Differential display implicates cyclophilin A in adult cortical plasticity

Removal of retinal input from a restricted region of adult cat visual cortex leads to a substantial reorganization of the retinotopy within the sensory-deprived cortical zone. Little is known about the molecular mechanisms underlying this reorganization. We used differential mRNA display (DDRT-PCR) to compare gene expression patterns between normal control and reorganizing visual cortex (area 17-18), 3 days after induction of central retinal lesions. Systematic screening revealed a decrease in the mRNA encoding cyclophilin A in lesion-affected cortex. In situ hybridization and competitive PCR confirmed the decreased cyclophilin A mRNA levels in reorganizing cortex and extended this finding to longer postlesion survival times as well. Western blotting and immunocytochemistry extended these data to the protein level. In situ hybridization and immunocytochemistry further demonstrated that cyclophilin A mRNA and protein are present in neurons. To exclude the possibility that differences in neuronal activity per se can induce alterations in cyclophilin A mRNA and protein expression, we analyzed cyclophilin A expression in the dorsal lateral geniculate nucleus (dLGN) of retinally lesioned cats and in area 17 and the dLGN of isolated hemisphere cats. In these control experiments cyclophilin A mRNA and protein were distributed as in normal control subjects indicating that the decreased cyclophilin A levels, as observed in sensory-deprived area 17 of retinal lesion cats, are not merely a reflection of changes in neuronal activity. Instead our findings identify cyclophilin A, classically considered a housekeeping gene, as a gene with a brain plasticity-related expression in the central nervous system.     

Shifting retinal maps in the development of the lateral geniculate nucleus

During development, the bilateral projections from each eye to subcortical visual structures in the mammal initially overlap throughout the majority of the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) before retracting to their separate territories. It has been shown in the ferret that during this period the larger contralateral retinal projection to both the dLGN and SC is retinotopically organised. By making small retinal lesions, and then anterogradely labelling the remaining retinofugal pathway from one eye, this study demonstrates that on the day of birth there is a superficial region of the dLGN in which the retinotopic map cannot be demonstrated. This region may be the presumptive C laminae. Further, by making small lesions in the temporal retina it has been shown that the smaller ipsilateral projection is also retinotopically organised before it retracts. Large lesions confined to the nasal retina had no effect on the pattern of label in the ipsilateral dLGN. Consequently, the ipsilateral projection which fills the nucleus at this stage must arise from the temporal retina. Because of this, the process of segregation requires that the retinotopic maps from each eye shift in relation to one another, and the borders of the nucleus to form the adult pattern.     

Alterations in GAP-43 and Synapsin Immunoreactivity Provide Evidence for Synaptic Reorganization in Adult Cat Dorsal Lateral Geniculate Nucleus following Retinal Lesions

Growth-associated protein-43 (GAP-43) and synapsin were used as molecular markers for synaptic reorganization in the adult cat visual system following sensory deprivation. Small binocular retinal lesions (central 10°) were made with a xenon light photocoagulator in adult cats. One, 3, 5 and 7 weeks after induction of the lesion, the neuropil levels of synapsin and GAP-43 in the dorsal lateral geniculate nucleus (dLGN) and area 17 were determined by immunocytochemistry. GAP-43 displayed a moderately low basal level in the dLGN of normal adult cats. The parvocellular C layers and the interlaminar plexi were characterized by higher immunoreactivity for GAP-43. Lesion-induced alterations were observed in all layers: GAP-43 immunoreactivity increased in the part of the dLGN representing central vision. This increase was maximal 3 weeks after the lesion. Under our experimental conditions, sensory deprivation did not significantly alter GAP-43 levels in the visual cortex. The changes in synapsin immunoreactivity were also restricted to the dLGN. In this nucleus, synapsin immunoreactivity decreased in all layers in the part subserving central vision 1 week after lesion. By 3 weeks after lesion, the level of synapsin had already returned to normal. This study provides evidence for a capacity for structural remodelling in primary sensory brain areas such as the dLGN throughout adult life. The observed changes in GAP-43 and synapsin in the dLGN suggest that synaptic reorganization is induced by retinal lesions. Normalization of synaptic density and activity could be important for the survival of the partially deafferented geniculate neurons.     

Retinal lesions induce fast intrinsic cortical plasticity in adult mouse visual system

Neuronal activity plays an important role in the development and structural–functional maintenance of the brain as well as in its life‐long plastic response to changes in sensory stimulation. We characterized the impact of unilateral 15° laser lesions in the temporal lower visual field of the retina, on visually driven neuronal activity in the afferent visual pathway of adult mice using in situ hybridization for the activity reporter gene zif268. In the first days post‐lesion, we detected a discrete zone of reduced zif268 expression in the contralateral hemisphere, spanning the border between the monocular segment of the primary visual cortex (V1) with extrastriate visual area V2M. We could not detect a clear lesion projection zone (LPZ) in areas lateral to V1 whereas medial to V2M, agranular and granular retrosplenial cortex showed decreased zif268 levels over their full extent. All affected areas displayed a return to normal zif268 levels, and this was faster in higher order visual areas than in V1. The lesion did, however, induce a permanent LPZ in the retinorecipient layers of the superior colliculus. We identified a retinotopy‐based intrinsic capacity of adult mouse visual cortex to recover from restricted vision loss, with recovery speed reflecting the areal cortical magnification factor. Our observations predict incomplete visual field representations for areas lateral to V1 vs. lack of retinotopic organization for areas medial to V2M. The validation of this mouse model paves the way for future interrogations of cortical region‐ and cell‐type‐specific contributions to functional recovery, up to microcircuit level.     

Topography of primary visual centres in the brain of the chick,Gallus gallus

In a previous paper (Ehrlich and Mark,'83b) the primary visual centres of the chick were described. In this paper patterns of retinotopy are examined by means of silver degeneration and autoradiographic techniques following discrete laser lesions of the retina. Well-defined and complete retinotopic maps are found in each of the following visual centres: tectum, lateral anterior thalamus, lateroventral geniculate nucleus, superficial synencephalic nucleus, ectomammillary nucleus, and tectal grey. In the dorsolateral anterior thalamus, pars lateralis, and external nucleus there is some evidence of a retinotopic innervation, yet not as well defined as those nuclei mentioned previously. Retinotopic maps were not observed in other retinorecipient regions. These include the ventrolateral thalamus, dorsolateral anterior thalamus, magnocellular part and rostrolateral part, pretectal optic area, and diffuse pretectal nucleus. Within the lateroventral geniculate, lateral anterior thalamus, superficial synencephalic nuclei, and the tectal grey, the ventral to dorsal retinal axis is mapped along the rostrocaudal axis, the reverse of the orientation seen in the tectum, which may have implications for explanations of how retinotopic maps are formed. The poor retinotopy in dorsolateral anterior thalamus, lateral part, is discussed with respect to the view that it may be the avian homologue of the mammalian lateral geniculate nucleus.     

Thalamo-cortical organization of the visual system in the cat

Thalamo-cortical relationships in the visual system of the cat were studied by the method of retrograde degeneration. Localized lesions limited to area 17 result in degeneration only in the dorsolateral geniculate body; cell changes are marked in 3 laminae (A, A₁, B), mild in nucleus interlaminaris centralis and minimal in nucleus interlaminaris medialis. Lesions limited to areas 18 and 19 are followed by marked degeneration in the medial interlaminar nucleus, mild in the other laminae; in addition, the lateral part of the posterior thalamic nucleus (ventral or inferior pulvinar) is also atrophied. Following large striate lesions which marginally involved areas 18 and 19, there is also mild, localozed degeneration in the anteroventral and reticular thalamic nuclei. Whin cortical lesions are limited to the convexity of the suprasylvian gyri, degeneration is present in the lateral aspect of laminae A, A₁, B and nucleus interlaminaris centralis and in the medial part of the posterior nucleus, in addition to lateral dorsal, lateral posterior and pulvinar nuclei. Lesions in the ectosylvian gyri result in slight but definite degeneration in the lateral part of lamina A of the dorsal lateral geniculate, but nothing in the posterior nucleus. The geniculate projections to areas 17, 18 and 19, to the suprasylvian and ectosylvian gyri all show a rostrocaudal organization. The geniculostriate projection is also topographically organized in a mediolateral manner. Thus, the geniculocortical projection in the cat is not striate specific but spreads over the occipito-temporal cortex at least as far as the acoustic areas of the ectosylvian gyri. In this species the dorsal lateral geniculate body is not a unitary structure but is a complex of nuclei, all of which receive retinal fibers, and the cortical projections of which overlap those of the posterior, lateral dorsal, lateral posterior, pulvinar, medial geniculate, reticular and anterior thalamic nuclei.     

Acute methamphetamine-induced zif/268, preprodynorphin, and preproenkephalin mRNA expression in rat striatum depends on activation of NMDA and kainate/AMPA receptors

This study tested the role of N-methyl-d-aspartate and kainate/AMPA receptors in mediating mRNA expression of the immediate early gene zif/268 and the opioid peptide genes preprodynorphin and preproenkephalin in rat forebrain following a single injection of methamphetamin. At 3 h after acute methamphetamine [4 mg/kg, intraperitoneally (IP)], quantitative in situ hybridization histochemistry revealed that zif/268 mRNA expression was increased in the dorsal striatum (caudoputamen) and in the sensory cortex. Preprodynorphin was increased in both dorsal and ventral striatum (nucleus accumbens) and preproenkephalin was increased in the dorsal striatum. Pretreatment with (±)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) (10 mg/kg, IP), an N-methyl-d-aspartate receptor antagonist, blocked the methamphetamine-induced zif/268 mRNA expression in the striatum and in the region of sensory cortex representing the upper limb and nose. 6,7-Dinitro-quinoxaline-2,3-dione (DNQX) (100 mg/kg, IP), a kainate/AMPA receptor antagonist, did not reduce the ability of methamphetamine to induce zif/268 mRNA in striatal and cortical neurons. Furthermore, both antagonists caused a parallel blockade of methamphetamine-stimulated preproenkephalin mRNA expression in the dorsal and ventral striatum but did not significantly affect methamphetamine-stimulated preproenkephalin mRNA expression. CPP and DNQX reduced basal levels of zif/268 mRNA in cortical and striatal neurons but did not affect the constitutive expression of the two opioid mRNAs in the striatum. Neither antagonist had a significant effect on methamphetamine-induced demonstrate that both N-methyl-d-aspartate and kainate/AMPA receptor-mediated glutamatergic transmission is linked to modulation of the methamphetamine-stimulated opioid peptide gene expression in rat forebrain. Furthermore, N-methyl-d-aspartate receptors participate in methamphetamine-stimulated zif/268 expression.     

Morphology of visual sector thalamic reticular neurons in the macaque monkey suggests retinotopically specialized,parallel stream‐mixed input to the lateral geniculate nucleus

The thalamic reticular nucleus (TRN) is a unique brain structure at the interface between the thalamus and the cortex. Because the TRN receives bottom‐up sensory input and top‐down cortical input, it could serve as an integration hub for sensory and cognitive signals. Functional evidence supports broad roles for the TRN in arousal, attention, and sensory selection. How specific circuits connecting the TRN with sensory thalamic structures implement these functions is not known. The structural organization and function of the TRN is particularly interesting in the context of highly organized sensory systems, such as the primate visual system, where neurons in the retina and dorsal lateral geniculate nucleus of the thalamus (dLGN) are morphologically and physiologically distinct and also specialized for processing particular features of the visual environment. To gain insight into the functional relationship between the visual sector of the TRN and the dLGN, we reconstructed a large number of TRN neurons that were retrogradely labeled following injections of rabies virus expressing enhanced green fluorescent protein (EGFP) into the dLGN. An independent cluster analysis, based on 10 morphological metrics measured for each reconstructed neuron, revealed three clusters of TRN neurons that differed in cell body shape and size, dendritic arborization patterns, and medial‐lateral position within the TRN. TRN dendritic and axonal morphologies are inconsistent with visual stream‐specific projections to the dLGN. Instead, TRN neuronal organization could facilitate transmission of global arousal and/or cognitive signals to the dLGN with retinotopic precision that preserves specialized processing of foveal versus peripheral visual information. J. Comp. Neurol. 525:1273–1290, 2017. © 2016 Wiley Periodicals, Inc.     

Neurofilament protein: a selective marker for the architectonic parcellation of the visual cortex in adult cat brain.

In this immunocytochemical study, we examined the expression profile of neurofilament protein in the cat visual system. We have used SMI-32, a monoclonal antibody that recognizes a nonphosphorylated epitope on the medium- and high-molecular-weight subunits of neurofilament proteins. This antibody labels primarily the cell body and dendrites of pyramidal neurons in cortical layers III, V, and VI. Neurofilament protein-immunoreactive neurons were prominent in 20 visual cortical areas (areas 17, 18, 19, 20a, 20b, 21a, 21b, and 7; posteromedial lateral, posterolateral lateral, anteromedial lateral, anterolateral lateral, dorsal lateral, ventral lateral, and posterior suprasylvian areas; anterior ectosylvian, the splenial, the cingulate, and insular visual areas; and the anterolateral gyrus area). In addition, we have also found strong immunopositive cells in the A laminae of the dorsal part of the lateral geniculate nucleus (dLGN) and in the medial interlaminar nucleus, but no immunoreactive cells were present in the parvocellular C (1-3) laminae of the dLGN, in the ventral part of the LGN and in the perigeniculate nucleus. This SMI-32 antibody against neurofilament protein revealed a characteristic pattern of immunostaining in each visual area. The size, shape, intensity, and density of neurofilament protein-immunoreactive neurons and their dendritic arborization differed substantially across all visual areas. Moreover, it was also obvious that several visual areas showed differences in laminar distribution and that such profiles may be used to delineate various cortical areas. Therefore, the expression of neurofilament protein can be used as a specific marker to define areal patterns and topographic boundaries in the cat visual system.     

Connections of the multiple visual cortical areas with the lateral posterior-pulvinar complex and adjacent thalamic nuclei in the cat

The present report describes the patterns of cat thalamocortical interconnections for each of the 13 retinotopically ordered visual areas and additional visual areas for which no retinotopy has yet emerged. Small injections (75 nl) of a mixture of horseradish peroxidase and [3H]leucine were made through a recording pipette at cortical injection sites identified by retinotopic mapping. The patterns of thalamic label show that the lateral posterior-pulvinar complex of the cat is divided into three distinct functional zones, each of which contains a representation of the visual hemifield and shows unique afferent and efferent connectivity patterns. The pulvinar nucleus projects to areas 19, 20a, 20b, 21a, 21b, 5, 7, the splenial visual area, and the cingulate gyrus. The lateral division of the lateral posterior nucleus projects to areas 17, 18, 19, 20a, 20b, 21a, 21b, and the anterior medial (AMLS), posterior medial (PMLS), and ventral (VLS) lateral suprasylvian areas. The medial division of the lateral posterior nucleus projects to areas AMLS, PMLS, VLS, and the anterior lateral (ALLS), posterior lateral (PLLS), dorsal (DLS) lateral suprasylvian areas, and the posterior suprasylvian areas. In addition, many of these visual areas are also interconnected with subdivisions of the dorsal lateral geniculate nucleus (LGd). Every retinotopically ordered cortical area (except ALLS and AMLS) is reciprocally interconnected with the parvocellular C layers of the LGd. The medial intralaminar nucleus of the LGd projects to areas 17, 18, 19, AMLS, and PMLS. Finally, each cortical area (except area 17) receives a projection from thalamic intralaminar nuclei. These results help to define the pathways by which visual information gains access to the vast system of extrastriate cortex in the cat.     

Topography of cells responding with long latencies to flashes and cells projecting to layer I of area 17 in the rat dorsal lateral geniculate nucleus

After deposition of horseradish peroxidase (HRP) crystals on the surface of cortical area 17 in 7 adult Wistar albino rats only relay cells were found to be labelled retrogradually in the caudal two thirds of the dorsal lateral geniculate nucleus (dLGN). From these results it can be concluded that cells projecting to the cortical layer I are concentrated in this part of the nucleus. Single cell recordings in the rat dLGN have shown that cells responding to light stimuli with long latencies are very frequent in this part of the dLGN, likewise. These findings support the idea that W-cells dominate in the caudal region of the rat dLGN, which is characterized by a population of relatively small geniculo-cortical relay neurons (GCR-neurons).     

Capacity of the retinogeniculate pathway to reorganize following ablation of visual cortical areas in developing and mature cats

The purpose of the present study was to determine the pattern and density of retinal projections to the dorsal lateral geniculate nucleus (dLGN) following ablation of visual cortical areas in developing cats of different postnatal ages and in mature cats. The terminations of retinal projections to the dLGN were evaluated following the injection of tritiated amino acids into one eye. Regardless of age, a visual cortical ablation of areas 17 and 18 induces massive death of neurons within the regions of the dLGN that are linked topographically to the cortical areas removed. However, the pattern of retinal projections to these degenerated regions of the dLGN differs depending upon whether the cortical lesion is incurred early in postnatal life or in adulthood. Following ablation on the day of birth (P1), virtually all surviving cells were found in the C-complex of dLGN with only a token number in the A-laminae. Correspondingly, retinal projections were maintained to the C-complex of the nucleus and were barely detectable in the degenerated A-laminae. However, in cats in which areas 17 and 18 had been removed in adulthood (≥ 6 months of age) retinal projections were maintained to the A-laminae even though nearly all neurons in those laminae had degenerated. Moreover, a subgroup of animals that incurred area 17 and 18 ablations at P1 showed that the modification of retinal projections to the A-laminae occurs within the first postnatal month, and an additional subgroup showed that retinal projections become increasingly resistant to the degenerative events in the dLGN that follow ablation of areas 17 and 18 at progressively older ages during the first postnatal month. Furthermore, retinal inputs also respond, in an age-dependent way, to degeneration of neurous in the C-complex induced by extension of the cortical ablation to include extrastriate visua areas. © 1993 Wiley-Liss, Inc.     

A transient geniculo-extrastriate pathway in macaques? Implications for 'blindsight'

Both monkeys and cats receiving primary visual cortex lesions in infancy show better residual vision than animals sustaining similar damage in adulthood. In cats, the better recovery has been explained in part by stabilization of a transient pathway from the dorsal lateral geniculate nucleus (dLGN) to cortical visual area PMLS. To test the hypothesis that a similar transient pathway from the dLGN to dorsal extrastriate areas exists in primates and thus serves as a candidate for recruitment after early V1 damage, retrograde tracers were injected into areas MT, MST, and/or 7a of infant macaques. No evidence of a transient pathway from the dLGN to these areas was obtained, despite projections from the pulvinar and other extrastriate areas in all cases.     

mGluR5-dependent increases in immediate early gene expression in the rat striatum following acute administration of amphetamine

Metabotropic glutamate receptor 5 (mGluR5) is densely expressed in medium-sized spiny projection neurons of the rat striatum. Activation of mGluR5 increases intracellular Ca2+, resulting in Ca(2+)-dependent cellular responses. Acute administration of the psychostimulant amphetamine (AMPH) induces immediate early gene (IEG) expression in the striatum, which is considered an important molecular event for the development of striatal neuroplasticity related to the addictive properties of drugs of abuse. This study investigated the role of mGluR5 in the mediation of IEG expression in the rat striatum induced by a single dose of AMPH (4 mg/kg, i.p.) in vivo. We found that systemic administration of the mGluR5-selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg, i.p. reduced AMPH-stimulated c-fos mRNA levels in the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum as revealed by quantitative in situ hybridization. Similar results were observed in the three areas of cerebral cortex (cingulate, sensory, and piriform cortex). In contrast to c-fos mRNAs, AMPH-stimulated mRNA expression of another IEG, zif/268, was not significantly altered by the blockade of mGluR5 with MPEP in the entire striatum and the three areas of cortex. Treatment with MPEP alone had no effect on basal levels of c-fos and zif/268 mRNAs in the striatal and cortical areas. These results indicate that an mGluR5-dependent mechanism selectively contributes to c-fos expression in the striatum and cortex in response to acute exposure to AMPH.     

'Hidden lamination' in the dorsal lateral geniculate nucleus: the functional organization of this thalamic region in the rat

The cyto-and myeloarchitecture of the rat's dorsal lateral geniculate nucleus (dLGN) display none of the laminar features characteristic of this thalamic region in carnivores and primates. Despite this, the rodent's nucleus contains a segregation of functionally and ocularly distinct afferents--organizational properties manifested in the prominent lamination of these other mammalian forms. The rat's dLGN can be divided into two main regions: an inner core and an outer shell. The inner core contains two ocular laminae receiving direct retinotopic projections from the contralateral nasal and ipsilateral temporal retinae, mapping the contralateral visual hemifield. The outer shell receives a retinotopic projection from the complete contralateral retina only, the representation of the ipsilateral hemifield being extremely compressed at the medial edge of this lamina. The retinotopic maps in these three ocular laminae (contra, ipsi, contra) are in conjugate register, so that lines of projection course rostro-ventro-medially from the optic tract at the thalamic surface through these laminae. Three morphologically distinct retinal ganglion cell types project to the dLGN, and the axons of these ganglion cells are partially segregated within the optic tract in anticipation of their segregation within the nucleus, where they terminate at distinct locations along the lines of projection. Type I and III cells terminate in the inner core of the nucleus, while type II and III cells terminate in the outer shell. The outer shell also receives a direct projection from the superior colliculus. These characteristics of the afferent termination within the rat's dLGN support the view of a general mammalian plan for the organization of this thalamic region, and provide a basis for further experimentation to test speculations about potentially homologous subdivisions of this nucleus. Conclusions regarding functionally analogous pathways are proposed with less confidence, due to the paucity of definitive evidence for physiologically distinct cell classes. The type I cells in the rat's retina are the likely homologues of the cat's alpha-cell. Geniculocortical relay cells driven by them have properties similar to the cat's Y-cell. The inner core of the nucleus then may transmit information of a Y-like nature onto striate cortex. The outer shell of the rat's nucleus, a portion of which receives collicular as well as retinal innervation, may convey W-like information onto striate cortex. The rat's retinogeniculate projection appears to be lacking a beta-cell-like pathway that may subserve X-cell function altogether.     

Organization of corticothalamic projections from parietal cortex in cat

Corticothalamic projections from areas 5a, 5b, and 7 of cat parietal cortex were studied with autoradiographic techniques. Each cortical area was identified by its cytoarchitectural characteristics and the patterns of termination were related to the thalamic nuclear groups. Injections of 3H-leucine in cortical area 5a were associated with terminal labeling primarily in the spinal recipient zone of the ventral lateral nucleus (VLsp) and the medial division of the posterior group (POm). The corticothalamic projections of area 5a are loosely topographically organized; medial parts of 5a project heavily to rostral and lateral parts of VLsp and sparsely to POm, while lateral parts of 5a project to more medial and caudal parts of VLsp and heavily to POm. Cortical area 5b projects primarily to the rostral portions of the lateral posterior nucleus (LP). These projections also appear to be topographically organized. The part of area 5b on the marginal gyrus projects to more ventral parts of rostral LP, while area 5b on the middle suprasylvian gyrus projects to more dorsal and lateral parts of rostral LP. Cortical area 7 projects to LP and the pulvinar (Pul). Rostral parts of area 7 project heavily to dorsal and lateral parts of LP and lightly to Pul; more caudal portions of area 7 projects relatively more heavily to Pul. The reticular, central lateral, and paracentral nuclei also receive projections, especially from the suprasylvian gyrus. The results are discussed with regard to putative sensory response characteristics of these cortical areas and to general thalamocortical organization.     

The organization of the retinal projection to the dorsal lateral geniculate nucleus in pigmented and albino rats

The retina maps over the dorsolateral and posterior surfaces of the contralateral dorsal lateral geniculate nucleus of albino and pigmented rats, such that the upper retina projects posteriorly and the lower retina projects over the dorsolateral surface. Nasal retina projects ventrolaterally and temporal retina dorsomedially. From the surface, lines of projection (demonstrated after either localised retinal or visual cortical lesions) tend to run deep and anteriorly within the nucleus. The area covered by the uncrossed optic pathway differs between pigmented and albino rats. In the pigmented animals it extends no more than 60° from the edge of the visual field represented in the crossed projection, while in albino animals it extends as far as 100° or more. The uncrossed projection in the albino strains tends to be deficient posteriorly (upper retinal representation) and dorsomedially, and the overall density is less than that found in the pigmented animals. Small lesions localised in the extreme peripheral temporal retina of both albino and pigmented rats have always produced degeneration in the contralateral as well as ipsilateral geniculate, even though some of the lesions map only as far as, or within the peripheral 20° on the contralateral colliculus. Most of these lesions in the albino rats give two patches of uncrossed degeneration in the geniculate with at least one focus of degeneration in the contralateral geniculate. Those made in the lower temporal retina of the albino rat give in addition to the uncrossed patches, two areas of crossed degeneration in the geniculate separated by about 50° in terms of visual field projection. These double projection aberrations do not occur in pigmented animals. The suggestion is offered that in albino rats the retina of each eye maps totally on the contralateral lateral geniculate nucleus. However, the vertical midline of the visual field is represented at about 40–50° from the medial edge of the nucleus. A normal (but sparse) uncrossed pathway maps lateral to this position, but also maps in duplicate medial to it. In addition, it appears that some crossed axons may also be abnormal in their termination, and project to both sides of the vertical meridian. If the above interpretation is correct, the anomalies of the albino rat may derive from a confusion of attempting both field and retinotopic maps on the same nucleus. The possibility exists that a comparable total crossed retinogeniculate projection exists in pigmented animals but that the retina temporal to the vertical midline projection is restricted in its connection to a small band on the medial border of the contralateral nucleus.     

Retinal projections to the superior colliculus and dorsal lateral geniculate nucleus in the tammar wallaby (Macropus eugenii): I. Normal topography

The topography of retinal projections to the superior colliculus and dorsal lateral geniculate nucleus of a wallaby, the tammar (Macropus eugenii), was investigated by an anatomical method. Small laser lesions were made in the retinas of experimental animals, and the remaining retinal projections were visualized by means of horseradish-peroxidase histochemistry. The position of each lesion was correlated with the position of the filling defects in the terminal label. The whole of the retina projects to the contralateral superior colliculus. The nasal retina is represented caudally, and the temporal retina rostrally. The ventral retina is represented medially, and the dorsal retina laterally. There is a projection to the ipsilateral superior colliculus, but it is patchy and its topography could not be determined by this method. The retinotopic map in the contralateral dorsal lateral geniculate nucleus has the nasal retina represented rostrally and the temporal retina caudally in the nucleus. The dorsal retina is represented ventrally, and the ventral retina is represented dorsally. It appears that the whole of the retina projects contralaterally, and in addition the temporal retina projects ipsilaterally. The maps of visual space through the two eyes were shown to be in topographic register in the binocular region by making a deposit of HRP in the visual cortex. This resulted in a column of retrogradely labeled cells in the nucleus. This column crossed the laminae, which are innervated by the ipsilateral and contralateral eye at right angles.