Picture this, I am a young child, and I am taking the color blindness test with my eye doctor. “And what does this number say?” he asks me. “25,” I answer (I am embellishing here, I obviously don’t recall the exact discussion, or what the exact numbers of my test where). “And this?” he asks. “54,” I answer. I am doing well, I think, and passing this test, whatever it indeed may be for. “Okay,” he asks, getting to the last plate of the test, “What does this say?” The plate is just a jumble of dots.
I strain to see, thinking maybe there is some number there, just more obscure the other plates. Finally after my becoming near-hysterical trying to see something, the doctor puts the plates away. “What did it say?!” I say out loud, “WHAT DID IT SAY??!!” The doctor is getting ready for the next vision test, when he realizes how upset I am. “Hmmmf, that last one was blank” he mumbles to me under his breath.
So I have understandably wondered something all these years. Why is the last plate always just a jumble of dots? What is that suppose prove or establish? Are there actually people who see a number on that last one? And if they do, what does that prove?
Someone please tell me. I have wondered all these years.
I believe it is there to lull you into a false sense of security before the screen suddenly changes into something horrifying, accompanied by a dreadful row.
A color blind person would indeed see a number that a normally sighted person would not. There was an exhibit at a local mall that had a bunch of the color blindness tests. There was one that looked like it had zebra stripes on it (to me, at least), but people who were color blind would have been able to see a number.
I think I mentioned it in another thread, but I always enjoy remembering the time in grade 12, Bio 30, when some guys in the back of the class (you know the type), colluded to cheat on a colour blindness test. The front of the class never felt so good…
There are three types of “blobs in a circle” things in that sort of test.
It has a number you can see with normal vision, but can’t if you’re colorblind
It has a number you can’t see with normal vision, but you CAN if you’re colorblind (Sometimes 1 and 2 are combined - normal vision sees one number, colorblind another)
Random blobs.
Without seeing the actual circle-with-blobs there’s no way to know if it’s #2 or #3. In any case, there’s more than one type of colorblindness (this has been covered in other threads) so even among the colorblind who sees what varies.
How does item #2 work? I know that a #1 card works because the colorblind person can’t tell the difference between, say, red and green. But how can a colorblind person see something that a normally-sighted person can’t?
Simple. Make a symbol (like ‘25’) with alternating green and red dots. The regular person will not see this as 25, whereas the colour blind one, who has difficulty seeing the difference in these colours, will recognise the ‘25’ instantyaneously.
The last plate on the page linked to by Wumpus has a good example of a figure a person with red-green color blindness will see that a normally-sighted individual will not. People with normal color vision will see a 5 (I do), but those with R-G blindness will see a 2 instead.
Even color normal individuals can see the 2. If you look at the figure perifoveally (just move your eyes from the figure to the right) you will see a shift from 5 -> 2. This happens because LM opponency falls off in the periphery.
Also, remember that there are varying degrees of “color blindess”. I can see the difference between red and green, and I can see that if my color perception were a little different I would chose the “regular” numbers, but the “color blind” numbers always seem more distinct to me. I’m only a little bit RG color blind, if it wasn’t for those tests I wouldn’t even know it. Except my wife and I disagree sometimes about shirt colors. Ones I think are greenish-tinged brown she will call brownish-tinged green. I can SEE that the shirt is somewhat green, but to me it seems that brown predominates.
Man, I hate the Ishihara. I first took it in high school and found out I was colorblind. Total surprise and feelings of inadequacy. I have many pairs of green-tinted slacks, by the way, which I took to be varying shades of tan and brown. My wife now selects colors for me, as I’m hopeless at it. I also have a huge bone to pick with the electronics industry and everyone else who uses red lettering over black backgrounds. Can’t see it at all and it really pisses me off. Okay, done now.
There are different levels and types of color blindness, and the Ishiara test that’s being talked about is pretty good in a clinical setting of telling them apart. There are far more accurate and detailed tests, but few are fast enough to be used outside a laboratory. The Ishihara’s strength is that it’s set up so that different color perception comes up differently on the same chart. What might look like a “54” to a normal person might be a “28” to someone else. With a chart of results, you can check these off and get pretty accurate.
If you were to chart color perception, a person with normal color vision would typically show three peaks, one in the red, one in the green and one in the blue parts of the spectrum.
In a patient that is color anomolous, you’d still chart three peaks, but one of them is shifted to some degree. The greater amount of shift, the more innefective the eye is at checking color.
In a patient that is color deficient, they simply lack a color receptor completely. From my recollection, it’s most commonly the red receptor that’s missing since the green receptor is coded for several times in DNA, but the red only once.
A true color blind person has no color receptors at all, and these people are typcially legally blind, since you would lack cones at all, and rods are only capable of about 20/200 vision.
Blue-yellow color blindness is strangly much less common than red-green. Strange because only about 10% of color receptors code for blue at all. This fact is used to test for early stages of retinal disease at times, since a disease that will effect all cones equally will damage blue-yellow perception first.
