In 1966 Mark Mayzner and his colleagues at New York University showed people a five-letter word on a cathode-ray tube, one letter at a time, each letter in its proper place. When the letters arrived quickly and out of order, some of them were never seen. The word chair read as c a r, with gaps where the h and the i had been lit a moment before.
Mayzner called this sequential blanking. This page rebuilds the demonstration in ten steps, and each step changes one thing.
The black panels flash letters briefly. Each one runs once when you press its button. Nothing loops.
Start with one letter. Mayzner’s display was a Fairchild CRT whose phosphor faded within a few microseconds, driven by a PDP-7 computer that set each letter’s on time in steps of 50 microseconds. A letter stayed lit for as long as the program asked, and no longer.
Your screen works differently. It redraws the whole picture at a fixed rate, so a letter can only be lit for a whole number of frames. This page has timed your screen: measuring…
Drag the slider to ask for a duration, then press Flash. The strip under the panel compares what you asked for with what this screen can show.
Ask for 5 ms and you still get a whole frame. Every demo below snaps to durations this screen can produce, and tells you which ones it used.
Now light all five letters of chair one after another, left to right. Each letter turns off as the next one turns on, so only one letter is ever lit.
Start at the slow end and press Run. The letters step across the panel. Then drag the slider toward the fast end and run it again.
At the slow end the order is obvious. Near the fast end the steps blur into a sweep, and at the fastest setting the word seems to arrive all at once. Mayzner found the same thing: when the whole word took less than about a tenth of a second, observers saw five letters appear together.
At every speed, all five letters are seen.
Keep the letters, their places and the timing. Change only the order in which they light: H first, then I, then C, A and R. Every letter is still lit for the same time, in its own place.
The slider starts near Mayzner’s setting of 40 ms per letter, which is 200 ms for the whole word. Rest your eyes on the small mark above the row and press Run.
What did you read?
Run the panel once, then choose.
Mayzner’s observers reported c a r, with empty spaces where the h and the i belong. His own summary was that when the whole word takes about 200 ms, the visual system erases what arrived in the first 100 ms.
Switch the order back to C H A I R at the same speed and the two letters return. Nothing about the light changed except its order.
Blanking only happens inside a band of speeds. Mayzner found that when the whole word took less than about 100 ms, the five letters looked simultaneous. When it took more than about 300 ms, they looked like a sequence. Either way all five were seen. In between, with a peak near 200 ms, the first letters went missing.
What counts is the time for the whole word. With ten letters he halved the time per letter and the effect held.
The slider now holds every speed this screen can produce in that range, always in the order H I C A R. Run each one and say what you read. The shaded band on the strip marks where Mayzner found blanking.
What did you read?
Run the panel, then choose. Each answer is kept on the strip.
On this screen, a few of the settings fall inside the band. Mayzner’s computer could move the timing in steps of 50 microseconds, so he could place a setting anywhere in it.
So far the letters have sat as close together as the typeface allows. That matters. Watching an earlier version of this demonstration in 2013, Mayzner asked for the gaps to be as small as possible, and said that an inch or two between letters greatly weakens the effect.
His lab measured it with five points of light in place of letters. At one spacing, the two points lit first went missing. With the points a third to a half further apart, all five were seen.
Drag the slider to spread the letters, then run it. The order and timing stay as in step 3. The letters are a little smaller here to leave room.
What did you read?
Run the panel, then choose. Each answer is kept on the strip.
Not every study agrees. Irwin Pollack’s forced-choice tests in 1972, which asked observers to judge the tilt of small line elements, found that even spacing made little difference to the result.
If later letters erase earlier ones, their strength should matter, and it does. Mayzner reported that blanking disappears when the blanked letters are made brighter or the blanking letters are made dimmer.
Here H and I stay at full brightness while the slider dims C, A and R. Run it at each setting. At some point the H and the I should come back.
What did you read?
Run the panel, then choose. Each answer is kept on the strip.
The percentages are approximate. This page sets the level of the pixels and cannot measure the light your screen gives off.
The last step uses a different display. In 1970 Mayzner filled a small grid with one repeated letter and lit the cells in groups, each group for 20 ms, with a tenth of a second from one group to the next.
Start with two groups. Two letters in the middle row light first, then the three letters around them. The second group erases the first, and the row reads as three letters with two gaps.
Then add a third group: six letters above and below the three, lit last. Those six act on the three the way the three acted on the pair. With the three suppressed, the first pair comes back.
Look midway between the two marks, run each version a few times, and say whether you saw the first pair.
Did you see the two letters that lit first?
Run the panel, then choose. Each run takes one answer.
Two groups: no runs yet. Three groups: no runs yet.
All ten of Mayzner’s observers changed in this direction, though not equally. One of them barely lost the first pair to begin with.
He called the effect disinhibition, after a finding by Hartline and Ratliff in the eye of the horseshoe crab, where lighting a third receptor releases the first from the inhibition of the second.
Where do the missing letters go? In 1972 Irwin Pollack at the University of Michigan tested whether anything of them survives. He replaced the letters with small marks, each a row of nine dots, either flat or tilted. And he stopped asking observers what they saw. He made them choose.
His observers watched four rows flash one after another and had to pick the row whose marks were tilted. They chose as well when the tilted marks were blanked as when they were plainly visible, and sometimes better.
This is a shorter version, with one row and two choices. Five marks light in the order that blanks the second and fourth. Those two are either both flat or both tilted. Even if you saw only three marks, choose. The page keeps score.
Were the second and fourth marks tilted or flat?
Run the panel, then choose. Each run takes one answer.
Out of order: no trials yet. In order: no trials yet. Guessing gets about half.
Twenty trials are enough to tell luck from sight. Fifteen or more right would happen by luck about once in fifty tries.
Pollack’s observers said the unseen mark gave itself away by making its neighbours seem to turn. Something of the letter that was never seen still reaches the person who did not see it.
With ten letters Mayzner cut the time per letter to 20 ms, which kept the whole word at 200 ms. Some orders blank about half the letters, as before. One order does something else.
Light the two halves of somersault in alternation: S and S, then O and A, then M and U, and so on. Mayzner’s observers saw all ten letters, and all of them were convinced that the word had split, with an empty space one or two letters wide between the R and the second S. He called it sequential displacement.
Their first thought was that the display was spacing the letters unevenly. Then he covered every letter but the R and the S with masking tape, and the two sat side by side.
What happened to the word?
Run the panel, then choose. Each run takes one answer.
Size matters here. Mayzner saw the gap with letters about an inch tall, and measured it by holding a ruler against the tube: half an inch to an inch. When he shrank the letters to an eighth of an inch, the gap closed and three letters went missing instead.
Everything above was timed in frames, because a frame is the shortest thing this screen can show. Mayzner’s machine had no frames. A crystal clock ticking 20,000 times a second let the program hold a letter on the tube for any multiple of 50 microseconds, and the phosphor went dark within a few microseconds of the beam moving on.
Drag the slider to ask three machines for the same flash.
The 1968 machine, as Mayzner described it that year:
In 1968 the scarce part was the computer. By 2013 it was the display. The computer could count microseconds and the screen could only show frames.
The rebuild goes back to what Mayzner had, which is lights that turn on and off when told. Each letter is its own LED light box, switched by a transistor from a microcontroller’s hardware timer. The last job is to point a photodiode at each box and measure when the light is on, because the pin that asks for light is not the light.