Distortion in perceived image size accompanies flash lag in depth

The flash-lag effect - a misperception that a flashed object appears to lag behind a moving object despite their physical alignment - has mainly been investigated as a spatiotemporal offset. Here, we report that the flash-lag-in-depth effect is accompanied by an illusory change in the apparent size of the flashed object. We found a strong flash-lag-in-depth effect with a dot-defined square, whose motion in depth was signaled by changing retinal disparity (stereomotion), and a Gaussian blob that was flashed in the center of the square. Using the same stimulus, observers matched the apparent size of the flashed blob with a reference blob when the square moved with approaching or receding motion. Approaching motion of the square resulted in a reduction in the apparent size of the flashed blob, and an apparent enlargement of the flashed blob was induced by receding motion of the square. Additionally, this size effect substantially diminished, or was eliminated, when looming (change of size) instead of stereomotion was used to cue motion in depth of the square. The flashed-object size change that is induced by the moving square is not explained by simple predictions from projective geometry.