The effect of monocular depth cues on the detection of moving objects by a moving observer

As observers move through the environment, they must detect moving objects. Previously, we showed that observers can use 2D image speed to detect moving objects in the radial optic flow field generated by a moving observer. However, a difference in image speed may signal either a moving object or a depth difference between stationary objects. Adding depth information may remove this ambiguity. We tested observers' ability to detect a moving object in scenes that contained increasingly salient monocular depth cues. We simulated observer motion in a straight line at a speed of 3 m/sec toward a scene that consisted of a textured ground plane with 8 objects located 12 m from the observer. In two conditions the objects were featureless red disks (diameter: 0.4 m) located on the horizontal midline that were either separate from the ground plane (condition 1) or connected to it with a thin line, giving a cue to distance (condition 2). In condition 3, the objects were textured blocks (width: 0.55 m; height: 1.6 m) located on the ground plane, giving further cues to depth. In half the trials one object moved faster (or slower) than the other objects in the scene. The speed differences ranged from 10% to 100% of the image speed of non-targets. Each trial lasted 1 sec. Observers indicated with a key press whether or not the scene contained a moving object. Thresholds were computed by fitting the data with a sigmoidal curve and determining the percent speed difference that led to 75% accuracy. For the 6 subjects tested, the results show a significant effect of scene condition, with average threshold speed changes of 43%, 37% and 25% for conditions 1, 2 and 3, respectively. Thus, monocular depth cues aid the detection of moving objects by moving observers.