The Aperture Problem
Many of the motion sensitive neurons in the visual system, especially those early in the visual system, tend to have relatively small receptive fields. That is, they respond to only a very small part of the retina. They can only respond to motion that is local to, or on, the receptive field. Such neurons give information about "local" motion. Later in the visual system, the information from several of the neurons that respond to local motion in similar parts of the retina is integrated to allow us to perceive the global, or overall, motion of the image.
The local motion information from a single neuron is inherently ambiguous with respect to the global motion. That is, many different motions on the retina could cause the same response in a single motion sensitive neuron with a small receptive field. This is called the aperture problem.
The aperture problem can be demonstrated by looking at a moving image through a small hole -- the aperture. Different directions of motion can appear identical when viewed through an aperture. For example, look at the three moving lines shown below. When viewed through an aperture, most people perceive all three lines as moving from the upper left to the lower right. In reality, the lines are moving in different direction. You can verify this yourself by hiding the aperture by clicking on the "Hide" button. The only thing that physically changes when you hide the aperture is that you can see the entire line instead of just a portion of it.
How would you describe the blue object? Does it look like a pulsating plus sign? Or does it look like a plus sign that gets closer, then farther from you? It is neither. The object is seen through the aperture (gap) created by the tan rectangles.
Drag the slider next to "Aperture size" just a little tiny bit to the right to make the aperture just a bit bigger. When you can easily see the 90° corner passing through the aperture, your perception should change. If the blue object now looks like a rotating square that gets bigger (or closer) and then smaller (or farther away), you are closer to a veridical (truthful) perception but still not quite there.
Open the aperture as far as it will go (drag the slider all the way to the right) to see the true nature of the blue object. It is just a rotating square -- it isn't getting bigger (or closer) or smaller (or farther).
Now that you know that the blue object is just a rotating square, return the aperture to its smallest size. I get yet another perception -- a diamond getting bigger and smaller. I even get the subjective contours of the diamond and some bleed through of the blue diamond into the tan rectangles. That is, the corners of the tan rectangles near the blue object appear slightly darker and the darkened region grows and shrinks as the blue "diamond" appears to move closer and farther away.