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At least 1,045 records · Page 58Linked to original sources
PubMed1997

Brain-derived neurotrophic factor and nerve growth factor potentiate excitatory synaptic transmission in the rat visual cortex.

1. The effect of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) on excitatory synaptic transmission in the developing visual cortex was studied by whole-cell patch-clamp recordings from rat brain slices. 2. Both neurotrophins induced a rapid increase in the amplitude of impulse-evoked excitatory postsynaptic currents (EPSCs). BDNF also increased the frequency of spontaneous EPSCs. 3. Analysis of the currents revealed that alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components contributing to the EPSC peak amplitude were equally potentiated by the neurotrophins. 4. When synaptic transmission was studied by minimal stimulation of intracortical afferents, neurotrophins induced a decrease in the occurrence of release failures. 5. A number of neurones were insensitive to the effects of the neurotrophins, possibly related to the considerable heterogeneity of neuronal types and to the uneven distribution of neurotrophin receptors in the visual cortex. 6. The probability of neurotransmitter release represents a rapidly modifiable synaptic feature by which neurotrophins can potentiate the efficacy of excitatory synaptic transmission in the visual cortex.

Animals↗
PubMed1992

[The spatial-frequency characteristics of the neuronal receptor fields in area 21 of the cat visual cortex].

21 area's neurons of the visual cortex revealed obvious low-pass frequency characteristics ranging from 0.05 to 3.5 c/deg in cats. Responses of the neurons to different frequencies ranged from 0.05 to 0.89 c/deg in the left hemisphere and from 0.11 to 3.5 c/deg in the right one. The data obtained suggest that the RF's neurons of the area 21 make low-pass spatial-frequency description of visual images, the left hemisphere making a lower spatial-frequency description of the image than the right one.

Animals↗
PubMed1979

The role of pattern vision in the increase of visual cortex acetylcholinesterase after first exposure of dark reared rats to light.

Acetylcholinesterase (AChE) activity in homogenates from motor and visual cortex of rats reared for 7 weeks in darkness was compared to levels in normally reared and dark reared littermates exposed to 3-h periods of controlled illumination. AChE levels in the visual cortex of animals fitted with light-diffusing scleral contact occluders, and therefore lacking normally organized visual information, were elevated 17% over dark-reared animals, but were not significantly different from levels in animals exposed to lighting conditions permitting detailed pattern discrimination. There were no differences between conditions in motor cortex AChE. We conclude that higher order perceptual processing is not necessary for the light-stimulated increase in visual cortex AChE in the rat.

Acetylcholinesterase↗
PubMedDate not supplied

Fuzzy clustering of gradient-echo functional MRI in the human visual cortex. Part II: quantification.

Fuzzy cluster analysis (FCA) is a new exploratory method for analyzing fMRI data. Using simulated functional MRI (fMRI) data, the performance of FCA, as implemented in the software package Evident, was tested and a quantitative comparison with correlation analysis is presented. Furthermore, the fMRI model fit allows separation and quantification of flow and blood oxygen level dependent (BOLD) contributions in the human visual cortex. In gradient-recalled echo fMRI at 1.5 T (TR = 60 ms, TE = 42 ms, radiofrequency excitation flip angle [theta] = 10 degrees-60 degrees) total signal enhancement in the human visual cortex, ie, flow-enhanced BOLD plus inflow contributions, on average varies from 5% to 10% in or close to the visual cortex (average cerebral blood volume [CBV] = 4%) and from 100% to 20% in areas containing medium-sized vessels (ie, average CBV = 12% per voxel), respectively. Inflow enhancement, however, is restricted to intravascular space (= CBV) and increases with increasing radiofrequency (RF) flip angle, whereas BOLD contributions may be obtained from a region up to three times larger and, applying an unspoiled gradient-echo (GRE) sequence, also show a flip angle dependency with a minimum at approximately 30 degrees. This result suggests that a localized hemodynamic response from the microvasculature at 1.5 T may be extracted via fuzzy clustering. In summary, fuzzy clustering of fMRI data, as realized in the Evident software, is a robust and efficient method to (a) separate functional brain activation from noise or other sources resulting in time-dependent signal changes as proven by simulated fMRI data analysis and in vivo data from the visual cortex, and (b) allows separation of different levels of activation even if the temporal pattern is indistinguishable. Combining fuzzy cluster separation of brain activation with appropriate model calculations allows quantification of flow and (flow-enhanced) BOLD contributions in areas with different vascularization.

Adult↗
PubMed2000

Tilt aftereffects in a self-organizing model of the primary visual cortex.

RF-LISSOM, a self-organizing model of laterally connected orientation maps in the primary visual cortex, was used to study the psychological phenomenon known as the tilt aftereffect. The same self-organizing processes that are responsible for the long-term development of the map are shown to result in tilt aftereffects over short timescales in the adult. The model permits simultaneous observation of large numbers of neurons and connections, making it possible to relate high-level phenomena to low-level events, which is difficult to do experimentally. The results give detailed computational support for the long-standing conjecture that the direct tilt aftereffect arises from adaptive lateral interactions between feature detectors. They also make a new prediction that the indirect effect results from the normalization of synaptic efficacies during this process. The model thus provides a unified computational explanation of self-organization and both the direct and indirect tilt aftereffect in the primary visual cortex.

Adaptation, Ocular↗
PubMed1997

A tonic hyperpolarization underlying contrast adaptation in cat visual cortex.

The firing rate responses of neurons in the primary visual cortex grow with stimulus contrast, the variation in the luminance of an image relative to the mean luminance. These responses, however, are reduced after a cell is exposed for prolonged periods to high-contrast visual stimuli. This phenomenon, known as contrast adaptation, occurs in the cortex and is not present at earlier stages of visual processing. To investigate the cellular mechanisms underlying cortical adaptation, intracellular recordings were performed in the visual cortex of cats, and the effects of prolonged visual stimulation were studied. Surprisingly, contrast adaptation barely affected the stimulus-driven modulations in the membrane potential of cortical cells. Moreover, it did not produce sizable changes in membrane resistance. The major effect of adaptation, evident both in the presence and in the absence of a visual stimulus, was a tonic hyperpolarization. Adaptation affects a class of synaptic inputs, most likely excitatory in nature, that exert a tonic influence on cortical cells.

Adaptation, Physiological↗
PubMed1996

New images from human visual cortex.

Recent developments in imaging and histology have greatly clarified our understanding of the nature and organization of human visual cortex. More than ten human cortical visual areas can now be differentiated, compared with the approximately 30 areas described in macaque monkeys. Most human areas and columns described so far appear quite similar to those in macaque but distinctive species differences also exist. Imaging studies suggest two general information-processing streams (parietal and temporal) in human visual cortex, as proposed in macaque. Several human areas are both motion- and direction-selective, and a progression of motion-processing steps can be-inferred from the imaging data. Human visual areas for recognizing form are less well defined but the evidence again suggests a progression of information-processing steps and areas, beginning posterior to the human middle temporal area (or V5), and extending inferiorly then anteriorly. This is consistent with findings from macaque, and with human clinical reports.

Animals↗
PubMed1996

Effects of dark-rearing on the vascularization of the developmental rat visual cortex.

Cerebral vascular density corresponds to metabolic demand, which increases in highly active areas. External inputs play an important role in the modeling and development of the visual cortex. Experience-mediated development is very active during the first postnatal month, when accurate simultaneous blood supply is needed to satisfy increased demand. We studied the development of visual cortex vascularization in relation to experience, comparing rats raised in darkness with rats raised in standard conditions. The parameters measured were cortical thickness, vascular density and number of perpendicular vessels, constituting the first stage of cortical vascular development. Vessels were stained using butyryl cholinesterase histochemistry, which labels some neurons and microvascularization (vessels from 5 to 50 microns). Animals from both groups were sampled at 0, 7, 14, 21 and 60 days postnatal. Vascularization of the brain starts with vertically oriented intracortical vascular trunks whose density decreases notably after birth in rats reared in standard laboratory conditions. The most striking finding of our work is the significantly lower decrease in the number of these vessels in dark-reared rats. Our results also show that cortex thickness and vessel density are significantly lower in dark-reared rats. These results suggest that the absence of visual stimuli retards the maturation of the visual cortex including its vascular bed.

Aging↗
PubMed1992

Selection of intrinsic horizontal connections in the visual cortex by correlated neuronal activity.

In the visual cortex of the brain, long-ranging tangentially oriented axon collaterals interconnect regularly spaced clusters of cells. These connections develop after birth and attain their specificity by pruning. To test whether there is selective stabilization of connections between those cells that exhibit correlated activity, kittens were raised with artificially induced strabismus (eye deviation) to eliminate the correlation between signals from the two eyes. In area 17, cell clusters were driven almost exclusively from either the right or the left eye and tangential intracortical fibers preferentially connected cell groups activated by the same eye. Thus, circuit selection depends on visual experience, and the selection criterion is the correlation of activity.

Aging↗
PubMed1994

Postnatal development and laminar distribution of noradrenergic fibers in cat visual cortex.

Previous studies have indicated that adrenergic receptors show significant changes either in laminar distribution or in number during the critical period of kitten visual cortex development. In order to further investigate the postnatal development of this neurotransmitter system, especially in relation to the critical period, we used a polyclonal antibody against dopamine-beta-hydroxylase to localize noradrenaline-containing afferents in visual cortex of kittens of various ages from birth to adulthood. In young kittens, less than 2 weeks of age, noradrenergic fibers were sparse, short and randomly oriented, and were concentrated in layer I and in deep cortical layers V and VI. By postnatal day 40, the fibers were present throughout all cortical layers and exhibited higher densities in layers I, II, III, V and VI, with a band of lower staining in layer IV. While tangential fibers predominated in layers I, V and VI, relatively straight radial fibers traversed layers II and III. After postnatal day 40, we did not find major changes in the laminar distribution of adrenergic fibers. This developmental laminar distribution pattern of adrenergic fibers resembles that of the beta-adrenergic receptors that we and others have studied in kitten visual cortex, but differs from that of alpha-adrenergic receptors.

Afferent Pathways↗
PubMed1975

Effects of visual cortex lesions following recovery from monocular deprivation in the cat.

Six monocularly deprived (MD) and four normal cats were trained monocularly on two-choice form and pattern discriminations. MD cats trained through the initially deprived eye were able to learn the discriminations; however, they required many more trials than normals. Retention tests showed that MD cats have nearly perfect retention of the discriminations over periods of up to 4 months. With retention intervals of 6 months or longer, there is a tendency for the MD cats to show an initial drop in performance, particularly on more difficult discriminations. However, criterion performance typically was attained with considerable savings, indicating good retention even over these extended intervals. Following the preoperative training and retention testing, the cats received one of the three types of visual cortex lesions. Two MD cats received total visual cortex removal (areas 17, 18, and 19). This produced a complete postoperative loss of the discriminations with continued chance performance over 800--1000 trials. Two MD cats and two normal cats received removal of the monocular segment of area 17, with the central visual field projection region of area 17 and all of areas 18 and 19 remaining intact. This produced no loss of the discriminations in either normal or MD cats beyond what is expected on the basis of normal forgetting. Two MD cats and two normal cats received removal of areas 18, 19, and the central 5--10 deg. of the visual field projection in area 17. Postoperative retention was somewhat variable for both normal and MD cats. However, subsequent acquisition of the discriminations by both normal and MD cats was in sharp contrast to the prolonged deficits produced by total visual cortex lesions. These results indicate that one or more of visual cortical areas 17, 18, and 19 are involved in the recovery of visual discrimination capacities in MD cats. However, the monocular segment of striate cortex does not appear to be specially involved in this ability, as has been suggested by previous investigations. Possible mechanisms for the recovered visual capacities in MD cats are considered.

Animals↗
PubMed1984

Primary visual cortex in the brushtailed possum: receptive field properties and corticocortical connections.

The corticocortical connections and receptive field properties of primary or striate visual cortex of the brushtailed possum, Trichosurus vulpecula, have been examined. In this Australian marsupial species primary visual cortex has connections with four other visual areas in the occipital lobe. In these adjacent visual areas fibers from striate cortex terminate mainly in layers 3 and 4 and in some cases also in layers 1 and 2. In all four areas return connections to striate cortex originate predominantly in layers 2 and 3, and to a much lesser extent in layers 5 and 6. Interhemispheric connections of striate cortex are limited to the boundary of striate and peristriate cortex. In addition to its cortical connections, striate cortex makes reciprocal connections with the claustrum. Most neurons in striate cortex are highly binocular. Of our sample of 113 visually responsive neurons, only 30% were orientation selective. On the basis of these observations we have compared striate cortex of the marsupial brushtailed possum with striate cortices of the American marsupial opossum and those of placental mammals.

Animals↗
PubMed2005

Tilt aftereffect and adaptation-induced changes in orientation tuning in visual cortex.

The tilt aftereffect (TAE) is a visual illusion in which prolonged adaptation to an oriented stimulus causes shifts in subsequent perceived orientations. Historically, neural models of the TAE have explained it as the outcome of response suppression of neurons tuned to the adapting orientation. Recent physiological studies of neurons in primary visual cortex (V1) have confirmed that such response suppression exists. However, it was also found that the preferred orientations of neurons shift away from the adapting orientation. Here we show that adding this second factor to a population coding model of V1 improves the correspondence between neurophysiological data and TAE measurements. According to our model, the shifts in preferred orientation have the opposite effect as response suppression, reducing the magnitude of the TAE.

Adaptation, Physiological↗
PubMed2002

Ovarian hormones after postnatal day 20 reduce neuron number in the rat primary visual cortex.

Previous work from our lab has documented a sex difference in neuron number in the binocular region of the adult rat primary visual cortex (Oc1B), with males having 19% more neurons than females. In the present study, the role of developmental steroid hormones in the formation of this difference was explored. Male and female rats underwent neonatal hormone manipulation (female + testosterone or dihydrotestosterone; male + flutamide) followed by gonadectomy on postnatal day 20. Animals that did not undergo hormone manipulation were either gonadectomized or sham operated at day 20. Neuron number was quantified in the monocular (Oc1M) and binocular (Oc1B) subfields of the adult rat primary visual cortex using the optical disector technique. As adults, day 20 gonadectomized females, as well as females + testosterone and females + dihydrotestosterone, had significantly more neurons than intact females. There was no difference in neuron number between postnatal day 20 gonadectomized males, males + flutamide, and intact males. Also, intact males had significantly more neurons than intact females in both in Oc1M and Oc1B. It appears that ovarian steroids after day 20 are the primary cause of the lower number of neurons in the primary visual cortex of the female rat.

Androgen Antagonists↗
PubMed1994

Masked long-term potentiation in kitten visual cortex in vitro.

In slices from the visual cortex of kittens maintained in vitro, long-term potentiation of synaptic transmission following high frequency stimuli (10 Hz, 2 min) delivered at low to medium stimulus intensities (80 to 200 microA), is accompanied by changes of certain electrophysiological measures recorded intracellularly, such as long-lasting depolarization of membrane potential and decreased threshold to elicitation of an action potential. These parameters have never before been shown to be altered following high frequency stimulation in other systems widely used in studying synaptic plasticity, such as in hippocampal neurons. Another important difference between results from these two systems is that the amplitude of the excitatory post-synaptic potential is enhanced after high frequency stimulation in hippocampal neurons, whereas in striate cortex from young kittens, we observed a decrease. We demonstrate here that this decrease can be reversed to show enhancement from the original amplitude, upon clamp of membrane potential back to the voltage observed prior to stimulation. Thus, what appears to be "long-term depression" of synaptic transmission, as recorded extracellularly and represented by diminished flow of synaptic current, can be reversed by stepping membrane voltage back to the pre-high frequency stimulation level, to produce responses that then become consistent with long-term potentiation.

Animals↗
PubMedDate not supplied

Early development of visual cortex in human fetuses.

Prenatal development of visual cortex (area 17) was studied in human fetuses of 8-9, 13-15 and 16-18 weeks of gestation, with a view to analyse the early critical events. Under light microscope, five zones of development were seen in all the age groups. The total thickness of cortex of area 17 as well as that of its cortical plate was measured with the help of camera lucida. It was observed that the total thickness of the cortex increased with increase in age. Diversity in the shape, size, staining intensity and arrangement of neurons was noted in the different zones. Most of the cells were found to have a thin rim of cytoplasm and a prominent nucleus with multiple nucleoli. The cells in subventricular zone and cortical plate were regularly arranged in vertical rows while in other zones, they were irregularly scattered. Several mitotic figures were seen in the ventricular zone at 8-9 weeks but later they were also noticed in subventricular and intermediate zones. In the later ages the mitotic figures were observed to be fewer in the ventricular zone. No mitosis was seen in cortical plate at any age period.

Cell Division↗
PubMed1997

Relationships between local synaptic connections and orientation domains in primary visual cortex.

Combined optical imaging of ferret primary visual cortex in vivo and scanning laser photostimulation in brain slices were used to determine the spatial relationships between synaptic inputs onto individual neurons and the pattern of orientation columns. In the upper cortical layers, both excitatory and inhibitory inputs originated primarily from regions with orientation tuning similar to that of the recorded neurons; the shapes of the input tuning curves were indistinguishable. The orientation distributions of both types of inputs centered around the orientation of the recorded neurons, and no evidence for preferential cross-orientation inputs, either excitatory or inhibitory, was observed. These patterns of synaptic connectivity are most consistent with feedforward models for generation of orientation selectivity and are inconsistent with the patterns required by models based on cross-orientation inhibition.

Animals↗