Mammalian Visual Cortex Research Articles

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

Imprecise correlated activity in self-organizing maps of spiking neurons

How neurons communicate with each other to form effective circuits providing support to functional features of the nervous system is currently under debate. While many experts argue the existence of sparse neural codes based either on oscillations, neural assemblies or synchronous fire chains, other studies defend the necessity of a precise inter-neural communication to arrange efficient neural codes. As it has been demonstrated in neurophysiological studies, in the visual pathway between the retina and the visual cortex of mammals, the correlated activity among neurons becomes less precise as a direct consequence of an increase in the variability of synaptic transmission latencies. Although it is difficult to measure the influence of this reduction of correlated firing precision on the self-organization of cortical maps, it does not preclude the emergence of receptive fields and orientation selectivity maps. This is in close agreement with authors who consider that codes for neural communication are sparse. In this article, integrate-and-fire neural networks are simulated to analyze how changes in the precision of correlated firing among neurons affect self-organization. We observe how by keeping these changes within biologically realistic ranges, orientation selectivity maps can emerge and the features of neuronal receptive fields are significantly affected.

Visual Neuroscience: Hypercomplex Cells in the Arthropod Visual System

Newly described visual interneurons in flies have sophisticated receptive field properties reminiscent of neurons in the mammalian visual cortex. The cells are well-suited to compute motion of conspecific females that male flies aerially intercept.

Experience-Driven Axon Retraction without Binocular Imbalance in Developing Visual Cortex

Refinement of the neural circuit during brain maturation is regulated by experience-driven neural activity. In the mammalian visual cortex, monocular visual deprivation (MD) in the early postnatal life causes a significant loss of cortical responses to a deprived eye and the retraction of input axons serving the deprived eye. A competitive interaction between inputs serving both eyes has been supposed to underlie the effects of MD because the loss of cortical response is much weaker when both eyes are deprived of vision. Also, the input axons do not retract after binocular deprivation. Here, we report that uncorrelated activity between presynaptic and postsynaptic neurons can solely lead to the retraction of geniculocortical axons in the absence of activity imbalance between two inputs. We analyzed the morphology of geniculocortical axons in a pharmacologically inhibited visual cortex of animals with normal vision and of binocularly deprived animals. In the normal vision animals, the axonal arbors in the inhibited cortex showed robust retraction. On the other hand, the arbors in binocularly deprived animals remained mostly intact. These results suggest that a homosynaptic associative mechanism, rather than a heterosynaptic competition between inputs, may play an important role in experience-driven axon retraction.

Imprinting modulates processing of visual information in the visual wulst of chicks

BackgroundImprinting behavior is one form of learning and memory in precocial birds. With the aim of elucidating of the neural basis for visual imprinting, we focused on visual information processing.ResultsA lesion in the visual wulst, which is similar functionally to the mammalian visual cortex, caused anterograde amnesia in visual imprinting behavior. Since the color of an object was one of the important cues for imprinting, we investigated color information processing in the visual wulst. Intrinsic optical signals from the visual wulst were detected in the early posthatch period and the peak regions of responses to red, green, and blue were spatially organized from the caudal to the nasal regions in dark-reared chicks. This spatial representation of color recognition showed plastic changes, and the response pattern along the antero-posterior axis of the visual wulst altered according to the color the chick was imprinted to.ConclusionThese results indicate that the thalamofugal pathway is critical for learning the imprinting stimulus and that the visual wulst shows learning-related plasticity and may relay processed visual information to indicate the color of the imprint stimulus to the memory storage region, e.g., the intermediate medial mesopallium.

Functional degradation of visual cortical cells in aged rats

Functional degradation of mammalian visual cortex is associated with aging. It has been hypothesized that much of the decline might be mediated by a degradation of cortical inhibitory system during senescence. In the present work, we compared the properties of adaptation, onset latency and signal-to-noise ratio in primary visual cortex of young and old rats using extracellular single-unit techniques. The short-term synaptic plasticity of young and old rats was also studied using field potential recording techniques. We found significant increased adaptation, prolonged onset latency, lower signal-to-noise ratio and decreased short-term synaptic plasticity in aged rats. The results are in accordance with previously reported functional declines in old monkeys and old cats, indicating a universal mechanism of degradation in cortical function that accompanies old age in different mammalian species.

Visual Cortex Inspired Intelligent Contour Detection

The present paper proposes a model for intelligent image contour detection. The model is strongly based on the architecture and functionality of the mammalian visual cortex. A pixel-to-feature transformation is performed on the input image, the result of which is a set of abstract image features, instead of another set of pixels. The contouring task is performed by a vast and complex network of simple units of computation that work together in a parallel way. The use of a large number of such simple units allows a clear structure that can be implemented on a special hardware to allow constant time computation.

Subplate Neurons Regulate Maturation of Cortical Inhibition and Outcome of Ocular Dominance Plasticity

Synaptic plasticity during critical periods of development requires intact inhibitory circuitry. We report that subplate neurons are needed both for maturation of inhibition and for the proper sign of ocular dominance (OD) plasticity. Removal of subplate neurons prevents the developmental upregulation of genes involved in mature, fast GABAergic transmission in cortical layer 4, including GABA receptor subunits and KCC2, and thus prevents the switch to a hyperpolarizing effect of GABA. To understand the implications of these changes, a realistic circuit model was formulated. Simulations predicted that without subplate neurons, monocular deprivation (MD) paradoxically favors LGN axons representing the deprived (less active) eye, exactly what was then observed experimentally. Simulations also account for published results showing that OD plasticity requires mature inhibition. Thus, subplate neurons regulate molecular machinery required to establish an adult balance of excitation and inhibition in layer 4, and thereby influence the outcome of OD plasticity.

Development and plasticity‐related changes in protein expression patterns in cat visual cortex: A fluorescent two‐dimensional difference gel electrophoresis approach

During early postnatal brain development, changes in visual input can lead to specific alteration of function and connectivity in mammalian visual cortex. In cat, this so-called critical period exhibits maximal sensory-driven adaptations around postnatal day 30 (P30), and ceases toward adulthood. We examined the molecular framework that directs age- and experience-dependent plasticity in cat visual cortex, by comparing protein expression profiles at eye opening (postnatal day 10 (P10), when experience-dependent plasticity starts), the peak of the critical period (P30), and in adulthood. Using 2-D DIGE, we performed comparisons of P10-P30 and P30-adult brain protein samples. Sixty protein spots showed statistically significant intensity changes in at least one comparison. Fifty-one spots were identified using quadrupole-TOF MS/MS or LC-MS/MS, containing 37 different proteins. The progressive increase or decrease in protein expression levels could be correlated to age-dependent postnatal brain development. Four spots containing transferrin, 14-3-3 alpha/beta and cypin, showed maximal protein expression levels at P30, thereby showing a positive correlation to critical period plasticity. Western analysis indeed revealed a clear effect of visual deprivation on cypin expression in cat visual cortex. Our results therefore demonstrate the power of 2-D DIGE as a tool toward understanding the molecular basis of nervous system development and plasticity.

Patterns and Symmetries in the Visual Cortex and in Natural Images

As borders between different entities, lines are an important element of natural images. Indeed, the neurons of the mammalian visual cortex are tuned to respond best to lines of a given orientation. This preferred orientation varies continuously across most of the cortex, but also has vortex-like singularities known as pinwheels. In attempting to describe such patterns of orientation preference, we are led to consider underlying rotation symmetries: Oriented segments in natural images tend to be collinear; neurons are more likely to be connected if their preferred orientations are aligned to their topographic separation. These are indications of a reduced symmetry requiring joint rotations of both orientation preference and the underlying topography. This is verified by direct statistical tests in both natural images and in cortical maps. Using the statistics of natural scenes we construct filters that are best suited to extracting information from such images, and find qualitative similarities to mammalian vision.

Spatiotemporal Specificity of Neuronal Activity Directs the Modification of Receptive Fields in the Developing Retinotectal System

The precise temporal relation between pre- and postsynaptic activity modulates the strength of synaptic connections. Despite its extensive characterization in vivo and in vitro, the degree to which spike timing-dependent plasticity can shape receptive field properties is unclear. We use in vivo patch-clamp recordings of tectal neurons in developing Xenopus tadpoles to control the precise timing of action potentials with respect to the arrival of a subset of visual inputs evoked by local light stimulation on the retina. The pattern of visual inputs onto a tectal neuron was tracked over time by rapid reverse correlation mapping of receptive fields. Spike timing-dependent potentiation or depression of a subset of synapses reliably shifts the spatial receptive fields toward or away from the trained subregion of visual space, respectively. These results demonstrate that natural patterns of activity evoked by sensory stimuli play an instructive role in the developing nervous system.

The Impact of a Corticotectal Impulse on the Awake Superior Colliculus

Corticotectal (CTect) neurons of layer 5 are large and prominent elements of mammalian visual cortex, with thick apical dendrites that ascend to layer 1, "intrinsically bursting" membrane properties, and fast-conducting descending axons that terminate in multiple subcortical domains. These neurons comprise a major output pathway of primary visual cortex, but virtually nothing is known about the synaptic influence of single CTect impulses on the superior colliculus (SC). Here, we examine the distribution of monosynaptic currents generated in the superficial SC by spontaneous impulses of single CTect neurons. We do this by recording the spikes of CTect neurons and the field potentials that they generate through the depths of the SC. Methods of spike-triggered averaging and current source density analysis are then applied to these data. We show, in fully awake rabbits, that single CTect impulses generate potent, fast-rising monosynaptic currents in the SC similar to those generated in sensory cortex by specific thalamic afferents. These currents are focal in depth, precisely retinotopic, and highly dependent on the conduction velocity of the CTect axon. Moreover, we show that CTect synapses, like thalamocortical synapses, suffer a chronic state of depression in awake subjects that is modulated by preceding interspike interval. However, CTect neurons generated few "bursts," and postsynaptic responses in the SC were not significantly influenced by a shift from alert to an inattentive state (indicated by hippocampal EEG). Together, our results suggest that single CTect neurons may resemble thalamocortical neurons in their ability to serve as potent "drivers" of postsynaptic targets.

Attractor neural networks with patchy connectivity

The neurons in the mammalian visual cortex are arranged in columnar structures, and the synaptic contacts of the pyramidal neurons in layer II/III are clustered into patches that are sparsely distributed over the surrounding cortical surface. Here, we use an attractor neural-network model of the cortical circuitry and investigate the effects of patchy connectivity, both on the properties of the network and the attractor dynamics. An analysis of the network shows that the signal-to-noise ratio of the synaptic potential sums are improved by the patchy connectivity, which results in a higher storage capacity. This analysis is performed for both the Hopfield and Willshaw learning rules and the results are confirmed by simulation experiments.

The evolution of visual cortex in mammals: Genetic and epigenetic contributions to the phenotype

The evolution of visual cortex in mammals: Genetic and epigenetic contributions to the phenotype

Boundary Cue Invariance in Cortical Orientation Maps

We effortlessly perceive oriented boundaries defined by either luminance changes ('first-order' cues) or texture variations ('second-order' cues). Many neurons in mammalian visual cortex show orientation preference to both types of boundaries, but it is uncertain how they contribute to perceptual orientation cue-invariance at the neuronal population level. Using optical imaging in cat A 18, we observed highly similar orientation preference maps to first-order and a variety of second-order visual stimuli. Thus the neuronal representation of coarse-scale boundary orientation appears to be invariant to the characteristics (including local orientation) of the fine-scale textures by which those boundaries are defined. A common feature of second-order visual stimuli is that modulation shifts their Fourier energy for boundary orientation to the higher spatial frequencies of their constituent textures - our results suggest a common neural mechanism (demodulation) mediating visual processing of many kinds of texture boundary. The similarity between orientation maps to different stimuli implies that second-order responsive neurons are homogeneously distributed across the cortical surface. Such homogeneously cue-invariant orientation representation could provide a neural substrate for perceptual form-cue invariance, and reflect an optimal organization for encoding orientation information in natural scenes.

Columnar organization of the mammalian visual cortex and its vulnerability following lesion in adult cats

It is well known that in the mammalian visual cortex the neurons, sharing similar response properties, are grouped together into functional units, known as cortical columns. The orientation and ocular dominance columnar organization is a fundamental element for both the anatomical and physiological features of the visual cortex. Nonetheless, little is known about the functional restoration of matured columnar columns following injury. In the present study, the visual cortex of adult cats was studied electrophysiologically, whereas the primary goal of the study was to reveal the functional stability of the columns, disconnected from the main visual input. Experiments were performed on the primary visual cortex (area 17) of 13 anaesthetized and paralyzed adult cats. The columnar distortion was produced by surgical incision perpendicular to the cortical columns. The single unit activity was recorded from 1186 visual cells (experimental groups) in areas proximal and distal to the lesion and, compared to data, received from intact visual cortex (control group). The results indicate that most of the visually responsive cells were found to be selective to specific orientation in all experimental groups (75–100%) similar to the normal control group (78%). Moreover, the distribution of orientation-specific cells was very similar in all experimental and control groups (p > 0.05), as well as in both recording areas (p > 0.05). The percentage of binocular cells was significantly lower in all experimental groups (23–49%) in comparison to the control (80%). However, the distribution of the binocular cells revealed the significant similarity between the experimental and control groups (p > 0.05). An additional finding of the study is that the visual responsiveness of cells was significantly reduced in all experimental groups: only 28–49% of cells were found to be responsive following injury, as compared to 86% in normal control group (p < 0.001). The distribution of cells has also been analysed in accordance with their directional specificity and it has been found that the majority of cells in the experimental groups were found to be bias and non-specific to light stimuli (52–84%) as compared normal controls (21%) (p < 0.001). It has been concluded that, despite the fact that no improvement in visual function was found, the inherent structure of the disrupted cortical columns in the visual cortex was generally preserved. Therefore, the disruption of the columnar connection does not lead to remarkable distortion of the connectivity pattern on the whole, though it does reduce the responsiveness level there. It was concluded that the columnar structure for both orientation and ocular dominance is characterized by high stability, which enables visual processing with minimal brain connections.

Contrast response in visual cortex: Quantitative assessment with intrinsic optical signal imaging and neural firing

While previous studies showed that intrinsic optical signals spatially correspond with electrophysiological responses in mammalian visual cortex, the quantitative correspondence of their response strengths is open to question. Measurement of both signals' strength as functions of visual stimulus contrast provides an opportunity for quantitative comparison. Towards that end, the spatial and temporal properties of the optical signal impose important constraints upon quantification of its strength. We used intrinsic optical signal imaging and single unit recording to measure responses to drifting gratings at contrasts ranging from 10–80% in cat area 18. We calculated the average difference images for pairs of oppositely moving, or orthogonally oriented, gratings at each contrast and evaluated three different methods for quantifying optical signal strength. After about 2.5 s, the spatial patterns of optical images and the time course of their strength were contrast-invariant. This “space–time-contrast separability” for optical response implies a spatial uniformity of the optical contrast response functions, provides an objective basis to guide temporal averaging of optical signals, and validates a scalar metric of optical signal strength. Optically measured contrast response functions increase monotonically and saturate at high contrasts, qualitatively resembling those from single units. However, quantitative comparison reveals a nonlinear relationship with neural firing, such that the optical response reaches half of its maximum when the neural response has reached only around 20% of its maximum. This relationship suggests that intrinsic optical signals are relatively more sensitive to weak signals than neural firing.

Effects of age on latency and variability of visual response in monkeys

Visual function declines during normal aging. The neural mechanisms underlying age-related changes have just begun to be revealed. It has been demonstrated that retinal ganglion cells and cells in dorsal lateral geniculate nucleus (dLGN) in old monkeys are relatively normal. However, age-related functional degradations have been found at cellular level in mammalian visual cortex. Decreased orientation and direction selectivity of neurons in primary visual cortex (V1) have been reported in both aged monkeys and cats. Similar degradation of stimulus selectivity also occurs in secondary visual cortex (V2) in old monkeys. In addition, we have reported prolonged response latency in both V1 and V2 of old primates. Response latency of neuron can be affected by several factors, including conduction speed of the axons, synaptic integration, action potential initiation and the response variability. These factors may all contribute to delayed visual response in aged animals. For example, increased response variability will affect the temporal integration in synaptic transmission, resulting in prolonged response latency in the postsynaptic neurons. On the other hand, the evaluation of latency depends on the reproducibility of response. The more variable the spike train driven by external stimulus, the longer time will be needed to reach the peak in the post-stimulus time histogram (PSTH). Thus, a prolonged latency will be observed. In the present experiment, using monkeys in early stage of senescence as subjects, we studied the effects of age on response variability and its relationship with the prolonged latency, aiming to investigate possible neural mechanisms underlying the age-related alteration of response latency. Subjects for this study were 3 young adult (5―6 years old) and 3 old (21―22 years old) male rhesus monkeys (Macaca mullata). All experimental protocols were consistent with the guidelines of the Society for Neuroscience and National Institute of Health. Monkeys were examined ophthalmoscopically to ensure that they had no optical or retinal problems that would impair visual function. Conventional acute extracellcular single unit recording technique was used to record the activity of V1 neurons of anaesthetized and paralyzed monkeys. The receptive field properties of neurons were analyzed as described previously. Visual stimuli were presented on a tangent screen positioned 171 cm from the retina (3 cm corresponds to 1° of visual angle). The visual stimulus was a flashing square within receptive field with optimal size for each cell, which is commonly used in related studies. The luminance of the stimulus used was 12.3 cd/m for white and 0.95 cd/m for black. For each cell, we presented the identical visual stimulus for 50 times, with an ON period of 0.5 s and an OFF period of 3 s, and accumulated a PSTH that represents the probability of firing as a function of time. The neurons with low response variability fire at almost the same time in 50 presentations, resulting in a sharp rising branch in PSTH. In contrast, the neurons with high response variability will produce a peak with a relative flat rising branch. Then, the rising branch of the first peak in PSTH with an amplitude equal or larger than three times of spontaneous activity was fitted by a Gaussian curve (y=y0+Aexp(-((x-x0)/ ) ) whose half-width ( ) was taken as a measure of the trial-to-trial variability, and time offset (x0) as the response latency (Peak latency). We studied the visual responses of 90 V1 cells in young monkeys and 99 V1 cells in old monkeys. Fig. 1 represents four typical PSTHs. The cells recorded in aged animals showed significant prolonged response latencies and increasing response variabilities, compared with cells in young monkeys (Table 1, Fig. 2(a), (b)). The finding of prolonged latency in aged animals is consistent with our previous study. However, the subjects we used in the present experiment were 21―22 years old monkeys, while the monkeys in our previous study were 28―32 years old. Analysis based on the sex maturation and lifespan revealed that the degree of senescence of 21―22 years old monkeys approximately equals 60―70 years old human beings, and 28―32 years old monkeys correspond to 80―90 years old human beings. The present results have demonstrated that the prolonged response latency occurs during the early stage of aging. It provided possible neural mechanisms underlying the delay in visual signal analysis and slowness of reaction time for visual stimulus in old people. The average for young monkey cells was 2.82 ms. In comparison, V1 cells in aged monkeys showed significant increased value of (4.56 ms), suggesting higher variability of response. The increase in re

Quantitative immuno-electron microscopic analysis of nuclear respiratory factor 2 alpha and beta subunits: Normal distribution and activity-dependent regulation in mammalian visual cortex

The macaque visual cortex is exquisitely organized into columns, modules, and streams, much of which can be correlated with its metabolic organization revealed by cytochrome oxidase (CO). Plasticity in the adult primate visual system has also been documented by changes in CO activity. Yet, the molecular mechanism of regulating this enzyme remains not well understood. Being one of only four bigenomic enzymes in mammalian cells, the transcriptional regulation of this enzyme necessitates a potential bigenomic coordinator. Nuclear respiratory factor 2 (NRF-2) or GA-binding protein is a transcription factor that may serve such a critical role. The goal of the present study was to determine if the two major subunits of NRF-2, 2alpha and 2beta, had distinct subcellular distribution in neurons of the rat and monkey visual cortex, if major metabolic neuronal types in the macaque exhibited different levels of the two subunits, and if they would respond differently to monocular impulse blockade. Quantitative immuno-electron microscopy was used. In both rats and monkeys, nuclear labeling of alpha and beta subunits was mainly over euchromatin rather than heterochromatin, consistent with their active participation in transcriptional activity. Cytoplasmic labeling was over free ribosomes, the Golgi apparatus, and occasionally the nuclear envelope, signifying sites of synthesis and possible posttranslational modifications. The density of both subunits was much higher in the nucleus than in the cytoplasm for all neurons examined, again indicating that their major sites of cellular action is in the nucleus.

A possible mechanism of curvature coding in early vision

This paper addresses the issue of curvature coding in the mammalian visual system. Considering the successful account of cortical neurons as detectors of features of visual stimuli like orientation and spatial frequency among others, we discuss the role of curvature in visual processes, review some previous investigations on curvature and present a possible biologically plausible model of curvature detection. This model requires only orientation selective neurons (simple cells) and short-range connections, both known and well-documented features of the mammalian primary visual cortex. The dependence of the model on different parameters is explored and discussed.