Fig. 1: The visual stimuli consisted of NCS stimuli that evoked illusory luminance to form a drifting grating and control stimuli that either blocked the illusion (DBC stimuli) or were a real grating (LDG stimuli).

a An example of the achromatic NCS stimulus presented to mice. The stimulus consists of 9 sets of white concentric circles. Each set of concentric circles contains gray segments at different positions. As can be seen, the gray color is diffusing into the empty area between concentric circles. This generates an illusory grating that appears darker than the surrounding black background. Changing the location of the gray segments on the concentric circles will change the orientation of this illusory grating. b The schematic shows the physical and perceptual luminance in the four arbitrary areas bounded by red boxes and marked as areas 1, 2, 3, and 4 in panel a. Luminance differences in areas 3 and 4 make an illusory perceptual luminance difference between areas 1 and 2 due to the diffusion of the gray color from area 3 to area 1. The luminance values are arbitrary. c In the DBC stimulus condition, the concentric circles are constrained by an outer white”band,” which serves to block the diffusion of gray color while maintaining the presentation of the physical stimulus (i.e., the concentric circles). d A control stimulus that provided a real luminance-defined grating (LDG) was used to compare an actual grating with the illusory grating evoked by the NCS illusion in panel a. This grating was presented over a background compound of a steady concentric circle with a temporal and spatial frequency identical to the illusory grating generated by the NCS stimulus. The green ellipse shows an example of a V1 RF for comparing its size relative to the stimuli.