Is vision metric? Comment on Lappin and Love (1992).

In a recent paper, Lappin and Love (1992) showed that depth judgments, based on planar motion of a stimulus, are remarkably precise. This led them to the conclusion that perception of planar motion allows acquiring metric information about visual space. We show, however, that Lappin and Love's stimuli contained 2-D cues in the frontal plane that could subserve the subject's judgments. This means that high precision in this task does not necessarily imply metric structure of visual space.