A visual illusion induced by a pattern of black squares separated by narrow white channels, at the intersections of which illusory grey spots appear (see illustration). The illusion is believed to be caused by the fact that both ON and OFF regions of centre-surround receptive fields are stimulated by white light at the intersections, whereas when the gaze is fixed on other parts of the white channels, the edges of the black squares fall on the OFF regions and the white channels on the ON regions, producing stronger responses from both types of retinal neurons. Compare Ehrenstein brightness illusion. [Named after the German physiologist Ludimar Hermann (1838–1914) who first observed it in a design on a book cover and reported his observation in 1870]
Hermann grid. Illusory grey spots appear at the corners of the black squares.