Classical Receptive Field Research Articles

A laminar cortical model for 3D perception of slanted and curved surfaces and of 2D images: development, attention, and bistability

A model of laminar visual cortical dynamics proposes how 3D boundary and surface representations arise from viewing slanted and curved 3D objects and 2D images. The 3D boundary representations emerge from non-classical receptive field interactions within intracortical and intercortical feedback circuits. Such non-classical interactions within cortical areas V1 and V2 contextually disambiguate classical receptive field responses to ambiguous visual cues using cells that are sensitive to colinear contours, angles, and disparity gradients. Remarkably, these cell types can all be explained as variants of a unified perceptual grouping circuit whose most familiar example is a 2D colinear bipole cell. Model simulations show how this circuit can develop cell selectivity to colinear contours and angles, how slanted surfaces can activate 3D boundary representations that are sensitive to angles and disparity gradients, how 3D filling-in occurs across slanted surfaces, how a 2D Necker cube image can be represented in 3D, and how bistable 3D Necker cube percepts occur. The model also explains data about slant aftereffects and 3D neon color spreading. It shows how chemical transmitters that habituate, or depress, in an activity-dependent way can help to control development and also to trigger bistable 3D percepts and slant aftereffects. Attention can influence which of these percepts is perceived by propagating selectively along object boundaries.

The impact of suppressive surrounds on chromatic properties of cortical neurons.

Stimulation of the suppressive surround of a cortical neuron affects the responsivity and tuning of the classical receptive field (CRF) on several stimulus dimensions. In V1 and V2 of macaques prepared for acute electrophysiological experiments, we explored the chromatic sensitivity of the surround and its influence on the chromatic tuning of the CRF. We studied receptive fields of single neurons with patches of drifting grating of optimal spatial frequency and orientation and variable size, modulated along achromatic or isoluminant color directions. The responses of most neurons declined as the patch was enlarged beyond the optimal size (surround suppression). In V1 the suppression evoked by isoluminant gratings was less than one-half that evoked by achromatic gratings. Consequently, many cells were most sensitive to achromatic modulation when patches just covered the CRF but were most sensitive to isoluminant modulation when patches were enlarged to cover the suppressive surround. Non-oriented neurons that were strongly chromatically opponent generally lacked suppressive surrounds. In V2 most neurons showed equal surround suppression from isoluminant gratings and achromatic gratings. This makes the relative sensitivity of V2 neurons to achromatic and isoluminant gratings mainly independent of the size of the grating. We also measured the chromatic properties of the CRF in the presence of differently colored surrounds. In neither V1 nor V2 did the surround alter the chromatic tuning of the CRF. Cortical mechanisms sensitive to chromatic contrast seem to provide little input to the suppressive surrounds of V1 neurons but substantial input to those of V2 neurons.

Orientation bias of the extraclassical receptive field of the relay cells in the cat's dorsal lateral geniculate nucleus

The spatial properties of the extraclassical receptive fields (ECRF) of neurons responding to a stimulus restricted to it and its interaction with the classical receptive field (CRF) in visual information processing were investigated in 74 relay cells in the dorsal lateral geniculate nucleus (LGNd) of anesthetized cats. The results demonstrate that the ECRF of most relay cells in the LGNd responded preferentially to a grating stimulus of low spatial frequency through a mechanism of spatial summation. These biased cells showed a significant orientation bias which was relatively smaller than that of the CRF. The preferred orientations of the ECRF were not correlated with those of the CRF in most relay cells. The orientation biased ECRFs and CRFs interacted with each other in a non-linear way, resulting in a great diversity of response properties. Overall, the CRF played a more significant role than the ECRF in determining a cell's orientation bias and preferred orientation. The ECRF mostly showed a modulatory role mainly in suppressing and/or in partially facilitating the neural response to stimulation in the CRF although in some cases, the ECRF did determine a cell's responsiveness and orientation sensitivity. These results suggest that the ECRF might contribute to the ability of the LGNd neurons to detect some complex features such as texture segmentation and provide a subcortical contribution to the integrative field of visual cortical cells through receiving inputs from retinal ganglion cells with similar orientation biased extended surrounds [Neuroscience 98 (2000) 207].

Time Course and Time-Distance Relationships for Surround Suppression in Macaque V1 Neurons

Iso-orientation surround suppression is a powerful form of visual contextual modulation in which a stimulus of the preferred orientation of a neuron placed outside the classical receptive field (CRF) of the neuron suppresses the response to stimuli within the CRF. This suppression is most often attributed to orientation-tuned signals that propagate laterally across the cortex, activating local inhibition. By studying the temporal properties of surround suppression, we have uncovered characteristics that challenge standard notions of surround suppression. We found that the latency of suppression depended on its strength. Across cells, strong suppression arrived on average 30 msec earlier than weak suppression, and suppression sometimes arrived faster than the excitatory CRF response. We compared the relative latency of CRF response onset and offset with the relative latency of suppression onset and offset. Response onset was delayed relative to response offset in the CRF but not in the surround. This is not the expected result if neurons targeted by suppression are like those that generate it. We examined the time course of suppression as a function of distance of the surround stimulus from the CRF and found that suppression was predominantly sustained for nearby stimuli and predominantly transient for distant stimuli. By comparing the latency of suppression for nearby and distant stimuli, we found that orientation-tuned suppression could effectively propagate across 6 - 8 mm of cortex at approximately 1 m/sec. This is considerably faster than expected for horizontal cortical connections previously implicated in surround suppression. We offer refinements to circuits for surround suppression that account for these results and describe how feedback from cells with large CRFs can account for the rapid propagation of suppression within V1.

The Receptive-Field Organization of Simple Cells in Primary Visual Cortex of Ferrets under Natural Scene Stimulation

The responses of simple cells in primary visual cortex to sinusoidal gratings can primarily be predicted from their spatial receptive fields, as mapped using spots or bars. Although this quasilinearity is well documented, it is not clear whether it holds for complex natural stimuli. We recorded from simple cells in the primary visual cortex of anesthetized ferrets while stimulating with flashed digitized photographs of natural scenes. We applied standard reverse-correlation methods to quantify the average natural stimulus that invokes a neuronal response. Although these maps cannot be the receptive fields, we find that they still predict the preferred orientation of grating for each cell very well (r = 0.91); they do not predict the spatial-frequency tuning. Using a novel application of the linear reconstruction method called regularized pseudoinverse, we were able to recover high-resolution receptive-field maps from the responses to a relatively small number of natural scenes. These receptive-field maps not only predict the optimum orientation of each cell (r = 0.96) but also the spatial-frequency optimum (r = 0.89); the maps also predict the tuning bandwidths of many cells. Therefore, our first conclusion is that the tuning preferences of the cells are primarily linear and constant across stimulus type. However, when we used these maps to predict the actual responses of the cells to natural scenes, we did find evidence of expansive output nonlinearity and nonlinear influences from outside the classical receptive fields, orientation tuning, and spatial-frequency tuning.

Non-classical receptive field mediates switch in a sensory neuron's frequency tuning.

Animals have developed stereotyped communication calls to which specific sensory neurons are well tuned. These communication calls must be discriminated from environmental signals such as those produced by prey. Sensory systems might have evolved neural circuitry to encode both categories. In weakly electric fish, prey and communication signals differ in their spatial extent and frequency content. Here we show that stimuli of different spatial extents mimicking prey and communication signals cause a switch in the frequency tuning and spike-timing precision of electrosensory pyramidal neurons, resulting in the selective and optimal encoding of both stimulus categories. As in other sensory systems, pyramidal neurons respond only to stimuli located within a restricted region of space known as the classical receptive field (CRF). In some systems, stimulation outside the CRF but within a non-classical receptive field (nCRF) can modulate the neural response to CRF stimulation even though nCRF stimulation alone fails to elicit responses. We show that pyramidal neurons possess a nCRF and that it can modulate the response to CRF stimuli to induce this neurobiological switch in frequency tuning.

Gain control from beyond the classical receptive field in primate primary visual cortex.

Gain control is a salient feature of information processing throughout the visual system. Heeger (1991, 1992) described a mechanism that could underpin gain control in primary visual cortex (V1). According to this model, a neuron's response is normalized by dividing its output by the sum of a population of neurons, which are selective for orientations covering a broad range. Gain control in this scheme is manifested as a change in the semisaturation constant (contrast gain) of a V1 neuron. Here we examine how flanking and annular gratings of the same or orthogonal orientation to that preferred by a neuron presented beyond the receptive field modulate gain in V1 neurons in anesthetized marmosets (Callithrix jacchus). To characterize how gain was modulated by surround stimuli, the Michaelis-Menten equation was fitted to response versus contrast functions obtained under each stimulus condition. The modulation of gain by surround stimuli was modelled best as a divisive reduction in response gain. Response gain varied with the orientation of surround stimuli, but was reduced most when the orientation of a large annular grating beyond the classical receptive field matched the preferred orientation of neurons. The strength of surround suppression did not vary significantly with retinal eccentricity or laminar distribution. In the marmoset, as in macaques (Angelucci et al., 2002a,b), gain control over the sort of distances reported here (up to 10 deg) may be mediated by feedback from extrastriate areas.

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: ∼2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: ∼0.025° visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Spectral integration in A1 of awake primates: neurons with single- and multipeaked tuning characteristics.

We investigated modulations by stimulus components placed outside of the classical receptive field in the primary auditory cortex (A1) of awake marmosets. Two classes of neurons were identified using single tone stimuli: neurons with single-peaked frequency tuning characteristics (147/185, 80%) and neurons with multipeaked frequency tuning characteristics (38/185, 20%), referred to as single- and multipeaked units, respectively. Each class of neurons was further studied using two-tone paradigms in which the frequency, intensity, and timing of the second tone were systematically varied while a unit was driven by the first tone placed at a unit's characteristic frequency (CF) if it was single-peaked or at one of multiple spectral peaks if it was multipeaked. The main findings were: 1) excitatory spectral peaks in the frequency tuning of the multipeaked units were often harmonically related. 2) Multipeaked units showed facilitation in their responses to combinations of two harmonically related tones placed at the spectral peaks of their frequency tuning. The two-tone facilitation was strongest for the simultaneously presented tones. 3) In 76 of 113 single-peaked units studied using the two-tone paradigm, facilitatory and/or inhibitory modulations by distant off-CF tones were observed. This distant inhibition differed from flanking (or side-band) inhibitions near CF. 4) In single-peaked units, the distant off-CF inhibitions were dominated by tones at frequencies that were harmonically related to the CF of a unit, whereas the facilitation by off-CF tones was observed for a wide range of frequencies. And 5) in both single- and multipeaked units, sound levels of two interacting tones determined whether the two tones produced excitation or inhibition. The largest facilitation was achieved by using two tones at their corresponding preferred sound levels. Together, these findings suggest that extracting or rejecting harmonically related components embedded in complex sounds may represent fundamental signal processing properties in different classes of A1 neurons.

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.

Coding of Natural Scenes in Primary Visual Cortex

Natural scene coding in ferret visual cortex was investigated using a new technique for multi-site recording of neuronal activity from the cortical surface. Surface recordings accurately reflected radially aligned layer 2/3 activity. At individual sites, evoked activity to natural scenes was weakly correlated with the local image contrast structure falling within the cells' classical receptive field. However, a population code, derived from activity integrated across cortical sites having retinotopically overlapping receptive fields, correlated strongly with the local image contrast structure. Cell responses demonstrated high lifetime sparseness, population sparseness, and high dispersal values, implying efficient neural coding in terms of information processing. These results indicate that while cells at an individual cortical site do not provide a reliable estimate of the local contrast structure in natural scenes, cell activity integrated across distributed cortical sites is closely related to this structure in the form of a sparse and dispersed code.

Orientation and spatiotemporal tuning of cells in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii.

The metatherians (marsupials) have been separated from eutherians (placentals) for approximately 135 million years. It might, therefore, be expected that significant independent evolution of the visual system has occurred. The present paper describes for the first time the orientation, direction and spatiotemporal tuning of neurons in the primary visual cortex of an Australian marsupial, the wallaby Macropus eugenii. The stimuli consisted of spatial sinusoidal gratings presented within apertures covering the classical receptive fields of the cells. The neurons can be classified as those with clear ON and OFF zones and those with less well-defined receptive field structures. Seventy-percent of the total cells encountered were strongly orientation selective (tuning functions at half height were less than 45 degrees ). The preferred orientations were evenly distributed throughout 360 degrees for cells with uniform receptive fields but biased towards the vertical and horizontal for cells with clear ON-OFF zones. Many neurons gave directional responses but only a small percentage of them (4%) showed motion opponent properties (i.e. they were excited by motion in one direction and actively inhibited by motion in the opposite direction). The median peak temporal tuning for cells with clear ON-OFF zones and those without were 3 Hz and 6 Hz, respectively. The most common peak spatial frequency tuning for the two groups were 2 cycles per degree and 0.5 cycles per degree, respectively. Spatiotemporal tuning was not always the same for preferred and antipreferred direction motion. In general, the physiology of the wallaby cortex was similar to well studied eutherian mammals suggesting either convergent evolution or a highly conserved architecture that stems from a common therian ancestor.

Spatial organization and magnitude of orientation contrast interactions in primate V1.

We have explored the spatial organization of orientation contrast effects in primate V1. Our stimuli were either concentric patches of drifting grating of varying orientation and diameter or grating patches displaced in x-y coordinates around a central patch overlying the classical receptive field (CRF). All cells in the sample exhibited response suppression to iso-oriented stimuli exceeding the CRF. Changing the outer stimulus orientation revealed five response patterns: 1) orientation alignment suppression (17% of cells)-a suppressive component tuned to the same orientation as the cell's optimal, 2) orientation contrast facilitation (63%)-responses to orientation contrast stimuli exceeded those to the center stimulus alone, 3) nonorientation specific suppression (3%), 4) mixed general suppression and alignment suppression (14%), and 5) orientation contrast suppression (14%)-cross-oriented stimuli evoked stronger suppression than iso-oriented stimuli. Thus most cells (94%) showed larger responses to orientation contrast stimuli than to iso-oriented stimuli, and over one-half showed orientation contrast facilitation. There appeared to be a spatially structured organization of the zones driving the different response patterns with respect to the CRF. Nonorientation-specific suppression and orientation contrast suppression were predominantly evoked by orientation contrast borders located within the CRF, and orientation contrast facilitation was mainly driven by surround stimuli lying outside the CRF. This led to different response patterns according to border location. Zones driving orientation contrast facilitation were not necessarily contiguous to, nor uniformly distributed around, the CRF. Our data support two processes underlying orientation contrast enhancement effects: a simple variation in the strength of surround suppression drawing on the fact that surround suppression is tuned to the same orientation as the CRF and a second process driven by orientation contrast that enhanced cells' responses to CRF stimulation by either dis-inhibition or orientation contrast facilitation. We suggest these processes may serve to enhance response levels to salient image features such as junctions and corners and may contribute to orientation pop-out.

Selectivity and spatial distribution of signals from the receptive field surround in macaque V1 neurons.

The responsiveness of neurons in V1 is modulated by stimuli placed outside their classical receptive fields. This nonclassical surround provides input from a larger portion of the visual scene than originally thought, permitting integration of information at early levels in the visual processing stream. Signals from the surround have been reported variously to be suppressive and facilitatory, selective and unselective. We tested the specificity of influences from the surround by studying the interactions between drifting sinusoidal gratings carefully confined to conservatively defined center and surround regions. We found that the surround influence was always suppressive when the surround grating was at the neuron's preferred orientation. Suppression tended to be stronger when the surround grating also moved in the neuron's preferred direction, rather than its opposite. When the orientation in the surround was 90 degrees from the preferred orientation (orthogonal), suppression was weaker, and facilitation was sometimes evident. The tuning of surround signals therefore tended to match the tuning of the center, though the tuning of the surround was somewhat broader. The tuning of suppression also depended on the contrast of the center grating-when the center grating was reduced in contrast, orthogonal surround stimuli became relatively more suppressive. We also found evidence for the tuning of the surround being dependent to some degree on the stimulus used in the center-suppression was often stronger for a given center stimulus when the parameters of the surround grating matched the parameters of the center grating even when the center grating was not itself of the optimal direction or orientation. We also explored the spatial distribution of surround influence and found an orderly relationship between the orientation of grating patches presented to regions of the surround and the position of greatest suppression. When surround gratings were oriented parallel to the preferred orientation of the receptive field, suppression was strongest at the receptive field ends. When surround gratings were orthogonal, suppression was strongest on the flanks. We conclude that the surround has complex effects on responses from the classical receptive field. We suggest that the underlying mechanism of this complexity may involve interactions between relatively simple center and surround mechanisms.

Scene segmentation and attention in primate cortical areas V1 and V2.

The responses of many neurons in primary visual cortex are modulated by stimuli outside the classical receptive field in ways that may contribute to integrative processes like scene segmentation. To explore this issue, single-unit neuronal responses were recorded in monkey cortical areas V1 and V2 to visual stimuli containing either a figure or a background pattern over the receptive field. Figures were defined either by orientation contrast or by illusory contours. In all conditions, the stimulation over the RF and its nearby surround was identical. Both figure types enhanced the average population response in V1 and V2. For orientation contrast figures, enhancement averaged 50% in V2 and 30% in V1; for illusory contour figures, the enhancement averaged 24% in V2 and 18% in V1. These differences were statistically significant for figure type but not for visual area. In V2, the latency of enhancement to illusory contour-defined figures was longer than that to orientation-defined figures. Neuronal responses were recorded while the monkey performed a directed-attention task. Enhancement to both figure types was observed even when attention was directed away from the figure. Attention slightly enhanced responses in V2, independent of figure type, but did not affect responses in V1. There was no discernible effect of attention on background firing rate in either V1 or V2. These results suggest that scene segmentation is a distributed process, in which neuronal signals at successive stages of the visual hierarchy and over time increasingly reflect the global structure of the image. This process occurs independent of directed visual attention.

Circuits for Local and Global Signal Integration in Primary Visual Cortex

Contrast-dependent changes in spatial summation and contextual modulation of primary visual cortex (V1) neuron responses to stimulation of their receptive field reveal long-distance integration of visual signals within V1, well beyond the classical receptive field (cRF) of single neurons. To identify the cortical circuits mediating these long-distance computations, we have used a combination of anatomical and physiological recording methods to determine the spatial scale and retinotopic logic of intra-areal V1 horizontal connections and inter-areal feedback connections to V1. We have then compared the spatial scales of these connectional systems to the spatial dimensions of the cRF, spatial summation field (SF), and modulatory surround field of macaque V1 neurons. We find that monosynaptic horizontal connections within area V1 are of an appropriate spatial scale to mediate interactions within the SF of V1 neurons and to underlie contrast-dependent changes in SF size. Contrary to common beliefs, these connections cannot fully account for the dimensions of the surround field. The spatial scale of feedback circuits from extrastriate cortex to V1 is, instead, commensurate with the full spatial range of center-surround interactions. Thus these connections could represent an anatomical substrate for contextual modulation and global-to-local integration of visual signals. Feedback projections connect corresponding and equal-sized regions of the visual field in striate and extrastriate cortices and cover anisotropic parts of visual space, unlike V1 horizontal connections that are isotropic in the macaque. V1 isotropic connectivity demonstrates that anisotropic horizontal connections are not necessary to generate orientation selectivity. Anisotropic feedback connections may play a role in contour completion.

Feedback from V1 and inhibition from beyond the classical receptive field modulates the responses of neurons in the primate lateral geniculate nucleus.

It is well established that the responses of neurons in the lateral geniculate nucleus (LGN) can be modulated by feedback from visual cortex, but it is still unclear how cortico-geniculate afferents regulate the flow of visual information to the cortex in the primate. Here we report the effects, on the gain of LGN neurons, of differentially stimulating the extraclassical receptive field, with feedback from the striate cortex intact or inactivated in the marmoset monkey, Callithrix jacchus. A horizontally oriented grating of optimal size, spatial frequency, and temporal frequency was presented to the classical receptive field. The grating varied in contrast (range: 0-1) from trial to trial, and was presented alone, or surrounded by a grating of the same or orthogonal orientation, contained within either a larger annular field, or flanks oriented either horizontally or vertically. V1 was ablated to inactivate cortico-geniculate feedback. The maximum firing rate of LGN neurons was greater with V1 intact, but was reduced by visually stimulating beyond the classical receptive field. Large horizontal or vertical annular gratings were most effective in reducing the maximum firing rate of LGN neurons. Magnocellular neurons were most susceptible to this inhibition from beyond the classical receptive field. Extraclassical inhibition was less effective with V1 ablated. We conclude that inhibition from beyond the classical receptive field reduces the excitatory influence of V1 in the LGN. The net balance between cortico-geniculate excitation and inhibition from beyond the classical receptive field is one mechanism by which signals relayed from the retina to V1 are controlled.

Clustered Organization of Neurons with Similar Extra-Receptive Field Properties in the Primary Visual Cortex

The primary visual cortex is organized into clusters of cells having similar classical receptive field (CRF) properties. Nonclassical, extra-receptive fields (ERFs) can either inhibit or facilitate the response elicited by stimulation within the CRF. Here, we report that in the primary visual cortex of cat, neurons with similar inhibitory or facilitatory ERF properties are also grouped into clusters. These clusters are randomly distributed in all cortical layers, with no detectable relationship with orientation and ocular dominance columns. This functional organization of neurons with respect to ERF properties may allow an efficient processing of global visual information.

Disinhibition Outside Receptive Fields in the Visual Cortex

By definition, the region outside the classical receptive field (CRF) of a neuron in the visual cortex does not directly activate the cell. However, the response of a neuron can be influenced by stimulation of the surrounding area. In previous work, we showed that this influence is mainly suppressive and that it is generally limited to a local region outside the CRF. In the experiments reported here, we investigate the mechanisms of the suppressive effect. Our approach is to find the position of a grating patch that is most effective in suppressing the response of a cell. We then use a masking stimulus at different contrasts over the grating patch in an attempt to disinhibit the response. We find that suppressive effects may be partially or completely reversed by use of the masking stimulus. This disinhibition suggests that effects from outside the CRF may be local. Although they do not necessarily underlie the perceptual analysis of a figure–ground visual scene, they may provide a substrate for this process.

Disinhibition outside receptive fields in the visual cortex.

By definition, the region outside the classical receptive field (CRF) of a neuron in the visual cortex does not directly activate the cell. However, the response of a neuron can be influenced by stimulation of the surrounding area. In previous work, we showed that this influence is mainly suppressive and that it is generally limited to a local region outside the CRF. In the experiments reported here, we investigate the mechanisms of the suppressive effect. Our approach is to find the position of a grating patch that is most effective in suppressing the response of a cell. We then use a masking stimulus at different contrasts over the grating patch in an attempt to disinhibit the response. We find that suppressive effects may be partially or completely reversed by use of the masking stimulus. This disinhibition suggests that effects from outside the CRF may be local. Although they do not necessarily underlie the perceptual analysis of a figure-ground visual scene, they may provide a substrate for this process.