Have you ever tested your digit span?
Read the 8-digit string above, then close your eyes and say it out loud. Were you successful? Try again with each member of this list. Better yet, get a friend to read each string out loud, and then say it back to them.
Digit span seems like a useless ability. Why would you want to test it? Well, surprisingly, digit span is highly correlative with general intelligence. And, as it turns out, 8 digits is a pretty good span. So, if you were successful at the task above, chances are you’re a pretty smart fellow.
How could a task as simple as repeating a digit string predict one’s IQ? Well, digit span tests working memory, which refers to the amount of information that can be stored in one’s brain at any given time. If someone can process more thoughts at a time—juggle more balls, so to speak—he will be able to make deeper connections and think more abstractly. Note that working memory can store all kinds of information—sounds, images, and even ideas. How did you complete the digit span task? You might have just repeated the sounds of the numbers. Or, maybe you pictured shapes of the numbers in your mind. Or maybe you used a combination of these and other techniques. It’s not too hard to imagine that one who can process many sights and sounds simultaneously will excel at activities we commonly deem as requiring intelligence.
Those with memory capacity significantly better than normal are known as mnemonists. Solomon Shereshevskii, known in much of the literature simply as “S,” was a famous mnemonist in the psychology world. As the story goes, S was an otherwise normal man who took up journalism after a failed music career. One day, S was reporting on a talk given by neuropsychologist Alexander Luria. After the talk, a slightly insulted Luria approached S, wondering why he, unlike his fellow reporters, hadn’t taken any notes. S replied that he didn’t need to, and proceeded to recite the entire talk, word for word! Luria was stunned. So was S, who, until then, had thought that everyone had his gift. (1)
Luria searched for limits to S’s memory, but even after years of study, found virtually none. S could effortlessly memorize huge matrices, complex mathematical formulae, and even poems in a foreign language, all within minutes. (2) Remember the digit span task from earlier? S’s span was 70—at least that’s the highest Luria ever tested. 15 years later, Luria found his old digit lists and, looking to test S’s long-term memory, asked him to repeat some of the strings. S was able to repeat the lists, forward and backward, without hesitation! (1)
Why exactly was S’s memory so good? Luria never really found out (in fact, no scientist really knows how the brain stores memories, and attempts to find a specific location for memory storage have been unsuccessful). But an important factor in S’s superb memory was definitely his unique brand of synesthesia. Synesthesia is a phenomenon in which a stimulus from one sensory pathway evokes an experience in another sense. For example, one might see colors upon hearing music, or feel a texture upon eating food. Of course, the synesthete doesn’t actually think he’s seeing those colors before his eyes, but still perceives them.
S’s synesthesia was quite interesting indeed. He describes the numbers as follows:
“Take the number 1. This is a proud, well-built man; 2 is a high-spirited woman; 3 a gloomy person; 6 a man with a swollen foot; 7 a man with a moustache; 8 a very stout woman—a sack within a sack. As for the number 87, what I see is a fat woman and a man twirling his moustache.” (1)
In fact, S had at least 6 different types of synesthesia, triggered by 4 different inducers.
It’s no wonder that a mind with senses so interconnected had some amazing properties!
I actually have a relatively common type of synesthesia known as grapheme-color synesthesia. For me, every single letter and number evokes a color. Each color is highly specific, and never changes. For example, R has always been a dark turquoise tinged with gray. I’m not sure how my synesthesia started, or if I always had it. I do remember seeing, in pre-school, a banner containing the digits ABC 123, colored as such (or at least so I remember). Note that each digit was colored in the “right” color. This poster could have triggered my grapheme-color association, or at least brought it out of latency. Or, maybe my associations existed before seeing that poster. In this case, maybe the digits were colored “correctly” by coincidence, or maybe I’m just remembering falsely that they were colored correctly. How should I know−I’m not S!
Much like S, though, I didn’t know that my synesthesia was unusual until I was about 16, when I casually asked a friend what color 3 was for him, or something, and he looked back incredulously.
Also similarly to S, but to a much lesser extent, my synesthesia helps with memory. I often can’t quite put my finger on a word, or can’t quite work out a word’s spelling, but I remember its colors. Those colors alone will often be enough to jog my memory. Synesthesia also helps with the digit span task. At 8 digits, I’m usually able to recite the string based on its sound alone. But with 9 digits, I have to break up the string into threes. For the first three, my memory is mostly tactile: I whisper the digits back, and remember the feeling of saying them. For the second three, I remember the shapes of the numbers, and of course the colors associated with those shapes. And for the last three, I remember the sound alone. 9 is a great digit span, and I’m fairly confident I wouldn’t have that span were it not for my synesthesia.
This, then, brings me to what’s perhaps the strangest part of this whole story. Digit span measures working memory, which correlates with intelligence. S’s digit span was 70. So was he a genius? Actually, S scored completely average on every IQ test he ever took (2). It’s possible that S possessed a latent intellect, which was prevented from surfacing by his overwhelming propensity to daydream. Having never forgotten a memory, he could spend hours simply playing back moments in his head. Also, his extensive synesthesia left him prone to distraction. For example:
“One time I went to buy some ice cream … I walked over to the vendor and asked her what kind of ice cream she had. ‘Fruit ice cream,’ she said. But she answered in such a tone that a whole pile of coals, of black cinders, came bursting out of her mouth, and I couldn’t bring myself to buy any ice cream after she had answered in that way.” (1)
So, it’s easy to imagine that whatever intelligence S possessed might have been stifled by his overly-active mind. On the other hand, maybe he was simply an average man with an exceptional memory. This would indicate that, while memory and intelligence are often linked, they’re not one and the same. In any case, memory, and intelligence in general, remain some of the most elusive and mysterious concepts in neuroscience.
- The Mind of a Mnemonist: A Little Book about a Vast Memory by Alexander Luria