Direct and indirect retinal input into degenerated dorsal lateral geniculate nucleus after striate cortical removal in monkey: implications for residual vision

Summary We removed the striate cortex of one cerebral hemisphere in a macaque monkey, causing almost total retrograde degeneration of the corresponding dorsal lateral geniculate nucleus (dLGN) and extensive transneuronal degeneration of ganglion cells in the corresponding hemi-retina of each eye. The rare surviving geniculate projection neurons were retrogradely labelled by horseradish peroxidase (HRP) from extra-striate cortex and retinogeniculate terminals were labelled by an intraocular injection of HRP. Retinal terminals in the degenerated dLGN made synaptic contact exclusively with the dendrites of interneurons immunopositive for -aminobutyric acid (GABA) in both parvocellular and magnocellular regions of dLGN. As well as being postsynaptic to retinal terminals these vescicle-containing dendrites were pre- and postsynaptic to other similar dendrites, and presynaptic to relay cells. Surviving labelled projection neurons received retinal input indirectly, via both the GABA-immunopositive interneurons and GABA-immunonegative terminals characteristic of those from the superior colliculus. In the degenerated, as opposed to the normal dLGN, about 20% of retinal terminals were GABA-immunopositive and GABA-immunoreactivity was prominently elevated in the ganglion and amacrine cell layers of the degenerated half of the retina. The optic nerve also contained numerous GABA-immunopositive axons but very few such axons were found in a normal optic nerve processed in identical manner. The surviving pathways from the retina must underlie the visual abilities that survive striate cortical removal in monkeys and human patients and may involve the degenerated dLGN as well as the mid-brain.     

The effects of unilateral cortical and tectal lesions on retinal ganglion cells in rats

Summary The ganglion cell layer of the retina was examined for retrograde transneuronal degeneration after removing the striate cortex unilaterally in infant or adult rats. No significant degeneration occurred, even after a survival time of 15 months, and the rat is therefore unlike other mammals in which the phenomenon has been studied. A possible explanation that most optic axons bifurcate in rats and that the tectal branch can sustain the ganglion cell after the branch to the dorsal lateral geniculate nucleus has degenerated following removal of striate cortex was ruled out by the demonstration that combined unilateral removal of striate cortex and superior colliculus in adults was similarly ineffective. Unilateral removal of the superior colliculus alone also failed to affect ganglion cells of adult rats but produced conspicuous degeneration in infants. The greater vulnerability of the infantile developing visual system casts doubt on the common assumption that the effects of brain damage are less severe in infants than adults.     

Transneuronal retrograde degeneration of retinal ganglion cells after damage to striate cortex in macaque monkeys: selective loss of P beta cells

We examined the retinae of two monkeys whose left striate cortex had been removed eight years previously and compared the transneuronally degenerated hemiretina of each eye with the normal hemiretina, and with the retinae of normal monkeys. All retinae were prepared as whole mounts. One from each pair was stained with Cresyl Violet; the other was reacted for horseradish peroxidase two days after placing pellets of the enzyme in the optic nerve. Measurements of ganglion cell density in the Nissl-stained retina of the contralateral right eye showed that approximately 80% of retinal ganglion cells were missing in the central 30 degrees of the degenerated hemiretinae. More peripherally the percentage loss was less extensive. Measurements of cell soma size and dendritic field size of peroxidase-labelled classified surviving cells in the degenerated temporal hemiretina of the ipsilateral eye showed them to be morphologically normal. In comparison with the normal hemiretina, however, the mean soma size at three selected eccentricities was larger than normal, suggesting selective loss of smaller ganglion cells. Classification of peroxidase-labelled ganglion cells in the normal and degenerated hemiretinae revealed that the population of P beta cells was reduced by as much as 85% in the degenerated region. There was comparable change in the density of P alpha or P gamma cells. The degeneration of the great majority of P beta cells, which are believed to be the morphological substrate of ganglion cells with small and colour-opponent receptive fields, must set limits on the visual sensitivity and discrimination that survive damage to striate cortex.     

Alterations of retinal inputs following striate cortex removal in adult monkey

Summary The morphology of the retina and central retino-recipient nuclei was studied in two monkeys that had undergone total bilateral striate cortex removal as adults. These animals had been behaviorally tested for two years after lesioning and had demonstrated significant recovery of pattern vision. Light and electron microscopy and autoradiography were done on the central retino-recipient nuclei following a monocular intravitreal injection of ³H-proline.Light microscopic analysis of retinal ganglion cell number showed a 30% loss in the parafoveal retina due to retrograde trans-synaptic degeneration.The most striking central change in retinal axon distribution was in the dorsal lateral geniculate nucleus (dLGN) where the parvocellular but not the magnocellular region showed a marked reduction in retinal input. Despite the loss of almost all dLGN neurons through retrograde degeneration, at the EM level both parvocellular and magnocellular regions contained islands of neuropil made up of retinal and several other types of synaptic terminals as well as small dendrites and pale unidentified processes. Approximately equal numbers of retinal terminals were identified by EM autoradiography in both regions of dLGN, which did not explain the apparent differences in labeling between the two regions seen in the light microscope.A second change in central retinal pathways was found in the olivary pretectal nucleus where a significant loss of retinal input also occurred. A third change, but in the opposite direction, was found in the pregeniculate nucleus (PGN) where the area of retinal terminals appeared enlarged. The remaining central retino-recipient nuclei had the same distribution of retinal input as the control animals.Supported in part by NIH Research Grants EY-01208, EY-01730 and EY-07013, and in part by HD02274     

Contrast sensitivity in rats with increased or decreased numbers of retinal ganglion cells

Summary Contrast thresholds were measured electrophysiologically on striate cortex in normal rats and in rats in which either the superior colliculi were removed bilaterally or unilaterally at 5 days of age, or one eye was removed on the day of birth. Despite the fact that the collicular ablation leads to the degeneration of more than half the retinal ganglion cells, contrast sensitivity was normal in this group, with the possible exception of sensitivity at very low spatial frequencies below 0.1 c/deg. The result is strong evidence that retinal ganglion cells which project to thalamus as well as to mid-brain escape the degenerative effects of neonatal mid-brain lesions. The contrast sensitivity of neonatally operated one-eyed rats was significantly and substantially better than that of normal rats tested monocularly. The increased sensitivity was greatest in the cortex ipsilateral to the remaining eye. This supernormal sensitivity is presumably related to the increase in the number of ganglion cells in the remaining eye, especially those projecting ipsilaterally from the temporal retina and which show a five-fold expansion of their terminal zone in the thalamus.     

Large retinal ganglion cells in the rat: their distribution and laterality of projection

Summary The distributions of the ipsilaterally and contralaterally projecting large ganglion cells in the retina of the rat were determined, using the retrograde transport of Horseradish peroxidase (HRP) following injections into one optic tract. Labelled large retinal ganglion cells occur throughout the contralateral retina and throughout the temporal crescent of the ipsilateral retina, but there is a noticeable decrease in their density in the contralateral retina's temporal crescent. This retinal region was identified in these same retinae by injecting a retrogradely transported flourescent tracer into the optic tract opposite that receiving the HRP. The density of large retinal ganglion cells increases in both the contralateral retina and the ipsilateral temporal crescent in the upper temporal periphery such that, together, these two populations of large cells combine to produce a peak density centred on the retinal representation of the visual field's vertical midline. This peak density of large retinal ganglion cells must therefore be further peripheral than the peak density for the total population of retinal ganglion cells, since all evidence indicates that the latter is positioned nasal to the vertical midline's representation. This was verified in one rat, in which the density distribution of the total population of retinal ganglion cells was determined and compared with the distribution of the large cell population. The results suggest that the rat possesses a specialized retinal focus of large ganglion cells for viewing the visual field directly in front of the animal.     

Cortical and subcortical influences on the nucleus of the optic tract of the opossum

In the present work we propose a new phylogenetic hypothesis for the role played by cortical and subcortical afferents to the nucleus of the optical tract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum (Didelphis aurita) using the following experimental approaches: (i) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the ipsilateral striate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence of the marker in the thalamus and mesencephalon; and (iv) single-unit recordings in the nucleus of the optic tract both before and after ablation of the ipsilateral visual cortex. The main results are: (i) there is a strong inhibitory reciprocal effect upon the nucleus of the optic tract following stimulation of its contralateral counterpart; (ii) electrophysiological and anatomical data imply that the visual cortex does not project directly to the nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies in the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of the entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model proposed here for the role of the interactions between the nuclei of the optic tract under optokinetic stimulation. The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeared only recently in phylogenesis. In more primitive mammals, such as the opossum, subcortical interactions are thought to play a relatively important role. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, such as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex.     

Cortical and subcortical influences on the nucleus of the optic tract of the opossum

In the present work we propose a new phylogenetic hypothesis for the role played by cortical and subcortical afferents to the nucleus of the optical tract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum (Didelphis aurita) using the following experimental approaches: (i) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the ipsilateral striate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence of the marker in the thalamus and mesencephalon; and (iv) single-unit recordings in the nucleus of the optic tract both before and after ablation of the ipsilateral visual cortex. The main results are: (i) there is a strong inhibitory reciprocal effect upon the nucleus of the optic tract following stimulation of its contralateral counterpart; (ii) electrophysiological and anatomical data imply that the visual cortex does not project directly to the nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies in the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of the entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model proposed here for the role of the interactions between the nuclei of the optic tract under optokinetic stimulation.The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeared only recently in phylogenesis. In more primitive mammals, such as the opossum, subcortical interactions are thought to play a relatively important role. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, such as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex.     

The ipsilateral field representation in the striate cortex of the opossum

Summary Reference axes for the visuotopic study of the opossum's striate cortex were estimated from corresponding binocular response fields using multi-unit recording. These central binocular axes (CBA) were derived from experimental data based on the concept that corresponding receptive fields for each eye should be mostly in register under natural conditions. Vertical reference meridians, orthogonal to these axes, define a contralateral and an ipsilateral field for each eye with respect to the recording site. An ipsilateral field representation was observed for both eyes in the striate cortex at the transition zone with peristriate. Maximal values for the center and border of ipsilateral receptive fields were, respectively, 8 and 20 degrees for the contralateral eye and 6 and 14 degrees for the ipsilateral eye. An equivalent ipsilateral field representation was found in animals that had the anterior commissure cut prior to the recording session. This suggests that the ipsilateral field of both eyes may be represented in the striate cortex via the ipsilateral optic tract. Additionally, it was observed that the region of higher ganglion cell density in the retina shows a flattened distribution and that the CBA intersects the retina at the temporal aspect of this region.     

Specific transcellular carbocyanine-labelling of rat retinal microglia during injury-induced neuronal degeneration

The present work employed a new technique for labelling phagocytizing microglia in the axotomized retinal of adult rats. Transection axotomy was performed within the intraorbital segment of the optic nerve, and the fast-transported, vital fluorescent carbocyanine dyes DiI and 4Di-10ASP were deposited at the ocular stump of the nerve in order to retrogradely prelabel the ganglion cells which were destined to die. Optic nerve transection resulted in progressive degradation of ganglion cell axons, perikarya and dendrites within the retina and in release of fluorescent material which was then incorporated into cells identified as microglia but not into other cells of the retina. Incorporation of labelled material into microglia occurred only when the ganglion cells degenerated and not when the non-lesioned ganglion cells were labelled from the superior colliculus. Double-staining of microglia with both dyes helped to compare the pattern of labelling for each dye. After progression of ganglion cell degeneration, microglia displayed a staggered, bilaminated distribution within the ganglion cell layer and within the inner plexiform layer. Fluorescent microglia were not found within the deeper layers of the retina indicating that transneuronal degeneration and subsequent labelling of microglial cells do not occur. The results show that one major function of microglia within the ganglion cell and inner plexiform layers of the lesioned retina is to remove debris produced after degradation of neurons.     

Optic fiber development between dual transplants of retina and superior colliculus placed in the occipital cortex

Summary Fetal rat retina was excised from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats in combination with and adjacent to: 1) the appropriate portion of the superior colliculus to serve as a specific target tissue in an attempt to stimulate an outgrowth of optic fibers from the isolated retinal transplant; 2) a sample of the medial thalamus to provide a alternate tissue not normally recipient to the retinofugal projection, thereby serving as a control to further test for the target specificity of the axonal outgrowth from the retinal transplant. After 5–30 days of development samples of transplant and surrounding host cortex were removed and subjected to light and electron microscopic study.The fetal retina has been shown to develop successfully while located in the occipital cortex but this tissue does not form a significant number of optic fibers nor does it demonstrate the normal quantity of ganglion cells, (Matthews et al. 1981). A dual transplant of fetal retina and medial thalamus demonstrates the major types of neurons and sensory elements in the retina as well as concentrations of large neurons in the thalamus. Examination of these transplants with a silver stain, as well as electron microscopy, revealed a network of axons coursing within the transplanted thalamic neuropil but few axons traversing the interface between the retinal and thalamic transplant. Additionally, no axons coursed out of the retina into the surrounding host cortex.Similar studies of dual transplants of retina and superior colliculus revealed marked concentrations of fibers projecting from the periphery of the retinal tissue, extending deep into the adjacent superior colliculus and merging with the tangle of axons found within this tissue. It should be emphasized, however, that such outgrowths from the retina were clearly restricted to those portions in apposition to the transplanted tectum. No axons were found extending from the opposite surface of the retinal transplant in contact with the host cortex.A quantitative analysis of ganglion cell populations in the transplant demonstrated that these were somewhat reduced in all retinal transplants with the exception of those regions located adjacent to a transplant of superimental enlargement of a peripheral target organ during early stages of development reduces the amount of spontaneous neuronal degeneration resulting from a failure to establish functional connections with the available postsynaptic sites in the target.Our experiments indicate that the use of multiple transplants may provide a useful model system for further exploration of the relationship between developing CNS neurons and tissues within the CNS or the periphery which receive their input.     

The crossed projection from the temporal retina to the dorsal lateral geniculate nucleus in the rat

The crossed projection from the temporal crescent in the rat's retina was studied by producing a discrete retinal lesion in one eye and examining the dorsal lateral geniculate nucleus and superior colliculus contralateral to the lesion for anterograde degeneration products. The position of this crossed degeneration was described in relation to the uncrossed retinal termination in the same structures by injecting the opposite eye with [³H]proline and processing the tissue for autoradiography. The location of the retinal lesion in relation to the temporal cresent was identified by injecting the dorsal lateral geniculate nucleus ipsilateral to the lesioned eye with a fluorescent tracer, to retrogradely label the ipsilaterally projecting retinal ganglion cells in the lesioned eye.

Retinal lesions that were histologically verified to be restricted to the temporal crescent produced crossed degeneration in the superior colliculus at its rostral border, in accord with this projection's published visual topography. These same lesions consistently yielded a very circumscribed and sparse amount of degeneration in the contralateral dorsal lateral geniculate nucleus at its dorsomedial border, abutting the optic tract dorsally and the lateroposterior nucleus medially. The degeneration bore no consistent relationship to the position of the uncrossed retinal terminal field, which is situated further 9ventrally in the dorsal lateral geniculate nucleus; rather, this crossed temporal projection terminated in the outer shell of the nucleus along its medial border.

This crossed temporal retinogeniculate projection, together with the crossed projection from nasal retina, forms a continuous map of the complete contralateral retina in the outer shell of the dorsal lateral geniculate nucleus, likely to arise from a population of retinal ganglion cells possessing small soma sizes. This dorsomedial part of the rat's dorsal lateral geniculate nucleus, receiving a crossed projection from the temporal retina, may by similar to the cat's lamina 3 in the medial interlaminar nucleus of its retinogeniculate pathway. This result clarifies the homologous subdivisions of the dorsal lateral geniculate nucleus in the rodent and feline thalamus.     

The distribution of displaced ganglion cells in the retina of the pigeon

Displaced ganglion cells in the pigeon's retina, at the inner margin of the inner nuclear layer, were labelled by retrograde axonal transport of horseradish peroxidase (HRP). Large HRP injections were made in order to fill all the retinal projection sites in the thalamus and midbrain. The distribution of labelled cells was studied in retinal whole mounts incubated with tetramethyl benzidine (TMB) substrate for HRP. A maximum of 5,300 HRP labelled displaced ganglion cells was found. They were concentrated in a band of retina centred on the horizontal meridian, with high density areas (of about 110 cells/mm2) near the area centralis and in the mid-temporal retina. This is a different distribution to that of ganglion and inner nuclear layer cells; these are concentrated in the area centralis and red field. The orientation of retinal maps was checked by ophthalmoscopic measurements of the angle of the pecten to the horizontal in alert pigeons; this was found to be approximately 70 degrees. The array of displaced ganglion cells, studied by nearest neighbour distributions, was irregular and nearly random, which is consistent with a system of low spatial acuity. In the central retina only the cell bodies and not the dendrites of small displaced ganglion cells (7.5 microns diameter) were labelled; towards the periphery large displaced ganglion cells (16 microns diameter) with 2-5 radially arranged primary dendrites were found.     

Different effects of intracranial and intraorbital section of the optic nerve on the functional responses of rat retinal ganglion cells

Summary A lesion to the optic nerve of adult mammals leads to the retrograde degeneration and finally to the death of injured retinal ganglion cells. In this study, we have evaluated the effects induced by different sites of axotomy on the functional changes occurring in the retinal ganglion cells after optic nerve section. We have investigated the functional properties of retinal ganglion cells of adult rats by recording the retinal responses to patterned stimuli (pattern electroretinogram) after unilateral section of the optic nerve at two different levels: intraorbital and intracranial. The results show that the site of lesion of the optic nerve affects the time of disappearance of the pattern electroretinogram. The pattern electroretinogram takes longer to be degraded after an intracranial section than an intraorbital section.     

Ganglion cell death within the developing retina: A regulatory role for retinal dendrites?

The effects of neonatal midbrain lesions on populations of retinal ganglion cells with ipsilateral or contralateral projections were investigated in hooded rats with the use of horseradish peroxidase. After bilateral lesions of the superior colliculus performed at birth, the number of contralaterally projecting ganglion cells is reduced but the number of ipsilaterally projecting cells is increased. Bilateral tectal lesions performed 5 days after birth reduce the number of both contralaterally and ipsilaterally projecting ganglion cells. Unilateral tecto-pretectal lesions performed at birth lead to extensive retrograde degeneration of contralaterally projecting ganglion cells in the opposite retina; but both the ipsilateral terminal fields of the same retina and its population of ipsilaterally projecting ganglion cells are increased. Cells located at the border between the temporal crescent and the nasal areas of the retina opposite an unilateral tecto-pretectal lesion were found to have their dendrites pointing towards the severely depleted nasal areas more frequently than in normal rats.These observations suggest that competitive interactions between retinal dendrites may play a role in regulating ganglion cell death in the developing retina. The increased population of ipsilaterally projecting ganglion cells would reflect the survival of neurones which would otherwise normally degenerate, resulting from reduced local interactions as a consequence of the massive removal of neighbouring contralaterally projecting cells.     

Bifurcating axons of retinal ganglion cells terminate in the hypothalamic suprachiasmatic nucleus and the intergeniculate leaflet of the thalamus

At least some retinal axons afferent to the hypothalamic suprachiasmatic nucleus (SCN; a circadian oscillator) bifurcate in the optic chiasm (O.E. Millhouse, Brain Res., 137 (1977) 351-355). The termination site(s) of the axonal branch that continues in the optic tract is unknown. Injection of the fluorescent tracer, True Blue, into the SCN and the fluorescent dye, Nuclear Yellow, into the lateral geniculate complex resulted in the labeling of individual retinal ganglion cells with both tracers. However, only Nuclear Yellow injections which included the intergeniculate leaflet (IGL) resulted in double-labeled ganglion cells in the retinae. These results indicate that individual retinal ganglion cells innervate both the hypothalamic SCN and the IGL of the thalamus by means of divergent axonal collaterals. Moreover, neurons of the IGL are afferent to the SCN, thereby forming a complex circuit within which photic information from the same retinal ganglion cell may influence the SCN both directly and after thalamic processing.     

The retinal origin of uncrossed optic nerve fibres in rats and their role in visual discrimination

Summary An attempt was made to sever the optic chiasma in the mid-sagittal plane in 12 rats. This was successful in 8 animals. Provided there was no additional substantial damage to the uncrossed optic fibres the rats were able to relearn an intensity discrimination and to learn or relearn an orientation discrimination, although optokinetic following was abolished. The results unequivocally demonstrate that the uncrossed optic fibres can mediate two kinds of visual discrimination.The area of retina giving rise to the uncrossed fibres was determined from the position of undegenerated retinal ganglion cells in each eye following section of the chiasma, and in one eye of 4 rats in which one optic tract was entirely or extensively destroyed. The ganglion cells giving rise to the uncrossed optic fibres occupy about 40 degrees of the temporal retina, corresponding to the binocular overlap in the visual field.     

Aberrant axonal paths in regenerated goldfish retina and tectum opticum following intraocular injection of ouabain

Following ouabain-induced degeneration, the neural retina and the retinotectal axons regenerate. The pathways of regenerated retinal ganglion cell axons in retina and in tectum are visualized by labeling with horseradish peroxidase (HRP) applied to the optic nerve. In retina, the axons exhibit highly abnormal courses, including extensive fascicle crossing, hairpin loops and circular routes. In tectum, retinal axon fascicles are not neatly aligned in a normal fascicle fan. Instead, long and short fascicles are mixed, and take erratic routes, crossing each other and crossing the tectal equator.     

Retinal projections to the accessory optic system in pigmented and albino ferrets (Mustela putorius furo)

We investigated if a reduced specificity of the retinal projection to the accessory optic system might be responsible for the loss of direction selectivity in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) and, in consequence of this, the optokinetic deficits in albino ferrets. Under electrophysiological control we performed dual tracer injections into the NOT-DTN and the medial terminal nucleus (MTN). Retrogradely labelled ganglion cells were found in the visual streak, the dorsal, and the ventral retina both after injections into the NOT-DTN and the MTN indicating that both nuclei receive input from the same retinal regions. The distribution and spacing of labelled ganglion cells did not differ between pigmented and albino ferrets. However, retinal ganglion cells projecting simultaneously to both the NOT-DTN and the MTN occurred only in albino ferrets. These results suggest that a reduced specificity of the projection pattern of direction specific ganglion cells may contribute to the loss of direction selectivity in the NOT-DTN in albino ferrets.     

The retino-geniculo-cortical pathway in Callithrix. II. The geniculo-cortical projection

Summary After monocular injections of tritiated tracer precursors and transneuronal transport of tritiated compounds, a continuous band of radioactivity in layer IV of striate cortex of Callithrix jachus (Callithricidae, New World primates) indicated a complete desegregation of the crossed and uncrossed retino-cortical pathways. Comparison with the organization of these pathways in other primates suggests that a segregation of the retino-cortical pathways of opposite ocularities into alternating ocular dominance columns in striate cortex has developed independently in the different primate lines. The significance is discussed of the weak labeling of the region of area 17 representing the central retina as compared to the representation of the peripheral retina.