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Figure 3-7. AceReader Pro presenting target words
With this particular experiment, nobody experiences the attentional blink every time. For instance, if you're already good at speed-reading or it's easy to guess the sentences in the text document as they come up, it probably won't work. We're using this software to simulate the controlled RSVP experiment, which uses random letters. Doing it this way isn't as reliable.
That said, it worked for me about half the time, and I can only describe the attentional blink itself as a peculiar experience. At about five words a second (300 words a minute), I wasn't overwhelmed by having to read every word and decide whether it was one of my targetsbut I was certainly on the cusp of being overwhelmed. I had to sustain a high level of concentration on the screen.
The first word jumped out at me, as I expected it would. OK, I'd recognized that one; now I could look out for the next. But the next word I remember reading properly was four places after. I'd somehow missed my second target. What had occurred in between was my attentional blink. Thinking back, I could remember the sensation of having seen my second word on the screen, but somehow, although I'd seen it, I hadn't twigged that it was actually my target. My memory was distinctly less visual and sure than for the first word, and all I could really remember, for the duration of the blink, was the feeling of doing two things at once: processing the first target and trying to keep up with the fresh words on-screen. If I hadn't been able to stop and figure out why I hadn't noticed my second target, knowing it had to have flashed up, I would've missed it completely.
How It Works
Clearly the attentional blink does exist. The half-second recovery time after noticing a target has been shown many times in experiments. Like attention in general, however, precisely how it arises in the brain is still subject to research.
One strong theory assumes there's a limited amount of attention to go round, which is rapidly transferred from one letter to the next in the rapid serial visual presentation task. Due to the amount of processing each letter needsto see if it's white or if it's the Xand the speed of change of letters, attention is forced to operate at maximum capacity. When the white letter, the first target, is spotted, additional attentional resources are suddenly needed to lift it to a level of conscious awareness. These extra resources have to come from somewhere, and the process of raising one's awareness takes time; for that period of time, new incoming letters aren't given as much attention as they really need.
That's not to say new letters aren't given any attention at all, and that's where the analogy with eye blinking breaks down. Eye blinks shut off vision almost completely, but attentional blinks just reduce the probability of spotting a target during the blink. The success rate for spotting the second target, the X, dips to its minimum of 50% if the second target occurs a quarter of a second (250 ms) after the first target and then gradually recovers as the half-second plays out.
In this view, it's not so much that the second target doesn't get seen at all, it's that it gets processed but there just isn't enough attentional resource to go around and so it isn't brought up to conscious awareness. Additional, random letters keep coming in and claim the processing resource for themselves, and so you never notice that second target.
Two pieces of evidence back this up. First, the processing demand contributed by the random letters is essential for the attentional blink to show up. If the letters aren't there, or instead something that is easily ignored is used (like blocks of random colors, perhaps), they don't act as a processing drain. The second target is seen as easily as the first target in that case.
Second, although the second target may never reach conscious awareness, it can still influence the subconscious mind. There's an effect called priming, in which seeing a word once will make it, or a related word, easier to notice the second time [Hack #81] . So, for example, in the RSVP task, if shown the word "doctor," the subsequent word is faster and easier to spot if it's the word "doctor" or "nurse."1 It turns out that the second target, even if it isn't consciously noticed, can prime the next item. This means that the items shown during the attentional blink reach the level of processing required for meaning, at least, and aren't just discarded. The limited-resources-for-attention theory appears to be a good one: there's just not enough attention to lift two items to awareness in quick succession.
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Think of the attentional blink next time you're looking along a bookshelf for particular titles or down a list of names for people you know. I've had experiences looking down lists when I miss one of the names I'm after time after time, only to look againslower the second timeand see it was shortly after another name that had jumped out at me each time for some other reason.
End Note
1. An excellent review paper on the subject, especially the priming effect, is: Shapiro, K. L., Arnell, K. M., & Raymond, J. E. (1997). The attentional blink. Trends in Cognitive Science, 1(8), 291-296.
See Also
· Two good introductions to the general topic of attention are: Styles, E. A. (1997). The Psychology of Attention. Hove: U.K.: Psychology Press. And: Pashler, H. (1998). The Psychology of Attention. Cambridge, MA: MIT Press.
We don't memorize every detail of a visual scene. Instead, we use the world as its own best representationcontinually revisiting any bits we want to think about. This saves the brain time and resources, but can make us blind to changes.
Both our vision [Hack 14] and attention [Hack #34] have far coarser resolutions than we'd have thought. What's more, there are gaps in our vision across time [Hack #17] and in space [Hack #16], but our brains compensate for these gaps and knit together a rather seamless impression of the world.
And this gapless impression is utterly convincing. Most of the time we don't even realize that there are holes in the information we're getting. And so we believe we experience more of the world than we actually do. There are two possibilities as to what's going on here. The first is that we build a model inside our heads of the world we can see. You can test to see whether this is the case.
Imagine you are looking at a picture. There's a flicker as the picture disappears and appears again. What's different? If we made and kept a full internal representation of the visual world inside our heads, it would be easy to spot the difference. In theorybefore memory decay set init should be as easy as comparing two pictures (before and after) side by side on a page. But it isn't.
So that puts paid to the first possibility. The other is that you don't build a full internal model of what you're seeing at allyou just think you do. The illusion is maintained by constant sampling as you move your eyes around, a part of what is called active vision [Hack #15] . After all, why bother to store information about the world in your head when the information is freely available right in front of your very eyes?
The proof of the pudding for active vision is testing the consequence that, if true, you should find it very difficult to spot changes between two scenes, even with just a short flicker in between. Since most of the two separated images aren't stored in memory, there's no way to compare them. And, true enough, spotting any difference is very difficultso hard, in fact, that the phenomenon's been labeled change blindness.
3.8.1. In Action
You can try an animated GIF demo, which we made, at http://www.mindhacks.com/book/44/changeblindness.gif, both frames of which are shown in Figure 3-8. Shown side by side, the difference between the two versions of this picture is obvious.
Figure 3-8. The difference is easy to spot when you're allowed to look at both versions of the "same" picture at once1
But if you don't know what you're looking for, it can be impossible to spot. Load the images in the following URLs and have a look. If you're finding the first one hard, have a look at the man's noseyou can be looking right at the change in the image and still not spot it for a frustratingly long time.
· http://nivea.psycho.univ-paris5.fr/ASSChtml/couple.gif (an animated GIF)
· http://www.usd.edu/psyc301/Rensink.htm (a Java applet)
3.8.2. How It Works
You need the momentary blink between the pictures so you are actually forced to compare the two pictures in memory rather than noticing the change as it happens. Interestingly enough, the blink doesn't actually even need to cover the feature that's changing, as another demonstration at http://nivea.psycho.univ-paris5.fr/ASSChtml/dottedline.gif shows. Rather than blanking out the entire image, distracting patterns momentarily appear overlaid on it to divert your attention from the change.
You're just as blind to the altering feature when patterns flash up, even though the picture as a whole remains present the entire time. It's enough that your attention is momentarily distracted from picking up on the change, forcing you to rely on your memory for what the scene looked like half a second agowe're not talking long-term memory here.
3.8.3. In Real Life
This isn't just lab theory. Change blindness can help you pull some great tricks outside of the lab and without the aid of a computer. A classic experiment by Daniel Simons and Daniel Levin2 is a perfect example. One of the pair would stop a passerby to ask for directions. In the midst of the kindly passerby's attempt at giving directions, two men would carry a door between the experimenter and passerby. During this distraction, the experimenter switched places with his colleague, who was a different height and build, sounded different, and was wearing different clothes. Despite these blatant differences, a full half of the people they tried this on didn't notice any difference between the man who started asking for directions and the man who finished listening to them.
3.8.4. End Notes
1. The road markings on the right of the picture change location.
2. Simons, D. J., & Levin, D. T. (1998). Failure to detect changes to people during a real-world interaction. Psychonomic Bulletin and Review, 5, 644-649.
3.8.5. See Also
· Daniel Simons' lab provides a nice collection of movies they've used to demonstrate change blindness (http://viscog.beckman.uiuc.edu/djs_lab/demos.html).
· J. Kevin O'Regan has a great talk entitled "Experience Is Not Something We Feel but Something We Do: a Principled Way of Explaining Sensory Phenomenology, with Change Blindness and Other Empirical Consequences" (http://nivea.psycho.univ-paris5.fr/ASSChtml/ASSC.html).
What you pay attention to determines what you see, so much so that you can miss things that are immensely obvious to otherslike dancing gorillas, for instance.
Attention acts as a kind of filter, directing all resources to certain tasks and away from others. Nowhere is the impact of attention on what you actually see more evident than in the various experiments on inattention blindness.
Inattention blindness comes up when you're focusing as much attention as you can on a particular task and trying really hard to ignore distractions. It's the name given to the phenomenon of not noticing those distractions, however blatant and bizarre they become. In the most famous experiment on this subject, subjects had to watch a video of a crowd playing basketball. Concentrating on a spurious task, a good number of them were completely blind to the gorilla that walked into view halfway through the game.
3.9.1. In Action
You can watch the basketball video used in the gorilla experiment by Daniel Simons and Christopher Chabris.1 Find it from the University of Illinois Visual Cognition Lab's page at http://viscog.beckman.uiuc.edu/media/mindhacks.html.2
OK, because you know what's going to happen, this isn't going to work for you, but here's the procedure anyway. Watch the basketball game, and count the number of passes made by the team in white shirts only. Find a friend and set her on the task.
If you were a subject in this experiment for real, counting those passes, what happens next would be completely unexpected: a woman in a gorilla suit walks through the group playing the game and stands in the middle of the screen before walking off again. About half the observers tested in Simons and Chabris's experiment missed the gorilla.
3.9.2. How It Works
Following the passes in the game and counting only some of them is a difficult task. There are two balls and six players, everyone's moving around, and the balls are often obscured. It's all your brain can do to keep up.
Actually, there's a little too much to keep up with. The bottleneck is in visual short-term memory, where the results of visual processing have to be stashed while the actual analysislooking for passes by players in white shirtshappens.
Visual short-term memory , or VSTM, can hold only a small amount of information. Its capacity is limited to the equivalent of about four objects. Now, there are tricks we can use to temporarily increase the size of short-term memory. Repeating a word over and over can lengthen the time it's remembered, for example. When two researchers at Vanderbilt University, J. Jay Todd and René Marois, performed experiments to measure the size of short-term memory,3 they devised their task in such a way that tricks weren't possible. Not only did subjects taking part have to do the memory experimentlooking at a pattern of colored dots and answering a question on it a second laterthey also had to speak numbers out loud for the duration, preventing the word repeating trick from being used. While the full load of the experiment was on VSTM, Todd and Marois looked at their subjects' brain activity using functional magnetic resonance imaging [Hack #4], a technique that produces images in which busy parts of the brain show up brighter.
What they found was a small area on the back surface, in a region called the posterior parietal cortex where the activity increased as the pattern presented in the experiment became more complex. They could see that, as the pattern contained more colored dots, the brain activity grew proportionatelybut only up to four dots. At that point, not only did activity reach its peak, but performance in the short-term memory task did too. This points to a real capacity limit in VSTM.
The capacity is a major factor in counting passes in the basketball game too. There's simply too much going on. That's where attention comes in. Attention is our everyday word for the mechanisms that prioritize processing of some objects, making sure they get into VSTM, and suppress irrelevant information. In this case, when you're watching the gorilla video you have no choice but to pay attention only to fast-moving people dressed in white and concentrate on the ball and whatever it goes behind. That automatically means you're discarding information about slow-moving objects, especially those colored blacklike the gorilla.
So when the dark gorilla suit slowly walks into the game, not only is your attention elsewhere, but also your visual processing system actively throws away information about it, to ensure the short-term memory doesn't get swamped. You don't even perceive the gorilla, despite the ball going behind it so that you're looking through it at some points.
To add a little proof to the pudding: when Simons and Chabris asked viewers to count the passes of the team dressed in black, they became significantly more likely to notice the gorilla, as this time the observation of it wasn't being activity discarded by the brain.
This example shows in a fun way just how powerfully attention affects our perception. It's also an example of the moment-by-moment way we allocate attention, picking some things to focus on and some to ignore, and how this is determined within an overall scheme of the priorities we've set ourselves. Psychologists call this the attentional set, which is the keyword phrase to use if you'd like to find out more.
3.9.3. End Notes
1. Simons, D. J., & Chabris, C. F. (1999). Gorillas in our midst: Sustained inattentional blindness for dynamic events. Perception, 28, 1059-1074. You can get a copy at http://viscog.beckman.uiuc.edu/reprints/index.php.
2. The Visual Cognition Lab (http://viscog.beckman.uiuc.edu/djs_lab) has additional research and demonstrations on inattentional blindness and on related topics.
3. Todd, J. J., & Marois, R. (2004). Capacity limit of visual short-term memory in human posterior parietal cortex. Nature, 428, 751-754.
The act of focusing on just one object goes hand in hand with actively suppressing everything you have to ignore. This suppression persists across time, in a phenomenon called negative priming.
In the story "The Boy Who Cried Wolf," the young shepherd repeatedly claims a wolf has come to attack his flock. There's no wolf there. The boy just enjoys seeing all the villagers run up the hill, coming to save him and the sheep. The villagers, naturally, get a bit annoyed at getting panicked and trying to scare off the nonexistent wolf, so when they hear the boy cry, "Wolf!" again in the middle of the night, they don't bother getting up. But this time there is a wolf. Oh dear. I could say the boy learns his lesson, but he doesn't: he gets eaten. Morality tale, very sad, etc.
Negative priming is the tiniest psychological root of "The Boy Who Cried Wolf." A stimulus, such as a color, a word, a picture, or a sound acts like the cry of "Wolf!" The brain acts as the villagers did, and it has an inhibition to responding to meaningless cries, and this kicks in after only one cry. But nobody gets eaten.
3.10.1. In Action
Negative priming can be picked up only in experiments with careful timing and many trialsit's a small-scale effect, but it's been demonstrated in many situations.
Look at the flash card in Figure 3-9, and say what the gray picture is as fast as you can. Speak it out loud.
Figure 3-9. An example negative priming flash card
Now look at Figure 3-10, and do the same: name the gray picture, out loud, as quickly as possible.
Figure 3-10. The next flash card in the sequence
You may find the picture in the second flash card slightly harder to make out, although really you need a controlled situation to pick up the reaction time difference. Both cards have a gray drawing to pick out and a black drawing to ignore, and you suppress both the black ink and the black image in order to ignore it. If, as is the case here, the image you have to identify in the second flash card is the same as the one you had to ignore in the first, you'll take a little longer about it. Your brain is acting like the second time the villagers hear the boy shouting "Wolf!"they still get out of bed, but it takes slightly longer to pull their clothes on.
3.10.2. How It Works
Negative priming has been found in situations much wider than when two colored pictures overlap. In that case, it's one of the pictures that has been negatively primed. But if you set up the experiment so one feature is selected at the expense of another, you can get negative priming for color, location, or shape. All it requires is for a feature to have been in your visual field but actively ignored, then subsequently that feature will take slightly longer to attend to.
What's curious is, in the flash cards used earlier, you're concentrating on the ink color (gray or black), thus ignoring the black ink...but the negative priming occurs for the picture itself too. You've not even had to consciously ignore the distracter picture, because you can just look past the black ink, but it gets suppressed anyway.
In a more extreme way, this is what is happening in inattention blindness [Hack #40] . You're concentrating on a certain set of features (white T-shirts, fast-moving), so you implicitly ignore anything that's colored black and is slow-movingand that's why an ape walking across the basketball game gets blanked. You're ignoring the features, not the objects themselves.
Looking at the ape in the basketball game [Hack #41] is a good way to figure out what negative priming is happening for. Attention's resources are scarce, and we simply don't have enough to allow them to be consumed by every event that comes along. We need to be able to avoid being distracted by the ape if we're concentrating on basketball. It's the ability to suppress perceptions that makes actions truly voluntary.1
What's happening is that attention is being allocated to one set of features, then selectively disabled for potential disrupters. This inhibition function is pretty indiscriminate too, so any feature that's being discarded gets added to the ignore pile, whether it's relevant to the task at hand or not. It looks like a piano? Ignore. It's in black ink? Ignore. Contextual information, whether you focus on it or not, is inhibited, and you'll take longer to notice it when you next have to. Features stay on the ignore pile for much longer than they have to. Traces can be found not just seconds, but days and even weeks later.2
(Incidentally, this also provides evidence that weat least initiallyperceive objects as bundles of features that can be separately perceived and inhibited.)
In a sense, negative priming is performing a similar selection function to focusing attention. It narrows down the quantity of perceptions that reach conscious awareness and can be responded to. In everyday life, you can see echoes of information filtering in action: you soon learn what noises foreshadow your car breaking down and which aren't relevant. And then, of course, there's the boy and the villagers. But these are long timescale, large effects. What's surprising is negative priming uses the same strategy, acting very quickly and almost entirely automatically. The narrowing down of information starts here, at the moment-to-moment and completely preconscious level.
3.10.3. End Notes
1. Pipper, S. P., Howard, L. A., & Houghton, G. (1999). Action-based mechanisms of attention. In G. W. Humphreys, J. Duncan, & A. Treisman (eds.), Attention, Space and Action, 232-247. Oxford University Press.
2. Treisman, A. (1999). Feature binding, attention and object perception. In G. W. Humphreys, J. Duncan, & A. Treisman (eds.), Attention, Space and Action, 91-111. Oxford: Oxford University Press.
3.10.4. See Also
· Damian, M. F. (2000). Semantic negative priming in picture categorization and naming. Cognition, 76, B45-B55. This typical, and interesting, negative priming experiment is also available online (http://eis.bris.ac.uk/~psmfed/papers/np.html).
· May, C. P., Kane, M. J., & Hasher, L. (1995). Determinants of negative priming. Psychological Bulletin, 118, 35-54. This paper explores the mechanisms behind negative priming in depth (http://www.psych.utoronto.ca/~hasher/abstracts/may_95.htm).
· Unfortunately, I can't understand Japanese so can't comment on the content of the negative priming intro I found online (http://www.l.u-tokyo.ac.jp/AandC/HLV/DataBase/NP/intro.html), but the sample flash cards overlapping green and red line drawings are perfect examples of how to negatively prime certain objects.
Some of the constraints on how fast we can task-switch or observe simultaneously aren't fixed. They can be trained by playing first-person action video games.
Our visual processing abilities are by no means hardwired and fixed from birth. There are limits, but the brain's nothing if not plastic. With practice, the attentional mechanisms that sort and edit visual information can be improved. One activity that requires you to practice lots of the skills involved in visual attention is playing video games.
So, what effect does playing lots of video games have? Shawn Green and Daphne Bavelier from the University of Rochester, New York, have researched precisely this question; their results were published in the paper "Action Video Game Modifies Visual Attention,"1 available online at http://www.bcs.rochester.edu/people/daphne/visual.html#video.
Two of the effects they looked at we've talked about elsewhere in this book. The attentional blink [Hack #39] is that half-second recovery time required to spot a second target in a rapid-fire sequence. And subitizing is that alternative to counting for very low numbers (4 and below), the almost instantaneous mechanism we have for telling how many items we can see [Hack #35] . Training can both increase the subitization limit and shorten the attentional blink, meaning we're able to simultaneously spot more of what we want to spot, and do it faster too.
3.11.1. Shortening the Attentional Blink
Comparing the attentional blink of people who have played video games for 4 days a week over 6 months against people who have barely played games at all finds that the games players have a shorter attentional blink.
The attentional blink comes about in trying to spot important items in a fast-changing sequence of random items. Essentially, it's a recovery time. Let's pretend there's a video game in which, when someone pops up, you have to figure out whether it's a good guy or a bad guy and respond appropriately. Most of the characters that pop up are good guys, it's happening as fast as you can manage, and you're responding almost automaticallythen suddenly a bad one comes up. From working automatically, suddenly the bad guy has to be lifted to conscious awareness so you can dispatch him. What the attentional blink says is that the action of raising to awareness creates a half-second gap during which you're less likely to notice another bad guy coming along.
Now obviously the attentional blinkthis recovery timeis going to have an impact on your score if the second of two bad guys in quick succession is able to slip through your defenses and get a shot in. That's a great incentive to somehow shorten your recovery time and return from "shoot bad guy" mode to "monitor for bad guys" mode as soon as possible.
3.11.2. Raising the Cap on Subitizing
Subitizingthe measure of how many objects you can quantify without having to count themis a good way of gauging the capacity of visual attention. Whereas counting requires looking at each item individually and checking it off, subitizing takes in all items simultaneously. It requires being able to give a number of objects attention at the same time, and it's not easy; that's why the maximum is usually about four, although the exact cap measured in any particular experiment varies slightly depending on the setup and experimenter.
Green and Bavelier found the average maximum number of items their non-game-playing subjects could subitize before they had to start counting was 3.3. The number was significantly higher for games players: an average of 4.9nearly 50% more.
Again, you can see the benefits of having a greater capacity for visual attention if you're playing fast-moving video games. You need to be able to keep on top of whatever's happening on the screen, even when (especially when) it's getting stretching.
3.11.3. How It Works
Given these differences in certain mental abilities between gamers and nongamers, we might suspect the involvement of other factors. Perhaps gamers are just people who have naturally higher attention capacities (not attention as in concentration, remember, but the ability to keep track of a larger number of objects on the screen) and have gravitated toward video games.
No, this isn't the case. Green and Bavelier's final experiment was to take two groups of people and have them play video games for an hour each day for 10 days.
The group that played the classic puzzle game Tetris had no improvement on subitizing and no shortened attentional blink. Despite the rapid motor control required and the spatial awareness implicit in Tetris, playing the game didn't result in any improvement.
On the other hand, the group that played Medal of Honor: Allied Assault (Electronic Arts, 2002), an intense first-person shooter, could subitize to a higher number and recovered from the attentional blink faster. They had trained and improved both their visual attention capacity and processing time in only 10 days.
3.11.4. In Real Life
Green and Bavelier's results are significant because processes like subitizing [Hack #35] are used continuously in the way we perceive the world. Even before perception reaches conscious attention, our attention is flickering about the world around us, assimilating information. It's mundane, but when you look to see how many potatoes are in the cupboard, you'll "just know" if the quantity fits under your subitization limit and have to count themusing conscious awarenessif not.
Consider the attentional blink, which is usually half a second (for the elderly, this can double). A lot can happen in that time, especially in this information-dense world: are we missing a friend walking by on the street or cars on the road? These are the continuous perceptions we have of the world, perceptions that guide our actions. And the limits on these widely used abilities aren't locked but are trainable by doing tasks that stretch those abilities: fast-paced computer games.
I'm reminded of Douglas Engelbart's classic paper "Augmenting Human Intellect"2 on his belief in the power of computers. He wrote this in 1962, way before the PC, and argued that it's better to improve and facilitate the tiny things we do every day rather than attempt to replace entire human jobs with monolithic machines. A novel-writing machine, if one were invented, just automates the process of writing novels, and it's limited to novels. But making a small improvement to a pencil, for example, has a broad impact: any task that involves pencils is improved, whether it's writing novels, newspapers, or sticky notes. The broad improvement brought about by this hypothetical better pencil is in our basic capabilities, not just in writing novels. Engelbart's efforts were true to this: the computer mouse (his invention) heightened our capability to work with computers in a small, but pervasive, fashion.
Subitizing is a like a pencil of conscious experience. Subitizing isn't just responsible for our ability at a single task (like novel writing), it's involved in our capabilities across the board, whenever we have to apply visual attention to more than a single item simultaneously. That we can improve such a fundamental capability, even just a little, is significant, especially since the way we make that improvement is by playing first-person shooter video games. Building a better pencil is a big deal.
3.11.5. End Notes
1. Green, C. S., & Bavelier, D. (2003). Action video game modifies visual attention. Nature, 423, 534-537.
2. Engelbart, D. (1962). Augmenting human intellect: a conceptual framework located at http://www.bootstrap.org/augdocs/friedewald030402/augmentinghumanintellect/ahi62index.html.
Audition is a specialized sense for gathering information from the fourth dimension.
If vision lets you see where something is, hearing tells you when it is. The time resolution of audition is way above that of vision. A cinema screen of 24 images a second looks like a constant display, rather than 24 brief images. A selection of 24 clicks a second sounds like a bunch of clicksthey don't blur into a constant tone.
4.2.1. In Action
Listen to these three sound files:
· 24 clicks per second, for 3 seconds (http://www.mindhacks.com/book/44/24Hz.mp3; MP3)
· 48 clicks per second, for 3 seconds (http://www.mindhacks.com/book/44/48Hz.mp3; MP3)
· 96 clicks per second, for 3 seconds (http://www.mindhacks.com//book/44/96Hz.mp3; MP3)
At a frequency of 24 frames per second, film blurs into a continuous image. At 24 clicks per second, you perceive the sound as separate clicks. At four times that rate, you still hear the sound as discontinuous. You may not be able to count the clicks, but you know that the sound is made up of lots of little clicks, not one continuous hum. Auditory "flicker" persists up to higher frequencies than visual flicker before it is integrated to a continuous percept.
4.2.2. How It Works
Specialization for timing is evident in many parts of the auditory system. However, it is the design of the sound receptor device (the ears) that is most crucial. In the eye, light is converted to neural impulses by a slow chemical process in the receptor cells. However, in the ear, sound is converted to neural impulses by a fast mechanical system.
Sound vibrations travel down the ear canal and are transmitted by the tiny ear bones (ossicles) to the snail-shaped cochlea, a piece of precision engineering in the inner ear. The cochlea performs a frequency analysis of incoming sound, not with neural circuitry, but mechanically. It contains a curled wedge, called the basilar membrane, which, due to its tapering thickness, vibrates to different frequencies at different points along its length. It is here, at the basilar membrane, that sound information is converted into neural signals, and even that is done mechanistically rather than chemically. Along the basilar membrane are receptors, called hair cells. These are covered in tiny hairs, which are in turn linked by tiny filaments. When the hairs are pushed by a motion of the basilar membrane, the tiny filaments are stretched, and like ropes pulling open doors, the filaments open many minute channels on the hairs. Charged atoms in the surrounding fluid rush into the hair cells, and thus sound becomes electricity, the native language of the brain. Even movements as small as those on the atomic scale are enough to trigger a response. And for low frequency sounds (up to 1500 cycles per second), each cycle of the sound can trigger a separate group of electrical pulses. For higher frequencies, individual cycles are not coded, just the average intensity of the cycles. The cells that receive auditory timing input in the brain can fire at a faster rate than any other neurons, up to 500 times a second.
This arrangement means that the auditory system is finely attuned to frequency and timing information in sound waves. Sounds as low as 20 Hz (1 Hz is one beat per second), and as high as 20,000 Hz can be represented. The timing sensitivity is exquisite; we can detect periods of silence in sounds of as little as 1 millisecond (thousandths of a second). Compare this with your visual system, which requires exposure to an image for around 30 milliseconds to report an input to consciousness. Furthermore, thanks to the specialized systems in the ear and in the brain, timing between the ears is even more exquisite. If sound arrives at one ear as little as 20 microseconds (millionths of a second) before arriving at the other, this tiny difference can be detected [Hack #45] . For perspective, an eye blink is in the order of 100,000 microseconds, 5000 times slower.
Although vision dominates many other senses in situations of conflicting information [Hack #53], given the sensitivity of our ears, it is not surprising that audition dominates over vision for determing the timing of events
We use this sensitivity to timing in many waysnotably in enjoying music and using the onset of new sounds to warn us that something has changed somewhere.