Spatial correlations and Grid Illusions

The Hermann8 grid illusion occurs when a dark background is 8Ludimar Hermann (1838–1914),
German speech scientist [and,
incidentally, coiner of the word
format; see chapter 2...
covered by a lightcoloured
grid. At the points of intersection
between grid lines, dark spots appear transiently, disappearing when
they are looked at directly.
A variant on Hermann’s grid, called the scintillating grid, was
discovered by Elke Lingelbach. The only alteration to Hermann’s
grid is to draw white circles at the intersections; this causes the
perception of a grid of lights flickering (scintillating) on and off.
Interestingly, while the Hermann grid illusion can appear with a
single intersection, it seems to be necessary to have at least a 3×3
grid for the scintillating effect to be perceived.