Contralateral Retina Research Articles

Displaced ganglion cells and the accessory optic system of pigeon

The central projection and retinal distribution of displace ganglion cells (DGC's) are described for the pigeon. Discrete, localized injections of horseradish peroxidase (HRP) into the nucleus of the basal optic root (nBOR) complex labeled as many as 4,800 DGC's in the contralateral retina. The greatest densities of DGC's were observed in the more peripheral regions of the middle and inferior temporal regions of the retina, with lowest densities occurring in the inferior nasal, red field, and foveal areas. Large HRP injections of the tectal lobes, which did not include the pretectal, accessory optic (nBOR), hypothalamic, or thalamic visual nuclei, labeled only ganglion cells within the ganglion cells layer. An HRP injection centered within the nucleus lentiformis mesencephali, also including portions of the optic tectum and optic tract, labeled only ganglion cells within the ganglion cell layer of the contralateral retina. DGC's thus appear to be the primary, if not exclusive, source of retinal afferents to the nBOR complex in pigeon. The observed retinal distribution of DGC's indicates that the areas of retina with the greatest density of cells in the receptor layer, inner nuclear layer, and ganglion cell layer are relatively devoid of DGC's. Since the nBOR complex projects directly upon the vestibulocerebellum and oculomotor nuclei, DGC's would thus appear to be involved in neural circuits that mediate oculomotor reflexes and visuomotor behavior.

Some cortical and subcortical fiber projections to the accessory optic nuclei in the cat

The terminal nuclei of the accessory optic system receive a dense fiber projection from the contralateral retina and have recently been implicated in visuo-vestibular aspects of oculomotor function. The present anatomical study, based on anterograde tracer experiments with 3H-labeled amino acids or horseradish peroxidase, provides evidence for a diverse set of non-retinal inputs to the accessory optic nuclei arising in both the neocortex and the brainstem. Corticofugal fiber projections to the terminal nuclei originate in several areas of the visual cortex; those from the medial Clare-Bishop area may be especially dense. Subcortical inputs to the terminal nuclei include fiber projections arising in ( a) the pretectal region; ( b) the ventral mesencephalon, probably including the substantia nigra; and ( c) more dorsal or caudal parts of the mesencephalic reticular formation. The source of the pretectal projection to the terminal nuclei appears to be the nucleus of the optic tract or the underlying posterior pretectal nucleus. Fiber projections from the ventral mesencephalic tegmentum were labeled by injections infiltrating the substantia nigra, but these deposits all involved, at least marginally, either interstitial cells of the transpeduncular limb of the accessory optic tract or the perinigral tegmentum. The part of the midbrain reticular formation giving rise to an accessory optic projection was not precisely delimited, but it appears to lie outside the substantia nigra proper. Although these afferent projections to the accessory optic nuclei appear individually to be much weaker than the input from the contralateral eye, their presence clearly suggests that transmission of visual information through the terminal nuclei may involve a complex modulation of retinal input by non-retinal pathways.

An analysis of the retinal afferents to the cat's medial interlaminar nucleus and to its rostral thalamic extension, the "geniculate wing".

The retinal afferents to the medial interlaminar nucleus and to its rostro-dorsal extension at the edge of the pulvinar have been studied in cats by fiber degeneration and autoradiographic methods. Fiber degeneration following section of one optic nerve shows two distinct retinal inputs: One is coarse-fibered and goes to the medial interlaminar nucleus itself; the second, which is fine-fibered, goes to the rostral and medial borders of the medial interlaminar nucleus and continues into the pulvinar. The regions in receipt of these fine fibers have been called the "geniculate wing". The topography of retinal representations and the degree of binocular overlap within the medial interlaminar nucleus and the wing have been studied by combining intraocular injections of 3H proline with local lesions of the injected eye, or with removal of the non-injected eye. In the medial interlaminar nucleus three distinct laminae are recognizable and are particularly clearly shown in horizontal sections. Rostrally and medially, lamina 1 maps the contralateral nasal retina as a mirror reversal of lamina A. Posterior and lateral to this, lamina 2 maps the ipsilateral temporal retina as a mirror reversal of lamina A1. Lamina 3 lies closest to the optic tract and receives crossed afferents from the temporal retina. In this lamina, which is the smallest, vertical retinal dimensions map as in the other layers, but we were unable to determine the mapping of the horizontal dimensions. In the geniculate wing, as in the other geniculate layers, vertical lines of the visual field are mapped as corresponding vertical diencephalic dimensions; horizontal retinal dimensions are mapped as horizontal lines in the wing, with central retinal areas represented furthest from the optic tract. In the geniculate wing the contralateral nasal and ipsilateral temporal retina are mapped with considerable binocular overlap. The crossed temporal retina has no demonstrable representation in the wing.

The organization of the lateral posterior nucleus of the golden hamster after neonatal superior colliculus lesions.

The reorganization of the adult hamster's lateral posterior nucleus after neonatal superior colliculus lesions was studied using primarily light and electron microscopic degeneration techniques. Two types of experiments were conducted. First, the distributions of the remaining afferents from the contralateral superior colliculus, the cotralateral retina, and the ipsilateral posterior neocortex were determined using the Fink-Heimer ('67) technique. Normally the projections from the contralateral superior colliculus and retina are sparse and restricted to small areas in the rostrolateral subdivision. After neonatal lesions of the ipsilateral colliculus, however, these two minor projections greatly increase in density and expand to share a common border. In contrast, the normal projection from the posterior neocortex is dense throughout the rostrolateral subdivision. After a neonatal colliculus lesion, however, this projection is greatly decreased in the region occupied by the optic tract terminals. Second, the ultrastructural organizatin of the rostrolateral subdivision was studied in adult animals whhich had received neonatal colliculus lesions. Normally, this region is characterized by synaptic clusters in which numerous medium-sized terminals (M-terminals), almost all from the ipsilateral colliculus, synapse around the shaft of a large central dendrite. The contralateral colliculus and retina normally contribute only a few M-terminals. After a neonatal colliculus lesion, typical clusters still form, but now the expanded projections from the contralateral colliculus and retina contribute numerous M-terminals. The cortex does not contribute M-terminals in either normal or experimental animals. These results suggest that the afferents to the rostrolateral subdivision normally compete for synaptic space. The various factors that might be involved in determining the outcome of such competition are discussed.

The normal organization of the lateral posterior nucleus of the golden hamster.

As a first step in analyzing the influence of various afferent projections on the development of the hamster lateral posterior nucleus, its normal organization was studied using both light and electron microscopic techniques. Rostrolateral, rostromedial, and caudal subdivisions were identified. The rostrolateral subdivision receives dense projections from the ipsilateral superior colliculus and posterior neocortex, as well as sparser, more restricted projections from the contralateral colliculus and retina. The ipsilateral colliculus is by far the major source of medium-sized (M)terminals with round vesicles. These terminals synapse around the shafts of large central dendrites to form distinctive synaptic clusters. The contralateral colliculus and retina contribute a few M-terminals to the clusters. In contrast, axons from the posterior neocortex form very large (RL-)terminals with round vesicles from the posterior neocortex form very large (RL)terminals with round vesicles which synapse onto numerous appendages of single proximal dendrite, are surrounded by glial lamellae, and rarely participate in the clusters. Axons from all four sources also form small (RS)terminals with round vesicles which synapse on the shafts of small dendrites. Finally, F-terminals with flat or pleomorphic vesicles form symmetric synaptic contacts both within and outside the clusters. The only identified projection to the rostromedial subdivision is from the ipsilateral posterior neocortex, which contributes RL- and RS-terminals. F-terminals are also found, but neither M-terminals nor synaptic clusters are present. The caudal subdivision also receives RL- and RS-terminals from the ipsilateral posterior neocortex. Small inputs from the ipsilateral and contralateral colliculi are present, but their axons form only RS-terminals. No M-terminals or synaptic clusters are found. These results indicate that a large neonatal superior colliculus lesion would eliminate the vast majority of the M-terminals in the synaptic clusters of the ipsilateral lateral posterior nucleus. In subsequent studies (Crain and Hall, '80 a,b,c), we will examine how the remaining inputs from the retina, contralateral superior colliculus, and posterior neocortex contribute to the synaptic organization when it develops after such a lesion.

The organization of afferents to the lateral posterior nucleus in the golden hamster after different combinations of neonatal lesions.

After a neonatal lesion of the ipsilateral superior colliculus, the projections to the lateral posterior nucleus from the contralateral superior colliculus and retina expand their terminal fields until they share a common border. In the first experiment described in this paper, we removed both superior colliculi at birth and used the Fink-Heimer method to show that the optic tract projection could expand even further and enter the region which would have been occupied by the terminals of the crossed colliculus projection. Similarly, in the second experiment, we showed that the crossed collicular projection could be increased even more if the contralateral eye as well as the ipsilateral colliculus was removed at birth. Another result of a neonatal superior colliculus lesion is that the projection from the optic tract shares a border with the posterior neocortical projection. In the third experiment, we removed both the ipsilateral superior colliculus and the posterior neocortex at birth and demonstrated that the optic tract projection expanded more than after an ipsilateral colliculus lesion alone. Our results support the hypotheses that the projections from the ipsilateral and contralateral superior colliculi and the retina compete for synaptic space in the lateral posterior nucleus, and that a similar competition between the retinal and cortical projections may also occur.

Interactions between optic fibres controlling the locations of their terminals in the goldfish optic tectum

Removal of the caudal half of a goldfish optic tectum induces optic fibres from the entire contralateral retina to terminate retinotopically within the remaining half. This compression has been viewed by some as the result of competition between the fibres and by others as a consequence of changes, induced by the surgery, in tectal labels guiding fibres to terminal sites. To distinguish between these possibilities, the time-course of compression has been measured by electrophysiological mapping of the visual projection. In some fish, fibres terminating in the rostral half-tectum remained intact when the caudal half was removed. In others, the optic nerve was cut at the time of tectal surgery: even after its regeneration into a half-tectum, optic terminals were first detected in the regions they normally occupy. The subsequent reorganization was gradual and retinotopic order was maintained. However, it was slower where some fibres had never been cut. In a third series the nerve was cut 18 days before the tectal halving to reveal any dependence of compression on progressive changes in the halved tectum; but its time-course from nerve section was found to be independent of the time within the regeneration period at which the tectum was halved. In a fourth series the nerve was cut at the time of tectal halving and then cut again after 85--97 days when compression was complete to reveal any permanent change in the halved tectum. No change was evident: the previous compression did not preclude subsequent regeneration of an uncompressed projection and its gradual recompression as before. In a fifth series, repeated crushing of fibres normally ending in the missing caudal tectum temporarily prevented compression among the remainder, while crushing of fibres destined for rostral tectum caused transposition of the remaining projection to the rostral half. Surgically induced changes in the labels which are thought to guide growing fibres to their normal tectal regions do not account for these results. Indeed, this guidance persists unchanged for fibres regenerating a second time after compression. Since compression is delayed while certain fibres are withheld, it appears instead to be the direct result of competition between the fibres. The maintenance of retinotopic order in compression, despite unchanged tectal guidance, may require selective interactions between fibres from different retinal regions which could contribute to the refinement of the normal visual projection.

Behavioral and electrophysiological effects of unilateral optic tract section in ordinary and siamese cats

In ordinary cats, section of one optic tract produced a complete contralateral hemianopsia in both eyes. Single-unit recordings showed a normal representation of the contralateral nasal retina and ipsilateral temporal retina in the SC on the side of the intact optic tract. In addition, in the rostral portion of this SC there was a representation of a small portion of the contralateral temporal retina. This portion was apposed to the vertical meridian and its width was at most 6 degrees. In the anterior half of the SC on the side of the optic tract section, despite the interruption of any direct optic input, there was an extensive representation of the ipsilateral nasal retina and the contralateral temporal retina. This indirect visual input to the SC ipsilateral to the optic tract section was absent in a cat with a section of the forebrain commissures. In Boston Siamese cats, section of one optic tract led to a virtually complete blindness in the eye contralateral to the section, whereas the other eye retained a full visual field, although the responsiveness of the temporal retina beyond 20 degrees from the vertical meridian was reduced. Similarly, the nasal hemiretina and most of the temporal hemiretina on the side of the section were represented in the opposite SC, whereas stimulation of the eye contralateral to the section could not drive SC units. There was some evidence that the visual field of the eye on the side of the section could at least in part be represented in the SC on the same side. The findings indicate that the crossed projections from temporal hemiretina in the ordinary cat, and the uncrossed projections from temporal hemiretina in the Siamese cat are insufficient by themselves to sustain visual orientation and to drive SC neurons. Each half of the visual field in the ordinary cat, and the field of each eye in the Siamese cat, can be represented in the ipsilateral SC via across-the-midline, indirect connections.

Changes in the uncrossed retinotectal projection after removal of the other eye at birth

THE connections between the eyes and the brain are so arranged as to give an orderly representation in the brain, of the visual field. The mechanisms involved in this ordering have been extensively studied, particularly in the retinotectal pathway of submammalian vertebrates1–3. In these species the optic nerves decussate almost completely, retinal ganglion cells in one eye connecting only with the opposite tectum. In mammals, however, there is partial decussation at the optic chiasma, so that the superior colliculus receives optic fibres from both eyes. Binocular representation poses a problem for the ordering of retinotectal projections. If one point on the tectum is to correspond to only one point in visual space, seen through both eyes, the mapping rules onto the tectum must have opposing polarities in the two eyes, along the nasotemporal axis. This is shown in Fig. 1 : a rostral movement in the colliculus corresponds to a temporal movement on the contralateral retina but a nasal movement on the ipsilateral retina. Little is known about the factors determining the topography of the direct ipsilateral projection to the mammalian superior colliculus. Anatomical studies in rodents have shown that neonatal removal of one eye induces an increased projection from the remaining eye to the ipsilateral colliculus4,5. These aberrant uncrossed optic fibres are derived from all regions of the retina and apparently have a distribution appropriate for a normal contralateral projection, that is, the nasotemporal retinal axis is represented caudorostrally6. The only published physiological study investigating the topography (in enucleated rats) confirmed the anatomy as regards the nasotemporal axis but, surprisingly, found the dorsoventral axis reversed over most of the colliculus ipsilateral to the remaining eye7. Here I present results on the enucleated hamster showing that the colliculus ipsilateral to the remaining eye contains a double representation of the visual field displaying mirror-image polarity along the nasotemporal axis.

Specific projection of displaced retinal ganglion cells upon the accessory optic system in the pigeon (Columbia livia).

In the pigeon, the nucleus of the basal optic root, a component of the accessory optic system, projects directly upon the vestibulo-cerebellum. This nucleus receives a prominent projection composed of large-diameter retinal axons, known as the basal optic root. The cells of origin of this tract were identified using horseradish peroxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7) as a retrograde marker. Injections of horseradish peroxidase confined primarily to the basal optic root nucleus labeled displaced ganglion cells of the contralateral retina. Cell sizes were 18-30 micronm and the dendrites of these cells were confined to the first stratum of the inner plexiform layer. Approximately 3700 displaced ganglion cells were labeled after injections of horseradish peroxidase into basal optic root. In contrast, no displaced ganglion cells were labeled after injections of horseradish peroxidase into the optic tectum, which labeled only cells in the ganglion cell layer proper. These findings indicate that displaced ganglion cells constitute a unique population of retinal neurons that give rise to a bisynaptic pathway directed to the cerebellum via the nucleus of the basal optic root. These displaced ganglion cells may play a major role inoculomotor reflexes.

Specific patterns of neuronal connexions involved in the control of the rabbit's vestibulo‐ocular reflexes by the cerebellar flocculus.

1. In anaesthetized albino rabbits, the occurrence of Purkinje cell inhibition on canal-ocular reflexes was surveyed with a reflex testing method. 2. Test reflexes were elicited by electrical stimulation of the semicircular canals. The results were appaised by recording potentials and tension from extraocular muscles. Twelve reflexes were defined in terms of the receptor canal and the effector muscle. 3. Conditioning electrical stimuli were applied to the flocculus, the inferior olive, and optic pathways at the retinae, optic chiasm, pretectal area and upper medulla. 4. The conditioning stimulation at the ipsilateral flocculus induced depression in six of the twelve canal-ocular reflexes; four of the six arose from the anterior canal and the remaining two from the horizontal canal. 5. The effect of stimulation of the contralateral inferior olive was similar to that of the ipsilateral flocculus, though less clear in two of the four reflexes from the anterior canal because of a contaminating effect. 6. The two reflexes from the horizontal canal were depressed by stimulation of the ipsilateral optic pathway which reached the ipsilateral flocculus via the contralateral pretectal area and inferior olive. 7. The four reflexes from the anterior canal were affected by stimulation of optic pathways in a different manner from each other. One was depressed from the contralateral retina via the ipsilateral pretectal area, while another was depressed from the ipsilateral retina via the contralateral pretectal area, though only occasionally. The third reflex was depressed from the ipsilateral pretectal area but not from the retina. The fourth was affected from neither the retina nor the pretectal area. 8. On the basis of latency measurements, it was concluded that the depression of canal-ocular reflexes was due to inhibition of relay neurones of the testing reflexes by flocculus Purkinje cells which were activated either directly, or indirectly through olivocerebellar climbing fibre afferents. 9. The above conclusion was supported by the observation that the depression induced by stimulation of the inferior olive and optic pathways was abolished by acute destruction of the ipsilateral flocculus. 10. The possible functional significance of the specific patterns of connexions from flocculus Purkinje cells to canal-ocular reflex pathways is discussed, and specialization among flocculus Purkinje cells in relationship with vestibulo-ocular reflexes is postulated.

Effects of laser photocoagulation on adenine nucleotides in rabbit retinas.

High-pressure liquid chromatography was used to provide a qualitative biochemical assessment of laser effects on retinal metabolism. The adenosine triphosphate-adenosine diphosphate ratio and the Atkinson energy charge are considerably increased in laser-treated rabbit retinas compared to the contralateral untreated retinas.

Cortical suppression of the ritino-collicular pathway in the monocularly deprived cat.

1. In nine cats monocularly deprived from birth, the responses of single neurones in the superior colliculus contralateral to the deprived eye were studied. 2. In six animals most units could be driven by visual stimuli only through the ipsilateral (experienced) eye despite the fact that this colliculus receives a major input from the contralateral (deprived) retina. 3. Immediately following removal of visual cortex, including areas 17, 18 and 19, the collicular units could be driven by the deprived eye. 4. We conclude that the cortex must exert a powerful suppression of the retino-collicular input, and we argue that this suppression occurs in normal as well as in monocularly deprived animals. 5. In three animals the retinal input from the deprived eye was not suppressed but instead dominated many collicular celld, apparently to the exclusion of the cortical input from the experienced eye.

Ectopic neurons and aberrant connections during neural development.

During the normal development of the nucleus of origin of centrifugal fibers to the chick retina (the so-called isthmo-optic nucleus) a significant number of neurons are misrouted during their migration from the neural epithelium in which they are generated. Like the cells in the isthmo-optic nucleus, these ectopic neurons can be identified by the retrograde transport of horseradish peroxidase (donor: hydrogen peroxide oxidoreductase, EC 1.11.1.7) injected into the eye. Between 500 and 600 such cells have been identified on one side of the brain of 12 to 15-day-old embryos, but only about 10% of this number is seen beyond hatching. In addition, during development some of the cells in the nucleus send their axons to the ipsilateral retina, whereas in post-hatched animals they all project to the contralateral retina. The number of such neurons with aberrant axons is greatly increased (in some cases by as much as two orders of magnitude) in chicks from which one eye was removed early in development. Although normally about 60% of the neurons in the isthmo-optic nucleus degenerate between the 13th and 17th days of incubation, they can almost all be labeled by intraocular injections of horseradish peroxidase on the 12th day, indicating that the axons of virtually all the cells, including those neurone that subsequently die, reach the retina.

Identification of corticotectal cells of the visual cortex of cats by means of horseradish peroxidase

Two primary sources of visual input to superior colliculus (SC) are known to be the contralateral retina and the ipsilateral visual cortex 14. Visual cortical areas comprising areas 17, 18, 19 and the Clare-Bishop area give rise to corticotectal pathways which terminate in the superficial laminae of SC 4,14. Corticotectal projections from area 17, the best studied of them all, display a topographical organization 4,1°,11 and play a crucial role in determining certain receptive field properties of the collicular cells to which they project 17. Thus, removal of area 17 leads to a marked decrease in the number of cells which are binocularly driven; to a marked reduction in the percentage of neurons showing directional selectivity to moving stimuli, and to an increase in the responsiveness of many neurons to stationary stimuli turned on or of~14,17.

The primary optic projections in pigmented and albino guinea pigs: an experimental degeneration study

Summary An eye was enucleated in each of 6 pigmented and 6 albino guinea pigs. Six to 8 days later the animals were killed and their brains were prepared by the Nauta and Nissl methods in order to study the course and terminal distribution of the degenerating optic fibers. Several major differences were demonstrated between the pigmented and albino guinea pigs in the terminal distribution of the crossed and uncrossed optic fibers. First, it was shown that in the pigmented animals the ventral and dorsal lateral geniculate nuclei, the nucleus tractus opticus, the nuclei praetectalis anterior and posterior, and the superior colliculus all receive inputs from both retinae; whereas in the albino animals these same nuclei could be found to receive organized inputs only from the contralateral retina. Second, it was found that the pattern of the contralateral retinal input to the ventral and dorsal lateral geniculate nuclei, but not to the pretectal nuclei, the superior colliculus and the terminal accessory optic nuclei, differs between the two strains of animals. In both the pigmented and albino guinea pigs it was shown that the accessory optic system is composed entirely of crossed optic fibers and consists of two fiber groups (inferior and superior fasciculi) and 3 terminal nuclei (dorsal, lateral and medial terminal nuclei). It was also shown that in both strains of animals the degenerating optic fibers that lie in and adjacent to the ventral and dorsal lateral geniculate nuclei and the pretectal nuclei course within two distinct fiber systems oriented perpendicularly to each other.

Centrifugal control of the avian retina. III. Effects of electrical stimulation of the isthmo-optic tract on the receptive field properties of retinal ganglion cells

Single unit recording techniques were used in decerebrate, immobilized domestic chicks to investigate the effects of electrical stimulation of the peripheral end of the severed isthmo-optic tract on the receptive field properties of ganglion cells in the contralateral retina. Tract stimulation always worked to enhance ganglion cell visual responses and was never observed to depress them. However, activation of the tract did not itself generate ganglion cell firing and centrifugal effects were only apparent when the retinal cells were receiving visual stimulation. Visual stimuli such as large, centred spots which were normally ineffective in driving ganglion cells because they enroached on the inhibitory surround, often produced activity when combined with tract stimuli. Experiments with spots and annuli suggest that this centrifugal enhancement probably results from suppression of the inhibitory surround mechanisms (disinhibition) since in such cases tract stimuli were only effective when paired with both spot and annulus, and not with either target alone. Occasionally, the centrifugal input seemed to increase ganglion cell excitability through a direct effect on central excitatory mechanisms (facilitation), since in these cases tract stimulation produced considerably increased firing to small, centred spots whilst failing to exert any appreciable effect on responses to either the annulus alone, or the annulus in combination with the spot. The possible functional significance of these findings is discussed.

Centrifugal control of the avian retina. IV. Effects of reversible cold block of the isthmo-optic tract on the receptive field properties of cells in the retina and isthmo-optic nucleus

Single unit recording techniques were used in decerebrate, immobilised domestic chicks, to investigate the effects of reversible cold block of the isthmo-optic tract on the receptive field properties of cells in the isthmo-optic nucleus and contralateral retina. Tract cooling had no clear-cut effects on the visual responses of either retinal, or isthmo-optic, units to a variety of moving targets or to stationary spots of light of various sizes turned on and off in their receptive field centres. These negative findings are tentatively taken to suggest that the centrifugal system is effectively decoupled for many targets, though there may be other kinds of visual stimuli (excluded here for technical reasons) whose retinal responses normally do incur centrifugal modification; indeed, at least one functional suggestion is made in this regard. In 4 retinal units out of 25 successfully tested, the firing to large spots turned on in the receptive field was much augmented after the recent passage of a dark edge moving forwards through the visual field, an effect which was abolished by cooling the isthmo-optic tract. It is suggested that this effect is mediated through the centrifugal system and may have functional significance in aiding the animal's normal visual search in shadowed areas. A general functional hypothesis is advanced, based upon the findings in the present series of experiments and the literature relating to other systems, proposing that some centrifugal systems may have evolved specifically to deal with the sensory consequences of the animal's own movements. It is suggested that in some cases this might operate merely to minimise the disruption of sensory processing due to the animal's movements; in others, the centrifugal contribution might be much more elaborate, facilitating some positive aspect of sensory exploration, and hence incorporated as feedforward in the animal's normal search strategies.

Effect on the superior colliculus of cortical removal in visually deprived cats.

ONE of the principal goals of analyses of the processing of sensory information has been an understanding of the receptive field properties of central neurones in terms of the convergence of afferents from more peripheral levels. Although it is well known that most cell groups receiving peripheral input also receive inputs descending from higher centres, there have been relatively few studies of descending effects on the organization of the receptive field1. We are concerned here with the superficial layers of the superior colliculus which receive optic nerve fibres, chiefly from the contralateral retina, and descending input from the ipsilateral visual cortex2. Most collicular cells in the normal cat are binocularly driven by slits, bars or edges and are directiorially selective, that is they respond much better to movement of a visual stimulus in one direction than to movement in the opposite direction3. After the visual cortex is removed, collicular cells respond to the same types of visual stimuli, but most cells are driven only by the contralateral eye. The cells also lose their directional selectivity and respond equally well to all directions of stimulus movement4. It therefore seems clear that some features of collicular cells depend on the cortex. The work described here with visually deprived animals demonstrates in another way the influence of the cortex on collicular cells.

The primary optic projections in the rabbit. An experimental degeneration study.

AbstractThe brains of ten rabbits (4 pigmented and 6 albino) sacrificed 6–21 days after unilateral ocular enucleation were prepared by Nauta and Nissl methods and used to study the projection and terminal destribution of degenerating optic axons. The retina is bilaterally represented in the alpha sectors of both ventral and dorsal lateral geniculate nuclei (LGv and LGd), in n. tractus opticus (NTO), in the dorsolateral portions of nn. praetectalis anterior (NPA) and posterior (NPP), and in the superior colliculus (SC). The retina is unilaterally represented in the contralateral terminal accessory optic nuclei.The mode of termination of crossed and uncrossed retinal projections endows the LGd with a concealed laminar organization. Four laminae are recognized and numbered consecutively from lateral to medial. Lamina one is extensive and formed entirely of crossed retinal input. The remaining three laminae occupy the medial zone of the LGd. Of these, laminae two and four are innervated predommantly by the ipsilateral retina while lamina three is supplied mainly by the contralateral retina. Uncrossed retinal innervations of LGv, NTO, NPA, NPP and SC are scant and overlapped entirely by crossed retinal inputs. Chiasmal decussation of the optic nerve is 90–95% complete with the uncrossed component, and specifically the uncrossed retino‐LGd and ‐SC projections, being larger in pigmented as compared with albino rabbits.