The principal aim of the research is to assess the validity of a model of the process underlying the perceived constancy of shape and distance. The model is an explication of the invariance algorithms for perceived shape and size. The general procedure is to determine whether the timing and demands of a mask in a backward masking have distinctive effects on the relationship between perceived shape and slant and between perceived size and distance. The masking effects will be used to assess the necessity for postulating the various stages in our model and for identifying their characteristics.