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At least 1,441 records · Page 80Linked to original sources
PubMed1993

A model for feature linking via collective oscillations in the primary visual cortex.

A neural network model for explaining experimentally observed neuronal responses in cat primary visual cortex is proposed. In our model, the basic functional unit is an orientation column which is represented by a large homogeneous population of neurons modeled as integrate-and-fire type excitable elements. The orientation column exhibits spontaneous collective oscillations in activity in response to suitable visual stimuli. Such oscillations are caused by mutual synchronization among the neurons within the column. Numerical simulation for various stimulus patterns shows that as a result of activity correlations between different columns, the amplitude and the phase of the oscillation in each column depend strongly on the global feature of the stimulus pattern. These results satisfactorily account for experimental observations.

Animals↗
PubMed1994

Differences in the induction of Fos protein in cat visual cortex during and after the critical period.

The aim of this study was to compare the inducibility of Fos protein, in terms of magnitude and laminar distribution in visual cortex, between young (5-week-old) and adult cats. Immunohistochemical methods were used to detect Fos protein in visual and frontal cortex of young and adult cats who experienced brief (1 or 4 h) visual experience after a 1 week period of total darkness. In the 5-week-old kittens, densely stained Fos-immunoreactive neurons were found throughout all visual cortical layers as a result of 1 h of visual experience. In the adults, immunoreactive cells were concentrated in supra- and infragranular layers and only very faint labeling was found in layer IV. Immunoreactivity in young kittens persisted at much greater dilutions of primary antibody than in adults, suggesting a difference in the magnitude of induction between ages. The inductions were markedly greater in visual than in frontal cortex, where only scattered immunopositive cells were seen at the highest antibody concentration. The induction of Fos protein in visual cortex was transient and largely disappeared by 4 h at both ages. The differences between ages, both in the magnitude and laminar distribution of immunoreactivity, are inconsistent with a simple explanation of Fos inductions in terms of neural activity level. Rather, these results suggest a relation between Fos inducibility and the level of plasticity in visual cortex.

Aging↗
PubMed2001

Pairing-induced changes of orientation maps in cat visual cortex.

We have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps.

Animals↗
PubMed2006

Investigation of MR signal modulation due to magnetic fields from neuronal currents in the adult human optic nerve and visual cortex.

Neuronal currents produce weak transient magnetic fields, and the hypothesis being investigated here is that the components of these parallel to the B0 field can potentially modulate the MR signal, thus providing a means of direct detection of nerve impulses. A theory for the phase and amplitude changes of the MR signal over time due to an external magnetic field has been developed to predict this modulation. Experimentally, a fast gradient-echo EPI sequence (TR = 158 ms, TE = 32.4 ms) was employed in an attempt to directly detect these neuronal currents in the adult human optic nerve and visual cortex using a 280-mm quadrature head coil at 1.5 T. A symmetrical intravoxel field distribution, which can be plausibly hypothesized for the axonal fields in the optic nerve and visual cortex, would result in phase cancellation within a voxel, and hence, only amplitude changes would be expected. On the other hand, an asymmetrical intravoxel field distribution would produce both phase and amplitude changes. The in vivo magnitude image data sets show a significant nerve firing detection rate of 56%, with zero detection using the phase image data sets. The percentage magnitude signal changes relative to the fully relaxed equilibrium signal fall within a predicted RMS field range of 1.2-2.1 nT in the optic nerve and 0.4-0.6 nT in the visual cortex, according to the hypothesis that the axonal fields create a symmetrical Lorentzian field distribution within the voxel.

Action Potentials↗
PubMed1980

Relationships of the visual cortex in the marsupial brush-tailed possum, Trichosurus vulpecula, a horseradish peroxidase and autoradiographic study.

The ascending and descending relationships of visual cortex in Trichosurus were determined using HRP and autoradiographic methods. The visual thalamus, LGNd and LP, was found to project to three cytoarchitecturally distinct areas of cortex with each of these regions displaying differing patterns of connectivity with other centres. The striate and peristriate cortices received homotypically organized projections from LGNd as well as projections from both divisions of LP. The posterior parietal cortex received projections from both LGNd and LP as well as from the latero-intermediate, posterior, ventroanterior and intralaminar thalamic nuclei. The cortical projection fields of LGNd and LP were co-extensive. Descending projections to thalamic centres were reciprocal with the exception of a descending only projection to the thalamic reticular nucleus. The striatogeniculate projection was homotypically organized as was the striatocollicular projection which extended only to the superficial layers of that nucleus. The posterior parietal projection to the superior colliculus involved the deeper layers of that nucleus. The entire visual cortex projected to the pretectum and to the pontine nuclei. The organization of visual cortex in Trichosurus differs from that reported in the American marsupial, Didelphis, in that the LGNd and LP complexes possess a very broad and co-extensive cortical projection, a finding substantially different to what has been reported in placental mammals as well. The organization of thalamic relationships with the posterior parietal cortex would also appear to be unusual with this region receiving convergent projections from at least seven separate thalamic nuclei.

Animals↗
PubMed1994

Simulation of visual cortex development under lid-suture conditions: enhancement of response specificity by a reverse-Hebb rule in the absence of spatially patterned input.

In this report, I show that a reverse-Hebb synaptic modification rule leads to the enhancement of response specificity of simulated visual cortex neurons in the absence of spatial patterning of the afferent activity. Although it is clear that receptive fields in the visual cortex can be modified by experience, many studies have shown a substantial increase of response specificity in cats deprived of pattern vision by lid suture, leading some to conclude that receptive field properties are essentially hard-wired. The hard-wired vs. experience-dependent controversy can be resolved by assuming that while Hebb-type plasticity is responsible for developmental synaptic changes, the organization of presynaptic activity which exists under conditions of visual deprivation is sufficient to drive the neurons towards greater specificity (Linsker 1986a-c; Miller 1989, 1992; Miller et al. 1989). As a reverse-Hebb rule enhances response specificity by balancing the push-pull system of ON- and OFF-center afferents, the sufficient condition is that the activity of ON- and OFF-center retinal ganglion cells be negatively correlated, a condition which will be met by diffuse illumination as seen through sutured eyelids. Unlike the models of Linsker and Miller and colleagues, which are based on a standard-Hebb rule, the model presented here does not require the presence of a "Mexican hat" spatial patterning of the afferent correlations, which has not been observed experimentally.

Animals↗
PubMed1996

The distribution of brain-derived neurotrophic factor and its receptor trkB in parvalbumin-containing neurons of the rat visual cortex.

We analysed the distribution of brain-derived neurotrophic factor (BNDNF) and its receptor trkB in the adult rat visual cortex, paying particular attention to a GABAergic neuronal subpopulation - the parvalbumin-positive cells. We found expression of trkB in the cell body and apical dendrite of pyramidal neurons and in the cell body of non-pyramidal neurons. Double labelling experiments revealed extensive colocalization of parvalbumin and trkB immunoreactivity in non-pyramidal neurons. Interestingly, the trkB-positive pyramidal neurons appeared surrounded by parvalbumin-labelled boutons. The use of double immunohistochemistry and in situ hybridization histochemistry showed that parvalbumin-positive neurons express trkB mRNA. BDNF mRNA was found in several cells. Coexpression of BDNF mRNA and parvalbumin immunoreactivity was extremely rare. These data strongly suggest that BDNF synthesized by cortical neurons acts as a postsynaptically derived factor for parvalbumin-positive neurons in the adult rat visual cortex.

Animals↗
PubMed2000

Attentional suppression of activity in the human visual cortex.

We have used fMRI to examine the nature of the changes that occur in the human visual cortex when an observer attends to a particular location in the visual image. Previous studies have shown that the magnitude of the response to a visual stimulus is increased when the observer attends to the stimulus. We show that, in addition, attention to a particular location results in a widespread suppression of activity levels at all other locations. This suggests that a key mechanism of attentional modulation may be that spontaneous (baseline) levels of neural activity are adjusted in a position-dependent manner across the entire visual field.

Attention↗
PubMed2005

Reduction of GFAP induced by long dark rearing is not restricted to visual cortex.

A key component of the astrocyte cytoskeleton is the glial fibrillary acidic protein (GFAP), which plays an essential role in neuron/astrocyte interactions. Environmental conditioning, such as visual experience manipulation, can affect neuronal and/or glial plasticity in specific brain areas. Previous work from our laboratory showed that short light deprivation throughout the period of GFAP maturation does not influence the expression profile of GFAP in mouse visual cortex; however, it was strong enough to affect neuronal phenotype. It was suggested that visual experience controls the maturation of the neuronal circuitry in this brain area. Therefore, to see whether the modifications of neuronal activity induced by light deprivation affect the maintenance of normal astrocytic phenotype, the dark rearing protocol was extended until the adult life. GFAP-immunoreactive cells were dramatically affected, showing an 80% decrease in number. In addition, GFAP protein level exhibited a 50% reduction, while its mRNA remained unaffected. Besides the visual cortex, two other areas of the brain not directly involved in vision, the hippocampus and the motor cortex, were chosen as internal controls. Unexpectedly, also in these areas, astrocytes were affected by light deprivation. The present results show that lack of visual experience for long periods of time deeply affects glial phenotype not only in visual areas but also in brain regions not directly involved in sensory processing.

Animals↗
PubMedDate not supplied

Prenatal development of calbindin D-28K in human visual cortex.

The distribution of the calcium-binding protein calbindin D-28K (CB) was investigated in human fetal primary visual cortex. CB is present in Cajal-Retzius cells of layer I, in sparse neurons of the ventricular and intermediate zones (VZ, IZ), and in tangential fibres in IZ by 15 weeks (W) of gestation. Cajal-Retzius cells lose their staining by 30W. CB appears in layers II-VI mainly from 26W, following an inside-outside sequence. Until 34W, CB labelling is in somata and neuropil located primarily in layers IVA, IVC and V. Then reactive perikarya and puncta increase in layers II-IVA and deep IVB and C, but are reduced in infragranular layers from 34W to term. From 30W positive somata form clusters in the cell-rich bands in layers IV and V and labelled neuropil in layers III and IV has a periodic pattern from 34W. Also from 34W, numerous lightly reactive pyramidal cells are present in layers II to IVA in primary, but not secondary, visual cortex. Our results show precocious expression of CB before full laminar differentiation of the cortex and that some of this expression is transient.

Calbindins↗
PubMed2005

VGluT2 immunochemistry identifies thalamocortical terminals in layer 4 of adult and developing visual cortex.

A vesicular glutamate transporter, VGluT2, has been suggested to be the transporter utilized in the thalamocortical pathway. We examined the reliability of this marker in identifying and discriminating thalamic terminals in adult and developing ferret visual cortex. We studied brain sections stained for the transporter protein and/or anterogradely filled thalamocortical or intracortical axons, by using light, confocal, and electron microscopy. Under light microscopy, VGluT2 immunoreactivity (ir) in adult animals [past postnatal day (P)90] and in neonatal animals as early as P27 formed a dense band in layer 4 and appeared as scattered puncta in layers 6 and 1. Confocal dual-labeling analyses of P46 and adult striate cortices indicated that VGluT2 was present in thalamocortical axons, suggesting that thalamic projections utilize this transporter during postnatal development as well as adulthood. In contrast, extracellularly filled intracortical axons failed to colocalize with VGluT2-ir, suggesting that no significant terminal population originating in cortex contained VGluT2 in layer 4. Electron microscopic analysis revealed that, in adult layer 4, VGluT2-ir was present in large terminals, forming asymmetric synapses. Similar to anterogradely labeled thalamocortical terminals, VGluT2-ir synaptic terminals were different from their unlabeled counterparts in terms of terminal area (0.6 vs. 0.3 microm), synaptic length (486 vs. 353 nm), and preference for synapsing on spines (77% vs. 59%). Moreover, no significant differences were found between VGluT2-ir and anterogradely labeled thalamocortical terminals. Comparable similarities were also demonstrated at P46. These results indicate that thalamocortical terminals in layer 4 of visual cortex utilize VGluT2 and suggest that this marker can be used to identify thalamic axons specifically in adult and developing animals.

Animals↗
PubMedDate not supplied

Distribution of neuropeptide Y immunoreactivity in human visual cortex and underlying white matter.

Immunocytochemical techniques have been used to study neuropeptide Y (NPY) distribution in the human visual cortex (Brodman's areas 17, 18 and 19) NPY cell bodies belong mostly to inhibitory (multipolar and bitufted) but also to excitatory (bipolar and some pyramidal) neuronal types. Their distribution is similar in the three cortical areas studied: 20 to 40% of the NPY perikarya are located in the cortical gray matter, mostly in the deep layers, while the remaining 60 to 80% are located in the underlying white matter. Immunoreactive NPY processes form a rich network of intersecting fibers throughout the entire visual cortex. A superficial plexus (layers I and II) and a deep plexus (deep layer V and layer VI) of NPY fibers are present in areas 17, 18 and 19. In area 17, an additional well developed plexus is present in layers IVb and IVc. These plexuses receive branches from long parallel fibers arising from deep cortical layers or underlying white matter and terminating in superficial layers. Local or extrinsic NPY terminals wind around vessels in the cortex as well as in the white matter, and either penetrate them or form clusters of club endings on their walls. Our results suggest a role for NPY in human visual circuitry and in cortical blood flow regulation.

Aged↗
PubMed2004

Surface stimulation of the brain with a prototype array for a visual cortex prosthesis.

We are developing a neural prosthesis to electrically stimulate the visual cortex to restore basic visual perceptions to blind patients. The effects on cortical excitation of different stimulus configurations using a prototype electrode array are presented. Cats underwent a bilateral craniotomy to expose the cortex. An array for brain stimulation was placed on the surface of the right hemisphere. Cortical stimulation was undertaken in a variety of configurations while measuring the evoked responses that propagated through transcallosal pathways, at a homologous region on the contralateral hemisphere. Cortical excitation elicited by stimulation with a particular paradigm could be assessed by measuring the spatial spread and amplitudes of evoked responses in the contralateral hemisphere. Results from this transcallosal model have allowed us to examine the spatial and amplitude effects of cortical stimulation with our prototype electrode array and will aid in developing a neuroprosthesis for blind patients.

Animals↗
PubMed1992

[An improvement in the cognitive characteristics of monkeys induced by an antioxidant: the neurophysiological correlates in the visual cortex].

Administration of oximetacyl considerably improved the cognitive characteristics of the processes of delayed visual recognition, increased auto- and cross-correlation coefficients in the responses of groups of neurons. The data obtained suggests nootropic properties of oximetacyl and participation of visual cortex of the monkey brain in realisation of these properties for improvement of the cognitive characteristics.

Acoustic Stimulation↗
PubMed2005

Spatiotemporal characteristics of form analysis in the human visual cortex revealed by rapid event-related fMRI adaptation.

The integration of local elements to coherent forms is at the core of understanding visual perception. Accumulating evidence suggests that both early retinotopic and higher occipitotemporal areas contribute to the integration of local elements to global forms. However, the spatiotemporal characteristics of form analysis in the human visual cortex remain largely unknown. The aim of this study was to investigate form analysis at different spatial (global vs. local structure) and temporal (different stimulus presentation rates) scales across stages of visual analysis (from V1 to the lateral occipital complex-LOC) in the human brain. We used closed contours rendered by Gabor elements and manipulated either the global contour structure or the orientation of the local Gabor elements. Our rapid event-related fMRI adaptation studies suggest that contour integration and form processing in early visual areas is transient and limited within the local neighborhood of their cells' receptive field. In contrast, higher visual areas appear to process the perceived global form in a more sustained manner. Finally, we demonstrate that these spatiotemporal properties of form processing in the visual cortex are modulated by attention. Attention to the global form maintains sustained processing in occipitotemporal areas, whereas attention to local elements enhances their integration in early visual areas. These findings provide novel neuroimaging evidence for form analysis at different spatiotemporal scales across human visual areas and validate the use of rapid event-related fMRI adaptation for investigating processing across stages of visual analysis in the human brain.

Adaptation, Psychological↗
PubMed1991

Differential effects of monocular deprivation on glutamic acid decarboxylase and type II calcium-calmodulin-dependent protein kinase gene expression in the adult monkey visual cortex.

Increases in immunocytochemically detectable type II calcium-calmodulin-dependent protein kinase (CaM II kinase) and decreases in immunocytochemically detectable glutamic acid decarboxylase (GAD) are known to occur in the visual cortex of adult monkeys following brief periods of monocular visual deprivation. In the present study, GAD and CaM II kinase gene expression was investigated under these conditions. The polymerase chain reaction (PCR) was used to generate species-specific cDNA clones that were used to make antisense RNA probes. A second form of CaM II kinase alpha, CaM II kinase alpha-33, which contains an additional phosphorylation consensus sequence, was identified. In situ hybridization in normal visual cortex revealed a complex sublaminar organization of GAD-expressing cells within layers IVC and VI and a distribution of CaM II kinase alpha-expressing cells that was greatest in layers II, III, IVB, and VI. In situ hybridization in the cortex from animals that had been monocularly deprived revealed enhanced CaM II kinase mRNA levels in deprived-eye columns of layer IVC and, associated with the deprived eye, cytochrome oxidase-stained periodicities in other layers. In layer IV, the enhancement of labeling in deprived-eye stripes was, on average, 16% greater than in normal-eye stripes. By contrast, GAD, mRNA levels appeared unchanged in all layers, suggesting a posttranscriptional regulatory mechanism.

Amino Acid Sequence↗
PubMed1968

Binocularly driven neurons in visual cortex of split-chiasm cats.

In cats with midsagittal section of the optic chiasm, some visual cortex neurons can be driven not only by the ipsilateral eye, through the direct geniculocortical pathways, but also by the contralateral eye, through the opposite visual cortex and corpus callosum. The receptive fields and the response characteristics observed upon stimulation of the contralateral eye are very similar to those observed upon stimulation of the ipsilateral eye; the two monocular receptive fields of a given cell lie in corresponding points of heteronymous halves of the visual field in close contact with the vertical meridian, thus adding in visual space and forming a binocular receptive area which crosses the vertical meridian and extends equally on either side of it.

Animals↗
PubMed1978

Some Golgi data on visual cortex of the rhesus monkey.

The rhesus monkey's visual cortex was studied on Golgi material. The terminal arborization of the geniculate fibres and non-specific vertical fibres have been analysed. The interneurons (intrinsic neurons) of the area described in detail and classified on the basis of their axonal and dendrite arborizations. The stellate neurons in layer IV are discussed.

Animals↗