Hermann Grid Optical Illusion and The Rebelling Dots of Reality

In 1870, Ludimar Hermann reported an optical illusion that has been popularized thereafter after his name. In this Hermann Grid illusion, we see either white grid upon the background of complete black or black grid on the background of complete white. If we watch the grid, we see blobs of darkness or blobs of white smudges appearing at the intersections of the white or black grid respectively that are more prominent at peripheral view. A number of approaches to explain this illusion have been ventured in course of time. In this article, we will take the endeavor to explain it from the angle of familiarity with the previous associated memories of grid views. To summarize our view, we can say that we see Hermann grid illusion because of the impossibility of the Hermann grid in reality.

A few observations on Hermann Grid stimuli

The Hermann Grid is made up of a series of vertical and horizontal bars. The Hermann Grid Illusion consists in the brightness of the intersections appearing different from that of the sections between intersections in spite of the luminance being the same. In the case of a light grid on a dark background the intersections tend to appear darker than the parts between intersections. It is here pointed out, in two different ways, that the stimulus power is less for the parts of the grid located at intersections than for parts of the grid between intersections. This is all in the stimuli and does not depend on vision or the visual system. Were we to assume that a stronger stimulus gives a brighter appearance this would make the parts between intersections appear brighter than the parts of the grid at intersections. This would be consistent with the Hermann Grid Illusion.

The Curved Grid Non-Illusions

The generally accepted explanation of the Hermann grid illusion is Baumgartner’s hypothesis that the illusory effect is generated by the response of retinal ganglion cells with concentric ON-OFF or OFF-ON receptive fields. To challenge this explanation, some simple distortions to the grid lines were introduced that make the illusion disappear totally, while all preconditions of Baumgartner’s hypothesis remained unchanged. Psychophysical experiments in which the distortion tolerance was measured showed the level of distortion at which the illusion disappears at a given type of distortion for a given subject. Statistical analysis shows that the distortion tolerance is independent of grid-line width within a wide range and of the type of distortion, except when one side of each line remains straight. The conclusion is the main cause of the Hermann grid illusion is the straightness of the edges of the grid lines. Similar results have been obtained in the scintillating grid.

The Scintillating Grid

The scintillating grid was first presented at the European Conference on Visual Perception in Tübingen in 1995. At the time, the prevailing explanation of the Hermann grid illusion was in terms of the arrangement of the receptive fields on the retina. The minor modification of having a grey instead of a white grid with white dots at the intersections produced a strikingly new and powerful illusion. Instead of the white dots, dark black dots are seen to “blink” and appear even darker in intensity than the black printer’s ink of the square. They then disappear again to reappear immediately somewhere else. The dark diagonals that appear from time to time fitted more to low-pass filtering. The study of low-pass filtered Hermann grids led to the discovery of the scintillating grid and several variants. To this day this illusion still has not been fully explained.

The Tilted Hermann Grid Illusion: ‘Illusory Spots’ versus ‘Phantom Bands’

The Hermann grid illusion became a cause célèbre, when it was reported that small figural changes from straight to curved bars abolish the dark illusory spots. We demonstrate that this is not an all-or-none effect; rather, the visual system tolerates some tilt/curviness. We transformed straight and curved Hermann grids to rhombic Motokawa grids by gradually tilting the horizontal bars. Initially we observed only dark illusory spots, then dark spots combined with phantom bands traversing the rhomb along the minor axis, and finally dark phantom bands only. This shows that two kinds of illusions can coexist in the same grid pattern.