Crossing the line: Estimations of line length in the Oppel-Kundt illusion

Abstract

In the Oppel-Kundt illusion, one of the oldest and least understood geometrical visual illusions, a line subdivided by a series of short orthogonal ticks appears longer than an identical line without these. Paradoxically, bisecting a long line with a single tick leads to perceived shortening of the line. We have systematically investigated the effects of adding 1 to 12 ticks on perceived line length and results suggest that at least three mechanisms must be at work: (a) bisection, which reduces perceived length; (b) a filled extent effect, which is also apparent in the von Helmholtz illusion, though no satisfactory explanation for it exists; and (c) a local contour repulsion effect of the penultimate tick upon the perceived position of the end tick, but this effect, though significant, is too small to explain the Oppel-Kundt illusion in its entirety.

The open-object illusion: size perception is greatly influenced by object boundaries

“…Moreover, there was some evidence that the reversed pattern is also true: the inclusion of inner boundaries makes the object looks smaller. This finding seems in contrast to the "Helmholtz square illusion" or the "Oppel-Kundt illusion", according to which filled spaces appear larger than unfilled spaces (Mikellidou, & Thompson, 2014;Thompson & Mikellidou, 2011). However, this underestimation effect was not as large as the open-object illusion, and additional investigation is needed for establishing its robustness.…”

“…Clearly, many important questions remained to be answered. For example, is the effect unique to objects or it is related to distorted space segmentation (e.g., Mikellidou & Thompson, 2014)? Can the effect be generalized to 3D or curved objects?…”

“…Many people have the sense that Bopen spaces^are bigger than closed ones, or that open windows make the room feels bigger. By contrast, it has also been shown that filled spaces are sometimes perceived as bigger than unfilled spaces, as demonstrated, for example, in the "Oppel-Kundt illusion" (e.g., Mikellidou, & Thompson, 2014). The role of boundaries in size perception is even less clear when it comes to objects, and thus the present study aimed at investigating how the boundaries of an object and, particularly, the absence of these boundaries, impact its size perception.…”

An Orientation Dependent Size Illusion Is Underpinned by Processing in the Extrastriate Visual Area, LO1

“…The two different phases of the stimulus were used to prevent afterimages. To create prominent illusion, each rectangular stimulus consisted of seven lines, with a duty cycle of 0.9 (thin lines on a gray background) and a spatial frequency of 0.91 cycles/degree ( Mikellidou & Thompson, 2013 , 2014 ; Thompson & Mikellidou, 2011 ). We spatially jittered the stimuli by up to 0.1° to prevent afterimages providing cues to physical size changes between successive trials.…”

An effect of continuous contextual filling in the filled‑space illusion

“…It has been demonstrated that a stimulus with a certain number of evenly allocated identical fillers induces a considerably stronger illusion than that with an irregular distribution (Lewis 1912, Noguchi 2003 or with many fillers fused into one continuous unit (Bailes 1995, Bertulis andBulatov 2001). At the same time, the region of the illusion maximum was relatively flat and varied between studies: the greatest effect was found for the number of fillers from 11 to 23 (Spiegel 1937), 9 to 14 (Piaget and Osterrieth 1953), 4 to 13 (Bulatov et al 1997), 11 to 13 (Wackermann and Kastner 2010), and 8 to 12 (Mikellidou and Thompson 2014). If the stimulus elements differed in shape or size then the effect of the illusion was substantially diminished (Obonai 1954, Wackermann and Kastner 2009, Wackermann 2012a.…”