Spatial Parameters Affecting the Hermann Grid and the Scintillating-Grid Illusions

The Hermann grid and the scintillating grid are examples of brightness-contrast illusions. The scintillating grid consists of a Hermann grid with bright disks about 1.4 times the diameter of the grid elements superimposed upon the intersections. The resulting scintillation effect can be described as striking illusory spots, darker than the background, perceived as flashing within the bright disks with each flick of the eye. Although the scintillation effect is different from the classical Hermann grid illusion, one prerequisite for its occurrence is the presence of a grid eliciting the Hermann grid illusion (Schrauf et al, Vision Research in press). At the 12th ECVP we showed that the Hermann grid illusion can be reduced by bright diagonals within the squares of the grid which have the effect of suppressing the 45°/135° components of the Fourier spectrum (Lingelbach et al, 1989 Perception 18 536 – 537). In a pilot study we successively reduced the length of the diagonals, and found that this suppression works well even if the diagonals are reduced to small tips near the grid intersections. According to Baumgartner's receptive-field account (Baumgartner, 1960 Pflügers Archiv 272 21 – 22), such elements should cause additional inhibition, thus decreasing the neuronal discharge rate and corresponding illusory percept. This is not the case: if any illusion is perceived, it is considerably weaker. The strength of the scintillation effect is similarly reduced by this manipulation. Thus the present results cannot easily be accounted for by Baumgartner's model.