Bifurcation of subcortical afferents to visual areas 17, 18, and 19 in the cat cortex

A strong direct link from the layer 3/4 border to layer 6 of cat primary visual cortex

The cat primary visual cortex (V1) is a cortical area for which we have one of the most detailed estimates of the connection 'weights' (expressed as number of synapses) between different neural populations in different layers (Binzegger et al in J Neurosci 24:8441-8453, 2004). Nevertheless, the majority of excitatory input sources to layer 6, the deepest layer in a local translaminar excitatory feedforward loop, was not accounted for by the known neuron types used to generate the quantitative Binzegger diagram. We aimed to fill this gap by using a retrograde tracer that would label neural cell bodies in and outside V1 that directly connect to layer 6 of V1. We found that more than 80% of labeled neurons projecting to layer 6 were within V1 itself. Our data indicate that a substantial fraction of the missing input is provided by a previously unidentified population of layer 3/4 border neurons, laterally distributed and connecting more strongly to layer 6 than the typical superficial layer pyramidal neurons considered by Binzegger et al. (Binzegger et al in J Neurosci 24:8441-8453, 2004). This layer 3/4 to layer 6 connection may be a parallel route to the layer 3 - layer 5 - layer 6 feedforward pathway, be associated with the fast-conducting, movement-related Y pathway and provide convergent input from distant (5-10 degrees) regions of the visual field.

Refinement of the Primate Corticospinal Pathway During Prenatal Development

Perturbation of the developmental refinement of the corticospinal (CS) pathway leads to motor disorders. While non-primate developmental refinement is well documented, in primates invasive investigations of the developing CS pathway have been confined to neonatal and postnatal stages when refinement is relatively modest. Here, we investigated the developmental changes in the distribution of CS projection neurons in cynomolgus monkey (Macaca fascicularis). Injections of retrograde tracer at cervical levels of the spinal cord at embryonic day (E) 95 and E105 show that: (i) areal distribution of back-labeled neurons is more extensive than in the neonate and dense labeling is found in prefrontal, limbic, temporal, and occipital cortex; (ii) distributions of contralateral and ipsilateral projecting CS neurons are comparable in terms of location and numbers of labeled neurons, in contrast to the adult where the contralateral projection is an order of magnitude higher than the ipsilateral projection. Findings from one largely restricted injection suggest a hitherto unsuspected early innervation of the gray matter. In the fetus there was in addition dense labeling in the central nucleus of the amygdala, the hypothalamus, the subthalamic nucleus, and the adjacent region of the zona incerta, subcortical structures with only minor projections in the adult control.

The Mouse Cortical Connectome, Characterized by an Ultra-Dense Cortical Graph, Maintains Specificity by Distinct Connectivity Profiles

The inter-areal wiring pattern of the mouse cerebral cortex was analyzed in relation to a refined parcellation of cortical areas. Twenty-seven retrograde tracer injections were made in 19 areas of a 47-area parcellation of the mouse neocortex. Flat mounts of the cortex and multiple histological markers enabled detailed counts of labeled neurons in individual areas. The observed log-normal distribution of connection weights to each cortical area spans 5 orders of magnitude and reveals a distinct connectivity profile for each area, analogous to that observed in macaques. The cortical network has a density of 97%, considerably higher than the 66% density reported in macaques. A weighted graph analysis reveals a similar global efficiency but weaker spatial clustering compared with that reported in macaques. The consistency, precision of the connectivity profile, density, and weighted graph analysis of the present data differ significantly from those obtained in earlier studies in the mouse.

Synaptic Organization of the Neuronal Circuits of the Claustrum

The claustrum, a poorly understood subcortical structure located between the cortex and the striatum, forms widespread connections with almost all cortical areas, but the cellular organization of claustral circuits remains largely unknown. Based primarily on anatomical data, it has been proposed that the claustrum integrates activity across sensory modalities. However, the extent to which the synaptic organization of claustral circuits supports this integration is unclear. Here, we used paired whole-cell recordings and optogenetic approaches in mouse brain slices to determine the cellular organization of the claustrum. We found that unitary synaptic connections among claustrocortical (ClaC) neurons were rare. In contrast, parvalbumin-positive (PV) inhibitory interneurons were highly interconnected with both chemical and electrical synapses. In addition, ClaC neurons and PV interneurons formed frequent synaptic connections. As suggested by anatomical data, we found that corticoclaustral afferents formed monosynaptic connections onto both ClaC neurons and PV interneurons. However, the responses to cortical input were comparatively stronger in PV interneurons. Consistent with this overall circuit organization, activation of corticoclaustral afferents generated monosynaptic excitatory responses as well as disynaptic inhibitory responses in ClaC neurons. These data indicate that recurrent excitatory circuits within the claustrum alone are unlikely to integrate across multiple sensory modalities. Rather, this cellular organization is typical of circuits sensitive to correlated inputs. Although single ClaC neurons may integrate corticoclaustral input from different cortical regions, these results are consistent with more recent proposals implicating the claustrum in detecting sensory novelty or in amplifying correlated cortical inputs to coordinate the activity of functionally related cortical regions. Significance statement: The function of the claustrum, a brain nucleus found in mammals, remains poorly understood. It has been proposed, based primarily on anatomical data, that claustral circuits play an integrative role and contribute to multimodal sensory integration. Here we show that the principal neurons of the claustrum, claustrocortical (ClaC) projection neurons, rarely form synaptic connections with one another and are unlikely to contribute to broad integration within the claustrum. We show that, although single ClaC neurons may integrate corticoclaustral inputs carrying information for different sensory modalities, the synaptic organization of ClaC neurons, local parvalbumin-positive interneurons within the claustrum, and cortical afferents is also consistent with recent proposals that the claustrum plays a role in detecting salient stimuli or amplifying correlated cortical inputs.

Comparative organization of the claustrum: what does structure tell us about function?

The claustrum is a subcortical nucleus present in all placental mammals. Many anatomical studies have shown that its inputs are predominantly from the cerebral cortex and its outputs are back to the cortex. This connectivity thus suggests that the claustrum serves to amplify or facilitate information processing in the cerebral cortex. The size and the complexity of the cerebral cortex varies dramatically across species. Some species have lissencephalic brains, with few cortical areas, while others have a greatly expanded cortex and many cortical areas. This evolutionary diversity in the cerebral cortex raises several questions about the claustrum. Does its volume expand in coordination with the expansion of cortex and does it acquire new functions related to the new cortical functions? Here we survey the organization of the claustrum in animals with large brains, including great apes and cetaceans. Our data suggest that the claustrum is not always a continuous structure. In monkeys and gorillas there are a few isolated islands of cells near the main body of the nucleus. In cetaceans, however, there are many isolated cell islands. These data suggest constraints on the possible function of the claustrum. Some authors propose that the claustrum has a more global role in perception or consciousness that requires intraclaustral integration of information. These theories postulate mechanisms like gap junctions between claustral cells or a “syncytium” to mediate intraclaustral processing. The presence of discontinuities in the structure of the claustrum, present but minimal in some primates, but dramatically clear in cetaceans, argues against the proposed mechanisms of intraclaustral processing of information. The best interpretation of function, then, is that each functional subdivision of the claustrum simply contributes to the function of its cortical partner.

Convergence of thalamic and cortical pathways in cat auditory cortex

Cat auditory cortex (AC) receives input from many thalamic nuclei and cortical areas. Previous connectional studies often focused on one connectional system in isolation, limiting perspectives on AC computational processes. Here we review the convergent thalamic, commissural, and corticocortical projections to thirteen AC areas in the cat. Each input differs in strength and may thus serve unique roles. We compared the convergent intrinsic and extrinsic input to each area quantitatively. The intrinsic input was almost half the total. Among extrinsic projections, ipsilateral cortical sources contributed 75%, thalamic input contributed 15%, and contralateral sources contributed 10%. The patterns of distribution support the division of AC areas into families of tonotopic, non-tonotopic, multisensory, and limbic-related areas, each with convergent input arising primarily from within its group. The connections within these areal families suggest a form of processing in which convergence of input to an area could enable new forms of integration. In contrast, the lateral connections between families could subserve integration between categorical representations, allowing otherwise independent streams to communicate and thereby coordinating operations over wide spatial and functional scales. These patterns of serial and interfamilial cooperation challenge more classical models of organization that underestimate the diversity and complexity of AC connectivity.

Visual and motor connectivity and the distribution of calcium-binding proteins in macaque frontal eye field: implications for saccade target selection

The frontal eye field (FEF) contributes to directing visual attention and saccadic eye movement through intrinsic processing, interactions with extrastriate visual cortical areas (e.g., V4), and projections to subcortical structures (e.g., superior colliculus, SC). Several models have been proposed to describe the relationship between the allocation of visual attention and the production of saccades. We obtained anatomical information that might provide useful constraints on these models by evaluating two characteristics of FEF. First, we investigated the laminar distribution of efferent connections from FEF to visual areas V4 + TEO and to SC. Second, we examined the laminar distribution of different populations of GABAergic neurons in FEF. We found that the neurons in FEF that project to V4 + TEO are located predominantly in the supragranular layers, colocalized with the highest density of calbindin- and calretinin-immunoreactive inhibitory interneurons. In contrast, the cell bodies of neurons that project to SC are found only in layer 5 of FEF, colocalized primarily with parvalbumin inhibitory interneurons. None of the neurons in layer 5 that project to V4 + TEO also project to SC. These results provide useful constraints for cognitive models of visual attention and saccade production by indicating that different populations of neurons project to extrastriate visual cortical areas and to SC. This finding also suggests that FEF neurons projecting to visual cortex and SC are embedded in different patterns of intracortical circuitry.

Branched projections in the auditory thalamocortical and corticocortical systems

Branched axons (BAs) projecting to different areas of the brain can create multiple feature-specific maps or synchronize processing in remote targets. We examined the organization of BAs in the cat auditory forebrain using two sensitive retrograde tracers. In one set of experiments (n=4), the tracers were injected into different frequency-matched loci in the primary auditory area (AI) and the anterior auditory field (AAF). In the other set (n=4), we injected primary, non-primary, or limbic cortical areas. After mapped injections, percentages of double-labeled cells (PDLs) in the medial geniculate body (MGB) ranged from 1.4% (ventral division) to 2.8% (rostral pole). In both ipsilateral and contralateral areas AI and AAF, the average PDLs were <1%. In the unmapped cases, the MGB PDLs ranged from 0.6% (ventral division) after insular cortex injections to 6.7% (dorsal division) after temporal cortex injections. Cortical PDLs ranged from 0.1% (ipsilateral AI injections) to 3.7% in the second auditory cortical area (AII) (contralateral AII injections). PDLs within the smaller (minority) projection population were significantly higher than those in the overall population. About 2% of auditory forebrain projection cells have BAs and such cells are organized differently than those in the subcortical auditory system, where BAs can be far more numerous. Forebrain branched projections follow different organizational rules than their unbranched counterparts. Finally, the relatively larger proportion of visual and somatic sensory forebrain BAs suggests modality specific rules for BA organization.

Neurochemically defined cell types in the claustrum of the cat

The claustrum is a subcortical structure reciprocally and topographically connected with all sensory and motor domains of the cerebral cortex. Previous anatomical and electrophysiological data suggested that most cells in the claustrum are large neurons that both receive cortical input and project back to cortex, forming excitatory connections with their cortical targets. These data have been interpreted to imply a relay function for the claustrum, with information from different functional cortical domains remaining segregated. The possibility that the claustrum might mediate a more "global" function has been recently been developed by Crick and Koch [Crick, F. C., Koch, C., 2005. What is the function of the claustrum? Philos. Trans. R. Soc. Lond., B Biol. Sci. 360, 1271-1279]. We have reexamined the anatomical substrate for information processing in the claustrum of the cat by analyzing the patterns of immunoreactivity to calcium-binding proteins, GAD, serotonin, nNOS and the glutamate transporter EAAC1. We found multiple neurochemically defined cell types, suggesting multiple classes of projection neurons and interneurons. Each class was found throughout the entire claustrum, in all functionally defined subdivisions. Many neurons in the claustrum were surrounded by parvalbumin, calretinin, GAD or nNOS immunoreactive terminals, suggesting that many neurons of the claustrum make extensive intraclaustral connections. The entire claustrum also receives a serotonergic input. The identification of multiple neurochemical cell classes, their distribution and the extent of their dendritic arborizations relative to functional compartments suggest a substrate for information processing in the claustrum that may allow integration of information across functional subdivisions.

Laminar processing in the visual cortical column

Sensory regions of neocortex are organized as arrays of vertical columns composed of cells that share similar response properties, with the orientation columns of the cat's visual cortex being the best known example. Interest in how sensitivity to different stimulus features first emerges in the columns and how this selectivity is refined by subsequent processing has fueled decades of research. A natural starting point in approaching these issues is anatomy. Each column traverses the six cortical layers and each layer has a unique pattern of inputs, intrinsic connections and outputs. Thus, it makes sense to explore the possibility of corresponding laminar differences in sensory function, that is, to examine relationships between morphology and physiology. In addition, to help identify general patterns of cortical organization, it is useful to compare results obtained from different sensory systems and diverse species. The picture that emerges from such comparisons is that each cortical layer serves a distinct role in sensory function. Furthermore, different cortices appear to share some common strategies for processing information but also have specialized mechanisms adapted for the demands of specific sensory tasks.

Crossmodal audio-visual interactions in the primary visual cortex of the visually deprived cat: a physiological and anatomical study

Blind individuals often demonstrate enhanced non-visual perceptual abilities. Neuroimaging and transcranial magnetic stimulation experiments have suggested that computations carried out in the occipital cortex may underlie these enhanced somatosensory or auditory performances. Thus, cortical areas that are dedicated to the analysis of the visual scene may, in the blind, acquire the capacity to participate in other sensory processing. However, the neural substrate that underlies this transfer of function is not fully characterized. Here we studied the synaptic and anatomical basis of this phenomenon in cats that were visually deprived by dark rearing, either early visually deprived after birth (EVD), or late visually deprived after the end of the critical period (LVD); data were compared with those obtained in normally reared cats (controls). The presence of synaptic and spike responses to auditory stimulation was examined by means of intracellular recordings in area 17 and the border between areas 17 and 18. While none of the cells recorded in control and LVD cats showed responses to sound, 14% of the cells recorded in EVD cats showed both subthreshold synaptic responses and suprathreshold spike responses to auditory stimuli. Synaptic responses were of small amplitude, but well time-locked to the stimuli and had an average latency of 30+/-12ms. In an attempt to identify the origin of the inputs carrying auditory information to the visual cortex, wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected in the visual cortex and retrograde labeling examined in the cortex and thalamus. No significant retrograde labeling was found in auditory cortical areas. However, the proportion of neurons projecting from supragranular layers of the posteromedial and posterolateral parts of the lateral suprasylvian region to V1 was higher than that in control cats. Retrograde labeling in the lateral geniculate nucleus showed no difference in the total number of neurons between control and visually deprived cats, but there was a higher proportion of labeling in C-laminae in deprived cats. Labeled cells were not found in the medial geniculate nucleus, a thalamic relay for auditory information, in either control or visually deprived cats. Finally, immunohistochemistry of the visual cortex of deprived cats revealed a striking decrease in pavalbumin- and calretinin-positive neurons, the functional implications of which we discuss.

Tonotopic and heterotopic projection systems in physiologically defined auditory cortex

Combined physiological and connectional studies show significant non-topographic extrinsic projections to frequency-specific domains in the cat auditory cortex. These frequency-mismatched loci in the thalamus, ipsilateral cortex, and commissural system complement the predicted topographic and tonotopic projections. Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), were electrophysiologically characterized by their frequency organization. Next, either cholera toxin beta subunit or cholera toxin beta subunit gold conjugate was injected into frequency-matched locations in each area to reveal the projection pattern from the thalamus and cortex. Most retrograde labeling was found at tonotopically appropriate locations within a 1 mm-wide strip in the thalamus and a 2-3 mm-wide expanse of cortex (approximately 85%). However, approximately 13-30% of the neurons originated from frequency-mismatched locations far from their predicted positions in thalamic nuclei and cortical areas, respectively. We propose that these heterotopic projections satisfy at least three criteria that may be necessary to support the magnitude and character of plastic changes in physiological studies. First, they are found in the thalamus, ipsilateral and commissural cortex; since this reorganization could arise from any of these routes and may involve each, such projections ought to occur in them. Second, they originate from nuclei and areas with or without tonotopy; it is likely that plasticity is not exclusively shaped by spectral influences and not limited to cochleotopic regions. Finally, the projections are appropriate in magnitude and sign to plausibly support such rearrangements; given the rapidity of some aspects of plastic changes, they should be mediated by substantial existing connections. Alternative roles for these heterotopic projections are also considered.

Quantitative analyses of principal and secondary compound parieto-occipital feedback pathways in cat

The purpose of our study was to quantify the magnitude of principal and secondary pathways emanating from the middle suprasylvian (MS) region of visuoparietal cortex and terminating in area 18 of primary visual cortex. These pathways transmit feedback signals from visuoparietal cortex to primary visual cortex. (1) WGA-HRP was injected into area 18 to identify inputs from visual structures. In terms of numbers of neurons, feedback projections to area 18 from MS sulcal cortex (areas PMLS, AMLS and PLLS) comprise 26% of inputs from all visual structures. Of these neurons, between 21% and 34.9% are located in upper layers 2-4 and the dominant numbers are located in deep layers 5 and 6. Areas 17 (11.8%) and 19 (11.2%) provide more modest cortical inputs, and another eight areas provide a combined total of 4.3% of inputs. The sum of neurons in all subcompartments of the lateral geniculate nucleus (LGN) accounts for another 34.8% of the input to area 18, whereas inputs from the lateral division of the lateral-posterior nucleus (LPl) account for the final 11.9%. (2) Injection of tritiated-((3)H)-amino acids into MS sulcal cortex revealed substantial direct projections from MS cortex that terminated in all layers of area 18, but with a markedly lower density in layer 4. Projections from MS cortex to both areas 17 and 19 are of similar density and characteristics, whereas those to other cortical targets have very low densities. Quantification also revealed minor-to-modest axon projections to all components of LGN and a massive projection throughout the LP-Pul complex. (3) Superposition of the labeled terminal and cell fields identified secondary, compound feedback pathways from MS cortex to area 18. The largest secondary pathway is massive and it includes the LPl nucleus. Much more modest secondary pathways include areas 17 and 19, and LGN. The relative magnitudes of the secondary pathways suggest that the one through LPl exerts a major influence on area 18, whereas the others exert more modest or minor influences. MS cortex in the contralateral hemisphere also innervates area 18 directly. These data are important for interpreting the impact of deactivating feedback projections from visuoparietal cortex on occipital cortex.

Two different types of Y cells in the cat lateral geniculate nucleus

The Y pathway in the cat visual system is traditionally viewed as a single channel that originates in the retina. However, most Y cells from the contralateral retina diverge to innervate two different layers of the lateral geniculate nucleus, suggesting a possible channel split: YC (Y geniculate cell in layer C) and YA (Y geniculate cell in layer A). We tested the functional significance of this anatomical divergence by comparing the response properties of simultaneously recorded YC and YA geniculate cells with overlapping receptive fields. Our results demonstrate that YC and YA cells significantly differ in a large number of temporal and spatial parameters including response latency, response transiency, receptive-field size, and linearity of spatial summation. Furthermore, for some of these parameters, the differences between YC and YA cells are as pronounced as the differences between Y and X cells in layer A. These results along with results from previous studies strongly suggest that Y retinal afferents diverge into two separate channels at the level of the thalamus.

Auditory Neurons with Transitory Axons to Visual Areas Form Short Permanent Projections

The kitten's auditory cortex (including the first and second auditory fields AI and AII) is known to send transient axons to either ipsi- or contralateral visual areas 17 and 18. By the end of the first postnatal month the transitory axons, but not their neurons of origin, are eliminated. Here we investigated where these neurons project after the elimination of the transitory axon. Eighteen kittens received early (postnatal day (pd) 2 - 5) injections of long lasting retrograde fluorescent traces in visual areas 17 and 18 and late (pd 35 - 64) injections of other retrograde fluorescent tracers in either hemisphere, mostly in areas known to receive projections from AI and AII in the adult cat. The middle ectosylvian gyrus was analysed for double-labelled neurons in the region corresponding approximately to AI and AII. Late injections in the contralateral (to the analysed AI, AII) hemisphere including all of the known auditory areas, as well as some visual and 'association' areas, did not relabel neurons which had had transient projections to either ipsi- or contralateral visual areas 17 - 18. Thus, AI and AII neurons after eliminating their transient juvenile projections to visual areas 17 and 18 do not project to the other hemisphere. In contrast, relabelling was obtained with late injections in several locations in the ipsilateral hemisphere; it was expressed as per cent of the population labelled by the early injections. Few neurons (0 - 2.5%) were relabelled by large injections in the caudal part of the posterior ectosylvian gyrus and the adjacent posterior suprasylvian sulcus (areas DP, P, VP). Multiple injections in the middle ectosylvian gyrus relabelled a considerably larger percentage of neurons (13%). Single small injections in the middle ectosylvian gyrus (areas AI, AII), the caudal part of the anterior ectosylvian gyrus and the rostral part of the posterior ectosylvian gyrus relabelled 3.1 - 7.0% of neurons. These neurons were generally near (<2.0 mm) the outer border of the late injection sites. Neurons with transient projections to ipsi- or contralateral visual areas 17 and 18 were relabelled in similar proportions by late injections at any given location. Thus, AI or AII neurons which send a transitory axon to ipsi- or contralateral visual areas 17 and 18 are most likely to form short permanent cortical connections. In that respect, they are similar to medial area 17 neurons that form transitory callosal axons and short permanent axons to ipsilateral visual areas 17 and 18.

Bihemispheric Collateralization of the Cortical and Subcortical Afferents to the Rat's Visual Cortex

A fluorescent dye (usually fast blue or rhodamine tagged latex microspheres) was injected into cortical area 17 (or area 17 and the lateral part of area 18b) of adult and juvenile (15 - 22 day old) Sprague-Dawley albino rats. Another fluorescent dye (usually diamidino yellow) was injected into cortical areas 17, 18a and 18b of the opposite hemisphere. The injections involved only the cortical grey matter. After postinjection survival of 2 - 14 days the distribution of retrogradely labelled mesencephalic and prosencephalic cells was analysed. Both small and large injections labelled retrogradely a substantial number of cells in specific and nonspecific dorsal thalamic nuclei (lateral geniculate, lateral posterior, ventromedial, several intralaminar nuclei and nucleus Reuniens) as well as a small number of cells in the preoptic area of the hypothalamus and the mesencephalic ventral tagmental area (VTA). While labelled thalamic cells contained only the dye injected into the ipsilateral cortex, a small proportion of hypothalamic and VTA cells was labelled with the dye injected into the contralateral cortex. Virtually none of the cells in these areas were double labelled with both dyes. Both small and large injections labelled cells in the ipsilateral telencephalic magnocellular nuclei of the basal forebrain and the caudal claustrum. A substantial minority of labelled cells in these structures was labelled by the dye injected into the contralateral cortex. Furthermore, a small proportion (about 1%) of claustral cells projecting to the ipsilateral cortex were double labelled with both dyes. In several cortical areas ipsilateral to the injected area 17, associational neurons were intermingled with commissural neurons projecting to the contralateral visual cortex. A substantial proportion of associational neurons projecting to ipsilateral area 17 also projected to the contralateral visual cortex (associational-commissural neurons). Thus, in visual area 18a, the associational-commissural neurons were located in all laminae, with the exception of lamina 1 and the bottom of lamina 6, and constituted about 30% of the neurons projecting to ipsilateral area 17. In paralimbic association area 35/13, associational-commissural neurons were located in lamina 5 and constituted about 20% of neurons projecting to ipsilateral area 17. In the limbic area 29d, the associational-commissural neurons were located in laminae 4, 5 and the upper part of lamina 6 and constituted about 10% of the associational-commissural neurons projecting to ipsilateral area 17. In oculomotor area 8, double-labelled neurons were located in lamina 5 and constituted about 10% of the neurons projecting to ipsilateral area 17. Thus, it appears that the axons of mesencephalic and diencephalic neurons projecting to the visual cortex do not send collaterals into both hemispheres. The bihemispheric projection to the rat's visual cortex originates almost exclusively in the retinotopically organized cortical area 18a and in integrative cortical areas 35/13, 29d and 8.

Bihemispheric Axonal Bifurcation of the Afferents to the Visual Cortical Areas during Postnatal Development in the Rat

Numerous cortical neurons in the juvenile and adult rat project to visual areas of both hemispheres whereas the vast majority of subcortical structures projecting to the visual cortex send strictly ipsilateral projections (Dreher et al., 1990). In the present study, the authors have sought to determine whether this pattern of axonal bifurcation in the connectivity of the visual areas undergoes a change during postnatal development. Two retrograde fluorescent dyes were used, fast blue (FB) and diamidino yellow (DY). Large multiple injections of one of the dyes were placed in all visual areas of one hemisphere and a small injection of the other dye was placed in area 17 of the opposite hemisphere. Labelled neurons were observed in subcortical and cortical structures on the side of the small injection. The experiments were performed on ten neonatal albino rat pups aged between 3 and 12 postnatal days (p.n.d.) at the time of injection and the results were compared with those obtained in the juvenile and adult animals, as reported in the preceding paper. In the thalamus of newborn animals, neurons belonging to nuclei located away from the midline send strictly ipsilateral cortical projections. However, in the midline nuclei of the intralaminar thalamic complex, a small region of overlap was observed between neurons projecting ipsilaterally and neurons projecting contralaterally in animals aged less than 9 postnatal days. In addition, in these neonatal animals a small number of bilaterally projecting neurons was detected in this region of overlap. In all other subcortical structures examined (ventral tegmental area, diagonal band of Broca, claustrum), the laterality of the projection was the same in the newborn and the adult animals. In particular, in the claustrum of neonatal animals, as in adult animals, there was a large contingent of contralaterally projecting neurons and only a very small number of bilaterally projecting neurons. The results in the cortex contrast with those observed in subcortical structures. Whereas ipsilaterally projecting neurons were distributed in a broadly similar way in newborn and adult animals, the laminar and areal distribution of contralaterally projecting neurons in newborn animals clearly differed from those observed in the adult animals. Furthermore, double labelled neurons were more numerous in animals aged less than 12 days than in adults. The proportions of such bilaterally projecting neurons were computed with respect to the numbers of neurons sending ipsilateral projections to area 17. These proportions are constant at all ages in the claustrum and cortical area 8. In areas 18a, 29 and 35 on the other hand, the proportions of bilaterally projecting neurons increase after 5 days and reach a peak in the period extending from 9 to 11 days of age when more than half of the neurons projecting ipsilaterally also send an axonal branch to the contralateral cortex. In cortical areas 29 and 35, this peak is followed by a sudden drop to the adult level at 12 postnatal days, whereas the return to the adult level is gradual in area 18a. These results demonstrate that, in subcortical structures and in cortical area 8, the laterality of the afferent connections to the visual cortex does not change during postnatal development. By contrast, cortical areas 18a, 29 and 35 go through a stage when numerous cells send bifurcating connections to both hemispheres. The timing of the decrease in proportions of bilaterally projecting neurons in these areas suggests that numerous neurons retract their callosal axonal branch when the adult pattern of callosal connectivity is established at 9 - 11 days of age.

Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

We studied the connections of eleven auditory cortical areas with the claustrum and the endopiriform nucleus in the cat, by means of cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase, or biotinylated dextran amines. Unlike previously accepted reports, all auditory areas have reciprocal connections with the ipsi- and contralateral claustrum, though they differ in strength and/or topography. The areas that send the strongest projections are the intermediate region of the posterior ectosylvian gyrus and the insular cortex, followed by the primary auditory cortex and the dorsal portion of the posterior ectosylvian gyrus. The high degree of convergence of cortical axons in the intermediate region of the claustrum, arising from tonotopic and nontonotopic areas, suggests that claustral neurons are unlikely to be well tuned to the frequency of the acoustic stimulus. Corticoclaustral axons from any given area cover territories largely overlapping with those occupied by the claustrocortical neurons projecting back to the same area. The location of cortically projecting neurons in the claustrum matches the position of the target cortical area in the cerebral hemisphere, both rostrocaudally and dorsoventrally. These findings suggest that the intermediate region of the claustrum integrates inputs from all auditory cortical areas, and then sends the result of such processing back to every auditory cortical field. On the other hand, the endopiriform nucleus, a limbic-related structure thought to play a role in the acquisition of conditioned fear, would process mostly polymodal information, since it only receives projections from the insular and temporal cortices.

Structure and dynamics of receptive fields in the visual cortex of the cat (area 18) and the influence of GABAergic inhibition

Receptive fields (RFs) in the visual cortex are characterized by spatiotemporal profiles that have been described in detail for area 17 simple cells. In this study, we analyse spatial and temporal RF properties of simple and complex cells in layer II/III of area 18 of the anaesthetized adult cat, using the reverse correlation method with brief 50 ms presentations of flashing bright and dark bars. Stimuli were presented with preferred orientation as previously determined by moving bars. Simple cell RFs were characterized by spatially and temporally separable ON and OFF subfields, while in complex cells ON and OFF subfields were superimposed. To discriminate possible contributions of GABAergic inhibition to RF structure and response dynamics in area 18, we have used three-barrelled micropipettes for single cell recordings and microiontophoresis, and have documented ON and OFF responses before, during and after application of bicuculline methiodide for blockade of GABAA receptors. During blockade of GABAergic inhibition, the stimulus-induced and resting discharge frequency increased, and in about 50% of the cells both ON and OFF subfields changed significantly in space and/or time in a reversible manner. In space, blockade of inhibition widened RF subfields, whereas in time, it shortened the duration of the excitatory cell response in simple and complex cells. ON and OFF subfields separated in space and time (simple cells), or time (complex cells) became less isolated or even superimposed. The results indicate substantial local inhibitory processing contributing to spatiotemporal RF properties in layers II/III of area 18 of the cat.

Ascending projections of simple and complex cells in layer 6 of the cat striate cortex

Receptive field properties vary systematically across the different layers of the cat striate cortex. Understanding how these functional differences emerge requires a precise description of the interlaminar connections and the quality of information that they transmit. This study examines the contribution of the two physiological types of neuron in layer 6, simple and complex, to the cortical microcircuit. The approach was to make whole-cell recordings with dye-filled electrodes in vivo to correlate visual response property with intracortical projection pattern. The two simple cells we stained projected to layer 4, as previously reported (Gilbert and Wiesel, 1979; Martin and Whitteridge, 1984). Six of the eight complex cells that we labeled projected to the superficial layers, a pathway not previously described in the cat. The remaining two cells targeted the infragranular layers. Layer 4 is dominated by simple cells, whereas layers 5 and 2+3 are mainly composed of complex cells (Hubel and Wiesel, 1962; Gilbert, 1977). Hence, our results indicate that the ascending projections of simple cells in layer 6 target other simple cells. In parallel, the ascending projections of a population of complex cells in layer 6 favor other complex cells. Anatomical experiments in several species (Lund and Boothe,1975; Burkhalter,1989; Usrey and Fitzpatrick, 1996; Wiser and Callaway, 1996) had also demonstrated that layer 6 gives rise to two separate intracortical pathways. Pooling the results of these anatomical studies with our own suggests a common feature of the laminar organization: cells that project to different intracortical targets have distinct functional characteristics.

Cortical projections of the parvocellular laminae C of the dorsal lateral geniculate nucleus in the cat: an anterograde wheat germ agglutinin conjugated to horseradish peroxidase study

The areal and laminar distributions of the projection from the parvocellular part of laminae C of the dorsal lateral geniculate nucleus (Cparv) were studied in visual cortical areas of the cat with the anterograde tracing method by using wheat germ agglutinin conjugated to horseradish peroxidase. A particular objective of this study was to examine the central visual pathways of the W-cell system, the precise organization of which is still unknown. Because the Cparv in the cat is said to receive W-cell information exclusively from the retina and the superior colliculus, the results obtained would provide an anatomical substrate for the W-cell system organization in mammals. The results show that the cortical targets of the Cparv are areas 17, 18, 19, 20a, and 21a and the posteromedial lateral suprasylvian (PMLS) and ventral lateral suprasylvian(VLS) areas. In area 17, the projection fibers terminate in the superficial half of layer I; the lower two-thirds of layer III, extending to the superficial part of layer IV; and the deep part of layer IV, involving layer Va. These terminations form triple bands in area 17. The projection terminals in layer I are continuous, whereas those in layers III, IV, and Va distribute periodically, exhibiting a patchy appearance. In areas 18 and 19, the projection fibers terminate in the superficial half of layer I and in the full portions of layers III and IV, forming double bands. In these areas, the terminals in layer I are continuous, whereas those in layers III and IV distribute periodically, exhibiting a patchy appearance. In area 20a, area 21a, PMLS, and VLS, projection fibers terminate in the superficial part of layer I, in part of layer III, and in the full portion of layer IV, although they are far fewer in number than those seen in areas 17, 18, and 19. The present results demonstrate that the Cparv fibers terminate in a localized fashion in both the striate and the extrastriate cortical areas and that these W-cell projections are quite unique in their areal and laminar organization compared with the X- and Y-cell systems.

Subcortical input to the smooth and saccadic eye movement subregions of the frontal eye field in Cebus monkey

We have recently identified two functional subregions in the frontal eye field (FEF) of the Cebus monkey, a smooth eye movement subregion (FEFsem) and a saccadic subregion (FEFsac). The thalamic input to these two subregions was studied and quantified to gain more information about the influence of the cerebellum and basal ganglia on the oculomotor control mechanisms of the cerebral cortex. A recent study using transneuronal transport of virus demonstrated that there are neurons in the basal ganglia and cerebellum that project to the FEFsac with only a single intervening synapse (Lunch et al., 1994). In the present study, we concentrated on the thalamic input to the FEFsem to define possible basal ganglia-thalamus-cortex and cerebellum-thalamus-cortex channels of information flow to the FEFsem. We localized the functional subregions using low threshold microstimulation, and retrogradely transported fluorescent tracers were then placed into the FEFsem and FEFsac. The neurons that project to the FEFsem are distributed in (1) the rostral portion of the ventral lateral nucleus, pars caudalis, (2) the caudal portion of the ventral lateral nucleus, pars caudalis, (3) the mediodorsal nucleus, (4) the ventral anterior nucleus, pars parvocellularis, and (5) the ventral anterior nucleus, pars magnocellularis. In contrast, the large majority of neurons that project to the FEFsac are located in the paralaminar region of the mediodorsal nucleus. The FEFsac and FEFsem thus each receive neural input from both basal ganglia-receiving and cerebellar-receiving cell groups in the thalamus, but each receives input from a unique combination of thalamic nuclei.

Lateral division of the lateral posterior region: connections with area 18 in cats

To establish the topography of the lateral division of the lateral posterior region (LP1) projections to area 18, up to five different anatomical tracers were injected in separate rostrocaudal locations in area 18 of four adult cats, and patterns of retrogradely labeled LP1 cells were identified. LP1 inputs to area 18 arose from both caudal and rostral nuclei and were topological, organized in patterns that indicate that lower visual space is represented anteroventrally, and more central and upper visual space is represented caudodorsally. In the caudal LP1 nucleus, patches of labeled cells formed bands that ran parallel to the medial and lateral LP1 borders and encompassed medial portions of the nucleus. In rostral LP1, the patches of labeled cells formed clusters giving the connections with area 18 a more modular appearance, and were nearer the lateral LP1 border. Injections made nearest area centralis representations in area 18 labeled more neurons than injections in cortex representing more peripheral visual space. Also, neighboring injections in area 18 labeled overlapping patches of cells, but no double-labeled cells were observed. These findings are consistent with previous conclusions based on electrophysiological mapping studies, that two retinotopically organized nuclei constitute LP1.

Role of directed growth and target selection in the formation of cortical pathways: prenatal development of the projection of area V2 to area V4 in the monkey

In experiments combining retrograde tracers and histochemistry, we have looked at the prenatal development of the cortical pathway linking areas V2 and V4. Transient expression of acetylcholinesterase in fetal area V2 reveals the separate compartments that project to V4 (temporal directed pathway) and V5 (parietal directed pathway). During early stages of pathway formation, V2 neurons projecting to area V4 are clustered in the appropriate compartments. During the phase of rapid axonal growth, there is a selective increase of connections originating from the appropriate compartments leading to a strongly clustered organization at the peak of connectivity. During this phase, injections involving the white matter also showed clustering, but this was somewhat reduced in comparison to that of gray matter injections. The growth phase is followed by an elimination phase during which there is a tendency for a preferential loss of intercluster connections, which may sharpen the early formed pattern. These results demonstrate the primary role of axonal guidance and target recognition mechanisms followed by a limited extent of selective elimination during the formation of functional cortical pathways in the primate isocortex. Compared to previous findings, these results suggest that the developmental restriction of callosal connections is not a universal model of cortical development. In the present report, the directed growth and early specification of feed-forward connections contrast with the prolonged remodelling of monkey feedback projections, suggesting two distinct developmental strategies of pathway formation in the monkey.

Evidence for greater sight in blindsight following damage of primary visual cortex early in life

This review compares the behavioral, physiological and anatomical repercussions of lesions of primary visual cortex incurred by developing and mature humans, monkey and cats. Comparison of the data on the repercussions following lesions incurred earlier or later in life suggests that earlier, but not later, damage unmasks a latent flexibility of the brain to compensate partially for functions normally attributed to the damaged cortex. The compensations are best documented in the cat and they can be linked to system-wide repercussions that include selected pathway expansions and neuron degenerations, and functional adjustments in neuronal activity. Even though evidence from humans and monkeys is extremely limited, it is argued on the basis of known repercussions and similarity of visual system organization and developmental sequence, that broadly equivalent repercussions most likely occur in humans and monkeys following early lesions of primary visual cortex. The extant data suggest potentially useful directions for future investigations on functional anatomical aspects of visual capacities spared in human patients and monkeys following early damage of primary visual cortex. Such research is likely to have a substantial impact on increasing our understanding of the repercussions that result from damage elsewhere in the developing cerebral cortex and it is likely to contribute to our understanding of the remarkable ability of the human brain to adapt to insults.

The lateral posterior-pulvinar complex modulation of stimulus-dependent oscillations in the cat visual cortex

It has been suggested that binding coherent targets depends on the capacity of excited cortical cells to fire in synchrony at approximately 40 Hz. However, the origin of stimulus-related cortical oscillations is still not clear. We hypothesized that 40 Hz oscillations might propagate to the visual cortex from the lateral posterior-pulvinar complex (LP-P) whose cells send fibers to the visual cortex and have a tendency to exhibit oscillations. To test our hypothesis, we recorded single unit activity in areas 17 and 18 of anaesthetized cats. The activity of neurons which showed oscillations evoked by optimal visual stimuli was analysed before, during and after a reversible inactivation of the LP-P with GABA. Such inactivation was found to markedly modify the strength of oscillatory activity of cortical neurons whose visual responses were affected by LP-P blockade. In contrast, the oscillation frequencies of cortical neurons were not modified by such inactivation. However, in some cells (three of nine), oscillatory activity was found to be completely abolished by injection of GABA into the LP-P. Collectively, these findings demonstrate that inputs from the LP-P play a key role in modulating the oscillatory activity of visual cortex neurons. Assuming that cortical neurons utilize oscillatory activity to encode perceptual aspects of the visual stimulus, our findings underscore the contribution of the LP-P in this process.

Trends in the anatomical organization and functional significance of the mammalian thalamus

The last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states.

Estimates of the convergence of association projections from area 17 to rostral area 18 in the cat depend on injection site size

To estimate the degree of convergence of corticocortical projections from area 17 to the rostral part of area 18 in the cat, we injected the retrograde tracer diamidino yellow either at single or multiple adjacent sites in area 18. We measured the diameters of the individual or larger composite injections and the diameters of the labelled regions in area 17; from these values we estimated convergence factors. Small injections led to underestimates of the overall convergence of this pathway. A maximal value for convergence (about 3 mm) was obtained with larger composite injections (above about 3 mm in diameter). These results indicate that some area 17-to-18 projections may arborize widely in area 18. Their detection may require that injected tracer covers a large portion of their arbors.

Areas PMLS and 21a of cat visual cortex are not only functionally but also hodologically distinct

In several cats, paired visuotopically matched injections of retrogradely transported fluorescent dyes, diamidino yellow (DY) and fast blue (FB), were made into two visuotopically organized, functionally distinct extrastriate cortical areas, the posteromedial lateral suprasylvian area (PMLS area) and area 21a respectively. After an appropriate survival time, the numbers of thalamic, claustral and cortical cells which were single-labelled with each dye as well as the numbers of cells in these structures labelled with both dyes (double-labelled cells) were assessed. The clear majorities of thalamic cells projecting to PMLS area (DY labelled cells) and to area 21a (FB labelled cells) were located in the ipsilateral lateral posterior-pulvinar complex with smaller proportions located in the laminae C and the medial intralaminar nucleus of the ipsilateral dorsal lateral geniculate nucleus and several nuclei of the rostral intralaminar thalamic group. Despite the fact that DY labelled (PMLS-projecting) and FB labelled (area 21 a-projecting) cells in all thalamic nuclei were well intermingled, only 1-5% of retrogradely labelled thalamic cells projected to both areas (cells double-labelled with both dyes). Small proportions of retrogradely labelled cells were located in the ipsilateral and to a lesser extent the contralateral dorsocaudal claustra. The proportions of claustral neurons retrogradely labelled with both dyes varied from 4 to 9%. Over half of the cortical neurons labelled retrogradely from area 21a or PMLS area were located in the supragranular layers of the ipsilateral area 17, with smaller proportions located in the supragranular layers of the ipsilateral areas 18 and 19 and even smaller proportions located in mainly but not exclusively, the infragranular layers of the ipsilateral areas 21b and 20a. Again despite strong spatial intermingling of neurons labelled with DY and these labelled with FB, the proportions of associational cortical neurons double-labelled with both dyes were small (2 to 5.5%). Finally, small proportions of neurons retrogradely labelled with DY or FB were located, mainly but not exclusively, in the supragranular layers of the contralateral areas 17, 18, 19 and 21a. Again, the proportions of the double-labelled neurons in the contralateral cortices were small (1-4.5%). Thus, the present study indicates that despite the fact that the diencephalic and telencephalic inputs to the visuotopically corresponding parts of area 21a and PMLS area originate from the same nuclei, areas and layers, the two areas receive their afferents from the largely separate populations of neurons.

Corticothalamic projections from layer V cells in rat are collaterals of long-range corticofugal axons

The vast majority of corticothalamic (CT) axons projecting to sensory-specific thalamic nuclei arise from layer VI cells but intralaminar and associative thalamic nuclei also receive, to various degrees, a cortical input from layer V pyramidal cells. It is also well established that all long-range corticofugal projections reaching the brainstem and spinal cord arise exclusively from layer V neurons. These observations raise the possibility that the CT input from layer V cells may be collaterals of those long-range axons projecting below thalamic level. The thalamic projections of layer V cells were mapped at a single cell level following small microiontophoretic injections of biocytin performed in the motor, somatosensory and visual cortices in rats. Camera lucida reconstruction of these CT axons revealed that they are all collaterals of long-range corticofugal axons. These collaterals do not give off axonal branches within the thalamic reticular nucleus and they arborize exclusively within intralaminar and associative thalamic nuclei where they from small clusters of varicose endings. As layer V cells are involved in motor commands everywhere in the neocortex, these CT projections and their thalamic targets should be directly involved in the central organization of motor programs.

Topography of developing thalamic and cortical pathways in the visual system of the cat

Adult patterns of connectivity could emerge during development by a process of selective elimination from an earlier, more widespread, connectivity. We have addressed this issue by examining the topography of developing projections to area 17 in the cat. At different postnatal ages, paired injections of the retrograde tracers diamidino yellow and fast blue were made in area 17. Interinjection separations were carefully controlled and the spatial distribution of the two populations of labelled neurones investigated. Projections to the striate cortex from the lateral geniculate nucleus, area 18, as well as connections intrinsic to area 17 were analysed quantitatively with a graphic method that uses a two-dimensional model of the projection. This allows two parameters of the projection to be calculated: the divergence (the spatial extent of area 17 contacted by an infinitely small region of an afferent structure) and the convergence (the extent of an afferent structure that projects to an infinitely small region of area 17). During postnatal development, the bulk of the connections making up the geniculostriate and corticocortical pathways showed no variation either in their convergence and divergence. However, the projection of area 18 to area 17 and the intrinsic area 17 connections (but not the geniculostriate projection) in the 3-15-day-old kittens were each found to contain a small subpopulation of widely scattered neurones with widespread axonal trajectories. These results, showing that many initially formed connections display a high degree of topographical order, are discussed in terms of the control mechanisms specifying axonal trajectories during development.

The role of ipsilateral and contralateral inputs from primary cortex in responses of area 21a neurons in cats

Neuronal responses in cat visual area 21a were analyzed when the primary visual cortex (areas 17 and 18) was deactivated by cooling. Ipsilateral and contralateral cortices were deactivated separately. Results established that (1) cooling the ipsilateral primary cortex diminished the activity of all area 21a cells and, in 30%, blocked responsiveness altogether, and (2) cooling the contralateral primary cortex initially increased activity in area 21a cells but, with further cooling, reduced it to below the original level although only 9% of cells ceased responding. These findings were then compared to earlier results in which bilateral deactivation of the primary cortex greatly reduced and, in most cases, blocked the activity of area 21a cells (Michalski et al., 1993). Despite the response attenuation following cooling of the primary visual cortex (either ipsilateral or contralateral), neurons of area 21a retained their original orientation specificity and sharpness of tuning (measured as the half-width at half-height of the orientation tuning curve). Direction selectivity also tended to remain unchanged. We concluded that for area 21a cells (1) the ipsilateral primary cortex provides the main excitatory input; (2) the contralateral primary cortex supplies a large inhibitory input; and (3) the nature of orientation specificity, sharpness of orientation tuning, and direction selectivity are largely unaffected by removal of the ipsilateral hemisphere excitatory input or the contralateral hemisphere inhibitory input.

Capacity of the retinogeniculate pathway to reorganize following ablation of visual cortical areas in developing and mature cats

The purpose of the present study was to determine the pattern and density of retinal projections to the dorsal lateral geniculate nucleus (dLGN) following ablation of visual cortical areas in developing cats of different postnatal ages and in mature cats. The terminations of retinal projections to the dLGN were evaluated following the injection of tritiated amino acids into one eye. Regardless of age, a visual cortical ablation of areas 17 and 18 induces massive death of neurons within the regions of the dLGN that are linked topographically to the cortical areas removed. However, the pattern of retinal projections to these degenerated regions of the dLGN differs depending upon whether the cortical lesion is incurred early in postnatal life or in adulthood. Following ablation on the day of birth (P1), virtually all surviving cells were found in the C-complex of dLGN with only a token number in the A-laminae. Correspondingly, retinal projections were maintained to the C-complex of the nucleus and were barely detectable in the degenerated A-laminae. However, in cats in which areas 17 and 18 had been removed in adulthood (> or = 6 months of age) retinal projections were maintained to the A-laminae even though nearly all neurons in those laminae had degenerated. Moreover, a subgroup of animals that incurred area 17 and 18 ablations at P1 showed that the modification of retinal projections to the A-laminae occurs within the first postnatal month, and an additional subgroup showed that retinal projections become increasingly resistant to the degenerative events in the dLGN that follow ablation of areas 17 and 18 at progressively older ages during the first postnatal month. Furthermore, retinal inputs also respond, in an age-dependent way, to degeneration of neurons in the C-complex induced by extension of the cortical ablation to include extrastriate visual areas.

Orientational influences of layer V of visual area 18 upon cells in layer V of area 17 in the cat cortex

We examined the orientation tuning curves of 86 cells located in layer V of area 17, before, during, and after focal blockade of a small (300-microns diameter) region of near-retinotopic register in layer V of area 18 of quantitatively established orientation preference. Such focal blockade revealed three distinct populations of area 17 layer V cells-cells with decreased responses to stimuli of some orientations (21%), cells with increased responses to stimuli of some orientations (43%), and cells unaffected by the focal blockade (36%). These effects were clearcut, reproducible, and generally directly related to the known receptive field properties of the cell recorded in area 18 at the center of the zone of blockade. These effects were also analyzed in terms of alterations in orientation bandwidth in the cells in area 17 as a result of the blockade-bandwidth increases (22%) and decreases (24%) were found; however, these changes were essentially unrelated to the measured receptive field properties. Inhibitory and excitatory effects were most pronounced when the regions in areas 17 and 18 were of like ocular dominance and were of similar orientation preference. Inhibitory effects (suggesting a normally excitatory input) were most dependent upon the similarity of receptive fields; excitatory effects (suggesting a normally inhibitory input) were less heavily dependent.

The incidence of bifurcation among corticocortical connections from area 17 in the developing visual cortex of the cat

In newborn kittens, cells in the striate cortex (visual area 17) that project to area 18 (part of extrastriate cortex) are distributed with uniform density in the superficial and in the deep layers. During postnatal weeks 2-3, some of these corticocortical connections are removed to generate an adult-like projection in which association cells are clustered mainly in the superficial layers of area 17. Axonal elimination, without cell death, is the major factor sculpting patches of corticocortical cells in superficial layers. In adult cats, few cells in area 17 (approximately 5%) have axons that bifurcate to multiple extrastriate areas. We have studied the possibility that the early exuberant innervation of area 18 by neurons in area 17 is largely from the transient collaterals of axons that also project to other visual areas. Kittens aged 2-21 days were each injected with a pair of retrogradely transported tracers, either diamidino yellow and fast blue, or diamidino yellow and a carbocyanine dye, at retinotopically corresponding points in area 18 and either area 19 or the posteromedial lateral suprasylvian cortex (PMLS). As for injections in area 18, those in area 19 and PMLS in kittens aged < or = 5 days labelled cells in continuous bands in area 17; in older kittens neurons projecting from area 17 to extrastriate regions were in patches, mainly in superficial layers. In each animal, the labelling from the two injections overlapped by 51-92%.(ABSTRACT TRUNCATED AT 250 WORDS)

Processing of form and motion in area 21a of cat visual cortex

Extracellular recordings from single neurons have been made from presumed area 21a of the cerebral cortex of the cat, anesthetized with N2O/O2/sodium pentobarbitone mixture. Area 21a contains mainly a representation of a central horizontal strip of contralateral visual field about 5 deg above and below the horizontal meridian. Excitatory discharge fields of area 21a neurons were substantially (or slightly but significantly) larger than those of neurons at corresponding eccentricities in areas 17, 19, or 18, respectively. About 95% of area 21a neurons could be activated through either eye and the input from the ipsilateral eye was commonly dominant. Over 90% and less than 10% of neurons had, respectively, C-type and S-type receptive-field organization. Virtually all neurons were orientation-selective and the mean width at half-height of the orientation tuning curves at 52.9 deg was not significantly different from that of neurons in areas 17 and 18. About 30% of area 21a neurons had preferred orientations within 15 deg of the vertical. The mean direction-selectivity index (32.8%) of area 21a neurons was substantially lower than the indices for neurons in areas 17 or 18. Only a few neurons exhibited moderately strong end-zone inhibition. Area 21a neurons responded poorly to fast-moving stimuli and the mean preferred velocity at about 12.5 deg/s was not significantly different from that for area 17 neurons. Selective pressure block of Y fibers in contralateral optic nerve resulted in a small but significant reduction in the preferred velocities of neurons activated via the Y-blocked eye. By contrast, removal of the Y input did not produce significant changes in the spatial organization of receptive fields (S or C type), the size of the discharge fields, the width of orientation tuning curves, or direction-selectivity indices. Our results are consistent with the idea that area 21a receives its principal excitatory input from area 17 and is involved mainly in form rather than motion analysis.

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat

Recordings were made from single neurons in area 18 of anesthetized cats (N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate) in which one optic nerve had been pressure blocked to selectively block conduction in the largest (Y-type) fibers. Cortical neurons were stimulated visually via the normal eye or via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields such as their spatial organization (S or C cells), orientation tuning, and the presence and strength of end-zone inhibition appear to be unaffected by removal of the Y input. By contrast, the removal of the Y input resulted in a small but significant reduction in the size of the discharge field and in the direction-selectivity index. In three respects, peak response discharge rate, eye dominance, and velocity sensitivity, removal of the Y input had strong and highly significant effects. Thus, the mean peak discharge frequency of responses evoked by the stimulation of binocular neurons via the Y-blocked eye was significantly lower than that of responses evoked by the stimulation via the normal eye. Accordingly, the eye-dominance histogram was shifted markedly towards the normal eye (more so than in the homologous experiment conducted on area 17-Burke et al., 1992). Finally, the mean preferred velocity of responses of cells activated via the normal eye was in the vicinity of 145 deg/s, whereas for cells activated via the Y-blocked eye the value was about 35 deg/s. Overall, the results of the present study imply that (1) apart from Y-type excitatory input there are significant excitatory non-Y-inputs to area 18; these inputs at least partially consist of indirect X-type input relayed via area 17; (2) in neurons of area 18 that receive both Y-type and non-Y-type excitatory inputs, the Y-type input has a major influence on strength of the response and velocity sensitivity and a lesser influence on the direction selectivity and size of the discharge fields; and (3) area 18 contains mechanisms determining such receptive-field properties as S- or C-type organization, orientation tuning, and direction selectivity which can be accessed either by the Y input or by non-Y input.

Spatial and temporal coherence in cortico-cortical connections: a cross-correlation study in areas 17 and 18 in the cat

Visual cortical areas are richly but selectively connected by "patchy" projections. We characterized these connections physiologically with cross-correlograms (CCHs), calculated for neuron pairs or small groups located one each in visual areas 17 and 18 of the cat. The CCHs were then compared to the visuotopic and orientation match of the neurons' receptive fields (RFs). For both spontaneous and visually driven activity, most non-flat correlograms were centered; i.e. the most likely temporal relationship between spikes in the two areas is a synchronous one. Although spikes are most likely to occur simultaneously, area 17 spikes may occur before area 18 or vice versa, giving the cross-correlogram peak a finite width (temporal dispersion). Cross-correlograms fell into one of three groups according to their full-width at half peak height: 1-8 ms (modal width, 3 ms), 15-65 ms (modal width 30 ms), or 100-1000 ms (modal width 400 ms). These classificatory groups are nonoverlapping; the three types of coupling appeared singly and in combination. Neurons whose receptive fields (RFs) are nonoverlapping or cross-oriented may yet be coupled, but the coupling is more likely to be the broadest type of coupling than the medium-dispersed type. The sharpest type of coupling is found exclusively between neurons with at least partially overlapping RFs and mostly between neurons whose stimulus orientation preferences matched to within 22.5 deg. The maximum spatial dispersion observed in the RFs of coupled neurons compares well with the maximum divergence seen anatomically in the A18/A17 projection system. We suggest three different mechanisms to produce each of the three different degrees of observed spatial and temporal coherence. All mechanisms use common input of cortical origin. For medium and broad coupling, this common input arises from cell assemblies split between both sides of the 17/18 projection system, but acting synchronously. Such distributed common-input cell assemblies are a means of overcoming sparse connectivity and achieving synaptic transmission in the pyramidal network.

Visuotopic organization of corticocortical connections in the visual system of the cat

It has recently been demonstrated that, in contrast with the retinogeniculocortical projection, the corticocortical connections in the cat present a high degree of convergence and divergence. This suggests that some corticocortical connections link nonvisuotopically corresponding regions. Using fine-grain electrophysiological mapping and anatomical tracing, we have set out to test this possibility by placing a small injection of retrograde tracer in area 17 and by comparing the extent of visual field encoded in the region of area 18 containing labeled cells and that represented in the uptake zone. The results demonstrate that the size of the labeled region on the surface of area 18 is independent of eccentricity and that, despite its anisotrophy, this region of labeling encodes a broadly circular region of visual field that is larger than that encoded in the uptake zone of the tracer in area 17. For example, in the representation of lower visual field, a virtual point in area 17 that encodes a visual field region 4 degrees in diameter receives afferents from a region of area 18 encoding a region 11 degrees wide. Examination of the density of labeled cells in the labeled zone in area 18 reveals that the highest density is observed in a region in visuotopic correspondence with the injection site. However, high labeling density is also occasionally found in patches that do not represent the same visual field region as the injection site. Many receptive fields of neurons recorded in the labeled zone in area 18 only partially overlap or fail to overlap the visual field region encoded by the injection site. The results also demonstrate that the extent of visual field encoded in the labeled zone in area 18 is the same as that represented in the region of intrinsic labeling in area 17. It is suggested that cortical afferents coming from several cortical areas and converging on a column of cells in area 17 cover the same extent of visual field and that this cortical network constitutes the structural basis for the modulatory regions of the receptive field as well as the synchronization of neurons in different cortical areas.

Organization of association projections from area 17 to areas 18 and 19 and to suprasylvian areas in the cat's visual cortex

Cells in area 17 that are labelled by single, discrete injections of retrogradely transported tracers into extrastriate visual areas are discontinuously distributed in dense patches. In this study we made multiple, closely spaced injections of fluorescent dyes into extrastriate areas, to generate large deposits that would reveal whether the distributions of corticocortical cell bodies in area 17 are truly patchy or appear clustered only after small injections. By injecting a different tracer into each extrastriate area, or group of areas, we examined the spatial relationships between the populations of association cells. All deposits of tracers in areas 18, 19, or suprasylvian cortex, irrespective of size, label cells in a series of clusters in topographically related parts of area 17. We conclude that the complete populations of cells in area 17 that project to areas 18, 19, and the lateral suprasylvian cortex are all genuinely distributed in a patchy fashion. There appears to be a complex relationship between the sets of association cells projecting to different extrastriate regions: they do not completely overlap, only partially, and share some cortical zones but not others. In these experiments, only tiny percentages (2-5%) of labelled cells in the overlapping regions were filled with both tracers, suggesting that very few association cells in area 17 project to more than one of the extrastriate areas we studied. By comparing the dimensions of each injection site and of the labelled region in area 17, we estimated the extent of the convergence from area 17 to areas 18, 19, and posteromedial suprasylvian areas in retinotopic terms. The functional convergence was very similar in these pathways.

The local domain for divergence of subcortical afferents to the striate and extrastriate visual cortex in the common marmoset (Callithrix jacchus): a multiple labelling study

In the common marmoset (Callithrix jacchus), the cortical projection from the pulvinar and other diencephalic structures into the striate and prestriate cortex was investigated with various fluorescent retrograde tracers. Single cortical injections as well as multiple injections at distances of 1-2 mm with one tracer into an extended but coherent cortical region were applied. Fields with multiple injections were placed so that they touched each other (minimal distances 2 to 3 mm). Retrogradely labelled cells in the LGN and/or the pulvinar were arranged in coherent columns, volumes or slabs, but cell volumes resulting from neighbouring cortical injections overlapped at their border (for details of the thalamo-cortical topography see the companion paper Dick et al. (1991]. Double labelled cells (dl) were only found in the zones of overlap of the cell volumes labelled by the respective tracers. The relative number of dl-cells in these overlap zones was 6.2 +/- 3.1%. The dl-frequency was the same in the various nuclei of the pulvinar and the LGN. In the main layers of LGN, dl-cells were found only in the overlap zone of two injection fields into area 17, but a few dl-cells were found in interlaminar cells after injections into area 17 and 18. Maximal cortical distances between injection fields which produced dl in the pulvinar, were 3 to exceptionally 4 mm but dl was highest at injection distances less than or equal to 2.5 mm and decreased sharply at wider distances. Such overlap zones were concerned with identical or overlapping regions of visual field representation in the cortex and probably also in the pulvinar. Although in individual experiments up to four different tracers were injected into different striate/prestriate regions, often embracing the same visual field representation, individual cells in the pulvinar showed dl from maximally only two tracers injected into neighbouring cortical regions. We conclude that dl in the posterior thalamic projection nuclei is determined essentially by cortical distance and thus reflects the local domain of branching of thalamo-cortical afferents. Pruning of such branches during development may further restrict bifurcating axons to identical visual field representations, but representation of identical visual field regions in different visual areas is not, per se, a sufficient condition for dl. It is not found if such regions are further apart from each other than the typical local domain of 2-3 mm, exceptionally up to 4 mm in one experiment after injections into area 17 and MT.(ABSTRACT TRUNCATED AT 400 WORDS)

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.