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At least 343 records · Page 19Linked to original sources

PubMed1998

Axotomy-induced neuronal death and reactive astrogliosis in the lateral geniculate nucleus following a lesion of the visual cortex in the rat.

Following a unilateral lesion of the visual cortex (cortical areas 17, 18, and 18a) in adult rats, neurons in the ipsilateral dorsal lateral geniculate nucleus (LGN) are axotomized, which leads to their atrophy and death. The time course of this neuronal degeneration was studied quantitatively, and the astroglial response was examined with glial fibrillary acidic protein immunohistochemistry. More than 95% of the neurons in the ipsilateral LGN survive during the first 3 days following a lesion of the visual cortex. However, in the next 4 days, massive neuronal death ensues, reducing the number of surviving neurons to approximately 33% of normal by the end of the first postoperative week. Between 2 weeks and 24 weeks postoperatively, the number of neurons present in the LGN declines very gradually from 34% to 17% of normal. Three days after a lesion of the visual cortex, the mean cross-sectional areas of ipsilateral LGN neurons are 13% smaller than normal (87%). By 1 week after the operation, surviving LGN neurons have atrophied to 66% of their normal area. Subsequently, the size of surviving neurons declines slowly to approximately 50% of normal at 24 weeks after the cortical lesion. Astrocytes in the ipsilateral LGN also react to cortical damage. At 1 day after a lesion of the visual cortex, glial fibrillary acidic protein immunoreactivity in the LGN is almost undetectable, but a distinct increase in immunoreactivity is seen at 3 days. Immunoreactivity peaks between 1 week and 2 weeks postoperatively and, thereafter, remains intense for at least 24 weeks. Thus, following a lesion of the visual cortex, the somata of neurons in the LGN remain essentially normal morphologically for about 3 days before the onset of rapid atrophy and death. Moreover, most of the neural cell death that occurs in the LGN after axotomy takes place in the last half of the first postoperative week.

Animals↗

PubMed2005

Reversible blockade of experience-dependent plasticity by calcineurin in mouse visual cortex.

Numerous protein kinases have been implicated in visual cortex plasticity, but the role of serine/threonine protein phosphatases has not yet been established. Calcineurin, the only known Ca2+/calmodulin-activated protein phosphatase in the brain, has been identified as a molecular constraint on synaptic plasticity in the hippocampus and on memory. Using transgenic mice overexpressing calcineurin inducibly in forebrain neurons, we now provide evidence that calcineurin is also involved in ocular dominance plasticity. A transient increase in calcineurin activity is found to prevent the shift of responsiveness in the visual cortex following monocular deprivation, and this effect is reversible. These results imply that the balance between protein kinases and phosphatases is critical for visual cortex plasticity.

Aging↗

PubMed1991

Regressive changes among corticocortical neurons projecting from the lateral suprasylvian cortex to area 18 of the kitten's visual cortex.

The postnatal development of corticocortical neurons projecting from the medial bank of the lateral suprasylvian cortex to area 18 of the kitten's visual cortex was examined using retrograde fluorescent tracers. Area 18 was injected in young kittens aged nine days or less and in older kittens aged 30 days or more. Many of the injected kittens were perfused with fixative four to five days later, but some of the youngest were killed after longer survival periods of 35-50 days (long-survival animals). Labelled neurons in the medial bank of the lateral suprasylvian cortex were densely distributed in both superficial layers (II and III) and deep layers (V and VI) in the kittens injected less than nine days postnatal, irrespective of whether survival was short or long, but they were found almost exclusively in layers V and VI in the old, short-survival animals. Only in the group of old kittens did we find a clear topographical arrangement of projections in the rostrocaudal direction and a correlation between the rostrocaudal lengths of the injection sites and labelled areas. In the other two groups, for a similarly sized injection site, the labelled areas were much longer rostrocaudally than in the old, short-survival kittens, and occupied roughly the posterior two-thirds of the medial bank of the lateral suprasylvian cortex, irrespective of the positions of the injections. In the frontal plane, topography was unclear in all groups. These findings demonstrate that there is considerable postnatal refinement of the projection from the medial bank of the lateral suprasylvian cortex to area 18. This involves a loss of connections originating from superficial layers and a decrease of convergence with the appearance of topography. Our results from long-survival kittens suggest that most of the early exuberant population of corticocortical neurons projecting from the medial bank of the lateral suprasylvian cortex to area 18 survive beyond the first postnatal month but undergo axonal elimination during this period.

Aging↗

PubMed1989

Responses of lateral geniculate neurons that survive long-term visual cortex damage in kittens and adult cats.

Damage to visual cortex (areas 17-19) in kittens or adult cats produces severe retrograde degeneration of neurons in the dorsal lateral geniculate nucleus (LGN). However, some neurons survive in otherwise degenerated portions of the LGN after a visual cortex lesion at any age. Previous studies have shown that there are well-defined differences in potential retinal inputs, soma size, synaptic connections, outputs, and physiological properties of output targets of the surviving LGN cells in cats that received visual cortex damage at different ages. The present experiment investigated the relationships between these differences and the responses of surviving LGN neurons to visual stimulation. Recordings were made from surviving neurons in the degenerated A- and C-layers of the LGN in cats that had received a visual cortex lesion on the day of birth, at 8 weeks of age, or as adults (survival was 11.5-36 months). Normal adult cats were studied for comparison. The visual receptive field was mapped, and tests were carried out to classify each cell as X, Y, or W. In addition, quantitative methods were used to assess response amplitude, strength of receptive-field surround inhibition, spatial-frequency tuning to drifting or counterphased sine-wave gratings, and response to nondominant-eye stimulation for each cell. We found that surviving cells in all LGN layers respond to light, have normal receptive-field organization, and have normal eye dominance following a lesion at any age tested. In addition, gross retinotopic organization of the LGN is normal. However, 2 main abnormalities were observed following a lesion at all 3 ages. First, there is a reduction in the percentage of X cells in the A layers, from 62% in normal LGNs to about 15% in degenerated LGNs. Second, many surviving cells in both the A- and C-layers have abnormally large receptive-field centers. Other differences that were observed between normal A-layer cells and surviving A-layer cells could be attributed to the loss of X cells. These results indicate that cells within a structure that shows severe retrograde degeneration after brain damage can maintain relatively normal function and can take part in potentially important residual neural pathways. Previous studies indicate that these residual pathways can show both anatomical and physiological compensation for the brain damage, and the present findings bear on the consequences and mechanisms of this compensation.

Animals↗

PubMed2001

Functional retinotopy of monkey visual cortex.

The operations of primary visual cortex generate continuous representations of orientation, ocular dominance, and retinotopy that, to fit in two dimensions, organize at separate but overlapping scales (e.g., 20-500 microm, 200 microm to 5 mm, and 2-33 mm). Where their scales overlap, these organizations interact; iso-orientation contours cross ocular dominance columns at right angles, and ocular dominance columns distort retinotopy near the V1/V2 border. To explore these interactions, we developed an optical technique for visualizing retinotopy in vivo that allows us to analyze it in relation to ocular dominance and orientation patterns. Our results show local retinotopic distortions in every region of macaque V1 that we examine, including regions far from the V1/V2 border. They also show a consistent relation between local axes of distortion and ocular dominance slabs, which they intersect at angles of approximately 90 degrees. A further correlation is provided by retinotopic maps from New World primates that show less distortion (9 vs 60%) in two species characterized by an absence of pronounced ocular dominance columns. Retinotopic maps from these New World primates also revealed an unexpected tilt of the vertical midline representation that diverged from the V1/V2 border by an angle of approximately 20 degrees. Overall, these results suggest a general tendency for slab-based organizations to distort retinotopy by representing the same part of space more than once in adjacent slabs.

Animals↗

PubMed2004

The prefrontal cortex shows context-specific changes in effective connectivity to motor or visual cortex during the selection of action or colour.

The role of the prefrontal cortex remains controversial. Neuroimaging studies support modality-specific and process-specific functions related to working memory and attention. Its role may also be defined by changes in its influence over other brain regions including sensory and motor cortex. We used functional magnetic imaging (fMRI) to study the free selection of actions and colours. Control conditions used externally specified actions and colours. The prefrontal cortex was activated during free selection, regardless of modality, in contrast to modality-specific activations outside prefrontal cortex. Structural equation modelling (SEM) of fMRI data was used to test the hypothesis that although the same regions of prefrontal cortex may be active in tasks within different domains, there is task-dependent effective connectivity between prefrontal cortex and non-prefrontal cortex. The SEM included high-order interactions between modality, selection and regional activity. There was greater coupling between prefrontal cortex and motor cortex during free selection and action tasks, and between prefrontal cortex and visual cortex during free selection of colours. The results suggest that the functions of the prefrontal cortex may be defined not only by selection-specific rather than modality-specific processes, but also by changing patterns of effective connectivity from prefrontal cortex to motor and sensory cortices.

Adult↗

PubMed2005

Orientation tuning, but not direction selectivity, is invariant to temporal frequency in primary visual cortex.

The activity of neurons in primary visual cortex is influenced by the orientation, contrast, and temporal frequency of a visual stimulus. This raises the question of how these stimulus properties interact to shape neuronal responses. While past studies have shown that the bandwidth of orientation tuning is invariant to stimulus contrast, the influence of temporal frequency on orientation-tuning bandwidth is unknown. Here, we investigate the influence of temporal frequency on orientation tuning and direction selectivity in area 17 of ferret visual cortex. For both simple cells and complex cells, measures of orientation-tuning bandwidth (half-width at half-maximum response) are approximately 20-25 degrees across a wide range of temporal frequencies. Thus cortical neurons display temporal-frequency invariant orientation tuning. In contrast, direction selectivity is typically reduced, and occasionally reverses, at nonpreferred temporal frequencies. These results show that the mechanisms contributing to the generation of orientation tuning and direction selectivity are differentially affected by the temporal frequency of a visual stimulus and support the notion that stability of orientation tuning is an important aspect of visual processing.

Action Potentials↗

PubMed1991

Temporal interactions in the cat visual system. I. Orientation-selective suppression in the visual cortex.

The perception of a visual contour depends on the spatial and temporal context in which it is viewed. Interactions between visual contours are believed to underlie a wide range of perceptual phenomena, including geometric illusions and aftereffects, contrast adaptation, and visual masking. The physiological mechanisms that might underlie such interactions were studied in the visual cortex of the cat by recording responses of single neurons to pairs of brief stationary stimuli that were separated in time. The results revealed a long-lasting, orientation-selective suppression, termed "paired-pulse suppression," which was strongest at the cell's preferred orientation, but which was more broadly tuned for orientation than the excitatory response of the cell. Although the strength and duration of the suppression varied widely, some degree of response reduction was present in most cells studied. The function of this suppression may be to regulate the gain with which visual inputs are transmitted to cortical neurons, thus preventing response saturation and positive feedback.

Anesthesia↗

PubMed1988

Noradrenergic sprouting and beta-adrenergic receptor binding in the lateral geniculate nucleus of rats after unilateral visual cortex ablation.

One and two weeks after unilateral visual cortex (VC) ablation beta-adrenergic receptor binding is increased in the lateral geniculate nucleus (LGN) of both sides. 6 month later beta-receptor binding in the LGN is decreased again and no differences to untreated control animals are detectable. Using the glyoxylic acid fluorescence method for the visualization of amines a transient increase in the density of noradrenergic fibers in the dorsal part of the ipsilateral LGN can be demonstrated with a maximum 2 weeks after VC ablation. With longer survival time noradrenergic fibre density in the ipsilateral dorsal LGN (LGNd) decreases again and one year after the operation only few fluorescent fibres can be observed in the lateral part of the LGN compared to untreated control animals. Histologically an increased gliosis in the ipsilateral LGNd develops following removal of the visual cortex. In addition, degenerative changes in nerve fibers and terminals as well as neuronal degenerative changes are present and are most pronounced in the medial part of the ipsilateral dorsal LGN. Electron microscopically degenerating terminals in the dorsal part of the ipsilateral LGN can be identified as cortical afferents. Using potassium permanganate fixation typical noradrenergic axons with small dense core vesicles can be demonstrated in the LGN. In the denervated LGNd (i.e. the ipsilateral LGNd after visual cortex ablation) axon terminals with dense core vesicles appear exhibiting the structural peculiarities of growth cones seen during ontogenesis. They could be regarded as ultrastructural equivalent of the newly formed noradrenergic sprouts.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed2003

Interindividual and interspecies variations of the extrastriate visual cortex.

Functional homology between human and macaque visual cortices has provided an important cue to functional subdivisions of the human visual cortex, but it is unclear beyond V1. We estimated the sizes and the visual field eccentricity functions of the extrastriate visual areas of human brains using MRI and fMRI measurements to analyze the interindividual and interspecies variations. We found distinctive features of the area fraction values relative to V1 and the visual field eccentricity functions beyond V2 between the human and the macaque visual cortices. This suggests that selection on color-form and stereoscopic vision, associated with processing and manipulating socio-visual stimuli, may generate variations of the architecture of the extrastriate visual cortex beyond V2.

Adult↗

PubMed1975

Behavioral study of the visual cortex of Galago senegalensis.

An ablation study of the visual cortex of Galago senegalensis was undertaken in the hope of finding clues about the evolution of primate visual cortex. Removal of area 17 resulted in a profound sensory loss manifested by, first, the failure to discriminate between simple patterns; second, a deficit in localizing objects; third, a deficiency in tracking moving objects; and fourth, symptoms attributable to a deficiency in depth perception, such as misreaching and inaccurate jumping. Thus, the effects of ablating area 17 are similar in bushbabies and monkeys. In contrast, minimal sensory loss is produced by ablating area 17 in squirrels or tree shrews. This difference between primates and other mammals may depend on differences in the extent of the cortical target of the tecto-pulvinar path; in Galago and perhaps in all primates, more of the extrastriate visual cortex is entirely dependent on area 17. Removal of the ventral temporal cortex resulted in a loss of learned visual discriminations and in retardation in learning new visual discriminations. These symptoms seem related to the inferotemporal syndrome in monkeys.

Animals↗

PubMed1990

Arrangement of ocular dominance columns in human visual cortex.

The arrangement of the ocular dominance columns in the human primary visual cortex was studied by examining cytochrome oxidase activity in autopsy specimens of occipital lobes obtained from two patients who became blind in one eye before death. By artificially flattening the cortex before processing, it was possible to reconstruct the pattern formed by the ocular dominance columns throughout most of the primary visual cortex. The columns form a mosaic of irregular parallel stripes about 500 microns to 1000 microns wide (right eye column plus left eye column measures 1 to 2 mm), oriented at right angles to the boundary of the primary visual cortex. The columns are wider near the boundary of the primary visual cortex and within the representation of the peripheral visual field, the ocular dominance columns of the ipsilateral eye become fragmented until they disappear altogether at the border of the monocular crescent representation. The arrangement of ocular dominance columns in the human visual cortex is very similar to the pattern reported in the macaque monkey, although the columns in humans are wider.

Aged↗

PubMed2004

Localization of activity-dependent changes in blood volume to submillimeter-scale functional domains in cat visual cortex.

We have examined whether blood volume changes induced by neural activation are controlled precisely enough for us to visualize the submillimeter-scale functional structure in anesthetized and awake cat visual cortex. To activate the submillimeter-scale functional structures such as iso-orientation domains in the cortex, visual stimuli (gratings) were presented to the cats. Two methods were used to examine the spatial precision of blood volume changes including changes in total hemoglobin content and changes in plasma volume: (i) intrinsic signal imaging at the wavelength of hemoglobin's isosbestic point (569 nm) and (ii) imaging of absorption changes of an intravenously injected dye. Both measurements showed that the visual stimuli elicited stimulus-nonspecific and stimulus-specific blood volume changes in the cortex. The former was not spatially localized, while the latter was confined to iso-orientation domains. From the measurement of spatial separation of the iso-orientation domains, we estimated the spatial resolution of stimulus-specific blood volume changes to be as high as 0.6 mm. The changes in stimulus-nonspecific and -specific blood volume were not linearly correlated. These results suggest the existence of fine blood volume control mechanisms in the capillary bed in addition to global control mechanisms in arteries.

Anesthesia↗

PubMed1997

Visual cortex: a cat's-eye view of the visual system.

Optical imaging data show that the cat's visual cortex contains patches of cells that respond to low spatial and high temporal frequencies; outside the patches, cells respond to high spatial and low temporal frequencies. The results suggest a possible anatomical substrate for psychophysically defined spatial frequency channels.

Animals↗

PubMed1998

[Image processing in the primary visual cortex].

INTRODUCTION: Area 17 or the primary visual area forms the first link in the chain of cerebral analysis of a visual image. The neurones forming the primary visual cortex are characterized by the extreme precision of their connections, functional specialization and hierarchic organization. The spatial precision of the connections within the system for vision permit retinotopic representation in the visual cortex, so that each point of the retina is projected into a specific area of the cortex. The cortical neurones which analyze the characteristics of the image situated in a precise zone of the visual field are themselves organized into a basic functional unit known as a hypercolumn. Within each hypercolumn there are various columnar cell systems with receptive fields having similar characteristics. Thus, each hypercolumn is made up of multiple orientation columns, two ocular dominance columns and 'blob' regions. All these systems permit the analysis of different aspects of the image. The neurones belonging to the orientation columns are sensitive to the orientation, spatial frequency and movement of a visual stimulus; those of the 'blob' regions to colour, and the binocular neurones of the ocular dominance columns to depth. Within each column, the hierarchical pattern of neurone interconnections determines the successive appearance of cells with receptive fields having new properties.

Geniculate Bodies↗

PubMed1997

Spatio-temporal frequency domains and their relation to cytochrome oxidase staining in cat visual cortex.

Spatial and temporal frequencies are important attributes of the visual scene. It is a long-standing question whether these attributes are represented in a spatially organized way in cat primary visual cortex. Using optical imaging of intrinsic signals, we show here that grating stimuli of different spatial frequencies drifting at various speeds produce distinct activity patterns. Rather than observing a map of continuously changing spatial frequency preference across the cortical surface, we found only two distinct sets of domains, one preferring low spatial frequency and high speed, and the other high spatial frequency and low speed. We compared the arrangement of these spatio-temporal frequency domains with the cytochrome oxidase staining pattern, which, based on work in primate striate cortex, is thought to reflect the partition of the visual cortex into different processing streams. We found that the cytochrome oxidase blobs in cat striate cortex coincide with domains engaged in the processing of low spatial and high temporal frequency contents of the visual scene. Together with other recent results, our data suggest that spatiotemporal frequency domains are a manifestation of parallel streams in cat visual cortex, with distinct patterns of thalamic inputs and extrastriate projections.

Animals↗

PubMed1989

The development of basal forebrain projections to the rat visual cortex.

The development of the basal forebrain projections to the visual cortex of the rat were studied using retrograde tracer techniques. Injections of wheat germ agglutinin-horseradish peroxidase placed in the visual cortex of newborn animals resulted in labelling of neurons throughout the basal forebrain nuclei. Although at this time the overall distribution of retrogradely labelled cells within the basal forebrain appeared similar to that seen in the adult, cells were smaller and weakly stained. It was only at the end of the second postnatal week that the somata of stained neurons showed sizes and staining intensity comparable to the adult. This precedes or coincides with the reported significant increases in cortical and basal forebrain ChAT activity and the first detection of ChAT-labelled fibres in this cortical area. These data suggest an important developmental point around the end of the second postnatal week that may correspond to the time when a significant number of cholinergic axons first appear within the visual cortex. They also suggest that the cholinergic projections to the visual cortex develop late in comparison with the thalamic and other subcortical afferents in this cortical area.

Aging↗

PubMed2003

Activity-dependent change in the protein level of brain-derived neurotrophic factor but no change in other neurotrophins in the visual cortex of young and adult ferrets.

Neurotrophins are suggested to play a role in activity-dependent plasticity of visual cortex during the critical period of postnatal development. Thus, the concentration of neurotrophins in the cortex is expected to change with development and/or with alteration in neuronal activities. To test this, we measured protein levels of nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and neurotrophin-4/5 in visual cortex of young (postnatal day 38-46, at the peak of the critical period) and adult ferrets with two-site enzyme-immunoassay systems. Measurements were carried out also in somatosensory cortex, hippocampus and cerebellum as control. With development the level of brain-derived neurotrophic factor did not significantly change, while those of the other neurotrophins changed in the visual cortex. A blockade of visual inputs for 24 h by an injection of tetrodotoxin into both eyes significantly decreased brain-derived neurotrophic factor protein level in the visual cortex, but not in the other regions in both young and adult ferrets. On the other hand, no significant decrease was seen in the protein level of the other neurotrophins in the visual cortex of young and adult ferrets. A monocular injection of tetrodotoxin in young ferrets resulted in the reduction of brain-derived neurotrophic factor by approximately half that by binocular injection. The degree of the decrease in the contralateral cortex to the injected eye was significantly larger than that in the ipsilateral cortex, reflecting that the contralateral eye is dominantly represented in the cortex in ferrets. Blockade of cortical neuronal activities by a GABA(A) receptor agonist led to a remarkable reduction of brain-derived neurotrophic factor protein in the visual cortex. These results suggest that the level of brain-derived neurotrophic factor protein in visual cortex is regulated by activities of cortical neurons.

Aging↗