Quantitative comparison of retinal synapses in the dorsal and ventral (parvicellular) C laminae of the cat dorsal lateral geniculate nucleus

Three physiological classes of retinal ganglion cell project to the cat dorsal lateral geniculate nucleus (DLGN). The dorsal laminae A, A1, and magnocellular C receive X and Y retinal input, whereas the ventral parvicellular laminae C1 and C2 receive predominantly W input. We have compared quantitatively the retinal synaptic terminals of the dorsal and ventral laminae to determine whether there are morphological differences in the terminals that correspond to their different response properties. Anterogradely labeled retinal synaptic terminals in all laminae contained pale mitochondria and large, round synaptic vesicles. However, retinal terminals with pale mitochondria varied in size and synaptic organization in different laminae. The terminals in the A laminae were, on average, quite large and made numerous contacts with conventional dendritic profiles and with profiles that themselves contained synaptic vesicles (F2 profiles). The terminals in lamina C that contained pale mitochondria had a smaller overall mean area. Terminals with pale mitochondria in C1 and C2 were almost all small and synapsed with F2 profiles less frequently than did terminals in the A laminae or in lamina C. These results provide quantitative evidence that visual areas receiving W-type retinal input contain smaller retinal terminals and have a different synaptic organization from that of laminae receiving X and Y input.     

Variations in the retinal synapses of the cat superior colliculus revealed using quantitative electron microscope autoradiography

This study has examined the retinal synapses of the cat superior colliculus using electron microscope autoradiography and morphometric techniques. The depth of each retinal synapse was measured using a computer-based EM plotter. The area, perimeter, and synapse contact density of selected synapses were calculated using a computer-based digitizer. Pale mitochondria were found to be an accurate cytological marker of retinal input to the colliculus. Fifty-eight percent of pale mitochondria terminals were labeled in the colliculus contralateral to eye injections. Ten percent of pale mitochondria terminals were labeled in the ipsilateral colliculus. A few labeled terminals contained dark mitochondria. The labeled retinal terminals in the contralateral colliculus were concentrated in a 60 μm wide dense band at the top of the superficial gray layer. They were also found within the deep superficial gray and upper optic layers. This distribution corresponded exactly to a larger population of pale mitochondria terminals. The cross-sectional area and synaptic contact density of selected pale mitochondria terminals varied with depth. Within the upper superficial gray, the terminals were small (mean area= 1.26 μ²) and high contact densities (mean= 0.25 per μm). These small terminals were also found deeper within the colliculus. Below the upper subdivision of the superficial gray, some labeled terminals were much larger and had lower contact densities. These results suggest there may be two subpopulations of retinal terminal in the cat superior colliculus: (1) small terminals with scalloped contours and complex synaptic relationships which may correspond to W-type input; and (2) larger terminals with simpler synaptic relationships which are distributed deeper and may correspond to Y-type input.     

Cytoarchitectonic and connectional organization of the ventral lateral geniculate nucleus in the cat

The ventral lateral geniculate nucleus is a small extrageniculate visual structure that has a complex cytoarchitecture and diverse connections. In addition to small-celled medial and lateral divisions, we cytoarchitectonically defined a small-celled dorsal division. A large-celled intermediate division intercalated between the three small-celled divisions, which we divided into medial and lateral intermediate subdivisions. In WGA-HRP injection experiments, the different cytoarchitectonic divisions were shown to have connections with different nuclei. The medial division was reciprocally connected to the pretectum and projected to the superficial layers of the superior colliculus and the intralaminar nuclei. The medial intermediate division received projections from the intermediate layer of the superior colliculus and the lateral and interpositus posterior cerebellar nuclei, and projected to the intermediate layer of the superior colliculus, the periaqueductal gray of midbrain, and the intralaminar nuclei. The lateral intermediate divisions received projections from the pretectum, the intermediate layer of the superior colliculus, and the lateral and interpositus posterior cerebellar nuclei, and projected to the pretectum, superficial layers of the superior colliculus, and the pulvinar. The lateral division received projections from superficial layers of the superior colliculus and had reciprocal connections with the pretectum. The dorsal division received projections from the pretectum and had reciprocal connections with the periaqueductal gray of midbrain. The different cytoarchitectonic divisions of the ventral lateral geniculate nucleus are thus suggested to play different functional roles related to vision, eye and head movements, attention, and defensive reactions.     

The serotoninergic fibers in the dorsal lateral geniculate nucleus of the cat: distribution and synaptic connections demonstrated with immunocytochemistry

The distribution, morphology, and synaptic contacts of serotoninergic fibers were studied with immunocytochemical methods in the lateral geniculate complex of the cat. The serotonin-immunoreactive fibers are diffusely distributed throughout the main laminae of the dorsal lateral geniculate nucleus (dLGN) and the perigeniculate nucleus (PGN) and reach a particular density in the ventral lateral geniculate nucleus (vLGN). The labeled fibers are in most cases very thin and sometimes varicose. There is no obvious order in their distribution pattern except that they sometimes partially encircle the unlabeled cell bodies of the dLGN. The synaptic connections of the serotoninergic fibers were investigated mainly in the A laminae of the dLGN. Few synaptic complexes were found, most of them with asymmetric morphology. The postsynaptic elements were small dendritic profiles. Perisomatic serotoninergic fibers were seen, but no convincing synaptic contacts were found between labeled fibers and cell somata. In the dLGN, serotoninergic profiles were almost exclusively confined to the extraglomerular neuropile. In the PGN serotoninergic fibers also contacted dendritic profiles and formed asymmetrical synapses, but as in the geniculate, synaptic specializations were very rare.     

ON/OFF organization in the cat lateral geniculate nucleus: sublaminae vs. columns

The hypothesis was tested that ON and OFF cells in the A layers of the cat's lateral geniculate nucleus (l.g.n.) form separate ON and OFF columns, and new evidence for their sublaminar distributions is presented. Evidence for columns was sought in previously published results from 184 electrode tracks oriented parallel to the layers and in new experiments, based on multiple, closely spaced electrode tracks oriented vertically to the layers. The tracks parallel to the layers were analyzed in two ways. First, the distances between cells of like-sign and unlike-sign were measured. Second, the signs of response of cells above and below one another, from pairs of tracks made simultaneously in the same layer, were compared. The analyses, which included computer simulations of the experiments based on columnar and noncolumnar models, provided no evidence for columns. The results from the closely spaced, vertical tracks also failed to show columns but confirmed that the organization of ON and OFF cells in the layers is fundamentally sublaminar.     

Afferent projections to the ventral lateral geniculate nucleus in the cat

Horseradish peroxidase (HRP) injections were made into the dorsal lateral geniculate nucleus (LGN_d) and ventral lateral geniculate nucleus (LGN_v) of the cat in order to define afferent projections to LGN_v. These were found from the superior colliculus, contralateral LGN_v, dorsal median raphe nucleus, locus coeruleus, ipsilateral pretectum, and various portions of visual cortex. While many cortical areas project to LGN_v (17, 18, 19, 21 and lateral suprasylvian), the heaviest input arises from areas 17 and 20. The cell bodies of origin are in layer 5 in contrast to layer 6 which projects to LGN_d.     

The organization of retinal maps within the dorsal and ventral lateral geniculate nuclei of the rabbit

The cytoarchitectonic subdivisions in the rabbit's dorsal and ventral lateral geniculate nuclei have been related to the several retinal maps that can be defined in terms of the distribution of retinal axons within these nuclei. Destruction of different retinal sectors was combined with intravitreal injections of ³H-proline, so that the distribution of fiber degeneration and autoradiographic label in the geniculate nuclei could be used to define the retinal maps in each nucleus, and to compare the two nuclei with each other. The two nuclei show surprisingly similar patterns of organization. Each is made up of a laminated alpha sector that curves around a relatively cell-sparse beta sector. Two morphologically distinct layers of each alpha sector receive contralateral retinal afferents and between these there is a small region in receipt of ipsilateral afferents. In each nucleus, the lines of projection that represent single points in visual space pass perpendicular to the layers of the alpha sector and continue an almost straight course into the beta sector. Quantitative comparisons of the retinal maps show that the relative volumes devoted to the representation of segments of the visual field are approximately the same in the two nuclei.     

Immunohistochemical organization of the ventral lateral geniculate nucleus in the tree shrew.

The ventral lateral geniculate nucleus (vLGN) of the tree shrew (Tupaia belangeri) was differentiated into multiple subdivisions (dorsal cap, intergeniculate leaflet, parvicellular segment, and internal and external magnocellular laminae, the latter being further divisible into a lateral and medial division) on the basis of retinal projections, immunochemistry, and histochemistry. Retinal projections traced with intravitreal injections of wheat germ agglutinin conjugated horseradish peroxidase revealed direct bilateral input to all subregions of the vLGN, except for the internal magnocellular lamina (which received only contralateral input) and the parvicellular segment (which was not retinorecipient). Furthermore, retinal inputs clearly distinguished the relatively heavily retinorecipient intergeniculate leaflet from the less prominently labeled dorsal cap. Immunohistochemical localization of Neuropeptide Y (NPY) perikarya revealed their prominence in the intergeniculate leaflet and the external magnocellular laminae with a concentration along the optic tract. NPY immunoreactive fibers were seen in all but the parvicellular subregion. Gamma amino butyric acid immunoreactivity was seen throughout the vLGN, but was most concentrated in the dorsal cap and the magnocellular laminae, followed by the intergeniculate leaflet. Histochemical studies of cytochrome oxidase and nicotinamide adenosine dinucleotide phosphate (NADPH)-diaphorase localization revealed similar patterns of dense reactivity within the external magnocellular lamina, intergeniculate leaflet and dorsal cap, and somewhat less dense, but substantial reactivity in the internal magnocellular lamina. Within the external magnocellular lamina, cells reactive for cytochrome oxidase were noted in the lateral portion bordering the optic tract, whereas those specific for NADPH-diaphorase were dispersed throughout the lamina. Poor reactivity for both histochemical markers was evident in the parvicellular segment. Overall, the markedly different patterns of retinal input and neurochemical organization between the subdivisions of the tree shrew vLGN suggest their involvement in diverse functions. Furthermore, the basic similarity of the organization of the tree shrew vLGN to that of the taxonomically unrelated ground squirrel may indicate a common mammalian scheme.     

Cerebellar projections to the ventral lateral geniculate nucleus and the thalamic reticular nucleus in the cat

The ventral lateral geniculate nucleus (LGNv) is a retinorecipient part of the ventral thalamus and in cats, it consists of medial (M), medial intermediate (IM), lateral intermediate (IL), lateral (L), and dorsal (D) subdivisions. These subdivisions can be differentiated not only by their cytoarchitecture, but also by their connectivity and putative functions. The LGNv may play a role in visuomotor gating, in that there is evidence of cerebellar afferent projections to the intermediate subdivisions. The cerebellar posterior interpositus (IP) and lateral (LC) nuclei are known to project to IM and IL, but the specifics of these projections are unclear. We hypothesized that the IP and LC project differentially to IM and IL. To evaluate LGNv innervation by the deep cerebellar nuclei, we injected the tract‐tracer wheat germ agglutinin‐horseradish peroxidase (WGA‐HRP) into several different regions of the LGNv and cerebellar nuclei of adult cats in either sex. Small injections into the middle and posterior LGNv retrogradely labeled cells in the ventral part of the IP. However, injections in the anterior regions of the LGNv, with or without diffusion into the thalamic reticular nucleus (Re), retrogradely labeled cells in the ventral part of both the IP and the LC. Confirmatory injections into the IP and LC produced terminal‐like labeling distributed in IM, IL, and Re; injections mostly localized to the LC resulted in labeling mainly in IM and Re. We concluded that the IP projects to IL whereas the LC projects to IM and Re.     

Distribution of synapses in the lateral geniculate nucleus of the cat: Differences between laminae A and A1 and between relay cells and interneurons

Laminae A and A1 of the lateral geniculate nucleus in the cat are generally considered to be a structurally and functionally matched pair of inputs from two eyes, although there are subtle light microscopic and physiological differences. The present study aims to display ultrastructural differences between these two laminae based on electron microscopic observances on the connectivity patterns of their afferents onto two main cell types: relay cells, and interneurons present in this nucleus. In a design of population measurement from randomized sample areas in laminae A and A1 from six brains, all synaptic contacts made by three terminal types of the geniculate nucleus were identified, and a number of relative distribution properties were analyzed. When the A-laminae were considered as a homogeneous structure, the distribution of the three terminal types on geniculate cells was similar to previously reported results, confirming the validity of the sampling strategies used; RLP (retinal) terminals provided one-fifth of all synapses, whereas RD (from cortex and brainstem) and F (inhibitory) types constituted one-half and one-third, respectively. The relay cells alone received a similar composition of afferents. However, interneurons alone received approximately equal amounts of synapses from the three sources. Similar analyses comparing the distributions in lamina A and A1 revealed that RD and F terminals, but not RLP terminals, innervate these two laminae differently; more RD and fewer F terminals were found in lamina A1. This difference was also present in the distribution of terminals on relay cells alone, but not on interneurons. These results suggest that (1) retinal terminals form a significantly larger fraction of the input to interneurons than to relay cells; correspondingly, cortex and brainstem provide a smaller fraction of all inputs to interneurons than to relay cells; and (2) laminae A and A1 are not strictly equivalent projection sites of the two retinae. The results are discussed in relation to the Y-cell subpopulation in lamina A1 that is involved in corticotectal, as well as corticogeniculate circuits, as opposed to Y-cells of lamina A that are involved in only the latter. J. Comp. Neurol. 390:247–255, 1998. © 1998 Wiley-Liss, Inc.     

The distribution of acetylcholinesterase in the lateral geniculate nucleus of the cat and monkey

The distribution of acetylcholinesterase (AChE) has been examined histochemically in the lateral geniculate nucleus (LGN) of the cat and the monkey, and in the cat visual cortex. It was found that in the cat, AChE is most concentrated in laminae A and A₁. Lamina C-proper possessed a weak band of AChE in its ventral part. Only restricted patches of activity were observed in the medial interlaminar nucleus. Laminae C_1–3 and the central interlaminar nucleus possessed very little AChE. This pattern of enzyme distribution suggests that in the cat LGN, AChE activity coincides with the sites of neurophysiologically recorded X-cells, which are predominantly found in laminae A and A₁ and are scarce in the C laminae and the medial interlaminar nucleus. The presence of AChE over neurones in layer VI of both areas 17 and 18 of the cerebral cortex in the cat suggests the corticothalamic pathway as one possible source of geniculate AChE activity.In the monkey LGN, AChE activity was observed in the parvocellular and magnocellular layers. The activity was greatest in the magnocellular layers, which are believed to contain neurons driven predominantly by retinal Y-cells. Thus, for this species the correlation between AChE activity and X-cells does not seem to hold.     

Monoclonal antibody Cat-301 identifies Y-cells in the dorsal lateral geniculate nucleus of the cat

In mammalian visual pathways, information is carried in parallel channels from the retina through the visual thalamus to visual cortex. The cat's visual pathway comprises at least three major channels that begin with the X, Y, and W ganglion cells in the retina. In the dorsal lateral geniculate nucleus (LGN) of the thalamus, neurons in the X, Y, and W channels receive input from their retinal counterparts and can be discriminated from one another on the basis of their anatomical and physiological properties. The search for molecular properties that might correlate with anatomically or physiologically defined classes of neuron has been a major area of research in recent years. Monoclonal antibody Cat-301 recognizes a neuronal surface-associated proteoglycan in many areas of the mammalian central nervous system. In the cat LGN Cat-301 immunoreactivity is restricted to a subset of neurons. We show here that the distribution, size, morphology, and cortical projection pattern of Cat-301-positive LGN neurons match those previously described for Y-cells. Taken together with our previous studies of the development of immunoreactivity and the sensitivity of Cat-301 staining to visual deprivation, these studies suggest that Cat-301 specifically recognizes Y-cells in the cat LGN. These results indicate that neurons within a physiologically and anatomically defined cell class share a molecular property. They further suggest that differences in molecular traits may reflect, and possibly subserve, differences in anatomical and physiological characteristics.     

Organization of cholinergic synapses in the cat's dorsal lateral geniculate and perigeniculate nuclei

In the preceding article, we showed that cholinergic fibers originating from the brainstem reticular formation provide a dense innervation of the lateral geniculate nucleus. In this report we describe the ultrastructure of these fibers and their relations with other elements in the neuropil of the lateral geniculate nucleus. Cholinergic fibers were labeled with an antibody to choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh). In the A-laminae of the lateral geniculate nucleus, ChAT + profiles are small and contain tightly packed, mostly round vesicles. Some end in encapsulated synaptic zones where they form asymmetrical synaptic contacts with processes of both projection cells and interneurons. Others form synapses upon the shafts of dendrites. Of the four classical types of vesicle-containing profiles identified by Guillery (Z. Zellforsch. Mikrosk. 96:1-38, '69; Vision Res. [Suppl.] 3:211-227, '71), ChAT + profiles most closely resemble RSD profiles (Round vesicles, Small profile, Dark mitochondria). However, as a population, ChAT + profiles can be distinguished from the unlabeled population of RSD profiles because they are larger in size, contain more mitochondria, and make synapses with smaller postsynaptic membrane specializations. Each of these differences is statistically significant and together they indicate that ChAT + profiles are a distinct morphological type of synaptic profile. ChAT + profiles in the perigeniculate nucleus resemble those found in the lateral geniculate nucleus; they also make synapses with obvious postsynaptic thickenings.     

The proportion of synapses formed by the axons of the lateral geniculate nucleus in layer 4 of area 17 of the cat

The connection between the dorsal lateral geniculate nucleus (dLGN) and area 17 of the cat is a classical model for studying thalamocortical relations. We investigated the proportion of asymmetric synapses in layer 4 of area 17 of cats formed by axons of the dLGN, because this is an important morphological parameter in understanding the impact of dLGN axons on their target neurons. Although the present consensus is that this proportion is small, the exact percentage remains in doubt. Most previous work estimated that the thalamus contributes less than 10% of excitatory synapses in layer 4, but one estimate was as high as 28%. Two issues contribute to these widely different estimates, one being the tracers used, the other being the use of biased stereological approaches. We have addressed both of these issues. Thalamic axons were labeled in vivo by injections of biotinylated dextran amine into the A lamina of the dLGN of anesthetized cats. After processing, the brain was cut serially and prepared for light and electron microscopy. The density of asymmetric synapses in the neuropil and the density of synapses formed by labeled dLGN boutons were measured by using an unbiased sampling method called the physical disector. Our counts indicate that, in the fixed cat brain, there are 5.9 × 10⁸ ± 0.9 × 10⁸ asymmetric synapses per cubic millimeter of layer 4 in area 17, and the dLGN input provides only 6% of all asymmetric synapses in layer 4. The vast majority of synapses of layer 4 probably originate from other neurons in area 17. J. Comp. Neurol. 516:264–276, 2009. © 2009 Wiley‐Liss, Inc.     

Synaptic delay in the lateral geniculate nucleus of the cat

Unitary, presynaptic spike potentials were observed in single cell recordings from the dorsal lateral geniculate nucleus of the cat. In 11 cells, spontaneous S potentials (extracellularly recorded excitatory postsynaptic potentials) were preceded at a fixed interval by a small wave (the 'T' potential). In another 14 cells, a T potential, although not detected in single traces, was revealed by averaging 20-100 samples synchronized to the peak of the S potential. Provided the field response was not too large a T potential could also be detected in the response to a stimulus to the optic nerve. The T potential would appear to be the spike potential of the afferent optic axon which is excitatory to the geniculate cell because it precedes the S potential at a very exact interval and also follows the corresponding retinal ganglion cell spike at a very exact interval and because the interval between T potential and S potential is reversibly decreased by cooling with a temperature coefficient characteristic of synapses. T potentials ranged in amplitude from 8 to 134 microV and were all predominantly positive-going suggesting a failure of the nerve impulse to invade fully the terminals of the optic nerve. The time from the positive peak of the T potential to the start of the S potentials was taken as a good measure of the synaptic delay. The T-S interval averaged 0.29 ms (+/- 0.045 ms S.D.).     

The synaptic organization of glomeruli in the magnocellular and parvocellular laminae of the lateral geniculate nucleus in the monkey

The synaptic organization of glomeruli in the magnocellular laminae of the lateral geniculate nucleus in the macaque monkey has been compared quantitatively with that of glomeruli in the parvocellular laminae using a sampling technique based on single sections. It was found that retinal boutons establish comparable total numbers of synapses in glomeruli in single sections in all laminae, but a relatively higher proportion of them are upon presynaptic dendrites in the magnocellular laminae. In addition, more synaptic contacts are made by presynaptic dendrites in the magnocellular laminae, and consequently in a given section glomeruli in these laminae have on average more synapses in them. Neuronal somata in the magnocellular and parvocellular laminae were studied with the light- and electron-microscopes and there were no complex laminated bodies encountered.     

Visual function in the ventral lateral geniculate nucleus of the cat

The visual receptive fields of 293 single units in the ventral lateral geniculate nucleus of the cat were studied. In addition to the wide variety of types described by others, a group of units responding differentially to color was identified that included units responding particularly to blue and others with opponent color properties. Some units with spontaneous firing and without definite visual receptive fields were inhibited by stimulation of the optic chiasm (OX). A study of latency of firing to OX stimulation suggested that these cells were driven by retinal ganglion cells of the W type. One-third of all units studied were binocularly driven.     

Zonal organization of the ventral lateral geniculate nucleus in the cat: cholera toxin mapping.

A zonal organization of the ventral lateral geniculate nucleus (LGNv) in the cat was studied with the bidirectional tracing method by using the subunit b of cholera toxin (CTb) as the marker. The prime objectives of the present study were to examine precise distribution of terminals of the retinal afferents to the LGNv, and to correlate it with the origins and terminations of the other central connections of the nucleus. The results obtained are summarized as follows: (1) the LGNv of the cat is divided into 3 zones according to the terminations of retinal afferents: lateral, intermediate, and medial; (2) the lateral zone receives afferents from the retina in which ipsi- and contralateral fibers terminate in a complementary fashion. According to the density of labeling of retinal terminals, the lateral zone is further divided into several areas. It also receives fibers from the visual related cortex; (3) the intermediate zone, which does not receive fibers from the retina nor the visual cortex, is reciprocally connected with the midbrain, primarily with the superficial layers of the superior colliculus (SC), and gives rise to thalamic projections to the nucleus centralis lateralis bilaterally; and (4) the medial zone, from which commissural fibers arise, receives afferents bilaterally from the retina, and sends fibers to the SC bilaterally. These results suggest that the LGNv of the cat has three different zones. Functional participation of each zone has been discussed.     

An electron microscopic comparison of the medial interlaminar nucleus and the a laminae in the dorsal lateral geniculate nucleus of the cat

The medial interlaminar nucleus (MIN) of the cat's dorsal lateral ge-niculate nucleus was studied under the light and electron microscopes and compared with the A laminae. At the light level, MIN has more axons and a lesser cell packing density than the A laminae. Examined at the electron microscopic level, MIN could not qualitatively be distinguished from the A laminae. When quantitative counts of the profiles containing synaptic ves-icles were made, MIN had less F profiles and more RSD profiles per unit area than the A laminae. Structure/function correlations suggest that additional F terminals may mediate nondominant eye inhibition and/or a greater amount of inhibition on X-cells versus Y-cells.     

Inhibitory circuitry involving Y cells and Y retinal terminals in the C laminae of the cat dorsal lateral geniculate nucleus

We previously established (Datskovskaia et al. [2001] J Comp Neurol 430:85-100) that roughly 40% of Y retinal terminals contact interneurons in the A lamina of the dorsal lateral geniculate nucleus (dLGN) of the cat. However, we did not establish whether the dendritic terminals of interneurons postsynaptic to Y retinal terminals subsequently contact Y thalamocortical cells. To begin to address this issue, we examined the synaptic targets of Y retinal terminals in the magnocellular C lamina of the dLGN, which is populated almost exclusively by Y thalamocortical cells and interneurons. We utilized material generated from our previous work, in which we injected the superior colliculus with biotinylated dextran amine to backfill the geniculate branches of Y retinogeniculate axons in the dLGN. Sections prepared for electron microscopy were stained for gamma aminobutyric acid (GABA) to distinguish interneurons from thalamocortical cells. We found that the majority of profiles postsynaptic to Y retinal axons were the GABA-negative dendrites of thalamocortical cells (116/200, 58%). The remainder were GABA-positive dendrites of interneurons (84/200, 42%), many of which contained vesicles (F2 profiles; 54/200, 27%). In addition, we examined the synaptic targets of F2 profiles and found that almost all contacts of F2 profiles in the magnocellular C lamina were made onto the GABA-negative dendrites of thalamocortical cells (199/200, 99.5%). Thus, Y retinogeniculate axons contact interneurons and interneurons contact Y thalamocortical cells in the magnocellular C lamina of the dLGN. This indicates that interneurons are involved in modulation of the Y pathway.