given that the above type of illusion also implies an illusory change in luminance, since it implies expanding darkness, it is possible to probe the impact of the illusion not only via an observer’s phenomenology, consisting of conscious verbal reports, but also by examining a concurrent, involuntary, physiological index: the diameter of the eye pupil.

at the root of these bright/dark illusions is the fact that, in general, the perception of light is not straightforwardly related to physical parameters; hence, the visual system relies on ecological regularities or constraints to generate perceptual hypotheses that, in most instances, achieve pragmatically the behavioral success of vision. 

frontiers science news

this illusion, new to science, is strong enough to trick our reflexes