Purpose: We performed psychophysical experiments to determine the rules governing the perception of transparency in additive square-wave plaids. Methods: Subjects were presented with a randomized sequence of square-wave plaids of varying grating speed, grating orientation and plaid intersection luminance. The two gratings were symmetrically oriented about vertical, with fixed and equal period and duty-cycle. Presentations lasted two seconds, with a three second inter-trial interval. Subjects were asked whether the stimulus appeared to be transparent or coherent. Results: Our experimental results suggest that the perception of transparency is primarily governed by the pattern speed and the grating speed. In particular, when the pattern speed exceeds a certain critical speed (Sc), the plaid is more likely to be seen as transparent. Furthermore, when the grating speed exceeds the critical speed, subjects report being unable to make clear judgments. This result is illustrated in the idealized diagram of subject response versus pattern speed (Sp) and grating speed (Sg) shown to the right. Further studies suggest that varying the luminance of the plaid intersections (see Stoner, et. al., 1990) seems to affect the percept of transparency only when the pattern speed is close to the critical speed. Conclusions: The existence of such a critical speed suggests that the human visual system may have a perceptual preference for slower speeds. This data and the original data of Stoner, et. al. is consistent with a fairly simple energy-based model for velocity computation in which the representation of velocity is speed-limited.
