The Neural Control of Vision
O. Illusions

4a. The illusion is fully effective when the pre-adapting stimulus has the same luminance and chrominance as the background on which the black squares appear. Under these conditions the ON-center retinal ganglion cells do not discharge as had been proposed.

Since preadapting stimulus is identical to background of display, ON-center ganglion cells produce little response. Yet under these conditions the smudges are fully visible. You can test yourself on this in the next display.