Showing papers on "Orientation column published in 2005"

Predicting the orientation of invisible stimuli from activity in human primary visual cortex

Abstract: Humans can experience aftereffects from oriented stimuli that are not consciously perceived, suggesting that such stimuli receive cortical processing. Determining the physiological substrate of such effects has proven elusive owing to the low spatial resolution of conventional human neuroimaging techniques compared to the size of orientation columns in visual cortex. Here we show that even at conventional resolutions it is possible to use fMRI to obtain a direct measure of orientation-selective processing in V1. We found that many parts of V1 show subtle but reproducible biases to oriented stimuli, and that we could accumulate this information across the whole of V1 using multivariate pattern recognition. Using this information, we could then successfully predict which one of two oriented stimuli a participant was viewing, even when masking rendered that stimulus invisible. Our findings show that conventional fMRI can be used to reveal feature-selective processing in human cortex, even for invisible stimuli.

Receptive field structure varies with layer in the primary visual cortex.

Abstract: Here we ask whether visual response pattern varies with position in the cortical microcircuit by comparing the structure of receptive fields recorded from the different layers of the cat's primary visual cortex. We used whole-cell recording in vivo to show the spatial distribution of visually evoked excitatory and inhibitory inputs and to stain individual neurons. We quantified the distribution of 'On' and 'Off' responses and the presence of spatially opponent excitation and inhibition within the receptive field. The thalamorecipient layers (4 and upper 6) were dominated by simple cells, as defined by two criteria: they had separated On and Off subregions, and they had push-pull responses (in a given subregion, stimuli of the opposite contrast evoked responses of the opposite sign). Other types of response profile correlated with laminar location as well. Thus, connections unique to each visual cortical layer are likely to serve distinct functions.

Laminar Organization of Response Properties in Primary Visual Cortex of the Gray Squirrel (Sciurus carolinensis)

Abstract: The gray squirrel (Sciurus carolinensis) is a diurnal highly visual rodent with a cone-rich retina. To determine which features of visual cortex are common to highly visual mammals and which are re...

Localization of Activity-dependent Changes in Blood Volume to Submillimeter-scale Functional Domains in Cat Visual Cortex

Abstract: We have examined whether blood volume changes induced by neural activation are controlled precisely enough for us to visualize the submillimeter-scale functional structure in anesthetized and awake cat visual cortex. To activate the submillimeter-scale functional structures such as iso-orientation domains in the cortex, visual stimuli (gratings) were presented to the cats. Two methods were used to examine the spatial precision of blood volume changes including changes in total hemoglobin content and changes in plasma volume: (i) intrinsic signal imaging at the wavelength of hemoglobin’s isosbestic point (569 nm) and (ii) imaging of absorption changes of an intravenously injected dye. Both measurements showed that the visual stimuli elicited stimulus-nonspecific and stimulus-specific blood volume changes in the cortex. The former was not spatially localized, while the latter was confined to iso-orientation domains. From the measurement of spatial separation of the iso-orientation domains, we estimated the spatial resolution of stimulus-specific blood volume changes to be as high as 0.6 mm. The changes in stimulus-nonspecific and -specific blood volume were not linearly correlated. These results suggest the existence of fine blood volume control mechanisms in the capillary bed in addition to global control mechanisms in arteries.

Feedback connections to ferret striate cortex: Direct evidence for visuotopic convergence of feedback inputs

Abstract: Interareal feedback connections are a fundamental aspect of cortical architecture, yet many aspects of their organization and functional relevance remain poorly understood. Previous studies have investigated the topography of feedback projections from extrastriate cortex to macaque area 17. We have extended this analysis to the ferret. We made restricted injections of cholera toxin B (CTb) into ferret area 17 and mapped the distribution of retrogradely labeled cells in extrastriate cortex. In addition to extensive label spreading within area 17, we found dense cell label in areas 18, 19, and 21 and the suprasylvian cortex and sparser connections from the lateral temporal and posterior parietal cortex. We made extensive physiological assessments of magnification factors in the extrastriate visual cortex and used these measures to convert the spread of labeled cortex in millimeters into a span in degrees of visual field. We also directly measured the visuotopic extents of receptive fields in the regions containing labeled cells in cases in which we made both CTb injections and physiological recordings in the same animals; we then compared the aggregate receptive field (ARF) of the labeled region in each extrastriate area with that of the injection site. In areas 18, 19, and 21, receptive fields of cells in regions containing labeled neurons overlapped those at the injection site but spanned a greater distance in visual space than the ARF of the injection site. The broad visuotopic extent of feedback connections is consistent with the suggestion that they contribute to response modulation by stimuli beyond the classical receptive field. J. Comp. Neurol. 487:312–331, 2005. © 2005 Wiley-Liss, Inc.

Spontaneous symmetry breaking in self–organizing neural fields

Abstract: We extend the theory of self-organizing neural fields in order to analyze the joint emergence of topography and feature selectivity in primary visual cortex through spontaneous symmetry breaking. We first show how a binocular one-dimensional topographic map can undergo a pattern forming instability that breaks the underlying symmetry between left and right eyes. This leads to the spatial segregation of eye specific activity bumps consistent with the emergence of ocular dominance columns. We then show how a 2-dimensional isotropic topographic map can undergo a pattern forming instability that breaks the underlying rotation symmetry. This leads to the formation of elongated activity bumps consistent with the emergence of orientation preference columns. A particularly interesting property of the latter symmetry breaking mechanism is that the linear equations describing the growth of the orientation columns exhibits a rotational shift-twist symmetry, in which there is a coupling between orientation and topography. Such coupling has been found in experimentally generated orientation preference maps

Functional impact of primary visual cortex deactivation on subcortical target structures in the thalamus and midbrain.

Abstract: The functional relationships between the primary visual cortex and its major subcortical target structures have long been a subject of interest. We studied these relationships by using localized cooling deactivation to silence portions of primary visual cortex and measuring 2-deoxyglucose (2DG) uptake to assess neural activity in subcortical and midbrain targets. We focused analysis on the largest subcortical targets of primary visual cortex: the superior colliculus (SC), the dorsal lateral geniculate nucleus of the thalamus (dLGN), and the lateral division of the lateral posterior nucleus of the thalamus (LPL). We found that localized cooling of different regions of primary visual cortex caused specific decreases in 2DG uptake in target structures such that the location of 2DG decrease varied according to joint retinotopy, and the magnitude of the decreases in target structures was associated with the amount of cooled cortex. In addition, we found that the impact of cortical cooling was more profound on the SC than on the dLGN. The functional impact of cortical deactivations on the LPL was weak for small deactivations but approximated the impact on the SC when deactivations were large. We discuss these findings in terms of neural circuits and in terms of drivers and modulators. J. Comp. Neurol. 488:414–426, 2005. © 2005 Wiley-Liss, Inc.

Course 12 - Symmetry Breaking and Pattern Selection in Visual Cortical Development

Abstract: The chapter discusses the description of the development of orientation preference columns in terms of a dynamics of abstract order parameter fields and connects this description to the theory of Gaussian random fields, and examines how the theory of Gaussian random fields is used to obtain quantitative information on the generation and motion of pinwheels, in the two dimensional pattern of visual cortical orientation columns. The chapter also discusses the symmetry based approach used to derive this prediction to study also the kind of patterns to which the map will asymptotically converge and the interactions essential for the stabilization of different kinds of solutions. An exposition of an appropriate perturbation method called weakly nonlinear analysis for the problem of orientation column formation is provided. Using this method, a class of generalized Swift–Hohenberg models for the formation of patterns of contour detecting neurons during visual cortical development is constructed. In this model class, permutation symmetry of the model equations satisfies the requirement that the visual cortex develops selectivity for all contour orientations. Long-range interactions are found to be essential for the stability of realistic solutions.

Visual shape recognition with contour propagation

Abstract: A neural architecture is presented that encodes the visual space inside and outside of a shape. The contours of a shape are propagated across an excitable neuronal map and fed through a set of orientation columns, thus creating a pattern which can be viewed as a vector field. This vector field is then burned as synaptic, directional connections into a propagation map, which will serve as a “shape map”. The shape map identifies its own, preferred input when it is translated, deformed, scaled and fragmented, and discriminates other shapes very distinctively. Encoding visual space is much more efficient for shape recognition than determining contour geometry only.

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

Abstract: 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,...

Anesthesia and tangential package of neurons sensitive to cruciform figures in the cat striate cortex.

Abstract: Local blood supply to the brain is known to depend on the changes in neuronal activity. The functional brain imaging methods developed in recent decades made it possible to study the fine organization of brain functions on the basis of changes in local circulation (positron emission tomography, functional magnetic resonance imaging, and mapping by intrinsic signal). Optical mapping of the cerebral cortex [2, 3, 11] is based on differences in optical characteristics between oxyhemoglobin and its reduced form, deoxyhemoglobin. Local decrease in the concentration of oxyhemoglobin and increase in the concentration of deoxyhemoglobin in the cerebral tissue results from the activation of neurons accompanied by an increase in their oxygen consumption. These changes in the concentrations of two hemoglobin forms can be measured using a sensitive CCD camera. Recording this intrinsic signal, one can construct functional maps of the brain surface where activated regions look darker. Mapping upon visual stimulation of different types allowed the arrangement of different functional modules of the cerebral cortex (orientation columns and hypercolumns and eye dominance bands) [5]. Earlier electrophysiological experiments showed that about half of neurons of area 17 of the cat visual cortex were most sensitive to intersections and branching of line segments [7–10] rather than their orientation [4]. It remains unknown whether the detectors of cruciform figures are included in the classic orientation columns or in other cortical modules. Therefore, the purpose of our study was to analyze the tangential package of neuron detectors of second-order shapes (grids, crosses, and corners) in the cat primary visual cortex with the use of optical mapping of the cerebral cortex by intrinsic signal. We performed 12 acute experiments with anesthetized adult cats immobilized with a myorelaxant. The animals were under artificial ventilation and were intraarterially or intravenously infused with Ringer solution containing glucose. The degree of anesthesia and the general state of the animals were monitored by measuring body temperature, arterial pressure, and CO 2 content of the expired air. The optical access to the cortex was ensured as follows. After trephination and the removal of the dura mater, a cylindrical chamber was fixed over area 17 (Fig. 1a), filled with silicon oil, and hermetically covered with glass to preclude respiratory and pulse shifts of the cortex. A highly sensitive CCD camera (Fig. 1b) with a small depth of focus, equipped with a light source (Fig. 1c), was focused at a depth of 700 μ m from the cortex surface to reduce artifacts related to the blood flow through large vessels on the brain surface.

Disentangling simple from complex cells.

Abstract: such single-cell map precision instantiated in the spatial arrangement of neural connections? For orientation tuning, at least, several lines of evidence suggest that an orientationspecific bias in the spatial arrangement of feed-forward axon arbors plays a significant role12,13. But the scale of precision in axonal arrays—that is, the degree of bias in the axon arbors that converge at a given point in the cortex—remains unclear. Moreover, it is not yet known how the broad and overlapping dendritic processes of cortical neurons sample from this array, as well as those supplied by other axonal populations, and how they do so in a fashion that yields such precision. At a more fundamental level, the function served by cortical maps remains a puzzle—one that is deepened further by Okhi and colleagues’ observations on the microarchitecture of orientation tuning in rat visual cortex. Despite the presence of neurons that are well tuned for orientation and/or direction of motion in rat visual cortex, there is no sign of the orderly arrangements that are so apparent in cat visual cortex: the orientation preferences of nearby cells are uncorrelated. The lack of maps in the rat is consistent with results from other studies in rodents and indicates that the orderly mapping of stimulus features is not a prerequisite for constructing tuned cortical responses14. As the authors suggest, orderly mapping might contribute to differences in tuning sharpness, but this seems a modest gain for such a major difference in cortical architecture. Perhaps the significance lies less in generating the mapped properties and more in how additional dimensions of the visual scene are encoded in the responses of individual neurons. Although they respond similarly to grating stimuli, adjacent neurons in cat area 18 are likely to differ in their response to a variety of other properties of the stimulus (for example, phase, contrast or temporal properties). Such diversity of functionally distinct cell types may be significantly reduced in rodent visual cortex, obviating advantages for connectivity or pooling that clustering of neurons according to similar properties may endow7,15. Whatever the reason for such species differences, new approaches for simultaneously visualizing the properties of large numbers of individual neurons in functioning circuits promise to add appreciably to our understanding of the complexities of cortical architecture. Exploring how the world is mapped in the brain just got a lot more exciting.

Receptive Field and Feature Map Formation in the Primary Visual Cortex via Hebbian Learning with Inhibitory Feedback

Abstract: A linear neural network is proposed for mamalian vision system in which backward connections from the primary visual cortex (V1) to the lateral geniculate nucleus play a key role. The backward connections control the flow of information from the LGN to V1 in such a way as to maximize the rate of transfer of information from the LGN to V1. The application of hebbian learning to the forward and backward connections causes the formation of receptive fields which are sensitive to edges, bars, and spatial frequencies of preferred orientations. Receptive field types in V1 are shown to depend on the density of the afferent connections in the LGN. Orientational preferences are organized in the primary visual cortex by the application of lateral interactions during the learning phase. Change in the size of the eye between the immature and mature animal is shown be an important factor in the development of V1 organization. The orgainization of the mature network is compared to that found in the macaque monkey by several analytical tests.