Use the next two pairs of slides to understand what is meant by motion ambiguity when the eye is the frame of reference. Use the arrow keys to go back and forth between the next slides. This is what happens when something in the world moves relative to an eye that is not moving.

Use the arrow keys to go back and forth between the next slides. This is what happens when something in the world is not moving, but the eye is moving over it.

In BOTH CASES, the black dot is on the left side of the eye and then the right side and back again. Back and forth. From standpoint of the eye, the same thing happened in both cases. That is what is meant by the ambiguity of motion – the point appearing on the right and left could happen as a result of either situation. The ambiguity is that when a point is experienced on different parts of the eye, there is no geometric difference between the case where the point moved (but the eye was still) and the case where the eye moved (but the point was still).

So what? Factually, we usually know the difference between when we are moving and when things in the world are moving. If motion at the eye is fundamentally ambiguous, how do we tell the difference between object motion and self-movement?

In the case where we showed the eye moving, as external observers, we could see that the eye was moving because its outline occupied different positions on the slide. As perception scientists, we are trying to explain the experience of the perceiver, from the perceiver’s point of view. It is commonly assumed that vision starts at the eye. If vision starts at the eye, then we (as theorists) have to treat both of our cases as fundamentally the same. Where is the point relative to the eye? And the eye relative to the point?

The usual assumption, again, is that the geometry is ambiguous. Another way to say this is that the cases are symmetric, meaning they are the same. In order to treat the two cases as different we need a way to break the symmetry. The usual way to break the symmetry is to appeal to the physiology and psychology of intention, along with prediction.

Thus If I INTEND to move my eye to the left, and the point I am looking at does not move, then the point should end up on the right side of my eyeball. If my prediction is correct, then I can conclude that my eye moved. If I DO NOT INTEND to move my eye, but the point moves from the left to the right sides, then I can conclude that the point in the world moved. One version of this theory is called EFFERENCE COPY THEORY. The word “efference” refers to neural signals moving OUT toward the muscles. The opposite, afference, refers to neural signals moving from the world IN toward the central nervous system. The following slide is a quote from a prominent perception researcher in a 2013 article.

“When making a saccade from one location to another, the location of every feature in the visual world moves to a new location. We do not notice these changes in location because, in programming eye movements, the visual system creates an efference copy and compares this model with the resulting sensory input.” p. 478 Proffitt, D. R. (2013). An Embodied Approach to Perception : By What Units Are Visual Perceptions Scaled? Perspectives on Psychological Science 8, 474 – 483. Note -- A “saccade” is an eye movement of the kind that jumps from one place to another.

The Ecological Approach Denies the Initial Assumption that the eyeball is the proper frame of reference. The Ecological Approach, after James Gibson, and elaborated early on by David N. Lee, takes the world as its starting point. It says that we live INSIDE a large envelope consisting of sky and ground, meeting at the horizon and that THIS, not the eyeball, is the fundamental frame of reference. If we examine what happens when we move inside this envelope, we find that when we move, there is GLOBAL OPTIC FLOW. When aspects of the world move, but we do not, there is LOCAL CHANGE. Thus, the two cases are NOT symmetric and there is NO AMBIGUITY between the geometry of self-motion and object motion. Therefore, no special theory is required to explain how we can break the symmetry of an ambiguity because the cases are not ambiguous to begin with.