The perception of shrinking in apparent motion

Apparent motion was produced using two triangular patterns of different sizes, each exposed for 100 msec, with a 50-msec interstimulus interval and 200-msec recycle interval. The triangles were aligned either on center or on the midpoints of the bases. Experiment 1, filled, outline, and three-dot triangles were viewed over four backgrounds: a blank illuminated field, and texture gradients constructed from horizontal lines, perspective lines, or a combination of these (full texture). In Experiment 2, outline and dot triangles were presented in one of three orientations: base down, base right, and base up over a blank background. Subjects made two forced-choice responses: apparent size was categorized as shrinking or not shrinking, and apparent motion was categorized as motion in depth or motion in a fixed frontal plane. The type of alignment was the major determiner of responses. When the midpoints of the base were aligned, the predominant response described a shrinking object in a fixed-position in depth. When the centers were aligned, the predominant response described an object of constant size moving in depth.