Zombie Fud

Hack 22. Depth Matters

2.11.1.1 In action
Close one eye and have a look at the two shaded blocks side by side in Figure 2-15. If you had to decide which block appears to be visually closer, which would you choose? The black block seems to separate and appear forward from the gray block. It is as if our mind wants it to be in front.

Figure 2-15. Which block appears closer?

2.11.1.2 How it works
The reason for this experience of depth, based on light-dark value differences, is atmospheric perspective and the science is actually quite simple. Everywhere in the air are dust or water particles that partially obscure our view of objects, making them appear dull or less distinct. Up close, you can’t see these particles, but as the space between you and an object increases, so do the numbers of particles in the air. Together these particles cause a gradual haze to appear on distant objects. In the daytime, this haze on faraway objects appears to be colored white or blue as the particles scatter the natural light. Darker objects separate and are perceived as foreground and lighter ones as background. At night, the effect is the same, except this time the effect is reversed: objects that are lit appear to be closer, as shown in Figure 2-16. So as a general rule of thumb, an object’s intensity compared to its surroundings helps us generate our sense of its position. Even colors have this same depth effect because of comparative differences in their value and chroma. The greater the difference in intensity between two objects, the more pronounced the sense of depth separation between them.

Figure 2-16. At night, lit objects appear closer

So how does intensity relate to attention? One view is that we pay more attention to objects that are closer, since they are of a higher concern to our physical body. We focus on visually intense objects because their association with the foreground naturally causes us to assign greater importance to them. Simply put, they stand out in front.

2.11.1.3 In real life
Since weather can affect the atmosphere’s state, it can influence perceived depth: the more ambient the air particles, the more acute the atmospheric perspective. Hence, a distance judged in a rainstorm, for example, will be perceived as further than that same distance judged on a clear, sunny day.

2.11.2. Known Size
How do we tell the distance in depth between two objects if they aren’t the same?

We all know that if you place two same-size objects at different distances and look at them both, the object further away appears smaller. But have you ever been surprised at an object’s size when you see it for the first time from afar and discover it is much bigger up close? Psychologists call this phenomenon size gradient and known size. Size gradient states that as objects are moved further away, they shrink proportionally in our field of view. From these differences in relative size, we generate a sense of depth. This general rule holds true, but our prior knowledge of an object’s size can sometime trip us up because we use the known size of an object (or our assumptions of its size) to measure the relative size of objects we see.

Being aware of a user’s knowledge of subjects and objects is key if comparative size is an important factor. Many visual communication designers have discovered the peril of forgetting to include scale elements in their work for context reference. A lack of user-recognizable scale can render an important map, diagram, or comparative piece completely useless. An unexpected change in scale can disorientate a useror, if employed right, can help grab attention.

2.11.2.1 In action
Have a look at the mouse and elephant in Figure 2-17. We know about their true relative sizes from our memory, even though the mouse appears gigantic in comparison.

Figure 2-17. An elephant and a mouseyou know from memory that elephants are bigger

But what about Figure 2-18, which shows a mouse and a zerk (a made-up animal). Since we’ve never seen a zerk before, do we know which is truly bigger or do we assume the scale we see is correct?

Figure 2-18. A zerk and a mousesince a zerk is made up, you can use only comparison with the mouse to judge size

2.11.2.2 How it works
Our knowledge of objects and their actual size plays a hidden role in our perception of depth. Whenever we look at an object, our mind recalls memories of its size, shape, and form. The mind then compares this memory to what we see, using scale to calculate a sense of distance. This quick-and-dirty comparison can sometimes trip us however, especially when we encounter something unfamiliar. One psychologist, Bruce Goldstein, offers a cultural example of an anthropologist who met an African bushman living in dense rain forest. The anthropologist led the bushman out to an open plain and showed him some buffalo from afar. The bushman refused to believe that the animals were large and said they must be insects. But when he approached them up close, he was astounded as they appeared to grow in size, and attributed it to magic. The dense rain forest and its limitations on viewing distance, along with the unfamiliar animal, had distorted his ability to sense scale.

2.11.2.3 In real life
Some designers have captured this magic to their benefit. The movie industry has often taken our assumptions of known size and captivated us by breaking them, making the familiar appear monstrous and novel. For example, through a distortion of scale and juxtaposition, we can be fooled into thinking that 50-foot ants are wreaking havoc on small towns and cities.

2.11.3. End Notes
Bardel, W. (2001). “Depth Cues for Information Design.” Thesis, Carnegie Mellon University (http://www.bardel.info/downloads/Depth_cues.pdf).

Street sign symbols courtesy of Ultimate Symbol Inc. (http://www.ultimatesymbol.com).

2.11.4. See Also
Goldstein, E. B. (1989). Sensation & Perception. Pacific Grove: Brooks/Cole Publishing.

Ware, C. (1999). Information Visualization. London: Academic Press.

Tufte, E. (1999). Envisioning Information. Cheshire: Graphics Press.

Braunstein, M. L. (1976). Depth Perception Through Motion. London: Academic Press.

Reagan, D. (2000). Human Perception of Objects. Sunderland: Sinauer Assoc.

William Bardel

Taken From : Mind Hacks

Hack 22. Depth Matters

Our perception of a 3D world draws on multiple depth cues as diverse as atmospheric haze and preconceptions of object size. We use all together in vision and individually in visual design and real life.

Our ability to see depth is an amazing feature of our vision. Not only does depth make what we see more interesting, it also plays a crucial, functional role. We use it to navigate our 3D world and can employ it in the practice of visual communication design to help organize what we see through depth’s ability to clarify through separation1.

Psychologists call a visual trigger that gives us a sense of depth a depth cue. Vision science suggests that our sense of depth originates from at least 19 identifiable cues in our environment. We rarely see depth cues individually, since they mostly appear and operate in concert to provide depth information, but we can loosely organize them together into several related groups:

Binocular cues (stereoscopic depth, eye convergence)

With binocular (two-eye) vision, the brain sees depth by comparing angle differences in the images from each eye. This type of vision is very important to daily life (just try catching a ball with one eye closed), but there are also many monocular (single-eye) depth cues. Monocular cues have the advantage that they are easier to employ for depth in images on flat surfaces (e.g., in print and on computer screens).

Perspective-based cues (size gradient, texture gradient, linear perspective)

The shape of a visual scene gives cues to the depth of objects within it. Perspective lines converging/diverging or a change in the image size of patterns that we know to be at a constant scale (such as floor tile squares) can be used to inform our sense of depth.

Occlusion-based cues (object overlap, cast shadow, surface shadow)

The presence of one object partially blocking the form of another and the cast shadows they create are strong cues to depth. See [Hack #20] for examples.

Focus-based cues (atmospheric perspective, object intensity, focu

Greater distance usually brings with it a number of depth cues associated with conditions of the natural world, such as increased atmospheric haze and physical limits to the eye’s focus range. We discuss one of these cues, object intensity, next.

Motion-based cues (kinetic depth, a.k.a. motion parallax)

As you move your head, objects at different distances move at different relative speeds. This is a very strong cue and is also the reason a spitting cobra sways its head from side to side to work out how far away its prey is from its position.

There isn’t room to discuss all of these cues here, so we’ll look in detail at just two depth cues: object intensity and known size (a cue that is loosely connected to the prespective-based cue family). More information on depth cues and their use in information design can be found in the references at the end of this hack.

2.11.1. Object Intensity
Why do objects further away from us appear to be faded or faint? Ever notice that bright objects seem to attract our attention? It’s all about intensity.

If we peer into the distance, we notice that objects such as buildings or mountains far away appear less distinct and often faded compared to objects close up. Even the colors of these distant objects appear lighter or even washed out. The reason for this is something psychologists call atmospheric perspective or object intensity. It is a visual cue our minds use to sense depth; we employ it automatically as a way to sort and prioritize information about our surroundings (foreground as distinct from background).

Designers take advantage of this phenomenon to direct our attention by using bold colors and contrast in design work. Road safety specialists make traffic safety signs brighter and bolder in contrast than other highway signs so they stand out, as shown in Figure 2-14. You too, in fact, employ the same principle when you use a highlighter to mark passages in a book. You’re using a depth cue to literally bring certain text into the foreground, to prioritize information in your environment.

Figure 2-14. Important street signs often use more intense colors and bolder contrast elements so they stand out from other signage2

Taken From : Mind Hacks

Hack 21. Objects Move, Lighting Shouldn’t

2.10.2. How It Works
Your brain constructs an internal 3D model of a scene as soon as you look at one, with the influence of shadows on the construction being incredibly strong. You can see this in action in the first movie: your internal model of the scene changes dramatically based solely on the position and motion of a shadow.

I feel bad saying “internal model.” Given that most of the information about a scene is already in the universe, accessible if you move your head, why bother storing it inside your skull too? We probably store internally only what we need to, when ambiguities have been involved. Visual data inside the head isn’t a photograph, but a structured model existing in tandem with extelligence, information that we can treat as intelligence but isn’t kept internally.

T.S.

The second movie shows a couple more of the assumptions (of which there are many) the brain makes in shadow processing. One assumption is that darker coloring means shadow. Another is that light usually comes from overhead (these assumptions are so natural we don’t even notice they’ve been made). Both of these come into play when two-dimensional shapesordinary picturesappear to take on depth with the addition of judicious shading [Hack #20] .

Based on these assumptions, the brain prefers to believe that the light source is keeping still and the moving object is jumping around, rather than that the light source is moving. And this despite all the cues to the contrary: the lighting pattern on the floor and walls, the sides of the box being lit up in tandem with the shifting shadowthese should be more than enough proof. Still, the shadow of the ball is all that the brain takes into account. In its quest to produce a 3D understanding of a scene as fast as possible, the brain doesn’t bother to assimilate information from across the whole visual field. It simplifies things markedly by just assuming the light source stays still.

It’s the speed of shadow processing you have to thank for this illusion. Conscious knowledge is slower to arise than the hackish-but-speedy early perception and remains influenced by it, despite your best efforts to see it any other way.

2.10.3. End Note
Zigzagging ball animation thanks to D. Kersten (University of Minnesota, U.S.) and I. Bülthoff (Max-Planck-Institut für biologische Kybernetik, Germany)

2.10.4. See Also
The Kersten Lab (http://gandalf.psych.umn.edu/~kersten/kersten-lab) researches vision, action, and the computational principles behind how we turn vision into an understanding of the world. As well as publications on the subject, their site houses demos exploring what information we can extract from what we see and the assumptions made. One demo of theirs, Illusory Motion from Shadows (http://gandalf.psych.umn.edu/~kersten/kersten-lab/images/kersten-shadow-cine.mov), demonstrates how the assumption that light sources are stationary can be exploited to provide another powerful illusion of motion.

Kersten, D., Knill, D., Mamassian, P., & Buelthoff, I. (1996). Illusory motion from shadows. Nature, 379(6560), 31.

Taken From : Mind Hacks

Hack 21. Objects Move, Lighting Shouldn’t

Moving shadows make us see moving objects rather than assume moving light sources.

Shadows get processed early when trying to make sense of objects, and they’re one of the first things our visual system uses when trying to work out shape. [Hack #20] further showed that our visual system makes the hardwired assumption that light comes from above. Another way shadows are used is to infer movement, and with this, our visual system makes the further assumption that a moving shadow is the result of a moving object, rather than being due to a moving light source. In theory, of course, the movement of a shadow could be due to either cause, but we’ve evolved to ignore one of those possibilitiesrapidly moving objects are much more likely than rapidly moving lights, not to mention more dangerous.

2.10.1. In Action
Observe how your brain uses shadows to construct the 3D model of a scene. Watch the ball-in-a-box movie at:

http://gandalf.psych.umn.edu/~kersten/kersten-lab/images/ball-in-a-box.mov (small version)

http://gandalf.psych.umn.edu/~kersten/kersten-lab/demos/BallInaBox.mov (large version, 4 MB)

If you’re currently without Internet access, see Figure 2-12 for movie stills.

The movie is a simple piece of animation involving a ball moving back and forth twice across a 3D box. Both times, the ball moves diagonally across the floor plane. The first time, it appears to move along the floor of the box with a drop shadow directly beneath and touching the bottom of the ball. The second time the ball appears to move horizontally and float up off the floor, the shadow following along on the floor. The ball actually takes the same path both times; it’s just the path of the shadow that changes (from diagonal along with the ball to horizontal). And it’s that change that alters your perception of the ball’s movement. (Figure 2-12 shows stills of the first (left) and second (right) times the ball crosses the box.)

Figure 2-12. Stills from the “ball-in-a-box” movie

Now watch the more complex “zigzagging ball” movie (http://www.kyb.tue.mpg.de/links/demo.html; Figure 2-13 shows a still from the movie), again of a ball in motion inside a 3D box.

Figure 2-13. A still from the “zigzagging ball” movie1

This time, while the ball is moving in a straight line from one corner of the box to the other (the proof is in the diagonal line it follows), the shadow is darting about all over the place. This time, there is even strong evidence that it’s the light sourceand thus the shadowthat’s moving: the shading and colors on the box change continuously and in a way that is consistent with a moving light source rather than a zigzagging ball (which doesn’t produce any shading or color changes!). Yet still you see a zigzagging ball.

Taken From : Mind Hacks

Hack 20. Fool Yourself into Seeing 3D

2.9.3. In Real Life
Given pop-out is so strong, it’s not surprising we often use the shading trick to produce it in everyday life.

The 3D beveled button on the computer desktop is one such way. I’ve not seen any experiments about this specifically, but I’d speculate that Susan Kare’s development of the beveled button in Windows 3.0 (http://www.kare.com/MakePortfolioPage.cgi?page=6) is more significant than we’d otherwise assume for making more obvious what to click.

My favorite examples of shade from shading are in Stuart Anstis’ lecture on the use of this effect in the world of fashion (http://psy.ucsd.edu/~sanstis/SAStocking.htm). Anstis points out that jeans faded white along the front of the legs are effectively artificially shadowing the sides of the legs, making them look rounder and shapelier (Figure 2-10). The same is true of stockings, which are darker on the sides whichever angle you see them from.

Figure 2-10. Shaded jeans add shape to legs

Among many examples, the high point of his presentation is how the apparent shape of the face is changed with makeupor in his words, “painted-on shadows.” The with and without photographs (Figure 2-11) demonstrate with well-defined cheekbones and a sculpted face just how compelling shape for shading really is.

Figure 2-11. With only half the face in makeup, the apparent shape difference is easy to see

2.9.4. End Notes
Kleffner, D. A., & Ramachandran, V. S. (1992). On the perception of shape from shading. Perception and Psychophysics, 52(1), 18-36.

Actually, more detailed experiments show that the brain’s default light source isn’t exactly at the top of the visual field, but to the top left. These experiments detailed in this paper involve more complex shadowed shapes than circles and testing to see whether they pop out or appear indented when immediately glanced. Over a series of trials, the position of the assumed light source can be deduced by watching where the brain assumes the light source to be. Unfortunately, why that position is top left rather than top anywhere else is still unknown. See Mamassian, P., Jentzsch, I., Bacon, B. A., & Schweinberger, S. R. (2003). Neural correlates of shape from shading. NeuroReport, 14(7), 971-975.

Taken From : Mind Hacks

Hack 20. Fool Yourself into Seeing 3D

How do you figure out the three-dimensional shape of objects, just by looking? At first glance, it’s using shadows.

Looking at shadows is one of many tricks we use to figure out the shape of objects. As a trick, it’s easy to foolshading alone is enough for the brain to assume what it’s seeing is a real shadow. This illusion is so powerful and so deeply ingrained, in fact, that we can actually feel depth in a picture despite knowing it’s just a flat image.

2.9.1. In Action
Have a look at the shaded circles in Figure 2-8, following a similar illustration in Kleffner and Ramachandran’s “On the Perception of Shape from Shading.”1

Figure 2-8. Shaded figures give the illusion of three-dimensionality

I put together this particular diagram myself, and there’s nothing to it: just a collection of circles on a medium gray background. All the circles are gradient-filled black and white, some with white at the top and some with white at the bottom. Despite the simplicity of the image, there’s already a sense of depth.

The shading seems to make the circles with white at the top bend out of the page, as though they’re bumps. The circles with white at the bottom look more like depressions or even holes.

To see just how strong the sense of depth is, compare the shaded circles to the much simpler diagram in Figure 2-9, also following Kleffner and Ramachandran’s paper.

Figure 2-9. Binary black-and-white “shading” doesn’t provide a sense of depth

The only difference is that, instead of being shaded, the circles are divided into solid black and white halves. Yet the depth completely disappears.

2.9.2. How It Works
Shadows are identified early in visual processing in order to get a quick first impression of the shape of a scene. We can tell it’s early because the mechanism it uses to resolve light source ambiguities is rather hackish.

Ambiguities occur all the time. For instance, take one of the white-at-top circles from Figure 2-8. Looking at it, you could be seeing one of two shapes depending on whether you imagine the shape was lit from the top or the bottom of the page. If light’s coming from above, you can deduce it’s a bump because it’s black underneath where the shadows are. On the other hand, if the light’s coming from the bottom of the page, only a dent produces the same shading pattern. Bump or dent: two different shapes can make the same shadow pattern lit from opposite angles.

There’s no light source in the diagram, though, and the flat gray background gives no clues as to where the light might be coming from. That white-at-top circle should, by rights, be ambiguous. You should sometimes see a bump and sometimes see a dent.

What’s remarkable is that people see the white-at-top circles as bumps, not dents, despite the two possibilities. Instead of leaving us in a state of confusion, the brain has made a choice: light comes from above.2

Assuming scenes are lit from above makes a lot of sense: if it’s light, it’s usually because the sun is overhead. So why describe this as a hackish mechanism?

Although the light source assumption seems like a good one, it’s actually not very robust. Try looking at Figure 2-8 again. This time, prop the book against a wall and turn your head upside-down. The bumps turn into dents and the dents turn into bumps. Instead of assuming the light comes from high up in the sky, your brain assumes it comes from the top of your visual field.

Rather than spend time figuring out which way up your head is and then deducing where the sun is likely to be, your brain has opted for the “good enough” solution. This solution works most, not all, of the time (not if you’re upside-down), but it also means the light source can be hardcoded into shape perception routines, allowing rapid processing of the scene.

It’s this rapidity that allows the deduction of shape from shadows to occur so early in processing. That’s important for building a three-dimensional mental scene rather than a flat image like a photograph. But the shaded circles have been falsely tagged as three-dimensional, which gives them a compelling sense of depth.

What’s happened to the shaded circles is called “pop-out.” Pop-out means that the circles jump out from the background at youthey’re easier to notice or give attention to than similar flat objects. Kleffner and Ramachandran, in the same paper as before, illustrate this special property by timing how long it takes to spot a single bump-like circle in a whole page of dents. It turns out to not matter how many dents are on the page hiding the bump. Due to pop-out, the bump is immediately seen.

If the page of bumps and one dent is turned on its side, however, spotting the dent takes much longer. Look one more time at Figure 2-8, this time holding the book on its side. The sense of depth is much reduced and, because the light-from-above assumption favors neither type of circle, it’s pretty much random which type appears indented and which appears bent out of the page. In fact, timings show that spotting the one different circle is no longer immediate. It takes longer, the more circles there are on the page.

The speed advantage for pop-out is so significant that some animals change their coloring to avoid popping out in the eyes of their predators. Standing under a bright sun, an antelope would be just like one of the shaded circles with a lit-up back and shadows underneath. But the antelope is dark on top and has a white belly. Called “countershading,” this pattern opposes the shadows and turns the animal an even shade, weakening the pop-out effect and letting it fade into the background.

Taken From : Mind Hacks

Hack 19. Release Eye Fixations for Faster Reactions

It takes longer to shift your attention to a new object if the old object is still there.

Shifting attention often means shifting your eyes. But we’re never fully in control of what our eyes want to look at. If they’re latched on to something, they’re rather stubborn about moving elsewhere. It’s faster for you to look at something new if you don’t have to tear your eyes awayif what you were originally looking at disappears and then there’s a short gap, it’s as if your eyes become unlocked, and your reaction time improves. This is called the gap effect.

2.8.1. In Action
The gap effect can be spotted if you’re asked to stare at some shape on a screen, then switch your gaze to a new shape that will appear somewhere else on the screen. Usually, switching to the new shape takes about a fifth of a second. But if the old shape vanishes shortly before the new shape flashes up, moving your gaze takes less time, about 20% less.

It has to be said: the effecton the order of just hundredths of a secondis tiny in the grand scheme of things. You’re not going to notice it easily around the home. It’s a feature of our low-level cognitive control: voluntarily switching attention takes a little longer under certain circumstances. In other words, voluntary behavior isn’t as voluntary as we’d like to think.

2.8.2. How It Works
We take in the world piecemeal, focusing on a tiny part of it with the high-resolution center of our vision for a fraction of a second, then our eyes move on to focus on another part. Each of these mostly automatic moves is called a saccade [Hack #15] .

We make saccades continuouslyup to about five every secondbut that’s not to say they’re fluid or all the same. While you’re taking in a scene, your eyes are locked in. They’re resistant to moving away, just for a short time. So what happens when another object comes along and you want to move your eyes toward it? You have to overcome that inhibition, and that takes a short amount of time.

Having to overcome resistance to saccades is one way of looking at why focusing on a new shape takes longer if the old one is still there. Another way to look at it is to consider what happens when the old shape disappears. Then we can see that the eyes are automatically released from their fixation, and no longer so resistant to making a saccadewhich is why, when the old shape disappears before the new shape flashes up, it’s faster to gaze-shift. In addition, the disappearing shape acts as a warning signal to the early visual system (”There’s something going on, get ready!”), which serves to speed up the eyes’ subsequent reaction times. It’s a combination of both of these factorsthe warning and the eyes no longer being held back from movingthat results in the speedup.

2.8.3. In Real Life
Just for completeness, it’s worth knowing that the old point of fixation should disappear 200 milliseconds (again, a fifth of a second) before the new object appears, to get maximum speedup. This time is used for the brain to notice the old object has vanished and get the eyes ready to move again. Now, in the real world, objects rarely just vanish like this, but it happens a lot on computer screens. So it’s worth knowing that if you want someone to shift his attention from one item to another, you can make it an easier transition by having the first item disappear shortly before the second appears (actually vanish, not just disappear behind something, because we keep paying attention to objects even when they’re temporarily invisible [Hack #36] ). This will facilitate your user’s disengagement from the original item, which might be a dialog box or some other preparatory display and put her into a state ready for whatever’s going to need her attention next.

2.8.4. See Also
Taylor, T. L., Kingstone, A., & Klein, R. M. (1998). The disappearance of foveal and non-foveal stimuli: Decomposing the gap effect. Canadian Journal of Experimental Psychology, 52(4), 192-199.

Taken From : Mind Hacks

Hack 18. When Time Stands Still

Our sense of time lends a seamless coherence to our conscious experience of the world. We are able to effortlessly distinguish between the past, present, and future. Yet, subtle illusions show that our mental clock can make mistakes.

You only have to enjoy the synchrony achieved by your local orchestra to realize that humans must be remarkably skilled at judging short intervals of time. However, our mental clock does make mistakes. These anomalies tend to occur when the brain is attempting to compensate for gaps or ambiguities in available sensory information.

Such gaps can be caused by self-generated movement. For example, our knowledge about how long an object has been in its current position is compromised by the suppression of visual information [Hack #17] that occurs when we move our eyes toward that objectwe can have no idea what that object was actually doing for the time our eyes were in motion. This uncertainty of position, and the subsequent guess the brain makes, can be felt in action by saccading the eyes toward a moving object.

2.7.1. In Action
Sometimes you’ll glance at a clock and the second hand appears to hang, remaining stationary for longer than it ought to. For what seems like a very long moment, you think the clock may have stopped. Normally you keep looking to check and see that shortly afterward the second hand starts to move again as normalunless, that is, it truly has stopped.

This phenomenon has been dubbed the stopped clock illusion. You can demonstrate it to yourself by getting a silently moving clock and placing it off to one side. It doesn’t need to be an analog clock with a traditional second hand; it can be a digital clock or watch, just so long as it shows seconds. Position the clock so that you aren’t looking at it at first but can bring the second hand or digits into view just by moving your eyes. Now, flick your eyes over to the clock (i.e., make a saccade [Hack #15] ). The movement needs to be as quick as possible, much as might happen if your attention had been grabbed by a sudden sound or thought [Hack #37] ; a slow, deliberate movement won’t cut it. Try it a few times and you should experience the “stopped clock” effect on some attempts at least.

Whether or not this works depends on exactly when your eyes fall on the clock. If your eyes land on the clock just when the second hand is on the cusp of moving (or second digits are about to change), you’re less likely to see the illusion. On the other hand, if your eyes land the instant after the second hand has moved, you’re much more likely to experience the effect.

2.7.2. How It Works
When our gaze falls on an object, it seems our brain makes certain assumptions about how long that object has been where it is. It probably does this to compensate for the suppression of our vision that occurs when we move our eyes [Hack #17] . This suppression means vision can avoid the difficult job of deciphering the inevitable and persistent motion blur that accompanies each of the hundred thousand rapid saccadic eye movements that we make daily. So when our gaze falls on an object, the brain assumes that object has been where it is for at least as long as it took us to lay eyes on it. Our brain antedates the time the object has been where it is. When we glance at stationary objects like a lamp or table, we don’t notice this antedating process. But when we look at a clock’s second hand or digits, knowing as we do that they ought not be in one place for long, this discord triggers the illusion.

This explanation was supported and quantified in an experiment by Keilan Yarrow and colleagues at University College, London and Oxford University.1 They asked people to glance at a number counter. The participants’ eye movements triggered the counter, which then began counting upward from 1 to 4. Each of the numerals 2, 3, and 4 was displayed for 1 second, but the initial numeral 1 was displayed for a range of different intervals, from 400 ms to 1600 ms, starting the moment subjects moved their eyes toward the counter. The participants were asked to state whether the time they saw the numeral 1 was longer or shorter than the time they saw the subsequent numerals. Consistent with the stopped clock illusion, the participants consistently overestimated how long they thought they had seen the number 1. And crucially, the larger the initial eye movement made to the counter, the more participants tended to overestimate the duration for which the initial number 1 was visible. This supports the saccadic suppression hypothesis, because larger saccades are inevitably associated with a longer period of visual suppression. And if it is true that the brain assumes a newly focused-on target has been where it is for at least as long as it took to make the orienting saccade, then it makes sense that longer saccades led to greater overestimation. Moreover, the stopped clock illusion was found to occur only when people made eye movements to the counter, not when the counter jumped into a position before their eyesagain consistent with the saccadic suppression explanation.

You’ll experience an effect similar to the stopped clock illusion when you first pick up a telephone handset and get an intermittent tone (pause, beeeep, pause, beeeep, repeat). You might find that the initial silence appears to hang for longer than it ought to. The phone can appear dead and, consequently, the illusion has been dubbed the dead phone illusion.

The clock explanation, however, cannot account for the dead phone illusion since it doesn’t depend on saccadic eye movement.2 And it can’t account, either, for another recent observation that people tend to overestimate how long they have been holding a newly grasped object,3 which seems like a similar effect: the initial encounter appears to last longer.

One suggestion for the dead phone illusion is that shifting our attention to a new auditory focus creates an increase in arousal, or mental interest. Because previous research has shown that increased arousalwhen we’re stressed, for instancespeeds up our sense of time, this could lead us to overestimate the duration of a newly attended-to sound. Of course, this doesn’t fit with the observation mentioned before, that the stopped clock illusion fails to occur when the clock or counter moves in front of our eyessurely that would lead to increased arousal just as much as glancing at a clock or picking up a telephone.

So, a unifying explanation for “when time stands still” remains elusive. What is clear is that most of the time our brain is extraordinarily successful at providing us with a coherent sense of what happened when.

2.7.3. End Notes
Yarrow, K., Haggard, P., Heal, R., Brown, P., & Rothwell, J. C. (2001). Illusory perceptions of space and time preserve cross-saccadic perceptual continuity. Nature, 414(6861), 302-305.

Hodinott-Hill, I., Thilo, K. V., Cowey, A., & Walsh, V. (2002). Auditory chronostasis: Hanging on the telephone. Current Biology, 12, 1779-1781.

Yarrow, K., & Rothwell, J. C. (2003). Manual chronostasis: Tactile perception precedes physical contact. Current Biology, 12(13), 1134-1139.

Christian Jarrett

Hack 17. Glimpse the Gaps in Your Vision

Our eyes constantly dart around in extremely quick movements called saccades. During each movement, vision cuts out.

Despite the fact that the eye has a blind spot, an uneven distribution of color perception, and can make out maximal detail in only a tiny area at the center of vision, we still manage to see the world as an uninterrupted panorama. The eye jumps about from point to point, snapshotting high-resolution views, and the brain assembles them into a stunningly stable and remarkably detailed picture.

These rapid jumps with the eyes are called saccades, and we make up to five every second. The problem is that while the eyes move in saccade all visual input is blurred. It’s difficult enough for the brain to process stable visual images without having to deal with motion blur from the eye moving too. So, during saccades, it just doesn’t bother. Essentially, while your eyes move, you can’t see.

2.6.1. In Action
Put your face about 6 inches from a mirror and look from eye to eye. You’ll notice that while you’re obviously switching your gaze from eye to eye, you can’t see your own eyes actually movingonly the end result when they come to rest on the new point of focus. Now get someone else to watch you doing so in the mirror. They can clearly see your eyes shifting, while to you it’s quite invisible.

With longer saccades, you can consciously perceive the effect, but only just.

Hold your arms out straight so your two index fingers are at opposite edges of your vision. Flick your eyes between them while keeping your head still. You can just about notice the momentary blackness as all visual input from the eyes is cut off. Saccades of this length take around 200 ms (a fifth of a second), which lies just on the threshold of conscious perception.

What if something happens during a saccade? Well, unless it’s really bright, you’ll simply not see it. That’s what’s so odd about saccades. We’re doing it constantly, but it doesn’t look as if the universe is being blanked out a hundred thousand times a day for around a tenth of a second every time.

Saccadic suppression may even be one of the ways some magic tricks work. We know that sudden movements grab attention [Hack #37] . The magician’s flourish with one hand grabs your attention, and as your eyes are moving, you aren’t able to see what he does with the other hand to pull off the trick.

N.H.

2.6.2. How It Works
Saccadic suppression exists to stop the visual system being confused by blurred images that the eye gets while it is moving rapidly in a saccade. The cutout begins just before the muscles twitch to make the eyes move. Since that’s before any blur would be seen on the retina, we know the mechanism isn’t just blurred images being edited out at processing time. Instead, whatever bit of the brain prepares the eyes to saccade must also be sending a signal that suppresses vision. Where exactly does that signal come from? That’s not certain yet.

One recent experiment proves that suppression definitely occurs before any visual information gets to the cortex. This isn’t the kind of experiment that can be done at home, unfortunately, as it requires transcranial magnetic stimulation (TMS). TMS [Hack #5] essentially lets you turn on, or turn off, parts of the brain that are close enough to the surface to be affected by a magnet. The device uses rapid electromagnetic pulses to affect the cells carrying signals in the brain. Depending on the frequency of the pulses, you can enhance or suppress neuronal activity.

Kai Thilo and a team from Oxford University1 used TMS to give volunteers small illusionary spots, called phosphenes, in their vision.

When phosphenes were made at the retina, by applying TMS to the eye, saccadic suppression worked as normal. During a saccade, the phosphenes disappeared, as would be expected. The phosphenes were being treated like normal images on the retina. But when the spots were induced later in visual processing, at the cortex, saccades didn’t affect them. They appeared regardless of eye movements.

So, suppression acts between the retina and the cortex, stopping visual information before the point where it would start entering conscious experience. Not being able to see during a saccade isn’t the same kind of obstruction as when you don’t see because your attention is elsewhere. That is what happens during change blindness [Hack #40] you don’t notice changes because your attention is engaged by other things, but the changes are still potentially visible.

Instead, saccadic suppression is a more serious limitation. What happens during a saccade makes it nowhere near awareness. It’s not just that you don’t see it, it’s that you can’t.

2.6.3. End Note
Thilo, K. V., Santoro, L., Walsh, V., & Blakemore, C. (2004). The site of saccadic suppression. Nature Neuroscience, 7(1), 13-14.

2.6.4. See Also
Saccadic suppression also lies behind the stopped clock illusion [Hack #18] .

Taken From : Mind Hacks

Hack 16. Map Your Blind Spot

Find out how big your visual blind spot is and how your brain fills the hole so you don’t notice it.

Coating the back of each eye are photoreceptors that catch light and convert it to nerve impulses to send to the brain. This surface, the retina, isn’t evenly spread with receptorsthey’re densest at the center and sparse in peripheral vision [Hack #14] . There’s also a patch that is completely devoid of receptors; light that falls here isn’t converted into nerve signals at all, leaving a blind spot in your field of viewor actually two blind spots, one for each eye.

2.5.1. In Action
First, here’s how to notice your blind spot (later we’ll draw a map to see how big it is). Close your left eye and look straight at the cross in Figure 2-6. Now hold the book flat about 10 inches from your face and slowly move it towards you. At about 6 inches, the black circle on the right of the cross will disappear, and where it was will just appear grey, the same color as the page around it.

Figure 2-6. A typical blind spot pattern

You may need to move the book back and forth a little. Try to notice when the black circle reappears as you increase the distance, then move the book closer again to hide the circle totally. It’s important you keep your right eye fixed on the cross, as the blind spot is at a fixed position from the center of vision and you need to keep it still to find it.

Now that you’ve found your blind spot, use Jeffrey Oristaglio and Paul Grobstein’s Java applet at the web site Serendip (http://serendip.brynmawr.edu/bb/blindspot; Java) to plot its size.

The applet shows a cross and circle, so, as before, close your left eye, fix your gaze on the cross, and move your head so that the circle disappears in your blind spot. Then click the Start button (at the bottom of the applet) and move your cursor around within the blind spot. While it’s in there, you won’t be able to see it, but when you can (only just), click, and a dot will appear. Do this a few times, moving the cursor in different directions starting from the circle each time.

Again, be careful not to move your head, and keep focused on the cross. You’ll end up with a pattern like Figure 2-7. The area inside the ring of dots is your blind spot.

Figure 2-7. Matt’s blind spot mapped

Here’s a fun way of playing with your blind spot. In a room of people, close one eye and focus on your index finger. Pick a victim and adjust where your finger is until your blind spot makes his head disappear and the background takes its place. Not very profitable, but fun, and not as obvious as making as if to crush his head between your thumb and index finger.

T.S.

2.5.2. How It Works
The blind spot for each eye corresponds to a patch on the retina that is empty of photoreceptors. With no photoreceptors, there’s nothing to detect light and turn it into information for use by the visual system, hence the blind spot.

Each receptor cell is connected to the brain via a series of cells that aggregate the signal before reporting it to the brain by an information-carrying fiber called an axon (see [Hack #9]). Bizarrely, the part of the photoreceptor responsible for detecting light is behind the fibers for carrying the information into the brain. That’s rightthe light-sensitive part is on the side furthest from the light. Not only does this seem like bad design, but also it means that there has to be a gap in surface of the retina where the fibers gather together to exit the eyeball and run to the brainand that exit point is the blind spot.

At first sight, there doesn’t appear to be any particular reason for this structure other than accident. It doesn’t have to be this way. If the light-detecting parts of the cells were toward the light, you wouldn’t need a blind spot; the fibers could exit the eye without interrupting a continuous surface of photoreceptors on the retina.

Can we be sure that this is a bug and not a feature? One bit of evidence is that in the octopus eye it was done differently. The eye evolved independently in octopuses, and when it did, the retinal cells have the photoreceptors in front of the nerve fibers, not behind, and hence no blind spot.

Conversely, there are benefits to the arrangement of the human retina: it allows a good blood supply close to the retina to both nourish the photoreceptors and help metabolize debris that accumulates there. Both orientations of the retina have their advantages.

We don’t normally notice these two great big holes in our field of vision. Not only do our eyes move around so that there’s no one bit of visual space we’re ignoring, but the blind spots from the two eyes don’t overlap, so we can use information from one eye to fill in the missing information from the other.

However, even in situations in which the other eye isn’t providing useful information and when your blind spot is staying in the same place, the brain has evolved mechanisms to fill in the hole.1 This filling in is why, in the demostration above, you see a continuous grey background rather than a black hole.

2.5.3. Hacking the Hack
The Cheshire Cat experiment (http://www.exploratorium.edu/snacks/cheshire_cat.html; full instructions) shows a really good interaction of the blind spot, the filling-in mechanisms and our innate disposition to notice movement competing against our innate disposition to pay attention to faces. With a blank wall, a mirror, and a friend, you can use your blind spot to give yourself the illusion that you can slowly erase your friend’s head until just her smile remains.

2.5.4. End Note
“Seeing More Than Your Eye Does” (http://serendip.brynmawr.edu/bb/blindspot1.html) is a fun tour through the capabilities of your blind spot (the link at the bottom of each page’s article will lead you to the next page). It demonstrates how your brain uses colors and patterns in the area surrounding the blind spot to make a good guess of what should be in the blind spot itself and will report that to your conscious mind.

2.5.5. See Also
Ramachandran, V. S. “Blind Spots.” Scientific American, May 1992, 86-91.

Ramachandran, V. S., & Gregory, R. L. (1991). Perceptual filling in of artificially induced scotomas in human vision. Nature, 350, 699-702.

There is an interesting discussion of the blind spot, filling in, and what that implies for the nature of experience in Daniel Dennett’s Consciousness Explained, 344-366. Boston: Little, Brown and Co., 1991.

Taken From : Mind Hacks