Showing papers on "Orientation column published in 2003"

Role of Subplate Neurons in Functional Maturation of Visual Cortical Columns

Abstract: The subplate forms a transient circuit required for development of connections between the thalamus and the cerebral cortex. When subplate neurons are ablated, ocular dominance columns do not form in the visual cortex despite the robust presence of thalamic axons in layer 4. We show that subplate ablation also prevents formation of orientation columns. Visual responses are weak and poorly tuned to orientation. Furthermore, thalamocortical synaptic transmission fails to strengthen, whereas intracortical synapses are unaffected. Thus, subplate circuits are essential not only for the anatomical segregation of thalamic inputs but also for key steps in synaptic remodeling and maturation needed to establish the functional architecture of visual cortex.

Anatomical Substrates for Functional Columns in Macaque Monkey Primary Visual Cortex

Abstract: In this review we re-examine the concept of a cortical column in macaque primary visual cortex, and consider to what extent a functionally defined column reflects any sort of anatomical entity that subdivides cortical territory. Functional studies have shown that columns relating to different response properties are mapped in cortex at different spatial scales. We suggest that these properties first emerge in mid-layer 4C through a combination of thalamic afferent inputs and local intracortical circuitry, and are then transferred to other layers in a columnar fashion, via interlaminar relays, where additional processing occurs. However, several properties are not strictly columnar since they do not appear in all cortical layers. In contrast to the functional column, an anatomically based cortical column is defined most clearly in terms of the reciprocal connections it makes, both via intra-areal lateral connections and inter-areal feedback/feedforward pathways. The column boundaries are reinforced by interplay between lateral inhibition spreading beyond the column boundary and disinhibition within the column. The anatomical column acts as a functionally tuned unit and point of information collation from laterally offset regions and feedback pathways. Thalamic inputs provide the highcontrast receptive field sizes of the column’s neurons, intra-areal lateral connections provide their low contrast summation field sizes, and feedback pathways provide surround modulation of receptive fields responses.

Emergent properties of layer 2/3 neurons reflect the collinear arrangement of horizontal connections in tree shrew visual cortex.

Abstract: The superficial layers of primary visual cortex, unlike layer 4, have an extensive network of long-range horizontal connections linking sites of similar orientation preference. To identify possible functional consequences of this distinct anatomy, we compared the receptive field properties of layers 2/3 and 4 neurons in tree shrew primary visual cortex with electrophysiological recordings. We found that elongated receptive fields, strong orientation tuning, and length summation (properties predicted by the anatomy of the horizontal connections) are present in layer 2/3 neurons, but not in layer 4 neurons. We further characterized the summation fields of layer 2/3 neurons and found axis and orientation-specific facilitation that matched the distribution of horizontal connections. The functional signature of horizontal connections was also evident in the population response of layer 2/3 neurons; the intrinsic signal activation pattern elicited by an array of collinear Gabor elements was significantly stronger than that elicited by a noncollinear array. Furthermore, our results showed that this enhancement of population response was achieved without compromising spatial resolution along the collinear axis, providing stimulus-specific facilitation without filling in between stimuli. Taken together, these results suggest that horizontal connections play a significant role in shaping the visual responses of layer 2/3 neurons.

Organization of the visual cortex in human albinism.

Abstract: In albinism there is an abnormal projection of part of the temporal retina to the visual cortex contralateral to the eye. This projection, together with the normally routed fibers from nasal retina, provides a cortical hemisphere with visual input from more than the normal hemifield of visual space. In many mammalian models of albinism, a possible sensory mismatch in the visual cortex is avoided either by reorganization of the thalamocortical connections to give the abnormal input an exclusive cortical representation, or by the abnormal input being substantially suppressed. In this study we examine, with fMRI, how the human visual cortex topographically maps its input in albinism. We find that the input from temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas. The abnormal routing of temporal fibers is not total, with the line of decussation shifting to between 6 and 14° into temporal retina. Our results indicate that the abnormal input to visual cortex in human albinism does not undergo topographic reorganization between the thalamus and cortex. Furthermore, the abnormal input is not significantly suppressed in either striate or extrastriate areas. The topographic mapping that we report in human does not conform, therefore, to the commonly observed patterns in other mammals but takes the form of the “true albino” pattern that has been reported rarely in cat and in the only other individual primate studied.

The Primate visual system

Abstract: Introduction: A Brief Overview of the Primate Visual System. The K, P, and M Pathways from Retina to Cortex. The Pulvinar Complex. The Development of Neuron Response Properties in Primary Visual Cortex. The Second Visual Area, V2. The Superior Colliculus. Early Visual Areas: V2, V3, DM, DL, and MT. Plasticity of Visual Cortex in Adults. Processing Hierarchies in Visual Cortex. Visuomotor Areas in Frontal and Parietal Cortex. Specializations of Human Visual Cortex. Maps of the Visual Field in the Cerebral Cortex of Primates: Functional Organization and Significance. Face Expertise and Category Specialization in the Human Occipitotemporal Cortex. Motion Processing in Human Visual Cortex. The Functional Organization of Monkey Inferotemporal Cortex. Comparative Studies of Pyramidal Neurons in Visual Cortex of Monkeys. Feedback Connections: Splitting the Arrow.

Mexican hats and pinwheels in visual cortex

Abstract: Many models of cortical function assume that local lateral connections are specific with respect to the preferred features of the interacting cells and that they are organized in a Mexican-hat pattern with strong “center” excitation flanked by strong “surround” inhibition. However, anatomical data on primary visual cortex indicate that the local connections are isotropic and that inhibition has a shorter range than excitation. We address this issue in an analytical study of a neuronal network model of the local cortical circuit in primary visual cortex. In the model, the orientation columns specified by the convergent lateral geniculate nucleus inputs are arranged in a pinwheel architecture, whereas cortical connections are isotropic. We obtain a trade-off between the spatial range of inhibition and its time constant. If inhibition is fast, the network can operate in a Mexican-hat pattern with isotropic connections even with a spatially narrow inhibition. If inhibition is not fast, Mexican-hat operation requires a spatially broad inhibition. The Mexican-hat operation can generate a sharp orientation tuning, which is largely independent of the distance of the cell from the pinwheel center.

Independent Projection Streams from Macaque Striate Cortex to the Second Visual Area and Middle Temporal Area

Abstract: The interareal wiring of the neocortex is usually depicted as a network of single point-to-point connections, often side-stepping the possibility that some neurons may project to multiple cortical areas. The prevalence of such neurons is unknown; if they are abundant, cortical circuits are more likely to be connectionally diffuse. We used a dual-tracer approach to determine whether single neurons in the macaque primary visual cortex (V1) project to two extrastriate areas, the second visual area (V2) and the middle temporal area (MT). We found two large intermingled groups of single-labeled neurons in layer 4B of V1 projecting independently to either V2 or MT. A third, sparser group of double-labeled neurons projected to both areas; we termed these manifold neurons. We also found that MT-projecting cells were distributed indiscriminately with respect to cytochrome oxidase compartment in layer 4B, revealing a subpopulation that provides a potential source of patch input from V1 to MT. The results demonstrate that primary sensory cortices can use multiple projection strategies to distribute signals to higher areas, and suggest that feedforward projections may route signals with more specificity than feedback pathways.

Receptive Fields and Response Properties of Neurons in Layer 4 of Ferret Visual Cortex

Abstract: The ferret has become a model animal for studies exploring the development of the visual system. However, little is known about the receptive-field structure and response properties of neurons in t...

Efficacy of retinal spikes in driving cortical responses.

Abstract: How does a single retinal ganglion cell (RGC) affect the firing of simple cells in the visual cortex? Although much is known of the functional connections between the retina and the lateral geniculate nucleus (LGN) and between LGN and visual cortex, it is hard to infer the effect of disynaptic connections from retina to visual cortex. Most importantly, there is considerable divergence from retina to LGN, so cortical neurons might be influenced by ganglion cells through multiple feedforward pathways. We recorded simultaneously from ganglion cells in the retina and cortical simple cells in the striate cortex with overlapping receptive fields and evaluated disynaptic connections with cross-correlation analysis. In all disynaptically connected pairs, the retinal receptive field center and overlapping cortical subregion always shared the same sign (either both ON or both OFF). Connected pairs were similar in other respects, such as relative position and timing of their receptive fields, and thus obeyed the same rules of connectivity found previously for retinothalamic and thalamocortical connections. We found that a single RGC directly contributed on average to3% of the activity of its cortical target. The relative timing of pairs of spikes from the retinal cell affected their efficacy in driving the cortical cell. When two retinal spikes were closely spaced (10 msec), the second spike was several times more likely to drive the cortical target. The relative magnitude of this disynaptic paired spike enhancement was considerably larger than has been found previously for retinogeniculate and geniculocortical connections. The amplified paired spike enhancement from retina to cortex ensures that signal transmission from retina to cortex is particularly effective when the retina fires a series of closely spaced action potentials.

Propagating waves in visual cortex: a large-scale model of turtle visual cortex.

Abstract: This article describes a large-scale model of turtle visual cortex that simulates the propagating waves of activity seen in real turtle cortex. The cortex model contains 744 multicompartment models of pyramidal cells, stellate cells, and horizontal cells. Input is provided by an array of 201 geniculate neurons modeled as single compartments with spike-generating mechanisms and axons modeled as delay lines. Diffuse retinal flashes or presentation of spots of light to the retina are simulated by activating groups of geniculate neurons. The model is limited in that it does not have a retina to provide realistic input to the geniculate, and the cortex and does not incorporate all of the biophysical details of real cortical neurons. However, the model does reproduce the fundamental features of planar propagating waves. Activation of geniculate neurons produces a wave of activity that originates at the rostrolateral pole of the cortex at the point where a high density of geniculate afferents enter the cortex. Waves propagate across the cortex with velocities of 4 μm/ms to 70 μm/ms and occasionally reflect from the caudolateral border of the cortex.

Local signals from beyond the receptive fields of striate cortical neurons.

Abstract: We examined in anesthetized macaque how the responses of a striate cortical neuron to patterns inside the receptive field were altered by surrounding patterns outside it. The changes in a neuron's response brought about by a surround are immediate and transient: they arise with the same latency as the response to a stimulus within the receptive field (this argues for a source locally in striate cortex) and become less effective as soon as 27 ms later. Surround signals appeared to exert their influence through divisive interaction (normalization) with those arising in the receptive field. Surrounding patterns presented at orientations slightly oblique to the preferred orientation consistently deformed orientation tuning curves of complex (but not simple) cells, repelling the preferred orientation but without decreasing the discriminability of the preferred grating and ones at slightly oblique orientations. By reducing responsivity and changing the tuning of complex cells locally in stimulus space, surrounding patterns reduce the correlations among responses of neurons to a particular stimulus, thus reducing the redundancy of image representation.

The Representation of Retinal Blood Vessels in Primate Striate Cortex

Abstract: The blood vessels that nourish the inner retina cast shadows on photoreceptors, creating “angioscotomas” in the visual field. We have found the representations of angioscotomas in striate cortex of the squirrel monkey. They were detected in 9 of 12 normal adult animals by staining flatmounts for cytochrome oxidase activity after enucleation of one eye. They appeared as thin profiles in layer 4C radiating from the blind spot representation. Angioscotomas can be regarded as a local form of amblyopia. After birth, when light strikes the retina, photoreceptors beneath blood vessels are denied normal visual stimulation. This deprivation induces remodeling of geniculocortical afferents in a distribution that corresponds to the retinal vascular tree. Angioscotoma representations were most obvious in monkeys with fine ocular dominance columns and were invisible in monkeys with large, well segregated columns. In monkeys without columns, their width corresponded faithfully to the inducing retinal shadow, making it possible to calculate the minimum shadow required to produce a cortical representation. The “amblyogenic threshold” was calculated as the fraction of the pupil area eclipsed to trigger remodeling of geniculocortical afferents. It was found to be constant over retinal eccentricity, vessel size, and shadow size. Ambliogenic shadows only three to four cones wide were sufficient to generate a cortical representation, testifying to the remarkable precision of the cortical map. The representations of retinal blood vessels separated by only 0.65° were resolvable in the cortex, yielding an upper limit on cortical resolution of 340 μm in layer 4C.

Universality in visual cortical pattern formation

Abstract: During ontogenetic development, the visual cortical circuitry is remodeled by activity-dependent mechanisms of synaptic plasticity. From a dynamical systems perspective this is a process of dynamic pattern formation. The emerging cortical network supports functional activity patterns that are used to guide the further improvement of the network’s structure. In this picture, spontaneous symmetry breaking in the developmental dynamics of the cortical network underlies the emergence of cortical selectivities such as orientation preference. Here universal properties of this process depending only on basic biological symmetries of the cortical network are analyzed. In particular, we discuss the description of the development of orientation preference columns in terms of a dynamics of abstract order parameter fields, connect this description to the theory of Gaussian random fields, and show how the theory of Gaussian random fields can be used to obtain quantitative information on the generation and motion of pinwheels, in the two dimensional pattern of visual cortical orientation columns.

Cortical connections and early visual function: intra- and inter-columnar processing.

Abstract: While it is widely assumed that the long-range horizontal connections in V1 are present to support contour integration, there has been only limited consideration of other possible relationships between anatomy and physiology (the horizontal connections) and visual function beyond contour integration. We introduce the possibility of other relationships directly from the perspective of computation and differential geometry by identifying orientation columns in visual physiology with the (unit) tangent bundle in differential geometry. This suggests abstracting early vision in a space that incorporates both position and orientation, from which we show that the physiology is capable of supporting a number of functional computations beyond contour integration, including texture-flow and shading-flow integration, as well as certain relationships between them. The geometric abstraction emphasizes the role of curvature, which necessitates a coupled investigation into how it might be estimated. The result is an elaboration of layer-to-layer interactions within an orientation column, with non-linearities possibly implemented by shunting inhibition. Finally, we show how the same computational framework naturally lends itself to solving stereo correspondence, with binocular tangents abstracting curves in space.

Cortical Columns: A Multi-parameter Examination

Abstract: Columnar structure in the cerebral cortex has been demonstrated in numerous studies. However, in the visual system, it is not clear from imaging, basic physiological and anatomical approaches how multiple stimulus parameters are related within columns. We have analyzed recordings from pairs of neurons in the striate cortex of the cat using various spatial and temporal parameters. We find that most parameters are clustered within inferred columns with the exception of spatial phase. Diversity of phase could be useful for serial processing in central visual pathways.

Functional organization of the cat visual cortex in relation to the representation of a uniform surface

Abstract: Neuronal activity in the early visual cortex has been extensively studied from the standpoint of contour representation. On the other hand, representation of the interior of a surface surrounded by a contour is much less well understood. Several studies have identified neurons activated by a uniform surface covering their receptive fields, but their distribution within the cortex is not yet known. The aim of the present study was to obtain a better understanding of the distribution of such neurons in the visual cortex. Using optical imaging of intrinsic signals, we found that there are a group of surface-responsive regions located in area 18, along the area 17/18 border, that tend to overlap the singular points of the orientation-preference map. Extracellular recordings confirmed that neurons responsive to uniform plane stimuli are accumulated in these regions. Such neurons also existed outside the surface-responsive regions around the singular points. These results suggest that there exists a functional organization related to the representation of a uniform surface in the early visual cortex.

Visual Processing in the Primate Brain

Abstract: The visual system is the most widely studied and perhaps the best understood mammalian sensory system. Not only have the details of its anatomical features been well described, but the behavior of it neurons have also been characterized at many stages of the neural pathway. This chapter focuses on the visual system of nonhuman primates and deals with the way the primate visual system performs the analysis of various attributes of the visual image and then integrates these attributes into a percept of a visual scene. It shows how such fundamental dimensions of visual stimuli as spatial and temporal variations in luminance and chromaticity are first encoded at the level of the retina, and the manner in which the encoding of other more complex stimulus features, such as motion, complex form and depth, emerge at the level of visual cortex. Finally, modulation of visual cortical activity by such cognitive phenomena as memory and attention are discussed. Keywords: dorsal visual stream; lateral geniculate nucleus; neurophysiology; nonhuman primates; parietal cortex; retina; striate cortex; temporal cortex; ventral visual stream

Neural Responses to the Uniform Surface Stimuli in the Visual Cortex of the Cat

Abstract: In the early stages of the visual cortex (areas 17 and 18), it is well known that neurons respond to bar and contour stimuli. However, several recent studies reported that some neurons in this stage respond to uniform plane stimuli that cover the classical receptive fields [1, 2]. In order to better understand the neuronal mechanisms underlying surface representation in the early visual stages, we explored the distribution of activation to the uniform plane stimuli by using optical imaging of intrinsic signals and extracellular recordings from areas 17 and 18 of anesthetized cats. We also intended to examine the distribution of such activities in relation to the orientation preference map in the same areas.

Assembly of Receptive Fields in Cat Visual Cortex

Abstract: The origin of orientation selectivity in the responses of simple cells in cat visual cortex serves as a model problem for understanding cortical circuitry and computation. The feedforward model of Hubel and Wiesel [1] posits that this selectivity arises simply from the spatial organization of the receptive fields of thalamic inputs synapsing on each simple cell. Much evidence, including a number of recent intracellular studies, supports a primary role of the thalamic inputs in determining simple-cell response properties including orientation tuning. And yet, while the feedforward model seems to explain the broad outline of simple-cell properties, there are number of detailed aspects of the behavior of simple cells that have appeared not to be accounted for by the feedforward model. These properties include contrast invariance of orientation tuning, the exact relationship between receptive field geometry and orientation tuning, and the dynamics of orientation tuning. The apparent failures of the feedforward model have prompted the development of a class of models that rely on feedback circuitry within the cortex: Properly arranged feedback from excitatory connections within an orientation column and inhibitory connections to adjacent columns can account for many of the properties that the feedforward models miss. The feedforward and feedback models are different enough in character that they make radically different predictions about the nature of the computation performed by the cortex, the feedforward models acting more like passive filters, and the feedback models more actively shaping the representation of the retinal image. I will review a series of experiments designed to test these two models, and present evidence that the feedforward models, with little modification, can account in detail for the behavior of cortical simple cells.

Contribution of Excitation and Inhibition to the Stimulus Size- and Orientation-Tuning of Response Modulation by the Receptive Field Surround in the Cat Primary Visual Cortex

Abstract: The response of neurons in the primary visual cortex (V1) is modulated by the stimulus presented at the surround of the classical receptive field (CRF), and the predominant effect of this surround stimulation is suppressive [1–5]. The strength of the surround suppression is dependent on the relationship of stimulus parameters, such as orientation and spatial frequency, between grating stimuli at the inside and outside of the CRF [1, 2]. Although properties of the surround suppression have been well characterized, input mechanisms underlying the stimulus-context-dependent response modulation and the size-tuning properties of V1 neurons have not been clarified yet. There is evidence that the surround suppression in VI is both sub-cortical [5] and intracortical [2, 4] in origin.

VEP in the definition of eloquent cortical areas

Abstract: Publisher Summary This chapter discusses the usefulness and limitations of visual evoked potentials (VEP) for the definition of visual cortex. Repetitive visual stimuli elicit visual evoked potentials; patterned stimuli and unpatterned achromatic stimuli. Patterned stimuli can be applied monocularly or confined to one hemifield or quadrant. Various physical parameters define the occurrence and latencies of the visually evoked potentials, and need to be standardized to yield reproducible and comparable results. Unpatterned stimuli elicited by flashes of light are more variable than response to patterned stimuli. A series of alternating negative and positive waves can be recorded over the midoccipital region, of which wave IV is the most conspicuous positivity and probably analogous to P100. Two recent studies using magnetoencephalography (MEG) and functional magnetic resonance imaging demonstrated that the generator of P100 is located in the striate area; the primary visual cortex. Using a current dipole model and superimposition on magnetic resonance imaging (MRI) images, it could be demonstrated that the generator of P100m was located at the lateral bottom of the calcarine fissure, comprising a smaller part of the striate cortex than that proposed by the cruciform model that locates the generator over a wider area of the striate cortex including the calcarine and interhemispheric cortices.