Transient expression of synaptic zinc during development of uncrossed retinogeniculate projections

The transition metal zinc is an essential dietary constituent that is believed to serve an important intercellular signaling role at certain excitatory synapses in the central nervous system. In the present study, we used histochemical techniques to investigate the distribution of synaptic zinc during postnatal development of retinogeniculate projections in rats. From postnatal day (P) 1 until P-21, the pattern of zinc histochemical staining in the dorsal lateral geniculate nucleus (LGNd) precisely matched the distribution of axon terminals from the ipsilateral eye that were labeled by anterograde transport of horseradish peroxidase. Regions of the LGNd that contained only crossed axons were devoid of zinc staining. Abnormalities in the distribution of uncrossed retinogeniculate projections in albino versus pigmented rats were paralleled by identical variations in localization of synaptic zinc. Unilateral enucleation on P-10 was followed within 5 days by loss of zinc staining in the LGNd ipsilateral to the removed eye without affecting staining in the contralateral nucleus. Finally, the ability to detect zinc histochemically in the LGNd ceased at approximately P-24. These findings provide evidence that zinc is sequestered within synaptic boutons of a subpopulation of retinal ganglion cells whose axons terminate on the ipsilateral side of the brain. The duration of zinc staining overlaps with the major period of axonal remodeling in the LGNd, suggesting that synaptically released zinc may play a role in postnatal refinement of retinogeniculate projections.     

Effects of monocular deprivation on the morphology of retinogeniculate axon arbors in a primate

Previous studies of the monocularly deprived (lid-sutured) primate (Galago crassicaudatus) have shown that magnocellular (M) and parvocellular (P) lateral geniculate nucleus (LGN) cells that receive input from the deprived eye are smaller than counterparts that receive input from the nondeprived eye; deprived koniocellular (K) cells show wide variability in size, but they do not differ from their nondeprived counterparts (Casagrande and Joseph, '80). Although deprivation results in cell-size changes, the physiological properties of deprived LGN cells do not change from normal (that is, P cells have normal X-like properties, M cells have normal Y-like properties, and K cells have normal W-like properties). Because of these findings, we were interested in determining how the morphology of retinogeniculate axon arbors is affected by deprivation. To this end, 104 horseradish-peroxidase-filled retinogeniculate arbors from galagos deprived from birth to maturity were completely reconstructed within the binocular segment of the LGN. These arbors were qualitatively and quantitatively compared with 56 arbors reconstructed from normal galagos as part of another study (Lachica and Casagrande, '88). Our main findings are as follows. Deprived M and P arbors are affected by deprivation in the same general manner: compared with normal arbors, they are altered in shape (rather than being round or columnar, respectively, both groups have terminals that are elongated parallel to laminar borders); they are smaller in area, and they have fewer boutons but innervate the LGN with a greater density of boutons. K arbors are affected by deprivation in the same manner, but less severely. Finally, our results show that nondeprived arbors are also affected by eyelid suture. Specifically, all nondeprived arbor groups are smaller in area than normal and possess more boutons/mm3. We interpret these changes in the morphology of deprived retinogeniculate axons to suggest that abnormal competitive interactions begin by affecting primarily immature LGN cells and their axons and that the retinogeniculate axons presynaptic to these cells experience secondary degenerative effects. Our results also show that similar manipulations of visual experience can result in changes that are not necessarily comparable across species such as cats and primates.     

Prenatal development of retinogeniculate projections in the rabbit: an HRP study

The prenatal development of the rabbit's retinal projections to the dorsal lateral geniculate nucleus (dLGN) was studied by using anterograde axonal transport of HRP injected intraocularly. Further, the ontogenesis of the dLGN's alpha and beta sectors was studied. Fetuses aged embryonic day 18 (E18) to E29 were examined. Gestation in the rabbit is 30-31 days. On E18 the future dorsal lateral and medial geniculate nuclei appear as a continuous strip of cells along the lateral margin of the dorsal thalamus. On E21 labelled retinal fibers are invading the lateral margin of the dLGN contralateral, but not ipsilateral, to an injected eye. At this age the dorsal lateral and medial geniculate nuclei are separating. By E23 contralateral fibers occupy the entire presumptive alpha sector, while ipsilateral fibers are invading the caudal half of the sector, overlapping the contralateral fibers. At this age the alpha and beta sectors begin to differentiate. On E25 contralateral fibers are more densely distributed throughout the alpha sector and the ipsilateral fibers are concentrated dorsally within the caudal three-quarters of the sector. By E27 contralateral fibers begin to withdraw from a medial zone of the alpha sector, while ipsilateral fibers remain densest in this zone and begin to withdraw from more lateral and caudal aspects of the sector; contralateral fibers, but not ipsilateral fibers, invade the beta sector. At this age the alpha and beta sectors acquire an adult-like appearance. By E29 the contralateral fibers vacate the beta sector and the medial zone of the dLGN and the ipsilateral fibers are restricted to this zone. Thus, 1 or 2 days before birth, the locations of the ipsilateral and contralateral retinal projections to the dLGN resemble those seen in the adult. The early overlapping projections of ipsilateral and contralateral retinal fibers within the dLGN and their eventual segregation in the fetal rabbit are consistent with the development of these projections in other mammalian orders. Further, the brief invasion of the beta sector by the contralateral fibers resembles the transient occupation of the carnivores' perigeniculate nucleus by developing retinal fibers. In addition, direct comparisons of temporal and spatial events during retinal innervation of the dLGN and the superior colliculus indicate several developmental differences between the two nuclei.     

Heterogeneity of retinogeniculate axon arbors

The retinogeniculate synapse transmits information from retinal ganglion cells (RGC) in the eye to thalamocortical relay neurons in the visual thalamus, the dorsal lateral geniculate nucleus (dLGN). Studies in mice have identified genetic markers for distinct classes of RGCs encoding different features of the visual space, facilitating the dissection of RGC subtype‐specific physiology and anatomy. In this study, we examine the morphological properties of axon arbors of the BD‐RGC class of ON‐OFF direction selective cells that, by definition, exhibit a stereotypic dendritic arbor and termination pattern in the retina. We find that axon arbors from the same class of RGCs exhibit variations in their structure based on their target region of the dLGN. Our findings suggest that target regions may influence the morphologic and synaptic properties of their afferent inputs.     

Influence of the cortico-geniculate pathway on response properties of cat lateral geniculate neurons

The influence of the cortico-geniculate pathway on identified X and Y lateral geniculate cells was studied by reversibly cooling visual cortical areas 17 and 18. The majority (86.5%) of cells changed their response to visual stimulation when cortex was inactivated, and both X- and Y-cells were modulated by the cortical input. The influence of the visual cortex was complex, with both excitatory and inhibitory actions. Furthermore, the mechanism underlying the basic center-surround receptive field organization was influenced.     

A quantitative study of the complex laminated body in the lateral geniculate nucleus of the cat

A quantitative study has been made of the proportions and distribution of cells with complex laminated bodies (CLBs) in the lateral geniculate nucleus of the cat. They develop between 55 and about 70 days postnatal and are distributed irregularly across the medio-lateral extent of lamina A. There was no indication that the proportion of cells with CLBs is higher in the region of lamina A where the area centralis is represented, but the proportion in the binocular segment as a whole was higher than for the monocular segment. In kittens reared with unilateral or bilateral eyelid suture the proportions of cells with CLBs was above normal in the deprived laminae and below normal in the undeprived laminae.     

Effects of convergent strabismus on the development of physiologically identified retinogeniculate axons in cats

We have studied the effects of surgically induced convergent strabismus (esotropia) on the morphological development of retinogeniculate X and Y axon arbors in cats. Single axons were recorded in the lateral geniculate nucleus or in the optic tract adjacent to the nucleus, classified physiologically, and injected intracellularly with horseradish peroxidase. The arbors of recovered axons were compared with X and Y axon arbors from normally reared adult cats. Our data demonstrate that while X axon arbors are relatively normal, the arbors of Y axons are profoundly affected by rearing with strabismus. Y axons, whether originating from the deviated or the nondeviated eye, have substantially smaller arbors and fewer boutons in the A-laminae of the lateral geniculate nucleus compared to Y axons in normal cats. The C-lamina terminations of contralaterally projecting Y axons in the strabismic cats are unaffected. These results suggest that the postnatal development of retinogeniculate Y axon arbors in the A-laminae is strongly influenced by abnormalities in postnatal visual experience. Furthermore, the present data suggest that, in addition to intraocular competitive interactions between X and Y axons previously proposed to account for the effects of other rearing conditions, interactions between afferents from the two eyes must also be involved in the development of at least Y axons.     

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.     

A reassessment of the role of activity in the formation of eye-specific retinogeniculate projections

In all mammalian species the projections from the two eyes to the dorsal lateral geniculate nucleus of the thalamus terminate in separate layers or territories. This mature projection pattern is refined early in development from an initial state where the inputs of the two eyes are overlapping. Here I discuss the results of studies showing that the formation of segregated eye-specific retinogeniculate projections involves activity-mediated binocular competition. I conclude that while retinal activity undoubtedly is involved in this process, the results of recent studies cast doubt on the prevalent notion that retinal waves of activity play an instructional role in the formation of segregated retinal projections.     

Anatomy of macaque fovea and spatial densities of neurons in foveal representation

Fine visual sampling in the macaque depends on the high density of cone outer and inner segments in the fovea. Cone pedicles, at the opposite, presynaptic end of the cone, are absent from the center of the fovea. Both ends of the cones, inner segments and pedicles, are closely packed within their respective monolayers, but the spatial density of foveal pedicles is lower because foveal pedicles are wider than inner segments. Because there is one pedicle for every inner segment, and because pedicles are wider than inner segments, increase in eccentricity finds increasing lateral displacement of the cone's pedicle from its inner segment. Further increase of eccentricity finds inner segment density falling below pedicle density, and so this lateral displacement declines. By 2-3 mm from the center, inner segments catch up with pedicles. Additional lateral displacements, of bipolar cells from pedicles and ganglion from bipolar cells, are largest for central-most elements and fall steeply with eccentricity. By taking into account all of these lateral displacements, the eccentricity of the cone inner segment(s) associated with a ganglion cell was determined, as was the area of inner segments represented by a unit area in the ganglion cell layer. Then raw ganglion cell densities were transformed to densities comparable to densities of inner segments and of cells in dorsal lateral geniculate nucleus. On average there appears to be close to 2 ganglion cells for each cone in the central fovea out to about 2.5 degrees. Thus, the density of foveal ganglion cells is sufficient to allow each red and each green cone to connect to 2 midget ganglion cells, and each blue cone to connect to 1 ganglion cell. Furthermore, there appears to be a single dorsal lateral geniculate cell for each ganglion cell.     

Morphology of single, physiologically identified retinogeniculate Y-cell axons in the cat following damage to visual cortex at birth

It has been reported previously that neurons in the dorsal lateral geniculate nucleus (LGN) of cats with neonatal damage to visual cortex (KVC cats) have receptive fields that are abnormally large and that the receptive fields of these neurons sometimes do not appear to conform to the normal retinotopic order in the LGN. A primary aim of this study was to determine if these physiological abnormalities are related to inappropriate patterns of retinogeniculate connections. We therefore have analyzed the terminal arbors of retinogeniculate axons in adult cats that had received a lesion of visual cortex (areas 17, 18, and 19) on the day of birth. Single retinogeniculate axons were characterized physiologically and injected intracellularly with horseradish peroxidase. Consistent with earlier reports that neonatal removal of visual cortex results in a retrograde loss of retinal X-cells, all of the retinogeniculate axons that we recorded were from Y-cells. While the visual responses of these Y-cell axons were normal, the morphology of their terminal arbors in the LGN was abnormal. Retinal Y-cell axons in KVC cats have terminal fields in the A laminae of the LGN that are as large or larger than those of normal Y-cells. However, since the LGN in KVC cats is severely degenerated, single Y-cell arbors occupy a proportional volume of the LGN that is 12 times greater than normal. Thus an early lesion of visual cortex produces a severe mismatch between retinogeniculate axon arbor size and target size. Also, despite the normal size of retinogeniculate axon arbors in KVC cats, the number and density of terminal boutons are greatly decreased. Thus our morphological results suggest that the unusually large receptive fields of LGN cells in KVC cats and the relative lack of retinotopic precision in the LGN are due, at least in part, to anomalies in the relative size and distribution of retinogeniculate axon arbors that develop after neonatal removal of visual cortex.     

Functional impact of primary visual cortex deactivation on subcortical target structures in the thalamus and midbrain

The functional relationships between the primary visual cortex and its major subcortical target structures have long been a subject of interest. We studied these relationships by using localized cooling deactivation to silence portions of primary visual cortex and measuring 2-deoxyglucose (2DG) uptake to assess neural activity in subcortical and midbrain targets. We focused analysis on the largest subcortical targets of primary visual cortex: the superior colliculus (SC), the dorsal lateral geniculate nucleus of the thalamus (dLGN), and the lateral division of the lateral posterior nucleus of the thalamus (LPL). We found that localized cooling of different regions of primary visual cortex caused specific decreases in 2DG uptake in target structures such that the location of 2DG decrease varied according to joint retinotopy, and the magnitude of the decreases in target structures was associated with the amount of cooled cortex. In addition, we found that the impact of cortical cooling was more profound on the SC than on the dLGN. The functional impact of cortical deactivations on the LPL was weak for small deactivations but approximated the impact on the SC when deactivations were large. We discuss these findings in terms of neural circuits and in terms of drivers and modulators.     

Development of the Pathway From the Reticular and Perireticular Nuclei to the Thalamus in Ferrets: A Dil Study

This study examines the connections of the thalamic reticular and perireticular cell groups in developing ferrets. Small crystals of Dil (1,1′-dioctadecyl-3, 3, 3′, 3′-tetramethylindocarbocyanine perchlorate) were implanted into either the dorsal thalamus or the cerebral cortex of aldehyde-fixed prenatal and postnatal ferret brains. A small implant of Dil into the presumptive lateral geniculate nucleus during early prenatal development [between embryonic day 23 (E23) and E25] reveals many retrogradely labelled cells in the reticular nucleus. At E40, just before birth, the number of cells retrogradely labelled in the reticular nucleus has become reduced compared to earlier prenatal implants, whether from small or large implants of Dil into the lateral geniculate nucleus. By postnatal day 7, an adult-like pattern of retrograde labelling is seen in the reticular nucleus; at this age, a small implant of Dil limited to the lateral geniculate nucleus retrogradely labels a discrete group of cells located in the caudal regions of the reticular nucleus. In the internal capsule, adjacent to the reticular nucleus, there are two distinct groups of neurons. One group, called the large-celled perireticular zone (LPR), enters the internal capsule very early in development (from E25; Mitrofanis, J., Eur. J. Neurosci., 6 , 253–263, 1994) and is not labelled from the lateral geniculate nucleus at any developmental stage. Small implants of Dil into presumptive visual and somatosensory cortices shows that the LPR lies in a distinct region of the primordial internal capsule. Corticothalamic and thalamocortical axons turn sharply in the region of the LPR, whilst corticospinal and corticobulbar axons pass straight through the LPR on towards their more caudal targets. Later, after both sets of axons have reached their targets, the LPR is not seen in the internal capsule. The other group of cells in the internal capsule, called the small-celled perireticular zone (SPR), forms a distinct band of cells lying midway between the reticular nucleus and the globus pallidus. These cells enter the internal capsule much later in development, at about E40. Unlike the cells in the LPR, cells in the SPR are retrogradely labelled after an implant of Dil into the lateral geniculate nucleus, and there are many which remain in the adult (Clemence, A. E. and Mitrofanis, J., J. Comp. Neurol., 322 , 167–181, 1992).     

Organization of retinogeniculate projections in turtles of the genera Pseudemys and Chrysemys

Organization of retinal projections to the dorsal lateral geniculate complex in turtles has been studied by means of light and electron microscopic axon tracing techniques. Orthograde degeneration studies with Fink-Heimer methods following restricted retinal lesions show the entire retina has a topologically organized projection to the contralateral dorsal lateral geniculate complex. The nasotemporal axis of the retina projects along the rostrocaudal axis of the geniculate complex; the dorsoventral axis of the retina projects along the dorsoventral axis of the geniculate complex. The projection to the ipsilateral dorsal lateral geniculate complex originates from the ventral, temporal and nasal edges of the retina. The nasotemporal axis of the ipsilateral retina projects along the rostrocaudal axis of the geniculate complex. It was not possible to determine the orientation of the dorsoventral axis of the ipsilateral retina on the geniculate complex. Light microscopic autoradiographic tracing experiments and electron microscopic degeneration experiments show the retinogeniculate projection has a laminar organization. Retinogeniculate terminals are found in both the neuropile and cell plate throughout all three subnuclei of the dorsal lateral geniculate complex but have a distinctive distribution in each subnucleus. In the subnucleus ovalis, they are frequent in both the neuropile and cell plate which forms the rostral pole of the complex. In the dorsal subnucleus, they are most prevalent in the outer part of the neuropile layer, less frequent in the inner part of the neuropile, and rare in the cell plate. In the ventral subnucleus, they are frequent in the outer part of the neuropile but are also common in the inner part of the neuropile and cell plate. These observations point to several principles of geniculate organization in turtles. First, the complex receives projections from the entire contralateral retina and a segment of the ipsilateral retina. It thus has monocular and binocular segments that together receive a topologically organized representation of the binocular visual space and the contralateral monocular visual space. Second, the three geniculate subnuclei receive information from different, specialized regions of the retina and visual space. Subnucleus ovalis receives information from the frontal binocular visual field. The ventral subnucleus receives information from the caudal binocular field. The dorsal subnucleus receives input from the contralateral monocular field. Third, there is a lamination of retinal inputs in the geniculate complex which differs in character within the three subnuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Organization of corticogeniculate projections in the turtle, Pseudemys scripta

The efferent pathways from the visual cortex to the dorsal lateral geniculate complex of turtles have been studied by using the orthograde and retrograde transport of horseradish peroxidase (HRP). Injections of HRP in the lateral thalamus retrogradely label neurons throughout the visual cortex. The majority of labeled neurons have somata in layer 2 of the lateral part of dorsal cortex (D2); a minority have somata in layer 3. Labeled neurons in layer 2 tend to have vertically oriented, fusiform somata and dendrites that ascend into layer 1. Labeled neurons in layer 3 have fusiform somata and dendrites, both oriented horizontally. Injections of HRP in visual cortex orthogradely label corticofugal axons. Those projecting to the lateral geniculate complex course laterally from the visual cortex, pass through the striatum (occasionally bearing varicosities), and enter the diencephalon in the ventral peduncle of the lateral forebrain bundle. Individual axons leave the ventral peduncle and run dorsally in the transverse plane, entering the dorsal lateral geniculate complex from its ventral edge. They continue dorsally, principally in the cell plate of the geniculate complex, where they bear varicosities.     

Depression of retinogeniculate synaptic transmission by presynaptic D(2)-like dopamine receptors in rat lateral geniculate nucleus

Extraretinal projections onto neurons in the dorsal lateral geniculate nucleus (dLGN) play an important role in modifying sensory information as it is relayed from the visual thalamus to neocortex. The dLGN receives dopaminergic innervation from the ventral tegmental area; however, the role of dopamine in synaptic transmission in dLGN has not been explored. In the present study, whole cell recordings were obtained to examine the actions of dopamine on glutamatergic synaptic transmission. Dopamine (2-100 microm) strongly suppressed excitatory synaptic transmission in dLGN relay neurons that was evoked by optic tract stimulation and mediated by both N-methyl-d-aspartate and non-N-methyl-d-aspartate glutamate receptors. In contrast, dopamine did not alter inhibitory synaptic transmission arising from either dLGN interneurons or thalamic reticular nucleus neurons. The suppressive action of dopamine on excitatory synaptic transmission was mimicked by the D(2)-like dopamine receptor agonist bromocriptine (2-25 microm) but not by the D(1)-like receptor agonist SKF38393 (10-25 microm). In addition, the dopamine-mediated suppression was antagonized by the D(2)-like receptor antagonist sulpiride (10-20 microm) but not by the D(1)-like receptor antagonist SCH23390 (5-25 microm). The dopamine-mediated decrease in evoked excitatory postsynaptic current amplitude was accompanied by an increase in the magnitude of paired-pulse depression. Furthermore, dopamine also reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents. Taken together, these data suggest that dopamine may act presynaptically to regulate the release of glutamate at the retinogeniculate synapse and modify transmission of visual information in the dLGN.     

Retinofugal projections following early lesions of the visual cortex in the ferret

Extensive lesions of the occipital cortex comprising the developing occipital visual areas and beyond in young ferrets (postnatal day 5) are followed by massive, but incomplete, degeneration of the lateral geniculate (LGN) and lateralis posterior (LP) nuclei of the thalamus, and minor volumetric reduction of the superior colliculus. Retinal projections (revealed by intraocular tracer injections), while reduced, remain confined to their territories of normal termination, both in the adult and throughout development. Comparisons with other mammalian species point to several common features in the developmental plasticity of retinofugal pathway.     

Superior colliculus connections with visual thalamus in gray squirrels (Sciurus carolinensis): evidence for four subdivisions within the pulvinar complex

As diurnal rodents with a well-developed visual system, squirrels provide a useful comparison of visual system organization with other highly visual mammals such as tree shrews and primates. Here, we describe the projection pattern of gray squirrel superior colliculus (SC) with the large and well-differentiated pulvinar complex. Our anatomical results support the conclusion that the pulvinar complex of squirrels consists of four distinct nuclei. The caudal (C) nucleus, distinct in cytochrome oxidase (CO), acetylcholinesterase (AChE), and vesicular glutamate transporter-2 (VGluT2) preparations, received widespread projections from the ipsilateral SC, although a crude retinotopic organization was suggested. The caudal nucleus also received weaker projections from the contralateral SC. The caudal nucleus also projects back to the ipsilateral SC. Lateral (RLl) and medial (RLm) parts of the previously defined rostral lateral pulvinar (RL) were architectonically distinct, and each nucleus received its own retinotopic pattern of focused ipsilateral SC projections. The SC did not project to the rostral medial (RM) nucleus of the pulvinar. SC injections also revealed ipsilateral connections with the dorsal and ventral lateral geniculate nuclei, nuclei of the pretectum, and nucleus of the brachium of the inferior colliculus and bilateral connections with the parabigeminal nuclei. Comparisons with other rodents suggest that a variously named caudal nucleus, which relays visual inputs from the SC to temporal visual cortex, is common to all rodents and possibly most mammals. RM and RL divisions of the pulvinar complex also appear to have homologues in other rodents.     

Evidence of sub-collicular auditory projections to the medial geniculate nucleus in the cat: An autoradiographic and horseradish peroxidase study

Connections of a posteromedial region of the ventral nucleus of the lateral lemniscus were examined in the cat using the autoradiographic tracing method. This sub-collicular region previously had been shown, using retrograde transport of horseradish peroxidase, to send axons to the superior colliculus¹⁰. The autoradiographic findings revealed that many axons from the posteromedial region of the ventral nucleus of the lateral lemniscus that entered the superior colliculus continued into the midbrain reticular formation. Moreover, other axons traced rostral to the inferior colliculus into the thalamus ended in the medial geniculate nucleus, bilaterally. Experiments in which horseradish peroxidase was placed in the medial geniculate nucleus retrogradely labeled the large neurons in the posteromedial region supporting the autoradiographic observations. Other sub-collicular regions also contained labeled cells in these cases, including the main body of the ventral nucleus of the lateral lemnicus and scattered cell groups around the superior olivary complex.