Similarity and lightness effects in Ebbinghaus illusion created by keyboard characters

Distinct Contributions of Alpha and Beta Oscillations to Context-Dependent Visual Size Perception

Previous studies have proposed two cognitive mechanisms responsible for the Ebbinghaus illusion effect, i.e., contour interaction and size contrast. However, the neural underpinnings of these two mechanisms are largely unexplored. The present study introduced binocular depth to the Ebbinghaus illusion configuration and made the central target appear either in front of or behind the surrounding inducers in order to disturb size contrast instead of contour interaction. The results showed that the illusion effect, though persisted, was significantly reduced under the binocular depth conditions. Notably, the target with a larger perceived size reduced early alpha-band power (8-13 Hz, 0-100 ms after stimulus onset) at centroparietal sites irrespective of the relative depth of the target and the inducers, with the parietal alpha power negatively correlated with the illusion effect. Moreover, the target with a larger perceived size increased the occipito-parietal beta-band power (14-25 Hz, 200-300 ms after stimulus onset) under the no-depth condition, and the beta power was positively correlated with the illusion effect when the depth conditions were subtracted from the no-depth condition. The findings provided neurophysiological evidence in favor of the two cognitive mechanisms of the Ebbinghaus illusion by revealing that early alpha power is associated with low-level contour interaction and late beta power is linked to high-level size contrast, supporting the claim that neural oscillations at distinct frequency bands dynamically support different aspects of visual processing.

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.

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.

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.

Outlined Objects in a Mach Pattern Create Illusions: Further Evidence of a Frequency Code for Size

In two experiments, new illusions of size were created using stimuli composed of nine trees, otoscopes, pens, hourglasses, hands, or mailboxes. The size of the nine objects varied across space in accordance with a Mach pattern. At the small side of the stimuli where there were three objects of identical size, the object at the inflection point was seen as smallest; at the large side of the stimuli where there were also three objects of equal size, the object at the inflection point was seen as largest. In Exp. 3, isolated objects and objects at the inflection points of the Mach stimuli were compared. When the stimuli were oriented to place the judged objects at the inflection points near the isolated object, figures with both pens and otoscopes at the small side of the pattern were underestimated and the figures of pens and otoscopes at the large side of the pattern were overestimated. These illusions resemble Brigner and Kauffman's Mach lines illusion and appear to result from interaction among visual neurons which are encoding the size of complex objects by frequency of their response.