Distinguishing object motion from observer motion

Figure 6.14: Two motions that cause equivalent movement of the image on the retina: (a) The eye is fixed and the object moves; (b) the eye moves while the object is fixed. Both of these are hard to achieve in practice due to eye rotations (smooth pursuit and VOR).

Figure 6.14 shows two cases that produce the same images across the retina over time. In Figure 6.14(a), the eye is fixed while the object moves by. In Figure 6.14(b), the situation is reversed: The object is fixed, but the eye moves. The brain uses several cues to differentiate between these cases. Saccadic suppression, which was mentioned in Section 5.3, hides vision signals during movements; this may suppress motion detectors in the second case. Another cue is provided by proprioception, which is the body's ability to estimate its own motions due to motor commands. This includes the use of eye muscles in the second case. Finally, information is provided by large-scale motion. If it appears that the entire scene is moving, then the brain assumes the most likely interpretation, which is that the user must be moving. This is why the haunted swing illusion, shown in Figure 2.20, is so effective.