Regulation of retinal ganglion cell axon arbor size by target availability: mechanisms of compression and expansion of the retinotectal projection

A Guide for the Multiplexed: The Development of Visual Feature Maps in the Brain

Neural maps are found ubiquitously in the brain, where they encode a wide range of behaviourally relevant features into neural space. Developmental studies have shown that animals devote a great deal of resources to establish consistently patterned organization in neural circuits throughout the nervous system, but what purposes maps serve beneath their often intricate appearance and composition is a topic of active debate and exploration. In this article, we review the general mechanisms of map formation, with a focus on the visual system, and then survey notable organizational properties of neural maps: the multiplexing of feature representations through a nested architecture, the interspersing of fine-scale heterogeneity within a globally smooth organization, and the complex integration at the microcircuit level that enables a high dimensionality of information encoding. Finally, we discuss the roles of maps in cortical functions, including input segregation, feature extraction and routing of circuit outputs for higher order processing, as well as the evolutionary basis for the properties we observe in neural maps.

The Impact of Ecological Niche on Adaptive Flexibility of Sensory Circuitry

Evolution and development are interdependent, particularly with regard to the construction of the nervous system and its position as the machine that produces behavior. On the one hand, the processes directing development and plasticity of the brain provide avenues through which natural selection can sculpt neural cell fate and connectivity, and on the other hand, they are themselves subject to selection pressure. For example, mutations that produce heritable perturbations in neuronal birth and death rates, transcription factor expression, or availability of axon guidance factors within sensory pathways can markedly affect the development of form and thus the function of stimulus decoding circuitry. This evolvability of flexible circuits makes them more adaptable to environmental variation. Although there is general agreement on this point, whether the sensitivity of circuits to environmental influence and the mechanisms underlying development and plasticity of sensory pathways are similar across species from different ecological niches has received almost no attention. Neural circuits are generally more sensitive to environmental influences during an early critical period, but not all niches afford the same access to stimuli in early life. Furthermore, depending on predictability of the habitat and ecological niche, sensory coding circuits might be more susceptible to sensory experience in some species than in others. Despite decades of work on understanding the mechanisms underlying critical period plasticity, the importance of ecological niche in visual pathway development has received little attention. Here, I will explore the relationship between critical period plasticity and ecological niche in mammalian sensory pathways.

Development and matching of binocular orientation preference in mouse V1

Eye-specific thalamic inputs converge in the primary visual cortex (V1) and form the basis of binocular vision. For normal binocular perceptions, such as depth and stereopsis, binocularly matched orientation preference between the two eyes is required. A critical period of binocular matching of orientation preference in mice during normal development is reported in literature. Using a reaction diffusion model we present the development of RF and orientation selectivity in mouse V1 and investigate the binocular orientation preference matching during the critical period. At the onset of the critical period the preferred orientations of the modeled cells are mostly mismatched in the two eyes and the mismatch decreases and reaches levels reported in juvenile mouse by the end of the critical period. At the end of critical period 39% of cells in binocular zone in our model cortex is orientation selective. In literature around 40% cortical cells are reported as orientation selective in mouse V1. The starting and the closing time for critical period determine the orientation preference alignment between the two eyes and orientation tuning in cortical cells. The absence of near neighbor interaction among cortical cells during the development of thalamo-cortical wiring causes a salt and pepper organization in the orientation preference map in mice. It also results in much lower % of orientation selective cells in mice as compared to ferrets and cats having organized orientation maps with pinwheels.

Regulation of ephrin-A expression in compressed retinocollicular maps

Retinotopic maps can undergo compression and expansion in response to changes in target size, but the mechanism underlying this compensatory process has remained a mystery. The discovery of ephrins as molecular mediators of Sperry's chemoaffinity process allows a mechanistic approach to this important issue. In Syrian hamsters, neonatal, partial (PT) ablation of posterior superior colliculus (SC) leads to compression of the retinotopic map, independent of neural activity. Graded, repulsive EphA receptor/ephrin-A ligand interactions direct the formation of the retinocollicular map, but whether ephrins might also be involved in map compression is unknown. To examine whether map compression might be directed by changes in the ephrin expression pattern, we compared ephrin-A2 and ephrin-A5 mRNA expression between normal SC and PT SC using in situ hybridization and quantitative real-time PCR. We found that ephrin-A ligand expression in the compressed maps was low anteriorly and high posteriorly, as in normal animals. Consistent with our hypothesis, the steepness of the ephrin gradient increased in the lesioned colliculi. Interestingly, overall levels of ephrin-A2 and -A5 expression declined immediately after neonatal target damage, perhaps promoting axon outgrowth. These data establish a correlation between changes in ephrin-A gradients and map compression, and suggest that ephrin-A expression gradients may be regulated by target size. This in turn could lead to compression of the retinocollicular map onto the reduced target. These findings have important implications for mechanisms of recovery from traumatic brain injury.

A reaction-diffusion model to capture disparity selectivity in primary visual cortex

Decades of experimental studies are available on disparity selective cells in visual cortex of macaque and cat. Recently, local disparity map for iso-orientation sites for near-vertical edge preference is reported in area 18 of cat visual cortex. No experiment is yet reported on complete disparity map in V1. Disparity map for layer IV in V1 can provide insight into how disparity selective complex cell receptive field is organized from simple cell subunits. Though substantial amounts of experimental data on disparity selective cells is available, no model on receptive field development of such cells or disparity map development exists in literature. We model disparity selectivity in layer IV of cat V1 using a reaction-diffusion two-eye paradigm. In this model, the wiring between LGN and cortical layer IV is determined by resource an LGN cell has for supporting connections to cortical cells and competition for target space in layer IV. While competing for target space, the same type of LGN cells, irrespective of whether it belongs to left-eye-specific or right-eye-specific LGN layer, cooperate with each other while trying to push off the other type. Our model captures realistic 2D disparity selective simple cell receptive fields, their response properties and disparity map along with orientation and ocular dominance maps. There is lack of correlation between ocular dominance and disparity selectivity at the cell population level. At the map level, disparity selectivity topography is not random but weakly clustered for similar preferred disparities. This is similar to the experimental result reported for macaque. The details of weakly clustered disparity selectivity map in V1 indicate two types of complex cell receptive field organization.

Large developing receptive fields using a distributed and locally reprogrammable address-event receiver

A distributed and locally reprogrammable address-event receiver has been designed, in which incoming address-events are monitored simultaneously by all synapses, allowing for arbitrarily large axonal fan-out without reducing channel capacity. Synapses can change the address of their presynaptic neuron, allowing the distributed implementation of a biologically realistic learning rule, with both synapse formation and elimination (synaptic rewiring). Probabilistic synapse formation leads to topographic map development, made possible by a cross-chip current-mode calculation of Euclidean distance. As well as synaptic plasticity in rewiring, synapses change weights using a competitive Hebbian learning rule (spike-timing-dependent plasticity). The weight plasticity allows receptive fields to be modified based on spatio-temporal correlations in the inputs, and the rewiring plasticity allows these modifications to become embedded in the network topology.

Early visual experience prevents but cannot reverse deprivation-induced loss of refinement in adult superior colliculus

The role of sensory experience in the development and plasticity of the visual system has been widely studied. It has generally been reported that once animals reach adulthood, experience-dependent visual plasticity is reduced. We have found that visual experience is not needed for the refinement of receptive fields (RFs) in the superior colliculus (SC) but instead is necessary to maintain them in adulthood (Carrasco et al., 2005). Without light exposure, RFs in SC of hamsters refine by postnatal day 60 as usual but then enlarge, presumably reducing visual acuity. In this study we examine whether a brief period of light exposure during early postnatal development would be sufficient to prevent RF enlargement in adulthood, and whether prolonged light exposure in adulthood could reverse the deprivation-induced increase in RF size. We found that an early postnatal period of at least 30 days of visual experience was sufficient to maintain refined RFs in the adult SC. Prolonged visual experience in adulthood could not reverse the RF enlargement resulting from long-term dark rearing, reflecting a loss of plasticity at this age. Our results suggest that, unlike in visual cortex, dark rearing does not indefinitely extend the critical period of plasticity in SC. Rather, there is a limited time window when early experience can protect RFs from the detrimental effects of visual deprivation in adulthood. These results contribute to understanding adult brain plasticity and argue for the importance of early visual experience in protecting the adult visual system.

Compensatory and transneuronal plasticity after early collicular ablation

Plasticity within the visual system was assessed in the quokka wallaby following unilateral superior collicular (SC) ablation at postnatal days (P) 8-10, prior to the arrival of retinal ganglion cell (RGC) axons. At maturity (P100), projections were traced from the eye opposite the ablation, and total RGC numbers were estimated for both eyes. Ablations were partial (28-89% of SC remaining) or complete (0-5% of SC remaining). Projections to the visual centers showed significant bilateral (P < 0.05) increases in absolute volume. Minor anomalous projections also formed within the deep, surviving non-retino-recipient layers of the ablated SC and via a small bundle of RGC axons recrossing the midline to innervate discrete patches in the SC contralateral to the lesion. Total absolute volume of projections did not differ between partial and complete ablations; moreover, values did not differ from normal (P > 0.05). Compared with normal, total RGC numbers were significantly (P < 0.05) reduced in the eye opposite the ablation but increased (P < 0.05) in the other eye. Consequently, the sum of the two RGC populations did not differ from normal (P > 0.05). As in rodents, the visual system in quokka compensates following injury by maintaining a set volume of arborization but does so by forming only minor anomalous projections. Furthermore, increased RGC numbers in the eye ipsilateral to the lesion indicate that compensation occurs transneuronally, thus maintaining total numbers of projecting neurons. The implication is that the visual system acts in concert following unilateral injury to maintain set values for RGC terminal arbors as well as their cell bodies.

Receptive field properties of near neighbor orientation selective neurons in the visual cortex: a modeling study

The primary visual cortex is organized into clusters of cells having similar receptive fields (RFs). A purely feedforward model has been shown to produce realistic simple cell receptive fields. The modeled cells capture a wide range of receptive field properties of orientation selective cortical cells. We have analyzed the responses of 78 nearby cell pairs to study which RF properties are clustered. Orientation preference shows strongest clustering. Orientation tuning width (hwhh) and tuning height (spikes/sec) at the preferred orientation are not as tightly clustered. Spatial frequency is also not as tightly clustered and RF phase has the least clustering. Clustering property of orientation preference, orientation tuning height and width depend on the location of cells in the orientation map. No such location dependence is observed for spatial frequency and RF phase. Our results agree well with experimental data.

NMDA receptor blockade in the superior colliculus increases receptive field size without altering velocity and size tuning

Neonatal brain injury triggers compensatory processes that can be adaptive or detrimental, but little is known about the mechanisms of compensation or how they might affect the response properties of neurons within the injured region. We have studied this issue in a rodent model. Partial ablation of the hamster superior colliculus (SC) at birth results in a compressed but complete visual field map in the remaining SC and a compensatory conservation of receptive field (RF) size and stimulus velocity and size tuning. The circuit underlying stimulus tuning in this system or its preservation after brain lesions is not known. Our previous work has shown that N-methyl-d-aspartate (NMDA) receptors are necessary for the development and conservation of RF size after partial SC ablation. In this study, we examined whether NMDA receptor function is also necessary for the development and conservation of stimulus velocity and size tuning. We found that velocity and size tuning were unaffected by chronic postnatal blockade of NMDA receptors and the resulting increases in RF size. Thus NMDA receptors in the SC are not necessary for the development of stimulus velocity and size tuning or in the compensatory maintenance of these properties following brain damage. These results suggest that stimulus velocity and size tuning may arise in the retina or from NMDA receptor-independent circuitry intrinsic to SC. The lack of conflict between NMDA receptor activity-dependent and -independent processes may allow conservation of some RF properties while others change during injury-induced or evolutionary changes in afferent/target convergence.

A cooperation and competition based simple cell receptive field model and study of feed-forward linear and nonlinear contributions to orientation selectivity

We present a model for development of orientation selectivity in layer IV simple cells. Receptive field (RF) development in the model, is determined by diffusive cooperation and resource limited competition guided axonal growth and retraction in geniculocortical pathway. The simulated cortical RFs resemble experimental RFs. The receptive field model is incorporated in a three-layer visual pathway model consisting of retina, LGN and cortex. We have studied the effect of activity dependent synaptic scaling on orientation tuning of cortical cells. The mean value of hwhh (half width at half the height of maximum response) in simulated cortical cells is 58 degrees when we consider only the linear excitatory contribution from LGN. We observe a mean improvement of 22.8 degrees in tuning response due to the non-linear spiking mechanisms that include effects of threshold voltage and synaptic scaling factor.

NMDA antagonists in the superior colliculus prevent developmental plasticity but not visual transmission or map compression

Partial ablation of the superior colliculus (SC) at birth in hamsters compresses the retinocollicular map, increasing the amount of visual field represented at each SC location. Receptive field sizes of single SC neurons are maintained, however, preserving receptive field properties in the prelesion condition. The mechanism that allows single SC neurons to restrict the number of convergent retinal inputs and thus compensate for induced brain damage is unknown. In this study, we examined the role of N-methyl-D-aspartate (NMDA) receptors in controlling retinocollicular convergence. We found that chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further enlargement in lesioned animals with compressed maps. The effect was linearly related to lesion size. These results suggest that NMDA receptors are necessary to control afferent/target convergence in the normal SC and to compensate for excess retinal afferents in lesioned animals. Despite the alteration in receptive field size in the APV-treated animals, a complete visual map was present in both normal and lesioned hamsters. Visual responsiveness in the treated SC was normal; thus the loss of compensatory plasticity was not due to reduced visual responsiveness. Our results argue that NMDA receptors are essential for map refinement, construction of receptive fields, and compensation for damage but not overall map compression. The results are consistent with a role for the NMDA receptor as a coincidence detector with a threshold, providing visual neurons with the ability to calculate the amount of visual space represented by competing retinal inputs through the absolute amount of coincidence in their firing patterns. This mechanism of population matching is likely to be of general importance during nervous system development.

Depletion of cortical target induced by prenatal ionizing irradiation: effects on the lateral geniculate nucleus and on the retinofugal pathways

Studies using neonatal surgical lesions to reduce the target area of the retina have supported the idea that developing axons show only a limited specificity in their targeting. This investigation tested whether retinogeniculate axons adjust for partial target depletion by repositioning of axons. We used adult Swiss mice exposed to gamma rays at the time when layer IV cells are generated in the ventricular zone (16 days of gestation). Nissl-stained brain sections were used for histological analyses in thalamus and cortex. Retinal ganglion cells were backfilled from the optic tract with horseradish peroxidase. Intraocular injections of horseradish peroxidase were used to study the retinal projections. In the posterior cortex there was a nearly complete absence of layer IV. The irradiated animals showed a 75% reduction of the dorsal lateral geniculate nucleus. The ventral division, superior colliculus, and other visually related nuclei were not affected. The loss in the ganglion cells (15.7%) was significant but clearly smaller than that observed in the dorsal lateral geniculate nucleus (75%). Therefore, the shrinkage of the dorsal lateral geniculate nucleus led to a reduction in the area available for retinal projections. Despite partial target loss, pattern of retinal projections did not differ from that of the controls. The effect on the dorsal lateral geniculate nucleus is discussed in the light of differences between prenatal and neonatal damage of the presumptive visual cortex. The absence of aberrant retinal projections suggests that repositioning of axons is not the first mechanism employed by retinal axons to match connections in numerically disparate populations.

Restoration of the retinofugal pathway

In a relatively short period of time covering the last 2 decades, regeneration of retinofugal axons has become one of most prominent experimental models in restorative neurobiology. There is now a significant knowledge both on the mechanisms governing retinal ganglion cell responses to transection of the optic nerve, and the subsequent cell-cell interactions accumulating in death of the neurons. In addition, retinofugal axons served as an excellent model to examine whether, and to conclude that these axons have remarkable abilities for re-growth. This last issue was of invaluable importance, because axons could regenerate in vivo, into peripheral nerve grafts, and last but not least within the white matter of the cut optic nerve. As it stands to date, the extremely complex aspects of axonal regeneration will probably be understood within the retinofugal pathway. Final elucidation of this delicate system will essentially lead to some revision of our knowledge concerning neurotraumatology and CNS-repair.

Development of the Xenopus laevis VIIIth cranial nerve: increase in number and area of axons of the saccular and papillar branches

Development of three branches of the VIIIth cranial nerve was examined in the anuran, Xenopus laevis. Sectioned tissue from the saccular, amphibian papillar, and basilar papillar branches of stage 52 larvae, 1 day postmetamorphosis juveniles, and 2-year adult animals was analyzed under the light microscope with a digital image analysis system. Numbers and cross-sectional areas of myelinated axons were measured in five to six nerve sections at each developmental age for each of the three branches. In all three branches, results show a significant increase in axon numbers between larval stage 52 and juvenile ages and negligible increase in axon number between the juvenile and adult ages. There were differences in the average number of axons between the saccular (704.4 +/- 39.5; n = 5), amphibian papillar (508.4 +/- 35.0; n = 5), and basilar papillar (316.0 +/- 7.0; n = 5) branches of adult animals. Myelinated axons increase at an estimated rate of 11.7, 15.1, and 6.2 axons per day for the saccular, amphibian papillar, and basilar papillar branches, respectively. Axonal cross-sectional areas increased throughout the developmental ages of this study, with the greatest increase taking place between juvenile and adult ages. In adult animals, 98% of axons in all three branches have diameters between 2-10 microns. Ratios of axons to hair cells in adult animals were estimated at 0.3, 1.1, and 5.3 for the sacculus, amphibian papilla, and basilar papilla, respectively. The higher axon to hair cell ratio correlates with the increasing acoustical frequency sensitivity of the end organ.

Factors controlling the dendritic arborization of retinal ganglion cells

The effects of changing retinal ganglion cell (RGC) density and availability of presynaptic sites on the development of RGC dendritic arbor in the developing chick retina were contrasted. Visual form deprivation was used to induce ocular enlargement and expanded retinal area resulting in a 20-30% decrease in RGC density. In these retinas, RGC dendritic arbors increased in a compensatory manner to maintain the inner nuclear layer to RGC convergence ratio in a way that is consistent with simple stretching; RGC dendritic arbors become larger with increased branch lengths, but without change in the total number of branches. In the second manipulation, partial optic nerve section was used to produce areas of RGC depletion of approximately 60% in the central retina. This reduction in density is comparable to the density of locations in the normal peripheral retina. In RGC depleted retinas, dendritic arbor areas of RGCs in the central retina grow to match the size of normal peripheral arbors. In contrast to the expanded case, two measures of intrinsic arbor structure are changed in RGC-depleted retinas; the branch density of RGC dendrites is greater, and the relative areas of the two arbors of bistratified cells are altered. We discuss the potential roles of retinal growth, local RGC density, and availability of presynaptic terminals in the developmental control of RGC dendritic arbor.

What do developmental mapping rules optimize?

Convergence ratios between pre- and postsynaptic cells in the visual system vary widely between cell classes, areas of the visual field, between individuals and between species. Proper stabilization of the convergence and divergence of single visual neurons is critical for visual integration generally, and for specific functions such as those of rod and cone pathways, or the center and peripheral regions of the visual field. In early development, retinal ganglion cells, target cells and all their processes are produced in excess and stabilize at certain mature values. The intent of the investigations described here is to determine what features of cell connectivity are stabilized over normal variability by these developmental processes and how such stabilization is accomplished, using the developing mammalian retinotectal system as an example. Orderly compression of the retinotopic map into a half tectum was induced by a partial tectal ablation at birth in hamsters, increasing the ratio of retinal ganglion cells to superior colliculus target cells. The convergence problem is solved in this case by undersampling the spatial array with respect to normal, preserving local spatial resolution, but potentially reducing sensitivity or introducing aliasing artifacts. Receptive field sizes of single neurons are indistinguishable from normal, and reduction of branching of presynaptic axon arbors is the mechanism of the remapping. Behaviorally, though the entire visual field is still represented in the remaining colliculus, the solution has a cost in decreased probability and increased latency to orient to visual stimuli, particularly in the peripheral visual field. The generality of this solution for retinal and other central convergence regulation problems is evaluated.

Changes in retinal arbors in compressed projections to half tecta in goldfish

In adult goldfish, electrophysiological studies have shown that the retinotectal projection reorganizes, following removal of half of the tectum, to form a complete but compressed projection over the remaining half tectum. As a result, each fiber terminates more rostrally than normal. Electron microscopic studies suggest a competition between retinal fibers for a fixed number of synaptic sites. The current study examines whether retinal arbors in the compressed projection are smaller than normal in extent or branching and whether the fiber paths in the tectum show the rostral movements and the search strategy that the retinal fibers use. The caudal half tectum was removed without cutting retinal fibers except those at the cut edge. At 3 to 19 months afterward, retinal fibers were labeled with horseradish peroxidase. In whole-mounted tecta, fibers and terminals were drawn under camera lucida and compared with normal arbors. The axonal paths were also traced across the tectum to their termination sites. At 3 to 6 months (early stages of compression), the arbors were rather normal in appearance, although they were actually significantly larger (23%) than normal in linear extent, arborized somewhat deeper and had fewer branches (18%). The fibers normally terminating in the rostral tectum followed normal stereotyped paths, whereas those cut at the edge had grown back and forth loops (apparent searching behavior) with little branching. By 10 months when compression is complete, arbors were significantly smaller than normal (19%), were arborizing significantly deeper, and had significantly fewer branches (19%). The differences were more pronounced in arbors of coarse and medium caliber than in fine caliber axons. The axons still ran in stereotyped fascicles, but included an extrafascicular portion that, unlike any axons in normals, turned back in a rostral direction before branching. This striking effect, present even in far rostral tectum, indicated that arbors had been forced to move rostrally to accomodate those from the ablated half. The small effect on arbor extent suggests that this is influenced by factors other than the magnification factor of the map, perhaps postsynaptic dendritic extent. The increased depth of termination is consistent with the increased thickness of the retinal terminal layer. The decreased number of branches is consistent with the conclusion that the remaining fixed number of synaptic sites shared among the full complement of retinal fibers should result in fewer synapses per retinal fiber.

Postnatal changes in the uncrossed retinal projection of pigmented and albino Syrian hamsters and the effects of monocular enucleation

Anterograde and retrograde tracing techniques have been used to study the uncrossed retinal projection in neonatal pigmented and albino Syrian hamsters. The total number of retinal ganglion cells projecting ipsilaterally peaks at postnatal days 2-4 (P2-P4) and declines to adult values by P12. The change in cell numbers has a similar time course in albino and pigmented animals. Although the population of uncrossed cells in the temporal retina of albino hamsters is always less than that in pigmented hamsters, no difference between the colour phases was found for the population of uncrossed cells in nasal retina. Differential cell death also contributes to the adult albino decussation pattern in hamsters: The relative loss of cells from temporal retina in albinos (72%) is greater than that in pigmented animals (56%). The additional loss in albinos does not appear to depend on binocular interactions: The same proportion (30%) of uncrossed cells is "rescued" from death by neonatal monocular enucleation in both colour phases. Flat-mount preparations showing the distribution of uncrossed fibres reveal that a distinct focus of terminals emerges in rostral superior colliculus, which is topographically appropriate for a binocular mapping, at the peak of uncrossed ganglion cell numbers (P4). Comparison of uncrossed terminal distributions and ganglion cell death reveals considerable refinement of the terminals prior to the main phase of cell death. Monocular enucleations performed some time after birth have a greater effect on uncrossed terminal distributions than on cell death. These observations suggest that independent mechanisms may be involved in the regulation of terminal distributions and of cell numbers in the developing uncrossed retinal pathways.

Morphology of retinal axons induced to arborize in a novel target, the medial geniculate nucleus. I. Comparison with arbors in normal targets

Ferret retinal axons can be induced to innervate the medial geniculate nucleus (MGN) by a combination of brain lesions early in development. Our previous work suggests that the retinal ganglion cells responsible for this plasticity are W cells. The present study continues this work with a morphological investigation of normal retinal ganglion-cell axons and retinal ganglion-cell axons induced to arborize in the MGN. Retinal axons were bulk filled with horseradish peroxidase placed in the optic tract, and individual axons were serially reconstructed from sagittal sections. The control population consisted of fine-caliber axons arborizing in the superior colliculus (SC) and in the ventral C laminae of the lateral geniculate nucleus (LGN) of normal ferrets. We also compared the axons in the MGN of lesioned ferrets to intracellularly filled X and Y axons from normal ferrets as reported by Roe et al. ([1989] J. Comp. Neurol. 288:208). We have found that the retino-MGN axons in the lesioned ferrets do not resemble X or Y axons in normal ferrets in axon diameter, arbor volume, bouton number, or bouton density. However, they do resemble the fine-caliber, presumed W axons arborizing in the C laminae of the LGN and in the SC of normal ferrets. Thus, this study, in combination with previous studies, suggests strongly that W retinal ganglion cells are responsible for the retinal input to the MGN in lesioned animals. In addition, we find that the retino-MGN axons are of two types, branched and unbranched, which may correspond to different subtypes of retinal W cells.