Irradiation: implications for theories of edge localization.

In a vernier alignment task, observers judged the relative positions of two blurred edges placed one above the other. If the two edges were given the same blur width and contrast polarity, their relative positions were perceived veridically. If the two edges were given different blur widths or opposite contrast polarities, reliable errors in perceived position were revealed, as if each edge were shifted into its dark phase by an amount proportional to its blur. The magnitude of shift varied from zero for sharply defined edges, to over 1 min arc for blurred edges. These illusory shifts in position are consistent with the well-known phenomenon of "irradiation". Implications for current models of edge localization are discussed.