Tiny Squares at the Hermann Grid Corners Can Completely Remove the Illusion

Abstract

A novel modification of the Hermann grid stimulus is demonstrated. It is shown that introduction of extremely tiny squares at the corners of the grid squares in the classical stimulus, keeping the position and orientation of the grid squares fixed, can reduce the strength and even completely wipe out the illusory dark spots. The novel perturbing stimulus was investigated further and a gray-level intensity threshold was measured for the tiny corner squares beyond which the illusory blobs disappear completely. It was also found that this threshold remains practically unchanged over a wide range of grid square size for an observer.

A few observations on Hermann Grid stimuli

“…1A) the brightness at the intersections tends to appear reduced relative to the brightness of the grid between the intersections. The Hermann Grid Illusion has been the topic of much interest (Hering, 1920;Baumgartner, 1960;Spillmann & Levine, 1971;Lanvin & Costall, 1978;Frisby, 1979;Levine et al, 1980;Troscianko, 1982;Lingelbach et al, 1985;Spillman, 1994;Palmer, 1999;Geier et al, 2004Geier et al, , 2008De Lafuente & Ruiz, 2004;Schiller & Carvey, 2005;Hamburger et al, 2012;Johnston & Timney, 2013;Roncato,et al, 2016;Bakshi & Ghosh, 2020). Interpretations of this illusion have focused on the visual system.…”