Boundary completion, contrast polarity, and the perception of illusory tilt

What we perceive as a unitary object can be the result of integrative processes that generate a whole from parts. Although this issue of visual perception has been widely explored, recent experimental findings demonstrate that our knowledge is still incomplete. In particular, the question whether contour binding is affected by the sign of contrast (contrast polarity) across edges requires more in-depth examination. Here we show the effects of edge bindings that originate from the merging of laterally displaced edges with the same contrast polarity. We have studied a particular context in which such effects may emerge: a checkerboard with a series of alternated dark and light shapes superimposed on the corners of the squares. The phenomenal observations and experimental findings support the theories according to which boundary completions are originated by phenomena of edge propagation within a 'field of completion' (eg Shipley and Kellman, 2003 Perception 32 985-999) adjacent to an edge ending. Our findings conform to the Shipley and Kellman theory that boundary completion results from the interaction of edges as well as from edges and shapes lacking in oriented contours, the latter serving as 'receiving units', anchoring the paths of activations generated by oriented edges. We propose to integrate this theory with the hypothesis that interactions sensitive to the contrast sign generate conjunction paths of edges that alter their perceived orientation. Based on this perspective we propose an alternative account for the Café Wall illusion that can be extended to other phenomena of orientation misperception and to a Café Wall inversion effect that has not been observed previously.