Delboeuf Illusion

The Delboeuf illusion is a phenomenon in which the perceived size of an object changes depending on the size of the surrounding shapes. In this optical illusion, typically examined using two concentric circles, a central circle of fixed size is perceived differently based on the size of the surrounding circle.

History

The illusion was first described in 1865 by Joseph Delboeuf. In his experiments, the diameters of concentric circles were compared. The results demonstrated that the size of the surrounding shape significantly influences the perceived size of the central circle. Circles of identical diameter appear larger when surrounded by a smaller outer circle and smaller when surrounded by a larger outer circle.

Perceptual Mechanisms

The Delboeuf illusion is rooted in the role of eye movements and visual attention processes. During measurement, the eyes are drawn toward or pulled away from the boundaries of the surrounding shapes, leading to a perception of diameter that differs from its actual physical measurement.

Coren and Girgus’s “pooling and storage” theory is one of the current approaches explaining this mechanism. According to this theory, visual information gathered in short-term sensory memory is processed by either reducing the perceived difference between diameters (assimilation) or exaggerating it (contrast). Assimilation effects dominate at small diameter ratios, while contrast effects prevail at larger ratios.^【1】

Example of the Delboeuf Illusion

Delboeuf Illusion and Other Illusions

The Delboeuf illusion is related to other optical-geometric illusions such as the Müller-Lyer and Ebbinghaus illusions. In the Ebbinghaus illusion, the perceived size of a central circle varies according to the size of surrounding circles. Similarly, the Delboeuf illusion demonstrates how contextual surroundings alter perception.

Role in Portion Perception

The Delboeuf illusion affects portion perception. Identical food portions are perceived as smaller when served on larger plates and larger when served on smaller plates. This phenomenon is a psychological factor influencing eating behavior and portion control.

Influencing Factors

The strength of the Delboeuf illusion depends on factors such as the size of the surrounding shapes, their proportions, duration of visual attention, and the distribution of eye movements.

Ratio of Inner Object Dimensions

The Delboeuf illusion emerges based on the ratio between the diameter of the inner circle and that of the surrounding circle. When the ratio is close, such as 3:2, the illusion produces an assimilation effect, causing the inner circle to appear larger than it is. When the ratio is larger, such as 5:1, a contrast effect dominates, making the inner circle appear smaller.

Eye Movements and Perceptual Orientation

According to Delboeuf’s theory, during a measurement task, the eyes move between the inner and outer boundaries. A tendency to remain within the inner circle’s boundary, termed “retention,” and a tendency to be drawn beyond the outer circle’s boundary, termed “attraction,” contribute to perceptual differences.

Stimulus Properties and Visual Contrast

The shape, color, brightness, and properties of surrounding stimuli can alter the direction and intensity of the illusion. However, the effect of brightness changes on the illusion is limited.

Timing and Fixation Duration

The magnitude and direction of the illusion can vary depending on the duration of gaze and the sequence of successive fixations. Assimilation effects are more prominent during brief single fixations, while contrast effects become dominant during longer or repeated fixations.

Applications

The Delboeuf illusion is a phenomenon used in psychology and neuroscience to reveal the contextual structure and limits of visual perception. It is considered in applications such as nutritional psychology and portion control to help develop healthier eating strategies.