Showing papers on "Orientation column published in 1999"

Dendritic asymmetry cannot account for directional responses of neurons in visual cortex.

Abstract: A simple model was proposed to account for the direction selectivity of neurons in the primary visual cortex, area V1. In this model, the temporal asymmetries in the summation of inhibition and excitation that produce directionality were generated by structural asymmetries in the tangential organization of the basal dendritic tree of cortical neurons. We reconstructed dendritic trees of neurons with known direction preferences and found no correlation between the small biases of a neuron's dendritic morphology and its direction preference. Detailed simulations indicated that even when the electrotonic asymmetries in the dendrites were extreme, as in cortical Meynert cells, the biophysical properties of single neurons could contribute only partially to the directionality of cortical neurons.

Topography of contextual modulations mediated by short-range interactions in primary visual cortex

Abstract: Neurons in primary visual cortex (V1) respond differently to a simple visual element presented in isolation from when it is embedded within a complex image. This difference, a specific modulation by surrounding elements in the image, is mediated by short- and long-range connections within V1 and by feedback from other areas. Here we study the role of short-range connections in this process, and relate it to the layout of local inhomogeneities in the cortical maps of orientation and space. By measuring correlation between neuron pairs located in optically imaged maps of V1 orientation columns we show that the strength of local connections between cells is a graded function of lateral separation across cortex, largely radially symmetrical and relatively independent of orientation preferences. We then show the contextual influence of flanking visual elements on neuronal responses varies systematically with a neuron's position within the cortical orientation map. The strength of this contextual influence on a neuron can be predicted from a model of local connections based on simple overlap with particular features of the orientation map. This indicates that local intracortical circuitry could endow neurons with a graded specialization for processing angular visual features such as corners and T junctions, and this specialization could have its own functional cortical map, linked with the orientation map.

Development of ocular dominance columns in the absence of retinal input

Abstract: The initial establishment of ocular dominance columns in visual cortex is believed to involve the segregation of overlapping geniculocortical axons into eye-specific patches based on patterns of correlated activity. However, we found that total removal of retinal influence early in visual development did not prevent segregation of geniculocortical axons into alternating stripes with periodicity normal for ocular dominance columns. Because the patterning of geniculocortical afferents resists this dramatic change in the level, source and pattern of spontaneous activity, we propose that formation of ocular dominance columns relies on molecular cues present on thalamic axons, cortical cells or both.

Asymmetric Suppression Outside the Classical Receptive Field of the Visual Cortex

Abstract: Areas beyond the classical receptive field (CRF) can modulate responses of the majority of cells in the primary visual cortex of the cat (). Although general characteristics of this phenomenon have been reported previously, little is known about the detailed spatial organization of the surrounds. Previous work suggests that the surrounds may be uniform regions that encircle the CRF or may be limited to the "ends" of the CRF. We have examined the spatial organization of surrounds of single-cell receptive fields in the primary visual cortex of anesthetized, paralyzed cats. The CRF was stimulated with an optimal drifting grating, whereas the surround was probed with a second small grating patch placed at discrete locations around the CRF. For most cells that exhibit suppression, the surrounds are spatially asymmetric, such that the suppression originates from a localized region. We find a variety of suppressive zone locations, but there is a slight bias for suppression to occur at the end zones of the CRF. The spatial pattern of suppression is independent of the parameters of the suppressive stimulus used, although the effect is clearest with iso-oriented surround stimuli. A subset of cells exhibit axially symmetric or uniform surround fields. These results demonstrate that the surrounds are more specific than previously realized, and this specialization has implications for the processing of visual information in the primary visual cortex. One possibility is that these localized surrounds may provide a substrate for figure-ground segmentation of visual scenes.

Neural correlates of perceived brightness in the retina, lateral geniculate nucleus, and striate cortex.

Abstract: Brightness changes can be induced in a static gray field by modulating the luminance of surrounding areas. We used this induction phenomenon to investigate the neural representation of perceived brightness. Extracellular recordings were made in striate cortex, the lateral geniculate nucleus (LGN), and the optic tract of anesthetized cats using stimuli that produced brightness induction. While a cell’s receptive field (RF) was covered by uniform gray illumination, the luminance of rectangular flanking regions was modulated sinusoidally in time, inducing brightness changes in the RF. We looked for a correspondence between the modulation of a cell’s response and stimulus conditions that did or did not produce perceptual changes in brightness. We found that the responses of retinal ganglion cell axons in the optic tract were never correlated with brightness. On the other hand, many neurons in striate cortex and a small fraction in the LGN responded in a phase-locked manner at the temporal frequency of the flank modulation, even though the flanks were 3–7° beyond the edges of the RF. Only in striate cortex were cells found that had responses correlated with brightness under all stimulus conditions. These findings suggest that brightness information is explicitly represented in the responses of neurons in striate cortex as part of a neural representation of object surfaces.

Topographic Organization of Human Visual Areas in the Absence of Input from Primary Cortex

Abstract: Recently, there has been evidence for considerable plasticity in primary sensory areas of adult cortex. In this study, we asked to what extent topographical maps in human extrastriate areas reorganize after damage to a portion of primary visual (striate) cortex, V1. Functional magnetic resonance imaging signals were measured in a subject (G.Y.) with a large calcarine lesion that includes most of primary visual cortex but spares the foveal representation. When foveal stimulation was present, intact cortex in the lesioned occipital lobe exhibited conventional retinotopic organization. Several visual areas could be identified (V1, V2, V3, V3 accessory, and V4 ventral). However, when stimuli were restricted to the blind portion of the visual field, responses were found primarily in dorsal extrastriate areas. Furthermore, cortex that had formerly shown normal topography now represented only the visual field around the lower vertical meridian. Several possible sources for this reorganized activity are considered, including transcallosal connections, direct subcortical projections to extrastriate cortex, and residual inputs from V1 near the margin of the lesion. A scheme is described to explain how the reorganized signals could occur based on changes in the local neural connections.

Increased Receptive Field Size in the Surround of Chronic Lesions in the Adult Cat Visual Cortex

Abstract: Visual cortical lesions destroy the target cells for geniculocortical fibers from a certain retinotopic region. This leads to a cortical scotoma. We have investigated the receptive fields of cells in the visual cortex before, 2 days and 2 months after focal ibotenic acid lesions in the adult cat visual cortex and have found signs of receptive field plasticity in the surroundings of the chronic but not the acute and subacute excitotoxic lesions. In the subacute state (first two days post lesion) receptive field sizes of cells at the border of the lesion were reduced in size or remained unchanged. Remapping of cortical receptive fields 2 months later revealed a number of cells with multifold enlarged receptive fields at the border of the lesion. The cells with enlarged receptive fields displayed orientation and direction selectivity like normal cells. The size increase appeared not specifically directed towards the scotoma; however, the enlarged receptive fields can reduce the extent of a cortical scotoma, since previously unresponsive regions of the visual field activate cortical cells at the border of the lesion. This late receptive field plasticity could serve as a mechanism for the filling-in of cortical scotomata observed in patients with visual cortex lesions.

Rewiring cortex: the role of patterned activity in development and plasticity of neocortical circuits.

Abstract: Visually driven activity is not required for the establishment of ocular dominance columns, orientation columns, and long-range horizontal connections in visual cortex, although spontaneous activity appears to be necessary. The role of activity may be instructive or simply permissive; evidence for an instructive role requires inquiry into the role of the pattern of activity in shaping cortical circuits. The few experiments that have probed the role of patterned activity include the effects of artificial strabismus, artificial stimulation of the optic nerve, and rewiring visual projections from the retina to the auditory thalamus and cortex. These experiments demonstrate that patterned activity is vital for the maintenance of thalamocortical, local intracortical, and long-range horizontal connections in cortex.

Maps of Central Visual Space in Ferret V1 and V2 Lack Matching Inputs from the Two Eyes

Abstract: In the visual cortex, the representation of central visual space is supplied by matching geniculate inputs that are driven exclusively by one eye or the other. In layer 4 of early visual areas (V1 in primates and V1 and V2 in cat), these inputs form a nearly uniform array of small ocular dominance domains, while preserving overall topographic order within the cortical map. In ferret, however, ocular dominance domains in different regions of the visual cortex are strikingly irregular in size and shape. The exceptionally large size of domains in some regions implies a departure from the usual visuotopic matching of inputs from the two eyes. Using optical-imaging, electrophysiological, and anatomical techniques, we show that this regional variation is attributable to exclusively monocular maps of the central portions of the ipsilateral visual field in V1 and the contralateral visual field in V2. In addition, we document a complex interdigitation of V1 and V2 that entails a discontinuity in the mapping of visual space and fragmentation of V2 into isolated cortical territories. We suggest that both the monocularity of these cortical maps and the visuotopic discontinuity along the V1–V2 border derive from asymmetries in the crossed and uncrossed retinal pathways.

Metabolic Mapping of Suppression Scotomas in Striate Cortex of Macaques with Experimental Strabismus

Abstract: Misalignment of the ocular axes induces double vision and rivalry. To prevent these unpleasant sensations, most subjects fixate preferentially with one eye and suppress entirely the deviating eye or else suppress portions of the visual field of either eye. To explore the mechanism of visual suppression, a divergent strabismus (exotropia) was induced in six normal, adult Macaca fascicularis by disinserting the medial rectus muscles. After 4-8 weeks, each animal was chaired to measure its exotropia and to determine its ocular fixation preference. Five of the monkeys developed a clearly dominant eye. It was injected with [(3)H]proline. Alternate sections from flat-mounts of striate cortex were then processed either for autoradiography to label the ocular dominance columns or for cytochrome oxidase (CO) to assess local metabolic activity. Two CO patterns were seen, often in the same cortex. The first consisted of thin dark columns alternating with wide pale columns. This pattern arose from reduced CO activity in the suppressed eye's monocular core zones and both eyes' binocular border strips. The second pattern consisted of thin pale bands from reduced metabolic activity in both eyes' border strips. The thin dark-wide pale CO pattern was more widespread in the three animals with a strong fixation preference. The dark CO columns usually fit in register with the ocular dominance columns of the fixating eye, suggesting that perception was suppressed in the deviating eye. In most animals, however, the correlation switched in peripheral cortex contralateral to the deviating eye, implying local suppression of the fixating eye's temporal retina (beyond 10 degrees), as reported in humans with divergent strabismus. In the two animals with a weak fixation preference, pale border strips were found within the central visual field representation in both hemispheres. This CO pattern was consistent with alternating visual suppression. These experiments provide the first anatomical evidence for changes in cortical metabolism that can be correlated with suppression scotomas in subjects with strabismus.

Orientation Constancy in Neurons of Monkey Visual Cortex

Abstract: We studied the effect of body tilt on the orientation selectivity of single neurons in the visual cortex of an alert monkey. The monkey performed a visual fixation task either in the upright position or with its whole body tilted about the naso-occipital (roll) axis by ±25° or ±30°. We determined the preferred stimulus orientation for 51 of 117 neurons in two or, if possible, three body positions (i.e. with the whole body upright, and tilted either left ear or right ear down). In striate cortex, most of the neurons were of a non-compensatory type, showing a change in the preferred orientation according to the body tilt and the estimated counterrolling of the eye. By contrast, about 40% of the neurons in prestriate cortex were of a compensatory type, preferring similar orientations in all body positions. This suggests that mechanisms which produce orientation constancy with respect to the direction of gravity are implemented at an early stage of cortical processing.

Organization of visual cortex in the northern quoll, Dasyurus hallucatus: evidence for a homologue of the second visual area in marsupials

Abstract: Two visual areas, V1 and V2 (first and second visual areas), appear to be present in the posterior neocortex of all eutherian mammals investigated so far. However, previous studies have not established whether an area homologous to V2 also exists in metatherian mammals (marsupials). Using electrophysiological techniques, we mapped the visual receptive fields of neurons in the striate and peristriate cortices of the northern quoll, an Australian marsupial. We found that neurons in a 2-mm-wide strip of cortex rostrolateral to V1 form a single, relatively simple representation of the complete contralateral hemifield. This area resembles V2 of eutherians in several respects: (i) neurons in the medial half of the peristriate area represent the lower visual quadrant, whereas those in the lateral half represent the upper visual quadrant; (ii) the vertical meridian of the visual field is represented adjacent to V1, while the visual field periphery is represented along the lateral and rostrolateral borders of the peristriate area; (iii) there is a marked anisotropy in the representation, with a larger magnification factor parallel to the V1 border than perpendicular to this border; and (iv) receptive fields of multiunit clusters in the peristriate cortex are much larger than those of cells in V1 at comparable eccentricities. The cortex immediately rostral and lateral to V2 did not respond to visual stimulation under our recording conditions. These results suggest that V1 and V2 together form a 'core' of homologous visual areas, likely to exist in all therian mammals.

Modeling the self-organization of directional selectivity in the primary visual cortex

Abstract: A model is proposed to demonstrate how neurons in the primary visual cortex could self-organize to represent the direction of motion. The model is based on a temporal extension of the self-organizing map where neurons act as leaky integrators. The map is trained with moving Gaussian inputs, and it develops a retinotopic map with orientation columns that divide into areas of opposite direction selectivity, as found in the visual cortex.

Functional organization of surgically created visual circuits.

Abstract: Lesions of cerebral targets of the retina in newborn hamsters, when combined with transection of lemniscal pathways to the primary auditory or somatosensory thalamic nuclei or the secondary thalamic visual nucleus, can induce the formation of permanent retinal pr ojections to the deafferented non-visual structures These projections are retinotopically organized and form functional synapses Consequently, neurons in the auditory or somatosensory cortices, which normally are not driven by visual stimuli, become visually responsive and have receptive field properties that ressemble, in several important ways, those of neurons in the visual cortex of normal animals The surgically-induced retino-thalamo-cortical pathways can mediate visually guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the thalamic dorsal lateral geniculate nucleus, is missing

Pattern formation in the developing visual cortex: Topological defects, their generation, motion, and annihilation

Abstract: The pinwheel-like arrangement of iso-orientation domains around orientation centers is a ubiquitous structural element of orientation preference maps in primary visual cortex. We show that activity-dependent self-organization of orientation preference implies that low densities of orientation centers develop through an initial overproduction and subsequent annihilation of pinwheels. Monitoring their density during development therefore offers a powerful novel approach to test whether orientation preference arises by activity-dependent mechanisms or is intrinsically predetermined. Observed interspecies differences in the density of orientation centers can be explained by a dynamic interaction of ocular dominance and orientation columns during development.

Neural field description of state-dependent receptive field changes in the visual cortex.

Abstract: Receptive elds in V1 have been shown to be wider during synchronized than during non-synchronized EEG states, where, in addition, they can shrink over time in response to ashed stimuli. In the present paper we employ a neural eld approach to describe the activity patterns in V1 analytically. Expressions for spatio-temporal receptive elds are derived and tted to experimental data. The model supports the idea that the observed RF-restructuring is mainly driven by statedependent LGN ring patterns (burst vs. tonic mode).

A Self-Organizing Model for the Development of Ocular Dominance and Orientation Columns in the Virtual Cortex

Abstract: A binocular model describing the ontogenetic development in the visual nervous system is presented. It consists of a set of deterministic differential equations which have been derived from an statistical approach. The evolution of the solution is led by the spontaneous generation of input activity, characterized in this model by its spatial and temporal decorrelation. The development of a connection depends on the output activity of both connected neurons; for this purpose, Hebbian and anti-Hebbian learning have been used. The model can explain some properties observed in natural brains such as the appearance of ocular domains and orientation selectivity in the V1 visual cortex development.

The role of activity in corpus callosum development in the visual cortex

Abstract: Many previous models have explained how ocular dominance- and orientation selective columns could develop in the visual cortex. However, few models have investigated the topographic arrangement of interhemispheric connections between primary visual areas. A model of the development of corpus callosum connections in cat visual cortex was presented by Hely (1999). The model showed how the observed hour-glass pattern of callosal receptive field (RF) positions arises as a consequence of the retinotopic mapping onto the cortex. In the cat, callosal connections only form between cells with visual field RFs located close to the vertical meridian. In contrast in rat visual cortex, callosal connections also form between cells with RFs in the peripheral visual field. This cannot be accounted for using a Hebbian rule based on spatial information alone. The current generic model of primary visual cortex was extended by including optic-flow/motion information. This extra input enabled cells with peripheral RFs fields in the medial cortex to make connections to the opposite hemisphere. The results from the model suggest that in some species motion information may affect the development of connections in primary visual cortex.