Hack76.To Be Noticed, Synchronize in Time

Hack 76. To Be Noticed, Synchronize in Time

We tend to group together things that happen at the same time or move in the same way. It's poor logic but a great hack for spotting patterns.

It's a confusing, noisy world out there. It's easier to understand the world if we perceive a set of objects rather than just a raw mass of sensations, and one way to do this is to group together perceptions that appear to have the same cause. The underlying assumptions involved manifest as the gestalt grouping principles, a set of heuristics used by the brain to lump things together (see [Hack #75] for the simplest of these, used for vision).

Perhaps the most powerful of these assumptions is termed common fate. We group together events that occur at the same time, change in the same way, or move in the same direction. Imagine if you saw, from far off, two points of light that looked a bit like eyes in the dark. You might think they were eyes or you could just put it down to a coincidence of two unrelated lights. But if the points of light moved at the same time, in the same direction, bounced with the characteristic bounce of a person walking, you'd know they were eyes. Using behavior over time allows you to stringently test spatial data for possible common cause. If the bouncing lights pass the common fate test, they're almost certainly a single object. Visual system tags this certainty by providing you with a correspondingly strong perceptual experience; if some things move together, it is almost impossible to see them as separate items instead of a coherent whole.

8.3.1. In Action

"IllusionMotion CaptureGrouping" (http://psy.ucsd.edu/chip/illu_mot_capt_grpng.html; a Real video requiring Real Player) demonstrates just how completely your perception of a single item is altered by global context and common fate. Watch the video for at least 30 seconds. At first you see just a dot blinking on and off next to a square. But then other dots are added in the surrounding area, and as the first dot blinks off, they all shift right. Now your unavoidable impression is of the first dot moving behind the square. The appearance of the other dots, and their behavior, gives your visual system correlations that are just too strong to ignore. The single dot is still blinking on and offyou just can't see it like that any more.

"A Time Gestalt Principle Example: Common Fate" (http://tepserver.ucsd.edu/~jlevin/gp/time-example-common-fate; a Java applet),1 shown in Figure 8-3, is an interactive demonstration of how your visual system deduces the shape of objects from movement, without any color or shading clues to help out.

Figure 8-3. When the circle hidden in the pattern is moving, it's clearly seen; printed like this, it's invisible

You see a shape with a static-like texture moving across a similarly randomized background. Click anywhere in the image to start and stop the demo. Frozen, there is no pattern to see; you see just a random mess. This is the real force of common fate. The correlations exist only across time, in movementit's only when the demo is moving that you can see an object among the noise.

8.3.2. How It Works

The gestalt grouping inferences are so preconscious and automatic that it's hard to imagine perceiving a world that the brain hasn't organized into objects. There's something very clever going on here; we are taking in very little information (only how the pattern changes over time), yet, in combination with an assumption that accidental correlations of visual patterns are unlikely, we construct a compelling perception of an object. In these demos, you just can't ignore the object. You are utterly unable to make yourself see a moving collection of dots instead of the shape in motion because the construction of the object is happening before the level of consciousness.

Common fate can lead to some sophisticated inferences. "Kinetic Depth" (http://www.biols.susx.ac.uk/home/George_Mather/Motion/KDE.HTML; a QuickTime video), just from a collection of moving lights, allows you to see an object with three-dimensional depth moving in a particular way. In this case, the pattern of dots causes you to see a sphere rotating on its axis.

What's really cute about this video is that there's an ambiguity in the visual informationyou can see the sphere rotating in one of two ways. Your visual system makes a choice for you, and you see some of the dots moving behind some of the others, which move in the opposite direction. The set you see as "in front" determines the direction in which you see the sphere rotating. If you watch for a while, your perception will switch and you'll see it reverse. You don't need to make any effort to for this to happen; it occurs naturally, probably due to some kind of adaptation process. Since you see the sphere rotating in one particular direction, the neurons that represent that perception will be active. Over time, they actively tune down their response, and the neurons that code for the other apparent rotation can now dominate. This kind of gain control [Hack #26] plays a similar role in motion aftereffects [Hack #25], in which neurons that are active for particular directions of movement down-regulate after being consistently stimulated and neurons active for the opposing direction take over and dominate our perception when the consistent moving stimulus is removed.

All these demonstrations show just how effective correlations over time are in molding our perception. And not just perceptionsynchronizing stimuli can actually alter your body image, where your brain believes your hands are [Hack #64], for instance. The heart of the thing is similarif two things correlate exactly, our perception treats them as part of the same object. For our brains, isolated inside the skull, perceived correlation is the only way we've ever had for deducing what sensations should be associated together as part of the same object.

Common fate can also draw inferences from points of light moving in much more complex ways than rotating spheres. For the case of biological motion [Hack #77], the visual system is specifically prepared to fit moving points into a schema based upon the human body to help perception of the human form. Alais et al. have suggested that the importance of common fate reflects a deeper principle of the brain's organization.² Neuroscientists talk about the binding problem, the question of how the brain correctly connects together all the information it is dealing with: all the things that are happening in different parts of the world, detected by different senses, whose component parts have properties represented in different cortical areas (such as color, contrast, sounds, and so on), all of which have to be knitted together into a coherent perception. The suggestion is that common fate reflects synchronization of neuron firingand that is this same mechanism that may underlie the brain's solution to the binding problem.

8.3.3. End Notes

1. Part of Jim Levin's "Gestalt Principles & Web Design" (http://tepserver.ucsd.edu:16080/~jlevin/gp). Applet developed by Adam Doppelt.

2. Alais, D., Blake, R., & Lee, S. (1998). Visual features that vary together over time group together over space. Nature Neuroscience. 1(2), 160-164.