Combination Vision Test
Combination Vision Test
[Caption above the drawing:] Combination Vision Test
[Below the caption is a circle formed by several hundred numeric digits from 0-9.]
[Caption below the panel:] If you can see one big number but not the other, you have synesthesia and colorblindness.

Synesthesia is a condition in which perception in one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. Common examples are experiencing colors when seeing numbers or words ( Grapheme-color synesthesia ), hearing tones or music while reading words or text, seeing sequences of numbers or month names in a distinct and fixed shape ( Number form ), etc. In 1608: Hoverboard Megan stands at the end of the Star Destroyer and wishes she had synesthesia so bad she can taste it...
Color-blindness is one of a number of conditions in which a person cannot distinguish certain pairs of colors that other people without color-blindness might find easy to distinguish. There are many different forms of color-blindness; the most common is an inability to separate the colors red and green.
There are two numbers embedded in the big circle of numbers, in a similar way to a common color perception test . But this test can not work for colors because it is just a black-and-white picture. Nobody can see it. However, the joke lies in the fact that those with one common form of synesthesia see colors associated with numbers. Randall implies that a synesthete will see colors connected to each number, and thus a color perception test will work after all - thus distinguishing synesthetes with color-blindness from those with normal color perception.
The comic playfully suggests that if you have synesthesia as well as colorblindness , then some of the colors might appear identical and so one number would not be visible, only leaving the other number.
The title text brings in two more conditions: diplopia , or double vision, and myopia , or near-sightedness. Those who are near-sighted sometimes see distant objects more clearly while squinting. Then they would be able to see the one large number still visible from the synesthesia/colorblindness combination, but because of double vision they see a second copy of it, hence two numbers that are the same.
If we color the numbers in the circle in a consistent way (and leave the 2, 3, 5, 7 and 9s black) we can reveal the large numbers:
The numbers are four and two, forming the number 42 , which is the famous " Answer to the Ultimate Question of Life, the Universe, and Everything ", according to the book The Hitchhiker's Guide to the Galaxy . The number 4 is formed by digits 2, 3, 5 and 7 (the single digit primes) while the number 2 is formed by digits 3, 5, 7 and 9 (the single digit odds, excluding 1).
For Randall's test to work (i.e. for either the large 4 or the large 2 in '42' to get lost in the noise to those with a given color-blindness), either the little number 2 or the little number 9 would have to be lost in the background noise. So, for example, if the background appeared in shades of red and the little number 2 was a shade of green, then the large number 4 would be less visible to those with red-green color-blindness than to others.
While it makes for a good joke, there are three reasons this kind of test wouldn't work in real life.
The first is that there is no one set of color-number associations seen by all synesthetes. So while some synesthete might see '2' as green and '0' as red (so a red-green color-blind person would lose anything made up of '2's against a background of '0's), others might see '2' as yellow and '0' as blue, or any other association imaginable.
The second reason it wouldn't work is that synesthetes do not (always) automatically see a 1:1 overlay of color on top of a number - they still need to read the number legibly. Randall's circle is very chaotic, so one wouldn't intuitively identify each single number. For a synesthete the color is produced after the number is recognized by the brain and lost when the focus shifts to the next number. However, some synesthetes may find if they pay attention to the numbers one by one they can make something out. However, as noted by a user in the discussion, who states that he has a type of synesthesia, he did indeed see the numbers ! Furthermore, in his blog's discussion section, one person commented they could see the large '2' but not the large '4' ! This was not because the person was colorblind, but because the '4' was mostly composed of numbers ('2's and '7's) whose colors blended in with the background, while the '2' contained an even mix of numbers, some of which (presumably '3's, '5's, and '9's) starkly stood out, making the large '2' easily visible. However, one could easily imagine this scenario pertaining to colorblindness: for example, a colorblind synesthete, in theory (although the third reason makes it clear why this would be extremely unlikely), might perceive most of the background numbers as shades of green (similar to the picture below) and see the '2's and '7's in shades of red, which would make it difficult to differentiate between the giant reddish '4' and the greenish background.
The third reason the test would not work is that color-blindness is an inability to distinguish colors of light hitting the retina, it's nonsensical to imagine a synesthete would perceive two separate colors that they cannot normally separate anyway. But again in the above mentioned link this particular person did see the colors in a way where people with red/green color-blindness might have a harder time seeing the 4 than the 2 in 42.
The next image shows all of the numbers, including 2, 3, 5, 7 and 9, colored in, in such a way as to ensure the number 42 is clearly visible to those with no particular blue-yellow color-blindness:
The "real problem" is actually that if a synesthete does indeed see the digits as colors that resolve into either one or two numbers, then what color would these new "color-numbers" then appear to be? If a synesthete could see both large numbers AND they appeared as the same color as the small numbers as soon the synesthete perceived the numbers, then what would this meta-synesthete see? The '4' would blend in with the background '4's, while the '2' would stand out (as '2' was not used in the background). Would that mean that as soon as they noticed the giant '4', it would suddenly disappear into the background? Is this sort of layered synesthesia even possible?
Keep in mind, as noted above, that synesthetes do not all see the same color-number associations. They also do not necessarily see every number in a different color, as depicted here, and may even see some numbers as purely black.
[Caption above the drawing:] Combination Vision Test
[Below the caption is a circle formed by several hundred numeric digits from 0-9.]
[Caption below the panel:] If you can see one big number but not the other, you have synesthesia and colorblindness.
