Hermann Grid Illusion

Smudges that aren't there

The demo shows a plain grid: black squares in rows and columns, separated by white lines. Every white line is pure white, with no gradient or grey anywhere. Yet at the crossings where four white lines meet, faint grey smudges seem to appear, especially at the crossings you aren't looking at directly. Look straight at one and its smudge disappears, while the others carry on flickering.

The textbook explanation

The classic account goes back to the wiring of the retina. Retinal ganglion cells, the neurons that carry signals from the eye to the brain, each respond to a small round patch of the visual field with a centre and a surrounding ring that work against each other: light on the centre excites the cell, light on the surround inhibits it. That arrangement sharpens edges and saves effort on large uniform areas.

A cell centred on a crossing has white on four sides of its surround, while a cell centred on a plain stretch of white line has white on only two sides, with black squares on the other two. More white in the surround means more inhibition, so the crossing produces a weaker signal and looks greyer. The account also explains why the smudge vanishes where you look: these receptive fields are smallest at the centre of vision, too small to take in the surrounding squares.

Why that's probably not the whole story

The neat retinal explanation has run into trouble. János Geier and colleagues showed that if the white lines are made slightly wavy, the smudges disappear, even though each crossing still has the same amount of white around it (Geier et al., 2008). Schiller and Carvey found further mismatches between what the retinal account predicts and what people see, and argued that the effect must arise in the visual cortex rather than the eye (Schiller & Carvey, 2005). What remains certain is that the smudges come from the brain's contrast processing working on a perfectly uniform white grid; exactly where that processing happens is still debated.

A grid in a textbook

The physiologist Ludimar Hermann described the effect in 1870. The usual story is that he noticed it in the grid-patterned illustrations of a book he was reading, though, as with many illusion origin stories, the details are hard to verify. It's still one of the standard ways of introducing receptive fields in perception courses. The related scintillating grid, with small white discs at the crossings, makes dark dots flash in and out even more strongly. For another grid that misleads the eye, see the café wall illusion.