A visual depiction of vision

Filed here, so I can use it next time I teach intro psychology:

What did we do before XKCD?

Color illusion -- too cool to believe

By far the most striking visual illusion I've ever seen. A little bit of color after-effect turns a black-and-white photograph into a vivid color photograph. You may have to do it a few times to convince yourself it is real.

Help Games with Words get a job!

As job application season comes around, I'm trying to move some work over from the "in prep" and "under revision" columns to the "submitted" column (which is why I'm working on a Sunday). There is one old project that's just waiting for more data before resubmission. I've already put up calls here for readers to participate, so you've probably participated. But if anyone is willing to pass on this call for participation to their friends, it would be much appreciated. I personally think this is the most entertaining study I've run online, but for whatever reason it's never attracted the same amount of traffic as the others, so progress has been slow.

You can find the experiment (The Video Test) here.

How good is your memory?

The average 20-29 year old scores a 2.5 on my Memory Test. How well can you do?

There are, of course, different types of memory. Most people think of 'memory' as an ability to recall facts and events from days or even years ago. This is what was destroyed in the famous amnesic H. M. However, H. M. was still able to remember new information for at least a few seconds; that is, his short-term ("working") memory was spared. There are also other types of memory, such as iconic memory, also knows as "sensory" memory. Moreover, memory for facts seems to dissociate from memory for skills ("know-how").

The Memory Test tests visual working memory.

Before you take the test, please do me one favor. If you want to test yourself multiple times, feel free to do so. But please check off the "have you done this experiment before" box. Failing to do this can screw up the data, so it's important.

What Does the Test Involve?

You try to remember four simple shapes for one second. Afterwards, you are shown a single shape. You have to decide if it is one of the four you were to remember. There are 40 trials, plus some practice trials.

A note about the practice: The practice trials are really, really hard. That is to get you warmed up, just like a runner tying weights to her ankles during her warm-up. The actual test is easier.

How is the Score Calculated?

On any given trial, you get the answer either right or wrong. We could just calculate what percentage you get right, but that would mean getting a score like "80%," which isn't very satisfying. 80% of what?

A formula developed by Nelson Cowan can be used to estimate how many of the shapes, on average, actually make it into your short-term memory store. The formula is this:

(% hits + % correct rejections - 1) / (Total number of objects)

A 'hit' means answering 'yes, this is one of the four objects,' when in fact that is the correct answer. A 'correct rejection' is saying 'no, this is not one of the four object,' when in fact it is not.

From the math, the score can run from -1/4, if you get every question wrong, to 4, if you get every question right (which has happened, but rarely). If you guessed at random, you should get half the questions right, in which case your score should be 0.

Keep in mind that this depends completely on the shapes. If the shapes are really hard to remember (as the practice shapes are), scores will be lower. If they are very easy, scores will be higher. What makes a shape easy is not just how complex it is, but how similar it is to the other shapes (how easy the shapes are to confuse with one another).

What Does the Score Mean?

You could have a higher or lower score for a number of reasons. For one thing, you might have guessed abnormally well or abnormally poorly. All tests are subject to a guessing effect. On average, guessing cancels itself out, but if the test is short enough and enough people take is, somebody is likely to get everything right (or wrong) just by chance.

Luck aside, a good score could mean that you have more "room" in your short-term memory. It might also mean you are better at avoiding interference. There are several types of interference in memory, and so you could be better at avoiding any one of them. You might also be better at paying attention, or you might have developed a useful strategy for success on this task. (That said, visual short-term memory does appear to be anywhere near as susceptible to strategies as verbal short-term memory.)

Remember one thing. This is not a clinical test. Though clinical tests for verbal short-term memory exist, I'm not sure there even are clinical tests for visual short-term memory. This is just for fun. Enjoy it.

Wait. How Do you Know What the Average Score Is?

The Memory Test is nearly identical to an experiment I ran previously. I used the data from that version to estimate what the scores will be on this version.

(Photo served from the National Geographic website)

Galileo -- Smarter than you thought

It is often said of cognitive scientists that we have, as a group, a memory that only stretches back about 10 years. This is for good reasons and bad. Methods change and improve constantly, constantly making much of the literature irrelevant. Then there is the fact that there is so much new work, it's hard to find time to read the old.

This is a shame, because some of the really old work is impressive for its prescience. A recent issue of Trends in Neurosciences carried an article on Galileo's work on perception. Most people then -- and probably most people now -- conceived of the senses as passing along an accurate representation of the world to your brain. We now know the senses are plagued by illusions (many of them actually adaptive).

Galileo was on to this fact. His study of the moon proved that perceptions of brightness are constantly subject to illusion. More generally, he noted -- contrary to the popular view -- that much of what we sense about the world is in a real sense an illusion. Objects exist, but colors and tastes in an important sense do not. It's worth presenting a few of the quotes from the article:

I say that, as soon as I conceive of a piece of matter, or a corporeal substance,...I do not feel my mind forced to conceive it as necessarily accompanied by such states as being white or red, bitter or sweet, noisy or quiet, or having a nice or nasty smell. On the contrary, if we were not guided by our senses, thinking or imagining would probably never arrive at them by themselves. This is why I think that, as far as concerns the object in which these tastes, smells, colours, etc., appear to reside, they are nothing other than mere names, and they have their location only in the sentient body. Consequently, if the living being were removed, all these qualities would disappear and be annihilated.

see also:

A wine's good taste does not belong to the objective determinations of the wine and hence of an object, even of an object considered as appearance, but belongs to the special character of the sense in the subject who is enjoying this taste. Colours are not properties of bodies to the inuition of which they attach, but are also only modifications of the sense of sight, which is affected in a certain manner by light.

Marco Piccolino, Nicholas J. Wade (2008). Galileo Galilei's vision of the senses Trends in Neurosciences, 31 (11)

The verbal memory hegemony

One fact about the world is that the most famous memory researchers did most of their work on verbal memory. Alan Baddeley and George Miller both come to mind -- and I doubt anybody can think of more famous memory researchers in the last 50 years.

Another fact about the world is that many researchers -- not necessarily Baddeley or Miller -- have assumed that anything discovered using memory tests involving words should apply to other forms of memory as well. To pick unfairly on one person, Cowan notes in his masterful paper "The magical number 4 in short-term memory" that out of several related experiments, one has results that diverge from the others. Cowan attempts an explanation but basically throws up his hands. He doesn't notice that of all the experiments discussed in that section, the divergent one was the only one to use visual rather than verbal stimuli.

Similarly, a reviewer of my paper which just came out complained that the results reported in the paper only "told us things we already knew." As evidence, the reviewer cited a number of other papers, all of which had investigated verbal rather than visual short-term memory.

As it happens, the results in this case were very similar to what had been reported previously for verbal memory. But it could have come out differently. That was the point of doing the experiment.

Partly because of this bias in favor of verbal materials, not enough is known about visual memory, though this has been changing in recent years, thanks in part to folks like Steve Luck, George Alvarez, Yuhong Jiang, Edward Vogel and several others.

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Cowan, N. (2001). The magical number 4 in short-term memory: a reconsideration of mental storage capacity. Behavioral and brain sciences, 24, 87-185.

Hartshorne, J.K. (2008). Visual working memory capacity and proactive interference. Public Library of Science One

Miller, G.A. (1956). The magical number seven, plus or minus two: some limits on our capacity for processing information. Psychological Review, 63, 81-97.

Results from an Experiment: The Time Course of Visual Short-Term Memory

The first experiment I ran on the Web has finally made it into print. Rather fittingly, it has been published in a Web-based journal: The Public Library of Science One.

Visual Memory is a Scrawny Creature

That experiment, The Time Course of Visual Short-Term Memory, was part of a larger study probing a fundamental question about memory: why is visual working (short-term) memory so lousy? In recent years, visual memory folk like Edward Vogel and George Alvarez have debated whether we can store as many as four items in visual memory, while on the other hand researchers looking more at verbal memory, such as Nelson Cowan, have been arguing over whether verbal memory can store only four items. There are memory tricks that can allow you to keep a hundred words in short-term memory; nobody has reported any similar tricks for visual memory.

There are many other ways in which visual memory is piddly compared to verbal memory, and I go into them in depth in the paper. Interestingly, previous researchers have not made much out of this difference, possibly because people seem to work on either visual memory or verbal memory, but not both.

Does Verbal Memory Explain the Differences between Humans and Apes?

One possibility that occurred to me is that if verbal memory in fact is considerably more robust and more useful than visual memory, that would endow verbal animals (i.e., adult humans) with significant advantages over non-verbal animals (e.g., human infants and all other animals). Just as writing has allowed some human cultures to supplement our limited memory capacity -- try doing a complicated math problem in your head; the real limitation is memory -- language could allow us to supplement limited non-verbal memory systems.

In fact, I found that many of the differences between adult humans on the one side and young children and apes on the other are found in tasks with large working memory demands. More examples are given in the paper, but this includes theory of mind tasks.

Is Verbal Memory Really Better?

Of course, this is fruitless speculation unless visual working memory is really inferior. The problem is that visual and verbal memory capacity is tested in somewhat different ways. The easiest way to test verbal memory capacity is to give people a list of words to remember and then ask them to repeat that list back (this forms an important part of many IQ tests).

This is obviously impossible with visual memory tests.

In a visual memory test, the participant is usually shown several images to remember. Then, after a delay, they are shown another image and asked if that is the same as one of the original images. Notice that you can be right 50% of the time just by guessing. Thus, to get a good measure, you need to do this many times.

Proactive Interference

This brings up the specter of proactive interference. I have written about proactive interference recently and won't belabor it here. The basic intuition is that if you do many trials of a memory test, it becomes hard to remember which stimuli were on which trial. So if you have been asked to remember circles of different colors, and then you are asked if the last trial contained a blue circle, you might remember that you have seen a blue circle recently but not remember if it was on the last trial or not.

So if visual working memory capacity tasks require many trials and verbal working memory tasks do not, one possible reason for the poor performance observed for visual working memory might be greater proactive interference.

Nope -- not proactive interference

The short version of the results of the published paper is that proactive interference does decrease measured capacity for visual working memory, but not by very much (about 15%). So it cannot account for the differences between visual and verbal working memory. The search must go on.

I hope to describe how the Web-based experiment contributed to this result in a future post. But interested readers can also read the paper itself. It is fairly short and reasonably non-technical.

Participate in current Web-based experiments at my lab by clicking here.

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Hartshorne, J.K. (2008). Visual working memory capacity and proactive interference. Public Library of Science One

How blind children learn the verb "see"

See is one of the most common words in English. For instance, while time, the most common English noun, gets 3,550,000,000 Google hits, see gets a very respectable 2,980,000,000. This compares well with talk (711,000,000) and eat (253,000,000). This means that blind children can't really avoid the verb altogether. In fact, look and see are among the very first verbs that blind children learn, just like sighted children.

So what do they think it means?

I probably can't answer the question completely, but here are some relevant research results:

When a sighted 3-year-old is asked to "look up," he will tilt their heads upwards, even if they are blindfolded. A blind 3-year-old raises her hands instead.

If told "You can touch that table, but don't look at it," the blind 3-year-old will lightly touch the table. If you later tell her she cal look at the table, she may explore all the surfaces of the table with her hands.

It's not likely that blind children are explicitly taught these meanings for these words, so they probably created what are very reasonable meanings for them.

(This research is summarized in Language and experience: Evidence from the blind child by Barbara Landau and Lila Gleitman.)

The DaVinci stereogram

A few posts ago, I described how to make stereograms. At the end of the post, I showed a second type of stereogram, in which an illusionary white box appears to float in front of a background of Xs, and I promised to explain how that one was done.

This type of stereogram, discovered by Nakayama and colleagues, is called a "DaVinci stereogram" in honor of the famous artist/engineer who worked out the logic centuries ago (though he didn't, to my knowledge, consider building any stereograms).

The idea works like this: Look at an object (such as your computer monitor). Your left eye can
"see around" the left side of the object a bit more than can your right eye, while your right eye can see more of what is behind the object than can your left eye. It turns out that this information alone is sufficient to induce a perception of depth.









Consider that final stereogram (reproduced here). In both images, there is a white box in the center. However, the left image (the one presented to the right eye, if you use the divergence method) has four extra Xs on the right side of the box, while the right image (the one presented to the left eye) has two extra Xs on the left side of the box. This results in the perception of a white box floating above the background.

Try this at home: Make your own stereogram

Have you ever wanted to make your own 3D movie? Your own Magic Eye Stereogram? This post will teach you to create (and see) your own 3D images.

Magic Eye Stereograms are a relatively new technology, but they grew out of the classic stereograms created in 1838 by Charles Wheatstone. For those of you who don't know what a stereogram is, the word broadly refers to a 3D-like image produced by presenting different images to each eye.

The theory is pretty straight-forward. Focus on some object in your room (such as your computer). Now close one eye, then the other. The objects in your field of vision should shift relative to one another. The closer or father from you they are (relative to the object you are focusing on), the more they should shift.

When you look at a normal photograph (or the text on this screen), this difference is largely lost. The objects in the picture are in the same position relative to one another regardless of which eye you are looking through. However, if a clever engineer rigs up a device so as to show different images to each eye in a way that mimics what happens when you look at natural scenes, you will see the illusion of depth.

For instance, she might present the drawing below on the left to your right eye, and the drawing on the right to your left eye:










If the device is set up so that each picture is lined up perfectly with the other (for instance, if each is in the center of the field of vision of the appropriate eye), you would see the colored Xs in the center at different depths relative to one another. Why? The green X shifts the most between the two images, so you know it is either the closest or the farthest away. Importantly, because it's farther to the left in the image shown to the right eye, it must be closer than the blue or red Xs.

You can demonstrate this to yourself using a pencil. Hold a pencil perfectly vertical a foot or two in front of your face. It should still look vertical even if you look with only one eye. Now, tilt the pencil so that the bottom part points towards your chest (at about a 45 degree angle from the floor). Close your right eye and move the pencil to the right or the left until the pencil appears to be perfectly vertical. Now look at the pencil with your right eye instead. It should appear to slope down diagonally to the left. That is exactly what is happening in the pictures above.

A device that would fuse these two images for you isn't hard to make, but it's even easier to learn how to fuse them simply by crossing your eyes. There are two ways of crossing your eyes -- making them point inwards towards your nose, and making them point outwards. One way will make the green X closer; one will make it farther away. I'll describe how to use the first method, because it's the own I typically use.

Look at the two images and cross your eyes towards your nose. This should cause each of the images to double. What you want to do is turn those four images into three by causing the middle two to overlap. This takes some practice. Try focusing on the Xs that form the rectangular frames of the images. Make each of those Xs line up exactly with the corresponding X from the frame of the other image. If you do this, eventually the two images should fuse into a single image, and you will see the colored Xs in depth. One tip: I find this harder to do on a computer screen than in print, so you might try printing this out.

That is the basic technique. You should be able to make your own and play around with it to see what you can do. For instance, this example has a bar pointing up out of the page, but you can also make a bar point into the page. You also might try creating more complicated objects. If you want, you can send me any images you make (coglanglab_AT_gmail_DOT_com), and I will post them (you can try including them as comments, but that is tricky).

One final tip -- you'll need to use a font that has uniform spacing. Courier will work. Times will not.

Finally, here's another stereogram that uses a completely different principle. If you can fuse these images, you should see an illusory white box floating in front of a background of Xs. In a future post, I'll explain how to make these.

Sure faces are special, but why?

Faces are special. There appears to be a dedicated area of the brain for processing faces. Neonates just a day or two old prefer looking at pictures of faces to looking at non-faces.

This has led many researchers to claim humans are born with innate knowledge about faces. Others, however, have claimed that these data are not the result of nature so much as nurture. Pawan Sinha at MIT attached a video camera to his infant child and let the tape roll for a few hours. He found that faces were frequently the most salient objects in the baby's visual field, and (I'm working from memory of a talk here) also found that a computational algorithm could fairly easily learn to recognize faces. Similarly, a number of researchers have claimed that the brain area thought to be specialized for face detection is in fact simply involved in detecting any object for which one has expertise, and all humans are simply face experts.

Things have seemed to be at an impass, but today, Yoichi Sugita from AIST spoke at both Harvard and MIT. The abstract itself was enough to catch everybody's attention:

Infant monkeys were reared with no exposure to any faces for 12 months. Before being allowed to see a face, the monkeys showed preference for human- and monkey faces in photographs. They still preferred faces even when presented in reversed contrast. But, they did not show preference for faces presented in upside-down. After the deprivation period, the monkeys were exposed first to human faces for a week. Soon after, their preference changed drastically. They preferred upright human faces but lost preference for monkey faces. Furthermore, they lost preference for human faces presented in reversed contrast. These results indicate that the interrelated features of the face can be detected without experience, and that a face prototype develops abruptly when flesh faces are shown.

Just to parse this: the monkeys were raised individually without contact with other monkeys. They did have contact with a human caregiver who wore a mask that obstructed view of the face. The point about not preferring upside down faces is important, as this is a basic feature of face processing.

This seems pretty decisive evidence for an innate face module in the brain, though one that requires some tuning (the monkeys' face preferences evolved with experience). However, Sugita apparently noted during the talk -- I heard this second-hand -- that perhaps the monkeys in question did in fact have some experience with faces prior to the face preference test; they could have learned by touching their own faces. This strikes me as a stretch, since that doesn't explain why they would become face experts.

Can you see this illusion?

Yesterday, Cognitive Daily posted a fairly compelling visual illusion. There is a while disk and a black disk. In the middle of each disk is a circle. The two circles go from black to white back to black in sequence.

Normally, the rules of perceptual grouping would cause you to see the two smaller disks blinking in unison as being related. However, in this case, due to the smaller disks being inside larger disks, most people see the disks blinking out of sequence. That is, you interpret the scene as a hole appearing in the left disk, then in the right disk, then in the left disk.

Both interpretations are correct. It's a matter of what your visual system focuses on. What interests me is, looking at the comments on this post, is that while the vast majority see the alternating blinking, some people only see the disks blinking in unison. One possibility is that they are misunderstanding what they were supposed to see. If there is some small percentage of people whose visual systems focus on different grouping principles, that could be very interesting and be useful in understanding perceptual grouping in the visual system.

So, if you only see the inner disks blinking in unison and don't get the alternation illusion, comment here or send an email to coglanglab_at_gmail.com.

Try out the illusion here.

Quantum Vision

Can quantum physics explain consciousness? The fact that the mind is instantiated in the physical brain has made it difficult for people to imagine how a physical object like the brain leads to conscious experience in similar ways that it becomes difficult to believe in free will. A number of people have hoped to find the solution in the indeterminacy of quantum physics.

There is a new hypothesis out from Efstratios Manousakis of Florida State University. The phenomenon that he is interested in understanding is binocular rivalry. In binocular rivalry, a different image is displayed to each of your eyes. Instead of seeing a mishmash of the two images, you tend to see one, then the other, then the first one again, ad infinitum. It's not possible to do a demonstration over the internet, but the experience is similar to looking at a Necker Cube, where you first see it popping out of the page, then receding from the page, then popping out, and so on. Notice that what your "eye" sees doesn't change. But your conscious experience does.

Manousakis has found that quantum waveform formulas describe this reasonably well. The question is whether they describe it well because the phenomenon is a quantum phenomenon or because there are two different phenomena for which the same formulas work. Keep in mind that binocular rivalry is something that can actually be seen with neuroimaging. That is, you can see the patterns in the brain change as the person first sees one image, then the other, etc. So if this is really a quantum effect, it is operating at a macro scale. New Scientist has an interesting article on this story this last week. It's not clear from the article if this is a problem Manousakis has thought about or not, and unfortunately his actual journal article isn't available on his website.

Visual illusions -- a compendium

Academic conferences involve a lot of long speeches. To liven things up, the Vision Sciences Society always has a visual illusions night. If you can't make it to their conference, I want to direct you to an incredible Web compendium of illusions.

Many of the illusions illustrate the point that I've made before, which is that what you see is usually, but not always, a reasonably accurate depiction of reality. One such is an illustration of motion-induced blindness. As you stare at an image with both stationary and moving parts, the stationary parts seem to flicker in and out of existence.

Another is the "stepping feet" illusion, which causes you to perceive time and space out of whack. Two small rectangles move across the screen. Sometimes, they seem to be going the same speed. Other times, they seem out of step with each other. In fact, they are always parallel; the "stepping" is your mind playing tricks with you.

One of my favorites is the "Rotating Snake" illusion. The image appears to be in constant motion, but in fact it is completely stationary.

If you want to be enlightened as well as stunned and amazed, the website provides detailed explanations of each illusion, as well as references to scientific papers investigating these phenomena. The main page of the website is here. One thing it does not explain, however, is why, although vision scientists spend their lives studying the visual world, are their websites always so ugly.

Visual memory -- does it even exist?

Researchers at Rochester recently reported that short-term memory for sign language words is more limited than for spoken words. In some sense, this is surprising. We've known for a long time now that sign languages recruit the same brain areas as spoken languages, so it stands to reason that many of the properties of sign languages would be similar to those of spoken languages, despite the obvious differences.

On the other hand, short-term visual memory is severely limited. If you give somebody a list of 7 spoken words, they can typically remember all of them. If you show somebody 7 objects, they cannot remember them. The textbooks say that you can only remember about 4 visual objects, but that turns out only to be true for very simple objects. In a series of experiments I ran (some of them online), the average person could remember only about 2 objects.

Even more striking is that visual short-term memory cannot be trained like verbal memory can be. A few people have learned to extend their verbal memory so that they could remember dozens of words at a time. However, nobody has been able to significantly improve their visual short-term memory (see a research report here).

Visual short-term memory is so incredibly limited that some vision scientists have wondered if it, in some sense, really exists. That is, they think that it may just be a biproduct of some other system (like our ability to imagine visual scenes), rather than a memory system in its own right. There is some sense to this. After all, what do you need short-term visual memory for? With verbal memory, it's obvious. You need to be able to remember the first part of a sentence while reading/hearing the rest of it. But why would you need to remember what you see over very short intervals?

Those who do not want to read a plug for my ongoing research should stop reading here.

I've been really fascinated by the limitations of short-term visual memory. I have run several experiments, one of which is still continuing. You can participate in it here.

What is the relationship between short-term and long-term memory?

In a textbook, you may see a description of memory in terms of stages. The first stage is iconic memory, which lasts just a few seconds, during which you can to some degree revive the perceptual experience you are trying to remember. Think of this almost like the afterglow of a bright flash of light.

Then comes short-term memory, which may or may not be also described as working memory (they aren't necessarily the same thing), which allows you to remember something for a short period of time by actively maintaining it. Anteriograde amnesics (like the guy in Memento) have intact short-term memory. What they don't have is long-term memory, which is basically recalling to mind something you haven't thought about in a while.

There are many aspects of the relationship between short-term memory and long-term memory that are still not clear. Over the last several months, Tal Makovski and I have been running a study trying to clarify part of this relationship.

We thought we had concluded it last week. I took the experiment offline, analyzed the results, wrote up a report and sent it to Tal. He wrote back with conclusions based on the data completely different from those that I had. Bascially, the results of two conditions are numerically different, but statistically there is no difference. He believes that if we had more participants, the difference would become statistically significant. I don't.

It's up to you, dear readers, to prove one of us wrong. The experiment is back online (click here for info; here to go straight to the test). It involves a quick visual short-term memory test, then you watch a video, after which you'll be quizzed on your memory for the video. It's by far the most entertaining of the experiments I've put online, mainly because the video is fantastic. It is Bill et John: Episode II, which was profiled in Slate. I've watched it easily a hundred times in the course of designing this study, and it's still fall-out-of-your-chair funny. Which is good, because it's nearly 10 minutes long, making the entire experiment take about 15 minutes.

Once again, you can find the experiment here. Once the results are available, I will post them on this blog and on the website.

One of my favorite illusions

Even if you've seen this beore, it's worth seeing again. The following link is to a video of two teams (white and black) playing a ball game. Your task is to watch the white team and carefully count how many times they pass the ball (concentration is important):

http://viscog.beckman.uiuc.edu/grafs/demos/15.html

Now, watch it again, paying attention to everything on the screen. Most people will notice something they did not see the first time when they were just watching the white team. (This is hard to do in a blog, without spoilers!)

This gets back to my previous posts about how attention affects how you perceive what you see. The professor that designed this, by the way, is Daniel Simons of the University of Illinois. He's produced a great deal of fantastic research; this is just one example. In a random small-world coincidence, he was the previous tenent of the office suite I worked in last year at Harvard.

Results from an experiment!!

In this post, I am very pleased to release the results of one of my very first Web-based experiments. This was the experiment on visual attention alluded to a few days ago. Web-based experimenting -- that is, what we do at the Cognition and Language Lab -- has a number of advantages over traditional lab-based experiments. The primary one, as far as I'm concerned, is the ability to easily test large numbers of participants. Typical cognition experiments require 8-20 participants. Testing some hypotheses, however, may require hundreds or even thousands of participants.

Recruiting 1000+ participants, explaining the experiment to them and recording their data is time-consuming and thus expensive. Recruiting a thousand participants on the Internet is not exactly easy, but it is easier than the alternatives. Testing a thousand participants through the Internet is a snap. Once the experiment is loaded on the Web page, there's nothing more for me to do.

Why might an experiment require large numbers of subjects? There are many reasons. The experiment I am about describe is one example.

Early this year, as I was setting up my first Web-based lab (the Visual Cognition Online Lab, now closed), Tal Makovski, then a post-doc at the same lab as me at Harvard, came to me with an idea.

One of Tal's research interests is visual attention. As I mentioned in that previous post, you don't pay attention to everything you see, and this can affect what parts of your environment you are aware of. Not long ago, Tal and his PhD advisor suggested that in some instances, trying to ignore an area in your visual field actually causes you to pay more attention to it (Tsal & Makovski, 2006). They called this the "White Bear Hypothesis." This name comes from the following "experiment":

Quick! Don't think about a white bear!

If you immediately thought about a white bear, you get the idea.

In the original study, participants were supposed to identify a quickly-presented stimulus, while ignoring a distractor. The stimulus always appeared in the same position, and the distractor always appeared in the same position. The participants knew where to look and what to ignore. It was particularly important to ignore the distractor, because it would otherwise throw the participant off. In one version of this type of task, the stimulus is either an H or a K, and your job is to say which one you see. The distractor is also either an H or a K. It requires some concentration to not accidentally identify the distractor instead of the stimulus. Again, the location is what sets the stimulus and distractor apart.

The subjects did this task over and over. On some trials, instead of the normal task, two dots would appear -- one in the distractor's location and one in another location that had not been important so far. Although these dots appeared simultaneously, participants said that the one in the distractor location appeared first. This is the result you would expect if participants were paying particular attention to the distractor location (despite the fact that they were supposed to be ignoring that location).

One problem with this experiment is this: The appearance of the dots was called a "suprise trial," but it happened many times during the course of the experiment. The first time, the participants might have been surprised, but after that, they knew that occasionally two dots would appear -- one in the distractor location -- and that they would need to report the order in which the dots appeared. This might encourage them to pay attention to the distractor location after all.

Why not do just one surprise trial per participant? The reason Tsal & Makovski repeated the surprise trials was to get statistical robustness. There is a reason that standardized tests like the SATs have more than one question; this produces a more stable and more nuanced result. The worry with Tsal & Makovski's study was that perhaps they had the equivalent of an SAT test with one question repeated a hundred times. The authors used a number of controls to try to eliminate this possibility, but the doubt still lingered.

Now with our new Web-based lab, Tal reasoned that we could "surprise" each participant only one time, and make up for the reduced amount of data by having many participants. That is exactly what we did.

There were 7 versions of the experiment (more about that below). A little over 500 people completed the final version. The participant was briefly presented with a letter in the middle of the screen. They were to press one key if the letter was an H or a K, and a different key for S or C. A distractor letter appeared near to the target, which either matched the target category (congruent trials) or did not not (incongruent trials). This was repeated a number of times (16 in the final version of the experiment). Not surprisingly, participants were significantly more accurate and significantly faster in the congruent condition than the incongruent condition. Scientifically, this was expected, but it was exciting nonetheless. By putting the experiment on the Web, we lost a lot of control over the timing of the display. Similarly, we can't get faithful reports of the participant's speed in responding. Many people had been skeptical that our program would have enough percision to successfully show this effect.

On the 17th trial, either a P or a Q flashed on the screen, either in the same position as the distractors had appeared or in a different position. Participants were then asked, "Did you see a P or a Q?"

Unfortunately for us, there was no significant difference in accuracy when the P or Q appeared in the distractor location (73.1%) or the new location (70.7%). The numbers go in the direction of the hypothesis, but statistically the two results are equivalent. With over 500 participants already tested, it is unlikely that testing more will make this difference significant.

What does that mean? There are two possibilities. One possibility is that Tsal & Makovski's original result was in fact due to the repetetive nature of their task.

Another possibility was that our new experiment wasn't sensitive enough. There are many ways this would be possible. The early versions of the task were either too fast (people couldn't see the P or Q regardless of its position) or too slow (everybody go the P and Q correct regardless of its position). If the distractor was too easy to ignore, that could mean we would not get an effect, so we adjusted the difficulty of that task. Etc. Perhaps, in the end, it was still too easy. Perhaps the two-dot suprise trial would have shown the effect, but the P/Q task does not. The possibilities are endless.

The story would have been better if we had made a major discovery. Unfortunately, this is a more typical: an inconclusive result. Still, I'm pretty happy with the outcome. In terms of the technical aspects, this was by far the most ambitious Web experiment I have run. Most Web experiments are surveys. I wasn't sure that they dynamic aspects of this experiment -- especially recording response time -- would even be possible. The fact that the distractor task (H/K vs. S/C) worked as expected is very encouraging.

This is the first of what I hope will be many posts describing results from the Web-based lab. This was one of the most abstract experiments we've run. The rest should be more lay-person-friendly. Completed experiments included tests of visual memory and writing abilities. Our ongoing experiments look at how people interpret language, how birth order affects personality, and how parents and children communicate.

Do you see what you see?

Descartes famously asked just how much one should trust one's own senses. Is it not possible that an extremely powerful and malicious demon could trick you into thinking you see things that you do not? (Even more subtly: could a demon trick you into thinking you think things that you do not?)

As it so happens, demons are not necessary, because we are deceived by our senses all the time. Daniel Schacter argued in the Seven Sins of Memory that human memory should not be understood as a function that is meant to keep a perfect record of what has happened, but as a tool that is meant to be used to promote survival. As such, the "seven sins of memory" (common memory "faults" like forgetting things) are often byproducts of useful features or are even useful features in and of themselves. For instance, having a memory junked full of useless details is not efficient; it is better to remember only that which is important. It cuts down on time speant searching for an important memory. Anyone who has dealt with a file cabinet full of documents nobody will ever need should recognize this insight. (Full disclosure: Dr. Schacter is a professor in the Harvard Psychology Department, where I am a graduate student.)

The same argument applies to the senses. The sense did not evolve in order to give us perfectly accurate representations of the world. They evolved in order to help us cope with the world. Often, the two goals are one and the same, but they are not always. Visual illusions are examples where our eyes do not give us faithful representations of the world, usually because of tricks our brains employ to make our vision more useful, not less.

Other aspects of our senses may not be inaccurate per se, but they are not the only solutions to the problems. The typical human eye, for instance, has three different types of cones, which optimally respond to three different wavelenghts of light: 565 nm, 535, nm and 440 nm, which our brain perceives as red, green and blue. It is important to note that "red," "green" and "blue" are perceptions that exist in our minds. As I understand it, our cones could preferentially respond to 566 nm, 536 nm, and 441 nm without our perception of red, green and blue being altered. We could only have two kinds of cones (as red-green colorblind people do), and thus have a more impoverished perception of color. We could have 4 types of cones, as some people do, and have a richer perception of color.

The point being made here is that our brains do not passively view the world, and our eyes do not just take photographs. What we "see" is a representation of the world that is faithful in some respects and unfaithful in others. Additionally, it is constantly touched-up. When a fashion photographer takes a photograph, she not only manipulates the lighting, the clothing, the angle, etc., all before the image reaches the camera, but after the picture is taken, the photograph is altered. This alteration -- removing of blemishes, fading this, enhancing that -- is not done randomly; it is done to improve the usefulness of the image -- usefulness in terms of selling product or magazines or whatever. The eye and brain are similar.

The same argument The New York Times just ran a fantastic piece touching on just this issue. It is perfectly possible for something to be square in your field of view, but for you to completely fail to see it. I have this happen with my keys all the time. This is at least partly due to the fact that you don't pay equal attention to everything in your line of sight. Your attention focuses in a particular place (or possibly places -- this is actually a very complicated area of research). Magicians use this fact, manipulating your attention so that you "see" what they want you to see. Thus, magicians and perception researchers often have much to say to each other. I had heard of symposiums like the one described in the Times article before. Without spoiling any of the great stories in it, I recommend you read the article before it gets archived and starts costing money.

We recently completed an experiment probing the allocation of attention. This was one of my first Web-based experiments. The results are in, and as soon as I have a chance, I'll share them here on this site. These will be the first results from our Web-based experiments we'll be sharing on line.