Morphology of geniculo-striate afferents in a prosimian primate

In order to analyze the arborization pattern of single axons innervating layer IV of striate cortex in Galago (a prosimian primate), injections of horseradish peroxidase were made into the white matter below visual cortex. Two distinct fiber types were identified which ramify within different subdivisions of layer IV. Fibers with large axons (type I) have relatively wide terminal arbors which ramify in layer IVa and lower layer III, while those with small axons (type II) arborize in restricted zones of layer IVb. Evidence from previous investigations suggests that terminals of these two fiber types represent the axon terminals of X-like and Y-like geniculate relay cells.     

Direct W-like geniculate projections to the cytochrome oxidase (CO) blobs in primate visual cortex: axon morphology.

The primate lateral geniculate nucleus (LGN) is composed of large, medium, and small cells located, respectively, in magnocellular (M), parvocellular (P), and specialized layers (intercalated and S-layers in simians, koniocellular (K) layers in prosimians). Several studies have examined the physiology and connections of M and P LGN cells and have concluded that they provide separate contributions to visual perception via separate pathways. Less is known about the structure and contributions of the small LGN cells. This study examined the distribution and structure of K LGN cell axons in the cortex of the prosimian, Galago crassicaudatus. Wheat germ agglutinin conjugated to horseradish peroxidase, or Phaseonlus vulgaris leucoaglutinin, was injected into the LGN K layers to demonstrate the overall axon projection pattern and the details of individual axons, respectively. Location of axons within striate cortex was specified relative to boundaries determined by Nissl or cytochrome oxidase (CO) stains on the same or adjacent sections. Our results show that K LGN axons end as single complex arbors within one CO blob zone in layer III; they never terminate in interblob zones. These axons also emit a collateral in layer I that arborizes more broadly and spans both CO blob and interblob zones. These data, together with data on K cell physiology and intralaminar cortical connections, suggest that the LGN small cell pathway could modulate the activity of the other two pathways in striate cortex and contribute directly to visual perception.     

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.     

W-like response properties of interlaminar zone cells in the lateral geniculate nucleus of a primate (Galago crassicaudatus)

Recent anatomical studies have suggested that the cells located in the interlaminar zones (ILZs) of the primate dorsal lateral geniculate nucleus (LGN) relay visual information from the retina to the striate cortex in a manner similar to that of W-cells in the LGN of cat. In the present study, we examined this idea directly by recording the response properties of single cells localized to the ILZs in the prosimian primate, Galago crassicaudatus. The properties of the cells in the ILZs were found to be physiologically distinct from the X-like and Y-like properties of the parvocellular and magnocellular LGN layers. Moreover, the small cells located in the interlaminar zones were physiologically similar to the W-like cells found in the specialized small-celled koniocellular layers in these primates. As is the case with the koniocellular layer cells, the ILZ cells exhibited a broad range of properties which, as a group, were distinguished by the following characteristics: the ILZ cells had long latencies to stimulation of the optic chiasm (mean, 3.95 ms) and to antidromic stimulation from striate cortex (mean, 3.31 ms) and had relatively large receptive-field centers (mean, 1.79 degrees). They also had low maintained discharge rates (5.5 spikes/s), relatively long response latencies to light (mean onset, 82 ms; peak, 112 ms) and low peak firing rates (59 spikes/s). Few (25%) had standard receptive-field organization (ON-center, OFF-surround, or vice versa). Only 29% responded well to sine-wave gratings and all were influenced by non-visual (auditory and tactile) stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Intrinsic connections of layer III of striate cortex in squirrel monkey and bush baby: Correlations with patterns of cytochrome oxidase

This study used biocytin and horseradish peroxidase (HRP) to examine the intrinsic connections of the cytochrome oxidase (CO) rich blob and CO poor nonblob zones within layer III of striate cortex in two primate species, nocturnal prosimian bush babies (Galago crassicaudatus) and diurnal simian squirrel monkeys (Saimiri sciureus). Our main objective was to determine whether separate classes of lateral geniculate nucleus (LGN) cells projected to separate superficial layer zones or layers in either species. There were three significant findings. First, we confirm that layer III consists of three sublayers, IIIA, IIIB, and IIIC in both species. Layer IIIA receives input from layers IIIB, IIIC, and V, with little or no input from LGN recipient layers IV and VI. Layer IIIB receives its input from nearly every cortical layer. Layer IIIC, receives input principally from layers IVα [which receives its input from magnocellular (M) LGN cells] and from layers V and VI. Taken together with other findings on the extrinsic connections of these layers, our data suggest that IIIA and IIIC provide output to separate hierarchies of visual areas and IIIB acts as a set of interneurons. Second, we find that, as in macaque monkeys, cells in both IVβ and IVα of bush babies and squirrel monkeys projct to layer IIIB, converging within the blobs. These results suggest that information from all LGN cell classes [parvocellular (P), M, and the Koniocellular (K) or their equivalents] may be integrated within the blobs. Thus, blobs in all of these primates may perform a function that transcends visual niche differences. Third, our data show a species specific difference in the connections of the IIIB nonblobs; nonblobs receive indirect input via IVα from the LGN M pathway in bush babies but receive indirect input via IVβ from the LGN parvocellular (P) pathway in squirrel monkeys. These findings indicate that the role of nonblob zones within striate cortex differs from that of blob zones and takes into account visual niche differences. © 1993 Wiley-Liss, Inc.     

Do the relative mapping densities of the magno- and parvocellular systems vary with eccentricity?

Two recent papers on the macaque visual system have concluded that in the lateral geniculate body the ratio of the number of cells in the magnocellular system to the number in the parvocellular system representing the same area of visual field increases by a factor of 20 between the fovea and the far periphery. In the primary visual cortex the relative cell densities of the 2 systems change little with eccentricity. These calculations therefore predict a 20-fold change in the relative densities of the inputs to the visual cortex from the 2 subdivisions of the lateral geniculate body. To test this prediction, we asked if the following vary with eccentricity: (1) the ratio of the number of magnocellular to parvocellular neurons innervating a given area of striate cortex and (2) the relative density, in the magno- and parvo-recipient sublaminae of layer 4C, of radioactivity transported from the eye to the cortex. Neither of these ratios showed any significant variation with eccentricity. These results seem to throw doubt on the contention that the ratio between the magnocellular and parvocellular layers of the number of cells per degree2 of visual field varies significantly with eccentricity.     

Organization of the feedback pathway from striate cortex (V1) to the lateral geniculate nucleus (LGN) in the owl monkey (Aotus trivirgatus)

The function of the corticogeniculate feedback pathway from the striate cortex (V1) to the lateral geniculate nucleus (LGN) in primates is not well understood. Insight into possible function can be gained by studying the morphology and projection patterns of corticogeniculate axons in the LGN. The goal of this research was to examine how corticogeniculate axons innervate the functionally specific (e.g., parvocellular [P], magnocellular [M], and koniocellular [K]) and eye-specific layers of the LGN. Pressure injections of biotinylated dextran were made into owl monkey V1, and the resulting labeled axons were reconstructed through serial sections of the LGN. All of the corticogeniculate axons, regardless of termination pattern, were thin with boutons en passant or at the ends of small stalks, as described in cats. Axons were found in all layers of the LGN, and two main patterns of innervation were observed. In the first pattern, axons terminated in individual M or P LGN layers. In the second pattern of innervation, axons terminated in pairs of functionally matched layers. Examples of this type were seen within pairs of M, P, or K layers. In most cases, both classes of axons contain arbors focused within the P or M layers but also had collateral side branches in neighboring K layers. Unlike corticogeniculate axons seen in the cat, corticogeniculate axons in the owl monkey maintained topographic innervation in the LGN layers that was consistent with receptive field sizes represented in V1. The patterns of layer projections along with the retinotopic match of corticogeniculate axons within the LGN suggest that in primates V1 can modulate activity in the LGN through functionally specific projections in a more tightly tuned retinotopic fashion than previously believed.     

The identification of relay neurons in the dorsal lateral geniculate nucleus of monkeys using horseradish peroxidase.

Intraaxonal retrograde transport of the protein horseradish peroxidase (HRP) was used to identify relay neurons in the dorsal lateral geniculate nucleus (LGN) of owl (Aotus trivirgatus) and rhesus (Macaca mulatta) monkeys. In both species, from 94.1-98.6% of the neurons within columns extending through both parvocellular and magnocellular layers were labeled following injection of HRP into striate cortex. Labeled neurons were also identified in the thin ventral-most S(0) Layers. Although most of the cells within the thin interlaminar regions in the LGN of both species were labeled following injections of HRP, many unlabeled neurons were identified within the large cell-rich interlaminar region (IL) between the internal parvocellular and internal magnocellular layers in the LGN of the owl monkey, suggesting that IL may be a specialized region containing a large number of intrinsic neurons. Finally, measurement of the cell diameters of neurons within the densely labeled areas in relay layers revealed that labeled and unlabeled neurons could not be distinguished on the basis of cell body size alone and that some of the smallest cells of the LGN project to striate cortex. These findings indicate that nearly all of the neurons of the main relay layers of the LGN in these two primates are relay cells and that the organization of the LGN in primates may differ significantly from that of other mammals with respect to the percentage of interneurons.     

Connections of higher order visual relays in the thalamus: a study of corticothalamic pathways in cats

Axonal markers injected into layers 5 and 6 of cortical areas 17, 18, or 19 labeled axons going to the lateral geniculate nucleus (LGN), the lateral part of the lateralis posterior nucleus (LPl), and pulvinar (P). Area 19 sends fine axons (type 1, Guillery [1966] J Comp Neurol 128:21-50) to LGN, LPl, and P, and thicker, type 2 axons to LPl and P. Areas 17 and 18 send type 1 axons to LGN, and a few type 1, but mainly type 2 axons to LPl and P. Type 1 and 2 axons from a single small cortical locus distribute to distinct, generally nonoverlapping parts of LP and P; type 1 axons have a broader distribution than type 2 axons. Type 2 axons, putative drivers of thalamic relay cells (Sherman and Guillery [1998] Proc Natl Acad Sci USA 95:7121-7126; Sherman and Guillery [2001] Exploring the thalamus. San Diego: Academic Press), supply small terminal arbors (100- to 200-microm diameter) in LPl and P, and then continue into the midbrain. Each thalamic type 2 arbor contains two terminal types. One, at the center of the arbor, is complex and multilobulated; the other, with a more peripheral distribution, is simpler and may contribute to adjacent arbors. Type 2 arbors from a single injection are scattered around and along "isocortical columns" in LPl, (i.e., columns that represent cells having connections to a common cortical locus). Evidence is presented that the connections and consequently the functional properties of cells in LP change along these isocortical columns. Type 2 driver afferents from a single cortical locus can, thus, be seen as representing functionally distinct, parallel pathways from cortex to thalamus.     

Neurochemical comparison of synaptic arrangements of parvocellular,magnocellular, and koniocellular geniculate pathways in owl monkey (Aotus trivirgatus) visual cortex

As in other primates, the lateral geniculate nucleus (LGN) of owl monkeys contains three anatomically and physiologically distinct relay cell classes, the magnocellular (M), parvocellular (P), and koniocellular (K) cells. M and P LGN cells send axons to the upper and lower tiers of layer IV, and K cells send axons to the cytochrome oxidase (CO) blobs of layer III and to layer I of primary visual cortex (V1). Our objective was to compare the synaptic arrangements made by these axon classes. M, P, and K axons were labeled in adult owl monkeys by means of injections of wheat germ agglutinin-horseradish peroxidase into the appropriate LGN layers. The neurochemical content of both pre- and postsynaptic profiles were identified by postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Our key finding is that the synaptic arrangements made by M, P, and K axons in owl monkey exhibit more similarities than differences. They are exclusively presynaptic, contain glutamate and form asymmetric synapses mainly with glutamate-positive dendritic spines. The majority of the remaining axons synapse with glutamatergic dendritic shafts. There are also differences between LGN pathways. M and P terminals are significantly larger and more likely to make multiple synapses than K axons, although M and P axons do not differ from each other in either of these characteristics. Of interest, a larger percentage of M and K axons than P axons make synapses with GABAergic dendritic shafts. Cells directly postsynaptic to M and K axons are known to exhibit orientation selectivity and, in some cases, direction selectivity. Cells postsynaptic to P axons do not show these properties, but instead tend to reflect their LGN inputs more faithfully; therefore, it is possible that these physiologic differences seen in the cortical cells postsynaptic to different LGN pathways reflect the differential involvement of inhibitory circuits.     

Laminar organization of receptive field properties in the dorsal lateral geniculate nucleus of the tree shrew (Tupaiaglis belangeri)

A laminar analysis of the receptive field properties of relay cells in the binocular region of the tree shrew dorsal lateral geniculate nucleus (LGN) found three main subdivisions. Lamina 1 (receiving ipsilateral eye input) and lamina 2 (contralateral) comprise a pair of layers that contain only ON-center neurons. Laminae 4 (contralateral) and 5 (ipsilateral) comprise a pair of layers with mostly OFF-center cells (86%). Laminae 3 and 6 (both contralaterally innervated) also form a distinct pair, although lamina 3 contains a mixture of cells with ON-centers (43%) or OFF-centers (57%), and lamina 6 contains mostly cells with ON-OFF centers and suppressive surrounds (81%). Cells located in the interlaminar zones resembled neurons in laminae 3 and 6. In comparison with the cells in the OFF-center laminae 4 and 5, the ON-center cells in laminae 1 and 2 had smaller, more elliptical receptive field centers with stronger responses to flashed visual stimuli. In addition, cells in the ipsilateral eye laminae 1 and 5 showed a greater change in center diameter, with eccentricity from the area centralis, than cells in the contralateral eye laminae 2 and 4. Principal components analysis using six receptive field properties (latency to optic chiasm stimulation, receptive field center diameter, maintained discharge rate, response onset latency, peak spike density, and phasic-tonic index) suggested that the cells in laminae 3 and 6 and the interlaminar zones are W-like. Principal components analysis of the same receptive field properties in laminae 1, 2, 4, and 5 did not reveal differences clearly related to X-like (parvocellular) and Y-like (magnocellular) categories. Ninety-seven percent of the cells tested for linearity of spatial summation in laminae 1, 2, 4, and 5 were linear. We conclude that the dominant organizational features of the tree shrew LGN are the ON-center, OFF-mter, and W pairs of layers that project to different regions within the striate cortex. © 1995 Wiley-Liss, Inc.     

Ordinal position and afferent input of neurons in monkey striate cortex

From the extracellular recording of single units in the monkey striate cortex and electrical stimulation at two selected sites in the optic radiations it was possible to estimate 1) the ordinal position of striate neurons (i.e., whether they received a monosynaptic, disynaptic or polysynaptic input from the thalamus) and 2) the nature of the afferent input to these neurons (i.e., whether it came from the magnocellular or parvocellular subdivision of the lateral geniculate nucleus (LGN)). Based on receptive field properties six major classes of striate neuron were identified--three which lacked orientation specificity (the ON-center, the OFF-center, and the ON/OFF or nonoriented (N-0) receptive fields) and three with orientation specific responses (the S, the C, and the B categories of receptive field). Units lacking orientation specificity were concentrated in laminae 4A, 4C beta and 6 while, for the cells with orientation specificity, C cells were found in laminae 4B and 6, B cells in 2/3 and 5, and S cells predominantly in laminae 2/3, 4C alpha, and 5. The results of electrical stimulation indicated that cell-to-cell transmission time in the monkey striate cortex is 1.5 msec, and latency measures showed that cells with a monosynaptic drive from the thalamus were confined to laminae 4 and 6 while disynaptically driven cells were found principally in upper lamina 4 (4A and 4B). No cell class was identified exclusively with a given ordinal position and there were many types of potential first-order neurons. The conduction time from one stimulating electrode to the next in the optic radiation was used to identify the afferent input to each striate neuron. The input to color-coded neurones was found to come exclusively from parvocellular layers while the C cells and two subclasses of the S cell (S2 and S3) were driven predominantly by the magnocellular subdivision. For other cell types (those with ON-center, N-0, and S1 receptive fields) the input came from either type of LGN neuron. The laminar distribution of neurons receiving a direct input from the magnocellular and parvocellular streams is in accord with the results of anatomical studies into the site of termination of the LGN input. The cell types receiving these direct inputs vary in the two streams so that the parvocellular input terminates on cells with ON-center and N-0 receptive fields in lamina 4C beta while the magnocellular input goes to cells with S, ON-center, N-0, and C receptive fields in lamina 4C alpha and the lower part of 4B. Consideration is given to the influence of these results on models for neural processing in monkey striate cortex and a comparison is drawn with the results of similar studies in the cat.     

Functional anatomy of macaque striate cortex. IV. Contrast and magno-parvo streams

Macaque monkeys were shown achromatic gratings of various contrasts during 14C-2-deoxy-d-glucose (DG) infusion in order to measure the contrast sensitivity of different subdivisions of primary visual cortex. DG uptake is essentially saturated at stimulus contrasts of 50% and above, although the saturation contrast varies with layer and with different criteria. Following visual stimulation with gratings of 8% contrast, stimulus-driven uptake was relatively high in striate layer 4Ca (which receives primary input from the magnocellular LGN layers), but was absent in layer 4Cb (which receives primary input from the parvocellular layers). In this same (magnocellular-specific) stimulation condition, striate layers 4B, 4Ca, and 6 showed strong stimulus-induced DG uptake, and layers 2, 3, 4A, and 5 showed only light or negligible uptake. By comparison to other cases that were shown stimuli of systematically higher contrast, and to a wide variety of DG cases shown very different stimuli, it is evident that information derived from the magnocellular and parvocellular layers in the LGN remains partially, or largely, segregated in its passage through striate cortex, and projects in a still somewhat segregated fashion to different extrastriate areas. The sum of all available evidence suggests that the magnocellular information projects strongly through striate layers 4Ca, 4B, and 6, with moderate input into the blobs in layers 2 + 3, and to blob-aligned portions of layer 4A. Parvocellular-dominated regions of striate cortex include both the blob and interblob portions of layers 2 + 3, 4A, 4Cb, and 5. Because the major striate input to V2 arrives from striate layers 2 + 3, and because the major striate input to MT originates in layer 4B and 6, it appears that area V2 receives information derived largely from the parvocellular LGN layers, and that area MT receives information derived mainly from the magnocellular layers.     

Synaptic and neurochemical characterization of parallel pathways to the cytochrome oxidase blobs of primate visual cortex

The primary visual cortex (V1) of primates is unique in that it is both the recipient of visual signals, arriving via parallel pathways (magnocellular [M], parvocellular [P], and koniocellular [K]) from the thalamus, and the source of several output streams to higher order visual areas. Within this scheme, output compartments of V1, such as the cytochrome oxidase- (CO) rich blobs in cortical layer III, synthesize new output pathways appropriate for the next steps in visual analysis. Our chief aim in this study was to examine and compare the synaptic arrangements and neurochemistry of elements involving direct lateral geniculate nucleus (LGN) input from the K pathway with those involving indirect LGN input from the M and P pathways arriving from cortical layer IV. Geniculocortical K axons were labeled via iontophoretic injections of wheat germ agglutinin-horseradish peroxidase into the LGN and intracortical layer IV axons (indirect P and M pathways to the CO-blobs) were labeled by iontophoretic injections of Phaseolus vulgaris leucoagglutinin into layer IV. The neurochemical content of both pre- and postsynaptic profiles was identified by postembedding immunocytochemistry for γ-amino butyric acid (GABA) and glutamate. Sizes of pre- and postsynaptic elements were quantified by using an image analysis system, BioQuant IV. Our chief finding is that K LGN axons and layer IV axons (indirect input from M and P pathways) exhibit different synaptic relationships to CO blob cells. Specifically, our results show that within the CO blobs: 1) all K cell axons contain glutamate, and the vast majority of layer IV axons contain glutamate with only 5% containing GABA; 2) K axons terminate mainly on dendritic spines of glutamatergic cells, while layer IV axons terminate mainly on dendritic shafts of glutamatergic cells; 3) K axons have larger boutons and contact larger postsynaptic dendrites, which suggests that they synapse closer to the cell body within the CO blobs than do layer IV axons. Taken together, these results suggest that each input pathway to the CO blobs uses a different strategy to contribute to the processing of visual information within these compartments. J. Comp. Neurol. 391:429–443, 1998. © 1998 Wiley-Liss, Inc.     

The termination of geniculocortical fibres in area 17 of the visual cortex in the macaque monkey

The termination of geniculocortical fibres within the different subdivisions of lamina IV in area 17 of the visual cortex of the monkey has been studied quantitatively with the electron microscope. In lamina IVCα the axon terminals of fibres coming from the magnocellular layers of the lateral geniculate nucleus (LGN) make significantly more synapses per bouton than those of fibres arising from the parvocellular layers and terminating in laminae IVA and IVCβ. In all parts of area 17 examined there was a clear difference in the relative proportions of multisynaptic geniculo-cortical boutons between the α and β divisions of lamina IVC. Calculations have shown that a single cell in the magnocellular laminae of the LGN may make about 6 times as many synaptic contacts within lamina IV of the visual cortex than one in the parvocellular laminae. It has also been estimated that there are at least 500 million geniculocortical boutons, or 1200 million synapses, in lamina IVCα and 1000 million boutons, 1200 million synapses, in lamina IVCβ for one hemisphere, giving an approximate total number of 1500 million boutons and 2400 million synapses.     

Changes in the distribution of geniculocortical projections following monocular deprivation in tree shrews

We examined changes in the percentages and distribution of labelled geniculate (LGN) cells following injection of HRP into the striate cortices of normal and monocularly deprived tree shrews. Results show that deprivation does not affect the percentage of labelled cells but does alter the distribution. The projection column of labelled cells is much narrower in deprived than in non-deprived layers suggesting that the axons of non-deprived LGN cells either extend across greater distances in cortex to reach the injection sites or have more peripheral terminal sites available for transport than deprived counterparts. This finding indicates that, although ocular input to tree shrew striate cortex is organized in the form of horizontal strips, not columns, binocular competition still results in alterations in the growth of geniculocortical axons.     

Stereologic analysis of the lateral geniculate nucleus of the thalamus in normal and schizophrenic subjects

Reduction of volume and neuronal number has been found in several association nuclei of the thalamus in schizophrenic subjects. Recent evidence suggests that schizophrenic patients exhibit abnormalities in early visual processing and that many of the observed perceptual deficits are consistent with dysfunction of the magnocellular pathway, i.e. the visual relay from peripheral retinal cells to the two ventrally located magnocellular layers of the lateral geniculate nucleus (LGN). The present study was undertaken to determine whether abnormalities in cell number and volume of the LGN are associated with schizophrenia and whether the structural alterations are restricted to either the magnocellular or parvocellular subdivisions of the LGN. Series of Nissl-stained sections spanning the LGN were obtained from 15 schizophrenic and 15 normal control subjects. The optical disector/fractionator sampling method was used to estimate total neuronal number, total glial number and volume of the magnocellular and parvocellular subdivisions of the LGN. Cell number and volume of the LGN in schizophrenic subjects were not abnormal. Volume of both parvocellular and magnocellular layers of the LGN decreased with age. These findings do not support the hypothesis that early visual processing deficits in schizophrenic subjects are due to reduction of neuronal number in the LGN.     

Some aspects of the organization of the lateral geniculate nucleus inGalago senegalensis revealed by using horseradish peroxidase to label relay neurons

Following injection of horseradish peroxidase into area 17 of the prosimian Galago senegalensis, columns of labeled neurons are seen in the dorsal lateral geniculate nucleus extending through all cell layers. Individual counts of the number of labeled and unlabeled neurons reveal that from 91 to 98% of all neurons within these densely labeled columns are labeled. These results indicate that most of the neurons within the LGN of the bushbaby project to striate cortex. Average diameter measurements of labeled and unlabeled cells within the labeled columns were used to determine whether cell layers could be separated into two types (parvocellular and magnocellular) or three types (small, medium, and large) on the basis of cell body size. These measurements indicate that the LGN of the bushbaby is composed of two layers each of small (layers 4 and 5), medium (layers 3 and 6), and large (layers 1 and 2) relay neurons. These observations are consistent with the conclusion that layers 1 and 2 in the LGN of Galago are homologous with the magnocellular layers, and layers 3 and 6 homologous with the parvocellular layers, identified in the LGN of New and Old World monkeys.     

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.