and GABA Immunoreactivity in Layers 8 and 9 of the Optic Tectum of Xenopus laevis

This study presents an ultrastructural analysis of layers 8 and 9 in the optic tectum of Xenopus laevis. Retinotectal axons were labelled with horseradish peroxidase and tectal cells were labelled with antibody to GABA. Four distinct axonal and dendritic structures were identified. GABA-negative axon terminals formed asymmetric synapses and were categorized as type a-1 (which included retinotectal axons), characterized by medium size synaptic vesicles and pale mitochondria, and type a-2 (non-retinotectal) with large vesicles and dense mitochondria. GABA-negative dendrites (type d) contained dense mitochondria, microtubules in the dendritic shafts, and dendritic spines devoid of microtubules. GABA-positive structures contained small synaptic vesicles and dense mitochondria. Some dendrites (type D) were not only postsynaptic but were also presynaptic elements, as defined by the presence of vesicles and distinct synaptic clefts with symmetric specializations. GABA-positive presynaptic structures were mostly located in vesicle-filled, bulbous extensions of dendritic shafts and usually terminated onto dendritic spines. Some type D dendrites were the middle element in serial synapses, with input from either GABA-positive or GABA-negative structures and output to GABA-negative structures. Retinotectal terminals were identified as one of the synaptic inputs to GABA-positive processes. Glia were characterized by granular cytoplasm and large mitochondria, often displaying a crystalline matrix structure. These results indicate that GABA-positive neurons are a prominent component of circuitry in the superficial layers of the tectum of Xenopus and that, as in mammals, they participate in serial synaptic arrangements in which retinotectal axons are the first element. These arrangements are consistent with complex processing of visual input to the tectum and a central role for inhibitory processes in the shaping of tectal responses.     

Development and maintenance of connectivity in the visual system of the frog. I. The effects of eye rotation and visual deprivation

Visual deprivation or disparity between the eyes was produced in frog embryos or larvae by occluding one eye with a skin graft or by rotating one eye. After metamorphosis, the projections from each eye to the ipsilateral and contralateral optic tectum were mapped electrophysiologically.After rotation of one eye in embryos or larvae, the projection from each eye to the ipsilateral tectum developed at the normal time during metamorphosis, but the ipsilateral and contralateral projections were incongruent. In some cases, surgical rotation of one eye in embryos resulted in growth of optic nerve fibers from either or both eyes directly to the ipsilateral optic tectum. The pattern of these anomalous retinotectal projections was determined by intrinsic developmental mechanisms and was not modified by visual stimulation.Skin grafts over one or both eyes did not alter the normal development of ipsilateral visuotectal projections during metamorphosis. However, long term monocular deprivation by a skin graft which covered an eye from before metamorphosis until 90–105 days after metamorphosis resulted in abnormally large ipsilateral visuotectal responsive fields of the normal eye as well as of the deprived eye. By contrast, no abnormality developed in monocular deprivation started after the onset of metamorphosis or if both eyes were occluded from before metamorphosis until 258 days after metamorphosis.These results show that neither patterned visual stimulation nor functional correspondence between the eyes is required for the initial development of ipsilatera visuotectal projections (which are subserved by intertectal connections) during metamorphosis. However, the maintenance of the ipsilateral projections requires symmetrical (but not patterned) stimulation of both eyes during the onset of metamorphosis.     

Relative number of cells projecting from contralateral and ipsilateral nucleus isthmi to loci in the optic tectum is dependent on visuotopic location: horseradish peroxidase study in the leopard frog.

The leopard frog optic tectum is the principal target of the contralateral retina. The retinal terminals form a topographic map of the visual field. The tectum also receives bilateral topographic input from a midbrain structure called nucleus isthmi. In this study we determined the relative strength of n. isthmi projections to different loci in the tectum. Horseradish peroxidase (HRP) was applied at single superficial tectal locations in a series of leopard frogs. The application sites were distributed across the tectum. Retrogradely filled cells were counted in ipsilateral and contralateral nucleus isthmi. Although all regions of the tectum receive input from both n. isthmi, the relative number of labeled cells in the two n. isthmi is dependent on visuotopic location. Input to the rostromedial tectum representing the visual field ipsilateral to the labeled tectum comes primarily from the contralateral n. isthmi. Input to the caudolateral tectum representing the visual field contralateral to the labeled tectum originates mostly from the ipsilateral n. isthmi. Tectal application sites representing the visual midline had approximately equal numbers of labeled cells in the two n. isthmi. The results are similar at postapplication survival times ranging from 2 to 14 days. Using application of HRP to rostral tectum and application of nuclear yellow to caudal tectum, we show that the anisotropy in isthmi labeling is not due to take up of these labels by isthmotectal fibers passing through the application sites that terminate elsewhere.     

Retinal distribution of ganglion cells which project to the ipsilateral optic tectum in Bufo matinus

The retinotectal projection in anura is mainly crossed, although a small proportion of optic axons projects to the ipsilateral tectum. Using the fluorescent carbocyanide dye, DiI, we mapped the retinal topography of ganglion cells which project to the ipsilateral tectum in adult Bufo marinus. DiI was injected into particular locations in the right tectum. After 10 days survival both the right and the left retinas were wholemounted and the number and retinal position of retrogradely filled ganglion cells were determined. The contralateral and ipsilateral cells were visuotopically distributed in the retina in the majority of experiments. However, in two cases cells were located in visuotopically disparate parts of the retina. The ipsilateral cells represented 3.7% of contralaterally projecting cells in the temporal retina. 0.1% in the nasal and dorsal retina and 0.6% of the ventral retina. The density of ipsilaterally projecting ganglion cells varied from a top of 25 cells/mm² in the temporal retina, 9 cells/mm² in the nasal, 3 cells/mm² in the dorsal to 11 cells/mm² in the ventral retina. The diversity of size and shape of retrogradely filled ganglion cells indicated that the ipsilateral population corresponded to a heterogeneous class of ganglion cell types. The functional significance of the direct ipsilateral retinotectal projection of the anuran visual system has yet to be elucidated. However, in light of the involvement of the indirect ipsilateral retinotectal projection in binocular vision, the direct pathway is likely to be associated with a retino-tecto-spinal circuit subserving postural adjustment to visually derived stimulation.     

Some observations on the early development of the optic tectum in the frog (Rana pipiens), with special reference to the effects of early eye removal on mitotic activity in the larval tectum

Following unilateral eye removal in Rana pipiens at embryonic stages 21–24 there is a statistically significant reduction in the numbers of mitoses found in the contralateral optic tectum at larval stage XIV amounting to approximately 16 per cent. Most of the reduction in mitotic activity occurs in the rostral third of the tectum and does not appear to involve its caudal pole, where cell proliferation is most vigorous. Since thymidine autoradiography indicates that the majority (if not all) of the cells being generated in the rostral third of the tectum at stage XIV are either glial or ependymal cells, it is suggested that the primary effect of eye removal on cell proliferation in the tectum is upon gliogenesis. Using the autoradiographic method for tracing axonal connections, it has been found that the axons of the retinal ganglion cells reach the tectum by embryonic stage 22, and by larval stage XIV have spread to all but its caudomedial region.     

Monocular and binocular optic inputs to salamander pretectal neurons: intracellular recording and HRP labelling study

Intracellular recordings and horseradish peroxidase injections were performed in the pretectum and adjacent tegmentum of Salamandra salamandra, while both optic nerves were electrically stimulated. In approximately half of the recorded units no spikes could be evoked but rather graded postsynaptic potentials. The latter type morphologically showed features of interneurons. From a total of 48 recorded units, nearly 60% were excited only by the contralateral optic nerve, whereas approximately 40% were binocular. For the most part (10/19) the binocular cells were excited by the contralateral and inhibited by the ipsilateral optic nerve. Fewer neurons (7/19) received excitatory inputs from both optic nerves. The latency distribution of the monocular cells shows a maximum of 20-30 ms. The same maximum exists for the contralateral inputs to the binocular cells, whereas the ipsilateral inputs to these units were nearly as frequent with latencies of 20-30 and 40-50 ms. Since neurons with the short ipsilateral latencies always had parts of their dendrites within the ipsilateral ocular projection field, a feature which was lacking in the cells with long ipsilateral latencies, it is possible that the longer latencies are due to indirect ipsilateral inputs. Efferents of labelled dorsal pretectal cells reach the contralateral pretectum via the posterior commissure, the basal optic neuropil of the accessory optic system and the tegmental white substance. More ventrally located cells often reach the pretectal and the basal optic neuropil with their dendrites. Axons of this type descend to the medulla oblongata via the medial longitudinal fasciculus.     

Thalamic fiber connections in a teleost (Sebastiscus marmoratus): visual somatosensory, octavolateral, and cerebellar relay region to the telencephalon

Fiber connections of the nucleus ventromedialis thalami (VM) of Schnitzlein (J. Comp. Neurol. 118:225-267, '62) in a teleost (Sebastiscus marmoratus) were examined by means of the horseradish peroxidase (HRP) tracing method. This nucleus receives fibers from the ipsilateral telencephalon (area dorsalis pars centralis), contralateral retina, contralateral VM, ipsilateral optic tectum, ipsilateral torus semicircularis, contralateral corpus cerebelli, contralateral sensory nucleus of the trigeminal nerve, bilateral bulbospinal reticular formation, contralateral obex region, and contralateral dorsal portion of upper spinal segments. In turn, axons arising from VM terminate in the dorsal telencephalic areas (pars centralis, pars dorsalis, and pars medialis) ipsilaterally, ventral telencephalic area (pars supracommissuralis) bilaterally, nucleus prethalamicus of Meader (J. Comp. Neurol. 60:361-407, '34) bilaterally, nucleus dorsomedialis thalami bilaterally, VM contralaterally, optic tectum bilaterally, torus semicircularis bilaterally, and nucleus lateralis valvulae ipsilaterally. Based on the cytoarchitecture and fiber connections, VM is subdivided into rostral and caudal components. The caudal part of VM in Sebastiscus is considered to be a multimodal thalamic complex that contains some cells that constitute the dorsal thalamus in other vertebrate groups.     

Development and maintenance of connectivity in the visual system of the frog. II. The effects of eye removal

The interdependence of retinotectal and intertectal connections in the ontogeny of the visual system of the frog (Rana pipiens) was studied. To remove the direct afferent input from retina to one side of the tectum one eye was taken out at different embryonic and larval stages of development. Animals were allowed to pass through metamorphosis, at which time intertectal connections linking the visual input to the two sides of the tectum normally develop. Responses to visual stimulation were then recorded with extracellular microelectrodes from both sides of the tectum.In all animals eye removal resulted in size reduction of the tectum on the side of the remaining eye. Eye removal prior to the onset of metamorphosis (stage XIX) resulted in gross abnormalities of visually evoked responses in the tectum ipsilateral to the remaining eye; receptive fields were much larger than normal, localization was poor, and responses fatigued rapidly upon repeated stimulation. Eye removal after the onset of metamorphosis, on the other hand, did not result in any detectable abnormalities of the ipsilateral visual responses; receptive fields were small and well localized. This normal ipsilateral visuotectal projection persisted indefinitely (up to 273 days) in the absence of one eye.We conclude that there is a Necessary interaction between the developing intertectal pathways and the afferent fibers linking retina and tectum. This interaction appears to be completed during metamorphosis. The data presented in this and the preceding paper¹² argue against any subsequent interactions between the retinotectal and intertectal connections. Furthermore, the two papers together suggest that the interactions taking place during metamorphosis are the result of intrinsic developmental mechanisms and are not altered by sensory stimulation.     

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.