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PubMedDate not supplied

[Activity of callosal neurons of the visual cortex in the cat].

Activity of 28 identified neurones of the visual cortex was recorded in cats immobilized by d-tubocurarine. Stimulation of the callosal body with a single stimulus or high-frequency train elicited a short-latency antidromic reaction of neurones in the visual cortex whose axons constitute the main part of the large cerebral commissure. Some commissural neurones responded to a single callosal stimulation by two action potentials the first one being antidromic, the second one being of long-latency postsynaptic origin. The second action potential was generated as a result of activation of axonal collaterals of the same neurone or the neighboring callosal neurones. More than a half of callosal neurones responded to a single stimulation of the lateral geniculate body by short-latency antidromic discharges and by long-latency postsynaptic reactions. These data indicate the existence of the systems of two-way neuronal connections, i.e. calloso-geniculate and geniculo-callosal ones.

Animals↗
PubMed2001

Rules of connectivity between geniculate cells and simple cells in cat primary visual cortex.

Hundreds of thalamic axons ramify within a column of cat visual cortex; yet each layer 4 neuron receives input from only a fraction of them. We have examined the specificity of these connections by recording simultaneously from layer 4 simple cells and cells in the lateral geniculate nucleus with spatially overlapping receptive fields (n = 221 cell pairs). Because of the precise retinotopic organization of visual cortex, the geniculate axons and simple-cell dendrites of these cell pairs should have overlapped within layer 4. Nevertheless, monosynaptic connections were identified in only 33% of all cases, as estimated by cross-correlation analysis. The visual responses of monosynaptically connected geniculate cells and simple cells were closely related. The probability of connection was greatest when a geniculate center overlapped a strong simple-cell subregion of the same sign (ON or OFF) near the center of the subregion. This probability was further increased when the time courses of the visual responses were similar. In addition, the connections were strongest when the simple-cell subregion and the geniculate center were matched in position, sign, and size. The rules of connectivity between geniculate afferents and simple cells resemble those found for retinal afferents to geniculate cells. The connections along the retinogeniculocortical pathway, therefore, show a precision that goes beyond simple retinotopy to include many other response properties, such as receptive-field sign, timing, subregion strength, and size. This specificity in wiring emphasizes the need for developmental mechanisms (presumably correlation-based) that can select among afferents that differ only slightly in their response properties.

Action Potentials↗
PubMed1989

The Role of Horizontal Connections in Generating Long Receptive Fields in the Cat Visual Cortex.

The cells in the primary visual cortex possess numerous functional properties that are more complex and varied than those seen in the cortical input. These properties result from the network of intrinsic cortical connections running across the cortical layers and between cortical columns. In the current study we relate the long receptive fields that are characteristic of layer 6 cells to the input that these cells receive from layer 5. The axons of layer 5 pyramidal cells project over long distances within layer 6, enabling layer 6 cells to collect input from regions of cortex representing large parts of the visual field. When layer 5 was locally inactivated by injection of the inhibitory transmitter GABA, layer 6 cells lost sensitivity over the portion of their receptive fields corresponding to the inactivated region of layer 5. This suggests that the extensive convergence in the projection from layer 5 to layer 6 is responsible for generating the long receptive fields characteristic of the layer 6 cells.

Journal Article↗
PubMed2005

Modulatory effects of low- and high-frequency repetitive transcranial magnetic stimulation on visual cortex of healthy subjects undergoing light deprivation.

The aim of the present study was to explore further the effects of light deprivation (LD) on visual cortex excitability. Healthy subjects reporting reliable induction of phosphenes by occipital transcranial magnetic stimulation (TMS) underwent 60 min of complete LD. Phosphene threshold (PT) was measured before (T0), after 45 min (T1) and 60 min (T2) of LD, and then every 10 min after light re-exposure until recovery to T0 values. Repetitive TMS (rTMS) (at 1 or 10 Hz) was applied in separate sessions during the last 15 min of LD. PTs significantly decreased after 45 min of LD. rTMS differentially modified the effects of 60 min LD on PTs depending on stimulation frequency. One hertz rTMS did not change the decreasing of PT values as observed in baseline condition, but significantly prolonged the time to recover T0 PT values after light re-exposure. By contrast, 10 Hz rTMS significantly increased PT and the time to recover T0 PT values after light re-exposure was shortened. The results of this study show that the modulatory effects of different rTMS frequencies on visual cortex critically depend on the pre-existing excitability state of inhibitory and facilitatory circuits, and provide novel insights into the neurophysiological changes that take place in the visual cortex following functional visual deafferentation.

Adult↗
PubMed1986

Cholinergic neurons and fibres in the rat visual cortex.

Choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was localized immunocytochemically in neurons and fibres in the rat visual cortex using a monoclonal antibody. ChAT-labelled cells were non-pyramidal neurons, primarily of the bipolar form, distributed in layers II through VI but concentrated in layers II & III. Their perikarya contained a large nucleus and a small amount of perinuclear cytoplasm. The somata and dendrites of all labelled cells received Gray's type I and type II synapses. ChAT-stained axons formed a dense and diffuse network throughout the visual cortex and particularly in layer V. Electron microscopy revealed that the great majority formed type II synaptic contacts with dendrites of various sizes, unlabelled non-pyramidal somata and, on a few occasions, with ChAT-labelled cells. However, a very small number of terminals appeared to form type I synaptic contacts. This study describes the morphological organization of the cholinergic system in the visual cortex, the function of which has been under extensive investigation.

Animals↗
PubMed1997

Development of orientation preference maps in area 18 of kitten visual cortex.

We investigated the development of orientation preference maps in the visual cortex of kittens by repeated optical imaging from the same animal. Orientation maps became detectable for the first time around postnatal day (P) 17 and improved continuously in strength unitl P30, the time at which their appearance became adultlike. During this developmental period the overall geometry of the maps remained unchanged, suggesting that the layout of the orientation map is specified prior to P17. Hence, before the visual cortex becomes susceptible to experience-dependent modifications its functional architecture is largely specified. This suggests that the initial development and layout of orientation preference maps are determined by intrinsic processes that are independent of visual experience. This conclusion is further supported by the result that orientation maps were well expressed at P24 in binocularly deprived kittens. Because the appearance of the first orientation-selective neurons and the subsequent development of orientation preference maps correlated well with the time course of the expression and refinement of clustered horizontal connections, we propose that these connections might contribute to the specification of orientation preference maps.

Animals↗
PubMed1995

Dark-rearing fails to affect the basal dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex.

The development of the cat's visual cortex is incomplete at birth and is influenced by the cat's early visual experience. We have previously demonstrated that the basal dendritic fields of layer 3 pyramidal cells grow substantially during the first 5 weeks after birth and that stripe-rearing affects their orientation. In this paper we determined the effects on these dendritic fields of visual deprivation (dark-rearing) during the first 3 months of life. The visual cortices of both normally reared and dark-reared cats were impregnated by the Golgi method, sectioned in the tangential plane and counterstained. The basal dendritic fields of completely impregnated pyramidal cells from layer 3 were drawn with the aid of a camera lucida, and compared in terms of number and length of primary dendrites, branching, size, elongation, and distribution of dendritic field orientations. Surprisingly, we observed no significant differences in any parameter measured. Thus, although stripe-rearing can specifically alter the orientation of the dendritic fields of the layer 3 pyramidal cells, and dark-rearing has been shown by others to alter the size of layer 4 stellate cells, dark-rearing failed to affect the dendritic fields of layer 3 pyramidal cells.

Animals↗
PubMed2000

Modulation of human visual cortex by crossmodal spatial attention.

A sudden touch on one hand can improve vision near that hand, revealing crossmodal links in spatial attention. It is often assumed that such links involve only multimodal neural structures, but unimodal brain areas may also be affected. We tested the effect of simultaneous visuo-tactile stimulation on the activity of the human visual cortex. Tactile stimulation enhanced activity in the visual cortex, but only when it was on the same side as a visual target. Analysis of effective connectivity between brain areas suggests that touch influences unimodal visual cortex via back-projections from multimodal parietal areas. This provides a neural explanation for crossmodal links in spatial attention.

Adult↗
PubMed1995

Long-range horizontal connections and their role in cortical reorganization revealed by optical recording of cat primary visual cortex.

The cortical 'point spread' (PS) is the area of cortex activated by a minimal visual stimulus. Here we use the PS to explore the functional role of lateral connectivity in normal cat primary visual cortex (V1) and its involvement in topographic reorganization of cortex following retinal lesions. We compared the distributions of PSs measured with optical recording, which reflects both spiking and subthreshold activity, with those measured with extracellular electrodes, which reveal spiking activity alone. The spiking PS represented only 5% of the area of activation shown in the optical PS, indicating that the remaining 95% was probably generated by subthreshold activation. The orientation dependence of the pattern of the subthreshold activation and its close match with orientation columns suggests that long-range horizontal connections radiating from the locus of spiking activity were responsible for the observed activation. The spike PS showed anisotropies and inhomogeneities that were related to the pattern of orientation columns and indicated distortions in the representation of visual space on the cortical surface. In the reorganized cortex the spike PS expanded, approximating the extent of the optical PS seen in normal cortex, and suggesting that reorganization was mediated by an unmasking of normally subthreshold activation to suprathreshold levels. The orientation map of the reorganized cortex showed a close match to that obtained before placing the lesion, despite the large shift in topography, supporting the idea that intrinsic horizontal connections were responsible for the remapping.

Action Potentials↗
PubMed1993

Fetal, neonatal and adult expression of benzodiazepine receptor subtypes in human visual cortex.

We have investigated the relative distributions of type I and type II benzodiazepine receptors in fetal, neonatal and adult human visual cortex using [3H]flunitrazepam and the triazolopyridazine, CL218872. Microdensitometry of autoradiograms show that while both benzodiazepine subtypes are found in visual area 17 early in life, the type II benzodiazepine receptors predominate. In adult human visual cortex comparable mounts of type I and type II subtypes occur in a laminar specific distribution concentrated in layers 4c and 2/3. The results may suggest a role for age-dependent expression of benzodiazepine receptor subtypes in visual cortex development and neuroplasticity.

Adult↗
PubMed1997

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex.

Direction selectivity of synaptic potentials in simple cells of the cat visual cortex. J. Neurophysiol. 78: 2772-2789, 1997. The direction selectivity of simple cells in the visual cortex is generated at least in part by nonlinear mechanisms. If a neuron were spatially linear, its responses to moving stimuli could be predicted accurately from linear combinations of its responses to stationary stimuli presented at different positions within the receptive field. In extracellular recordings, this has not been found to be the case. Although the extracellular experiments demonstrate the presence of a nonlinearity, the cellular process underlying the nonlinearity, whether an early synaptic mechanism such as a shunting inhibition or simply the spike threshold at the output, is not known. To differentiate between these possibilities, we have recorded intracellularly from simple cells of the intact cat with the whole cell patch technique. A linear model of direction selectivity was used to analyze the synaptic potentials evoked by stationary sine-wave gratings. The model predicted the responses of cells to moving gratings with considerable accuracy. The degree of direction selectivity and the time course of the responses to moving gratings were both well matched by the model. The direction selectivity of the synaptic potentials was considerably smaller than that of the intracellularly recorded action potential, indicating that a nonlinear mechanism such as threshold enhances the direction selectivity of the cell's output over that of its synaptic inputs. At the input stage, however, the cells apparently sum their synaptic inputs in a highly linear fashion. A more constrained test of linearity of synaptic summation based on principal component analysis was applied to the responses of direction-selective cells to stationary gratings. The analysis confirms that the summation in these cells is highly linear. The principal component analysis is consistent with a model in which direction selectivity in cortical simple cells is generated by only two subunits, each with a different receptive-field position and response time course. The response time course for each of the two subunits is derived for four analyzed cells. Each derived subunit is linear in spatial summation, suggesting that the neurons that comprise each subunit are either geniculate X-cells or receive their primary synaptic input from X-cells. The amplitude of the response of each subunit is linearly related to the contrast of the stimulus. The subunits are nonlinear in the time domain, however: the response to a stationary stimulus whose contrast is modulated sinusoidally in time is nonsinusoidal. The principal component analysis does not exclude models of direction selectivity based on more than two subunits, but such higher-order models would have to include the constraint that the extra subunits form a smooth continuum of interpolation between the properties derived from the two subunit solution.

Animals↗
PubMed2001

Intra- and inter-areal connections between the primary visual cortex V1 and the area immediately surrounding V1 in the rat.

We have qualitatively and quantitatively analysed the anatomical connections within and between rat primary visual cortex (V1) and the rim region surrounding area V1, using both ortho- and retrograde anatomical tracers (biotinylated dextran amine, biocytin, cholera toxin b subunit). From the analysis of the projection patterns, and with the assumption that single points in the rat visual cortex, as in other species, have projection fields made up of multiple patches of terminals, we have concluded that just two V1 recipient areas occupy the entire rim region: an anterolateral area, probably homologous with V2 in other mammals, previously named Oc2L, and a medial area, corresponding to Oc2M. A non-reciprocal projection from the anterolateral area to the medial area was identified. Small injections (300-600microm uptake zone diameter) of the anatomical tracers in area V1, or in the rim region, label orthograde intra-areal connections from each injection site to offset small patches. This is found in all regions of the rim and within at least the relatively expanded central dorsal field representation of V1. From the extent of these projections in V1 and the two rim regions, we have estimated that the neurons at the injection site send diverging laterally spreading projections to other neurons whose receptive fields share any part of the area included in the pooled receptive fields of the neurons at the injection site. Orthogradely labelled inter-areal feedforward projections from V1 to either rim region are estimated to diverge in their projections to neurons that share any part of the area of the pooled receptive fields of the V1 intra-areal connectional field of the same injection. The orthogradely labelled feedback projections to V1, from injection sites in either rim region, reach V1 neurons whose pooled receptive fields match those of the neurons in the rim injection site, i.e. with no divergence. Despite patchy anatomical connectional fields, our estimates indicate that visual space is represented continuously in the receptive fields of neurons postsynaptic to each intra- or inter-areal field of orthograde label. We suggest that, despite the absence of regularly mapped functions in rat V1 (e.g. regularly arranged orientation specificity), which in other species (e.g. primates and cats) relate to the patchy connectional patterns, the rat visual cortex intra- and inter-areal anatomical connections follow similar patterns and scaling factors to those in other species.

Animals↗
PubMed1992

Postnatal development of zinc-containing cells and neuropil in the visual cortex of the mouse.

The postnatal development of zinc-containing synaptic boutons and their cells of origin in the visual cortex of a pigmented mouse is described. Two phases can be distinguished. During the early phase zinc-containing neuropil is first apparent by postnatal day 3. By day 7 a light, but distinct neuropil staining sketches the primary and secondary visual cortices. The primary visual area contains light precipitate in layers V and VI as well as the monocular portion of layer II/III. The secondary visual areas contain slightly denser precipitate in layers II/III through VI. The transition to the second phase is marked by a large increase in precipitate density by day 11. Thereafter, the intensity of the neuropil staining increases to day 28, first in layer II/III and then in layer V, as the adult pattern of neuropil staining gradually develops. In the primary visual cortex precipitate is dense in layers II/III and V, moderate in layer VI, and sparse in layers I and IV. In the secondary visual areas the precipitate is dense in layers II/III and V and moderate in the lower portion of layer I and in layers IV and VI. Cells of origin of zinc-containing boutons are visible by the end of the second postnatal week in layer II/III of the secondary visual cortex. By 21 days of age the pattern of staining in the mature mouse is established, and cells in layers II/III and VI are labeled in both the primary and secondary visual cortices. The developmental sequence of zinc-containing cells and neuropil does not preclude an involvement of zinc in the postnatal regulation of NMDA receptor function.

Animals↗
PubMed1996

Experience-dependent plasticity of binocular responses in the primary visual cortex of the mouse.

An activity-dependent form of synaptic plasticity underlies the fine tuning of connections in the developing primary visual cortex of mammals such as the cat and monkey. Studies of the effects of manipulations of visual experience during a critical period have demonstrated that a correlation-based competitive process governs this plasticity. The cellular mechanisms underlying this competition, however, are poorly understood. Transgenic and gene-targeting technologies have led to the development of a new category of reagents that have the potential to help answer questions of cellular mechanism, provided that the questions can be studied in a mouse model. The current study attempts to characterize a developmental plasticity in the mouse primary visual cortex and to demonstrate its relevance to that found in higher mammals. We found that 4 d of monocular lid suture at postnatal day 28 (P28) induced a maximal loss of responsiveness of cortical neurons to the deprived eye. These ocular dominance shifts occurred during a well-defined critical period, between P19 and P32. Furthermore, binocular deprivation during this critical period did not decrease visual cortical responses, and alternating monocular deprivation resulted in a decrease in the number of binocularly responsive neurons. Finally, a laminar analysis demonstrated plasticity of both geniculocortical and intracortical connections. These results demonstrate that an activity-dependent, competitive form of synaptic plasticity that obeys correlation-based rules operates in the developing primary visual cortex of the mouse.

Animals↗
PubMed2000

The serotonin 5-HT2 receptor-phospholipase C system inhibits the induction of long-term potentiation in the rat visual cortex.

The effect of serotonin 5-HT2 receptor stimulation on long-term potentiation (LTP) in the primary visual cortex was investigated by using rat brain slices in vitro. Field potentials evoked by stimulation of layer IV were recorded in layer II/III. The 5-HT2 receptor agonist 1-(2,5-dimethyl-4-iodophenyl)-2-aminopropane (DOI) did not affect baseline synaptic potentials evoked by single-pulse test stimulation, but significantly inhibited the induction of LTP in a concentration-dependent manner (0.1-10 microM). The LTP-inhibiting effect of DOI (10 microM) was blocked by the 5-HT2,7 receptor antagonist ritanserin (10 microM), but not by the 5-HT1A receptor antagonist NAN-190 (10 microM) nor by the 5-HT3,4 receptor antagonist MDL72222 (10 microM). The inhibitory effect of DOI was also blocked by the phospholipase C inhibitor U73122, but not by its inactive analogue U73343. These results suggest that visual cortex LTP is inhibited by activation of the 5-HT2 receptor-phospholipase C system. In addition, the LTP-inhibiting effect of DOI was abolished by the presence of the GABAA receptor antagonist bicuculline (10 microM), suggesting that 5-HT2 receptor-mediated inhibition of visual cortex LTP is dependent on GABAergic inhibition.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed2004

A morphological basis for orientation tuning in primary visual cortex.

Feedforward connections are thought to be important in the generation of orientation-selective responses in visual cortex by establishing a bias in the sampling of information from regions of visual space that lie along a neuron's axis of preferred orientation. It remains unclear, however, which structural elements-dendrites or axons-are ultimately responsible for conveying this sampling bias. To explore this question, we have examined the spatial arrangement of feedforward axonal connections that link non-oriented neurons in layer 4 and orientation-selective neurons in layer 2/3 of visual cortex in the tree shrew. Target sites of labeled boutons in layer 2/3 resulting from focal injections of biocytin in layer 4 show an orientation-specific axial bias that is sufficient to confer orientation tuning to layer 2/3 neurons. We conclude that the anisotropic arrangement of axon terminals is the principal source of the orientation bias contributed by feedforward connections.

Animals↗
PubMed1976

Lateral geniculate relay of slowly conducting retinal afferents to cat visual cortex.

1. Lateral geniculate neurones of the cat were studied in terms of the latency for activation by local electrical stimulation of the retina, the latency of electrical activation from the visual cortex and properties of receptive fields. Most of the units were relay cells (antidromic activation from visual cortex) but a small proportion were trans-synaptically activated from the cortex. The latter group included units with on-off, on-centre or off-centra receptive fields. 2. Direct activation of lateral geniculate neurones from local electrical stimulation of retinal ganglion cells or their axons in the retina was identified by the sharpness of timing of the elicited impulses. This procedure revealed the existence of slowly conducting axons relaying in the lateral geniculate nucleus. 3. The distribution of latencies for direct activation from the retina was bimodal with an extended tail of long values. It is similar to the distribution of antidromic latencies of retinal ganglion cells following stimulation of the optic tract. 4. There was a tendency for geniculate neurones with fast input from the retina to have fast axons to the visual cortex and correspondingly for medium-speed and slow input. 5. The previous classification of geniculate receptive fields into sustained and transient types was extended to include commonly encountered 'brisk' and uncommonly encountered 'sluggish' varieties of each. The extension was based on visual properties and latency for direct electrical activation from the retina. Units with receptive fields differing from the familiar on-centre or off-centre concentric pattern were encountered rarely; they included colour-coded fields, local-edge-detectors and one edge-inhibitory off-centre type.

Animals↗
PubMed1983

The formation and maturation of synapses in the visual cortex of the rat. II. Quantitative analysis.

We have examined quantitatively the formation and maturation of synapses in the visual cortex of the rat. The density of the total number of synapses (synapses per 100 micron2 neuropil) as well as the densities of Gray's type I and type II contacts were estimated from photographic montages of coronal strips of visual cortex from rats of various postnatal ages. Histograms of synaptic density as a function of depth were prepared, and the mean values of the postsynaptic density length and vesicle number per terminal were estimated for the two synapse types at each age examined. During the first few days of life, synapses were concentrated in the subplate region. By the latter part of the second postnatal week they were present throughout the cortex and an adult-like distribution, in which the highest densities were present in the superficial layers, was achieved by day 14. The postsynaptic density length of the type I synapses remained relatively unchanged during development but that of the type II synapses was more variable. Specifically, it was significantly longer during the second and third postnatal weeks compared to earlier ages and to adult values. The mean number of vesicles per terminal for the two synapse types increased with age until day 28. Subsequently, it only increased slightly between days 28 and 90 for the type I synapses but decreased significantly for the type II synaptic contacts. At all ages examined, type I synapses formed the majority of synaptic contacts. The developmental pattern appeared to differ for the two synapse types. The density of type I synapses increased continuously during the first three weeks and achieved a mean value close to that of adult animals by day 20. In contrast, the density of type II synapses did not increase significantly until day 6, increased dramatically in the second and third postnatal weeks, and then declined markedly between days 20 and 90. The observed decrease in the density of type II synaptic contacts is a clear example of synapse elimination in the visual cortex.

Age Factors↗