Contour lightness and separation effects in the Ebbinghaus illusion

Effects of cortical distance on the Ebbinghaus and Delboeuf illusions

The Ebbinghaus and Delboeuf illusions affect the perceived size of a target circle depending on the size and proximity of circular inducers or a ring. Converging evidence suggests that these illusions are driven by interactions between contours mediated by their cortical distance in primary visual cortex. We tested the effect of cortical distance on these illusions using two methods: First, we manipulated retinal distance between target and inducers in a two-interval forced choice design, finding that targets appeared larger with a closer surround. Next, we predicted that targets presented peripherally should appear larger due to cortical magnification. Hence, we tested the illusion strength when positioning the stimuli at various eccentricities, with results supporting this hypothesis. We calculated estimated cortical distances between illusion elements in each experiment and used these estimates to compare the relationship between cortical distance and illusion strength across our experiments. In a final experiment, we modified the Delboeuf illusion to test whether the influence of the inducers/annuli in this illusion is influenced by an inhibitory surround. We found evidence that an additional outer ring makes targets appear smaller compared to a single-ring condition, suggesting that near and distal contours have antagonistic effects on perceived target size.

Thinking about time: identifying prospective temporal illusions and their consequences

Time is fundamentally abstract, making it difficult to conceptualize and vulnerable to mental distortions. Nine preregistered experiments identify temporal illusions that characterize prospective time judgments and corresponding consequences for decision making in a variety of domains. Using visual illusions as a grounding metaphor, studies 1-4 demonstrated that the temporal distance between two dates was perceived as closer together as those two dates were imagined further into the future (e.g., Vanishing Point); the length of a single day whether negative (e.g., a 12 h illness-Study 2a) or positive (e.g., 12 h with a good friend-Study 2b) was estimated to feel longer when embedded within a short versus long trip (e.g., the Delbouef Illusion); a 60 min activity was expected to go by more quickly when adjacent activities were 90 (vs. 30) min (e.g., Ebbinghaus Illusion); and a 9 + 1 day vacation was expected to be considerably lengthier than an 11-1 day vacation (e.g., Representational Momentum). Four additional studies explored moderating factors (Studies 5 and 6) and the impact of distortions on downstream non-time judgments including the forecasted emotional intensity of a negative event (Study 6), estimations of fair monetary compensation for lost time (Study 7), and willingness to make prosocial time commitments (Study 8). Implications for uncovering additional temporal illusions as well as practical applications for leveraging the relativity of prospective time to achieve desired cognitive and behavioral outcomes are discussed.

Ebbinghaus illusion depends more on the retinal than perceived size of surrounding stimuli

A stimulus surrounded by smaller/larger stimuli appears larger/smaller (Ebbinghaus illusion). We examined whether the Ebbinghaus illusion would depend on the retinal or perceived size of the surrounding stimuli. The flash-lag effect, where a flashed stimulus perceptually lags moving stimuli, was used to dissociate the retinal from perceived size of the surrounding stimuli. Two sets of four surrounding disks changed their size smoothly: one with larger disks shrinking, the other with smaller disks expanding. Two identical central disks were presented briefly at various timings relative to the moment when the surrounding disks were physically identical in their size (coincidence time). A significant flash-lag effect was observed for size change (Experiment 1). Participants reported the two central disks being in equal size when they appeared only slightly before the coincidence time. However, this asynchrony was not significantly different from zero and was significantly smaller than the perceptual delay expected from the flash-lag effect (Experiment 2). These results suggest that the Ebbinghaus illusion depends more on the retinal than perceived size of the surrounding stimuli.

Visual similarity modulates visual size contrast

Perception is relational: object properties are perceived in comparison to the spatiotemporal context rather than absolutely. This principle predicts well known contrast effects: For instance, the same sphere will feel smaller after feeling a larger sphere and larger after feeling a smaller sphere (the Uznadze effect). In a series of experiments, we used a visual version of the Uznadze effect to test whether such contrast effects can be modulated by organizational factors, such as the similarity between the contrasting inducer stimulus and the contrasted induced stimulus. We report that this is indeed the case: size contrast is attenuated for inducer-inducing pairs having different 3D shapes, orientations, and even - surprisingly - color and lightness, in comparison to equivalent conditions where these features are the same. These findings complement related work in revealing basic mechanisms for fine-tuning local interactions in space-time in accord to the global stimulus context.

Is the Ebbinghaus illusion a size contrast illusion?

The Ebbinghaus illusion, in which a central target surrounded by larger context figures looks smaller than when surrounded by smaller context figures, is usually classified as a size contrast illusion. Thus "size contrast" is the dominant account of this effect. However, according to an alternative "contour interaction" account this phenomenon has little to do with size contrast but is rather caused by distance-dependent attractive and repulsive interactions between neural representation of contours. Here evidence is presented against the size contrast account and consistent with the contour interaction account. Experiment 1 was a control study confirming that the illusion can be obtained using displays consisting only of squares, which are more convenient to manipulate than the standardly used circles. In Experiment 2, the standard configuration involving small context figures surrounding the target was compared to a novel configuration, which involved many "spread" small context figures. The illusory effect of the standard context was stronger than the illusory effect of the spread context, in accord with the prediction of the contour interaction account, and contrary to the prediction of the size contrast account. In Experiment 3 two novel configurations were used, based on standard and spread contexts. The results were in accord with the prediction of the contour interaction account, whereas the size contrast account had no prediction because the stimuli did not involve conventional size contrast. Additional aspects of the stimuli and an account of the illusion based on a perspective interpretation are also discussed.

The Ebbinghaus illusion in the gray bamboo shark (Chiloscyllium griseum) in comparison to the teleost damselfish (Chromis chromis)

This is the first study to comparatively assess the perception of the Ebbinghaus-Titchener circles and variations of the Delboeuf illusion in four juvenile bamboo sharks (Chiloscyllium griseum) and five damselfish (Chromis chromis) using identical training paradigms. We aimed to investigate whether these two species show similarities in the perceptual integration of local elements into the global context. The Ebbinghaus-Titchener circles consist of two equally sized central test circles surrounded by smaller or larger circles of different size, number and/or distance. During training, sharks and damselfish learned to distinguish a large circle from a small circle, regardless (i) of its gray level and (ii) of the presence of surrounding circles arranged along an outer semi-circle. During the subsequent transfer period, individuals were presented with variations of the Ebbinghaus-Titchener circles and the Delboeuf illusion. Similar to adult humans, dolphins, or some birds, damselfish tended to judge the test circle surrounded by smaller inducers as larger than the one surrounded by larger inducers (contrast effect). However, sharks significantly preferred the overall larger figure or chose indifferently between both alternatives (assimilation effect). These contrasting responses point towards potential differences in perceptual processing mechanisms, such as 'filling-in' or '(a)modal completion', 'perceptual grouping', and 'local' or 'global' visual perception. The present study provides intriguing insights into the perceptual abilities of phylogenetically distant taxa separated in evolutionary time by 200 million years.

Visual Search for Size-Defined Target Objects is Modulated by the Ebbinghaus Apparent-Size Illusion: Facilitatory and Inhibitory Effects of the Context Objects

Five experiments were carried out to investigate under which conditions the apparent size of objects is computed and exploited optimally in visual search for size-defined targets. Observers searched for a target test circle that was retinally larger than the distractor test circles, with both types of circles surrounded by context circles modulating the apparent size of the test circles (Ebbinghaus illusion). RTs were the faster the better test circles could be differentiated from the context circles, ie with smaller numbers of context circles, larger distances, and higher lightness (or colour) contrast between test and context circles. Apparent-size modulation had a strong influence on search RTs, resulting in faster RTs with smaller, and slower RTs with larger, context circles. A model assuming overall facilitatory effects of the apparent-size modulation and interference effects arising from decreasing test-context circle discriminability can explain the present results.

The Roles of Inducer Size and Distance in the Ebbinghaus Illusion (Titchener Circles)

Although the Ebbinghaus illusion is commonly used as an example of a simple size-contrast effect, previous studies have emphasised its complexity by identifying many factors that potentially influence the magnitude of the illusion. Here, in a series of three experiments, we attempt to simplify this complexity. In each trial, subjects saw a display comprising, on one side, a target stimulus surrounded by inducers and, on the other, an isolated probe stimulus. Their task was to indicate whether the probe appeared larger or smaller than the target. Probe size was adjusted with a one-up, one-down staircase procedure to find the point of subjective equality between probe and target. From these experiments, we argue that the apparent effects of inducer size are often confounded by the relative completeness of the inducing surround and that factors such as the similarity of the inducers and target are secondary. We suggest a simple model that can explain most of the data in terms of just two primary and independent factors: the relative size of the inducers and target, and the distance between the inducers and the target. The balance between these two factors determines whether the size of the target is underestimated or overestimated.

The Dynamic Ebbinghaus: motion dynamics greatly enhance the classic contextual size illusion

The Ebbinghaus illusion is a classic example of the influence of a contextual surround on the perceived size of an object. Here, we introduce a novel variant of this illusion called the Dynamic Ebbinghaus illusion in which the size and eccentricity of the surrounding inducers modulates dynamically over time. Under these conditions, the size of the central circle is perceived to change in opposition with the size of the inducers. Interestingly, this illusory effect is relatively weak when participants are fixating a stationary central target, less than half the magnitude of the classic static illusion. However, when the entire stimulus translates in space requiring a smooth pursuit eye movement to track the target, the illusory effect is greatly enhanced, almost twice the magnitude of the classic static illusion. A variety of manipulations including target motion, peripheral viewing, and smooth pursuit eye movements all lead to dramatic illusory effects, with the largest effect nearly four times the strength of the classic static illusion. We interpret these results in light of the fact that motion-related manipulations lead to uncertainty in the image size representation of the target, specifically due to added noise at the level of the retinal input. We propose that the neural circuits integrating visual cues for size perception, such as retinal image size, perceived distance, and various contextual factors, weight each cue according to the level of noise or uncertainty in their neural representation. Thus, more weight is given to the influence of contextual information in deriving perceived size in the presence of stimulus and eye motion. Biologically plausible models of size perception should be able to account for the reweighting of different visual cues under varying levels of certainty.

The Ebbinghaus Illusion: New Contextual Effects and Theoretical Considerations

Current accounts of the Ebbinghaus illusion emphasize either size contrast or contour interaction processes. To assess these alternatives, four variants of the Ebbinghaus figure were constructed using 1, 5, 9, or 13 small circles dispersed along the perimeter of larger contextual circles. 30 observers ranked the perceived size of the central circles and a single control circle. The rankings indicated that increasing the number of small circles reduced the perceived size of the central circle. The results parallel the effects of contextual arcs on the Ebbinghaus illusion and suggest that the mis-estimations of central circle size in Ebbinghaus figures result primarily from contour interactions.

Ebbinghaus Illusions with Disc Figures: Effects of Contextual Size, Separation, and Lightness

The Ebbinghaus illusion was produced using figures with four small or large contextual discs located either near or far from the central disc. For similar figures, the discs were either all black or all white; for dissimilar figures, black and white contextual and central discs were used in opposition. 48 observers, in equal numbers, were assigned to one of the four crossings of size and separation of the contextual discs and, using the converging method of limits, illusion magnitude scores for each Ebbinghaus configuration were obtained. The central disc appeared larger when bounded by small contextual discs and smaller when the contextual discs were more distant. Contrary to size contrast theory, uniformly colored discs did not generate greater illusions; instead, white central discs appeared larger than black ones regardless of contextual color. Collectively, the results indicated that contour interactions play a prominent role in producing the Ebbinghaus illusion.

Size and Direction of Distortion in Geometric-Optical Illusions: Conciliation between the Müller-Lyer and Titchener Configurations

Over the past few decades, different theories have been advanced to explain geometric-optical illusions based on various perceptual processes such as assimilation and/or contrast. Consistent with the contradictory effects of assimilation and contrast, Pressey's assimilation theory provided an explanation for the Müller-Lyer illusion, but failed to account for the Titchener (Ebbinghaus) illusion. A model that explains both Müller-Lyer and Titchener illusions according to a common underlying process may outline a unified explanation for a variety of geometric-optical illusions. In order to develop such a model, the concept of empty space is introduced as an area of the illusory figure that is not filled by line drawings. It was predicted that the magnitude of illusion would increase with the area of the empty space around the illusory figures. The effect of empty space on the magnitude of perceptual distortion was measured in Müller-Lyer figures, with outward arrowheads of different length. The results indicated an overestimation of the target stimulus in all of the figures. Nevertheless, consistent with the prediction of the present model, the horizontal line in the Müller-Lyer figure with the longest arrowheads appeared shorter than that with the shortest arrowheads, although the size contrast of these figures was the same. According to the analysis proposed in the present study, the area of empty space not only affects the magnitude of illusion but also serves as a contextual cue for the perceptual system to determine the direction of illusion (orientation). The functional relationships between the size contrast and empty space provide a common explanation for the Müller-Lyer, Titchener, and a variety of other geometric-optical illusions.

Size contrast and assimilation explained by the statistics of natural scene geometry

The term "size contrast and assimilation" refers to a large class of geometrical illusions in which the apparent sizes of identical visual targets in various contexts are different. Here we have examined whether these intriguing discrepancies between physical and perceived size can be explained by a visual process in which percepts are determined by the probability distribution of the possible real-world sources of retinal stimuli. To test this idea, we acquired a range image database of natural scenes that specified the location of every image point in 3-D space. By sampling the possible physical sources of various size contrast or assimilation stimuli in the database, we determined the probability distributions of the size of the target in the images generated by these sources. For each of the various stimuli tested, these probability distributions of target size in different contexts accurately predicted the perceptual effects reported in psychophysical studies. We conclude that size contrast and assimilation effects are a further manifestation of a fundamentally probabilistic process of visual perception.

Similarity and lightness effects in Ebbinghaus illusion created by keyboard characters

36 observers judged the size of a central S in variants of the Ebbinghaus figure having contextual Ss, $s, or Hs. When the figures were composed of similarly shaped elements, underestimation of the central S was obtained. Manipulations of lightness indicated that these underestimations were strongest for figures with gray contextual characters and a black central S and weakest for figures with black contextual characters and a gray central S. All black or all gray figures produced intermediate illusions. The data are consistent with Choplin and Medin's 1999 claim that figural properties rather than semantic similarity influences size contrast and further show that the visual processes underlying size contrast include interactions of contours.

Similarity of the perimeters in the Ebbinghaus illusion

Coren and Miller (1974) and Coren and Enns (1993) argued that the magnitude of the Ebbinghaus illusion is a function of the rated or conceptual similarity of the inducing objects to the test object. In three experiments, we examined the convergence between conceptual similarity and illusion magnitude. The first failed to find support for this parallel. Two further experiments yielded support for an alternative hypothesis that the magnitude of the Ebbinghaus illusion is a function of the similarity of the perimeters of the inducing object to the test object. The similarity of the centers had no effect. These results suggest that the information used to estimate size is computed earlier in the visual system than suggested by Coren and colleagues and apparently does not involve the use of conceptual information.