Retinotopische Beziehungen und Struktur rezeptiver Felder im Tectum opticum und Praetectum der Katze

Overrepresentation of the central visual field in the superior colliculus of the pigmented and albino ferret

We have examined the retinotopy in the superior colliculus of pigmented and albino ferrets using both anatomical and electrophysiological methods. While the distribution of contralaterally projecting retinotectal ganglion cells is characterized by the presence of an area centralis superimposed on a visual streak in both strains, the ipsilateral projection from temporal hemiretina is strongly reduced in albinos. In spite of the significantly altered retinotectal projection pattern, the collicular visual field map in the albino ferret reveals the same characteristics as in the pigmented animal with a strongly enlarged representation of the center of visual space. An areal comparison between retinotectal ganglion cell distribution and collicular areal magnification shows that the increase in areal magnification factor between the periphery and the representation of the central visual hemifield exceeds the corresponding increase in retinal ganglion cell density between peripheral retina and area centralis by a factor of three in pigmented and a factor of four in albino ferrets. The areal magnification factor of the representation of the retinal visual streak does not exceed the increase in retinotectal ganglion cell density. Thus, our results suggest that the representation of visual space in the superior colliculus of albino and pigmented ferrets does not simply follow the retinotectal ganglion cell density, but that there is an enhanced representation of the frontal central visual field. The possibility is discussed that the collicular visual field map may be determined either by both retinotectal and corticotectal projections or by the colliculus' intrinsic structure.

Pattern of retinotectal projection in the megachiropteran bat Rousettus aegyptiacus

The retinotopic organisation of the superior colliculus (SC) in the megachiropteran bat Rousettus aegyptiacus was examined with single and multi-unit recordings and by tracing the retrograde and anterograde transport of horseradish peroxidase (HRP) between the retina and the SC. The pattern of projection of the visual field onto the SC in Rousettus resembles the pattern found in most mammals. The whole of the contralateral visual field is represented and, in addition, a region of the ipsilateral visual field extending 25 degrees beyond the vertical 0 degree meridian. The ipsilateral visual field is represented binocularly in the most anterior 300-500 microns of the rostral pole of the SC. The contralateral visual field up to 25 degrees from the vertical meridian is represented through both eyes for the next 500-800 microns. The peripheral part of the contralateral visual field, 25 degrees-110 degrees from the vertical meridian is seen only by the nasal retina (the monocular crescent) of the contralateral eye and is represented in the caudal part of the SC. Following multiple injections of HRP into one SC, ganglion cells were labeled in both the nasal and temporal hemiretina of the contralateral eye. In the retina ipsilateral to the injection site, labeled cells were restricted to the temporal hemiretina. After injections of HRP into one eye, labeled terminals were found all over the contralateral SC, but in the ipsilateral SC they were restricted to a band that begins 300-500 microns caudal from the rostral pole and extends to the middle of the SC. These results suggest that in Rousettus, unlike the megachiropteran bats described by Pettigrew, Jamieson, Robson, Hall, McAnally, and Cooper (Philosophical Transactions of the Royal Society of London Series B 325:489-559, 1989), the retinotopic organisation of the SC is not primate-like, but follows the general mammalian scheme. As the retinotopic organisation of the SC is not consistent among the megachiropteran bats, the pattern of this projection may not be a useful indicator of their phylogenetic origins.

Distributed spatial coding in the superior colliculus: a review

This paper reviews evidence that the superior colliculus (SC) of the midbrain represents visual direction and certain aspects of saccadic eye movements in the distribution of activity across a population of cells. Accurate and precise eye movements appear to be mediated, in part at least, by cells of the SC that have large sensory receptive fields and/or discharge in association with a range of saccades. This implies that visual points or saccade targets are represented by patches rather than points of activity in the SC. Perturbation of the pattern of collicular discharge by focal inactivation modifies saccade amplitude and direction in a way consistent with distributed coding. Several models have been advanced to explain how such a code might be implemented in the colliculus. Evidence related to these hypotheses is examined and continuing uncertainties are identified.

The pupillary light reflex in normal and innate microstrabismic cats, II: Retinal and cortical input to the nucleus praetectalis olivaris

The anatomical substrate of the pupillary light reflex was investigated in normal and innate microstrabismic cats using anatomical methods as well as electrical stimulation. The bilateral retinal input to the nucleus praetectalis olivaris (NPO), the pretectal relay station in the subcortical pupilloconstrictor pathway, was identified to come from the ventral retina were the upper visual field is represented. Orthodromic electrical stimulation revealed that retinal information is transmitted to on-tonic neurons in the NPO mainly via slowly conducting axons probably originating from W- and X-type retinal ganglion cells. For the first time, a direct cortical input to on-tonic neurons in the NPO could be demonstrated. This cortical input originates from caudolateral parts of the occipital cortex. Putative input structures are those subdivisions of areas 19 and 20a where the upper part of the visual field is represented. A direct, predominantly contralateral projection with a weak ipsilateral component from NPO to the nucleus of Edinger-Westphal, and an interhemispheric connection between the NPOs could be demonstrated. With respect to the anatomical connections as described in this study, no differences between normal and innate microstrabismic cats could be found. The results are discussed with respect to the binocular summation of the pupillary light reflex and its reduction in subjects with impaired binocular vision.

The pupillary light reflex in normal and innate microstrabismic cats, I: Behavior and receptive-field analysis in the nucleus praetectalis olivaris

Neurons in the nucleus praetectalis olivaris (NPO) were antidromically identified by electrical stimulation of the nucleus of Edinger-Westphal (EW), the location of preganglionic pupilloconstrictor motoneurons. Electrical stimulation within the NPO leads to bilateral pupil constriction. Single neurons recorded in the NPO respond tonically to light stimuli, and their discharge frequency increases linearly with logarithmic increase in light intensity. This characteristic identifies NPO neurons as luminance detectors. They have large receptive fields mostly lying in the upper and contralateral quadrant of the visual field. Cats with impaired binocular vision show a significantly reduced binocular summation of the pupillary light reflex (BSP), i.e. the increase of pupil constriction during binocular illumination when compared to monocular illumination is less than in normal animals. The investigation of ocular dominance and subthreshold binocular interactions in the NPO of normal and innate microstrabismic cats revealed two possible mechanisms for BSP and its reduction in strabismic subjects. First, the percentage of neurons increasing their discharge rate by illuminating either eye is significantly reduced in the NPO of innate microstrabismic cats (6.6%) when compared to normal cats (22% of all neurons tested). Second, in most NPO neurons of normal cats the subthreshold influence of the ipsilateral eye leads to an increase in neuronal activity during binocular stimulation when compared to monocular stimulation of the contralateral eye (binocular summation). The subthreshold influence of the ipsilateral eye in most NPO neurons of microstrabismic cats, however, is inhibitory, i.e. the neuronal discharge rate during binocular stimulation is decreased when compared to monocular stimulation of the contralateral eye (binocular inhibition). However, there is no significant correlation between BSP and binocularity in the NPO in individual animals. This suggests that BSP may be additionally influenced by visual structures other than NPO.

The retinal projection to the cat pretectum

Retinal ganglion cells were labeled retrogradely by localized injections of HRP into different regions of the pretectum, tectum, and optic tract in 26 cats. Retinal projection zones in the pretectum were labeled anterogradely in the same cats by intravitreal injections of 3H-proline. This allowed the HRP injection sites to be located with respect to the retinal termination zones. The form of the projection zones from retina to pretectum was determined from serial reconstructions of either coronal or horizontal sections. The zones are best distinguished in horizontal sections, where they are seen as four roughly parallel strips on either side of the brain. They are more-or-less parallel to the anterior border of the tectum, and appear to traverse the entire width of the retinal projection to the tectum. Each zone is similar in form for the ipsilateral and contralateral projections, although the contralateral projection is thicker and denser. Binocular injections of 3H-proline showed that the projections from the two eyes were in register and did not interdigitate. Cells labeled by HRP injections in the anteromedial end of the pretectum were concentrated in the lower nasal quadrant of the contralateral retina, and the lower temporal quadrant of the ipsilateral retina. Posterolateral injections labeled cells in the upper quadrants. There is thus a rough retinotopic mapping along the elongated axis of the pretectum. When the distributions of ganglion cells labeled by HRP injections to different parts of the pretectum are combined, they show a concentration in both the visual streak and area centralis, and thereby reflect, at least qualitatively, the relative spatial distribution of the entire ganglion-cell population. About 85% of the retinal projection to the pretectum is contralateral. For all of the HRP injections, the spatial density of labeled cells was always low, accounting for no more than 3% of the total spatial density of ganglion cells in any retinal region. Several types of ganglion cells were labeled following injections to most regions of the pretectum; these included alpha, beta, and epsilon cells, as well as small-bodied cells showing a variety of morphologic forms. Alpha cells were labeled mainly from the anterolateral end of the pretectum, but other cell types were labeled from all injected regions. In the peripheral retina, 2% of the labeled cells were alpha cells, 32% were beta cells, 19% were epsilon cells, and the remaining 47% were small cells whose dendrites only occasionally filled to any significant extent.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal W-cell projections to the medial interlaminar nucleus in the cat: implications for ganglion cell classification

The perikaryal sizes and retinal distribution of ganglion cells labeled after small iontophoretic injections of horseradish peroxidase (HRP) into the medial interlaminar nucleus (MIN) were studied. Injections were also made into the LGNv and the C-laminae of the dorsal lateral geniculate nucleus (LGNd) for comparison. The results are consistent with suggestions that the MIN contains three approximately vertically oriented laminae which, from medial to lateral, receive their input from, respectively, contralateral nasal, ipsilateral temporal, and contralateral temporal retina. Each MIN lamina receives afferents from two distinct groups of retinal ganglion cells (1) cells with large somas (over 25 micron), coarse primary dendrites, large dendritic trees (500-900 micron in diameter), and coarse axons; (2) cells with medium-sized somas (14-20 micron), medium-caliber primary dendrites, large dendritic trees (350-700 micron), and fine axons. The large cells are clearly Y-cells or alpha cells, and they provide approximately 50% of the retinal input to all layers of the MIN. The medium-sized cells, which provide the remaining 50% of the retinal output in the MIN, are, we argue, W-cells, since they do not differ in soma size, dendritic morphology, axon caliber, or receptive field properties from medium-sized W-cells which project to other thalamic or midbrain structures. These results suggest two phylogenetic trends within the W-cell group: (1) the differentiation of thalamic and midbrain components; and (2) the further differentiation of ipsilateral and contralateral projections within the midbrain component. This latter division corresponds to the distinction between W1 and W2 cells described previously (Rowe and Stone, '77, '80).

Distribution of small and medium-sized ganglion cells in the cat's retina

The distributions of small and medium-sized ganglion cells in the cat's retina have been studied. The soma size ranges of ganglion cells projecting to the superior colliculus and the A-laminae of the dorsal lateral geniculate nucleus were determined by the retrograde transport of horseradish peroxidase. Confirming previous work, the results suggest a division of the overall soma population into large, medium and small ranges. The large somas are the cell bodies of alpha- or Y-cells; the small somas are the cell bodies of gamma- or W-cells, most of which project to the superior colliculus; and the medium-sized somas include the cell bodies of beta- or X-cells and of gamma- or W-cells, most of which project to the forebrain. The small-soma (collicular-projecting) cells have a strongly streaky distribution, i.e., the isodensity lines in a map of their distribution are markedly elongated horizontally. These cells form the principal component of the visual streak, especially in nasal retina. The medium-soma (forebrain-projecting) cells also show some streakiness, i.e., their isodensity lines are also elongated horizontally, but less markedly than for small cells. The results also show differences between nasal and temporal retina in the distributions of medium-sized and small somas. It is suggested that the patterns of distribution of small and medium cells may be reflected in the topography of visual centres of the midbrain and forebrain, respectively.

The retinal projection to the superior colliculus in the cat: a quantitative study with HRP

The projections of cat retinal ganglion cells to the superior colliculus (SC) were examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). Several injections of HRP were made in a single SC after the visual projection to the injection sites had been established physiologically. The HRP injections resulted in a homogeneous distribution of labelled ganglion cells in whole mount preparations of the retinae of both eyes. In the eye contralateral to the injected colliculus, ganglion cells with a crossed projection were labelled in both nasal and temporal retina; in the ipsilateral eye, ganglion cells with uncrossed projection were labelled only in the temporal retina. Analysis of the counterstained retinal whole mounts indicated that at least 50% of all ganglion cells in the nasal retina and 26% in the temporal retina have crossed projection to SC, and that 24% of all ganglion cells of the temporal retina have an uncrossed projection to the SC. The morphological classes of retinal ganglion cells have different patterns of crossed/uncrossed decussation and they participate in varying proportions in the retino-tectal projection. Almost all Alpha cells in the retina send axon collaterals to the SC. Probably only about 10% of the Beta cells project to the SC and at least 80% of all Gamma cells send axons to the SC.

Properties of cells projecting rostrally from the superficial layers of the cats superior colliculus

Cells projecting rostrally from the cat's superior colliculus were identified by antidromic activation from the posterior thalamus. These cells occurred principally in the stratum griseum superficiale and stratum opticum, although some were also encountered in deeper laminae. The rostrally projecting neurons of the superficial laminae formed a heterogeneous group with respect to axonal conduction velocity, receptive field dimensions and special response properties, such as directional and velocity sensitivity. No correlation was detected between receptive field size and estimated axonal conduction velocity in these units. Cells with the smallest receptive fields, were rarely excited antidromically from the thalamus.

Orientation of slit pupil and visual streak in the eye of the cat

The orientation of the visual streak of the cat's retina was compared to that of the long axis of the slit pupil in the same eye. In five paralyzed, anesthetized cats, the retinal projection to the superior colliculus was mapped with electrophysiological techniques. The orientation of the visual streak was estimated from the projection in visual space of the collicular region of high magnification which corresponds to the central projection of the streak. The angle by which the streak was tilted from absolute horizontal was always within one or two degree of the angle by which the pupil axis was tilted from absolute vertical. This relationship was confirmed in three of the animals in which small retinal lesions were placed a known distance from the histologically determined axis of the streak. From the visual coordinates of these lesions, an independent estimate of the streak's orientation was obtained. In each case, the tilt of streak axis from horizontal differed by no more than 0.5 degrees from the tilt of the pupil axis from vertical. The results support the hypothesis that planes containing the long axis of the cat's slit pupil are perpendicular to planes containing the long axis of the visual streak of the same eye.

Anatomical organization of retinotectal afferents in the cat: an autoradiographic study

The distribution of retinotectal afferents has been studied by autoradiography in 4 adult cats. The findings suggest that crossed and uncrossed retinal fibers terminate in a striking cluster-and-sheet pattern that varies systematically with respect to the retinotopic map of the colliculus. Following unilateral eye injection, labelling was most pronounced in the contralateral colliculus but a suprising volume of label appeared on the ipsilateral side in all cases in the form of dense clumps of silver grains separated by sparsely labelled zones. The contralateral projection appeared densest in the most superficial of the 3 laminae of the stratum griseum superficiale; appreciable labelling was present also in the middle lamina at all survival times used (23-72 h). Near the area centralis representation labelling in both contralateral tiers weakened markedly and local gaps appeared densest in the more dorsal band. Elsewhere, labelling in this dorsal band was generally dense, though sharply interrupted at the optic disc representation and in a curious, elongated lateral zone at mid-collicular levels. In the caudal half of the binocular zone rarefications or 'holes', about 200 mum wide, appeared in the more ventral tier between more densely labelled zones of roughly similar width. On the ipsilateral side, labelling was sparse or absent at the rostral and caudal collicular poles, and was also weak in the region of the area centralis representation save for occasional very superficial grain-clusters. Farther caudally, however, prominent approx. 200 mum wide 'puffs' of label marked the middle lamina of the superficial gray layer. The puffs were most regular in shape in the caudal half of the ipsilateral zone and these were spaced at roughly 200 mum intervals. Puffs lateral to the horizontal meridian representation tended to lie more dorsal than those medial to this line and some of the most lateral puffs at mid-collicular levels invaded the upper lamina of the superficial gray layer. The optic disc representation was marked by a column of label extending through the upper and middle laminae. Similar experiments in cat fetuses suggest that these staggered--and possible even complementary--patterns of crossed and uncrossed retinotectal projection are innate: ipsilateral 'puffs' of labelling and contralateral 'holes' appear in the superior colliculus at least one week before term, as does the ipsilateral filling-in and contralateral gap at the optic disc representation. These observations suggest that in the cat, a vertical as well as horizontal organization may characterize the superficial layers of the superior colliculus. The additional finding of a similar, interrupted puff-like pattern of labelling in the stratum griseum medium following injections in the region of the substantia nigra makes it likely that a somewhat comparable cluster-and-sheet organization may exist also in the deep collicular layers.