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PubMed1985

Neurogenesis of the cat's primary visual cortex.

The 3H-thymidine method of birth-dating was used to determine when the cells belonging to each of the principal cellular layers of the cat's primary visual cortex are generated. In order to detect systematic differences in the position of radioactively labeled cells following 3H-thymidine administration at different prenatal ages, a geometric method was devised to represent the distribution of labeled cells in the form of depth histograms. Results show that visual cortical neurogenesis occurs largely during the second half of gestation between embryonic day 31 (E31) and E57. Cells of layer 6 are generated early, between E31 and E38, whereas cells destined for successively more superficial layers are generated at progressively later times. Layer 4 cells, the principal targets of geniculocortical afferents, are generated between E37 and E44. In addition, a special population of cells embedded in the white matter below layer 6 was found to be produced throughout the week-long period immediately prior to the onset of layer 6 neurogenesis. Overall, this radial pattern of cortical neurogenesis closely resembles the inside-first, outside-last, spatiotemporal sequence of development described for the monkey's primary visual cortex (Rakic, '74). In addition to finding this pronounced gradient in the radial dimension, we were also able to detect a less pronounced gradient along the tangential dimension: neurons destined for any given layer in the anterior part of the cortex (inferior visual field representation) are generated slightly in advance of neurons destined for more posterior regions (superior visual field). However even our more quantitative histogram analysis failed to reveal a mediolateral (central to peripheral visual field) gradient within area 17. In the cat, layers 6, 5, and 4 each take about a week to be generated, although their total cell numbers and packing densities differ in the adult. About 2 weeks are required to produce the cells of layers 2 and 3 combined. Furthermore, we found that neurons belonging to different layers and different morphological classes can be generated simultaneously. This suggests that the identity of a cortical neuron is not solely a function of the time of neurogenesis.

Age Factors↗
PubMed1985

Small lesions in the primary visual cortex of rats cause a specific reorganization of associational connections.

Neuroplastic changes in associational connections were investigated 3 weeks after the intrinsic organization of the visual cortex of rats had been partially damaged by small cylindrical lesions (type I). These lesions caused the degeneration of short intracortical connections and associational connections that form patches in the primary and secondary visual areas. The resulting terminal degeneration disappeared within 20 days p.o. after which only some fiber degeneration was evident in the infragranular layers. Patches of terminal degeneration reappeared in the vicinity of the stab wounds, when the associational connections between the retrosplenial and the primary visual cortex had been secondarily interrupted by elongated lesions (type II), which penetrated the paramedian cortex and subcortical white matter. When type-II lesions were made in the intact cortex, patches of degeneration were absent, although in both cases some terminal degeneration was diffusely distributed in the primary visual cortex. Horseradish peroxidase (HRP) was applied to sites similar to those where type-I lesions were applied. In the intact cortex, HRP caused a granular labeling of numerous neurons in various positions including the retrosplenial cortex and patches of the postero-median visual cortex. HRP was also applied to type-I lesions that had been made 3 weeks earlier. In these cases, apparently HRP labeled the same subpopulations of neurons as it did in the intact cortex. However, a fraction of the labeled neurons showed a Golgi-like staining (e.g., 27% of the labeled neurons in the retrosplenial cortex) only when HRP was applied to stab wounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗
PubMed1979

Demonstration of a direct projection from the intralaminar central lateral nucleus to the primary visual cortex.

Orthograde autoradiographic and retrograde horseradish peroxidase (HRP) tracing techniques were used to demonstrate the existence of a direct projection from the central lateral nucleus of the intralaminar complex of the thalamus to the primary visual cortex of the cat. The projection is sparser than the projections from the thalamic non-specific nuclei to other cortical areas of the cat which have been described in the past [9,20,21]. The projection to primary visual cortex is most dense in cortical layers I and VI. It does not appear to have a well-defined topographical organization within this cortical area.

Afferent Pathways↗
PubMed1989

Intrinsic connections of rat primary visual cortex: laminar organization of axonal projections.

The organization of local projections within the rat primary visual cortex (area 17) was investigated by tracing fibers with HRP in in vitro brain slices. The projections from different layers showed distinct laminar patterns. Layer 4 made a strong, topographically precise, projection to lower layer 2/3; weaker projections extended laterally and terminated diffusely in layer 2/3 but also ran vertically to layers 5 and 6. The connections of lower and upper layer 2/3 were reciprocal and point-to-point. Within layer 2/3, a large number of fibers ran horizontally and terminated at variable distances from the injection site without making terminal clusters. The main output from layer 2/3 was to layer 5. The most prominent projections from the upper half of layer 5 were to layers 2/3 and 6; lower layer 5, in contrast, made wide-ranging, clustered projections to layer 1, the bottom of layer 2/3, and the top of layers 4 and 5. The patches were 130-160 micron wide and spaced apart by 230-260 micron. The main projection that arose from the superficial layer 6 terminated in layer 4 above the injection site. In contrast, lower layer 6 made clustered projections to the layer 3/4 border, extending up to 2 mm in the coronal plane. The patches were 190-220 micron wide and spaced apart by 320-390 micron. Additional projections went to the layer 5/6 border and layers 1 and 2. These results indicate that geniculocortical input is processed through interlaminar connections that are topographically precise, widespread, or patchy. These connectivity patterns suggest a role for these connections in the transformation of functional maps between layers; focused projections preserve the architecture of the layers of origin, and diverging or patchy projections rearrange this organization and form new maps in the target layers (Lund: Annu. Rev. Neurosci. 11:253-288, '88). However, only a few interlaminar connections show one of these patterns in isolation, making it difficult to assign a single function to a particular connection. We, therefore, tentatively conclude that projections terminating in layers 1-4, with the possible exception of the connection between upper layer 6 and layer 4, transform functional maps. In contrast, the topographically precise projections from upper to lower layers preserve functional maps. The specific role of these connections in the construction of receptive field properties, however, is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Laminar and synaptic organization of the projection from the thalamic nucleus centralis to primary visual cortex in the cat.

The projection from the nucleus centralis (an intralaminar thalamic nucleus) to the primary visual cortex was examined with anterograde and retrograde tracing techniques. After large injections of horseradish peroxidase into areas 17 and 18 almost one-half of the neurons in the nucleus centralis were retrogradely labeled. An injection of 3H-proline into the nucleus centralis led to sparse anterograde labeling in layers 5 and 6 of areas 17 and 18. Large injections of peroxidase-conjugated wheat germ agglutinin (WGA) into the nucleus centralis led to similar anterograde labeling of layers 5 and 6 and, in addition, to a band in layer 1. No retrogradely labeled cells were seen in areas 17 or 18. The WGA-labeled terminals in area 17 were examined in the electron microscope: they formed type 1 (asymmetric) synapses on dendritic spines. These observations suggest that the afferents from the nucleus centralis primarily contact pyramidal cells that project to subcortical targets. The findings are consistent with physiological studies suggesting that the nucleus centralis is involved in the modulation of cortical outflow with varying levels of arousal.

Afferent Pathways↗
PubMed1992

A computational model of vertical signal propagation in the primary visual cortex.

A computational model of the flow of activity in a vertically organized slab of cat primary visual cortex (area 17) has been developed. The membrane potential of each cell in the model, as a function of time, is given by the solution of a system of first order, coupled, non-linear differential equations. When firing threshold is exceeded, an action potential waveform is "pasted" in. The behavior of the model following a brief simulated stimulus to afferents from the dorsal lateral geniculate nucleus (dLGN) is explored. Excitatory and inhibitory post-synaptic potential (E and IPSP) latencies, as a function of cortical depth, were generated by the model. These data were compared with the experimental literature. In general, good agreement was found for EPSPs. Many disynaptic inhibitory inputs were found to be "masked" by the firing of action potentials in the model. To our knowledge this phenomenon has not been reported in the experimental literature. The model demonstrates that whether a cell exhibits disynaptic or polysynaptic PSP latencies is not a fixed consequence of anatomical connectivity, but rather, can be influenced by connection strengths, and may be influenced by the ongoing pattern of activity in the cortex.

Action Potentials↗
PubMed1979

Development of orientation selectivity in the primary visual cortex of normally and dark reared kittens. II. Models.

The development of orientation selectivity in the primary visual cortex is described by first-order kinetics between three functional compartments chained in a catenary mode. A first model is presented, in which two unidirectional kinetics with constant exchange coefficients, symmetrical in their effects, function in an alternating mode depending on the presence or absence of visual experience. The failure of this model to simulate the modifications induced by a delayed visual experience, when the exchange coefficients are identified to fit normal and dark rearing, supports the hypothesis that the maturation process consequent to interaction with visual environment is dependent on the date at which it is allowed to take place. A second model is then proposed, in which exchange coefficients are piecewise linear functions of time. In order to correctly predict the functional effects of restricted visual experience following prior dark rearing, it is assumed that visuomotor experience during the critical period permits the expression of a non-linear "modifiability gradient" which may have been masked up to this point by the absence of vision or eye movements.

Animals↗
PubMed1986

The relationship between the geniculocortical afferents and their cortical target cells during development of the cat's primary visual cortex.

To study the prenatal development of connections between the lateral geniculate nucleus (LGN) and the primary visual cortex in the cat, we have examined the relationship between the position of ingrowing afferents from the LGN and their target cells in cortical layers 4 and 6 at various times during the cat's 65 d gestation period and during the first 3 weeks of postnatal life. In 1 series of experiments, the method of transneuronal transport of intraocularly injected tritiated proline (3H-proline), followed by autoradiography, was used to label the developing geniculocortical pathway. In another series, the tritiated thymidine (3H-thymidine) method was employed to keep track of the cells destined for layers 4 and 6 by labeling them on their birthdates (layer 4: embryonic day (E) 37-43; layer 6: E31-36) (Luskin and Shatz, 1985b) and then charting their locations at subsequent times during development. The results of the 2 sets of experiments were compared at corresponding ages. By E39, many of the cells of cortical layer 6 have completed their migrations and are situated within the cortical plate immediately above the subplate. However, the transneuronal labeling pattern indicates that the geniculocotical afferents have not yet arrived within the vicinity of the future visual cortex, but rather are still en route and confined within the optic radiations of the telencephalon. By E42, a week after the first afferents can be detected in the radiations, substantial transneuronal label is found in the subplate immediately below future visual cortex. However, the overlying cortical plate is free of label. Over the next 2 weeks, geniculocortical axons continue to accumulate in the subplate zone, and, in addition, transneuronal label can be found in the marginal zone. By E55 a faint geniculocortical projection can be detected within the cortical plate, but only within its deeper half (future layers 5 and 6), and even then the major portion of the projection is still confined to the subplate. The absence of a projection to cortical layer 4 at these ages is remarkable in view of the results from our 3H-thymidine experiments, which indicate that by E57 the majority of cells destined to belong to layer 4 have already completed their migrations and assumed positions superficial to the cells of layers 5 and 6. By birth, a substantial geniculocortical projection to cortical layer 4 can be detected in the transneuronal autoradiographs.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1992

Cholinergic fiber loss occurs in the absence of synaptophysin depletion in Alzheimer's disease primary visual cortex.

The significance of cholinergic degeneration in Alzheimer's disease (AD) depends, in part, on whether it is an early event, possibly integral to the progression of the disease, or a late event, occurring only as a secondary effect of cortical degeneration. We have been studying the primary visual cortex in AD cases, on the assumption that the disease process may be retarded in this relatively-spared area, thus providing a 'window' on early AD. In this work, we have quantified acetylcholinesterase fiber density and the density of an immunohistochemical reaction for synaptophysin as measures of cholinergic and total synaptic loss, respectively, in the primary visual cortex of AD and control cases. Cholinergic fibers were depleted to 15% of control values, while synaptophysin density was not significantly altered. Cholinergic degeneration thus appears to occur in the absence of generalized synaptic loss in this area.

Acetylcholinesterase↗
PubMed1991

A computational model of the vertical anatomical organization of primary visual cortex.

A method for modeling anatomical connectivity for a vertically organized slab of cortical tissue in mammalian primary visual cortex has been developed. The modeled slab covers 500 x 500 microns of cortical surface and extends vertically throughout the full depth of the cortex. The model slab was divided into 6 laminae and neuronal somata were distributed in three dimensions through the slab in accordance with experimentally derived cell densities. Axonal and dendritic arborizations were modeled as line segments. A total of 17 morphological types of neurons were included. Connectivity was established based on proximity between axonal and dendritic arbors. There is good general agreement between the vertical distribution of connections generated by the model and the vertical distribution of synapses observed for cat area 17. In all layers, fewer connections were generated in the model than synapses in cat area 17. This is due, at least in part, to the exclusion of long range intracortical projections and sources of afferent input other than the dorsal lateral geniculate nucleus from the model. The connection scheme described here will be used in conjunction with a physiology model to model vertical signal flow, and will be expanded further to model receptive fields of cortical neurons.

Afferent Pathways↗
PubMed1975

A study of geniculate unit activity during cryogenic blockade of the primary visual cortex in the cat.

In cats under nembutal or chloralose anesthesia, unilateral temporary cryogenic blockades of the primary visual cortex were performed and their influence on unitary responses to restricted light spots was tested in the ipsilateral geniculate nucleus. No significant effect could thus be observed, at variance with previous positive results obtained by our group in structures like superior colliculus and pulvinar complex using the same experimental procedure.

Animals↗
PubMed1988

The primary visual cortex in the mouse: receptive field properties and functional organization.

Receptive field (RF) characteristics of cells in primary visual cortex of the mouse (C57B16 strain) were studied by single unit recording. We have studied the functional organization of area 17 along both the radial and tangential dimensions of the cortex. Eighty seven percent of the visual neurons could be classified according to their responses to oriented stimuli and to moving stimuli. Cells which preferred a flashed or moving bar of a particular orientation and responded less well to bars of other orientations or to spots, were classified as orientation selective (simple RF 23%, complex RF 18%). The majority of them were, moreover, unidirectional (24%). All orientations were roughly equally represented. Cells with oriented RFs were recorded mostly in the upper part of cortical layers II-III, where they appeared to be clustered according to their preferred orientation. Neurons that responded equally well to spots and bars of all orientations (46%) were classified as "non-oriented"; among these neurons there were several subcategories. Cells which responded equally well to spots and bars but preferred stimuli moving along one or both directions of a particular axis were classified as non oriented asymmetric cells (unidirectional 14%, bidirectional 4%). They were recorded mainly in supra- and infra-granular layers. Cells unaffected by stimulus shape and orientation which responded equally well to all directions of movement were classified as symmetric units. They had receptive field classified as ON (11%), OFF (1%), ON/OFF (11%), or were unresponsive to stationary stimuli (5%). These cells were mostly found in layer IV, in which they constituted the majority of recorded cells. There was no apparent correlation between the functional type and size of RFs. However, the greatest proportion of small RFs was found in layer IV. In the binocular segment of the mouse striate cortex, the influence of the contralateral eye predominated. Ninety five percent of cells in this segment were driven through the contralateral eye. However, 70% of cells were binocularly activated, showing that considerable binocular integration occurred in this cortical segment. Ocular dominance varied less along the radial than along the tangential dimension of the cortex.

Animals↗
PubMed1976

Influence of association cortex on penicillin discharges in the primary visual cortex.

The influence of visual association cortex on the pattern of neuronal activity in the lateral geniculate body, during penicillin-induced discharges in visula cortex, was studied. In a majority of those geniculate cells which exhibited a pre-penicillin discharge increment in activity, that increment was abolished or significantly reduced during focal hypothermia of the visual association cortex. The frequency of penicillin discharges in visual cortex was, in most cases, reduced during cooling of association cortex. These findings are briefly discussed in terms of corticothalamic interactions.

Animals↗
PubMed1989

Stimulation of brainstem nuclei in the cat: effect on neuronal activity in the primary visual cortex of relevance to cerebral blood flow and migraine.

The electrical activity of single neurons was recorded in the primary visual cortex of anesthetized cats in order to test the hypothesis that changes in their activity might be the cause of the changes in blood flow seen after stimulation of the brainstem and in migraine. Discharge evoked by optic radiation stimulation was facilitated by conditioning stimulation (6 pulses over 24 msec) of the locus coeruleus (LC), whereas no consistent effect was exerted on resting discharge. No clear difference could be demonstrated between the effect on resting discharge of short-duration stimulation of the LC, midbrain reticular formation and dorsal raphe nucleus (DRN). Stimulation of these structures at 5/sec for 10 sec was also studied to ascertain whether long-term changes in neuronal activity could be produced, and to indicate whether changes in cerebral blood flow previously described with these parameters of stimulation were secondary to alterations in metabolic demand due to increased or decreased bulk neuronal activity. The vast majority of single neurons were unaltered by these parameters of stimulation, suggesting that cortical blood flow changes following LC and DRN stimulation are due to a direct action on the cortical microcirculation.

Animals↗
PubMed1984

Primary visual cortex in the brushtailed possum: receptive field properties and corticocortical connections.

The corticocortical connections and receptive field properties of primary or striate visual cortex of the brushtailed possum, Trichosurus vulpecula, have been examined. In this Australian marsupial species primary visual cortex has connections with four other visual areas in the occipital lobe. In these adjacent visual areas fibers from striate cortex terminate mainly in layers 3 and 4 and in some cases also in layers 1 and 2. In all four areas return connections to striate cortex originate predominantly in layers 2 and 3, and to a much lesser extent in layers 5 and 6. Interhemispheric connections of striate cortex are limited to the boundary of striate and peristriate cortex. In addition to its cortical connections, striate cortex makes reciprocal connections with the claustrum. Most neurons in striate cortex are highly binocular. Of our sample of 113 visually responsive neurons, only 30% were orientation selective. On the basis of these observations we have compared striate cortex of the marsupial brushtailed possum with striate cortices of the American marsupial opossum and those of placental mammals.

Animals↗
PubMed1990

The laminar distribution and postnatal development of serotonin-immunoreactive axons in the cat primary visual cortex.

The laminar distribution and postnatal development of profiles immunoreactive to antibodies directed against serotonin (5-hydroxytryptamine, 5-HT) have been investigated in the primary visual cortex (striate cortex, area 17) of cats. In the adult cat, profiles with serotonin-like immunoreactivity consist exclusively of fibers which exhibit laminar differences in density and predominant orientation. Immunoreactive fibers are dense in layers I-III, less dense in layer V, and sparse in layers IV and VI. In layers I and VI the trajectories of these fibers are mainly tangential to the pial surface; in layers II-V they are predominantly radial and more irregular. The vast majority of immunoreactive fibers consists of fine axons with frequent small varicosities. In addition, there are a few thick axons. In 2-week-old cats, immunoreactive fibers are sparsely distributed through layers II-V. By 4 weeks, fiber density has decreased still further in layer IV and increased in layers I-III. By 6 weeks, the laminar pattern resembles that of adult cats except that fiber density is still lower than in adults. At three months of age, the mature pattern is established.

Aging↗
PubMed1986

The size of the zone of origin of callosal afferents projecting to the primary visual cortex contralateral to the remaining eye in rats monocularly enucleated at different postnatal ages.

The cortical zone from which callosal afferents projecting to the primary visual cortex (area Oc1) originate was studied in monocularly enucleated and normal rats. The extent of this cortical strip was determined by retrograde labeling with HRP and by measurement of its width in coronal sections. Albino rats were monocularly enucleated from the 23rd ontogenetic day to the 120th and iontophoretical injections into Oc1 contralateral to the remaining eye were done more than one year after enucleation. The width of the labeled strip of perikarya in the hemisphere ipsilateral to the remaining eye was largest in neonatally enucleated rats (about 1.1 mm) and declined with increasing age at which enucleation was performed. Additionally, the perikarya of callosal afferents in the hemisphere ipsilateral to the remaining eye in rats enucleated as young adults (90th and 120th ontogenetic day) were labeled in significantly wider strips (about 0.6 mm) than in unoperated control rats (about 0.4 mm).

Afferent Pathways↗
PubMed1990

Components of field potentials evoked by white matter stimulation in isolated slices of primary visual cortex: spatial distributions and synaptic order.

1. We have recorded profiles of the spatial distributions of extracellular field potentials in transverse slices of rat primary visual cortex. Responses were evoked by electrical stimulation near the white matter/layer VI border and sampled from layers I to V along the radial axis orthogonal to the laminae and intersecting the stimulation site ("on-beam" recording). To assess the activity of "horizontal" connections, we also recorded profiles along axes parallel to the cortical lamination ("off-beam" recording), usually in layer III. Overall, our goal was to extend understanding of the physiology and organization of neocortical circuitry and to provide a basis for comparisons of data from different experiments and experimenters when neocortical field potentials are used in studies of plasticity and pharmacology. 2. Responses were highly specific with respect to the cortical layers. We distinguish four major components: two kinds of population spike ("S1" and "S2") and two slower waveforms ("W1" and "W2"). The latter appear to represent flow of current in apical dendrites of the supragranular layers. Component W1, the earliest slow component, is a synaptically driven field potential dipole that is positive in layer I and negative in layer II. Based on estimates of current source densities (CSDs), we attribute this to entry of depolarizing current into dendrites and/or cell somata in layer II, ascending intradendritic current, and passive depolarization of inactive dendritic membrane in layer I. Component W1 rises during the 20 ms after stimulation and falls during the 50-100 ms thereafter. Component W2 is also positive in layer I but maximally negative in layer III. It rises for approximately 100 ms after stimulation and decays during the following 400-800 ms. 3. Component S1 does not depend on synaptic transmission because it persists during the application of glutamate receptor antagonists or medium that is low in Ca2+. This component is largest in layer III, radial to the site of stimulation. There, it is a negative deflection, typically 1-2 mV in amplitude and lasting roughly 2 ms, with a latency to peak between 2 and 4.5 ms. Component S1 is most likely a population spike due to synchronized firing of cell somata activated antidromically via unmyelinated efferent axons. 4. Component S2 is a short (less than 20 ms) burst of population spikes specifically in layer III. Individual S2 spikes closely resemble S1 spikes, and we propose that the same neuronal population generates both. However, S2 spikes require glutamatergic synaptic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

6-Cyano-7-nitroquinoxaline-2,3-dione↗