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PubMed2004

Good continuation with kinetic edges.

We examined the perceptual formation of extended contours from second-order kinetic-edges created by motion discontinuities. Paths were formed by spatially separate kinetic-edge elements, aligned along smooth paths, and embedded in randomly oriented noise elements. Path detection was severely degraded when the sign of motion contrast alternated along the path compared to when the same sign was used, or if random motion direction was assigned to each edge element, or if alternating opposite motion directions was used along the paths. Performance increased monotonically with the length of the path. Irrespectively of path curvature a fast temporal summation occurs within the first 200-400 ms and then levels off. Hence, the kinetic-edge grouping is relatively fast and a pure second-order process that senses whether the motion is globally in the same phase and direction along extended contours.

Form Perception↗
PubMed2005

Dynamic competition between contour integration and contour segmentation probed with moving stimuli.

Line-ends, corners and junctions are important singularities for form analysis, object recognition, depth ordering or motion processing. In this study, we investigate the extent to which processing the motion of line ends depends on the spatial configuration of their immediate surround. To that aim, we used two vertical collinear line segments, translating clockwise or anti-clockwise along a circular path, together with a direction discrimination task. Direction discrimination was measured independently for outer line-ends--at both segments extremities--and inner line-ends--in between collinear segments--using line segments partially occluded by invisible masks such that the direction of either inner or outer line-ends' motion was restricted to a sinusoidal translation along a horizontal axis, and thus irrelevant for the motion task. Under these conditions, access to the direction of inner line-ends is longer and more difficult than it is for outer line-ends. Subsequent experiments show that these effects depend on the degree of collinearity between line segments. Similar experiments were performed after volunteers took a dose of Lorazepam, a benzodiazepine that facilitates the fixation of GABA on GABAA receptors. The results show that the differences between the processing of inner and outer line-ends is reduced, suggesting that the effect of the surround is modulated by inhibitory mechanisms. Using a simple model, we propose that this effect can be explained by a competition between a segmentation process based on surround suppression and contour integration through long-range horizontal connections, at or prior to motion processing stages.

Adult↗
PubMed2004

Contour curvature polarity and surface interpolation.

Contour curvature polarity (i.e., concavity/convexity) is recognized as an important factor in shape perception. However, current interpolation models do not consider it among the factors that modulate the trajectory of amodally-completed contours. Two hypotheses generate opposite predictions about the effect of contour polarity on surface interpolation. Convexity advantage: if convexities are preferred over concavities, contours of convex portions should be more extrapolated than those of concave portions. Minimal area: if the area of amodally-completed surfaces tends to be minimized, contours of convex portions should be less extrapolated than contours of concave portions. We ran three experiments using two methods, simultaneous length comparison and probe localization, and different displays (pictures vs. random dot stereograms). Results indicate that contour polarity affects the amodally-completed angles of regular and irregular surfaces. As predicted by the minimal area hypothesis, image contours are less extrapolated when the amodal portion is convex rather than concave. The field model of interpolation [Fantoni, C., & Gerbino, W. (2003). Contour interpolation by vector-field combination. Journal of Vision, 3, 281-303. Available from http://journalofvision.org/3/4/4/] has been revised to take into account surface-level factors and to explain area minimization as an effect of surface support ratio.

Adult↗
PubMed2005

Recognizing partially visible objects.

In two experiments we measured object recognition performance as a function of delay. In Experiment 1 we presented half of an image of an object, and then the other half after a variable delay. Objects were subdivided into top versus bottom halves, left versus right halves, or vertical strips. In Experiment 2 we separated the low (LSF) and high spatial frequency (HSF) components of an image, and presented one component followed by the other after a variable delay. For both experiments, performance was worse with a 105ms delay between the presentations of the object components than when the two components were presented simultaneously. These results are consistent with predictions made by models that combine information at a relatively early stage in processing. In addition, the results revealed that object recognition performance is significantly better when the LSF sub-image preceded the HSF sub-image than when the HSF sub-image preceded the LSF sub-image, consistent with previous work suggesting that LSF information is processed prior to HSF in object recognition.

Adult↗
PubMed2006

Illusory color mixing upon perceptual fading and filling-in does not result in 'forbidden colors'.

A retinally stabilized object readily undergoes perceptual fading. It is commonly believed that the color of the apparently vanished object is filled in with the color of the background because the features of the filled-in area are determined by features located outside the stabilized boundary. Crane, H. D., & Piantanida, T. P. (1983) (On seeing reddish green and yellowish blue. Science, 221, 1078-1080) reported that the colors that are perceived upon full or partial perceptual fading can be 'forbidden' in the sense that they violate color opponency theory. For example, they claimed that their subjects could perceive "reddish greens" and "yellowish blues." Here we use visual stimuli composed of spatially alternating stripes of two different colors to investigate the characteristics of color mixing during perceptual filling-in, and to determine whether 'forbidden colors' really occur. Our results show that (1) the filled-in color is not solely determined by the background color, but can be the mixture of the background and the foreground color; (2) apparent color mixing can occur even when the two colors are presented to different eyes, implying that color mixing during filling-in is in part a cortical phenomenon; and (3) perceived colors are not 'forbidden colors' at all, but rather intermediate colors.

Adult↗
PubMed2006

Amodal completion impairs stereoacuity discrimination.

Visual stimulus configuration can influence elementary visual processes. We provide empirical evidence to demonstrate this effect in stereoscopic depth discrimination. Two vertically aligned bars were presented in stereo such that one of them was closer to the human observer. Observers discriminated which of the two was closest. In the first, "occluded" condition, a horizontal bar, positioned closest in depth to the observer, was added to the display such that the two vertical bars perceptually completed to form a whole by connecting together behind the horizontal bar. In the second, control condition, the horizontal bar was placed furthest away from the observer such that there was a visible gap between the two vertical bars, which could no longer complete perceptually. We measured observers' psychometric functions using the method of constant stimuli, and found that their discrimination sensitivity d' was smaller when the two vertical bars perceptually completed than when they did not. We used a simple model to illustrate that when the two vertical bars perceptually completed, they also tended to be perceived as coplanar in the fronto-parallel plane. This consequence of completion made it more difficult to discriminate any difference in depth between the two vertical bars.

Analysis of Variance↗
PubMed2006

Role of a circle's center in visual interpolation.

Basic geometric patterns like straight lines and circles seem fundamental to human perception and mental imagery. In this study we examined subjects' ability to interpolate circular curves-to derive the whole circle from an arc of 180 degrees or less. Specifically, we tested how the center point is utilized during such visual interpolation. Naturally, a mechanism that interpolates by extending the curvature of the visible arc will be unaffected by the presence or absence of the center point. On the other hand, a mechanism that achieves the same end by completing the circle from estimates of the center and radius will be significantly aided by the presence of the center. We found that when the visible arc was long (180 degrees), presenting the circle's center did not affect the precision with which subjects localized the invisible section. However, when the visible arc was relatively short (90 degrees or 45 degrees), displaying the center point significantly increased spatial precision. Thus, both computational mechanisms appear to exist.

Adult↗
PubMed2006

Visual interpolation is not scale invariant.

According to the scale-dependence hypothesis, the visual interpolation of contour fragments depends on the retinal separation of endpoints: as the retinal size of a partially occluded angle increases, the interpolated contour gradually deviates from the shortest connecting path and approaches the shape of the unoccluded angle. In the field model, as the retinal size increases the strength of good continuation increases while the strength of the minimal-path tendency decreases. To test the scale-dependence hypothesis--as well as other hypotheses connected to inclusion, support-ratio dependence, and extended relatability--we ran two experiments using the probe localization technique. Stimuli were regular polygons with rectilinear contours bounding symmetrically occluded angles. Retinal size was manipulated by changing viewing distance. Observers were asked to judge if a probe, briefly superposed on the occlusion region, was inside or outside the amodally completed angle. Retinal size strongly influenced the penetration of interpolated trajectories in the predicted direction. However, support ratio and interpolated angle size interacted with retinal size, consistently with the idea that unification factors are effective within a spatial window. We modified the field model to include the size of such a window as a new parameter and generated model-based trajectories that fitted empirical data closely.

Attention↗
PubMed2002

The perception of closed flat knots and completion by folding.

The aim of the present research was to study a well-defined set of line drawings that have never been analyzed before, and that are perceived as closed flat knots (CFKs). These knots are obtained by folding two-dimensional laminae. From this study it emerged that the perception of CFKs is always accompanied by a type of amodal completion, which had not previously been studied, and which we have called "completion by folding". It occurs when three quadrilaterals are so arranged that they look like a sheet of paper folded around one of them, partly occluded and partly occluding. Two experiments were carried out on this phenomenon revealing that a three-level stratification is produced in the completion by folding that facilitates unification between occluded figures, even when current models do not foresee such a perceptual solution, either because the distance between the two figures that become a unit is too big (experiment ) or because the slope between the two figures is not favorable (experiment 2). A third experiment, which took up the problem of the perception of CFKs, revealed the following: (1) Besides the prototype CFK, obtained by the interlacing of a rectangular lamina, there is a theoretically infinite class of CFKs. (2) The drawing of all the possible closed flat knots involves geometrically precise rules. (3) Not all feasible knots with these rules are seen as such. (4) Only the knots with characteristics of alignment and regularity are recognized as knots. (5) The closed flat knots are seen as the result of a transformation which the figure undergoes. This evidence constitutes a remarkable perceptual problem, discussed with reference to the recent theories of amodal completion.

Adolescent↗
PubMed1999

Subconfigurations of the human form in the perception of biological motion displays.

We report four experiments examining processes that contribute to the perception of point-light displays of human locomotion. In three experiments, we employed a simultaneous masking paradigm to examine the visual system's use of configural information in global analyses of biological motion displays. In the fourth experiment, we obtained descriptions of our stimulus displays from naive observers. Performance in both the detection and identification studies suggests that the visual system responded equivalently to figures exhibiting any organization of limbs that is consistent with the human form. Moreover, the subconfigurations best detected were also most likely to be described independently as depicting a human figure. Thus our findings provide evidence that the visual system can exploit characteristic subconfigurations of the human form in the perception of human locomotion.

Adult↗
PubMed2003

The shape of holes.

The shape of holes can be recognized as accurately as the shape of objects (Palmer, S. E. (1999). Vision science: photons to phenomenology. Cambridge, MA: MIT Press), yet the area enclosed by a hole is a background region, and it can be demonstrated that background regions are not represented as having shape. What is therefore the shape of a hole, if any? To resolve this apparent paradox, we suggest that the shape of a hole is available indirectly from the shape of the surrounding object. We exploited the fact that observers are faster at judging the position of convex vertices than concave ones (Perception 30 (2001) 1295), and using a figural manipulation of figure/ground we found a reversal of the relative speeds when the same contours were presented as holes instead of objects. If contours were perceived as belonging to the hole rather than the surrounding object then there would have been no qualitative difference in responses to the object and hole stimuli. We conclude that the contour bounding a hole is automatically assigned to the surrounding object, and that a change in perception of a region from object to hole always drastically changes the encoded information. We discuss the many interesting aspects of holes as a subject of study in different disciplines and predict that much insight especially about shape will continue to come from holes.

Form Perception↗
PubMed1977

Form perception in stumptail macaques following posterior parietal and lateral frontal lesions.

Monkeys with bilateral posterior parietal and lateral frontal ablations were compared with normal animals on a series of four tests for their ability to relate elements of a visual pattern on the basis of the Gestalt principle of proximity. The operated animals showed no clear impairment on these tests of perceptual grouping. The three groups of monkeys were then trained to discriminate two faces and tested for generalisation to different views of the same faces. The pariental-operated monkeys showed less generalisation than the other animals to lateral mirror-images of the training stimuli, but more generalisation to inverted mirror-images. It is concluded that any deficit in "gestalt" perception following such lesions must be at a relatively high level.

Animals↗
PubMed1987

A left hemisphere contribution to visuospatial processing.

Men with chronic, focal, unilateral missile injures of the brain--twenty-five with left hemisphere lesions (LH group) and twenty with right hemisphere lesions (RH groups)--and twenty-two control subjects were given two visuoperceptual and two visuospatial tests. The LH group was significantly impaired in relation to the control group on both the spatial tasks. A different pattern of dissociable perceptual and spatial deficits was found in the experimental groups: better preserved perceptual than spatial performance was observed more frequently in the LH group whereas the converse--relatively better spatial than perceptual performance--was more evident in the RH group. Double dissociations in performance on the two spatial tasks were found predominantly in the LH group. These findings suggest an important left hemisphere contribution to visuospatial processing and the possibility of a more focal representation of spatial abilities in the left hemisphere than in the right.

Aged↗
PubMed1996

Object-based neglect: an investigation of the contributions of eye movements and perceptual completion.

Eye movements made by a patient with object-based neglect and no hemianopia, R.R., were recorded while he viewed whole, half, and chimaeric objects. When whole or chimaeric objects were presented centred on his initial fixation point, and when right half objects were presented, R.R. restricted his saccades to the right sides of the images. R.R.'s verbal identifications were consistent with these scan paths, since he correctly reported the identities of whole objects, right half objects, and the right sides of chimaeric objects, but failed to report objects on the left sides of chimaerics. When a left half object was presented to the left of initial fixation, however, R.R. made left saccades to scan the image and correctly named these objects; hence his failure to scan the left sides of chimaeric object cannot simply be attributed to an inability to make contralesional saccades. We examined the possibility of perceptual completion by testing R.R. with sets of whole and half shapes presented tachistoscopically and asking him to report what he had seen. When R.R. viewed whole shapes he correctly reported seeing a whole shape on every trial. However, R.R. often reported seeing right half objects as whole, whereas he was much less prone to report left half objects as perceptually whole. We discuss the nature of this perceptual completion, and conclude that both scan paths and completion are consequences of object-based neglect, not its causes.

Cerebrovascular Disorders↗
PubMed2002

Brain activity related to integrative processes in visual object recognition: bottom-up integration and the modulatory influence of stored knowledge.

We report evidence from a PET activation study that the inferior occipital gyri (likely to include area V2) and the posterior parts of the fusiform and inferior temporal gyri are involved in the integration of visual elements into perceptual wholes (single objects). Of these areas, the fusiform and inferior temporal gyri were more activated by tasks with recognizable stimuli than by tasks with unrecognizable stimuli. We propose that the posterior parts of the fusiform and inferior temporal gyri, compared with the inferior occipital gyri, are involved in higher level integration, due to the involvement of re-entrant activation from stored structural knowledge. Evidence in favor of this interpretation comes from the additional finding that activation of the anterior part of the left fusiform gyrus and a more anterior part of the right inferior temporal gyrus, areas previously associated with access to stored structural knowledge, was found with recognizable stimuli, but not with unrecognizable stimuli. This latter finding also indicates: (i) that subjects may not refrain from (automatically) identifying objects even if they only have to attend to the objects' global shape, and (ii) that perceptual and memorial processes can be dissociated on both functional and anatomical grounds. No evidence was obtained for the involvement of the parietal lobes in the integration of single objects.

Adult↗
PubMed2001

Filling-in the details on perceptual fading.

We examined the perceptual disappearance (or 'filling in') of a peripheral target surrounded by dynamic texture. Targets defined by different visual attributes were used to explore the importance of target properties in determining the time-course of fading. Introducing luminance-, motion- or direction-contrast between the target and background increased the time-to-fade. For motion contrast, this was related to target visibility. Targets defined by a difference of texture from the background took longer to fade than those defined by a difference of motion. This might correspond to activity in different visual areas, or could be due to different visibilities in each case.

Adaptation, Physiological↗
PubMed2001

Target/surround asymmetry in perceptual filling-in.

Four experiments examined how differences in the properties of the target and surround affect the time required for perceptual filling-in. They examined differences in luminance, orientation, spatial frequency, and color. A larger target/surround difference delayed filling-in ('feature difference effect'). Interestingly, exchanging the target and surround properties significantly varied the time ('target/surround asymmetry'). Filling-in was facilitated when the target was brighter and closer to the vertical or horizontal than the surround. Little asymmetry was found in the frequency domain, while significant asymmetry was observed for specific color combinations. These effects are discussed with respect to edge adaptation, feature adaptation, balance of neural activities, and contextual modulation.

Analysis of Variance↗