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At least 289 records · Page 16Linked to original sources
PubMed1999

Increased activity in human visual cortex during directed attention in the absence of visual stimulation.

When subjects direct attention to a particular location in a visual scene, responses in the visual cortex to stimuli presented at that location are enhanced, and the suppressive influences of nearby distractors are reduced. What is the top-down signal that modulates the response to an attended versus an unattended stimulus? Here, we demonstrate increased activity related to attention in the absence of visual stimulation in extrastriate cortex when subjects covertly directed attention to a peripheral location expecting the onset of visual stimuli. Frontal and parietal areas showed a stronger signal increase during this expectation than did visual areas. The increased activity in visual cortex in the absence of visual stimulation may reflect a top-down bias of neural signals in favor of the attended location, which derives from a fronto-parietal network.

Attention↗
PubMed1992

Activity-dependent decrease in NMDA receptor responses during development of the visual cortex.

Plasticity of the developing visual system has been regarded as the best model for changes of neuronal connections under the influence of the environment. N-methyl-D-aspartate (NMDA) receptors are crucial for experience-dependent synaptic modifications that occur in the developing visual cortex. NMDA-mediated excitatory postsynaptic currents (EPSCs) in layer IV neurons of the visual cortex lasted longer in young rats than in adult rats, and the duration of the EPSCs became progressively shorter, in parallel with the developmental reduction in synaptic plasticity. This decrease in NMDA receptor-mediated EPSC duration is delayed when the animals are reared in the dark, a condition that prolongs developmental plasticity, and is prevented by treatment with tetrodotoxin, a procedure that inhibits neural activity. Application of L-glutamate to outside-out patches excised from layer IV neurons of young, but not of adult, rats activated prolonged bursts of NMDA channel openings. A modification of the NMDA receptor gating properties may therefore account for the age-dependent decline of visual cortical plasticity.

Action Potentials↗
PubMed1987

The pattern of ocular dominance columns in flat-mounts of the cat visual cortex.

Ocular dominance (OD) columns in the cat visual cortex were visualized with autoradiography after intravitreal injection of (3H)proline. Extending previous studies, a flat-mount technique was applied that enabled the analysis of the distribution of label throughout extensive regions of the visual cortex without requiring reconstruction from serial sections. OD-columns were confined to layer IV and consisted of isolated patches and short bands. The latter were parallel to each other and regularly spaced, the main trajectory being orthogonal to the 17/18 border. This pattern of the geniculo-cortical terminals was similar in the hemispheres ipsi- and contralateral to the injected eye. The mean periodicities of the OD-bands were virtually identical in the two hemispheres of the same animal: 850 microns and 830 microns in cat D1 and 770 microns and 800 microns in cat D2. However, the ipsilateral OD-columns appeared smaller, more heavily labeled and more sharply delineated than the contralateral columns.

Animals↗
PubMedDate not supplied

Electrophysiological studies of color processing in human visual cortex.

Electrophysiological recordings from human visual cortex were carried out with electrodes chronically implanted in 13 patients for localization of an epileptogenic focus. Visual evoked potentials (VEPs) elicited by red or blue checkerboard stimuli were recorded using an adaptation stimulus-test stimulus design in which color was the most salient feature. A "significant color effect," defined as a statistically significant effect of the adaptation stimulus on test stimulus VEPs evoked by the same or a different color, was determined for various cortical regions: medial lingual gyrus, 20%; lateral lingual gyrus, 38%; posterior fusiform gyrus, 50%; anterior fusiform gyrus, 0%; inferior temporal gyrus, 5%; occipital pole, 30%; lateral surface of non-visual cortex, 6%; inferior parietal and temporal cortex, 5%. The time course of the significant color effects suggests that wave length-selective neuronal activity occurs initially at the first stage of cortical processing in the medial lingual gyrus, followed by progressively later activation of the lateral lingual gyrus, the posterior fusiform gyrus, and the inferior temporal gyrus. In two patients, stimulation of the lateral lingual and fusiform gyri elicited color sensations in the contralateral half-field, whereas stimulation of the medial lingual and cuneate gyri evoked retinotopically appropriate quadrantic "shimmering" devoid of color. These results suggest that a region of inferior occipital cortex, primarily the posterior portion of the fusiform gyrus, is involved in color perception and may be homologous with area V4 in monkeys. There is also a region of dorsolateral surface cortex which exhibits a fairly high percentage of significant color effects and when stimulated may evoke sensations of color. This region may be the same as the dorsolateral region thought to be involved in selective attention to color.

Adult↗
PubMed1996

Activation of the primary visual cortex by Braille reading in blind subjects.

Primary visual cortex receives visual input from the eyes through the lateral geniculate nuclei, but is not known to receive input from other sensory modalities. Its level of activity, both at rest and during auditory or tactile tasks, is higher in blind subjects than in normal controls, suggesting that it can subserve nonvisual functions; however, a direct effect of non-visual tasks on activation has not been demonstrated. To determine whether the visual cortex receives input from the somatosensory system we used positron emission tomography (PET) to measure activation during tactile discrimination tasks in normal subjects and in Braille readers blinded in early life. Blind subjects showed activation of primary and secondary visual cortical areas during tactile tasks, whereas normal controls showed deactivation. A simple tactile stimulus that did not require discrimination produced no activation of visual areas in either group. Thus in blind subjects, cortical areas normally reserved for vision may be activated by other sensory modalities.

Blindness↗
PubMed1993

An interdigitated columnar mosaic of cytochrome oxidase, zinc, and neurotransmitter-related molecules in cat and monkey visual cortex.

There is considerable physiological evidence for the compartmentalization of mammalian visual cortex into functional columnar modules, representing features of visual information processing such as eye and orientation specificity. However, anatomical markers of visual cortical compartmentalization have been described only for primate visual cortex. In this report, we describe an interdigitated mosaic of four neuroactive molecules which demarcate two distinct columnar systems in the kitten visual cortex. Serotonin 1C receptors and synaptic zinc were found to demarcate columns within layer IV of kitten visual cortex, which were interdigitated with a second, patchy system characterized by increased levels of cytochrome oxidase and acetylcholinesterase. In primate visual cortex, as well as in the kitten, synaptic zinc was periodically distributed in a manner precisely complementary to cytochrome oxidase. These findings provide an anatomical framework on which unifying hypotheses of the functional organization of columnar systems in mammalian visual cortex can be built.

Acetylcholinesterase↗
PubMed2002

Cognitive processing in the primary visual cortex: from perception to memory.

The primary visual cortex is the first cortical area of the visual system that receives information from the external visual world. Based on the receptive field characteristics of the neurons in this area, it has been assumed that the primary visual cortex is a pure sensory area extracting basic elements of the visual scene. This information is then subsequently further processed upstream in the higher-order visual areas and provides us with perception and storage of the visual environment. However, recent findings show that such neural implementations are observed in the primary visual cortex. These neural correlates are expressed by the modulated activity of the late response of a neuron to a stimulus, and most likely depend on recurrent interactions between several areas of the visual system. This favors the concept of a distributed nature of visual processing in perceptual organization.

Animals↗
PubMed1999

Bidirectional, experience-dependent regulation of N-methyl-D-aspartate receptor subunit composition in the rat visual cortex during postnatal development.

In the visual cortex, as elsewhere, N-methyl-D-aspartate receptors (NMDARs) play a critical role in triggering long-term, experience-dependent synaptic plasticity. Modifications of NMDAR subunit composition alter receptor function, and could have a large impact on the properties of synaptic plasticity. We have used immunoblot analysis to investigate the effects of age and visual experience on the expression of different NMDAR subunits in synaptoneurosomes prepared from rat visual cortices. NMDARs at birth are comprised of NR2B and NR1 subunits, and, over the first 5 postnatal weeks, there is a progressive inclusion of the NR2A subunit. Dark rearing from birth attenuates the developmental increase in NR2A. Levels of NR2A increase rapidly (in <2 hr) when dark-reared animals are exposed to light, and decrease gradually over the course of 3 to 4 days when animals are deprived of light. These data reveal that NMDAR subunit composition in the visual cortex is remarkably dynamic and bidirectionally regulated by sensory experience. We propose that NMDAR subunit regulation is a mechanism for experience-dependent modulation of synaptic plasticity in the visual cortex, and serves to maintain synaptic strength within an optimal dynamic range.

Animals↗
PubMed2001

Intracellular calcium signals in the surround of rat visual cortex lesions.

Focal lesions of the visual cortex induce deafferentiation, excitotoxic cell death as well as functional reorganization in the surrounding tissue. The intracellular second messenger calcium is involved in a wide range of cellular responses including excitotoxicity and functional reorganization following cortical injuries. We investigated the intracellular calcium concentration [Ca2+]i in neurons of the visual cortex using fluorescence imaging of fura-2 signals in a slice preparation obtained from lesioned and sham-operated cortices. We observed an increase in resting and stimulus evoked [Ca2+]i in the surround of the lesion, which were mediated by NMDA and non-NMDA ionotropic glutamate receptors. This increase in [Ca2+]i might be an important factor for lesion-induced functional reorganization in the rat visual cortex.

Animals↗
PubMed2006

Retinotopic organization of primary visual cortex in glaucoma: Comparing fMRI measurements of cortical function with visual field loss.

Primary open angle glaucoma (POAG) is a progressive optic neuropathy characterized by retinal ganglion cell loss. Experimental primate glaucoma indicates neuronal degeneration of the lateral geniculate nucleus (LGN) and activity changes in the visual cortex (V1). Neuronal degeneration has also been shown in a post-mortem human study of the optic nerve, LGN and visual cortex. Functional magnetic resonance imaging (fMRI), a non-invasive means of inferring function-specific neuronal activity, provides an opportunity to evaluate glaucomatous changes in neuronal activity throughout the visual pathway in vivo. The purpose of this study is to demonstrate that the relationship between visual field loss in human POAG and the functional organization of V1 can be measured using novel fMRI analysis methods. Visual field defects were measured using standard automated perimetry (SAP). A retinotopic map of visual space was obtained for V1, and the retinotopy data was fit with a template. The template was used to project regions within the visual field onto a flattened representation of V1. Viewing through the glaucomatous vs. fellow eye was compared by alternately presenting each eye with a scotoma-mapping stimulus. The resulting blood oxygen level dependent (BOLD) fMRI response was compared to interocular differences in thresholds for corresponding regions of the visual field. The spatial pattern of activity observed in the flattened representation agreed with the pattern of visual field loss. Furthermore, the amplitude of the BOLD response was correlated on a pointwise basis with the difference in sensitivity thresholds between the glaucomatous and fellow eyes (r = 0.53, p < 0.0001). The BOLD signal in human V1 is altered for POAG patients in a manner consistent with the loss of visual function. FMRI of visual brain areas is a potential means for quantifying glaucomatous changes in neuronal activity. This should enhance our understanding of glaucoma, and could lead to new diagnostic techniques and therapies.

Animals↗
PubMed1994

Organization of contour from motion processing in primate visual cortex.

A major objective of visual processing is the segmentation of the scene into separate objects. Relative motion is one of the most salient segmentation cues. In man and monkey, we recorded visually evoked potentials to a stimulus, designed to signal the presence of relative motion processing. Relative motion specific response components were only elicited when human observers perceive contours from relative motion. Equivalent dipole source localization of the responses indicated the involvement of primary visual cortex in man. This was corroborated by intracortical recordings in awake monkey, where sources of the specific components are located within the supra- and infragranular layers of primary visual cortex. It is concluded that V1 does not merely provide an input stage to contour from motion processing, but that segmentation information, based on relative motion, is present at this early cortical level.

Animals↗
PubMed1979

Neurophysiological mechanisms of recovery from visual cortex damage in cats: properties of lateral suprasylvian visual area neurons following behavioral recovery.

Damage to visual cortical areas 17, 18, and 19 in the cat produces severe and long-lasting deficits in performance of form and pattern discriminations. However, with extensive retraining the animals are able to recover their ability to discriminate form and pattern stimuli. Recent behavioral experiments from this laboratory have shown that a nearby region of cortex, the lateral suprasylvian visual area (LS area), plays an important role in this recovery (Wood et al., 1974; Baumann and Spear, 1977b). The present experiment investigated the underlying neurophysiological mechanisms of the recovery by recording from single neurons in the LS area of cats which had recovered from long-term visual cortex damage. Five adult cats received bilateral removal of areas 17, 18, and 19. They were then trained to criterion on two-choice brightness, form, and pattern discriminations. Recording from LS area neurons was carried out after the behavioral training, from 3 to 7 months after the visual cortex lesions. The properties of these neurons were compared to those of LS area neurons in normal cats (Spear and Baumann, 1975) and in cats with acute or short-term visual cortex damage and no behavioral recovery (Spear and Baumann, 1979). The results showed that all of the changes from normal which were produced by acute visual cortex damage were also present after the behavioral recovery. Moreover, all of the response properties of LS area neurons which remain after acute visual cortex damage were present in similar form after the behavioral recovery. There was no evidence for any functional reorganization in the LS area concomitant with its role in the behavioral recovery. These results suggest that functional reorganization plays little or no role in recovery from visual cortex damage in adult cats. Rather, the recovery of form and pattern discrimination ability appears to be based upon the functioning of residual neural processes in the LS area which remain after the visual cortex damage.

Animals↗
PubMed2003

Visual deprivation effects on the s100beta positive astrocytic population in the developing rat visual cortex: a quantitative study.

After birth, exposure to visual inputs modulates cortical development, inducing numerous changes of all components of the visual cortex. Most of the cortical changes thus induced occur during what is called the critical period. Astrocytes play an important role in the development, maintenance and plasticity of the cortex, as well as in the structure and function of the vascular network. Dark-reared Sprague-Dawley rats and age-matched controls sampled at 14, 21, 28, 35, 42, 49, 56 and 63 days postnatal (dpn) were studied in order to elucidate quantitative differences in the number of positive cells in the striate cortex. The astrocytic population was estimated by immunohistochemistry for S-100beta protein. The same quantification was also performed in a nonsensory area, the retrosplenial granular cortex. S-100beta positive cells had adult morphology in the visual cortex at 14 dpn and their numbers were not significantly different in light-exposed and nonexposed rats up to 35 dpn, and were even higher in dark-reared rats at 21 dpn. However, significant quantitative changes were recorded after the beginning of the critical period. The main finding of the present study was the significantly lower astroglial density estimated in the visual cortex of dark-reared rats over 35 dpn as well as the lack of difference at previous ages. Our results also showed that there were no differences when comparing the measurements from a nonsensory area between both groups. This led us to postulate that the astrocytic population in the visual cortex is downregulated by the lack of visual experience.

Animals↗
PubMed2003

Response to contrast of electrophysiologically defined cell classes in primary visual cortex.

Information processing in the visual cortex is critically dependent on the input-output relationships of its component neurons. The transformation of synaptic inputs into spike trains depends in turn on the host of intrinsic membrane properties expressed by neurons, which define established electrophysiological cell classes in the neocortex. Here we studied, with intracellular recordings in vivo, how the electrophysiological cell classes in the primary visual cortex transform an increasing input, represented by stimulus contrast, into membrane depolarization and trains of action potentials. We used contrast as input because, regardless of their stimulus selectivity, primary visual cortical cells increase their firing rates in response to increases in luminance contrast. We found that both the spike rate response and the membrane potential response are best described by the hyperbolic ratio function when compared with linear, power, and logarithmic functions. In addition, both responses show similar parameter values and similar residual variance from the fits to all four functions. We also found that changes in membrane potential are similar, but firing rates differ strongly, between the established electrophysiological cell classes: fast spiking neurons show the highest firing rates, followed by fast rhythmic bursting, and regular spiking (RS) cells. In addition, among complex cells, RS cells from supragranular layers fired at higher rates than RS cells from infragranular layers. Finally, we show that the differences in firing rates between cell classes arise from differences in the slope of the relationship between membrane potential and spike rate.

Action Potentials↗
PubMed1999

Monocular focal retinal lesions induce short-term topographic plasticity in adult cat visual cortex.

Electrophysiological recording in primary visual cortex (VI) was performed both prior to and in the hours immediately following the creation of a discrete retinal lesion in one eye with an argon laser. Lesion projection zones (LPZs; 21-64 mm2) were defined in the visual cortex by mapping the extent of the lesion onto the topographic representation in cortex. There was no effect on neuronal responses to the unlesioned eye or on its topographic representation. However, within hours of producing the retinal lesion, receptive fields obtained from stimulation of the lesioned eye were displaced onto areas surrounding the scotoma and were enlarged compared with the corresponding field obtained through the normal eye. The proportion of such responsive recording sites increased during the experiment such that 8-11 hours post-lesion, 56% of recording sites displayed neurons responsive to the lesioned eye. This is an equivalent proportion to that previously reported with long-term recovery (three weeks to three months). Responsive neurons were evident as far as 2.5 mm inside the border of the LPZ. The reorganization of the lesioned eye representation produced binocular disparities as great as 15 degrees, suggesting interactions between sites in VI up to 5.5 mm apart.

Animals↗
PubMed2001

Developmental lesions of visual cortex influence control of reaching.

We examined visually guided reaching movements in a young adult (EW) who had extensive bilateral lesions in the visual cortex since birth. EW lacked a right occipital lobe and ventral portions of the left and had poor visual acuity (3/400), yet could point to visual targets as quickly as 9 controls with visual cortex lesions acquired in adulthood and 4 adults without neurological disease. However, EW's endpoint variability and hand movement path curvature were much greater, especially for left hand movements, in concert with large sensorimotor transformation errors. Experimental reduction of acuity (to 3/240 or worse) in the normal controls produced symmetric increases in endpoint variability but did not change hand path curvature, indicating that EW's impaired movements were not due to poor vision alone. Results suggest that visual cortex in early life supports the development of lifelong neural mechanisms for the planning and control of reaching movements.

Adult↗
PubMed1998

Independent component analysis of natural image sequences yields spatio-temporal filters similar to simple cells in primary visual cortex.

Simple cells in the primary visual cortex process incoming visual information with receptive fields localized in space and time, bandpass in spatial and temporal frequency, tuned in orientation, and commonly selective for the direction of movement. It is shown that performing independent component analysis (ICA) on video sequences of natural scenes produces results with qualitatively similar spatio-temporal properties. Whereas the independent components of video resemble moving edges or bars, the independent component filters, i.e. the analogues of receptive fields, resemble moving sinusoids windowed by steady Gaussian envelopes. Contrary to earlier ICA results on static images, which gave only filters at the finest possible spatial scale, the spatio-temporal analysis yields filters at a range of spatial and temporal scales. Filters centred at low spatial frequencies are generally tuned to faster movement than those at high spatial frequencies.

Models, Biological↗
PubMed2001

Subcellular localization of GABA(B) receptor subunits in rat visual cortex.

Although studies in the visual cortex have found gamma-aminobutyric acid B (GABA(B)) receptor-mediated pre- and postsynaptic inhibitory effects on neurons, the subcellular localization of GABA(B) receptors in different types of cortical neurons and synapses has not been shown directly. To provide this information, we have used antibodies against the GABA(B) receptor (R)1a/b and GABA(B)R2 subunits and have studied the localization of immunoreactivities in rat visual cortex. Light microscopic analyses have shown that both subunits are expressed in cell bodies and dendrites of 65-92% of corticocortically projecting pyramidal neurons and in 92-100% of parvalbumin (PV)-, calretinin (CR)-, and somatostatin (SOM)-containing GABAergic neurons. Electron microscopic analyses of immunoperoxidase- and immunogold-labeled tissue revealed staining in the nucleus, cytoplasm and cell surface membranes with both antibodies. Colocalization of both subunits was observed in all of these structures. GABA(B)R1a/b and GABA(B)R2 were concentrated in excitatory and inhibitory synapses and in extrasynaptic membranes. In GABAergic synapses, GABA(B)R1a/b and GABA(B)R2 were more strongly expressed postsynaptically on pyramidal and nonpyramidal cells than presynaptically. In type 1 synapses GABA(B)R1a/b and GABA(B)R2 was found in pre- and postsynaptic membranes. The nuclear localization of GABA(B)R1 and GABA(B)R2 subunits suggests a novel role for neurotransmitter receptors in controlling gene expression. The synaptic colocalization of GABA(B)R1 and GABA(B)R2 indicates that subunits form heteromeric assemblies of the functional receptor in inhibitory and excitatory synapses. Subunit coexpression in GABAergic synapses that include PV-containing and PV-deficient terminals suggests that pre- and postsynaptic GABA(B) receptor activation is provided by several different types of interneurons. The coexpression of both subunits in excitatory synapses suggests a role for GABA(B) receptors in the regulation of glutamate release and raises the question how these receptors are activated in the absence of pre-or postsynaptic GABAergic synaptic inputs to excitatory synapses.

Animals↗