Effects of spatial configuration on contrast detection

Simple reaction time and statistical facilitation: a parallel grains model

A race-like model is developed to account for various phenomena arising in simple reaction time (RT) tasks. Within the model, each stimulus is represented by a number of grains of information or activation processed in parallel. The stimulus is detected when a criterion number of activated grains reaches a decision center. Using the concept of statistical facilitation, the model accounts for many classical effects on mean simple RT, including those of stimulus area, stimulus intensity, stimulus duration, criterion manipulations, redundant stimuli, and the dissociation between intensity effects on simple RTs and temporal order judgments. The model is also consistent with distributional properties of simple RTs.

Perceptual grouping in the Ternus display: evidence for an 'association field' in apparent motion

We present psychophysical experiments designed to reveal the role of facilitative contour interactions (the so-called 'association field') in apparent motion. We use the Ternus display (a trio of horizontally aligned elements oscillating in apparent motion). This display is perceived in 'element' motion when interframe intervals (IFIs) are short, and in 'group' motion when IFIs are long. Using Gabor elements arranged collinearly or in parallel, IFI is varied to find group motion thresholds. Consistent with a role for collinearity in perceptual grouping, thresholds are lower for collinear displays. The collinear vs. parallel comparison is made while manipulating contrast, spatial frequency, eccentricity, phase, orientation jitter and element separation. Results show a clear effect of contrast not observed in lateral masking paradigms or in 'pathfinder' stimuli, with higher contrast promoting within-frame grouping, and evidence of facilitatory interactions among parallel elements (although over a smaller scale). The tendency for collinear displays to group more than parallel displays declined with eccentricity with no clear difference evident at 12 deg. These changes in group motion thresholds indicate changing association strengths among the elements and is accounted for in terms of an association field. Alternative accounts in terms of second-order collector units or visible persistence are considered but are not supported by the data.

Spatial coherence does not affect contrast discrimination for multiple Gabor stimuli

Gestalt grouping rules imply a process or mechanism for grouping together local features of an object into a perceptual whole. Several psychophysical experiments have been interpreted as evidence for constrained interactions between nearby spatial filter elements and this has led to the hypothesis that element linking might be mediated by these interactions. A common tacit assumption is that these interactions result in response modulation which disturbs a local contrast code. We addressed this possibility by performing contrast discrimination experiments using two-dimensional arrays of multiple Gabor patches arranged either (i) vertically, (ii) in circles (coherent conditions), or (iii) randomly (incoherent condition), as well as for a single Gabor patch. In each condition, contrast increments were applied to either the entire test stimulus (experiment 1) or a single patch whose position was cued (experiment 2). In experiment 3, the texture stimuli were reduced to a single contour by displaying only the central vertical strip. Performance was better for the multiple-patch conditions than for the single-patch condition, but whether the multiple-patch stimulus was coherent or not had no systematic effect on the results in any of the experiments. We conclude that constrained local interactions do not interfere with a local contrast code for our suprathreshold stimuli, suggesting that, in general, this is not the way in which element linking is achieved. The possibility that interactions are involved in enhancing the detectability of contour elements at threshold remains unchallenged by our experiments.

Effects of grouping in contextual modulation

Perception of a visual target and the responses of cortical neurons can be strongly influenced by a context surrounding the target. This observation relates to the fundamental issue of how cortical neurons code objects of the external world. In high-contrast regimes, embedding a target in an iso-oriented context reduces neural responses and deteriorates performance in psychophysical experiments. Performance from orthogonal surrounds is better than that from iso-oriented ones. This contextual interference is often postulated to be caused by long- or short-range interactions between neurons tuned to orientation. Here we show, using a new illusion called 'shine-through' as a sensitive psychophysical probe, that the orientation difference between target and context does not determine performance. Instead, contextual modulation depends on the overall spatial structure of the context. We propose that contextual suppression vanishes if the contextual elements are grouped to an independent and coherent object.

Attention-dependent brief adaptation to contour orientation: a high-level aftereffect for convexity?

In contrast to the abundant literature investigating how orientation coding depends on edges defined by various image features, relatively little is known about how coding of orientation might also depend on the two distinct functional roles that oriented edges commonly play. Oriented lines can delineate outline contours of a figure or they can form texture. The results of five experiments using orientation aftereffects measured with brief tests (27 ms, backward masked; adapt-to-test interval=201 ms) provided evidence that brief stimuli (<135 ms) selectively adapt coding of contour-line orientation rather than coding of line-texture orientation. Furthermore, parametric results revealed that the rapidly adapting aftereffects for contour orientation are characterized by (1) broad orientation tuning (peaking at +/-30 degrees to +/-50 degrees from test orientation), (2) indifference as to how the contours are defined (e.g. bright lines, high-pass-filtered lines, faint lines generated by the spatial inhomogeneity of visual sensitivity), (3) rapid saturation at low contrast energy, (4) strong modulation by selective attention, and (5) relative size tolerance. These characteristics appear to parallel those of cells in the high end of the visual form processing pathway (such as inferotemporal cortex). It is thus suggested that the rapidly adapting contour orientation aftereffects reported here may be mediated by high-level neural units that encode global configurations of orientation (e.g. convexity and concavity).

Integration of local features into a global shape

It is a well-established fact that the cortical filters selective for spatial frequency and orientation have a limited spatial extent. The present study investigated how information from local filters is integrated into global shapes. Specifically, we were interested in whether identification of a global pattern consisting of small oriented, spatially-bandpass features (Gabor patches) depends on the orientations of those features. The observer was presented with a C-like stimulus shape comprised of oriented Gabor elements on a blank background, and the performance measure was the threshold contrast for identifying the global orientation of the C-shape (four possible rotated orientations). Performance was significantly enhanced (i.e. contrast thresholds were lower) when the local features were aligned with the C-contour than when the features were orthogonal to it or they had mixed local orientations (randomly both aligned and orthogonal). Control experiments showed that the alignment of local features improves global pattern perception only when the features are close to each other. In contrast, a close spacing between the features interfered with global pattern perception when the local orientations were mixed.

A transition between eye and object rivalry determined by stimulus coherence

Two orthogonal patterns presented to the two eyes, respectively, are perceived as alternating in time, a phenomenon often assumed to reflect competition between neuronal activities corresponding to the two eyes, presumably in the primary visual cortex. Recent evidence supports a competition between neuronal activities corresponding to the two patterns (objects) at some higher cortical processing stage after inputs from the two eyes have converged. Here, using textures made of Gabor signals, we present psychophysical data showing that the level of visual processing at which competition takes place and is resolved, is determined by the degree of stimulus coherence. Moreover, depending on stimulus parameters, competition may occur at several levels of processing at the same time.

Measurement and modeling of center-surround suppression and enhancement

The apparent contrast of a central stimulus is affected by the presence of surrounding stimuli. For some stimulus conditions, the apparent contrast is suppressed and for other conditions the apparent contrast is enhanced. This report is intended to offer a coherent description of the stimulus factors that influence suppression and enhancement. Using a contrast-matching protocol, we measured the contrast dependence of center-surround interactions by systematically varying the suprathreshold contrasts of the central and surround gratings. Different spatial configurations of the surround stimuli were studied. Our results confirmed previous findings that (1) a surround stimulus could produce either contrast enhancement or contrast suppression depending on the balance of the central and surround contrasts; (2) suppression varied with the width of the surround stimulus and was strongly orientation-specific; and (3) enhancement was less sensitive to changes in surround configurations (in particular, enhancement did not depend on the colinearity of the central and surround gratings). Based on the experimental data, we developed a computational model to account for center-surround suppression and enhancement.

Contour into texture: information content of surface contours and texture flow

Both surface contours and texture patterns can provide strong cues to the three-dimensional shape of a surface in space. Many of the most perceptually salient texture patterns have a strong flowlike structure, resulting from the directional nature of the surface textures from which they project. Under the minimal assumption that an oriented surface texture is homogeneous, the texture flow on a developable surface can be shown to follow parallel geodesics of the surface. The geometry of texture flow is therefore equivalent to that of an important class of surface contours: those that project from parallel geodesics of a developable surface. I derive a set of differential equations that support the estimation of surface shape from geodesic surface contours under spherical perspective, for both parallel and nonparallel contours. For perfectly oriented textures, the equations apply directly to the integrated flow lines in a texture image. For weakly oriented textures, perspective projection distorts the projected orientation of flow lines away from the idealized case of pure contours; however, simulations show that for a large class of textures, these distortions will be small and limited largely to extreme surface poses. The geometrical analysis, along with a number of phenomenal demonstrations and psychophysical results, suggests that the human visual system co-opts shape from contour mechanisms to estimate surface shape from texture flow.

The function of dynamic grouping in vision

In this review we consider the logic and the evidence relating to the issue of dynamic grouping in human vision. Dynamic grouping is required when the visual system is creating novel descriptions, either because it is dealing with novel stimuli or it is providing information for novel purposes. In such cases, dynamic grouping provides a mechanism for discovering regularities in the data. We consider a number of examples, including grouping of visual information into surface descriptions and contour descriptions. The main issues that arise concern the configurability of the process and the effects of the propagation of local configurations. We then turn to the complex issue of visual search. Visual search allows the experimenter to establish something of the nature of pre-attentive visual descriptions and how these differ from attentive descriptions. We describe the basic results of visual search experiments in terms of the type of grouping involved. Finally, we consider the hypothesis that dynamic grouping is signalled by neuronal synchrony.

Seeing circles: what limits shape perception?

Humans are remarkably adept at judging shapes and discriminating forms. Forms and shapes are initially sampled by discrete localized visual filters (or receptive fields) in 'early' stages of visual processing. However, more complex higher level filters which integrate or pool information from many local filters may be needed to discern shapes. In order to understand the mechanisms that limit shape perception we asked observers to detect distortions in the shape of briefly presented circles constructed out of samples (Gabor patches, which are well matched to the early visual filters), and varied the radius of the circle, and the number and orientation of the samples. Our results show that shape perception is determined by two factors: the primary determinant is the separation between the samples; however, the orientation of the samples can modulate performance. At small separations, performance is best when the samples are aligned with the global shape, poorer when they are orthogonal, and intermediate when they are all horizontal or vertical. At larger separations these contextual differences disappear; however at all separations, performance is reduced when the orientations of the samples are mixed (i.e. each sample is randomly either aligned or orthogonal, or randomly either horizontal or vertical.). These results suggest an important role for sample separation in shape perception for sampled shapes and suggest that the mechanisms involved in feature binding may modulate the responses of the mechanisms underlying shape perception.

Probability summation for multiple patches of luminance modulation

When components of a compound pattern stimulate different visual mechanisms, psychophysical performance typically improves by a small amount consistent with probability summation amongst independent detectors. Here we extend previous summation experiments by (i) plotting full psychometric functions; and (ii) using compound stimuli with components that varied in up to three stimulus dimensions: spatial frequency (1, 4, 5 or 11 c/deg), orientation (0 degrees, +/-45 degrees ), and position. Stimulus components were isolated circular sine-phase patches of grating centred on up to four corners of an imaginary square surrounding a fixation-point. Combinations of component patches produced compound stimuli made from up to 16 components that differed in various combinations of the three stimulus dimensions. Other than when the spatial frequency was 11 c/deg, results were well described using: (i) probabilistic summation of individual psychometric functions; (ii) the Quick pooling formula; and (iii) the signal detection analysis for 2IFC developed by Tyler and Chen (2000) [Signal detection theory in the 2AFC paradigm: attention, channel uncertainty and probability summation (under review)]. We conclude that in general, nonlinear spatial summation is consistent with probabilistic summation across independent detecting mechanisms that vary in spatial frequency (a range of at least 1-5 c/deg), orientation (a range of 90 degrees ) and position (a range of at least 24 cycles at 4 c/deg). In further experiments, results were found to be consistent with probability summation for pairs of orthogonally oriented step-edge stimuli and a matrix of randomly oriented 11 c/deg sine-wave patches. This casts doubt on the generality of a recent suggestion that local interactions between colinearly oriented detectors within a spatial neighbourhood of around four cycles may contribute to nonlinear spatial summation [Bonneh & Sagi, 1998; Vision Research, 38, 3541-3553].

A Fraser illusion without local cues?

In the well-known Fraser illusion, a line composed of tilted elements itself appears tilted. The standard explanation of this illusion has been that the global orientation of the line is in some way influenced by the local orientation of the elements. The illusion was recreated using a texture composed of collinear Gabor stimuli, which were vertical. There was no local tilt. The illusory tilt was produced by gradually shifting the phase of the successive Gabors along each line. Although the 2D Fourier transform of this global pattern peaks at off-vertical orientations, the local energy of the patches is predominantly vertical. How does the visual system nevertheless pick up this global information? This can be explained by elongated linear filters, or a phase-tuned second-stage mechanism. We examined the first theory using a stereoscopic demonstration. When lines of opposite tilt are presented in the two eyes, they combine binocularly to produce stereoscopic slant. We tested whether the illusory tilts in the phase-shifted Gabors texture give stereoscopic slant, when opposite tilts are presented to the two eyes. They do not. Instead, stereoscopic depth is dominated by the local phase-disparity of the individual patches. This indicates that the illusion is not present at the stage of linear filters, which are input to stereo, but must involve second-stage interactions or collators.

Contrast integration across space

Contrast integration across space was studied in respect to stimulus extent and the spatial layout, using high-contrast stimuli. Contrast discrimination thresholds were measured (2AFC) by either increasing the size of a peripheral (2.4 degrees) Gabor signal (GS: lambda = 0.08 degree) or by increasing the number of GS elements in a circular arrangement. The supra-threshold mask (pedestal) was either increased with the target or fixed at maximal size and had 30% contrast. For stimuli with an increasing size of both the pedestal and the increment target, we find approximately constant discrimination thresholds. Contrast discrimination improved linearly on a log-log scale with slopes average of -1/4 (fourth-root summation) when the size of the Gabor target was increased but the mask was kept at maximal size, indicating contrast integration across space. Taken together, these results indicate balanced spatial integration of both contrast increment and pedestal, resulting object-size invariant contrast discrimination. Contrast discrimination was found to improve as well when the number of aligned Gabor elements was increased (both pedestal and increment), pointing to independent contrast normalization for disconnected (sparsely positioned) stimuli. The results indicate a complex pattern of spatial integration involved in contrast discrimination, possibly depending on image segmentation.

Configuration saliency revealed in short duration binocular rivalry

Supra-threshold spatial integration was studied by testing the saliency of multi-Gabor element configurations in short duration binocular rivalry (dichoptic masking) conditions. Dichoptic presentations allow for a competition between spatially overlapping supra-threshold stimuli that involve non-overlapping monocular receptive fields in the first stage of visual filtering. Different spatial configurations of Gabor patches (sigma = lambda = 0.12 degree) were presented to one eye (target) together with a bandpass noise presented to the other eye (mask). After a short rivalry period (120 ms) in which a dominance of one eye was established, a probe (a randomly positioned small rectangle of reduced contrast in the target) was presented for additional detection period (80 ms). Probe detection performance was measured (two-alternative-forced choice paradigm (2AFC) by finding the mask contrast leading to 79% correct response. Results show that configuration saliency is consistently expressed as dominance in short-duration binocular rivalry, with similar results obtained for longer durations (200 ms and continuous presentations). We find that textures of high-contrast randomly oriented patches are more dominant than uniform textures where the effect decreases and eventually reverses with decreasing of contrast. For supra-threshold contours, however, we find that smooth collinear contours are more dominant than 'jagged' ones, regardless of phase and contrast. These findings suggest principles underlying early lateral integration mechanisms based on contrast dependent inhibitory and excitatory connections. This mechanism could be based on iso-orientation surround (2D) inhibition and collinear (1D) facilitation, with inhibition being more effective at high contrasts.