Single cell responses from the optic tectum of the zebra finch (Taeniopygia guttata castanotis Gould)

Responses of neurons of the optic tectum, the prominent, highly laminated mesencephalic station of the tectofugal visual pathway in birds, to computer-generated and other visual stimuli were examined in zebra finches. Our study shows that the contralateral retina projects to the tectum in topographic order. The representation of the visual field is tilted against the horizon by 22°. The representation of the contralateral hemifield extends to the ipsilateral side by 15°. Most neurons have receptive fields with excitatory centres of different shapes and inhibitory surround. A new type of neuronal receptive field is described which has an excitatory centre and a surround which is movement sensitive and preferably excited by very small spots. The first type of neurons is mostly located in upper tectal layers, the latter only in deeper layers. Excitatory centre sizes increase with depth, and there is a tendency of smaller receptive fields in the foveal region. The representation of the frontal visual field does not show specializations which could be expected if it were used for fixation of grain during pecking. Our results are in accordance with previous behavioural experiments. Accepted: 30 April 1999     

Adaptive surround modulation in cortical area MT

Visual motion perception relies on two opposing operations: integration and segmentation. Integration overcomes motion ambiguity in the visual image by spatial pooling of motion signals, whereas segmentation identifies differences between adjacent moving objects. For visual motion area MT, previous investigations have reported that stimuli in the receptive field surround, which do not elicit a response when presented alone, can nevertheless modulate responses to stimuli in the receptive field center. The directional tuning of this "surround modulation" has been found to be mainly antagonistic and hence consistent with segmentation. Here, we report that surround modulation in area MT can be either antagonistic or integrative depending upon the visual stimulus. Both types of modulation were delayed relative to response onset. Our results suggest that the dominance of antagonistic modulation in previous MT studies was due to stimulus choice and that segmentation and integration are achieved, in part, via adaptive surround modulation.     

Reaching beyond the classical receptive field of V1 neurons: horizontal or feedback axons?

It is commonly assumed that the orientation-selective surround field of neurons in primary visual cortex (V1) is due to interactions provided solely by intrinsic long-range horizontal connections. We review evidence for and against this proposition and conclude that horizontal connections are too slow and cover too little visual field to subserve all the functions of suppressive surrounds of V1 neurons in the macaque monkey. We show that the extent of visual space covered by horizontal connections corresponds to the region of low contrast summation of the receptive field center mechanism. This region encompasses the classically defined receptive field center and the proximal surround. Beyond this region, feedback connections are the most likely substrate for surround suppression. We present evidence that inactivation of higher order areas leads to a major decrease in the strength of the suppressive surround of neurons in lower order areas, supporting the hypothesis that feedback connections play a major role in center-surround interactions.     

Seeing beyond the receptive field in primary visual cortex

Recent studies on the response properties of neurons in primary visual cortex emphasize the dynamics and the complexities of facilitatory and suppressive interactions between the receptive field center and surrounding areas of visual space. These observations raise new questions about the circuitry responsible for receptive field surround effects and their contribution to visual perception.     

Predictive coding: a fresh view of inhibition in the retina

Interneurons exhibiting centre--surround antagonism within their receptive fields are commonly found in peripheral visual pathways. We propose that this organization enables the visual system to encode spatial detail in a manner that minimizes the deleterious effects of intrinsic noise, by exploiting the spatial correlation that exists within natural scenes. The antagonistic surround takes a weighted mean of the signals in neighbouring receptors to generate a statistical prediction of the signal at the centre. The predicted value is subtracted from the actual centre signal, thus minimizing the range of outputs transmitted by the centre. In this way the entire dynamic range of the interneuron can be devoted to encoding a small range of intensities, thus rendering fine detail detectable against intrinsic noise injected at later stages in processing. This predictive encoding scheme also reduces spatial redundancy, thereby enabling the array of interneurons to transmit a larger number of distinguishable images, taking into account the expected structure of the visual world. The profile of the required inhibitory field is derived from statistical estimation theory. This profile depends strongly upon the signal: noise ratio and weakly upon the extent of lateral spatial correlation. The receptive fields that are quantitatively predicted by the theory resemble those of X-type retinal ganglion cells and show that the inhibitory surround should become weaker and more diffuse at low intensities. The latter property is unequivocally demonstrated in the first-order interneurons of the fly's compound eye. The theory is extended to the time domain to account for the phasic responses of fly interneurons. These comparisons suggest that, in the early stages of processing, the visual system is concerned primarily with coding the visual image to protect against subsequent intrinsic noise, rather than with reconstructing the scene or extracting specific features from it. The treatment emphasizes that a neuron's dynamic range should be matched to both its receptive field and the statistical properties of the visual pattern expected within this field. Finally, the analysis is synthetic because it is an extension of the background suppression hypothesis (Barlow & Levick 1976), satisfies the redundancy reduction hypothesis (Barlow 1961 a, b) and is equivalent to deblurring under certain conditions (Ratliff 1965).     

Coarse-to-fine changes of receptive fields in lateral geniculate nucleus have a transient and a sustained component that depend on distinct mechanisms

Visual processing in the brain seems to provide fast but coarse information before information about fine details. Such dynamics occur also in single neurons at several levels of the visual system. In the dorsal lateral geniculate nucleus (LGN), neurons have a receptive field (RF) with antagonistic center-surround organization, and temporal changes in center-surround organization are generally assumed to be due to a time-lag of the surround activity relative to center activity. Spatial resolution may be measured as the inverse of center size, and in LGN neurons RF-center width changes during static stimulation with durations in the range of normal fixation periods (250-500 ms) between saccadic eye-movements. The RF-center is initially large, but rapidly shrinks during the first ~100 ms to a rather sustained size. We studied such dynamics in anesthetized cats during presentation (250 ms) of static spots centered on the RF with main focus on the transition from the first transient and highly dynamic component to the second more sustained component. The results suggest that the two components depend on different neuronal mechanisms that operate in parallel and with partial temporal overlap rather than on a continuously changing center-surround balance. Results from mathematical modeling further supported this conclusion. We found that existing models for the spatiotemporal RF of LGN neurons failed to account for our experimental results. The modeling demonstrated that a new model, in which the response is given by a sum of an early transient component and a partially overlapping sustained component, adequately accounts for our experimental data.     

A system of insect neurons sensitive to horizontal and vertical image motion connects the medulla and midbrain

Summary The response properties and gross morphologies of neurons that connect the medulla and midbrain in the butterfly Papilio aegeus are described. The neurons presented give direction-selective responses, i.e. they are excited by motion in the preferred direction and the background activity of the cells is inhibited by motion in the opposite, null, direction. The neurons are either maximally sensitive to horizontal motion or to slightly off-axis vertical upward or vertical downward motion, when tested in the frontal visual field. The responses of the cells are dependent on the contrast frequency of the stimulus with peak values at 5–10 Hz. The receptive fields of the medulla neurons are large and are most sensitive in the frontal visual field. Examination of the local and global properties of the receptive fields of the medulla neurons indicates that (1) they are fed by local elementary motion-detectors consistent with the correlation model and (2) there is a non-linear spatial integration mechanism in operation.     

Eye morphology and visual acuity in the pipevine swallowtail (Battus philenor) studied with a new method of measuring interommatidial angles

Because of the important role sensory systems play in the behaviour of animals, information on sensory capabilities is of great value to behavioural ecologists in the development of hypotheses to explain behaviour. In compound eyes, interommatidial angles are a key determinant of visual acuity but methods for measuring these angles are often demanding and limited to live animals with a pseudopupil. Here we present a new technique for measuring interommatidial angles that is less demanding in terms of technology than other techniques but still accurate. It allows measurements in eyes without a pseudopupil such as dark eyes or even museum specimens. We call this technique the radius of curvature estimation (RCE) method. We describe RCE and validate the method by comparing results from RCE with those from pseudopupil analysis for the butterfly Asterocampa leilia. As an application of RCE we measure the eyes of the butterfly Battus philenor, a species whose visually guided behaviour is well known but whose eye structure and visual acuity are unknown. We discuss the results of the eye morphology in B. philenor in relation to their behaviour and ecology. We contend that RCE fills a gap in the repertoire of techniques available to study peripheral determinants of spatial resolution in compound eyes, because it can be applied on species with dark eyes. RCE then opens up for sampling a larger number of specimens, which, in combination with being able to use museum specimens, makes it possible to quantitatively test ecologically and evolutionarily driven hypotheses about vision in animals in a new way.     

Mathematical principles in afferent visual neurons: Differentiation,integration and transient proportionality related to receptive fields and shift-effect

The dual reciprocal and antagonistic organization of B- and D-neurons of the afferent visual system is obtained using differentiation and integration as mathematical equivalents of visual information processing by an impulse frequency code. The spatial and temporal derivatives lead to the transient responses. A constant and a time-dependent term proportional to the luminance distribution describe the sustained response components and the shift-effect of retinal on- and off-center ganglion cells. Receptive field properties of lateral geniculate cells and their antagonistic shift-effect are obtained by passing the retinal output, i.e. the difference between B- and D-neurons' activity, once again through the same operations. However, the factor of proportionality is applied to the retina alone. The surprisingly small difference between retinal and geniculate receptive field properties on the one hand and the dramatic change from a synergistic to an antagonistic shift-effect on the other hand are thereby explained. The theory offers an understanding of a a possible functional significance of the shift-effect as a mechanism of transientrestoration of visual information, which prevents the system from total fading by means of shifts of the retinal image, normally produced by eye movements.     

Electrophysiological and morphological characterization of the medulla bilateral neurons that connect bilateral optic lobes in the cricket,Gryllus bimaculatus

The medulla bilateral neurons (MBNs) in the cricket brain directly connect two optic lobes and have been suggested to be involved in mutual coupling between the bilateral optic lobe circadian pacemakers. Single unit analysis with intracellular recording and staining with Lucifer Yellow was carried out to reveal morphology and physiology of the MBNs. Neurons having a receptive field in the rostral part of the compound eye showed greater response and a higher sensitivity to light than those having receptive fields in the ventro-caudal or dorsal portions. The MBN showed diurnal change in their responsiveness to light; the light-induced response in the night was about 1.3, 5 and 2 times of that in the day in MBN-1s, -3s and -4s, respectively. These results suggest that the MBNs mainly encode the temporal information by the magnitude of light-induced responses. The differences in magnitude of light-induced responses and of daily change in photo-responsiveness among MBNs may suggest that each group of MBNs plays different functional role in visual and/or circadian systems.     

Sensory and behavioral properties of neurons in posterior parietal cortex of the awake, trained monkey.

Neurons in posterior parietal cortex of the awake, trained monkey respond to passive visual and/or somatosensory stimuli. In general, the receptive fields of these cells are large and nonspecific. When these neurons are studied during visually guided hand movements and eye movements, most of their activity can be accounted for by passive sensory stimulation. However, for some visual cells, the response to a stimulus is enhanced when it is to be the target for a saccadic eye movement. This enhancement is selective for eye movements into the visual receptive field since it does not occur with eye movements to other parts of the visual field. Cells that discharge in association with a visual fixation task have foveal receptive fields and respond to the spots of light used as fixation targets. Cells discharging selectively in association with different directions of tracking eye movements have directionally selective responses to moving visual stimuli. Every cell in our sample discharging in association with movement could be driven by passive sensory stimuli. We conclude that the activity of neurons in posterior parietal cortex is dependent on and indicative of external stimuli but not predictive of movement.     

Rabbit visual cortical neurons with simple and complex receptive fields

Receptive fields of neurons of the rabbit visual cortex selective for stimulus orientation were investigated. These receptive fields were less well differentiated than those of the analogous neurons of the cat visual cortex (large in size and circular in shape). Two mechanisms of selectivity for stimulus orientation were observed: inhibition between on and off zones of the receptive field (sample type) and oriented lateral inhibition within the same zone of the receptive field (complex type). Lateral inhibition within the same zone of the receptive field also took place in unselective neurons; "complex" selective neurons differed from them in the orientation of this inhibition. A combination of both mechanisms was possible in the receptive field of the same neuron. It is suggested that both simple and complex receptive fields are derivatives of unselective receptive fields and that "complex" neurons are not the basis for a higher level of analysis of visual information than in "simple" neurons.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 13–21, January–February, 1978.     

Eye movements modulate visual receptive fields of V4 neurons

The receptive field, defined as the spatiotemporal selectivity of neurons to sensory stimuli, is central to our understanding of the neuronal mechanisms of perception. However, despite the fact that eye movements are critical during normal vision, the influence of eye movements on the structure of receptive fields has never been characterized. Here, we map the receptive fields of macaque area V4 neurons during saccadic eye movements and find that receptive fields are remarkably dynamic. Specifically, before the initiation of a saccadic eye movement, receptive fields shrink and shift towards the saccade target. These spatiotemporal dynamics may enhance information processing of relevant stimuli during the scanning of a visual scene, thereby assisting the selection of saccade targets and accelerating the analysis of the visual scene during free viewing.     

Characterisation of columnar neurons and visual signal processing in the medulla of the locust optic lobe by system identification techniques

We describe visual responses of seventeen physiological classes of columnar neuron from the retina, lamina and medulla of the locust (Locusta migratoria) optic lobe. Many of these neurons were anatomically identified by neurobiotin injection. Characterisation of neuronal responses was made by moving and flash stimuli, and by two system identification techniques: 1. The first-order spatiotemporal kernel was estimated from response to a spatiotemporal white-noise stimulus; 2. A set of kernels to second order was derived by the maximal-length shift register (M-sequence) technique, describing the system response to a two-channel centre-surround stimulus. Most cells have small receptive fields, usually with a centre diameter of about 1.5°, which is similar to that of a single receptor in the compound eye. Linear response components show varying spatial and temporal tuning, although lateral inhibition is generally fairly weak. Second-order nonlinearities often have a simple form consistent with a static nonlinear transformation of the input from the large monopolar cells of the lamina followed by further linear filtering.Abbreviations LMC large monopolar cell - LVF long visual fibre - RF receptive field - SMC small monopolar cell - SVF short visual fibre     

Retinotopische Beziehungen und Struktur rezeptiver Felder im Tectum opticum und Praetectum der Katze

Zusammenfassung 1.Von 600 Neuronen des Colliculus superior und Praetectums der Katze wurde mit Stahlmikroelektroden abgeleitet und der Ableitort markiert. Die Lage der rezeptiven Felder wurde mit bewegten und stationären Lichtreizen bestimmt und dem Ableitort zugeordnet.2.Im Colliculus superior und Praetectum fanden sich richtungsspezifische und richtungsunspezifische Bewegungsneurone. Ein Teil der praetectalen Neurone reagierte richtungsspezifisch auf Bewegungen vom Tier weg und auf das Tier zu (S-Neurone).3.Innerhalb einer senkrecht zur Oberfläche des Colliculus verlaufenden Penetrationssäule nahm die Feldgröße bei gleichbleibender Feldposition mit zunehmender Tiefe zu. Zwischen Ableitort und Feldposition bestand eine systematische retinotopische Beziehung. Die Projektion des vertikalen O-Meridians des Gesichtsfeldes verlief im rostralen Drittel des Colliculus von medial nach lateral, die des horizontalen O-Meridians in der Mitte des Colliculus von rostral nach caudal. Das Projektionsschema eines Colliculus enthält einen nasalen Teil der ipsilateralen Gesichtsfeldhälfte.4.Im Praetectum verlief die Projektion vertikaler Meridiane am caudalen Ende von medial nach lateral und überlappte sich teilweise mit dem Projektionsgebiet des vertikalen O-Meridians im Colliculus. Die horizontalen Meridiane verliefen so von caudal nach rostral, daß das Projektionsschema des Praetectums spiegelbildlich zu dem des Colliculus superior angeordnet war. Dieses Projektionsschema galt nur für den Nucleus tractus optici und die Area praetectalis. Die übrigen praetectalen Kerne mit zum Teil sehr großen rezeptiven Feldern und spezifischen Reaktionsweisen erhielten keine retinotopische Projektion.5.Rezeptive Felder der oberflächennahen Schichten waren uniform on-, off-oder on-off strukturiert, Felder tiefergelegener Einheiten waren ungeordnet aus on-, off- und on-off Bezirken zusammengesetzt. Binocular erregbare Neurone zeigten für beide Augen gleiche Position und Struktur der rezeptiven Felder.6.Die Ergebnisse wurden mit den an anderen Tierarten erhobenen Befunden verglichen. Ihre mögliche funktionelle Bedeutung wurde diskutiert.

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Retinotopic relationship and structure of receptive fields in the optic tectum and pretectum of the cat

Summary 1.600 neurons of the cat's superior colliculus and pretectum were recorded and marked with stainless-steel microelectrodes. The position and structure of receptive fields were tested with stationary flickering and moving stimuli. The position of the stimuli in the visual field was determined by the direction of the lamp projecting the light-points because animal and lamp were arranged in a fixed relationship to the screen. The positions of the stimuli were described in a coordinate system based on the horizontal-and vertical zeromeridean of the retina.2.About 55% of tectal neurons are directionally selective and signal mainly movements directed to the periphery of the visual field. Neurons of the pretectum have the same response characteristics as neurons of the superior colliculus but in addition some are selectively responsive to movements towards the animal or away from it (S-neurons).3.Neurons in one functional column (diameter 0.5 mm, length 3.6 mm) perpendicular to the surface of the superior colliculus react to the same position and preferred direction of a moving stimulus. The size, complexity and directional selectivity of the receptive fields increase with the depth of the recorded neurons. The projection of the vertical zero-meridean passes across the rostral part of the colliculus but does not form the rostral border of the superior colliculus. The nasal part of the ipsilateral visual field projects to the most rostral part of the superior colliculus. The projection of the horizontal zeromeridean passes rostro-caudally in a nearly sagittal plane down the middle of the colliculus. Along this projection-line the resolving power is 13°/column in the caudal part and 6°/column in the rostral part of the superior colliculus. The size of the receptive fields increase with their excentricity in the visual field. (Average of field diameters: 26±13°).4.The diameter of receptive fields in the pretectum was 21±11°, except for a few very large fields (70° and larger). Along the medio-lateral axis of the pretectum there was a retinotopic organization identical to that in the colliculus. Along the caudo-rostral axis, the retinotopic organization was the mirror image of that in the colliculus. No retinotopic organization was observed in the so-called deep pretectal nucleus or in the nucleus of the posterior commissure. Neurons of these nuclei may represent more complex levels in the visual pathway.5.The more superficial neurons of the colliculus (0.1–1.8 mm deep) react mainly with uniform on-, off- or on-off responses to stationary flickered stimuli, i.e. their receptive fields (7–20° in diameter) are uniformly on-, off- or on-off. The deeper neurons (2 mm and deeper) have receptive fields (20–40° in diameter) with compound but not antagonistic structure. No receptive fields showed on- or off-inhibition. Binocularly driven neurons have the same position and structure of their receptive fields for both eyes.6.A survey of the literature reveals that all vertebrates so far investigated show small differences in the destination and retinotopic organization of their retinofugal fibre projections and in the types of tectal receptive fields. These differences seem to indicate an adaption to the development of binocular representation of the center of the visual field, of a specialized area of the retina and of a retino-cortical system.

     

Neurons with complex receptive fields in the stratum griseum centrale of the zebra finch (Taeniopygia guttata castanotis Gould) optic tectum

The electrophysiological and morphological features of visually driven neurons of the stratum griseum centrale of the zebra finch optic tectum were studied by extracellular recording and staining techniques. Stratum griseum centrale neuron responses are sustained in most cases. Receptive fields are big, up to 150 degrees of the visual field. The excitatory center (hot spot) varies in size from 1 degrees to 15 degrees. It can be mapped by small static stimuli, adapts slower than the surround, and has a shape comparable to the excitatory fields of upper-layer neurons. In contrast, the big surround shows responses only to small moving objects which elicit a typical pattern of alternating bursts and silent periods. Alternatively, the same stimuli elicit long-lasting bursts followed by strong adaption. Anatomically, stratum griseum centrale neurons are characterized by far reaching dendrites which terminate with "bottlebrush"-like endings in the upper retinorecipient layers. In addition, they are connected with retinorecipient structures by an interneuron located between layers 10 and 11. The role of the structure of inputs for the organization of the receptive fields is discussed.     

Neurons with visual receptive fields independent of eye position in the caudal section of the ventral bank of cat cruciate slucus

Neurons responding to tactile and visual stimulation were found in the caudal section of the cruciate slucus ventral bank in awake cats. Tactile receptive fields were located on the face, mainly around the mouth. Visual stimuli evoked a response when presented close to the tactile receptive field. It was found that the visual responses of these bimodal neurons located in layer VI of the cortex display spatial consistency. The position of these visual receptive fields remained constant through saccadic eye movements, while still linked to the tactile receptive field.Institute for Research into Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neifofiziologiya, Vol. 18, No. 6, pp. 800–805, November–December, 1986.     

Did neural pooling for night vision lead to the evolution of neural superposition eyes?

Observations of the infrared deep pseudopupil, optical determinations of the corneal nodal point, and histological methods were used to relate the visual fields of individual rhabdomeres to the array of ommatidial optical axes in four insects with open rhabdoms: the tenebrionid beetle Zophobas morio, the earwig Forficula auricularia, the crane fly Tipula pruinosa, and the backswimmer Notonecta glauca.The open rhabdoms of all four species have a central pair of rhabdomeres surrounded by six peripheral rhabdomeres. At night, a distal pigment aperture is fully open and the rhabdom receives light over an angle approximately six times the interommatidial angle. Different rhabdomeres within the same ommatidium do not share the same visual axis, and the visual fields of the peripheral rhabdomeres overlap the optical axes of several near-by ommatidia. During the day, the pigment aperture is considerably smaller, and all rhabdomeres share the same visual field of about two interommatidial angles, or less, depending on the degree of light adaptation. The pigment aperture serves two functions: (1) it allows the circadian rhythm to switch between the night and day sampling patterns, and (2) it works as a light driven pupil during the day.Theoretical considerations suggest that, in the night eye, the peripheral retinula cells are involved in neural pooling in the lamina, with asymmetric pooling fields matching the visual fields of the rhabdomeres. Such a system provides high sensitivity for nocturnal vision, and the open rhabdom has the potential of feeding information into parallel spatial channels with different tradeoffs between resolution and sensitivity. Modification of this operational principle to suit a strictly diurnal life, makes the contractile pigment aperture superfluous, and decreasing angular sensitivities together with decreasing pooling fields lead to a neural superposition eye.Abbreviations DPP deep pseudopupil - LMC large monopolar cell     

Symmetry between the visual B- and D-systems and equivalence of center and surround: Studies of light increment and decrement in retinal and geniculate neurons of the cat

The dual center surround organization of retinal and geniculate neurons in two antagonistic subsystems B and D, having on-center or off-center receptive fields and signalling brightness or darkness respectively, has been studied by local light increments and decrements. Intensity response functions obtained by the introduction and withdrawal of small center spots either brighter or darker than a common homogeneous field are similar in a given neuron, but the phasic responses are stronger in on-center neurons than in off-center neurons. Center size increments and decrements, however, lead to equal excitations in the B- and D-system, respectively, provided that both luminance steps start from the same level and are of equal size on a linear scale. Decrementing and incrementing the surrounding luminance of the same optimal center spots lead to equal surround responses in the two subsystems if the two luminance steps terminate at the same level. This lateral activation is elicited by light decrement in the B-system and by light increment in the D-system. Center and surround responses within a given subsystem are of comparable amplitude, but generally slightly stronger responses are elicited by optimal center increments (decrements) than by the equivalent surround decrements (increments) which lead to the same spatial contrast for B-(D-) neurons. The symmetry relations between the B- and D-system and the equivalence relations between center and surround in each subsystem hold for retinal and geniculate neurons. The difference between center and surround response latencies is about 9 ms in both subsystems at the retinal and 14 ms at the geniculate level. Stimulus response functions of on- and off-center neurons are unified on the basis of linear relative luminance scales.     

Functional architecture of long-range perceptual interactions

The pattern of lateral interactions in the primary visual cortex, which has emerged from recent studies, conforms to the grouping rules of similarity, proximity, smoothness and closure. The goal of this paper is to understand the perceptual salience of oriented elements that are specifically organized to form a smooth contour. An overview of recent studies, in combination with new experimental results, is presented here to emphasis the idea that visual responses depend on input from both the center and the surround of the classical receptive field (CRF). It is assumed that normal lateral interactions produce a neuronal network that is formed by two antagonistic mechanisms: (i) excitation, that is spatially organized along the optimal orientation (collinear), and is predominant near the contrast threshold of the neuron, and (ii) inhibition, that is less selective and is distributed diffusely around the cell's response field. Thus, the inputs from the CRF and the anisotropic surround are summated non-linearly. The specificity of the facilitation and suppression along the collinear direction suggests the existence of second-order elongated collinear filters, which may increase the response similarity between neurons responding to elongated stimulus, thus may enhance the perceptual salience of anisotropic configurations such as contours. This causal connection is particularly evident in amblyopes, where abnormal development of the network results in the abnormal perception of contours.