Ambiguous motion in a two-frame sequence.

We measured the ability of ten different observers to identify the direction of motion displacement of a 1.77 c/deg grating in a two-frame sequence. One subject showed virtually errorless performance, in agreement with Derrington and Cox [(1992) Vision Research, 32, 2191-2193], but most subjects found the task difficult and made many errors. One subject saw motion in the reverse of the actual direction of displacement. We conclude that the two-frame sequence contains motion signals in both directions, and that selective attention to the forward direction is necessary for errorless performance.