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Transients in the retina: the critique of the usual explanation of the Hermann grid illusion
ELTE, Kísérleti és Általános Pszichológiai Tanszék (Eötvös Loránd University, Budapest, General Psychological Department)
Abstract of the talk in the Nyolcadik Magyar Látás Szimpózium,
08th September, 2001
(Translated form the original Hungarian abstract by Mariann Hudák, 05. July, 2008)
The widely known, nearly 150-year-old Hermann grid illusion is a grid consisting of periodically recurring horizontal and vertical white lines on black background, or black lines on white background. Observing this figure, illusory spots are seen at the intersections of the grid lines. The usual explanation in the literature – and lately even in textbooks – is to be found in the existence of on-off or off-on receptive fields in the retina. On this basis, if, for example, an on-centred, off-surround circular-symmetric receptive field is placed into an intersection, a different sign will be received as the result of the occurring inhibition and stimulation, than in the inner parts of the lines or in the background.
To me, this explanation has always seemed unnatural and over-simplified, since it does not provide any answer to a number of questions. For instance, it can be deduced by a line of thought (and even modelled by computer) that if such a receptive field is continuously moved around the area of the intersection in different directions, then the modelled spot (contrary to the perceived illusion) will not be circular symmetric, it will rather be similar to the diamond shape in a pack of playing-cards.
The scintillating grid  illusion is a newer variant of the Hermann grid illusion, invented by Schrauf (et al) , published in 1997. He placed small white disks to the intersection areas of the grey grid on a black background, as the consequence of which a surprising phenomenon occurred: at the locations of the white disks, dynamically scintillating black dots appeared. This phenomenon, in my opinion, utterly overturns the above-mentioned oversimplified explanation of the Hermann grid illusion: on that basis, the illusory spots should be compensated by the proper selection of the intensity of the white disks. Instead, the spots start to scintillate.
Method: in my talk, I am investigating this set of issues by presenting different computer-generated variants of the Hermann-Schrauf grid. I have made several parameters adjustable, by which their effect on the illusion can be investigated. Adjustable parameters are: the period of the grid, the width of the grid lines, grids of random period, honeycomb-shaped grid (intersections of 120 degree), interruption of grid-lines. In case of each variant, the Schrauf disks can be placed to the intersection and they can be withdrawn.
Results: (i) the illusion disappears both in case of interrupted grid-lines and honeycomb-shaped grids. (ii) the Hermann grid illusion occurs in those and only those grids, in which the Schrauf illusion also occurs when placing disks in them.
Conclusion: (a) continuous and nearly perpendicular grid-lines are necessary for the illusion to occur, (b) both results (i) and (ii) confute the correctness of the usual explanation of the Hermann grid illusion: the phenomenon is caused by something entirely different, and not by the existence of on-off receptive fields in itself. The explanation might be found in retinal “transient overshoots”, whose investigation is a future task.
<<< Images presented in the symposium
 As far as I knew when writing my abstract, he was the first researcher to investigate this issue:
Schrauf, M., Lingelbach, B., Wist, E.R. (1997) "The scintillating grid illusion."
Vision Research 37, 1033-1038.
[2 ] Further references on the scintillating grid:
The first version of the scintillating grid version was published by Bergen at the ARVO 1985 conference:
Bergen, J.R. (1985) "Hermann´s grid: New and improved".Investigative Ophthalmology and Visual Science 26, suppl. 280
The story of the „rediscovery" of the scintillating grid can be found here:
Lingelbach, B & Ehrenstein, WH Jr (2002) "Das Hermann-Gitter und die Folgen". DOZ 5:13–20, www.leinroden.de/304herfold.htm
Schrauf, M., Lingelbach, B., Lingelbach, E., Wist, E.R. (1995) "The Hermann grid and the scintillation effect". Perception 24, suppl. 88-89
Schrauf, M., Ehrenstein, W.H., Wist, E.R. (1998) "Dynamic dependence of the
scintillating grid illusion: Equivalence and efferent motion conditions",
Perception 27, supplement (ECVP 1998, Oxford)
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