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PubMed1996

The development of topography in the visual cortex: a review of models.

The repetitive stochastic patterns of eye dominance and orientation preference found in the mammalian visual cortex have attracted much attention from theoretical neurobiologists during the last two decades. Reasons for this include the visually intriguing nature of the patterns and the fact that many aspects of their development seem likely to be dependent upon both spontaneous and visually driven patterns of neural activity. Understanding these processes holds out the promise that general theories of learning and memory may be derived from those found to be applicable to the visual cortex. It has turned out, in fact, that remarkably simple models, based on Hebbian synaptic plasticity, intracortical interactions and competitive interactions between cells and growing axons, have been able to explain much of the phenomenology. This article reviews the models of topographic organization in the visual cortex in a roughly historical sequence, beginning with von der Malsburg's paper 1973 paper in Kybernetik on self-organization of orientation selectivity. The principles on which each of the models is based are explained, and the plausibility of each model and the extent to which it is able to account for the relevant experimental data are evaluated. Attention is drawn to the underlying similarities and differences between the models and suggestions are made for future directions in research.

Journal Article↗

PubMed1994

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties.

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1993

Electrophysiological identification of horizontal synaptic connections in rat visual cortex in vitro.

The presence of intrinsic horizontal synaptic connections in rat visual cortex was explored electrophysiologically using in vitro slice preparations. Intracellular recordings were made from pyramidal neurons located in the superficial and deep layers. Electrical stimulation at the gray matter in the same or different layers but 0.8-2.7 mm apart from the recording site evoked compound synaptic potentials composed of excitatory and inhibitory postsynaptic potentials of fast and slow time courses. Glutamate blockers, DNQX (5 microM) and kynurenate (2 mM) reduced the excitatory postsynaptic potential (EPSP), and GABAB receptor antagonist, phaclofen (0.5 mM), abolished the inhibitory postsynaptic potential of the slow time course. EPSP of the fast time course followed 20 Hz repetitive stimulation in the medium of high Ca2+ (6.0 mM) and Mg2+ (4.0 mM) concentration, suggesting that this fast EPSP was monosynaptic. Conduction velocity of the fibers mediating the monosynaptic EPSP was estimated to be 0.15-0.55 m/s. These results provide physiological evidence for the horizontal synaptic connections in the rat visual cortex, which had been previously suggested by morphology.

Animals↗

PubMed1994

Receptive-field maps of correlated discharge between pairs of neurons in the cat's visual cortex.

1. To investigate the functional significance of temporally correlated discharge between nearby cells in the visual cortex, we obtained receptive-field maps of correlated discharge for 68 cell pairs in kittens and cats. Discharge from cell pairs was measured by a single extracellular electrode. A reverse correlation procedure was used to relate neural discharge to particular stimuli within a random sequence of briefly flashed bright and dark bars. Bicellular receptive fields (BRFs) were mapped by applying reverse correlation to approximately synchronous discharge from two cells. Unicellular receptive fields (URFs) were simultaneously mapped by separately applying reverse correlation to the discharge of each cell. 2. The receptive fields of the two neurons within each pair were initially studied by varying the orientation and spatial frequency of drifting sinusoidal gratings. After these tests a random sequence of appropriately oriented bars was used to evoke discharge suitable for reverse correlation analysis. For most cell pairs, the temporal pattern or strength of correlated discharge produced by such stimulation is different from that observed with stimulation by sinusoidal gratings. This indicates that visually evoked correlated discharge between nearby cells is stimulus dependent. 3. BRFs were classified according to their pattern of spatial sensitivity into three groups that roughly correspond to the single-cell receptive-field types of the lateral geniculate nucleus (LGN; center-surround) and visual cortex (simple and complex). These classifications were compared with the receptive-field types of the single cells within each pair. LGN-type and simple-type BRFs were only seen for pairs in which at least one of the cells was simple. Conversely, complex-type BRFs were only seen for pairs in which at least one of the cells was complex. 4. Because the reverse correlation procedure can be used to characterize the spatiotemporal receptive-field structure of simple cells, we were able to compare both the spatial and temporal properties associated with the URFs and BRFs of simple cell pairs. The spatiotemporal structure of the BRF of a simple-cell pair can largely be predicted on the basis of the two URFs. Although this prediction suggests the possibility that BRFs are stimulus artifacts, a shuffle procedure, in which multiple repetitions of random sequences were presented, verifies the neural origin of BRFs. BRFs emerge from specific neural pathways and are not simply a consequence of unicellular response preferences. 5. Five measures were derived from the reverse correlation analysis of simple-cell receptive fields: width, duration, optimal spatial and temporal frequency, and optimal velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1986

[Classification of neurons of the visual cortex of the guinea pig (Cavia porcellus). A Golgi study].

In the visual cortex of the guinea pig stellate neurons in L IV and neurons with short axons were studied with the Golgi-method. We found spiny multipolar neurons in L IV (stellate cells in L IV), large spinefree neurons (basket cells), spinefree neurons with vertical or horizontal axonal distribution, small neurons with dense local dendritic and axonal plexuses (neurogliaform cells), sparsely spined polarized neurons with ascending axons (Martinotti-cells), small sparsely spined cells in L IV, spinefree bipolar neurons with vertical axons (bipolar cells), neurons with chandelier axons (chandelier cells) and sparsely spined bitufted neurons with dense vertical axonal plexuses (double bouquet cells). The analysis of neurons in the visual cortex of the guinea pig was undertaken to establish their number and distribution. Comparing the neurons of two species of rodentia, the diurnal guinea pig and the night-active rat we could found a higher variability of neurons in the guinea pig and a higher diversity of neuronal structures, which allows a better differentiation of cell types (e.g. double bouquet cells).

Animals↗

PubMed1998

Effects of the metabotropic glutamate receptor antagonist MCPG on phosphoinositide turnover and synaptic plasticity in visual cortex.

The neurotransmitter glutamate, in addition to activating ligand-gated ion channels, also stimulates phosphoinositide (PI) hydrolysis in neurons by activating a group of G-protein-coupled metabotropic glutamate receptors (mGluRs). A role for mGluRs in synaptic plasticity originally was hypothesized based on the observation that the developmental decline in glutamate-stimulated PI turnover is well correlated with the decline in experience-dependent synaptic plasticity in visual cortex. Over the past few years, the compound alpha-methyl-4-carboxyphenylglycine (MCPG) has been widely used to test the role of PI-coupled mGluRs in a number of types of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), ocular dominance plasticity in visual cortex, and the neural plasticity underlying learning and memory. The conclusions of most of these studies were based on the assumption that MCPG blocks the actions of glutamate at PI-coupled mGluRs in the cerebral cortex. Here we show that this assumption is not valid in visual cortex. Although MCPG does antagonize the actions of the synthetic mGluR agonist 1S, 3R-aminocyclopentane-1,3-dicarboxylic acid, it fails to block PI turnover and changes in spike adaptation stimulated by glutamate, the endogenous mGluR ligand. In addition, we find that MCPG fails to block the NMDA receptor-dependent forms of LTP, LTD, and depotentiation in visual cortex.

Action Potentials↗

PubMed1988

Differential burst firing modes in neurons of the mammalian visual cortex in vitro.

The firing patterns of visual cortical neurons were studied by intracellular recording in in vitro guinea pig brain slices. On depolarization 57% of the cells exhibited tonic firing of action potentials while the remaining cells (43%) had a phasic component in their response. Phasic cells exhibited a large diversity in their burst characteristics as well as in the burst dependence on the membrane potential. Ionic conductances underlying burst generation appeared to be also diverse, thus bursting neurons in the visual cortex cannot be grouped in a single, homogeneous population.

Action Potentials↗

PubMed1997

Contrast dependence of contextual effects in primate visual cortex.

The responses of neurons in the visual cortex to stimuli presented within their receptive fields can be markedly modulated by stimuli presented in surrounding regions that do not themselves evoke responses. This modulation depends on the relative orientation and direction of motion of the centre and surround stimuli, and it has been suggested that local cortical circuits linking cells with similar stimulus selectivities underlie these phenomena. However, the functional relevance and nature of these integrative processes remain unclear. Here we investigate how such integration depends on the relative activity levels of neurons at different points across the cortex by varying the relative contrast of stimuli over the receptive field and surrounding regions. We show that simply altering the balance of the excitation driving centre and surround regions can dramatically change the sign and stimulus selectivity of these contextual effects. Thus, the way that single neurons integrate information across the visual field depends not only on the precise form of stimuli at different locations, but also crucially on their relative contrasts. We suggest that these effects reflect a complex gain-control mechanism that regulates cortical neuron responsiveness, which permits dynamic modification of response properties of cortical neurons.

Animals↗

PubMed2001

Independent parcellation of the embryonic visual cortex and thalamus revealed by combinatorial Eph/ephrin gene expression.

The visual cortex in primates is parcellated into cytoarchitectonically, physiologically, and connectionally distinct areas: the striate cortex (V1) and the extrastriate cortex, consisting of V2 and numerous higher association areas [1]. The innervation of distinct visual cortical areas by the thalamus is especially segregated in primates, such that the lateral geniculate (LG) nucleus specifically innervates striate cortex, whereas pulvinar projections are confined to extrastriate cortex [2--8]. The molecular bases for the parcellation of the visual cortex and thalamus, as well as the establishment of reciprocal connections between distinct compartments within these two structures, are largely unknown. Here, we show that prospective visual cortical areas and corresponding thalamic nuclei in the embryonic rhesus monkey (Macaca mulatta) can be defined by combinatorial expression of genes encoding Eph receptor tyrosine kinases and their ligands, the ephrins, prior to obvious cytoarchitectonic differentiation within the cortical plate and before the establishment of reciprocal connections between the cortical plate and thalamus. These results indicate that molecular patterns of presumptive visual compartments in both the cortex and thalamus can form independently of one another and suggest a role for EphA family members in both compartment formation and axon guidance within the visual thalamocortical system.

Animals↗

PubMed1999

Neuronal asymmetries in primary visual cortex of dyslexic and nondyslexic brains.

Dyslexic brains exhibit histologic changes in the magnocellular (magno) cells of the lateral geniculate nucleus, and consistent with these changes, dyslexics demonstrate abnormal visually evoked potentials and brain activation to magno-specific stimuli. The current study was aimed at determining whether these findings were associated with changes in the primary visual cortex with the prediction that magno components of this cortex would be affected. We measured cross-sectional neuronal areas in primary visual cortex (area 17) in dyslexic and nondyslexic autopsy specimens. There was a significant interaction between hemispheres and diagnostic category; ie, nondyslexic brains had larger neurons in the left hemisphere, whereas dyslexic brains had no asymmetry. On the other hand, cell layers associated with magno input from the lateral geniculate nucleus did not show consistent changes in dyslexic brains. Thus, there is a neuronal size asymmetry in favor of the left primary visual cortex in nondyslexics that is absent in dyslexic brains. This is yet another example of anomalous expression of cerebral asymmetry in dyslexia similar to that of the planum temporale, which in our view reflects abnormality in circuits involved in reading.

Adult↗

PubMed1982

The contribution of the cortical layers to the generation of the EEG: field potential and current source density analyses in the rabbit's visual cortex.

Intracortical spontaneous field potentials (EEG) were simultaneously recorded from the different layers of the rabbit's visual cortex using multielectrodes produced by thin-film technology. The signals were subjected to spectral analyses. Additionally, current source density analysis was applied to spontaneous delta waves. The results are based on 10 experiments. The spontaneous EEG of the rabbit's visual cortex consists mainly of slow delta waves. The spectral analytical studies of the delta band showed maximum power in layers I and II and in layers V and VI. Significant power minima were found in the middle layers IV and/or III. Essentially, the zones of power minima agree with the zones of minimum coherence and those zones where a phase reversal is observed. These properties of the intracortical spontaneous delta activity are best described as dipole-like. The spontaneous delta waves can be surface negative or surface positive; accordingly they show different source-sink distributions from which different generation mechanisms can be inferred. The possible mechanisms for the generation of these current source-sink distributions (dipoles) are discussed, taking into consideration the cortical anatomy. In many cases these discussions must be speculative because, for an unambiguous explanation of the observed phenomena, a much better knowledge of the cortical anatomy, the fibre connections with other cortical anatomy, the fibre connections with other cortical areas and with the deep cerebral structures is necessary.

Alpha Rhythm↗

PubMed1995

Physiological studies of visual cortex reorganization following cortical deafferentation in neonatal cats.

Whether restoration takes place in the visual cortex of neonates was physiologically studied in cortical cells of cats following their deafferentation. Deafferentation was performed by a parasagittal incision made in the visual cortex, separating the medial part of it from the thalamocortical and other visual fibers. Responsiveness (percentage of responsive cells) in the middle zone (the middle sector along the cortical incision) of the deafferented region was 82.5%, compared with 91.7% in the afferented (lateral to the incision) region (p = 0.5). In comparison, the responsiveness level was 32.3 and 81.3% (p < 0.05) in the respective zones of the similarly deafferented adult controls. The ocular dominance distribution and binocularity were almost normal in the deafferented region of the neonatally operated cats, whereas binocularity was remarkably diminished in the adult controls. Recovery was also found in the specificity of the cells to orientation and direction in the neonatally operated cats, but not in the adult-operated cats. Thus, functional reorganization of the columnar organizations takes place in the neonatally deafferented but not in the adult-operated cats.

Age Factors↗

PubMed1992

Rate coherence and event coherence in the visual cortex: a neuronal model of object recognition.

We propose a function-oriented model of the visual cortex. The model addresses an essential task of the visual system: to detect and represent objects. These are defined as sets, which reappear in the input with invariant inner relations. A network, incorporating an idealized description of anatomical and physiological data, is presented with a movie showing various moving objects. In the course of time, as a result of Hebbian plasticity, a connection scheme develops which embodies in its forward and lateral connections the information necessary to perform the operations involved in object recognition. We demonstrate that coherent neural activity can exploit this information. Two types of coherence have to be distinguished in this respect. Rate coherence performs invariance operations and association, while event coherence accomplishes segmentation tasks. The model reproduces and explains experimental findings made both in physiological recordings from the visual cortex and in psychophysical studies.

Animals↗

PubMed1997

T2 shortening in the visual cortex: effect of aging and cerebrovascular disease.

PURPOSE: To evaluate the effect of aging and cerebrovascular disease on T2 shortening in the visual cortex at MR imaging. METHODS: MR images of 72 neurologically normal subjects (45 men and 27 women, 35 to 92 years old) and 32 (13 men and 19 women, 54 to 92 years old) with cerebrovascular disease were evaluated retrospectively. On T2-weighted spin-echo images, the signal intensity of the visual, motor, and sensory cortices was divided into three grades and compared with the signal intensity of the frontal subcortical white matter. RESULTS: Decreased signal intensity (grade III) was rarely seen in the visual and sensory cortices of the neurologically normal subjects who were less than 60 years old. The signal intensity of the motor cortex decreased rapidly after the age of 50 years. At 61 to 70 years of age, 53% of these subjects had grade III intensity, and at age 71 years or older, 94% had reached grade III. The frequency of progression from grade I to grade III was lower in the visual cortex than in the motor cortex; 22% of these subjects had grade III appearance at age 61 to 70 years, and at age 71 years older, 56% had reached grade III. In patients with cerebrovascular disease who were older than 60 years of age, the frequency of grade III signal intensity in the visual cortex was almost equal to that in the neurologically normal subjects. CONCLUSIONS: T2 shortening in the visual cortex is frequently seen in neurologically normal older persons. These findings are compatible with a previously reported histochemical study of normal iron deposition in the visual cortex. Cerebrovascular disease has no effect on T2 shortening in the visual cortex.

Adult↗

PubMed1985

Substantial reduction of noradrenaline in kitten visual cortex by intraventricular injections of 6-hydroxydopamine does not always prevent ocular dominance shifts after monocular deprivation.

Ten kittens had cannulas inserted into their lateral ventricles for daily injections of 6-hydroxydopamine (6-OHDA). At 5-6 weeks of age one eye was sutured shut, and one week later recordings were made from the visual cortex to assay the ocular dominance of a sample of cells. In six kittens the injections of 6-OHDA were continued until the day before recording, while in four kittens the injections were stopped around the time of eye suture, on the assumption that continued injections of 6-OHDA over several days has effects that are not specific to the noradrenaline (NA) system and that the two procedures might show different results. In all animals the concentration of NA in the visual cortex near the site of recording was reduced by approximately 90%. In all animals the ocular dominance histograms recorded from the visual cortex were shifted so that the majority of cells (83 +/- 13%) were dominated by the open eye. There were no substantial differences between the two groups of experimental animals or between the experimental animals and two control animals that had cannulas implanted and ascorbate alone injected without 6-OHDA. We conclude that the concentration of NA in the visual cortex can be reduced substantially by injections of 6-OHDA into the lateral ventricle without preventing the shift in ocular dominance that usually occurs after suturing shut the eyelids of one eye.

Animals↗

PubMed1997

[Brain functional MRI of the visual cortex with echo planar imaging].

Brain functional MR imaging (fMRI) is a non invasive imaging method for detecting neural activity. We performed functional MRI of the visual cortex with gradient-echo echo planar imaging (GE-EPI) and spin-echo EPI (SE-EPI) using 1.5T MRI system. Visual stimuli was performed with a checkerboard patterns. Magnitude and temporal phase of correlation between each pixel's time-course and sine functions at the frequency of the stimulus was calculated. In all subjects, the activation area in visual cortex obtained from SE-EPI was smaller than that from GE-EPI. Temporal phase delay images from both GE-EPI and SE-EPI showed signal spread from the primary visual cortex to peripheral supplementary areas. Temporal phase analysis is important to discriminate the source of the hemodynamic response to neural activation in fMRI.

Echo-Planar Imaging↗

PubMed1990

Effect of the richness of the environment on neurons in cat visual cortex. I. Receptive field properties.

In a recent study, it was demonstrated that the number of synaptic contacts associated with flat vesicles (FS synapses) is higher in the visual cortex of cats raised in an enriched environmental condition (EC) compared to those reared in an impoverished condition (IC). Moreover, the size of the FS synaptic contacts is also affected by the richness of the animal's environment during development. Based on evidence that the vast majority of FS synapses are GABAergic (gamma-aminobutyric acid) and that many of the properties of visual cortex neurons are influenced by GABA-dependent mechanisms, it has been suggested that these morphological synaptic changes induced by the richness of the environment correlate with differences in cortical receptive field properties. In the present study, this has been explored by recording visual responses of area 17 cells in cats raised either in isolation (IC) or in a colony with ample environmental stimulation (EC). Enriched visual cortex contains a higher proportion of orientation selective cells and a lower proportion of orientation biased and unoriented cells. In addition, orientation tuning is significantly sharper in EC animals (mean bandwidth of responsive units is equal to 32 degrees) than in IC cats (mean bandwidth is equal to 38 degrees; P less than 0.001). This is mostly due to the greater incidence of orientation biased units in impoverished cortex (23% in EC and 41% in IC animals; P less than 0.01). Unit responsivity is significantly affected by the richness of the environment. We found that all units of the EC cortex were responsive to light stimuli. In contrast, 14% of the impoverished cells studied fail to increase their response to at least twice the standard deviation of the spontaneous activity and were judged as unresponsive. We suggest that the lower responsivity in IC visual units is related to the higher number of GABAergic synapses per IC neuron, while the broader selectivity in IC cortex might be due to a more diffuse distribution of the GABAergic inhibitory connections.

Animals↗

PubMed2002

Immediate early gene expression in the visual cortex of normal and dark reared cats: differences between fos and egr-1.

Northern blotting indicated that the level of induced fos was higher in normal than dark reared cat visual cortex (VC) at 5 weeks of age, comparable at 10 weeks, and higher in dark reared than normal cat visual cortex at 20 weeks. Fos egr-1, induction was higher in dark reared visual cortex at all ages. Thus, dark rearing has similar effects on fos, but not egr-1, expression as it has on physiological neuronal plasticity during the VC critical period.

Animals↗