The projection of optic fibers to the visual centers in the cat

Binocular response modulation in the lateral geniculate nucleus

The dorsal lateral geniculate nucleus of the thalamus (LGN) receives the main outputs of both eyes and relays those signals to the visual cortex. Each retina projects to separate layers of the LGN so that each LGN neuron is innervated by a single eye. In line with this anatomical separation, visual responses of almost all of LGN neurons are driven by one eye only. Nonetheless, many LGN neurons are sensitive to what is shown to the other eye as their visual responses differ when both eyes are stimulated compared to when the driving eye is stimulated in isolation. This, predominantly suppressive, binocular modulation of LGN responses might suggest that the LGN is the first location in the primary visual pathway where the outputs from the two eyes interact. Indeed, the LGN features several anatomical structures that would allow for LGN neurons responding to one eye to modulate neurons that respond to the other eye. However, it is also possible that binocular response modulation in the LGN arises indirectly as the LGN also receives input from binocular visual structures. Here we review the extant literature on the effects of binocular stimulation on LGN spiking responses, highlighting findings from cats and primates, and evaluate the neural circuits that might mediate binocular response modulation in the LGN.

Central pupillary light reflex circuits in the cat: I. The olivary pretectal nucleus

The central pathways subserving the feline pupillary light reflex were examined by defining retinal input to the olivary pretectal nucleus (OPt), the midbrain projections of this nucleus, and the premotor neurons within it. Unilateral intravitreal wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections revealed differences in the pattern of retinal OPt termination on the two sides. Injections of WGA-HRP into OPt labeled terminals bilaterally in the anteromedian nucleus, and to a lesser extent in the supraoculomotor area, centrally projecting Edinger-Westphal nucleus, and nucleus of the posterior commissure. Labeled terminals, as well as retrogradely labeled multipolar cells, were present in the contralateral OPt, indicating a commissural pathway. Injections of WGA-HRP into the anteromedian nucleus labeled fusiform premotor neurons within the OPt, as well as multipolar cells in the nucleus of the posterior commissure. Connections between retinal terminals and the pretectal premotor neurons were characterized by combining vitreous chamber and anteromedian nucleus injections of WGA-HRP in the same animal. Fusiform-shaped, retrogradely labeled cells fell within the anterogradely labeled retinal terminal field in the OPt. Ultrastructural analysis revealed labeled retinal terminals containing clear spherical vesicles. They contacted labeled pretectal premotor neurons via asymmetric synaptic densities. These results provide an anatomical substrate for the pupillary light reflex in the cat. Pretectal premotor neurons receive direct retinal input via synapses suggestive of an excitatory drive, and project directly to nuclei containing preganglionic motoneurons. These projections are concentrated in the anteromedian nucleus, indicating its involvement in the pupillary light reflex.

Dorsal lateral geniculate substructure in the long-evans rat: a cholera toxin B subunit study

The pigmented rat is an increasingly important model in visual neuroscience research, yet the lamination of retinal projections in the dLGN has not been examined in sufficient detail. From previous studies it was known that most of the rat dLGN receives monocular input from the contralateral eye, with a small island receiving predominantly ipsilateral projections. Here we revisit the question using cholera toxin B subunit, a tracer that efficiently fills retinal terminals after intra-ocular injection. We imaged retinal termini throughout the dLGN at 0.5 μm resolution and traced areas of ipsilateral and contralateral terminals to obtain a high resolution 3D reconstruction of the projection pattern. Retinal termini in the dLGN are well segregated by eye of origin, as expected. We find, however, that the ipsilateral projections form multiple discrete projection zones in three dimensions, not the single island previously described. It remains to be determined whether these subdomains represent distinct functional sublaminae, as is the case in other mammals.

Synaptic organization of thalamocortical axon collaterals in the perigeniculate nucleus and dorsal lateral geniculate nucleus

We examined the synaptic targets of large non-gamma-aminobutyric acid (GABA)-ergic profiles that contain round vesicles and dark mitochondria (RLD profiles) in the perigeniculate nucleus (PGN) and the dorsal lateral geniculate nucleus (dLGN). RLD profiles can provisionally be identified as the collaterals of thalamocortical axons, because their ultrastrucure is distinct from all other previously described dLGN inputs. We also found that RLD profiles are larger than cholinergic terminals and contain the type 2 vesicular glutamate transporter. RLD profiles are distributed throughout the PGN and are concentrated within the interlaminar zones (IZs) of the dLGN, regions distinguished by dense binding of Wisteria floribunda agglutinin (WFA). To determine the synaptic targets of thalamocortical axon collaterals, we examined RLD profiles in the PGN and dLGN in tissue stained for GABA. For the PGN, we found that all RLD profiles make synaptic contacts with GABAergic PGN somata, dendrites, and spines. In the dLGN, RLD profiles primarily synapse with GABAergic dendrites that contain vesicles (F2 profiles) and non-GABAergic dendrites in glomerular arrangements that include triads. Occasional synapses on GABAergic somata and proximal dendrites were also observed in the dLGN. These results suggest that correlated dLGN activity may be enhanced via direct synaptic contacts between thalamocortical cells, whereas noncorrelated activity (such as that occurring during binocular rivalry) could be suppressed via thalamocortical collateral input to PGN cells and dLGN interneurons.

The pretectum: connections and oculomotor-related roles

Research over the past two decades in mammals, especially primates, has greatly improved our understanding of the afferent and efferent connections of two retinorecipient pretectal nuclei, the nucleus of the optic tract (NOT) and the pretectal olivary nucleus (PON). Functional studies of these two nuclei have further elucidated some of the roles that they play both in oculomotor control and in relaying oculomotor-related signals to visual relay nuclei. Therefore, following a brief overview of the anatomy and retinal projections to the entire mammalian pretectum, the connections and potential roles of the NOT and the PON are considered in detail. Data on the specific connections of the NOT are combined with data from single-unit recording, microstimulation, and lesion studies to show that this nucleus plays critical roles in optokinetic nystagmus, short-latency ocular following, smooth pursuit eye movements, and adaptation of the gain of the horizontal vestibulo-ocular reflex. Comparable data for the PON show that this nucleus plays critical roles in the pupillary light reflex, light-evoked blinks, rapid eye movement sleep triggering, and modulating subcortical nuclei involved in circadian rhythms.

Organization of the neurons of origin of the descending pathways from the ferret superior colliculus

The superior colliculus (SC), through its descending projections to the brainstem and spinal cord, is involved in initiating sensory-driven orienting behaviors. Ferrets are carnivores that hunt both above and below ground using visual (and auditory) cues in the daylight but non-visual cues in darkness and in subterranean environments. The present investigation sought to determine whether the ferret SC shows organizational features similar to those found in other visually dominant animals (e.g. cats), or whether characteristics of colliculi from non-visually dominant animals (e.g. rodents) prevail. Injection of retrograde tracer into the identified targets of the colliculus (cervical spinal cord, the contralateral pontomedullary reticular formation, or the ipsilateral pontine reticular formation) labeled tectospinal, crossed tectoreticular, and ipsilateral tectoreticular neurons, respectively, within the adult ferret SC. Labeled tectospinal and crossed tectoreticular neurons were far outnumbered by neurons with ipsilateral reticular projections. Like those of their visually dominant relatives, ferret tectospinal neurons were well represented throughout the anterior-posterior extent of the SC and crossed tectoreticular neurons tended to be distributed more broadly across the intermediate gray layer than those of rodents. Thus, even though ferrets perform well as subterranean predators where non-visual cues initiate orienting behaviors, these anatomical characteristics indicate that their colliculi are organized similar to that of their visually dominant, carnivorous relatives.

Postnatal development of the retinal projection to the nucleus of the optic tract and accessory optic nuclei in the hooded rat

Retinal projections to the nucleus of the optic tract (NOT) and accessory optic nuclei (AON) were studied in the postnatal hooded rat after monocular injection of cholera toxin B subunit (CTB) into the vitreous chamber of the eye. At all postnatal ages, retinal axons were labeled sensitively; they revealed dense projections to the contralateral, and sparse but distinct projections to the ipsilateral, NOT and AON. The CTB labeling enabled the first delineation of the complete morphology of developing retinal axons in the ipsilateral NOT and AON. From postnatal day (P) 1 to P3, axons with complex growth cones were seen, and unbranched collaterals with simple growth cones increased and extended gradually. At P6, complex growth cones disappeared while branched collaterals with simple growth cones as well as small-sized varicosities increased. By P12 (two days before eye-opening) the adult-like pattern of terminal arbors appeared. The branched collaterals with tiny, small-sized varicosities present probably represented developing synaptic boutons. At P16 (after eye opening), the pattern of terminal arbors was well developed, almost to the same extent as in the adult. By contrast, a broadly distributed, transient retinal projection around NOT and AON was gradually eliminated; it started to disappear during the first few postnatal days, and was fully retracted by the time of eye-opening time to a pattern normal for the adult.

Y retinal terminals contact interneurons in the cat dorsal lateral geniculate nucleus

One of the largest influences on dorsal lateral geniculate nucleus (dLGN) activity comes from interneurons, which use the neurotransmitter gamma-aminobutyric acid (GABA). It is well established that X retinogeniculate terminals contact interneurons and thalamocortical cells in complex synaptic arrangements known as glomeruli. However, there is little anatomical evidence for the involvement of dLGN interneurons in the Y pathway. To determine whether Y retinogeniculate axons contact interneurons, we injected the superior colliculus (SC) with biotinylated dextran amine (BDA) to backfill retinal axons, which also project to the SC. Within the A lamina of the dLGN, this BDA labeling allowed us to distinguish Y retinogeniculate axons from X retinogeniculate axons, which do not project to the SC. In BDA-labeled tissue prepared for electron microscopic analysis, we subsequently used postembedding immunocytochemical staining for GABA to distinguish interneurons from thalamocortical cells. We found that the majority of profiles postsynaptic to Y retinal axons were GABA-negative dendrites of thalamocortical cells (117/200 or 58.5%). The remainder (83/200 or 41.5%) were GABA-positive dendrites, many of which contained vesicles (59/200 or 29.5%). Thus, Y retinogeniculate axons do contact interneurons. However, these contacts differed from X retinogeniculate axons, in that triadic arrangements were rare. This indicates that the X and Y pathways participate in unique circuitries but that interneurons are involved in the modulation of both pathways.

Making brain connections: neuroanatomy and the work of TPS Powell, 1923-1996

Dr. Thomas PS Powell was one of the founders of modern neuroanatomy. His career spanned an era that saw techniques for analyzing connections in the central nervous system dramatically increase in number and resolving power. In tracing the history of his research, one can see how the introduction of each new technique provided an incremental step in analytical capacity although eventually revealing its own limitations. Also evident is the extent to which prejudices born in the days of applying earlier techniques could continue to influence the interpretation of results obtained with new ones. Powell's contributions to neuroscience were extremely wide-ranging, encompassing investigations of the circuitry of the basal ganglia, corticofugal connections, topographic maps in sensory systems, central olfactory pathways, corticocortical and commissural connections, and pathways for sensory convergence in the cerebral cortex. From these investigations, made with tract tracing techniques, much existing knowledge of forebrain organization is derived. He was also one of the earliest investigators to use electron microscopy in the investigation of the central nervous system, and his electron microscopic studies on the olfactory bulb, thalamus, cerebral cortex, and basal ganglia laid, to a large extent, the foundations for all modern research on the synaptic circuitry of these structures. He was given to synthesizing data across systems in order to arrive at common principles of brain organization. A number of these syntheses have been sources of great interest and, occasionally, controversy.

Role of afferents in the development and cell survival of the vertebrate nervous system

1. During normal development of the vertebrate central nervous system, a considerable number of neurons die. The factors controlling which neurons die and which survive are not fully understood. 2. Target populations are known to maintain their innervating neurons. However, the role of afferents in maintaining their targets is still under review. 3. In the developing nervous system, deafferentation of a neuron population is difficult to achieve because plasticity (structural re-organization) can cause re-innervation of the area. Re-innervation alters, rather than removes, the afferent supply. 4. Afferent input is important for neuronal survival during development because deafferentation increases neuronal death by 20-30% and increasing input diminishes neuronal death. 5. Deafferented neurons die at the normal time for cell death for any given population. This occurs after the arrival of afferent axons but before the completion of connectivity and the onset of function. 6. Neuronal survival is maintained by any input, such as reinnervation by inappropriate fibres, but for optimal survival, morphological maturation and the acquisition of normal physiology, the correct input is required. 7. Afferents maintain their target neurons via a combination of electrical activity and delivery of trophic agents, which adjust intracellular calcium, thereby facilitating protein synthesis, mitochondrial function and suppressing apoptosis. 8. Evidence from animal and in vitro experiments indicates that afferents play an extremely important role in the survival of developing neurons in the immature vertebrate nervous system.

Overlapping ipsilateral and contralateral retinal projections to the lateral geniculate nucleus and superior colliculus in the cat: a retrograde triple labelling study

To analyze the relative proportion and distribution of retinal ganglion cells projecting ipsilaterally and contralaterally in the cat, large injections of the fluorescent tracers Fluoro Gold, Fast Blue, and Diamidino Yellow were made in the main layers of the lateral geniculate nucleus (LGN) and superior colliculus (SC). One tracer was injected in both the LGN and SC on one side, and the other two tracers were injected contralaterally, in the LGN and SC, respectively; labelled ganglion cells were charted on retinal whole mounts. Ganglion cells labelled from the LGN and SC were highly intermingled in both the ipsilateral and contralateral retinae. The adopted combinations of tracers allowed the detection of cells double labelled from the SC and LGN, supporting the occurrence of branched retino-thalamic axons to the SC. About one-fourth of the ganglion cells labelled from the LGN and SC was located in the eye ipsilateral to the injection. Retrograde labelling from the ipsilateral side was almost entirely confined to the temporal hemiretina. In the contralateral eye, labelled cells were mainly concentrated in the nasal hemiretina, but more than 10% were also detected in the temporal half of the retina. In the latter area, cells displaying the entire range of sizes of the retinal ganglion cells, labelled from the contralateral LGN and SC, were found throughout the entire hemiretina. However, more than 50% of such "wrong" projecting cells were grouped in a strip of 2 mm closest to the nasotemporal division. Control experiments, in which the tracers injections were restricted to the rostral and dorsal portions of the LGN to avoid optic tract contamination, consistently confirmed the occurrence and distribution of the "wrong" projecting cells in the temporal hemiretina. Thus, these latter cells are not grouped in a central strip, where ganglion cells would have the same chance of projecting to the same or to the opposite side, and sparsely distributed in the temporal periphery, as previously believed. Instead, the present findings indicate that the retinal ganglion cells of origin of contralateral projections are distributed more in a continuum, with a naso-temporal gradient of density, across the temporal hemiretina.

Orientation discrimination in the cat: its cortical locus II. Extrastriate cortical areas

Luminance-defined edges or bars are among the basic units of visual analysis: a "primitive" component of perception. We have utilized this stimulus in a psychophysical study of bar orientation discrimination in the cat before and after selective lesions in visual cortical areas. The cortices have been divided on the basis of their connectivity into three tiers. Tier I refers to areas 17 and 18, tier II includes areas that receive directly from tier I, and tier III includes those areas that receive directly from tier II. Previous studies (Vandenbussche et al. [1991] J. Comp. Neurol. 305:632-658) have shown that the discrimination of bar orientation depends heavily upon the integrity of areas 17 and 18 (tier I). The present study indicates that several extrastriate areas in tiers II and III contribute to this discrimination task. Our data suggest that the anterior medial lateral suprasylvian, the posterior lateral lateral suprasylvian (tier II), and the anterior lateral lateral suprasylvian (tier III) areas are most likely to contribute to bar orientation discrimination.

Topography and extent of visual-field representation in the superior colliculus of the megachiropteran Pteropus

It has been proposed that flying foxes (genus Pteropus) have a primate-like pattern of representation in the superficial layers of the superior colliculus (SC), whereby the visual representation in this structure is limited by the same decussation line that limits the retino-geniculo-cortical projection (Pettigrew, 1986). To test this hypothesis, visual receptive fields were plotted based on single- and multi-unit recordings in the SC of ten flying foxes. A complete representation of the contralateral hemifield was observed in the SC. Although the binocular hemifield of vision in Pteropus is 54 deg wide, receptive-field centers invaded the ipsilateral hemifield by only 8 deg, and the receptive-field borders by 13 deg. This invasion is similar to that observed at the border between visual areas V1 and V2 in the occipital cortex. The extent of the ipsilateral invasion was not affected by a lesion that completely ablated the occipital visual areas, thus suggesting that this invasion may be consequence of a zone of nasotemporal overlap in the retinal projections to the two colliculi. Neurones located in the superficial layers typically responded briskly to stimulation of both eyes, with a bias towards the contralateral eye. After cortical lesions the neuronal responses to the ipsilateral eye were depressed, and the ocular-dominance histograms shifted towards an even stronger dominance by the contralateral eye. However, cells located in the rostral pole of the SC remained responsive to the ipsilateral eye after cortical lesions. Responses in the stratum opticum and stratum griseum intermediale were more severely affected by cortical lesions than those in the stratum griseum superficiale. Our results demonstrate that the SC in flying foxes retain some generalized mammalian characteristics, such as the stronger direct projections of the contralateral eye and the location of the upper, lower, central, and peripheral representations in the SC. Nonetheless, the extent of visual representation in the SC demonstrates a specialized, primate-like pattern. These observations are consistent with the hypothesis that megachiropterans are members of a group that branched off early during the differentiation of primates from basal mammals.

Single retinal ganglion cells projecting bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals in the cat

In most mammals with frontalized eyes, retinal ganglion cells in the nasal or temporal retina send their axons to the contralateral or ipsilateral half, respectively, of the brain. Previous studies in the cat, however, have indicated a retinal region of "nasotemporal overlap" from which arise the retinal projections to both the contralateral and ipsilateral halves of the brain. The present study thus examined in the cat whether any retinal ganglion cells give rise to bifurcating axons that innervate both halves of the brain. By employing fluorescent retrograde double labeling, we investigated whether or not single retinal ganglion cells project bilaterally to the lateral geniculate nuclei or superior colliculi by way of axon collaterals. After Fast Blue was injected into the lateral geniculate nucleus on one side and Diamidino Yellow was injected contralaterally into the lateral geniculate nucleus, 100-200 ganglion cells in each retina were double labeled with both tracers. These double-labeled cells were distributed primarily in the temporal retina, including the region around the vertical meridian and, additionally, in the nasal retina. About 60-80% of the double-labeled cells had large cell bodies (more than 25 microns in diameter), and the others had medium-sized ones (15-25 microns in diameter). The pattern of distribution of double-labeled cells, which was observed after the combined injection into both superior colliculi, was similar to that seen after the combined injection into both lateral geniculate nuclei; more than 90% of double-labeled cells, however, were large.(ABSTRACT TRUNCATED AT 250 WORDS)

NADPH-d (dihydronicotinamide adenine dinucleotide phosphate diaphorase) activity in geniculo-tectal neurons of the rat

Retrograde transport of horseradish peroxidase (HRP) and dihydronicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry were used in order to determine whether NADPH-d-positive neurons of the ventral lateral geniculate nucleus (LGv) project to the superior colliculus (SC). Our results show that intensely stained NADPH-d neurons are restricted to the lateral half of the magnocellular division of LGv (LGv-MC), where they represent 50% of the total number of retrogradely labeled neurons. These findings indicate: (1) that LGv provides an important NADPH-d input to SC, and (2) that within the population of geniculo-tectal neurons, which constitute a morphologically well defined neuronal type, there are two different subclasses, one being NADPH-d positive and the other NADPH-d negative.

Single retinal ganglion cells sending axon collaterals to the bilateral superior colliculi: a fluorescent retrograde double-labeling study in the Japanese monkey (Macaca fuscata)

Single retinal ganglion cells projecting bilaterally to the superior colliculi (SC) by way of axon collaterals were revealed in the Japanese monkey (Macaca fuscata). After injecting Fast blue into the SC on one side and Diamidino yellow into the SC on the opposite side, some retinal ganglion cells were double-labeled with both tracers. Most of them were large cells (more than 25 microns in diameter), and were localized in a narrow strip around the vertical meridian of the retina on each side. This retinal area roughly corresponds to the reported strip of nasotemporal overlap, where both crossed and uncrossed retinofugal projections arise.

Structure of the starburst amacrine network in the cat retina and its association with alpha ganglion cells

To investigate indirect pathways to ganglion cells we studied the starburst amacrine cell network and its relationship to the alpha ganglion cell. Starburst cells were identified by an antiserum to choline acetyltransferase and alpha cells by injection of Lucifer yellow. The density of on and off starburst cells peaks at the area centralis and decreases with eccentricity by a factor of seven. The fine amacrine processes, interrupted by distinct varicosities, arborize in a planar fashion in the inner plexiform layer. The on network, at the junction of strata 3 and 4, and the off network, in stratum 2, have a similar appearance. Neighboring starburst processes run in intimate association to form a network of bundles. As bundles cross each other, loops of irregular size and shape are formed. The loops are smallest in the area centralis and increase by a factor of three towards the periphery; correspondingly, bundle length per unit area decreases with eccentricity. However, the number of varicosities/bundle length stays constant with eccentricity as does the number of processes per bundle. Segments of the starburst network associate over fairly long distances with dendrites of alpha ganglion cells. About 26% of the alpha ganglion dendritic tree shows such association, and this is significantly greater than would be expected if the alpha and starburst processes were independent. We conclude that the functional unit of the starburst cell is a linear bundle of processes and that the starburst network may connect synaptically to the alpha cell.

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.

Antinociceptive effect of agonists microinjected into the anterior pretectal nucleus of the rat

Electrical stimulation at many sites within the pretectal complex and adjacent structures of the rat dorsomedial thalamus yields antinociception. It is documented that no other site in this region evokes antinociception longer lasting than that obtained by stimulation of the anterior pretectal nucleus (APtN). The effects of agonists injected into different nuclei of the dorsomedial thalamus on the tail-flick reflex of rats in response to noxious heat was examined. All animals were submitted to intracerebral electrical stimulation and microinjection of agonists. It was confirmed that strong and long lasting antinociception followed brief (15 s), low intensity (35 microA rms) stimulation of the APtN. In addition, L-glutamate (3.5 and 7.0 micrograms), morphine (1.0 and 5.0 micrograms), and 5-hydroxytryptamine (5-HT; 2.5 and 5.0 micrograms), but not acetylcholine (5.0 micrograms), carbachol (2.5 micrograms), norepinephrine (5.0 micrograms), or dopamine (5.0 micrograms), induced dose-dependent antinociception when microinjected into the APtN. The effect of 5-HT was fully depressed by pretreating animals with methysergide (5 mg/kg, i.p.). A survey of the sites from which morphine and 5-HT induce antinociception revealed that in no site of the dorsomedial thalamus did the effect last longer than after microinjection into the APtN. It is concluded that antinociception evoked by stimulation of the APtN depends on the activation of neuronal cell bodies in the nucleus, and that 5-HT and endogenous opioids may play a physiological role as neurotransmitters mediating antinociception in the APtN.

Representation of the temporal raphe within the optic tract of the cat

The retinal topography of the cat's optic tract was determined by means of injections of the enzyme horseradish peroxidase (HRP) into the tract. This analysis was accomplished by the subtraction of all HRP injection sites not labeling a defined retinal area from those injection sites which resulted in ganglion cell labeling (Venn diagram analysis). Using this method, the following correspondences were demonstrated for the ipsilateral and contralateral projections: superior retina represented in medial optic tract; inferior retina in lateral tract; and area centralis in a dorsocentral location (which was part of a larger area representing the visual streak). The temporal raphe was represented in the ipsilateral tract as a band curving from the area centralis region toward the dorsomedial border of the tract. Contralateral fibers from a region superior to the optic disc were found to be displaced with respect to the general retinal representation in the optic tract and this appeared to be related to retinal development. The ratio of contralateral to ipsilateral fibers was determined and found to be nonuniform within the tract. Injection of HRP into the optic tract of the cat also allowed the axons from labeled retinal ganglion cells to be traced within the retina and optic disc. Axons from ganglion cells lying temporal to the raphe curve around the area centralis enter the optic disc on the lateral and inferior aspects. Ganglion cells lying nasal to the raphe send their axons more directly to enter the optic disc on its superior aspect. A schema is proposed whereby the retina is mapped onto the optic tract.

The cholinergic input to the superficial layers of the superior colliculus: an ultrastructural immunocytochemical study in the ferret

The cholinergic innervation of the superficial layers of the ferret's superior colliculus was investigated with a combination of electron microscopy and choline acetyltransferase immunohistochemistry. Cholinergic boutons in the superficial layers of the superior colliculus possess spherical vesicles and make predominantly asymmetrical synapses onto the profiles of small dendrites, as do the terminals of cortical and retinal axons. In most areas of the brain studied so far, cholinergic terminals tend to form synapses of the symmetrical variety.

The ventral lateral geniculate nucleus in the cat: thalamic and commissural connections revealed by the use of WGA-HRP transport

The present investigation was carried out to clarify the topographical details of both the origin and terminal site of the thalamic projections and the commissural connections of the ventral lateral geniculate nucleus (LGNv) in the cat by using bidirectional transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Thalamic projections: Unilateral injections of WGA-HRP into the LGNv produced orthograde labeling in the intralaminar nuclei bilaterally and in the lateralis posterior (LP) and the pulvinar (Pul) nucleus ipsilaterally. In the intralaminar nuclei the rostral part of the nucleus centralis lateralis (CL) was most densely labeled by orthogradely transported material, particularly in its dorsal and lateral large-celled portion. Other intralaminar nuclei--such as the nucleus paracentralis, centralis medialis, and centralis dorsalis--also were labeled bilaterally with ipsilateral predominance, but no labeling was detected in the caudal portion of the CL and the centromedian and parafascicular nuclei. In the Pul, labeling of terminal ramifications was found to be concentrated in a region just medial to the so-called retinorecipient zone of the Pul as a slim band of labeling inclining dorsoventrally. In the LP, fine labeled fibers were located in the lateral portion of the LP. Commissural connections: Commissural fibers crossed in the dorsal part of the posterior commissure and reached the most caudal part of the contralateral LGNv. Labeling in the contralateral LGNv was concentrated in the dorsomedial part of the medial zone that extends medially to the middle portion of the cerebral peduncle. Origins of the commissural connections arose mostly from the medial zone that roughly corresponds to the commissural terminal zone and partly from aberrant cells dispersed among optic tract fibers. From these results, together with the previous studies, it is concluded that although the cat's LGNv has connections with diverse structures in the central nervous system, the origin and terminal site of the connections are partially segregated within the nucleus, which suggests that the LGNv may contain functional subsystems.

The accessory optic system of the wallaby, Setonix brachyurus: anatomy in normal animals and after early unilateral eye removal

We have traced primary visual projections to nuclei of the accessory optic system in the mature wallaby, Setonix brachyurus, the "quokka," following unilateral intraocular injections of horseradish peroxidase. The organization of pathways and nuclei is similar to that of other marsupials and to that of eutherian mammals. The dorsal, lateral and medial terminal nuclei receive bilateral input, though nuclei ipsilateral to the injected eye are weakly labelled in comparison with their contralateral counterparts. We also report on the accessory optic system in animals which were unilaterally enucleated neonatally or at postnatal day 35. At maturity in enucleated animals, ipsilateral projections to all nuclei of the accessory optic system are more densely labelled than normal. This exuberance is more pronounced in neonatally enucleated animals than in those enucleated at the later stage.

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

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

The accessory optic system of the rabbit, cat, dog and monkey; a whole-mount HRP study

The three-dimensional fiber pathways of the accessory optic system in the rabbit, cat, dog and monkey were studied in whole-mounted preparations of the diencephalon and the midbrain, without sectioning, by anterograde labeling of retinal axons with horseradish peroxidase (HRP). HRP histochemical studies on alternate serial coronal sections were also performed. The rabbit accessory optic system exhibited two fasciculi (the inferior fasciculus, and the superior fasciculus consisting of the anterior fibers and the posterior fibers) and three terminal nuclei (the medial terminal nucleus, and the anterior and posterior portions of the lateral terminal nucleus), but lacked the dorsal terminal nucleus. In the cat and dog, only the posterior fibers of the superior fasciculus were detected. The inferior fasciculus and the anterior fibers of the superior fasciculus were absent. The medial terminal nucleus and the posterior portion of the lateral terminal nucleus were commonly observed in the cat and dog. The cat accessory optic system possessed the dorsal terminal nucleus, and the dog accessory optic system possessed the anterior portion of the lateral terminal nucleus. The monkey (Macaca fuscata) accessory optic system consisted of the posterior fibers of the superior fasciculus, but blacked the inferior fasciculus and the anterior fibers of the superior fasciculus. Most of the posterior fibers terminated in the well-developed posterior portion of the lateral terminal nucleus located on the upper surface of the cerebral peduncle. A small number of posterior fibers projected to the poorly-developed medial terminal nucleus. Based on these findings, species differences in the mammalian accessory optic system were discussed.

Projection from the pretectal nuclei to the dorsal lateral geniculate nucleus in the cat: a wheat germ agglutinin-horseradish peroxidase study

To study the projection from the pretectum to the dorsal lateral geniculate nucleus (LGNd) in the cat, we used anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Special attention was directed to the retinotopic maps of the pretectum and LGNd. Multiple restricted injections were made into different parts of the pretectum or LGNd. The pretectogeniculate pathway terminates mostly in the medial interlaminar nucleus (MIN) and layers A and A1, and to some extent in the lamina C within the ipsilateral LGNd. The lateral part of the nucleus of the optic tract (NTO) receives afferents from the superior retina, and the medial part of NTO and posterior pretectal nucleus (NPP) receives afferents from the inferior retina. There is no topographic organization in the retinal projection to the olivary pretectal nucleus (NOL). The lateral part of NTO projects ipsilaterally to the rostral portion of LGNd, which receives afferents from the superior retina. The medial part of NTO projects ipsilaterally to the caudal portion of LGNd, which receives afferents from the inferior retina. The NOL projects to all laminar parts of LGNd, ipsilaterally. The NPP projects largely to the ipsilateral MIN, which receives afferents from the pericentral and peripheral retina. These results suggest that similar parts of the retinotopic maps present in the pretectum and LGNd are connected.

Subcortical connections of the superior colliculus in the mustache bat, Pteronotus parnellii

The mustache bat, Pteronotus parnellii, depends on echolocation to navigate and capture prey. This adaptation is reflected in the large size and elaboration of brainstem auditory structures and in the minimal development of visual structures. The superior colliculus, usually associated with orienting the eyes, is nevertheless large and well developed in Pteronotus. This observation raises the question of whether the superior colliculus in the echolocating bat has evolved to play a major role in auditory rather than visual orientation. The connections of the superior colliculus in Pteronotus were studied with the aid of anterograde and retrograde transport of wheat germ agglutinin conjugated to HRP. These results indicate that the superior colliculus of Pteronotus is composed almost entirely of the layers beneath stratum opticum. The retinal projection is restricted to a very thin zone just beneath the pial surface. Prominent afferent pathways originate in motor structures, particularly the substantia nigra and the deep nuclei of the cerebellum. Sensory input from the auditory system originates in three brainstem nuclei: the inferior colliculus, the anterolateral periolivary nucleus, and the dorsal nuclei of the lateral lemniscus. The projections from these auditory structures terminate mainly in the central tier of the deep layer. The most prominent efferent pathways are those to medial motor structures of the contralateral brainstem via the predorsal bundle and to the ipsilateral midbrain and pontine tegmentum via the lateral efferent bundle. Ascending projections to the diencephalon are mainly to the medial dorsal nucleus and zona incerta. Thus, the superior colliculus in Pteronotus possesses well-developed anatomical connections that could mediate reflexes for orienting its ears, head, or body toward objects detected by echolocation.

Gamma-aminobutyrate-like immunoreactivity in the thalamus of the cat

Serial sections of the cat's thalamus were incubated with a purified antiserum raised against gamma-aminobutyric acid conjugated to bovine serum albumin by distilled glutaraldehyde. This serum has been extensively characterized and appears to react selectively with fixed gamma-aminobutyric acid in brain tissue treated with glutaraldehyde. Adjoining sections were stained with thionin and served as invaluable guides for a correct evaluation of the immunolabelling pattern. In the neuropil the intensity of the immunostaining varies considerably between thalamic nuclei and even between nuclear subdivisions. The neuropil staining appears particularly dense in the nuclei parataenialis, periventricularis, centralis medialis, reuniens, rhomboideus, habenularis lateralis, centrum medianum, parafascicularis, subparafascicularis, submedius, dorsal and ventral parts of the lateral geniculate body, the dorsal part of the medial geniculate body, the posterior complex, suprageniculate nucleus, pulvinar and parts of the lateral posterior nucleus. The pulvinar/lateralis posterior complex shows a particularly well-differentiated staining pattern which closely matches Updyke's [Updyke (1983) J. comp. Neurol. 219, 143-181] parcellation of this region. In several thalamic nuclei or subareas--and notably in those relay nuclei which are known to project upon non-primary sensory cortical areas--the immunostained neuropil is characterized by many puncta encircling an unstained profile. With few exceptions all thalamic nuclei displayed immunoreactive nerve cell bodies. Several examples were found of a mismatch between the number of such cells and the staining intensity of the neuropil. Thus the nuclei periventricularis, parafascicularis, subparafascicularis, parataenialis, limitans and centrum medianum although being very rich in neuropil staining have practically no immunostained perikarya. Rough estimates were made of the size and the proportion of gamma-aminobutyric acid labelled neurons in all major--and some minor--thalamic nuclei and their subdivisions. In some thalamic nuclei, notably the nuclei reticularis, anterior dorsalis, lateralis dorsalis, centralis lateralis, ventralis posterior and the dorsal lateral geniculate body, the population of immunoreactive neurons is distinctly heterogeneous with regard to soma size. The findings are discussed with regard to previous immunocytochemical studies of the distribution of gamma-aminobutyric acid and its synthesizing enzyme in the thalamus. Particular emphasis is put on the great species differences which appear to exist in this respect.

The accessory optic system in a prosimian primate (Microcebus murinus): evidence for a direct retinal projection to the medial terminal nucleus

The accessory optic system (AOS) was studied in the prosimian primate, Microcebus murinus, by using intraocular injections of the anterograde tracers 3H-proline and horseradish peroxidase (HRP). Retinal fibers were found to terminate bilaterally in all three mesencephalic AOS nuclei as defined by Hayhow ('66, J. Comp. Neurol. 126:653-672). In contrast to previous reports in primates, we find that both the ventral and dorsal divisions of the medial terminal nucleus (MTN) receive projections from the retina. The ventral MTN is composed of a compact triangular group of cells, situated at the medial base of the cerebral peduncle, rostral to the rootlets of the third cranial nerve. The dorsal MTN extends dorsomedial to the substantia nigra and is composed of characteristic fusiform cells embedded in a fibrous neuropil. Although the cells of the dorsal MTN intermingle somewhat with the nigral cells, the nucleus is clearly distinguished by cyto- and myeloarchitectural features. The large lateral terminal nucleus (LTN) receives a dense projection from the retina and forms a prominent bulge on the lateral surface of the cerebral peduncle. The dorsal terminal nucleus (DTN) is located between the brachia of the superior and inferior colliculi, near the origin of the superior fasciculus of the accessory optic tract (AOT). This fasciculus is composed of anterior, middle, and posterior branches. In addition, a ventral group of fibers, corresponding to the inferior fasciculus of the AOT previously described in nonprimates, was identified in all planes of section. The results confirm the existence of a common plan of AOS organization in mammals.

Response properties of single units in the accessory optic system of the dark-reared cat

The visual responses of single units in the lateral and dorsal terminal nuclei (LTN and DTN) of the accessory optic system (AOS) were studied in adult cats reared in total darkness. In the LTN of the normal cat equal numbers of cells prefer upward and downward vertical stimulus motion (previous results). While direction selectively continued to be a characteristic property of LTN and DTN units in dark-reared animals, the distribution of preferred and non-preferred directions of LTN cells was radically altered such that almost every LTN cell examined in the dark-reared cat preferred downward stimulus motion. In contrast, the distribution of preferred directions among DTN cells was largely unaffected by dark rearing. Both normal and dark-reared cat DTN cells responded best to stimuli moving horizontally toward the recorded hemisphere. The velocity preferences of DTN units of the dark-reared cat were, however, much slower than those of normal DTN units. LTN units responding to downward motion in dark-reared cats showed similar velocity preferences to those downward direction-selective LTN units in normal animals. Unlike the highly binocular responses of AOS cells encountered in the normal cat, the ocular dominance distribution obtained from units in the LTN and DTN of the dark-reared cat is completely monocular, favoring the contralateral eye. Thus, dark rearing renders the distribution of preferred directions most affected in the LTN, velocity preference most affected in the DTN and ocular dominance strongly affected in both nuclei. The physiological response properties of the dark-reared cat presented in this report bear a close resemblance to those we have described in the AOS of acutely decorticated animals (previous results). Data obtained from the dark-reared cat support our earlier suggestion that the visual cortex is a major source of upward direction selectively, high-velocity tuning and ipsilateral eye input for AOS cells. Some of the functional consequences of these findings are discussed in relation to frontal eye placement and optokinetic nystagmus.

Identification of ventral lateral geniculate nucleus cells projecting to the pretectum and superior colliculus in the cat

Among 235 histologically identified cells of the ventral lateral geniculate nucleus (LGV) in the cat, 66 responded antidromically to electrical stimulation of the pretectum (PT) and/or superior colliculus (SC): 22 projected to PT, 22 to SC and 22 to both sites. The LGV cells were innervated by optic tract fibers corresponding to axons of X- as well as W-type retinal ganglion cells.

Demonstration of ipsilateral retinocollicular projections in the tree shrew (Tupaia glis)

Ipsilateral retinocollicular projections labeled by anterograde transport of wheatgerm agglutinin-horseradish peroxidase (HRP) conjugate in the tree shrew were examined. For those animals in which this pathway was demonstrated (4 of 14) ipsilateral collicular labeling extended across approximately the anterior two-thirds of the colliculus, with the exception of the extreme rostral pole. Labeling was invariably punctuate and spaced at regular intervals in the lower stratum griseum superficiale. The laminar distribution and patchy terminations of ipsilateral projections are discussed in relation to two apparently independent pathways originating in the temporal retina, the crossed and uncrossed collicular pathways.

Nigral inhibitory termination on efferent neurons of the superior colliculus: an intracellular horseradish peroxidase study in the cat

Intracellularly recorded responses of deeper tectal neurons to stimulation of the substantia nigra and the cerebral peduncle were obtained to demonstrate the monosynaptic inhibitory nature of the nigrotectal pathway in the cat. We also employed antidromic stimulation (contralateral predorsal bundle and superior colliculus) and intracellular labeling with HRP to demonstrate which types of tectal efferent neurons are nigrorecipient. The response to nigral stimulation in 61% of the cells studied was a monosynaptic IPSP of short duration. Recovered HRP-labeled nigrorecipient neurons include X1 (large multipolar radiating), X2 (tufted), X4 (medium-size vertical), X5 (medium-size horizontal), T1 (medium-size trapezoid radiating), T2 (small ovoid vertical), I (small sparsely ramified), and A (small horizontal) neurons. Nigrorecipient cells participate in all four of the major efferent axonal systems of the deeper tectal layers: crossed descending (X and T neurons), ipsilateral descending (I and T neurons), ascending (A, X, and T neurons), and commissural (T neurons). EPSPs accompanied by long-lasting hyperpolarizing potentials were recorded from the remaining tectal neurons in response to stimulation of the substantia nigra, cerebral peduncle, and pericruciate cortex. Collision experiments indicate that at least part of the excitatory responses of tectal neurons to nigral and penduncular stimulation are mediated by corticotectal fibers traversing the cerebral peduncle and the substantia nigra. Excitatory effects of nigral, peduncular, and cortical stimulation were disclosed in X neurons including the non-nigrorecipient large vertical neurons of the X3 subgroup. Cortical excitatory and nigral inhibitory inputs converge only on X neurons (X1, X2, X4, X5). In this case, nigrally evoked IPSPs were preceded by a brief EPSP. Collectively, these results demonstrate the inhibitory termination of the nigrotectal pathway on a wide variety of deeper tectal efferent neurons. Such findings imply the versatility of the nigral involvement in tectal mechanisms of gaze control. We suggest that the substantia nigra pars reticulata contacts tectal neurons differing as to their response properties and shapes the signal carried by all the major tectofugal bundles.

The thalamic reticular nucleus of the adult rat: experimental anatomical studies

The thalamic reticular nucleus (TRN) is a sheet-like nucleus partially enclosing the dorsolateral and anterior aspects of the thalamus and traversed by the thalamo-cortical and cortico-thalamic fibre systems. This paper describes the cellular and synaptic organization of the TRN in adult albino rats on the basis of LM and EM studies of normal animals and experimental animals with injections of horseradish peroxidase (HRP) and/or lesions in various parts of the brain. Particular attention was paid to the dorso-caudal part of the TRN, which establishes connections with visual centres. LM-HRP preparations show that the neurons of TRN project only to ipsilateral dorsal thalamus; no labelled cell bodies were found in TRN after injections into the cortex or any part of the brain stem caudal to the thalamus. Small injections into dorsal thalamus result in a small cluster of labelled neurons and an associated patch of terminal label in TRN. The dorso-caudal part of the nucleus projects to the dorsal lateral geniculate nucleus, the ventro-caudal part to the medial geniculate nucleus and a large part of the nucleus anterior to the areas associated with the geniculate nuclei projects to the ventrobasal nucleus. No evidence was found for a widespread distribution of reticulo-thalamic axons and the connections between TRN and the dorsal lateral geniculate nucleus and between TRN and the ventrobasal nucleus show a fine-grain topographical organization with more rostral and dorsal parts of TRN projecting to more rostral and dorsal parts of the dorsal lateral geniculate and ventrobasal nuclei. The neurons of TRN are variable in size (range of somal diameters c. 10-20 micron), shape (cell bodies are most commonly ellipsoidal) and dendritic morphology (bitufted and bipolar arrangements most common), but no basis for subdividing them into more than one class was found with any of the techniques used. The cell body and dendrites are commonly aligned parallel to the surface of TRN and at right angles to the traversing fibre bundles. The dendrites do not branch extensively and are only moderately spinous. Long, hair-like spines corresponding to those described by Scheibel & Scheibel (1966) were not found: nor were dendritic bundles found to be as prominent in EM material as reported by these authors in LM-Golgi material. Plasma membranes of dendrites in small bundles and of contiguous somata were commonly in direct contact over large areas, but gap junctions between them were not seen.(ABSTRACT TRUNCATED AT 400 WORDS)

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 innervation of the inferior colliculus in adult cats: electron microscopic observations

A moderate number of degenerating synaptic boutons (DSB) in the pericentral nucleus of the inferior colliculus (IC) were identified electron microscopically following contralateral eye enucleation in adult cats. Most of the DSB measure 0.6-1 micron and a lower percent are larger (1-2.5 micron). Both types contain a slightly polymorph population of clear vesicles, mainly round and oval, and a very few are elongated. The DSB are involved in asymmetric axodendritic contacts. The present report provides evidence for a crossed retinofugal monosynaptic excitatory tract innervating the IC pericentral nucleus, a possible audio-visual integrative subcortical center.

The accessory optic system of rodents: a whole-mount HRP study

The three-dimensional fiber pathways of the accessory optic system in three species of rodents (rat, golden hamster, guinea pig) were examined on whole-mounted preparations of the diencephalon and the midbrain, without sectioning, by anterograde labeling of retinal axons with horseradish peroxidase (HRP). HRP histochemical studies on the serial coronal sections were also done. In this study, only the accessory optic system on the side contralateral to the eye injection of HRP was clearly detected. The rat accessory optic system consisted of the inferior fasciculus, the superior fasciculus, the medial terminal nucleus, the lateral terminal nucleus, and the dorsal terminal nucleus. After the inferior fasciculus arrived at the ventromedial border of the cerebral peduncle, some fibers from the inferior fasciculus ran caudally to the medial terminal nucleus. The remaining fibers from the inferior fasciculus further proceeded dorsocaudally on the surface of the cerebral peduncle and left the inferior fasciculus at various levels of the cerebral peduncle to be mixed up with the fibers from the superior faciculus. The golden hamster accessory optic system also consisted of the inferior fasciculus, the superior fasciculus, the medial terminal nucleus, the lateral terminal nucleus, and the dorsal terminal nucleus. However, all fibers of the inferior fasciculus ran caudally on the lateral surface of the hypothalamus or along the ventromedial border of the cerebral peduncle to terminate at the medial terminal nucleus. The guinea pig accessory optic system and rat accessory optic system were similar, but the posterior fibers of the superior fasciculus decreased in number, and the dorsal terminal nucleus and the posterior portion of the lateral terminal nucleus were not observed in the guinea pig accessory optic system.

Retinal projections to the pulvinar nucleus of the macaque monkey: a re-investigation using autoradiography

After a monocular injection of [3H]amino acid into the vitreous chamber of the eye, the distribution of retinal terminations in the pulvinar nucleus of the crab-eating monkey and pigtail macaque was studied by autoradiography. Two groups of labeled terminals were found in the bilateral inferior pulvinar nuclei: one small, dense group was located in the most rostral part of the nuclei and the other, composed of a few small clusters of the labeled terminals, was observed over the medial zone of the middle portion. The terminals were slightly predominant in the contralateral nucleus. A small amount of silver grains showing labeled retinofugal fibers was found in the dorsal surface of the thalamus just medial to the stria terminalis contralateral to the injected site, but termination of these fibers could not be traced in this study.

Development of the retinal pathway to the pretectum of the cat

The development of retinal projections to the pretectal complex of prenatal and early postnatal cats has been examined using the anterograde transport of horseradish peroxidase and tritiated amino acids. As early as embryonic day 38, the entire dorsal pretectum is penetrated by retinal ganglion cell axons. At this stage the bilateral complement of retinal efferents appears to be dispersed uniformly within the pretectal anlage. A week later, on embryonic day 46, indistinct foci of peroxidase reaction product can be discerned within 2 of the primordial nuclei: the nucleus of the optic tract and the olivary nucleus. By embryonic day 56, five distinct bilateral fields of retinal fiber termination are apparent within the following regions: (i) the nucleus of the optic tract; (ii) the pretectal olivary nucleus; (iii) the posterior pretectal nucleus; (iv) the anterior pretectal nucleus; and (v) the medial pretectal nucleus. Four days before birth, on embryonic day 61, crossed and uncrossed retinal arbors are partially segregated within the nucleus of the optic tract and the pretectal olivary nucleus. The early postnatal retinal connection to the pretectum has an overall pattern virtually indistinguishable from that of the mature cat. The ontogeny of the retinal influx to the pretectum is similar to that of the retinocollicular projection. However, the development of retinal projections to the pretectum and superior colliculus appears to lag behind those to the dorsal lateral geniculate nucleus. These differences may reflect temporal and spatial gradients in the maturation of three major classes of retinal ganglion cells.