Selective and invariant sensitivity to crosses and corners in cat striate neurons

Orientation Perception in Real and Virtual Environments

Spatial perception in virtual environments has been a topic of intense research. Arguably, the majority of this work has focused on distance perception. However, orientation perception is also an important factor. In this paper, we systematically investigate allocentric orientation judgments in both real and virtual contexts over the course of four experiments. A pattern of sinusoidal judgment errors known to exist in 2D perspective displays is found to persist in immersive virtual environments. This pattern also manifests itself in a real world setting using two differing judgment methods. The findings suggest the presence of a radial anisotropy that persists across viewing contexts. Additionally, there is some evidence to suggest that observers have multiple strategies for processing orientations but further investigation is needed to fully describe this phenomenon. We also offer design suggestions for 3D user interfaces where users may perform orientation judgments.

Novel Visual Illusions Related to Vasarely's ‘Nested Squares’ Show That Corner Salience Varies with Corner Angle

Vasarely's ‘nested-squares’ illusion shows that 90° corners can be more salient perceptually than straight edges. On the basis of this illusion we have developed a novel visual illusion, the ‘Alternating Brightness Star’, which shows that sharp corners are more salient than shallow corners (an effect we call ‘corner angle salience variation’) and that the same corner can be perceived as either bright or dark depending on the polarity of the angle (ie whether concave or convex: ‘corner angle brightness reversal’). Here we quantify the perception of corner angle salience variation and corner angle brightness reversal effects in twelve naive human subjects, in a two-alternative forced-choice brightness discrimination task. The results show that sharp corners generate stronger percepts than shallow corners, and that corner gradients appear bright or dark depending on whether the corner is concave or convex. Basic computational models of center – surround receptive fields predict the results to some degree, but not fully.

Complexity of Images: Experimental and Computational Estimates Compared

We tested whether visual complexity can be modeled through the use of parameters relevant to known mechanisms of visual processing. In psychophysical experiments observers ranked the complexity of two groups of stimuli: 15 unfamiliar Chinese hieroglyphs and 24 outline images of well-known common objects. To predict image complexity, we considered: (i) spatial characteristics of the images, (ii) spatial-frequency characteristics, (iii) a combination of spatial and Fourier properties, and (iv) the size of the image encoded as a JPEG file. For hieroglyphs the highest correlation was obtained when complexity was calculated as the product of the squared spatial-frequency median and the image area. This measure accounts for the larger number of lines, strokes, and local periodic patterns in the hieroglyphs. For outline objects the best predictor of the experimental data was complexity estimated as the number of turns in the image, as Attneave (1957 Journal of Experimental Psychology53 221–227) obtained for his abstract outlined images. Other predictors of complexity gave significant but lower correlations with the experimental ranking. We conclude that our modeling measures can be used to estimate the complexity of visual images but for different classes of images different measures of complexity may be required.

Mechanisms underlying the representation of angles embedded within contour stimuli in area V2 of macaque monkeys

We previously found that surprisingly many V2 neurons showed selective responses to particular angles embedded within continuous contours [M. Ito & H. Komatsu (2004)Journal of Neuroscience, 24, 3313-3324]. Here, we addressed whether the selectivity is dependent on the presence of individual constituent components or on the unique combination of these components. To reveal roles of constituent half-lines in response to whole angles, we conducted a quantitative model study after the framework of cascade models. Our linear-non-linear summation model implemented a few subunits selective to particular half-lines and was fitted to neuronal responses for each neuron. The study indicates that the best-fitting models well replicate the selectivity in the majority of V2 neurons and that the angle selectivity is dependent on a linear combination of responses to individual half-line components of the angles. The implication is that optimal angles are given by a combination of two preferred half-line components and the selectivity is sharpened by introducing suppression to non-preferred half-line components, rather than a specific facilitatory interaction between two preferred half-line components. The study indicates the participation of the gain control of responsiveness according to the number of half-line components. We also showed that the selectivity to acute angles depends on a combination of responses to one preferred component and weak responses to another component. Therefore, we concluded that the angle selectivity is dependent on selective responses to individual half-line components of the angles rather than a unique combination between them, whereas neurons could be selective to various angle widths at area V2.

Axon topography of layer IV spiny cells to orientation map in the cat primary visual cortex (area 18)

Our aim was to reveal the relationship between layer IV horizontal connections and the functional architecture of the cat primary visual cortex because these connections play important roles in the first cortical stage of visual signals integration. We investigated bouton distribution of spiny neurons over an orientation preference map using in vivo optical imaging, unit recordings, and single neuron reconstructions. The radial extent of reconstructed axons (14 star pyramidal and 9 spiny stellate cells) was ~1.5 mm. In the vicinity of the parent somata (<400 μm), boutons occupied chiefly iso-orientations, however, more distally, 7 cells projected preferentially to non-iso-orientations. Boutons of each cell were partitioned into 1-15 distinct clusters based on the mean-shift algorithm, of which 57 clusters preferred iso-orientations and 43 clusters preferred cross-orientations, each showing sharp orientation preference "tuning." However, unlike layer III/V pyramidal cells preferring chiefly iso-orientations, layer IV cells were engaged with broad orientations because each bouton cluster from the same cell could show different orientation preference. These results indicate that the circuitry of layer IV spiny cells is organized differently from that of iso-orientation dominant layer III/V cells and probably processes visual signals in a different manner from that of the superficial and deeper layers.

Recognition of lateralized halftone and outline images of everyday objects in conditions of masking

Recognition of the shapes of halftone and outline images of everyday objects in conditions of lateralized tachystoscopic presentation and different levels of noise masking (with "raindrops") by humans was studied. Mean group data for 15 subjects demonstrated significantly better recognition of outline images of everyday objects by the left hemisphere of the brain than the right at all levels of masking. Increases in masking produced gradual and significant degradation of recognition as compared with controls (recognition of unmasked figures). Recognition of outline images at all levels of masking was significantly better than recognition of halftone images of the same objects. In men, there were no significant differences between hemispheres either at different levels of masking or for different types of stimuli. The neurophysiological mechanisms and functional significance of these effects are discussed.

BOLD activation varies parametrically with corner angle throughout human retinotopic cortex

The Alternating Brightness Star (ABS) is an illusion that provides insight into the relationship between brightness perception and corner angle. Recent psychophysical studies of this illusion have shown that corner salience varies parametrically with corner angle, with sharp angles leading to strong illusory percepts and shallow angles leading to weak percepts. It is hypothesized that the illusory effects arise because of an interaction between surface corners and the shape of visual receptive fields: sharp surface corners may create hotspots of high local contrast due to processing by center-surround and other early receptive fields. If this hypothesis is correct, early visual neurons should respond powerfully to sharp corners and curved portions of surface edges. Indeed, the primary role of early visual neurons may be to localize the discontinuities along the edges of surfaces. If so, all early visual areas should show greater BOLD responses to sharp corners than to shallow corners. On the other hand, if corner processing is exclusively constrained to certain areas of the brain, only those specific areas will show greater responses to sharp vs shallow corners. To address this we explored the BOLD correlates of the ABS illusion in the human visual cortex using fMRI. We found that BOLD signal varies parametrically with corner angle throughout the visual cortex, offering the first neurophysiological correlates of the ABS illusion. This finding provides a neurophysiological basis for the previously reported psychophysical data that showed that corner salience varied parametrically with corner angle. We propose that all early visual areas localize discontinuities along the edges of surfaces, and that specific cortical corner-processing circuits further establish the specific nature of those discontinuities, such as their orientation.

Neurophysiological and simulation studies of striate cortex receptive field maps: the role of intracortical interneuronal interactions

Acute experiments on 27 adult anesthetized and immobilized cats investigated 101 on and off receptive fields in 67 neurons in visual cortex field 17 by mapping using single local stimuli presented sequentially at different parts of the visual field, as well as in combination with additional stimulation of the center of the receptive field. Both classical and combined mapping identified receptive fields with single receptive zones (63.4% and 29.3% respectively), along with fields consisting of several (2-5) excitatory and/or inhibitory zones (36.6% and 70.7%). We provide the first report of receptive fields with horseshoe, cross, and T shapes. Simulations of horizontal interneuronal interactions in the visual cortex responsible for the multiplicity of excitatory and inhibitory zones of receptive fields were performed. A role for cooperative interactions of neurons in this effect was demonstrated. The possible functional role of receptive fields of different types in extracting the features of visual images is discussed.

Characteristics of the responses of visual cortex neurons with sensitivity to bars or cross-shaped figures in cats

The magnitudes and latent periods of spike responses were recorded from 280 individual neurons tuned to the orientation of light bars or cross-shaped figures in the primary visual cortex (field 17) of the cat. In control experimental conditions, half of 195 cells preferred the bar (first group), the remainder preferring crosses (second group); the responses of neurons of the first group to bars and crosses were of similar magnitude, while in the second group, responses to crosses were significantly larger than responses to bars. The latent periods of responses to optimal bars in the first group of neurons were shorter than those in the second group, and became longer on exposure to crosses, while latent periods in the second group were shorter on exposure to crosses. In conditions of local bicuculline blockade of intracortical inhibition, about a quarter of 85 neurons were sensitive only to the bar, regardless of the presence or absence of inhibition. The remaining neurons were sensitive to crosses in at least one of the states and continued to have responses which were smaller in terms of absolute magnitude than the responses of group 1 neurons. The significance of these data for understanding the mechanisms of tuning of striate neurons to signal features and the temporal sequence of their operation is discussed.

The time course of disinhibition of visual cortex neurons and sensitivity to cross-shaped figures

The relationship between the responses of 74 neurons in field 17 of the cat cortex to presentation of cross-shaped figures flashing in their receptive fields and the asynchronicity with which the lines of the figures were presented were investigated. The cross sensitivity of neurons was studied with simultaneous, leading, and delayed activation of the disinhibitory zone of the receptive field in relation to the time at which its major excitatory and end-stopping inhibitory zones were stimulated. Two types of temporal interaction were identified between the receptive field zones determining cross sensitivity. In cells of the first type (14 of 23 cells), the response was maximal in conditions of simultaneous stimulation of the major and disinhibitory zones of the receptive field; neurons of the second type (nine of 23 cells) showed the opposite temporal relationship. Digital simulation showed that cross sensitivity in neurons of the first type was supported by disinhibition of end-stopping inhibition, while in neurons of the second type it depended on a combination of disinhibitory and convergence mechanisms.

Simulation studies of the role of intracortical inhibition in the formation of sensitivity to cross-shaped figures

The receptive fields of detector neurons for cross-shaped figures in the visual cortex were modeled in conditions of blockade of intracortical inhibition. The tuning of simulated neurons was compared with and without inhibition in the receptive field. In a simulated detector with convergence from two orientation detectors, acute tuning to the cross widened in the absence of inhibition, becoming invariant to the shape and orientation of the cross. A detector based on the disinhibition mechanism lost cross sensitivity when inhibition was blocked and became a detector for the orientation of a single bar. A model of a receptive field in which the inhibitory zones mask the tuning to a cross-shaped figure and in which blockade of inhibition affects only sensitivity is also proposed. We identified which of the properties of receptive field (configuration, location, zone weightings) allow them to simulate the properties of cat visual cortex field 17 neurons, these being sensitive to the shape and orientation of cross-shaped figures.

Dynamic changes in the tuning of striate neurons to the shapes of cross-shaped figures

Time slice analysis was used to study the dynamics of tuning to the shapes of cross-shaped figures flashing in the receptive fields of 83 neurons in the primary visual cortex (field 17) of the cat brain. Tuning was assessed in terms of the numbers of spikes in the overall response and its sequential 20-msec fragments. Only 11.7% of neurons produced reproducibly developing spike responses to a given shape (defined as the angle between the lines), i.e., had a preferred cross-shaped figure. In the remaining cases (88.3%), tuning of neurons to the shape of the cross showed dynamic changes. In 7.2% of cases, changes in the preferred shape of the cross occurred monophasically; changes were biphasic in 27.0% of cases, while in the remaining 54.1% of cases, the dynamics in changes in the preferred cross shape were undulatory. The tuning of receptive field zones is assessed as the cause of these effects and their difference from the previously observed dynamics of preferred orientations of single bars and cross-shaped figures; the functional significance of these effects is also discussed.

Nonlinear and higher-order approaches to the encoding of natural scenes

Linear operations can only partially exploit the statistical redundancies of natural scenes, and nonlinear operations are ubiquitous in visual cortex. However, neither the detailed function of the nonlinearities nor the higher-order image statistics are yet fully understood. We suggest that these complicated issues can not be tackled by one single approach, but require a range of methods, and the understanding of the crosslinks between the results. We consider three basic approaches: (i) State space descriptions can theoretically provide complete information about statistical properties and nonlinear operations, but their practical usage is confined to very low-dimensional settings. We discuss the use of representation-related state-space coordinates (multivariate wavelet statistics) and of basic nonlinear coordinate transformations of the state space (e.g., a polar transform). (ii) Indirect methods, like unsupervised learning in multi-layer networks, provide complete optimization results, but no direct information on the statistical properties, and no simple model structures. (iii) Approximation by lower-order terms of power-series expansions is a classical strategy that has not yet received broad attention. On the statistical side, this approximation amounts to cumulant functions and higher-order spectra (polyspectra), on the processing side to Volterra Wiener systems. In this context we suggest that an important concept for the understanding of natural scene statistics, of nonlinear neurons, and of biological pattern recognition can be found in AND-like combinations of frequency components. We investigate how the different approaches can be related to each other, how they can contribute to the understanding of cortical nonlinearities such as complex cells, cortical gain control, end-stopping and other extraclassical receptive field properties, and how we can obtain a nonlinear perspective on overcomplete representations and invariant coding in visual cortex.

Representation of angles embedded within contour stimuli in area V2 of macaque monkeys

Angles and junctions embedded within contours are important features to represent the shape of objects. To study the neuronal basis to extract these features, we conducted extracellular recordings while two macaque monkeys performed a fixation task. Angle stimuli were the combination of two straight half-lines larger than the size of the classical receptive fields (CRFs). Each line was drawn from the center to outside the CRFs in 1 of 12 directions, so that the stimuli passed through the CRFs and formed angles at the center of the CRFs. Of 114 neurons recorded from the superficial layer of area V2, 91 neurons showed selective responses to these angle stimuli. Of these, 41 neurons (36.0%) showed selective responses to wide angles between 60 degrees and 150 degrees that were distinct from responses to straight lines or sharp angles (30 degrees ). Responses were highly selective to a particular angle in approximately one-fourth of neurons. When we tested the selectivity of the same neurons to individual half-lines, the preferred direction was more or less consistent with one or two components of the optimal angle stimuli. These results suggest that the selectivity of the neurons depends on both the combination of two components and the responses to individual components. Angle-selective V2 neurons are unlikely to be specific angle detectors, because the magnitude of their responses to the optimal angle was indistinguishable from that to the optimal half-lines. We suggest that the extraction of information of angles embedded within contour stimuli may start in area V2.

The disinhibitory zone of the striate neuron receptive field and its sensitivity to cross-like figures

Acute experiments on immobilized anesthetized cats were used to confirm the suggestion that the sensitivity of many neurons on the primary visual cortex to cross-shaped, angular, and Y-shaped figures may be determined by the presence within their receptive fields of disinhibitory zones, which block end-stopping inhibition. A total of 55 neurons (84 functions, i.e.. on and off responses) were used for studies of sensitivity to crosses, and responses to single bars of different lengths were compared before and after stimulation of an additional lateral zone of the field (the presumptive disinhibitory zone), which was located in terms of responses to crosses. Seventeen of the 55 cells in which increases in the length of a single bar decreased responses, i.e., which demonstrated end-stopping inhibition, showed significant increases in responses (by an average factor of 2.06 +/- 0.16) during simultaneous stimulation of the lateral zone of the receptive field, which we interpreted as a disinhibitory effect on end-stopping inhibition. These data provide the first direct evidence for the role of end-stopping inhibition and its blockade by the disinhibitory zone of the receptive field in determining the sensitivity of some neurons in the primary visual cortex of the cat to cross-shaped figures.

Model studies of the mechanisms of tuning of visual cortex neurons to incomplete cross-shaped figures

Numerical simulation modeling of the receptive fields of visual cortex neurons able to detect cross-shaped figures with masked central or peripheral areas was performed. Receptive field models of two types were considered: those with antagonistic and cooperative interactions between the center and the periphery. Model neurons with receptive fields with reciprocal (antagonistic) interactions produced greater responses to peripheral or central crosses than to complete crosses. Studies using the model showed that the basis of this type of tuning could be provided by a disinhibition mechanism: blockade of the inhibitory zones in the center or periphery of the receptive field by activation of a lateral disinhibitory zone. A model with cooperative interactions between the center and periphery of the receptive field was also studied, in which responses to complete crosses were summed from the responses to the peripheral and central parts. Tuning of these model receptive fields was comparable to the sensitivity of real visual cortex neurons to the shape, size, and orientation of figures. The properties of model receptive fields (configuration, localization, and weightings of the various zones) allowing simulation of the properties of cat visual cortex field 17 neurons sensitive to the orientation and configuration of incomplete cross-shaped figures were identified.

Disinhibition as a mechanism for visual cortex neurons to tune to cross-shaped figures

A discrete simulation model of a receptive field selectively responding to cross-shaped figures, as seen in 40% of primary visual cortex neurons in the cat, was studied. The model was based on disinhibition of end-stop inhibition in the receptive field by the lateral disinhibition zone. These experiments showed that this mechanism can produce selective or, conversely, invariant tuning to the shape and orientation of cross-shaped figures and could underlie the high sensitivity of neurons to second-order image features.

Directional tunings independent of orientation in the primary visual cortex of the cat

A family of moving 'random-line' patterns was developed and used to study the directional tuning of 91 single units in cat primary visual cortex (V1). The results suggest that, in addition to the well-known orientation-dependent mechanism, there is also some kind of orientation-independent mechanism underlying the direction selectivity. The directional tuning of the neurons varies in accordance with the increase of orientation or non-orientation element in the stimulus.

Pattern and component motion selectivity in cortical area PMLS of the cat

Visual motion perception is one of the most prominent functions performed by the mammalian cerebral cortex. The moving images are commonly considered to be processed in two stages. The first-stage neurons are sensitive to the motion of one-dimensional orientated components, and their outputs are combined at the second stage to perceive the global motion of the whole pattern. Alternatively, the pattern motion may be signalled by monitoring a distinctive feature of the image, such as a line-end or a corner. In the present study, a series of 'random-line' patterns were used to measure the direction-tuning responses of 138 neurons in the posteromedial lateral suprasylvian area of the cat. The novel stimuli comprised identical thin line segments, with a length : width ratio no less than 10 : 1, which were moved perpendicularly or obliquely to their common orientation during the recordings. When the component lines were much shorter than the size of receptive field, the majority of cells were selective to the direction of pattern motion while only a small subset was sensitive to the direction of component motion. However, the response profiles of most cells became more component-motion selective with the increment of orientation element in stimulus by elongating the component lines in the patterns. These findings imply that the two-stage theory might be incomplete for modelling the visual motion analysis. Even at relatively low levels of the visual system, some kind of nonorientation-based processing may coexist with the orientation-sensitive processing in a dynamic competition, where one rises as the other falls depending upon the strength of the orientation element in the stimulus, so that under some circumstances it becomes possible to signal the veridical direction of pattern motion.

Tuning to Y-like figures in the cat striate neurons

Sensitivity to symmetric or asymmetric Y-like figures and crosses of different shapes and orientations flashed in the receptive field was studied in 101 neurons of the cat striate cortex (area 17) and compared with their orientation tuning to a single light bar. Selective sensitivity to the Y-like figure (figure/bar response ratio more than 1.25) was found in 78/101 neurons (77.2% of cases) and to the cross-in 54/101 units (53.4%). In 62.5% of neurons with sensitivity to both figures, sensitivity to the Y-like figure was higher than to a cross. Tuning to Y-like figure was typically (60%) selective to both its shape and orientation. The remaining Y-like selective neurons exhibited invariant tuning to orientation and/or shape of the figure. The preferred angles between two lines of Y-like figures were distributed in the range of 22.5-157.5 degrees with slight preference to 90 degrees, while crosses of 45 degrees and 90 degrees angles were preferable. Response magnitudes to a single bar, a Y-like figure and a cross were positively correlated. Possible mechanisms and functional implication of the striate sensitivity to Y-like figures are discussed.

Sensitivity of striate neurons to Y-like figures: experiment and simulation

Under stimulation of the receptive fields (RF) of neurons in the cat area 17 by flashing Y-like figures of different shape and orientation, the sensitivity to these figures was revealed in 72% of the studied cells, while 62% of units were sensitive to cross-like figures as well. Tuning to Y-like figures was typically selective to their shape and orientation, but in some cases it was invariant to these features. Response magnitudes to single bar, Y-like figure and cross were positively correlated. Simulation showed that the disinhibition might be a sufficient mechanism for effective detection of Y-like figures in a classical receptive field.

Image features selected by neurons of the cat primary visual cortex

The sensitivity of neurons in field 17 of the visual cortex in cats to cross-shaped, Y-shaped, and star-shaped figures flashing in the receptive field was studied. About 40% of the neurons studied (114 of 289) were found to generate large responses (with an average response factor of 3.06 +/- 0.32) to one of the figures flashing in the center of the receptive field, as compared with the responses produced to a single bar in the optimal orientation. Most of these neurons (72%) were selectively sensitive to the shape and orientation of figures; the remainder demonstrated some degree of tuning invariance to these properties. The latent periods of responses to figures were usually shorter than those of responses to bars. Tuning parameters for bars and figures were generally related: neurons with acute orientational tuning to a bar were usually highly selective to both the configuration and the orientation people figures. Separate or combined stimulation with crosses in the center and near periphery of the receptive fields demonstrated summation, antagonism, or the lack of any interaction between these zones in producing sensitivity to crosses. Local blockade of intracortical GABAergic inhibition by microiontophoretic application of bicuculline showed that in one third of the neurons studied, sensitivity to figures was generated or enhanced by inhibition in normal conditions, while one third of cells showed suppression by inhibition, and sensitivity in the remainder was independent of inhibition. These data show that reconsideration of existing concepts of the role of field 17 in selecting only first-order shape features of images (i.e., the orientations of single lines) is needed, since almost half the neurons in the cat primary visual cortex can efficiently detect second-order features (angles and line intersections).

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

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.

Interrelation of tuning characteristics to bar, cross and corner in striate neurons

Characteristics of responses and background activity, as well as of tuning to a single bar orientation and to cross or corner shape and orientation have been compared in one third (561174) of neurons in the cat striate cortex. Shortening of the response latency to cross vs bar, to corner vs bar and to corner vs cross was revealed in most of the units studied. Direct correlation between the response and tuning characteristics for bar, cross and corner was revealed: units with better tuning to one type of stimulus were typically better tuned to the other types of stimuli. At the middle cortical depth (700-1200 microm from the surface) we found a reliable improvement of response magnitude and latency, cross/bar response ratio and selectivity of tuning in comparison with more superficial and deeper layers. Although we could not find a direct correlation between characteristics of tuning to figures and the type of the receptive field (simple, complex or hypercomplex), our data pointed to a lower cross/bar ratio and selectivity of tuning in the units with small receptive fields. The functional implication of neuronal sensitivity to cross and corner and possible meaning of correlation between their functional characteristics are discussed.

Second-order features extraction in the cat visual cortex: selective and invariant sensitivity of neurons to the shape and orientation of crosses and corners

About 1/3 of neurons (52/174) studied in the cat striate cortex (area 17 or V1) gave a larger (by 3.2 times on average) response to a flashed cross or corner centered in the receptive field (RF) than to a single bar of optimal orientation. Most such neurons (71.4%) were found to be highly selective both to shape (angle between the lines) and to orientation of these figures. In the studied neuronal selection we also found all possible types of invariance of sensitivity to orientation and/or shape of these figures. We found neurons with selectivity to form of the figure and invariance to its orientation and vice versa. Some cells were found invariant both to form and orientation of the cross or corner but highly sensitive to flashing of any such figure in the RF. The role of RF center and surrounding area in sensitivity to cross figures was also studied in 44 additional units. Separated and combined stimulation of these zones revealed in different units summation, antagonism and absence of interaction of these zones by the selectivity index (figure/bar response ratio). Possible mechanisms of the described effects are discussed as well as their functional implication for second-order feature extraction in the visual cortex: selective or invariant sensitivity of neurons to the shape and orientation of the line-crossings.