Illusory boundary interpolation from local association field

We previously showed that interpolation between vertically misaligned luminance edges of same polarity of contrast is preferred to that between co-linear edges of opposite polarity of contrast, although it results in illusory tilt (Roncato and Casco, 2003). We here analyze the spatial conditions that produce this illusory binding of vertically misaligned edges of light and dark tiles, alternated in a row, and in counterphase with those in the rows above and below. We find that, independently of scale and number of tiles in a row, the illusion is perceived when the vertical misalignment of more than three tiles is smaller than or equal to 9' and the horizontal separation between co-linear edges is smaller than or equal to 13'. These short distances suggest that the underlying mechanism is local. Both our phenomenological and psychophysical results support the notion of a local short-range association field, selective to contrast polarity, which produces a binding solution different from that of a phase-independent long-range mechanism. We suggest that the occurrence of the illusion at local-level is a result of the activation, within a local short-range association field, of units with orientation different from that stimulated by the physical edges. These units are not inhibited when they are close, iso-oriented and co-linear, and the misaligned edges from which they propagate have the same contrast polarity. We found that horizontal and vertical spatial limits for the interpolation covary but not such that their ratio is fixed, indicating that the two edges can be connected by projections having a relatively wide range of orientations.