The medial terminal nucleus of the monkey: evidence for a 'complete' accessory optic system

Retinorecipient areas in the common marmoset (Callithrix jacchus): An image-forming and non-image forming circuitry

The mammalian retina captures a multitude of diverse features from the external environment and conveys them via the optic nerve to a myriad of retinorecipient nuclei. Understanding how retinal signals act in distinct brain functions is one of the most central and established goals of neuroscience. Using the common marmoset (Callithrix jacchus), a monkey from Northeastern Brazil, as an animal model for parsing how retinal innervation works in the brain, started decades ago due to their marmoset's small bodies, rapid reproduction rate, and brain features. In the course of that research, a large amount of new and sophisticated neuroanatomical techniques was developed and employed to explain retinal connectivity. As a consequence, image and non-image-forming regions, functions, and pathways, as well as retinal cell types were described. Image-forming circuits give rise directly to vision, while the non-image-forming territories support circadian physiological processes, although part of their functional significance is uncertain. Here, we reviewed the current state of knowledge concerning retinal circuitry in marmosets from neuroanatomical investigations. We have also highlighted the aspects of marmoset retinal circuitry that remain obscure, in addition, to identify what further research is needed to better understand the connections and functions of retinorecipient structures.

A simpler primate brain: the visual system of the marmoset monkey

Humans are diurnal primates with high visual acuity at the center of gaze. Although primates share many similarities in the organization of their visual centers with other mammals, and even other species of vertebrates, their visual pathways also show unique features, particularly with respect to the organization of the cerebral cortex. Therefore, in order to understand some aspects of human visual function, we need to study non-human primate brains. Which species is the most appropriate model? Macaque monkeys, the most widely used non-human primates, are not an optimal choice in many practical respects. For example, much of the macaque cerebral cortex is buried within sulci, and is therefore inaccessible to many imaging techniques, and the postnatal development and lifespan of macaques are prohibitively long for many studies of brain maturation, plasticity, and aging. In these and several other respects the marmoset, a small New World monkey, represents a more appropriate choice. Here we review the visual pathways of the marmoset, highlighting recent work that brings these advantages into focus, and identify where additional work needs to be done to link marmoset brain organization to that of macaques and humans. We will argue that the marmoset monkey provides a good subject for studies of a complex visual system, which will likely allow an important bridge linking experiments in animal models to humans.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.

The retinal projections to the ventral and dorsal divisions of the medial terminal nucleus and mesencephalic reticular formation in the Japanese monkey (Macaca fuscata): a reinvestigation with cholera toxin B subunit as an anterograde tracer

After a monocular injection of the cholera toxin B subunit (CTB) into the vitreous chamber of the eye, retinal projections to the medial terminal nucleus (MTN) of the accessory optic system (AOS) were studied in the Japanese monkey. The anterogradely transported tracer was visualized with the peroxidase antibody technique by using an anti-cholera toxin antibody. One small accumulation of the CTB-immunopositive retinofugal terminals was located in a small area just medial to the medial edge of the cerebral peduncle and anterior to the attachment of the oculomotor nerve, suggesting the existence of a ventral division of the MTN of the AOS. Caudally, one very small bundle of the retinofugal fibers extending dorsally from this accumulation was seen running along the medial edge of the cerebral peduncle and substantia nigra to the small region corresponding to the dorsal division of the MTN. A few small bundles of CTB-immunopositive retinal fibers were observed to leave the superior fasciculus of the AOS at various points. These fibers coursed medially through the cerebral peduncle and substantia nigra to reach some restricted areas of the mesencephalic reticular formation between the medial lemniscus and the substantia nigra.

Projections from visual areas of the cerebral cortex to pretectal nuclear complex, terminal accessory optic nuclei, and superior colliculus in macaque monkey

The purpose of this study was to analyze the projections from visually related areas of the cerebral cortex of rhesus monkey to subcortical nuclei involved in eye-movement control; i.e., the pretectal nuclear complex, the terminal nuclei of the accessory optic system (AOS), and the superior colliculus (SC). The anterograde tracer 3H-leucine was pressure injected bilaterally into the cortex of six monkeys (for a total of 12 cases) involving the primary visual cortex (area 17); the medial prestriate cortex (medial 18/19); dorsomedial area 19; the caudal portion of the cortex of the superior temporal sulcus, upper bank (cytoarchitectural area OAa) and lower bank (area PGa); the lower bank of the caudal lateral intraparietal sulcus (area POa); and the inferior parietal lobule (area 7). The results revealed that the pretectal nucleus of the optic tract received inputs from medial prestriate cortex, dorsomedial part of area 19, OAa, and PGa. The posterior pretectal nucleus received sparse projections from area 7 and the cortex lining the intraparietal sulcus (dorsomedial part of area 19 and POa). The pretectal olivary nucleus was targeted by neurons in cortex of dorsomedial area 19, and the anterior pretectal nucleus was targeted by neurons in both dorsomedial 19 and area 7. The nuclei of the AOS (dorsal terminal; lateral terminal; and interstitial nuclei of the superior fasciculus, posterior and medial fibers) received projections exclusively from areas OAa and PGa. Furthermore, in one case with PGa injection, the medial terminal nucleus, dorsal portion, was also labeled. The visual cortical areas studied projected differentially upon the SC laminae. The primary visual area 17 projected only to the superficial laminae, i.e., stratum zonale (SZ), stratum griseum superficiale (SGS), and stratum opticum (SO). On the other hand, the medial portion of the prestriate cortex and caudal OAa and PGa targeted the superficial and intermediate laminae, i.e., SZ, SGS, SO, and stratum griseum intermediale (SGI), whereas caudal area POa projected primarily to the intermediate layer SGI. Rostral area 7 (mainly 7b) neurons terminated in the stratum album intermediale (SAI); no SC terminals were found in a case in which caudal area 7 (mainly 7a) was injected.

Projections of the lateral terminal accessory optic nucleus of the common marmoset (Callithrix jacchus)

The connections of the lateral terminal nucleus (LTN) of the accessory optic system (AOS) of the marmoset monkey were studied with anterograde 3H-amino acid light autoradiography and horseradish peroxidase retrograde labeling techniques. Results show a first and largest LTN projection to the pretectal and AOS nuclei including the ipsilateral nucleus of the optic tract, dorsal terminal nucleus, and interstitial nucleus of the superior fasciculus (posterior fibers); smaller contralateral projections are to the olivary pretectal nucleus, dorsal terminal nucleus, and LTN. A second, major bundle produces moderate-to-heavy labeling in all ipsilateral, accessory oculomotor nuclei (nucleus of posterior commissure, interstitial nucleus of Cajal, nucleus of Darkschewitsch) and nucleus of Bechterew; some of the fibers are distributed above the caudal oculomotor complex within the supraoculomotor periaqueductal gray. A third projection is ipsilateral to the pontine and mesencephalic reticular formations, nucleus reticularis tegmenti pontis and basilar pontine complex (dorsolateral nucleus only), dorsal parts of the medial terminal accessory optic nucleus, ventral tegmental area of Tsai, and rostral interstitial nucleus of the medial longitudinal fasciculus. Lastly, there are two long descending bundles: (1) one travels within the medial longitudinal fasciculus to terminate in the dorsal cap (ipsilateral >> contralateral) and medial accessory olive (ipsilateral only) of the inferior olivary complex. (2) The second soon splits, sending axons within the ipsilateral and contralateral brachium conjunctivum and is distributed to the superior and medial vestibular nuclei. The present findings are in general agreement with the documented connections of LTN with brainstem oculomotor centers in other species. In addition, there are unique connections in marmoset monkey that may have developed to serve the more complex oculomotor behavior of nonhuman primates.

Macaque accessory optic system: I. Definition of the medial terminal nucleus

The organization of the accessory optic system (AOS) has been studied in the macaque monkey following intravitreal injections of tritiated amino acids in one eye. Retinal projections to the dorsal (DTN) and the lateral (LTN) terminal nuclei are identical to those previously described in other primate species. We observed an additional group of retinorecipient cells of the AOS, located between the cerebral peduncle and the substantia nigra, which we define as the interstitial nucleus of the superior fasiculus, medial fibers. In this report, we focus our attention on the medial terminal nucleus (MTN). Although a ventral division of this nucleus (MTNv) was not observed in the macaque, the retina projects to a group of cells in the midbrain reticular formation (MRF), which we argue to be homologous to the dorsal division of the MTN (MTNd). To provide evidence in support of this homology, the retinal projection to the MTNv and MTNd was also examined in 21 additional species from 11 orders of mammals including carnivores, marsupials, lagomorphs, rodents, bats, insectivores, tree shrews, hyraxes, pholidotes, edentates, and five additional species of primates. Whereas the retina projects to both ventral and dorsal divisions in all species studied, in haplorhine primates only the projection to the MTNd is conserved. The relative topological position of the MTNd in the MRF, dorsomedial to the substantia nigra and ventrolateral to the red nucleus, remains constant throughout the mammals. The trajectory of fiber paths innervating the MTNd is also similar in all species. In addition, the MTNd has comparable afferent and efferent connections with retina, pretectum, and vestibular nuclei in all species thus far studied. These results support the unequivocal conclusion that the MTNd is an unvarying feature of the mammalian AOS.

Macaque accessory optic system: II. Connections with the pretectum

Connections of the accessory optic system (AOS) with the pretectum are described in the macaque monkey. Injections of tritiated amino acids in the pretectum demonstrate a major contralateral projection to the dorsal (DTN), lateral (LTN), and medial (MTN) terminal nuclei of the AOS and a sparser projection to the ipsilateral LTN. Injections of retrograde tracers, Fast Blue (FB), or wheat germ agglutinin horseradish peroxidase (WGA-HRP) plus nonconjugated horseradish peroxidase (HRP) in the LTN show that the pretectal-LTN projection originates from two nuclei. The main source of pretectal efferents to the LTN is from the pretectal olivary nucleus (OPN) and is entirely contralateral. This projection, which appears unique to primates, originates from the large multipolar cells of the OPN. In addition to this projection, the nucleus of the optic tract (NOT) projects to the ipsilateral LTN, as in nonprimates. Injection of WGA-HRP in the pretectum shows a reciprocal predominantely ipsilateral projection from the LTN to the pretectum. Retinas were observed after injection of FB in the LTN. The retinal ganglion cells projecting to the AOS are mainly distributed near the fovea and in the nasal region of the contralateral eye, suggesting a nasotemporal pattern of decussation. The demonstration of a direct connection between LTN and OPN forces to a reconsideration of the functional role of the AOS. Previous descriptions of luminance responsive cells in the LTN support a possible participation of this nucleus in the control of the pupillary light reflex.

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.

The human accessory optic system

The accessory optic system (AOS) has been extensively studied among vertebrates, including primates. It has never clearly been identified in man, and it has not been considered functionally important by clinicians. Because of a lack of a suitable neuroanatomical tract-tracing technique, anatomical demonstration of a retinofugal pathway to the human AOS had previously not been feasible. A modified osmium impregnation method has been shown to permit the tracing of degenerated fibers in man even after long survival periods. This technique employs p-phenylene diamine (PPD) as a marker of myelin and products of axonal degeneration. We applied the PPD method in the examination of one monkey brain (Cynomolgus) and two human autopsy brains with previous visual system lesions. The lateral, dorsal, and medial terminal accessory optic nuclei and the interstitial nucleus of the superior fasciculus, posterior fibers (LTN, DTN, MTN, and inSEp) in the monkey and the LTN, the DTN, and the inSEp in the human all showed degenerated axons and preterminal axonal profiles indicative of direct retinal input. The ventral midbrain tegmentum including the MTN area was not available for study in either of the human brains. The accessory optic projections in both the monkey and human brains proved to be bilateral but primarily crossed. The human visual system thus shares similarities with the simian, in the location and number of the AOS fiber bundles and terminal nuclei and in the organization of the retinofugal projections to these nuclei.

Retinofugal projections in hedgehog-tenrecs (Echinops telfairi and Setifer setosus)

Using the autoradiographic tracing technique the retinal projections were studied in the tenrecs, Echinops telfairi and Setifer setosus (insectivora, tenrecidae). Bilateral projections were found to the n. suprachiasmaticus, the anterior hypothalamic area, the dorsal and ventral lateral geniculate bodies, the pretectal olivary nucleus and the superior colliculus. The contralateral projections were usually more intense than the ipsilateral ones except the retinohypothalamic connections. A partial segregation of the projection fields from both eyes was present in the dorsal and ventral lateral geniculate bodies. In the superior colliculus retinal fibers predominantly involved the stratum zonale and the upper portion of the stratum griseum superficiale on both sides. The projections to the deeper portion of the colliculi were rather faint, particularly on the ipsilateral side. Target areas receiving contralateral projections exclusively were the periamygdaloid area (labeled only in Setifer), the terminal accessory nuclei including the n. tractus optici and the inferior colliculus. The data are compared with other species. The most striking finding may concern the projection to the medial terminal nucleus being quite prominent in marsupials and most eutherian mammals (including the erinaceomorphous hedgehogs), but greatly reduced in tenrecs and primates.

Retinal ganglion cells projecting to the accessory optic system in the rat

The present data identify the distribution and morphological features of a homogeneous group of rat retinal ganglion cells. These cells were labelled after injection of either horseradish peroxidase or a fluorescent tracer, Fast Blue, into the medial terminal nucleus (MTN) of the accessory optic system. After retrograde fluorescent labelling, MTN-projecting retinal ganglion cells were intracellularly injected with Lucifer Yellow to reveal their complete dendritic morphology. There were on average 1,750 MTN-projecting cells fairly evenly distributed over the entire retinal ganglion cell layer. Their density ranged from 40-49 cells/mm2 in superior retina to 10-19 cells/mm2 towards the peripheral regions of both inferior and superior retina. The area of highest density formed a nasal-temporal band suggestive of a visual streak. Soma diameters ranged from 8.7 to 14.5 micron centrally and from 9.9 to 17.1 microns peripherally. Maximal dendritic field diameter ranged from 431 to 644 micron and averaged 516 micron with no obvious eccentricity dependence. The majority of MTN-projecting cells were bistratified. Dendrites stratified predominantly in the inner sublamina of the inner plexiform layer (IPL) with a varying number of branches from the remaining dendrites contained within the outer IPL, both strata presumably corresponding to the electrophysiologically determined on-off dichotomy. Cells projecting to the MTN were characterised by higher-order dendritic branching patterns that resulted in a dense dendritic tree with minor dendritic overlap. The slender dendrites had a beaded appearance and displayed spiny protrusions. The dendritic coverage of 5-6, stratification pattern, and overall morphological appearance of rat MTN-projecting cells renders them suitable candidates for on-direction--selective cells shown electrophysiologically to be linked with the MTN of the accessory optic system.

Vertical and horizontal visual whole-field motion differently affect the metabolic activity of the rat medial terminal nucleus

The metabolic activity of the medial terminal nucleus (MTN) of the Accessory Optic System was studied by means of the [14C]2-deoxyglucose (2-DG) method in Long-Evans rats exposed to moving and stationary visual stimuli. In particular we explored the rate of local cerebral glucose utilization (LCGU) and the spatial distribution of 2-DG uptake within MTN related to visual stimuli capable of triggering optokinetic nystagmus. It was found that increases in MTN metabolism accompanied the retinal slip signals evoked by whole-field visual patterns moving in the vertical as well as in the horizontal direction. At the same level of luminous flux neither the same but stationary pattern, nor constant, diffuse illumination were able to elicit comparable changes in MTN metabolic rates. The effects of vertical and horizontal motions differed, however, from each other. In binocular testing LCGU rates resulted significantly higher after vertically moving patterns and upon the same stimulus condition the spatial distribution of 2-DG matched very closely the spatial distribution of the retinal afferents and the cellular density within MTN, in sharp contrast with the diffuse spreading out of the label across the nucleus following horizontal motion. In monocular testings only the vertically moving patterns were able to increase LCGU rates significantly and then in contralateral MTN alone. However, comparison between the levels of glucose consumption measured in binocular and in monocular vision also showed the involvement of the uncrossed retinal path in relaying the retinal slip signals to MTN. No difference in LCGU and in spatial distribution of the label were finally observed in relation to the upward or to the downward direction of the moving pattern.

Accessory optic system of an anthropoid primate, the gibbon (Hylobates concolor): evidence of a direct retinal input to the medial terminal nucleus

The accessory optic system (AOS) was studied in an anthropoid primate by using anterograde transport of tritiated amino acids and autoradiographic techniques. The course of the accessory optic tract (AOT) and the retinal projection to the terminal nuclei are described in the gibbon and compared to that of other mammals. The AOT consists of a superior fasciculus, which includes both an anterior and a posterior fiber branch. An inferior fasciculus of the AOT is absent. In contrast to previous reports in haplorhine primates, which describe the AOS as consisting of only the dorsal (DTN) and the lateral (LTN) terminal nuclei, we find that in the gibbon, three cellular groups receive a bilateral projection, predominantly from the contralateral retina. According to cytoarchitecture and topographic location, two of these nuclei correspond to the DTN and the LTN. The third cellular group, situated dorsomedial to the substantia nigra, receives a distinct retinal projection and extends rostrocaudally for 2.0 mm in the mesencephalon. This nucleus is homologous to the dorsal division of the medial terminal nucleus (MTN) in other mammals. There was no evidence for a ventral division of the MTN, which in nonprimates is typically situated at the ventromedial base of the cerebral peduncle. Examination of brain morphology in primates suggests that the ventral division of the MTN has been displaced from its phylogenetically stable location in the medial part of the ventral midbrain to a more dorsal position. This shift appears to be a consequence of the overall morphological influences resulting from the relative enlargement of the pons in this region. The demonstration of a direct retinal projection to the MTN in the gibbon, as well as recent reports in other primates, indicates that a complete AOS consisting of three terminal nuclei is a feature common to all mammals.

A common mammalian plan of accessory optic system organization revealed in all primates

The accessory optic system (AOS), which was described as early as 1870 by Gudden, constitutes a distinct midbrain visual pathway in all classes of vertebrates. In non-primate mammals, retinal fibres of this system project to a set of three nuclei: the dorsal (DTN), the lateral (LTN) and the medial (MTN) terminal nuclei. Whereas all AOS cells respond to the slow motion of large visual stimuli, the neurons are tuned to complementary directions of movement: horizontal temporo-nasal direction for the DTN, vertical up and down for the LTN and vertical down for the MTN. It has thus been suggested that these nuclei establish a system of retinal coordinates for the detection of whole field motion. As the AOS provides direct and indirect pathways to both oculomotor and vestibular structures, each of these nuclei is thought to be an essential link in the co-ordination of eye and head movements in relation to movement within the visual-field. One problem for the generalization of this theory is that the medial terminal nucleus has never been found in primates. In this report we establish both the existence of this nucleus and its afferent input from the retina in all major groups of primates (prosimians, New and Old World monkeys and apes), indicating a common anatomical plan of organization of the AOS in mammals.

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.