Phenomenal transparency at X-junctions.

Phenomenal transparency was studied in a stimulus geometry that differs markedly from the conventional Metelli configuration, namely four squares that abut at a common vertex. In case of subjective transparency one perceives either a bipartite square ground overlaid with a uniform transparent rectangle, or a uniform square background overlaid with a pair of mutually orthogonal, uniform, transparent rectangular regions. Thus, the generic interpretations are limited to "left", "right", "lower", or "upper" transparent rectangles or (when no subjective transparency occurs) "mosaic". All transparent cases are congruent, whereas the Metelli configuration allows distinct Gestalt interpretations. This avoids interactions between Gestalt factors and subjective transparency per se. Formal analysis reveals that as in Metelli's case a number of ambiguous cases (e.g., "left" or "lower") are to be expected. In the experiment we included these ambiguous cases as additional response categories. Observers who differ markedly on the Metelli configuration are virtually indistinguishable under the quad-square configuration. Moreover, observers reliably categorise the ambiguous instances as such; thus, "multiple transparency" has to be reckoned with as a bona fide percept.