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PubMed1985

Evidence for active synapse formation or altered postsynaptic metabolism in visual cortex of rats reared in complex environments.

Animals placed in complex environments develop greater numbers of visual cortex synapses per neuron than animals housed in standard cages. Increased numbers of synapses could theoretically arise from (i) active formation of new synapses, or (ii) selective stabilization of constitutively produced synapses. The postsynaptic location of polyribosomal aggregates appears to be an indicator of newly forming synapses. In developmental synaptogenesis and adult reactive (to injury) synaptogenesis, polyribosomes are more frequently found at spine synapses and are more likely to appear in the spine head and stem. In the visual cortex of rats from complex environments, there was a greater frequency of spine synapses associated with polyribosomes, relative to rats from individual or group cages. Furthermore, a greater percentage of these spines had polyribosomes in the head and stem region. This suggests that synapses in this region may be actively induced by neural activity arising from the complex environment experience.

Animals↗

PubMed2000

Brain-derived neurotrophic factor causes cAMP response element-binding protein phosphorylation in absence of calcium increases in slices and cultured neurons from rat visual cortex.

Neurotrophins play a crucial role in the developmental plasticity of the visual cortex, but very little is known about the cellular mechanisms involved in their action. In many models of synaptic plasticity, increases in cytosolic calcium concentration and activation of the transcription factor cAMP response element-binding protein (CREB) are crucial factors for the induction and maintenance of long-lasting changes of synaptic efficacy. Whether BDNF modulates intracellular calcium levels in visual cortical neurons and the significance of this action for BDNF signal transduction is still controversial. We investigated whether CREB phosphorylation and calcium changes are elicited by acute BDNF presentation in postnatal visual cortical slices and cultures. We found that BDNF did not cause any calcium increase, but it induced robust CREB phosphorylation in neurons from both preparations. We further analyzed signal transduction and its dependency on calcium changes in cultured neurons. CREB phosphorylation required trkB activation because treatment with the trk inhibitor k252a completely blocked CREB phosphorylation. In agreement with the imaging experiments, we verified that calcium changes were not necessary for CREB activation because preincubation with BAPTA-AM did not diminish the level of CREB phosphorylation induced by BDNF stimulation. CREB phosphorylation was accompanied by gene expression, because we observed the upregulation of c-fos expression, which was also not affected by preincubation with BAPTA-AM. Finally, BDNF caused phosphorylation of mitogen-activated protein kinase (MAPK), and because the treatment with the MAPK inhibitor U0126 completely abolished CREB activation and c-fos upregulation, it is likely that both processes depend mainly on the MAP kinase pathway. These results indicate that MAPK and CREB, but not intracellular calcium, are important mediators of neurotrophin actions in the visual cortex.

Activating Transcription Factor 2↗

PubMed2000

Brain-derived neurotrophic factor expands ocular dominance columns in visual cortex in monocularly deprived and nondeprived kittens but does not in adult cats.

Segregation and stabilization of thalamocortical afferents to eye-specific patches, so-called "ocular dominance (OD) columns," in visual cortex are hypothesized to be based on activity-dependent competition for trophic factors such as brain-derived neurotrophic factor (BDNF) between afferents representing the two eyes during the critical period of postnatal development. To test this hypothesis we observed effects of an intracortical infusion of BDNF on OD columns in monocularly deprived kittens and also compared effects between normal kittens and adult cats. BDNF had a hypertrophic action on afferents irrespective of visual inputs so that it desegregated OD columns in the visual cortex of deprived and normal kittens, but this action was not seen in the adults, substantiating its hypothesized trophic role in plasticity of OD columns in the developing visual cortex.

Animals↗

PubMed1982

[Dynamics of the receptive fields of visual cortex and lateral geniculate body neurons in the cat].

The dynamics of receptive fields of the visual cortex and lateral geniculate neurons were investigated in acute experiments on unanesthetized immobilized cats. For this purpose a computer presented small local flashes in a random order to 100 points of the receptive field and changes in three-dimensional relief of activity within the field were evaluated every 20 ms. Regular dynamic reorganizations of registered receptive fields of all investigated neurons were observed both after switching on and off the light stimulus. After the latency a small region of weak responses appeared in the centre of field. Later on the recorded field enlarged gradually, reaching maximal size at 60-100 ms, and after that began to narrow and disappeared or fell to pieces. The central inhibitory zone of the field had the same dynamics. If the neuron responded to stimulation by generating two bursts of spikes, during the second one the receptive field changed in the same way. The effect was revealed under various conditions of light stimulation. Functional significance of these effects for orientational information coding by visual cortex neurons is discussed.

Adaptation, Physiological↗

PubMed1981

Organization of direction preferences in cat visual cortex.

Single unit recordings were made from the visual cortex of 5 adult cats. Visual stimuli were used to determine the stimulus orientation and direction of movement preferred by cortical cells. Analysis of the sequence of neurons recorded along each electrode penetration and their direction preferences indicates that neurons preferring similar directions of movement are clustered together in the cortex.

Animals↗

PubMed1990

Correlations between morphology and electrophysiology of pyramidal neurons in slices of rat visual cortex. II. Electrophysiology.

The aim of this study was to determine whether the different morphological classes of pyramidal neurons in layers 2/3 and 5 of rat visual cortex (Larkman and Mason, 1990) have particular electrophysiological properties. Neurons in in vitro slices of rat visual cortex were impaled with glass micropipettes containing horseradish peroxidase (HRP) and studied using current-clamp techniques prior to pressure injection of HRP into the neurons. On morphological grounds, cells stained in layer 2/3 were placed into a single class whereas layer 5 cells were divided into 2 classes. Cells in one of these classes had thick apical dendrites which arborized in layer 1, whereas the apical dendrites of cells in the other class were thinner and did not reach layer 1 (Larkman and Mason, 1990). Despite variation between individual cells of a single class, significant differences were found in the time constants, current/voltage relations, and repetitive firing behaviors of the 3 classes. Burst firing responses to injected current pulses were confined to the layer 5 cells with thick apical dendrites. These results add to those from other areas of the brain demonstrating that the electrophysiological properties of pyramidal neurons are heterogeneous. Furthermore, we have shown that distinctive intrinsic membrane properties of pyramidal neurons in visual cortex are correlated with different morphologies.

Action Potentials↗

PubMed2002

Gene expression patterns during enhanced periods of visual cortex plasticity.

During a critical period in its postnatal development the mammalian visual cortex displays susceptibility to experience-dependent alterations of neuronal response properties. Plasticity represents an integrated set of developmental processes controlled by a transcriptional hierarchy that coordinates the action of many genes. To illuminate the expression of these critical genes, we examined gene expression patterns of 18371 non-redundant cDNAs in the visual cortex of cats at birth, at eye opening, at the peak of the critical period of eye dominance plasticity and in the adult cat using filter-based cDNA arrays and software-based hierarchical cluster analysis. We identified a small set of genes that were selectively expressed during the peak of the critical period for plasticity. We further examined the patterns of expression of these genes by analyzing the gene expression pattern of dark-reared chronologically older animals that are known to retain this ocular dominance plasticity beyond the chronologically defined critical period. This additional cluster assessment allowed us to separate age-related changes in the patterns of gene expression from plasticity-related changes, thus identifying a subset of genes that we define as plasticity candidate genes. Those plasticity candidate genes that have previously characterized functions include participants in second messenger systems, in cell adhesion, in transmitter recycling and cytokines, among others. Comparison of cDNA array quantitation with reverse transcription-polymerase chain reaction showed almost identical expression profiles for three genes that we examined. The expression pattern of one identified gene, opioid binding cell adhesion molecule, from the cDNA array analysis, is also in agreement with immunocytochemical results. We conclude that the approach of high-density cDNA array hybridization can be used as a useful tool for examining a complex phenomenon of developmental plasticity since it is amenable to multiple developmental stage gene expression comparisons.

Animals↗

PubMed1992

Light regulates expression of brain-derived neurotrophic factor mRNA in rat visual cortex.

Specific sensory input has profound transient and long-lasting effects on the function of corresponding sensory cortical areas both during development and in adulthood. To study whether neurotrophic factors might play a role in such processes, we investigated the effects of light on the nerve growth factor and brain-derived neurotrophic factor (BDNF) mRNA levels in rat visual cortex. Keeping adult rats in the dark or preventing normal activity of retinal ganglion cells by intraocular injection of tetrodotoxin significantly decreased the levels of BDNF mRNA in the visual cortex but not in other cortical areas. Exposure to light after a period in darkness rapidly restored the mRNA to control levels. These alterations in visual input had no effect on nerve growth factor mRNA. The mRNA of trkB, the putative signal-transducing receptor unit for BDNF, was also decreased in darkness, although less than BDNF mRNA. BDNF mRNA levels increased in the visual cortex of newborn rats after eye-opening. This increase is retarded, although not completely abolished, by rearing the pups in darkness. Thus, the levels of BDNF mRNA are rapidly regulated by sensory input during development and in adulthood. BDNF may therefore play an important role in formation and in activity-dependent modulation of specific connections in the visual cortex.

Animals↗

PubMed1981

[Number of neurons and synapses in the visual cortex of different species].

The number of neurons under 1 mm2 of visual cortex (area 17) is about 200 000 in monkey and man, and it varies between 45 000 and 70 000 in non-primates which have been studied. The number per hemisphere increases with the surface of area 17, passing from less than 1 million in mouse to about 538 million in man. The number of synapses under 1 mm2 of visual cortex has been estimated by different authors at between 480 million (mouse) and 1270 million (rat) : the number per hemisphere increases with brain size from 32 billion in rat to 3 084 billion (x10(9)) in man. The number of synapses per neurons tends to be higher in species with fewer neurons per mm3. Our laminar study in monkey shows this correlation at the level of each lamina : those having the largest number of neurons per mm3 have the least number of synapses per neuron.

Animals↗

PubMed1996

Electrophysiological elucidation of pathways of intrinsic horizontal connections in rat visual cortex.

Cortical neurons receive synaptic inputs through both vertical and horizontal pathways. We made a systematic survey of the synaptic strength and intracortical pathways of intrinsic horizontal connections in rat visual cortex using intracellular recordings from alice preparations. Excitatory postsynaptic potentials were recorded from pyramidal neurons of layers 2/3 and layers 5/6 in response to electrical shocks applied to these layers at a lateral distance of 1.0 mm from the impaled neuron or to the underlying white matter. When the threshold intensity of stimulation to activate monosynaptic excitatory postsynaptic potentials was compared, the vertical input had a lower threshold than the horizontal inputs. The threshold intensity of the horizontal inputs was lower, and the amplitude of the responses was larger in sagittally sectioned slices than in coronal slices, suggesting that the horizontal synaptic connection in the rat visual cortex was stronger in the rostrocaudal than in the mediolateral direction. Tetrodotoxin puffs focally applied to gray matter between the stimulation and the recording sites caused a transient depression of excitatory postsynaptic potentials, which was selective to the input conveyed through the puffed area. This pharmacological dissection revealed that routes parallel to the cortical Iaminae in the same layer as the stimulation site mediated the largest part of excitation conduction of intrinsic connections. Obliquely ascending routes mediated almost half of all the detected inputs originating from a deep layer to the neurons in either layers 5/6 or layers 2/3, whereas the contribution of obliquely descending routes from layers 2/3 to layers 5/6 was small (25%). Our results present semi-quantitative data on the connection diagram of the intrinsic neuronal circuits in the rat visual cortex, which will provide the basis for further investigations of the roles of the intrinsic connections in information processing in rodent cerebral cortex.

Action Potentials↗

PubMed1979

An analysis of the variability of unit activity in the cat's visual cortex.

The spontaneous firing of single units in the cat's visual cortex, and their responses to repeated presentations of a constant visual stimulus, have been analysed quantitatively. Although the responses of most cells showed some habituation to the stimulus, this was less extensive than the fluctuations in responsiveness which occurred from one set of presentations to the next, over the course of several minutes. Fluctuations occurred largely independently in both the spontaneous and the stimulus-evoked firing of the cell. This suggests that separate mechanisms determine spontaneous and evoked firing. Quantitative estimates of the reliability (the reciprocal of the variability) of responses were higher if the peak firing rate of the cell was measured, rather than the total number of spikes fired by the cell. Thus, peak firing rate is more likely than total spike count to be the signal of biological significance.

Animals↗

PubMed2002

Magnetic responses of human visual cortex to illusory contours.

To examine the neural mechanism underlying illusory-contour perception, we measured the magnetic responses of the human visual cortex to an abutting-line grating inducing illusory contours (test stimulus) and a non-abutting-line grating (control stimulus) using the technique of magnetoencephalography (MEG). In the initial latency period of 60-80 ms, the MEG response to the test stimulus was nearly identical with that to the control stimulus, but in the subsequent period of 80-150 ms, the former was larger than the latter. The origin of the peak MEG response to the test stimulus was estimated to be in the vicinity of striate cortex/extrastriate visual cortex for two of the four subjects. These results suggest that, in accord with those of the previous electrophysiological and functional magnetic resonance imaging studies, illusory-contour signals are generated in the very early stage(s) of processing in the primate visual cortex.

Adult↗

PubMed1985

Noradrenaline and functional plasticity in kitten visual cortex: a re-examination.

A quantitative re-examination was made of the influence of noradrenergic depletion on the epigenesis of kitten visual cortex. Two methods were used to deplete noradrenaline at the cortical level: stereotaxically controlled injection of 6-hydroxydopamine (6-OHDA) in the coeruleus complex, from which the noradrenergic input to visual cortex arises; intraventricular injection of 6-OHDA. The latter chemical lesion also depleted dopamine levels in the brain. Lesion of the noradrenergic or catecholaminergic systems was performed neonatally or at an age of 3-4 weeks in kittens submitted to five different rearing procedures: normal rearing, dark rearing, monocular rearing, monocular exposure following dark rearing and monocular deprivation following normal rearing. Forty-two kittens between 3 and 12 weeks of age were used for this biochemical and electrophysiological study. Noradrenaline and dopamine levels were measured by a radioenzymatic method in the primary visual cortex of twenty-six kittens. A total of 1263 cells were recorded in area 17 of twenty-six kittens. Combined biochemical and electrophysiological data were obtained in ten 6-OHDA-lesioned kittens. Whatever the mode of chemical lesion used, cortical noradrenergic depletion failed to block either maturation or vision-dependent processes which are known to affect orientation selectivity and/or ocular dominance during the critical period. However, in some cases, the amplitude of the epigenetic functional modifications was slightly reduced in 6-OHDA-treated kittens. The cortical effects of monocular deprivation starting from the age of 5 weeks were studied quantitatively both in lesioned and intact kittens. Disappearance of noradrenaline in area 17 did not prevent the loss of binocularity in cortical cells. However, even when monocular occlusion had been maintained for 2 or 3 weeks in 6-OHDA-treated kittens, ocular dominance shifts were limited to a stage equivalent to that observed in the intact kitten after 5-8 days of monocular occlusion. The amplitude of this partial protective effect was found to be unrelated either to the delay following the chemical lesion, or to the level of noradrenaline remaining in lesioned kitten cortex. Although a putative gating role of noradrenaline cannot be excluded in the development of the intact animal, this report shows that its presence is not required for functional plasticity to occur in kitten area 17.

Animals↗

PubMed1996

Distribution of acetylcholinesterase in the developing visual cortex of neonatally hemidecorticate rats.

The present study investigated the postnatal establishment of the laminar pattern of acetylcholinesterase (AChE) activity in the visual cortex (Oc1) of normal and neonatally hemidecorticate rates. Rat pups received a hemidecortication on post-natal day (PND) 3 and sacrificed at three day intervals starting at PND-6 through PND-24. Laminar patterns of AChE activity in Oc1 are described qualitatively and quantitatively using optical densitometry. The postnatal development of the laminar distribution of AChE activity is similar in normal and hemidecorticate rats. In both cases, AChE activity is intense in layer I, in the deep layer III as well as in layer IV. This pattern is first detected at the end of the first postnatal week. AChE activity reaches a peak intensity during the third postnatal week and gradually declines to adult levels during the fourth postnatal week. Hemidecortication has no significant effect on the intensity of AChE activity measured in the visual cortex. Neonatal hemidecortication does not affect AChE activity levels, structure of AChE neurites or the laminar distribution pattern, nor does it affect the time course of the establishment of this pattern in Oc1 of the remaining cortex. These data do not support the hypothesis that massive cortical lesions in rats result in an increase in contralateral cholinesterase activity nor do they suggest terminal sprouting of basal forebrain projections to the visual cortex.

Acetylcholinesterase↗

PubMed1996

The lateral posterior-pulvinar complex modulation of stimulus-dependent oscillations in the cat visual cortex.

It has been suggested that binding coherent targets depends on the capacity of excited cortical cells to fire in synchrony at approximately 40 Hz. However, the origin of stimulus-related cortical oscillations is still not clear. We hypothesized that 40 Hz oscillations might propagate to the visual cortex from the lateral posterior-pulvinar complex (LP-P) whose cells send fibers to the visual cortex and have a tendency to exhibit oscillations. To test our hypothesis, we recorded single unit activity in areas 17 and 18 of anaesthetized cats. The activity of neurons which showed oscillations evoked by optimal visual stimuli was analysed before, during and after a reversible inactivation of the LP-P with GABA. Such inactivation was found to markedly modify the strength of oscillatory activity of cortical neurons whose visual responses were affected by LP-P blockade. In contrast, the oscillation frequencies of cortical neurons were not modified by such inactivation. However, in some cells (three of nine), oscillatory activity was found to be completely abolished by injection of GABA into the LP-P. Collectively, these findings demonstrate that inputs from the LP-P play a key role in modulating the oscillatory activity of visual cortex neurons. Assuming that cortical neurons utilize oscillatory activity to encode perceptual aspects of the visual stimulus, our findings underscore the contribution of the LP-P in this process.

Action Potentials↗

PubMedDate not supplied

The distribution of calcium-binding proteins in the lateral geniculate nucleus and visual cortex of a New World monkey, the marmoset, Callithrix jacchus.

Antibodies directed against the calcium-binding proteins, parvalbumin and calbindin, can be used to label distinct neuronal subgroups in the primate visual pathway. We analyzed parvalbumin immunoreactivity (P-IR) and calbindin immunoreactivity (C-IR) in the lateral geniculate nucleus (LGN) and visual cortex of the marmoset, Callithrix jacchus. We compared marmosets which were identified as having dichromatic or trichromatic color vision. Within the LGN, the density of P-IR neurones is highest in the parvocellular and magnocellular laminae, but C-IR neurones are found mainly in the koniocellular division of the LGN, that is, the interlaminar zones and S laminae. Not all interlaminar zone cells are C-IR. In the visual cortex, P-IR neurones are present in all laminae except lamina 1, in areas V1 and V2. Neurones which are strongly C-IR are mainly located in laminae 2 and 3 in V1 and V2. Lightly C-IR neurones are concentrated in lamina 4, and are more numerous in V1 than in V2. Quantitative analysis showed no differences in the density or distribution of IR neurones in either LGN or visual cortex when dichromat and trichromat animals were compared. We conclude that this functional difference is not associated with differences in the neurochemistry of calcium-binding proteins in the primary visual pathways.

Animals↗

PubMed1996

Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex.

Long-range spatial interactions in human visual cortex were explored using a lateral masking paradigm. Visual evoked potentials (VEPs) elicited by a Gabor signal presented in isolation or in the presence of two flanking high-contrast Gabor signals (masks) were measured. Response amplitude and phase were recorded for a vertically oriented test, for horizontal and vertical masks and for combinations of vertical tests and vertical or horizontal masks. The amplitudes and phases of the test alone and mask alone responses were added coherently to predict the amplitude for collinear and orthogonal lateral masking conditions. Additivity failures were taken as evidence for neural interactions. At a target-to-mask distance of 2 deg, VEP amplitude exceeded the linear prediction for test contrasts in the range of 8-16% for the collinear, co-axial target/mask combination. Measured response phase also led predicted response phase over the same range of contrast. The VEP amplitudes were less than the linear prediction in the orthogonal target/mask combination and measured response phase lagged the predicted phase. Significant facilitation occurred with collinear test/mask combinations up to at least 3 deg of separation (nine wavelengths). Co-oriented, but non-collinear test/mask combinations (oblique test and mask, horizontal test and mask) did not produce facilitation. Contrast gain thus appears to be set over considerable distances in a configuration-specific fashion.

Contrast Sensitivity↗

PubMed1997

Information in the neuronal representation of individual stimuli in the primate temporal visual cortex.

To analyze the information provided about individual visual stimuli in the responses of single neurons in the primate temporal lobe visual cortex, neuronal responses to a set of 65 visual stimuli were recorded in macaques performing a visual fixation task and analyzed using information theoretical measures. The population of neurons analyzed responded primarily to faces. The stimuli included 23 faces and 42 nonface images of real-world scenes, so that the function of this brain region could be analyzed when it was processing relatively natural scenes. It was found that for the majority of the neurons significant amounts of information were reflected about which of several of the 23 faces had been seen. Thus the representation was not local, for in a local representation almost all the information available can be obtained when the single stimulus to which the neuron responds best is shown. It is shown that the information available about any one stimulus depended on how different (for example, how many standard deviations) the response to that stimulus was from the average response to all stimuli. This was the case for responses below the average response as well as above. It is shown that the fraction of information carried by the low firing rates of a cell was large--much larger than that carried by the high firing rates. Part of the reason for this is that the probability distribution of different firing rates is biased toward low values (though with fewer very low values than would be predicted by an exponential distribution). Another factor is that the variability of the response is large at intermediate and high firing rates. Another finding is that at short sampling intervals (such as 20 ms) the neurons code information efficiently, by effectively acting as binary variables and behaving less noisily than would be expected of a Poisson process.

Action Potentials↗