Abstract
Purpose. Static perceptual transparency is known to affect the integration of motion signals in plaids and other similar stimuli (Adelson & Movshon, 1982; Stoner & Albright, 1990). We show that it can also affect ambiguous apparent motion, causing a change from vertical motion to horizontal motion. In addition, we show an interesting new brightness illusion. Stimuli. Our stimulus contained a rectangular occluder with two horizontal stripes extending across it. In one motion frame the left end of the upper stripe and the right end of the lower stripe extended slightly beyond the vertical edges of the occluder. In the other frame these protruding “tabs” were on the right end of the upper stripe and the left end of the lower stripe. The luminance of the tabs was very slightly higher or lower than that of the stripes lying within the occluder. The occluder appeared transparent when it had intermediate luminance relative to the background and the stripes (and tabs), otherwise it did not. Results. In the transparent condition the perceptual brightness difference between the tabs and the stripes was very small, whereas in the non-transparent condition was quite large. This is surprising given that it is generally difficult to obtain a strong brightness illusion with uniform targets that share an extended common boundary (cf., the Koffka ring). Further, when the transparent frames are alternated at roughly 3–5 frames per second, the long horizontal stripes appear to “pump” back and forth like two engine pistons. However, when the non-transparent frames are alternated in this way the tabs appear to break off from the ends of the stripes and move vertically, up and down. The parts of the stripes inside the rectangle appear to be stationary. This change in perceived motion may be related to the change in relative brightness. Conclusions. Perceptual transparency can strongly affect 1) the relative brightness of adjacent targets, and 2) the perception of apparent motion.