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

Abstract

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.

A new illusion of height and width: taller people are perceived as thinner

“…Another possibility is that the illusion of height and width that we have demonstrated is actually a variant of a size contrast illusion, of which the Ebbinghaus illusion (Titchener circles) is probably the most famous (although see Roberts, Harris, & Yates, 2005, for an alternative explanation). Here, the width and height of the stimulus serve as contrasting elements, in the same way that the large and small circles do in the Ebbinghaus illusion, with the smaller width, for example, inducing the height to look larger by contrast.…”

Computation of mean size is based on perceived size

“…The first had a conventional configuration (regular Ebbinghaus figure). The inducers in these figures were arranged to occupy 75% of an imaginary circumference to keep the relative completeness of the surrounding inducers constant (Roberts et al, 2005). Consequently, depending on target size, the number of inducers in each figure was varied from 4 to 12.…”

Perceptual Organization and Consciousness

“…We already mentioned simultaneous brightness contrast (Figure 2A) where the brightness of a stimulus is influenced by the brightness of the surround. Similar effects are seen in the tilt illusion where the orientation of a central grating appears to be tilted away from that of a surrounding annulus, contrast-suppression ( Figure 2B) where the contrast of a central stimulus surrounded by a high-contrast annulus appears to be reduced, and the Ebbinghaus illusion ( Figure 2C) where a stimulus appears larger or smaller depending on the size of and the distance to stimuli surrounding it (Roberts et al 2005). Other illusions, like the Ponzo and Mueller-Lyer illusions ( Figure 2D,E) and variants thereof, may affect the neural processes underpinning interpretation of three-dimensional distance (Gregory 2008).…”