Spatial distortions induced by dotted patterns

Prolonged steady fixation of parallel, radial, or concentric lines positioned close to each other generates phenomena of illusory oscillation, waving, and shimmering. Purkinje reported a first observation of these phenomena almost two centuries ago (review by Wade, 1977 Perception 6 407-433). Here we show that illusory waving and oscillation phenomena arise at first glance when dotted lines are either interposed between continuous lines, or positioned very close to straight edges. Phenomenological and individual observations indicate that dotted and linear patterns are independent sources of different illusory effects: the former of waving, the latter of shimmering/scintillating. The two effects enhance each other when dotted and continuous lines are alternated. Possible accounts of the phenomena are discussed. Waving is likely to originate from the perceived alteration of dot spacing, which in turn gives rise to depth cues and the consequent illusory deviation in depth. Shimmering and scintillating are attributed to imperfect binocular fusion, which can cause phenomena of brightness induction. A theory of the combination of the two illusory effects is put forward.