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PubMedDate not supplied

Propagating waves in visual cortex: a large-scale model of turtle visual cortex.

This article describes a large-scale model of turtle visual cortex that simulates the propagating waves of activity seen in real turtle cortex. The cortex model contains 744 multicompartment models of pyramidal cells, stellate cells, and horizontal cells. Input is provided by an array of 201 geniculate neurons modeled as single compartments with spike-generating mechanisms and axons modeled as delay lines. Diffuse retinal flashes or presentation of spots of light to the retina are simulated by activating groups of geniculate neurons. The model is limited in that it does not have a retina to provide realistic input to the geniculate, and the cortex and does not incorporate all of the biophysical details of real cortical neurons. However, the model does reproduce the fundamental features of planar propagating waves. Activation of geniculate neurons produces a wave of activity that originates at the rostrolateral pole of the cortex at the point where a high density of geniculate afferents enter the cortex. Waves propagate across the cortex with velocities of 4 microm/ms to 70 microm/ms and occasionally reflect from the caudolateral border of the cortex.

Algorithms↗
PubMed2001

Endogenous serotonin contributes to a developmental decrease in long-term potentiation in the rat visual cortex.

The primary visual cortex shows synaptic plasticity during a postnatal "critical period," and its plasticity declines with development. Indeed, we found a developmental decrease in the induction of long-term potentiation (LTP) in the rat visual cortex. In visual cortex slices obtained from 2- to 3-week-old rats, tetanic stimulation (100 Hz for 1 sec, twice at an interval of 30 sec) of the white matter reproducibly induced LTP of field potentials in layer II/III. However, in slices from 5-week-old rats, the same tetanic stimulation failed to induce LTP. We hypothesized that endogenous serotonin (5-HT) is responsible for the developmental decrease in visual cortex LTP, because the induction of visual cortex LTP was suppressed by the addition of exogenous 5-HT (10 microm) and because the amount of 5-HT in the visual cortex increased during development. To test this hypothesis, we investigated the effect of methysergide, a 5-HT receptor antagonist, on the induction of visual cortex LTP. When visual cortex slices from 5-week-old rats were perfused with 50 microm methysergide, tetanic stimulation of the white matter induced robust LTP in layer II/III. Furthermore, serotonergic neurons were lesioned by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT). LTP was induced in visual cortex slices from 5,7-DHT-treated, 5-week-old rats. These results suggest that the induction of visual cortex LTP in 5-week-old rats is suppressed by endogenous 5-HT. 5-HT may be a factor that determines a critical period for synaptic plasticity in the rat visual cortex.

5,7-Dihydroxytryptamine↗
PubMed2003

Two forms of synaptic plasticity with distinct dependence on age, experience, and NMDA receptor subtype in rat visual cortex.

In visual cortex, NMDA receptor (NMDAR) properties depend primarily on NR2A and NR2B subunits, and NR2 subunit composition changes with age and visual experience. We examined the roles of these NR2 subunits in activity-dependent long-term modification of synaptic responses, which were evoked in layer 2/3 cells by stimulation of layer 4 in rat visual cortical slices. We used theta-burst stimulation (TBS) of presynaptic fibers or low-frequency stimulation paired with postsynaptic depolarization, which has been commonly used to induce NMDAR-dependent long-term potentiation (LTP) in visual cortex. In pyramidal cells, however, TBS produced long-term depression (LTD) at inhibitory synapses rather than LTP at excitatory synapses. This was observed in association with LTP of extracellular field potentials that reflect postsynaptic potentials in a population of cells (field-LTP). This result is inconsistent with the previous view that field-LTP reflects LTP of excitatory connections. However, pairing stimulation produced LTP at excitatory synapses of pyramidal cells frequently during development but rarely in adulthood. In contrast, inhibitory LTD and field-LTP occurred similarly in both developing and mature cortex. Experiments using NR2B selective and NR2 subunit nonselective NMDAR antagonists demonstrated that NR2A- and NR2B-containing NMDARs contribute selectively to inhibitory LTD-field-LTP and excitatory LTP, respectively. In addition, we found that the developmental decline in the NR2B component was paralleled by a decline in the incidence of excitatory LTP, and these declines were both prevented by dark rearing. These results implicate NR2 subunit composition in the regulation of neocortical plasticity and demonstrate differential subunit regulation at inhibitory and excitatory connections.

Age Factors↗
PubMed2006

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.

Acoustic Stimulation↗
PubMed1994

Morphological analyses of NADPH-diaphorase/nitric oxide synthase positive structures in human visual cortex.

Human visual cortex was studied using NADPH-diaphorase histochemistry and nitric oxide synthase immunohistochemistry. Large, strongly stained, sparsely spined non-pyramidal cells (average soma diameter: 16 x 16 microns) occur in layers II-VI, but are commonest in layers II-III. Small weakly stained multipolar cells (average soma diameter 3.6 x 4 microns, stellate like cells) in layers II-VI are concentrated in layer IV of areas 17 and 18. The density of these cells, measured with a computer assisted microscopy system is less in area 18 than 17. Large, strongly stained, predominantly horizontal cells (average soma diameter 12 x 19 microns) are localized in the underlying white matter. Axons of the large, strongly NADPH-diaphorase positive cells are thin and unbranched with fine boutons. These axons ascend to layer I. The large, strongly stained cells in layers II-VI we identify as Martinotti neurons. In layer I parallel unbranched positive fibres with some fine boutons run horizontally and build dense axonal plexuses together with the axons of Martinotti neurons. Axons of presumed extrinsic origin are morphologically different from NADPH-diaphorase positive intrinsic fibres. They show thick varicosities running in different directions and forming a network in layers III-VI. Basket like formations of these fibres were frequently observed in layers IV, V and VI. Other fibres seem to innervate blood vessels. Nitric oxide synthase was also demonstrated immunohistochemically by a polyclonal rabbit nitric oxide synthase antiserum. The morphology and distribution of the immunostained cells correspond with those seen with NADPH-diaphorase histochemistry. Double labelling experiments confirm the colocalization of NADPH-diaphorase and nitric oxide synthase in all demonstrated cells. Immunohistochemical demonstration of glial fibrillary acidic protein has shown that astrocytes are not involved in the NADPH-diaphorase/NOS system in the human visual cortex.

Aged↗
PubMed2001

Development of cytochrome oxidase blobs in visual cortex of normal and visually deprived cats.

Cytochrome oxidase (CO) blobs are central to our understanding of the columnar organization and parallel pathways in primate and cat visual cortex. In primates, development of blobs and their relationship with other columnar features of the visual cortex begins pre-natally, before visual experience. In kittens, the supragranular layers differentiate post-natally, after eye opening, raising the possibility that visual experience may influence the development of blobs in cat V1. We have examined the development of blobs in unfolded and flattened sections through the visual cortex of normally reared, dark-reared, monocularly deprived and binocularly deprived kittens. Blobs were found in superficial layers of V1 of normally reared kittens as early as 2 weeks of age, although at this age the overall CO staining in V1 was lighter than in V2. By 6 weeks of age the blobs were adult-like. A patchy pattern of CO staining was also found in V2 of young kittens but not in adults. Visual experience was not necessary for expression of the blobs and monocularly deprived kittens had well developed blobs, indicating that strong Y cell drive is not necessary for the development of blobs in cat V1. CO blobs appear in kitten V1 very early in post-natal development and their expression is independent of visual experience, suggesting that they may be an intrinsic feature of V1 organization.

Age Factors↗
PubMed1992

A mathematical model for the self-organization of orientation columns in visual cortex.

The visual cortex contains regular arrangements of neurons responding to specific types of visual stimulation, such as ocular dominance columns and orientation columns. These columnar structures can be considered as the functional architecture for early visual information processing. The model of activity-dependent self-organization reported here demonstrates that both response properties of neurons to visual stimulation and the related columnar organization can be reproduced based on the competition between ON- and OFF-center inputs. Furthermore, the relationships between anatomical neural organization and its physiological response properties are clarified. This model also predicts new columnar organization linking the symmetry of receptive fields, which has never been examined. A hypercolumn, composed of orientation columns and symmetry columns, is suggested to serve as a basis for the representation of visual information.

Animals↗
PubMed2001

Mismatch between BDNF mRNA and protein expression in the developing visual cortex: the role of visual experience.

Brain-derived neurotrophic factor (BDNF) messenger RNA (mRNA) expression in the rat visual cortex of young and postnatal day 90 (P90) animals is developmentally regulated and influenced by visual experience. In the present paper we compared the expression of BDNF mRNA to the actual changes of BDNF protein occurring during postnatal development and verified whether BDNF protein distribution is controlled by visual activity. To achieve this aim we analysed BDNF mRNA and/or BDNF protein cellular distribution in the rat visual cortex at different postnatal ages by using immunohistochemistry and highly sensitive in situ hybridization. We found that before eye opening (P13), in all cortical layers a large number of visual cortical neurons contain BDNF mRNA with no detectable amount of BDNF protein. At later ages (P23 and P90), the number of BDNF-immunostained cells increases; most neurons are double labelled for BDNF mRNA and protein, and a small group of neurons is labelled only for BDNF protein. The cellular increase of BDNF immunolabelling is blocked in animals deprived of visual experience from birth (dark rearing), with a large population of neurons containing BDNF mRNA but not BDNF protein. This is similar to what is observed before eye opening. Exposure of dark-reared rats to a brief period (2 h) of light restores a good match between BDNF mRNA and BDNF protein cellular expression. We propose that visual experience controls the neuronal content of BDNF mRNA and BDNF protein in developing visual cortex.

Animals↗
PubMed2002

No ON-OFF maps in supragranular layers of ferret visual cortex.

Primary visual cortex contains functional maps of a number of stimulus properties including ocular dominance, orientation, direction, color, and spatial frequency. These maps must be organized with respect to each other and to a single continuous retinotopic map of visual space such that each stimulus parameter is represented at each point in space. In the ferret, geniculo-cortical inputs to cortical layer IV are segregated into ON- and OFF-center patches, suggesting the possibility that there might be an additional cortical map in this species. We have used optical imaging of intrinsic signals to search for ON-OFF maps in ferret visual cortical cells and have found none. This suggests that the high degree of ON-OFF segregation seen subcortically in the ferret may play a role in the development of visual cortical receptive fields rather than in adult cortical function.

Aminobutyrates↗
PubMed1981

Layer-specific labelling of cat visual cortex after stimulation with visual noise: a [3H]2-deoxy-d-glucose study.

Tritiated 2-deoxy-D-glucose (2-DG) was used to demonstrate layer specific uptake of 2-DG at the cellular level in the visual cortex of the cat after stimulation with different kinds of visual stimuli. Two-dimensional static Gaussian visual noise drifting across the visual field led to an increased accumulation of 2-DG in layers III and V as compared to the amount of radioactivity in layer IV. In unstimulated control tissue of visual cortex a homogeneous pattern of labelling was found. Horizontal bars moving vertically across the visual field increased the uptake in layer IV more than in all other layers. Analysis of the 2-DG uptake at the cellular level revealed that visual noise activated two bands of cells, one above and one below layer IV, whereas bar stimuli activated cells mainly in layer IV. Accumulation of 2-DG was always higher in the perikarya than in the surrounding neuropil. These results confirm the physiologically recorded properties of cells in different cortical layers.

Animals↗
PubMed1996

Convergence of magno- and parvocellular pathways in layer 4B of macaque primary visual cortex.

Early visual processing is characterized by two independent parallel pathways: the magnocellular stream, which carries information useful for motion analysis, and the parvocellular stream, which carries information useful for analyses of shape and colour. Although increasing anatomical and physiological evidence indicates some degree of convergence of the two streams, the pathway through layer 4B of primary visual cortex (VI) and on to higher cortical areas is usually considered to carry only magnocellular input. This is inferred from anatomical descriptions of local circuitry in V1, and functional studies of area MT, which receives input from layer 4B. We have directly measured the sources of local functional input to individual layer 4B neurons by combining intracellular recording and biocytin labelling with laser-scanning photostimulation. We found that most layer 4B neurons receive strong input from both magnocellular-stream-recipient layer 4Calpha neurons and parvocellular-stream-recipient layer 4Cbeta neurons. Thus higher cortical areas that receive input either directly or indirectly from layer 4B are likely to be more strongly influenced by the parvocellular pathway than previously believed.

Action Potentials↗
PubMed1999

Long term depression is expressed during postnatal development in rat visual cortex: a role for visual experience.

Long term forms of synaptic plasticity and in particular LTD/LTP are both present in the mammalian visual cortex. However, while LTP is not inducible in adulthood LTD can be elicited in the mature brain, but its developmental pattern is unknown. Aim of this work was to investigate whether LTD is expressed during postnatal development and if it is modulated by visual experience. To investigate these points we have used rat primary visual cortex slices taken at different stages of functional maturation process, i.e., postnatal day 17 (P17), P23 and P30-35. LTD was assessed by measuring the amplitude of extracellular field potentials recorded in cortical layers 2/3 and elicited by low frequency stimulation to the white matter. LTD was expressed at all ages investigated without significant differences between age groups. These data indicate that LTD developmental expression is not temporally related with the period of functional maturation of rat visual cortex. Dark rearing from birth to P23 resulted in a reduction of LTD amplitude while light deprivation from P17 to P30 did not affect LTD expression in comparison to age matched control values. We suggest that light imprinting is essential for a normal LTD expression during postnatal development.

Animals↗
PubMed1996

Global plasticity in adult visual cortex following reversal of visual input.

The reversal or displacement of the retinal image by prism spectacles leads to extreme disruption of visually guided behaviour, but after an extended period of visual transformation normal behaviour is gradually restored. It is unclear whether this adaptation involves a change in visual perception, the learning of new motor responses, a modification of the sensori-motor control system or a proprioceptive change in the perceived positions of the body parts. Here I describe the effects of visual field reversal on neuronal activity in the monkey visual cortex. After a few months of wearing reversing spectacles, some cells in the primary visual cortex began to respond to stimuli presented not only in the contralateral visual field but also in the ipsilateral field. These cells were not selective for orientation or direction of motion, but responded well to a light flash. This result suggests that adaptation to visual field reversal is mediated, at least in part, by a large-scale functional reorganization at an early stage in the visual processing pathway.

Adaptation, Physiological↗
PubMed1993

Monocular deprivation effects in the rat visual cortex and lateral geniculate nucleus are prevented by nerve growth factor (NGF). I. Visual cortex.

The effects of monocular deprivation done during the critical period are usually ascribed to competition between the two sets of monocular thalamic afferents taking place at cortical level. We have suggested that loss in competition for the deprived eye is explained by the lack of a neurotrophic factor, produced in the cortex and dependent on electrical activity. To test this hypothesis we have exogenously supplied nerve growth factor (NGF) to rats monocularly deprived (MD) during the critical period, and studied whether monocular deprivation still affected the functional and anatomical organization of the visual cortex. NGF is produced in the rat visual cortex during the critical period, and its expression, at least in the hippocampus, seems to be regulated by electrical activity. Ocular dominance distribution of area 17 neurons, visual acuity, and Parvalbumin immunoreactivity (Parva-LI) were determined in four sets of animals: normal rats, control untreated monocularly deprived rats, deprived rats treated with cytochrome c (to control for non-specific aspects of NGF treatment), and deprived rats treated with NGF. Parva-LI is an excellent marker for the effects of monocular deprivation on the functional organization of the rat visual cortex. We found that exogenous supply of NGF completely prevented the shift in ocular dominance distribution of visual cortical neurons, the loss of visual acuity for the deprived eye, and the strong reduction in Parva-LI induced by monocular deprivation in control rats.

Acetylcholinesterase↗
PubMed1993

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)

Amidines↗
PubMed1995

Functional magnetic resonance imaging of the primary visual cortex: evaluation of human afferent visual system.

The authors evaluated signal changes in the human primary visual cortex during visual stimulation using functional magnetic resonance imaging (MRI) scanner at 1.5 Tesla. Experiments were performed on 10 normal volunteers and 2 patients with homonymous hemianopsia. In the normal volunteers, a signal increase was observed on the bilateral primary visual cortex during hemifield stimulation. In one patient with homonymous hemianopsia after cerebral infarction, the signal change was clearly decreased on the affected side. In the other patient, who was recovering from multiple sclerosis to an almost normal visual field, the fMRI results were within normal limits. These results suggest that it is possible to map noninvasively the activation of the visual stimulation with a clinical MRI system, and that this test might be useful as an objective method of visual field examination.

Adult↗