Showing papers on "Orientation column published in 1997"

Retinotopic organization in human visual cortex and the spatial precision of functional MRI.

Abstract: A method of using functional magnetic resonance imaging (fMRI) to measure retinotopic organization within human cortex is described. The method is based on a visual stimulus that creates a traveling wave of neural activity within retinotopically organized visual areas. We measured the fMRI signal caused by this stimulus in visual cortex and represented the results on images of the flattened cortical sheet. We used the method to locate visual areas and to evaluate the spatial precision of fMRI. Specifically, we: (i) identified the borders between several retinotopically organized visual areas in the posterior occipital lobe; (ii) measured the function relating cortical position to visual field eccentricity within area V1; (iii) localized activity to within 1.1 mm of visual cortex; and (iv) estimated the spatial resolution of the fMRI signal and found that signal amplitude falls to 60% at a spatial frequency of 1 cycle per 9 mm of visual cortex. This spatial resolution is consistent with a linespread whose full width at half maximum spreads across 3.5 mm of visual cortex. In a series of experiments, we measured the retinotopic organization of human cortical area V1 and identified the locations of other nearby retinotopically organized visual areas. We also used the retinotopic organization of human primary visual cortex to measure the spatial localization and spatial resolution that can be obtained from functional magnetic resonance imaging (fMRI) of human visual cortex. Human primary visual cortex (area V1) is located in the

Correlated size variations in human visual cortex, lateral geniculate nucleus, and optic tract

Abstract: We have examined several components of the human visual system to determine how the dimensions of the optic tract, lateral geniculate nucleus (LGN), and primary visual cortex (V1) vary within the same brain. Measurements were made of the cross-sectional area of the optic tract, the volumes of the magnocellular and parvocellular layers of the LGN, and the surface area and volume of V1 in one or both cerebral hemispheres of 15 neurologically normal human brains obtained at autopsy. Consistent with previous observations, there was a two- to threefold variation in the size of each of these visual components among the individuals studied. Importantly, this variation was coordinated within the visual system of any one individual. That is, a relatively large V1 was associated with a commensurately large LGN and optic tract, whereas a relatively small V1 was associated with a commensurately smaller LGN and optic tract. This relationship among the components of the human visual system indicates that the development of its different parts is interdependent. Such coordinated variation should generate substantial differences in visual ability among humans.

The Perceptual Grouping Criterion of Colinearity is Reflected by Anisotropies of Connections in the Primary Visual Cortex

Abstract: An important step in the processing of visual patterns is the segmentation of the retinal image. Neuronal responses evoked by the contours of individual objects need to be identified and associated for further joint processing. These grouping operations are based on a number of Gestalt criteria. Here we report that connections in the visual cortex of the cat exhibit a highly significant anisotropy, preferentially linking neurons activated by contours that have similar orientation and are aligned colinearly. These anatomical data suggest a close relation between the perceptual grouping criterion of colinearity and the topology of tangential intracortical connections. We propose that tangential intracortical connections support perceptual grouping by modulating the saliency of distributed cortical responses in a context-dependent way. The present data are compatible with the hypothesis that the criteria for this grouping operation are determined by the architecture of the tangential connections.

Orientation Selectivity in Pinwheel Centers in Cat Striate Cortex

Abstract: In primary visual cortex of higher mammals neurons are grouped according to their orientation preference, forming "pinwheels" around "orientation centers." Although the general structure of orientation maps is largely resolved, the microscopic arrangement of neuronal response properties in the orientation centers has remained elusive. The tetrode technique, enabling multiple single-unit recordings, in combination with intrinsic signal imaging was used to reveal the fine-grain structure of orientation maps in these locations. The results show that orientation centers represent locations where orientation columns converge containing normal, sharply tuned neurons of different orientation preference lying in close proximity.

Disruption of orientation tuning visual cortex by artificially correlated neuronal activity

Abstract: In the primary visual cortex, the development of orientation selectivity is influenced by patterns of neural activity. The introduction of artificially correlated activity into the visual pathway (through synchronous activation of retinal ganglion cell axons in the optic nerve) substantially weakens the orientation selectivity of neurons in superficial and deep cortical layers. This is consistent with activity having an instructive role in shaping cortical neuron receptive field tuning properties.

Physiological properties of inhibitory interneurons in cat striate cortex.

Abstract: Physiological and morphological properties of identified interneurons in the striate cortex of the cat were studied in vivo by intracellular recording and staining with biocytin. In conformity with in vitro studies, these non-pyramidal fast spiking cells have very brief action potentials associated with a high rate of fall, and a large hyperpolarizing afterpotential. These cells show high discharge rates, little or no spike frequency adaptation in response to depolarizing current injection, as well as a diverse range of firing patterns. Three of the cells were labeled and were found to be aspiny or sparsely spiny basket cells, with bitufted or radial dendritic arrangements, in layers II-IV. Their axonal arborizations were more dense near their somata and extended horizontally or vertically. Of 13 visually responsive cells tested, the receptive field properties of six cells and the orientation and direction preferences of eight cells were determined. Five of the successfully mapped cells had simple receptive fields while one had a complex receptive field type. The orientation and direction tuning properties of the overlapping set of eight cells showed a broad spectrum ranging from unselective to tightly tuned. The majority exhibited a clear preference for orientation and none of the cells were clearly direction selective. Quantitative analysis of the temporal properties of the spike trains during visual stimulation and spontaneous activity revealed that these cells do not exhibit any significant periodic activity, and fired at rates that were well below their maximum in response to depolarizing current pulses.

Distortions of visuotopic map match orientation singularities in primary visual cortex

Abstract: The map of orientation columns in primary visual cortex (V1) is known to show strong local distortions, with a generally smooth progression of orientation preference across extended regions of cortex, interrupted by sharp jumps (fractures) and point singularities. The map of visual space on V1, in contrast, has been assumed to be locally smooth and isotropic. We find, on the contrary, that the map of visual space on cat V1 shows strong and systematic local distortions in register with inhomogeneities in the orientation map, with the rate of receptive field movement across cortex being largely proportional to the local rate of change of orientation. This suggests possible systematic local variations in the functional connectivity of short-range lateral connections that underlie local cortical processing.

Topographic receptive fields and patterned lateral interaction in a self-organizing model of the primary visual cortex

Abstract: A self-organizing neural network model for the simultaneous development of topographic receptive fields and lateral interactions in cortical maps is presented. Both afferent and lateral connections adapt by the same Hebbian mechanism in a purely local and unsupervised learning process. Afferent input weights of each neuron self-organize into hill-shaped profiles, receptive fields organize topographically across the network, and unique lateral interaction profiles develop for each neuron. The model suggests that precise cortical maps develop only if the initial receptive fields are topographically ordered or if they cover the whole receptive surface. It demonstrates how patterned lateral connections develop based on correlated activity, and explains why lateral connection patterns closely follow receptive field properties such as ocular dominance. The model predicts a dual role for lateral connections: to support self-organization of receptive fields, and to represent low-level Gestalt knowledge acquired during development of the cortex.

Organization of sensory cortex in a Madagascan insectivore, the tenrec (Echinops telfairi).

Abstract: We identified subdivisions of somatosensory cortex, and the borders and extents of auditory and visual cortex in Madagascan tenrecs (Echinops telfairi) by using microelectrode recording techniques and cortical myeloarchitecture. There was evidence for three distinct somatosensory fields. The primary somatosensory area (S1) contained an orderly representation of the contralateral body surface that stained darkly for myelin. Neurons were activated by light touch, and receptive fields were often small, especially for the snout. Immediately rostral to S1, a lightly myelinated rostral field (R) also contained a representation of the contralateral body, although the internal topography was not fully determined. Neurons in R responded to manipulations of body parts and tissue displacements. A small, moderately myelinated area lateral to S1 was termed PV/S2 because it possessed features that were similar to both the parietal ventral area (PV) and the second somatosensory area (S2) in other mammals. Neurons in PV/S2 responded to light tactile stimulation. A densely myelinated oval of cortex caudal to PV/S2, the auditory area (A), contained neurons that responded to clicks, and the densely myelinated caudomedial visual area (V) contained neurons that were activated by stimulation of one or both eyes. Some characteristics of V were similar to the primary visual area (V1) described in other mammals. A visual area located in rostromedial cortex (RV) contained neurons that were highly responsive to visual stimulation. Area RV may be a specialization of tenrecs or an elaboration of a visuomotor field that has been retained in most extant mammals. The results support the view that most of the neocortex of primitive mammals was composed of a few sensory areas. J. Comp. Neurol. 379:399–414, 1997. © 1997 Wiley-Liss, Inc.

Figure‐Ground Segregation at Contours: a Neural Mechanism in the Visual Cortex of the Alert Monkey

Abstract: An important task of vision is the segregation of figure and ground in situations of spatial occlusion. Psychophysical evidence suggests that the depth order at contours is defined early in visual processing. We have analysed this process in the visual cortex of the alert monkey. The animals were trained on a visual fixation task which reinforced foveal viewing. During periods of active visual fixation, we recorded the responses of single neurons in striate and prestriate cortex (areas V1, V2, and V3/V3A). The stimuli mimicked situations of spatial occlusion, usually a uniform light (or dark) rectangle overlaying a grating texture of opposite contrast. The direction of figure and ground at the borders of these rectangles was defined by the direction of the terminating grating lines (occlusion cues). Neuronal responses were analysed with respect to figure-ground direction and contrast polarity at such contours. Striate neurons often failed to respond to such stimuli, or were selective for contrast polarity; others were non-selective. Some neurons preferred a certain combination of figure-ground direction and contrast polarity. These neurons were rare both in striate and prestriate cortex. The majority of neurons signalled figure-ground direction independent of contrast polarity. These neurons were only found in prestriate cortex. We explain these responses in terms of a model which also explains neuronal signals of illusory contours. These results suggest that occlusion cues are used at an early level of processing to segregate figure and ground at contours.

Contribution of Area 17 to Cell Responses in the Striate‐recipient Zone of the Cat's Lateral Posterior‐Pulvinar Complex

Abstract: The cat's lateral posterior-pulvinar complex (LP-pulvinar) contains three main representations of the visual field. The lateral part of the LP nucleus (LPl or striate-recipient zone) is the only region of these extrageniculate nuclei which receives afferents from the primary visual cortex. We investigated the contribution of area 17 to the response properties (orientation and spatial frequency tuning functions) of LPl neurons by cooling or lesioning the visual cortex. Responses of 40 LPl cells were studied before, during and after the reversible cooling of the striate cortex. When tested for orientation, a total of 10 units out of 28 was affected (36%). For most of these cells (eight of 10), cooling the visual cortex yielded a reduction of the cells' visual responses without altering their orientation-selectivity (there was no significant change in the orientation tuning width). For only two cells, inactivation led to an increase in the response amplitude. Also, blocking the visual cortex never modified the direction-selectivity of LPl cells. When tested for spatial frequency, 12 neurons out of 33 were affected (36%) by the experimental protocol. In most cases, we observed a reduction in the responses at each spatial frequency tested, with no change in tuning bandwidth. For only three LPl cells, the effects of inactivation of the visual cortex were restricted to specific spatial frequencies, altering the profile of the spatial frequency tuning function. In five cats, removing area 17 reduced the proportion of visual neurons in LPl and the spared visually evoked responses were noticeably depressed. Despite the reduction in responsiveness, a few LPl receptive fields within the cortical scotoma were still sensitive to the orientation and/or direction of a moving stimulus. This last observation suggests that some properties in LPl could be generated either by circuits intrinsic to the LPl or by afferents from extrastriate cortical areas. Overall, these results indicate that projections from the visual cortex to the striate-recipient zone of the LP-pulvinar complex are mainly excitatory. Despite the strong impact of the area 17 projections, our data suggest that the extrastriate cortex could also play a role in the establishment of response properties in the cat's LPl.

Organization of somatosensory cortex and distribution of corticospinal neurons in the eastern mole (Scalopus aquaticus).

Abstract: The somatotopic organization of somatosensory cortex of the eastern mole (Scalopus aquaticus) was explored with multiunit microelectrode recordings from middle layers of cortex. The recordings revealed the presence of at least parts of two systematic representations of the body surface in the lateral cortex. One of the representations appears to be primary somatosensory cortex (S1), and it contained cytochrome oxidase dark regions, separated by light septa that formed isomorphs with some body parts. The rostral portion of this presumptive S1 cortex contained a face representation with a series of barrel-like cytochrome oxidase dark ovals that corresponded to the vibrissae on the snout. In caudolateral S1, light septa outline the palm and digits of the forepaw. Cortex caudal to S1, in the expected region of auditory cortex, responded to vibration, suggesting a modification of auditory cortex. Injections of wheat germ agglutinin-horseradish peroxidase into the cervical enlargement of the spinal cord revealed two dense foci of cortical cells that project to the spinal cord. The focus medial to the face region in S1 may correspond to primary motor cortex (M1). The second focus was coextensive with the somatosensory representation of the forelimb and the trunk in S1. The dense corticospinal projections from the forelimb representation of S1 and motor cortex may reflect sensorimotor specializations related to digging behaviors in moles.

Modelling binocular neurons in the primary visual cortex

Abstract: 1.1 Introduction Neurons sensitive to binocular disparity h a ve been found in the visual cortex of many mammals and in the visual wulst of the owl, and are thought to play a signiicant role in stereopsis Barlow et al. A number of physiologists have suggested that disparity might be encoded by a shift of receptive-eld position According to this position-shift model, disparity selective cells combine the outputs of similarly shaped, monocular receptive elds from diierent retinal positions in the left and right e y es. More recently, Ohzawa et al. (1990) and DeAngelis et al. (1991, 1995) have suggested that disparity sensitivity might instead be a result of interocular phase shifts. In this phase-shift model, the centers of the left-and right-eye receptive elds coincide, but the arrangement of receptive eld subregions is diierent. This chapter presents a formal description and analysis of a binocular energy model of disparity selectivity. According to this model, disparity selectivity results from a combination of position-shifts and/or phase-shifts. Our theoretical analysis suggests how one might perform an experiment to estimate the relative contributions of phase and position shifts to the disparity selectivity of binocular neurons, based on their responses to drifting sinusoidal grating stimuli of diierent spatial frequencies and disparities. We also show that for drifting grating stimuli, the binocular energy response (with phase and/or position shifts) is a sinusoidal function of disparity , consistent with the physiology of neurons in primary visual cortex (area 1

A Model for Orientation Tuning and Contextual Effects of Orientation Selective Receptive Fields

Abstract: We investigate a mean-field model which has previously been used to explain the response properties of orientation selective neurons in the primary visual cortex of monkeys and cats [2] Two mutually coupled orientation hypercolumns are setup as local amplifiers based on local recurrent excitation and inhibition We first investigate the individual hypercolumns The model correctly predicts contrast invariant tuning, but analytical and numerical results show that the contrast response functions of individual orientation columns do not saturate We therefore hypothesize that the cortical saturation effects found experimentally may be a consequence of the non-linear properties of single neurons rather than being an effect of different gains for inhibitory and excitatory cells [13] We then extend this model to cover non-classical receptive fields and contextual effects The model correctly predicts effective iso-orientation inhibition between hypercolumns As long as parameters are chosen to ensure contrast invariant orientation tuning, however, net cross-orientation facilitation emerges only, if cells of different orientation preference are connected across hypercolumns These results hint at deficiencies of this simple approach and suggest that contextual effects are mediated by populations of neurons, which are not take part of the local gain control

Spatial Receptive Fields of Single Neurons of Primary Auditory Cortex of the Cat

Abstract: Neurons in the primary auditory cortical field (AI) have been shown to be sensitive to the direction of a sound when the source is either in an anechoic free field (Middle- brooks et al, 1980; Rajan et al, 1990; Imig et al, 1990) or in anechoic virtual acoustic space (Brugge et al, 1994; 1996a,b) The spatial receptive fields obtained under these stimulus conditions are typically large in size at suprathreshold levels, often exceeding an acoustic hemifield; close to threshold their centers tend to lie on or near the acoustic axis How large receptive fields centered around the acoustic axis enable AI neurons to encode information about sound direction is not well understood, although it would appear that the time structure of the neuronal discharge within the receptive field plays a role (Mid- dlebrooks et al, 1994; Brugge et al, 1996) In this paper we review and extend our findings on directional sensitivity of isolated AI neurons to transient sound, employing conventional extracellular recording methods (Brugge et al, 1994,1996a) and a technique by which synthesized signals that mimic sounds coming from particular directions in space are delivered at the eardrums of Nembutal-anesthetized cats through a sealed and calibrated sound delivery system (Chan et al, 1993; Reale et al, 1996)

Topological singularities in cortical orientation maps: the sign theorem correctly predicts orientation column patterns in primate striate cortex

Abstract: Optical imaging methods have revealed the spatial arrangement of orientation columns across striate cortex, usually summarized in terms of two measurements at each cortical location: (i) a ‘best’ stimulus orientation, corresponding to the stimulus orientation that elicits a maximal response, and (ii) the magnitude of the response to the best orientation. This mapping has been described as continuous except at a set of singular points (also termed ‘vortices’ or ‘pinwheels’. Although prior work has shown that vortex patterns qualitatively similar to those observed in visual area 17 of the Macaque cortex can be produced by either band-pass or low-pass filtering of random vector fields, there has been to date little further topological characterization of the structure of cortical vortex patterns. Nevertheless, much theoretical work has been done in other disciplines on mappings analogous to the cortical orientation map. In particular, a recent theorem in the optics literature, termed the sign principle, stat...

Characterizing Neurons in the Primary Auditory Cortex of the Awake Primate Using Reverse Correlation

Abstract: While the understanding of the functional role of different classes of neurons in the awake primary visual cortex has been extensively studied since the time of Hubel and Wiesel (Hubel and Wiesel, 1962), our understanding of the feature selectivity and functional role of neurons in the primary auditory cortex is much farther from complete. Moving bars have long been recognized as an optimal stimulus for many visual cortical neurons, and this finding has recently been confirmed and extended in detail using reverse correlation methods (Jones and Palmer, 1987; Reid and Alonso, 1995; Reid et al., 1991; Ringach et al., 1997). In this study, we recorded from neurons in the primary auditory cortex of the awake primate, and used a novel reverse correlation technique to compute receptive fields (or preferred stimuli), encompassing both multiple frequency components and ongoing time. These spectrotemporal receptive fields make clear that neurons in the primary auditory cortex, as in the primary visual cortex, typically show considerable structure in their feature processing properties, often including multiple excitatory and inhibitory regions in their receptive fields. These neurons can be sensitive to stimulus edges in frequency composition or in time, and sensitive to stimulus transitions such as changes in frequency. These neurons also show strong responses and selectivity to continuous frequency modulated stimuli analogous to visual drifting gratings.

A ring model for spatiotemporal properties of simple cells in the visual cortex.

Abstract: A neural model is proposed for the spatiotemporal properties of simple cells in the visual cortex. In the model, several cortical cells are arranged on a ring, with mutual excitatory or inhibitory connections. The cells also receive excitatory inputs either from lagged and nonlagged cells of the lateral geniculate nucleus in one setting or from nonlagged cells in the other. Computer simulation shows that the cortical cells have spatiotemporally inseparable receptive fields in the former setting and separable fields in the latter; spatial profiles at a given time in the spatiotemporal fields are described with a Gabor function whose phase parameter varies regularly from 0 to 2π with rotation along the ring; the inseparable cells have directional selectivity as observed physiologically.

On the Anatomical Basis of Field Size, Contrast Sensitivity, and Orientation Selectivity in Macaque Striate Cortex: A Model Study

Abstract: Neurons in layer 4C in macaque striate cortex show a differential change in receptive field size and achromatic contrast sensitivity with depth, and exhibit orientation selective responses in the upper 4Cα sublayer. Using a computational model we first demonstrate that the observed change in receptive field size and contrast sensitivity can arise from a differential convergence of afferents from the P and M subdivisions of the lateral geniculate nucleus onto layer 4C spiny stellate cells - if one postulates that the two anatomically identified M1 and M2 subpopulations of the M afferents differentially project to different depth in the 4Cα subdivision. Number ratios and response properties of both M subpopulations are predicted and may now be tested experimentally. We then show that realistic orientation selective responses in upper 4Cα can emerge intracortically as a result of local lateral interactions, which are anisotropic, between spiny stellate cells and inhibitory interneurons. The model assumes that orientation bias and tuning are generated by the same cortical circuits and predicts a receptive field dynamics with an initial non orientation specific response.

[Gaze-position-dependent activities of striate cortex (V1) neurones of awake macaque monkeys].

Abstract: We recorded the activities of single neurons of the primary visual cortex in awake, behaving monkeys to test the influence of the position of gaze on cellular activity. Two monkeys (Macaca mulatta) were trained to fixate a small spot positioned sequentially at 25 locations on a viewing screen. About half (52%) of the neurons recorded showed a selective gaze field (GF), when monkey fixated at this field of view, the cell activities increased significantly. For the majority of the neurons, GF located at the contralateral field of view with respect to the hemisphere from which responses were recorded. The GF was usually found a few degrees peripheral to the related RF. Gaze-position-dependent neurons were found at different depths of the cortex, but mostly in the superficial and the deepest layers. The results indicate that the striate cortex neurons may code information about gaze position.

Geometry of Orientation Preference Map Determines Nonclassical Receptive Field Properties

Abstract: We propose a simple mechanism for the nonclassical receptive field property of orientation contrast sensitivity. Our model includes the long-range lateral connections linking cell populations of similar orientation preference, and the dynamics of local microcircuits which introduce a differential interaction whose sign depends on the post- and presynaptic activation. We demonstrate that the geometry of an orientation preference map determines the positions of cells sensitive for orientation contrasts, and we propose a simple statistical method to check the predictions of our model for experimentally given maps.

[Influence of visual stimuli on eye-position related activities of neurons in primary visual cortex (V1) of awake monkeys].

Abstract: Extracellular recordings were made in the primary visual cortex (V1) in two awake monkeys to test the influence of visual stimuli on the eye-position related activites of the neurons. While the monkeys gazed on a fixation point (FP) positioned sequentially at different locations on a TV screen, two types of visual stimuli were presented on the same screen: (1) A small light ring flashed repeatedly around the FP or (2) A prefered light bar shifted continuously across the cell's receptive field (RF). Both stimuli significantly enhanced the eye-position related activities and correspondingly increased the incidence of the eye-position dependent neurons. The results show that the integration of information on vision and on eye position may take place at quite the earliest stage of the visual cortices.

Multidimensional Filtering Inspired by Retino-cortical Projection: Application to Texture Segmentation

Abstract: We present a method for analysing frequency and orientation based on space-variant filtering (SVF). SVF response impulse depends on its spatial position. This filtering, inspired by retino-cortical map and orientation column of striate cortex, is based on a periodical projection. This special case of filtering requires less time to be computed than filter banks used for example in texture analysis. After the description of the method, the choice of the parameters which optimize the frequency cover are explained. Finally, an application for texture segmentation is presented. Results show that the high filtering selectivity makes the texture segmentation possible, even if these textures have close spectra.

Calculating conditions for the emergence of structure in self-organizing maps

Abstract: The self-organization of sensotopic maps in the brain has been modeled with various approaches, using models with linear [1, 2, 3, 4] and nonlinear [5, 6, 7] lateral interaction functions. In the realm of visual maps, all models are able to generate ocular dominance or orientation structure [8]. Models differ, however, with regard to more subtle effects, like the impact of input correlation on ocular dominance stripe width, the self-organization of oriented receptive fields from non-oriented stimuli or correlation functions, or the preferred angle of intersection between ocular dominance and orientation column systems. For a correct assessment of the behavior of particular models with regard to these or other phenomena, it is dangerous to rely on simulations only. Rather, analytic results on conditions for the pattern formation in map models are desirable. We present here a method for calculating such conditions for a map model with strong lateral nonlinearity, the high-dimensional version of Kohonen’s Self-Organizing Map (SOM). Using this method we then analyze two relevant models, a SOM-model for the development of orientation maps and a SOM-model for the development of ocular dominance maps.