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PubMed1995

A Bayesian framework for understanding texture segmentation in the primary visual cortex.

This paper presents a mathematical theory for understanding the computations involved in texture segmentation in the primary visual cortex. We propose that texture segmentation is a part of the early visual system's overall strategy to infer surfaces of objects in a visual scene. Based on this insight, we use the Bayesian inference paradigm to formulate the texture segmentation problem into a maximum a posteriori surface inference problem. The dynamical system for finding the optimal solution of this problem can be characterized by two concurrent and interactive processes: a gradual sharpening of the boundary signals and a simultaneous smoothing of the surface signals. The behavior of these dynamical processes was studied using both analytical and computational methods. We present some computational results and mathematical predictions. This theory suggests a novel framework for understanding the functional roles of the complex cells in the primary visual cortex.

Bayes Theorem↗
PubMed1983

Excitation of visual cortex neurons by local intracortical microstimulation.

The threshold current required for the excitation of visual cortex neurons in the vicinity (approximately 1 mm) of an intracortical stimulating electrode was measured as a function of the stimulus pulse duration in the anesthetized cat. For cortical neurons with latencies of activation from 0.4 to 3.4 ms and for stimulus pulse durations from 0.02 to 0.7 ms, the threshold current for all neurons tested decreased in an exponential fashion as the pulse width was increased. Rheobase current values (ampere-threshold) were 1.2 to 516 muA (mean 160 +/- 24 muA, N = 24) and chronaxie values were 0.07 to 0.79 ms (mean 0.217 +/- 0.036 ms, N = 24). When the quantity of charge required for neuronal excitation was calculated, a quasilinear relationship was found between threshold charge and stimulus pulse width. The minimum threshold charge (coulomb-threshold) occurred for the briefest pulse widths tested and were 2 to 86 nC (mean 36.4 +/- 4.4 nC, N = 24). When the pulse energy index was calculated (threshold current squared multiplied by the pulse width), the minimum pulse energy capable of generating an evoked response (a single action potential) occurred when the pulse width was approximately 80% greater than the chronaxie. These studies demonstrate that the predictions derived from A. V. Hill's classical theory of nerve excitation are to a first approximation obeyed by visual cortex neurons. For the three parameters analyzed as a function of stimulus pulse width, the pulse current is minimized at long pulse durations, the pulse charge is minimized at short pulse durations, and the pulse energy is minimized at pulse widths of intermediate value.

Animals↗
PubMed1994

Neocortical grafts receive functional afferents from the same neurons of the thalamus which have innervated the visual cortex replaced by the graft in adult rats.

Electrophysiological and anatomical studies were carried out in parallel to investigate the ability of lateral geniculate body neurons to regenerate axons damaged by the removal of the primary visual cortex and to innervate graft neurons functionally after transplantation of fetal neocortical tissue to a lesion cavity in the brain of adult rats. In electrophysiological experiments neurons of a large portion of the transplants (14/35) displayed visual responses with characteristics resembling closely those of normal primary visual cortex; these transplants also displayed a different degree of restoration of topographically organized visual field representations on them. To demonstrate anatomical regeneration of inputs from the host lateral geniculate body to the graft, injections of FluoroGold were made before grafting into the intact visual cortex for retrograde labeling of the lateral geniculate body neurons. After completion of the microelectrode recordings from the transplants a second dye, Bisbenzimide, was injected into the transplants. The rats with transplants whose neurons displayed responses to visual stimulations contained in the lateral geniculate body neurons with FluoroGold-labeled cytoplasm and Bisbenzimide-labeled nuclei. The presence of double-labeled neurons suggests that the same neurons, the axons of which have terminated in area 17 of the cortex, innervated the transplants functionally through the regeneration of damaged axons.

Animals↗
PubMed2002

Surround suppression in the human visual cortex: an analysis using magnetoencephalography.

The responses of neurons in the primate and cat primary visual cortices (V1s) to the stimuli within their classical receptive fields (CRFs) are markedly suppressed by the surrounding stimuli outside CRFs. In the present study, we show that a similar suppressive effect occurs for visually evoked magnetic responses in the human visual cortex. The initial peak amplitude of the magnetic response (at a latency of around 90 ms) to a test grating accompanied by high-contrast surround gratings was smaller than that for the test without the surround. Current source localization with a single dipole model indicated that the initial response originated from cortical activity near the occipital pole in the contralateral hemisphere to the visual stimulation. The peak amplitude for the test decreased with increasing surround contrast, and increased with increasing test contrast. The contrast dependence and the early development of the surround suppression were in agreement with the results of the V1 single-cell studies of monkeys and cats. We suggest that the surround suppression of the initial peak amplitude of the magnetic response may be ascribed to the inhibition of the neural activity at the early processing stage(s), presumably at V1, in the human visual cortex.

Adult↗
PubMed1989

Laminar distribution of benzodiazepine receptors in visual cortex of adult rat.

The characteristics and distribution of benzodiazepine receptors in individual layers of the visual cortex of adult rats were examined with the 3H-flunitrazepam binding technique employed on intact tissue slices. The different visual cortical layers were separated by cutting serial cryocut sections horizontally to the cortical surface and collecting the slices from each individual cortical layer under anatomical control. Highest benzodiazepine receptor densities were found in layers IV and VI. A moderate receptor density was detected in layer V (80% of highest density). The lowest receptor binding was observed in cortical layers I and II/III, still representing 66% of the highest receptor density. Binding affinities varied slightly between layers with dissociation constants somewhat higher for layers IV to VI in comparison to layers I and II/III. The distinct laminar pattern of benzodiazepine receptors in rat visual cortex suggests a differential neuromodulatory significance of these receptors in each individual cortical layer.

Animals↗
PubMed2003

[Effect of striate visual cortex topography on the parameters of image filtering].

A model of spatial-frequency filtering at the level of 4C layer of the striate visual cortex is proposed and based on the well-known literature data. The evidence on conformable character of representation of the visual field on the primary visual cortex and the suggestions concerning uniformity of horizontal connections of cortical neurones serve as the ground of the model. A correspondence between predictions of the model and results of the experiments with shape perception and size discrimination has been obtained.

Animals↗
PubMedDate not supplied

NMDA receptors in the visual cortex of young kittens are more effective than those of adult cats.

Acidic amino acids, such as glutamate and aspartate, are thought to be excitatory transmitters in the cerebral neocortex and hippocampus. Receptors for these amino acids can be classified into at least three types on the basis of their agonists. Quisqualate-preferring receptors and kainate-preferring receptors are implicated in the mediation of synaptic transmission in many regions including the hippocampus and visual cortex, whereas N-methyl-D-aspartate (NMDA)-preferring receptors are thought to be involved in modulating synaptic efficacy, for example in longterm potentiation, a form of synaptic plasticity in the hippocampus. In the visual cortex of the cat and monkey, it is well established that synaptic plasticity, estimated by susceptibility of binocular responsiveness of cortical neurons to monocular visual deprivation, disappears after the 'critical' period of postnatal development. Here we report that during the critical period in young kittens, a selective NMDA-receptor antagonist blocks visual responses of cortical neurons much more effectively than it does in the adult cat. This suggests that NMDA receptors may be involved in establishing synaptic plasticity in the kitten visual cortex.

Aging↗
PubMed1990

Protein kinase C activity and substrate (F1/GAP-43) phosphorylation in developing cat visual cortex.

Protein kinase C (PKC) and substrate proteins such as F1/GAP-43 have been previously implicated in the synaptic plasticity of long-term potentiation (LTP). As a first step in determining whether they participate in the plasticity observed during the critical period of visual cortex development, we have studied cytosol and membrane PKC activity as well as the endogenous phosphorylation of visual cortical proteins in cat cortical areas 17, 18 at postnatal days 1 and 3, weeks 1, 3, 5, 7, 9, 13, 28 and 51, and adult year 5. There was an 8.4 to 10.9 fold increase in cytosolic PKC activity relative to day 1 level during the critical period of synaptic plasticity (weeks 3-13) which then dramatically decreased back to 2.5-fold of day 1 level by week 51. This was near the adult level of cytosolic PKC. Since there was an increase of 1.8- to 2.1-fold in membrane PKC activity during the critical period, this argues against a PKC translocation event and for an increase in enzyme synthesis. Endogenous phosphorylation in the same visual cortex tissue revealed an increase in protein F1 phosphorylation during the critical period. This level of PKC substrate activity was maintained in the adult providing a mechanism for plasticity in adult cat visual cortex.

Aging↗
PubMed2005

Local connections to specific types of layer 6 neurons in the rat visual cortex.

Because layer 6 of the cerebral cortex receives direct thalamic input and provides projections back to the thalamus, it is in a unique position to influence thalamocortical interactions. Different types of layer 6 pyramidal neurons provide output to different thalamic nuclei, and it is therefore of interest to understand the sources of functional input to these neurons. We studied the morphologies and local excitatory input to individual layer 6 neurons in rat visual cortex by combining intracellular labeling and recording with laser-scanning photostimulation. As in previous photostimulation studies, we found significant differences in the sources of local excitatory input to different cell types. Most notably, there were differences in local input to neurons that, based on analogy to barrel cortex, are likely to project only to the lateral geniculate nucleus of the thalamus versus those that are likely to also project to the lateral posterior nucleus. The more striking finding, however, was the paucity of superficial layer input to layer 6 neurons in the rat visual cortex, contrasting sharply with layer 6 neurons in the primate visual cortex. These observations provide insight into differences in function between cortical projections to first-order versus higher-order thalamic nuclei and also show that these circuits can be organized differently in different species.

Animals↗
PubMed2005

Spontaneously emerging direction selectivity maps in visual cortex through STDP.

It is still an open question as to whether, and how, direction-selective neuronal responses in primary visual cortex are generated by feedforward thalamocortical or recurrent intracortical connections, or a combination of both. Here we present an investigation that concentrates on and, only for the sake of simplicity, restricts itself to intracortical circuits, in particular, with respect to the developmental aspects of direction selectivity through spike-timing-dependent synaptic plasticity. We show that directional responses can emerge in a recurrent network model of visual cortex with spiking neurons that integrate inputs mainly from a particular direction, thus giving rise to an asymmetrically shaped receptive field. A moving stimulus that enters the receptive field from this (preferred) direction will activate a neuron most strongly because of the increased number and/or strength of inputs from this direction and since delayed isotropic inhibition will neither overlap with, nor cancel excitation, as would be the case for other stimulus directions. It is demonstrated how direction-selective responses result from spatial asymmetries in the distribution of synaptic contacts or weights of inputs delivered to a neuron by slowly conducting intracortical axonal delay lines. By means of spike-timing-dependent synaptic plasticity with an asymmetric learning window this kind of coupling asymmetry develops naturally in a recurrent network of stochastically spiking neurons in a scenario where the neurons are activated by unidirectionally moving bar stimuli and even when only intrinsic spontaneous activity drives the learning process. We also present simulation results to show the ability of this model to produce direction preference maps similar to experimental findings.

Models, Neurological↗
PubMedDate not supplied

[Plastic properties of cat visual cortex neurons].

Prolonged changes in receptive fields of cells in the visual cortex were studied in different periods after ablation of projection connections of the cortex in the contralateral hemisphere. Cellular activity was recorded in non-anaesthetized immobilized cats in different periods (from 2 hours to 14 months) after ablation. It was found that 6-7 months after unilateral section of projection fibres in the cortex, photic sensitivity of neuronal elements markedly decreased in the field 17, whereas cells in fields 18 and 19 retained their responses to commonly used photic stimuli. The paper discusses the problem of different roles of retino-geniculo-cortical and retino-tecto-thalamo-cortical systems in the compensation of impaired visual function.

Animals↗
PubMedDate not supplied

Shaping of receptive fields in the visual cortex during retinal maturation.

We present a computational study of the formation of simple-cell receptive field patterns in the primary visual cortex. Based on the observation that the spatial frequency of the retinal filter increases postnatally, our results explain differences in the time course of the development of orientation selectivity in binocularly deprived and normally reared kittens. Development after eye-opening in normal animals is modelled by training with natural images, whereas in the case of binocular deprivation noise-like stimulation continues. Further, it is shown that different orientation selectivities are obtained for network models trained with natural images in contrast to random phase images of identical second order statistics. The latter finding suggests that higher-order statistics of the inputs influences development of primary visual cortex. Finally, we search for quantities that identify possible signatures of natural image statistics in order to specify the amount of constructiveness that visual experience has on the formation of receptive fields.

Aging↗
PubMed1989

Lateralization of opioid receptors in turtle visual cortex.

The opioid mu-agonist morphine, the delta-agonist [D-Ala2,D-Leu5]enkephalin (DADL) and the kappa-agonist bremazocine locally applied to the surface of turtle visual cortex inhibited the orthodromic evoked potential (EP; fast negative component N1). After application of the sigma-agonist SKF 10.047 the inhibition was followed by facilitation of EP. The lack of cross-desensitization to the inhibitory action of opioids upon EP indicates that the drugs exert their effects via different opioid receptors. Bremazocine and morphine predominantly inhibited the left cortical EP, whereas DADL was a potent inhibitor of the right cortical EP. Thus, the opioid receptors which modulate evoked electrical activity of the left visual cortex (LVC) apparently belong mostly to the kappa- and mu-type, while delta-receptors were predominantly responsible for the modulation of electrical activity in the right visual cortex (RVC). Besides, we have studied in vivo binding of the kappa-agonist [3H]ethylketocyclazocine ([3H]EKC) and the delta-agonist [3H]DADL to LVC and RVC. The binding was specific and could be accounted for by the interaction with membrane opioid receptors and/or specific uptake of these drugs by cortex cells. [3H]EKC and [3H]DADL exhibited a more effective binding to LVC and RVC membranes, respectively. The results obtained in vitro apparently indicate that LVC and RVC differ in the number of kappa- and delta-binding sites. Thus, turtle brain may have a side-specific mechanism for selective neurochemical regulation of neuron activity in LVC and RVC realized predominantly via kappa- and delta-receptors, respectively.

Animals↗
PubMed2002

Suppressive effects of receptive field surround on neuronal activity in the cat primary visual cortex.

Effects of sinusoidal grating stimulus presented outside the classical receptive field (CRF) on neuronal responses were studied in the primary visual cortex of anaesthetized cats. Among 101 cells electrophysiologically recorded, the predominant effect of the stimulus in the receptive field surround (SRF) was the suppression of responses to the CRF stimulation, and the SRF grating suppressed them up to 56% of the responses (44% suppression) to the CRF stimulus alone. The strong suppression was observed more often in layer II/III cells than in other layers and in complex cells more often than in simple cells. The modulatory effects by SRF stimulus might be enhanced by the cortical recurrent excitation particularly in the superficial layers. We also examined whether the modulation by the surround grating exhibits a differential effect according to the presence or absence of figure-ground segregation in the stimulus configuration. For this purpose, effects of stimulus configuration with orientation-, direction-contrast or relative spatial phase difference between CRF and SRF stimuli (figure-ground segregated configuration) were compared with those of uniform configuration of stimulus (non-segregated configuration). There was a population of cells, which exhibited significantly stronger suppression with non-segregated configuration than with figure-ground segregated configuration. Such differential modulation of response by the SRF stimulus in the primary visual cortex is a possible basis of perceptual figure-ground segregation.

Animals↗
PubMed2001

Abnormal activation in the visual cortex after corneal refractive surgery for myopia: demonstration by functional magnetic resonance imaging.

OBJECTIVE: To try to correlate subjective photophobic symptoms with visual pathway modifications (from the retinal image to the visual cortex) after refractive surgery by exploring brain activation on photic stimulation. DESIGN: Noncomparative case series. PARTICIPANTS: Four subjects reporting discomfort produced by luminance (glare, halos, starbursts, or a combination thereof) in one eye after laser in situ keratomileusis (LASIK) were enrolled. The contralateral myopic eye (control) had no visual impairment and had undergone LASIK without complications or had not had previous surgery. METHODS: Functional magnetic resonance imaging was performed during photic stimulation, delivered by an optical fiber, of the affected and unaffected eyes. RESULTS: Functional magnetic resonance imaging provided evidence that most subjective visual symptoms correlated with anatomic flap abnormalities are associated with a higher signal increase in the visual association cortices compared with a nonsymptomatic eye. CONCLUSIONS: Functional magnetic resonance imaging of the visual cortex may help in exploring the mechanisms involved in glare effects after refractive surgery.

Adult↗
PubMed2005

Quantitative morphology and postsynaptic targets of thalamocortical axons in critical period and adult ferret visual cortex.

Thalamocortical axons segregate into ocular dominance columns several weeks before the onset of critical period plasticity in ferret visual cortex, a stage characterized by anatomical changes in thalamic input as a consequence of abnormal visual stimulation. In search of possible anatomical correlates of this plasticity, we examined, at electron microscope resolution, the morphology and the synapsing and target selection properties of thalamic axons in ferret visual cortex during and after the critical period. Adult thalamocortical terminals visualized by anterograde tract-tracing display significantly larger cross-section areas than terminals at postnatal day (P) 35, P40, and P49 critical period ages. They are also significantly larger than nonthalamocortical terminals, which attain an adult-like size distribution by P40. The synaptic zones of adult thalamocortical terminals are significantly larger than those of critical period terminals. Perforated and invaginated synapses are encountered frequently on thalamic axons in both adulthood and the P40-49 age group. This result contradicts the view that synaptic perforations and spinules are indicative of a capacity for plasticity. It also suggests that at least some morphological features of thalamic terminals attain maturity on a developmental schedule that is independent of critical period plasticity. Connectivity properties of labeled axons, however, suggest an active role for thalamocortical axons in the critical period. In P40, P49, and adult brains, 23%, 17%, and 9%, respectively, of all thalamocortical synapses contact GABAergic interneurons, suggesting that thalamic input is more strongly involved in driving inhibitory circuits in young ages. Furthermore, thalamocortical axons in P35-49 brains form about 60% more synapses per axon length than in adult brains, suggesting that stabilization of thalamic synapses at the end of the critical period may be accompanied by a reduction of synaptic contacts, as well as a reorganization of postsynaptic circuit selectivity.

Age Factors↗
PubMed2000

Differential effects of neurotrophins on ocular dominance plasticity in developing and adult cat visual cortex.

In the present study we examine the influence of neurotrophins on experience-dependent synaptic rearrangement in developing and adult visual cortex. Brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF) was continuously infused into cortical area 18, and the functional architecture of the cortex was examined by use of optical and electrophysiological recording techniques. In kittens, BDNF infusion during monocular deprivation (MD) reversed the normally occurring ocular dominance (OD) shift towards the non-deprived eye so that the deprived eye dominated the BDNF-treated cortex after MD. Under conditions of equal activation of thalamocortical synapses, i.e. when animals were either subject to binocular deprivation (BD) or reared without deprivation, BDNF infusion did not disrupt binocularity of cortical units, but reversed the natural OD bias towards the contralateral eye in favour of the ipsilateral eye. In addition, BDNF treatment in kittens led to a loss of the orientation selectivity of cortical units irrespective of rearing conditions. In adult animals, BDNF influenced neither OD distributions nor orientation selectivity. The effect of NGF was markedly different. It was ineffective in kittens but in adult animals it caused a shift of OD towards the deprived eye when MD was combined with NGF infusion. However, in this case orientation selectivity was preserved. Thus, both neurotrophins have profound activity- and age-dependent effects on the functional architecture of the visual cortex. Moreover, our results indicate that simple substitution of neurotrophins in excess is unlikely to compensate for deprivation effects by preserving or restoring the normal functional architecture of the cortex.

Age Factors↗
PubMed2004

The primary visual cortex fills in color.

One of the most important goals of visual processing is to reconstruct adequate representations of surfaces in a scene. It is thought that surface representation is produced mainly in the midlevel vision and that area V1 (the primary visual cortex) activity is solely due to feedback from the midlevel stage. Here, we measured functional MRI signals corresponding to "neon color spreading": an illusory transparent surface with long-range color filling-in, one of the important mediums in reconstructing a surface. The experiment was conducted with careful controls of attention, which can send feedback signals from higher visual areas. Activity for filling-in was observed only in V1, whereas activity for illusory contours was observed in multiple visual areas. These results indicate that surface representation is produced by multiple rather than single processing.

Color↗