Motion

InDepth: Time and the Perception of Motion.
 1. Time and Change. 1. Time as a dimension of space-time. The concept of motion clearly is intimately tied up with the concept of time. In the figure below, I have "replaced" the up-down space dimension with time, under the assumption that all our motion is horizontal. Such a diagram is called a space-time diagram. Your world-line is the path you take through space-time. 2. Time and memory. Much of our idea about time involves memory. However, the perception of motion probably does not involve conscious memory but at least at the lowest level occurs in the retina (see below). 3. The arrow of time. The perception that there is a remembered past and an unknown future is related to what is called the arrow of time. It seems as if we can go anywhere in space, but must move forward in time at a fixed rate without choice. Part of our perception of time involves the statistical behavior of large systems, which seem to change from ordered to disordered states (e.g. weathering and decay of buildings, etc.) if left to their own devices.

 2. Motion: Physics. 1. Linear motion: meters/second. Linear motion is the measured motion of something with respect to a fixed grid of space. 2. Angular motion: degrees/second. Angular motion is the measured change in direction to a moving object, as seen from a fixed point in space. We sense angular motion when we use our eyes; however, our brains convert the angular motion into linear motion in the space around us. 3. Motion and geometrical perspective. Our 'calculation' of linear motion from angular motion involves both binocular disparity and ambiguous depth cues. Motion itself enhances the perception of depth via motion parallax and geometrical perspective (something distant will have a smaller angular speed than something close by). 4. Look, Lawrence, the bunny moved! Ultimately, our perception of motion is not conceptually different from that of color; we have to be told that something is moving to make the connections in our brains, just as we have to be told that an orange is orange.

 3. Motion: Perception. 1. The three ways motion enters visual perception: a. The Image of a moving object crosses the retina. b. The eye follows a moving object; the background crosses the retina. c. The head follows a moving object; the background crosses the retina. Each of these three ways provides different stimuli to the brain. But, because the background is assumed to be fixed and unmoving, the brain interprets the object to be moving in front of the background. 2. Eye and brain processing: a. Motion is analysed in a separate area of the visual cortex. b. Different cells respond to different motion directions. c. A neural model for direction detection: directional inhibition. d. Saccades and fixation. Rapid-eye-movement consists of very fast saccades (jumps in direction) and short fixations in one direction. There are typically 3-5 saccades per second. Each saccade lasts about 0.02-0.05 sec. No information is processed in the saccade. Successive fixations are compared to stabilize the image. Directional inhibition must subtract off the overall shift. 3. Is it motion? The eye does not 'see' something move; it checks each fixation image with the directional inhibition pattern provided by the previous fixation image. Persistence is not involved. Any blinking images are similar to fixation images, so the following artificial situations are interpreted as motion. a. Blinking lights on a marquee. b. The movies. c. Strobe lights. In this case, the brain has fun trying to 'align' successive strobe images, with sometimes peculiar results. 4. Ambiguous motion. movie a. All motion is relative. There is no fixed geometry in space. We can only experience motion of one object relative to another object. The brain doesn't really know which object, or both, is moving, because there is no such definition. b. Induced motion: you or the clouds? Sometimes, if enough of the background IS moving, one's brain interprets it as if he or she is moving. c. Autokinetic movement: solitary, dim objects. These objects seem to jump around because of rapid-eye-movement. 5. Motion afterimages: the waterfall effect, rotating spirals. After staring at a waterfall, everything else one looks at seems to be drifting upward. 6. More details are here.

Sample Questions

On a space-time diagram, be able to identify where on its world line an object is moving fastest, slowest.

Why is it unlikely that memory is involved in motion perception? Hint: how many processes are necessary to recall a memory?

You watch a soccer team race down the field. What kind of motion is your eye recording?

How does motion enhance depth perception?

What must happen to an object for us to detect its movement?

Is the conscious perception of motion an instantaneous property of objects or a sequential overlap of changing still positions?

How does the retina process motion?