Three-dimensional motion perception: comparing speed and speed change discrimination for looming stimuli

Judging the speed of objects moving in three dimensions is important in our everyday lives, because we interact with objects in a three-dimensional world. However, speed perception has been seldom studied for motion in depth, particularly when using monocular cues such as looming. Here, we compared speed discrimination, and speed change discrimination, for looming stimuli, to better understand what visual information is used for these tasks. For the speed discrimination task, we manipulated the distance and duration information available, to investigate if participants were specifically using speed information. For speed change discrimination, total distance and duration were held constant, hence they could not be used to successfully perform that task. We found speed change discrimination thresholds were consistently higher than those for speed discrimination. Evidence suggested that participants used a variety of cues to complete the speed discrimination task, not always solely relying on speed. Further, our data suggested that participants may switch between cues on a trial to trial basis. We conclude that speed change discrimination for looming is more difficult than speed discrimination, and that naive participants may not always exclusively use speed for speed discrimination.