Development of depth perception mediated by motion parallax in unidimensional projections of rotation in depth.

Motion parallax is a composite of five transformations demonstrated to be effective in adult judgments of rotation direction in polar motion projections of a horizontal row of dots rotating in depth. The effectiveness of these transformations as a function of age was tested by presenting six such motion projections to first graders (age = 6 years), seventh graders (age = 13 years), and college students (age = 19 years). Identical age functions were obtained for judged rotation direction from the four motion projections representing (1) Velocity, corresponding to the traditional definition of motion parallax as differential velocity, (2) Velocity plus differences between ratios of instantaneous displacement to instantaneous acceleration for dots on the near and far sides of the rotation axis ($\text{D}\text{A}$ Difference), (3) Velocity, $\text{D}\text{A}$ Difference, and a gradient across the row of $\text{D}\text{A}$ ratios, and (4) all transformations. First graders, unable to use horizontal transformations, performed at chance on these four projections, while older students made correct judgments. Order, separated from Velocity for the first time, resulted in chance performance at all ages, while Direction, also separated from Velocity for the first time, resulted in veridical judgments in only 4 of 24 college students.