There was a pretty direct correlation in Daneman and Carpenter (1980) between the then-new Reading Span Task and reading comprehension. This was compared to reading samples and scores on the verbal portion of the SAT. What's important about this particular study is that they were the first people to really concretise the notion that 1) people's working memory varied individually. Exactly how (same volume with different command centers, or different volumes same command center, or a combination?) is still being discussed. 2) It birthed the concept of "complex span" (Hitch, 2006). Simple span are basic tasks like: "remember this list of words: parsely, thyme, rosemary, sage" for a verbal span, "remember these numbers: 34, 4, 54" for a math span, and "remember these locations" for a visuo-spatial span. Daneman and Carpenter birthed the idea of processing into these tasks.
There are a variety of people who are studying how these tasks demand and tax memory. There is a significant bleed over from attentional, behavioral, and memory studies to accomplish this task. They look at everything from eye movements to ERP's to fMRI. I'm of the belief that you have a certain capacity for working memory that is divvied up depending on the task. That is why I can do complex long division, but not when I'm also watching TV or driving.
Oh don't get me started on memory and driving...that's a whole field unto itself.
But people our age (though I'm afraid I'm at the tail end of it) are at the peak of their working memory spans. What does that mean per se? Probably that we're more efficient at remembering what we need to remember, so we don't really need a higher-capacity working memory. Hence adults tend to be far worse at playing video games, but far better at using the internet.
So what does this all have to with N400, my first entry? Well that, my dear readers, will have to wait for another entry.
Daneman, M., Carpenter PA., (1980) Individual Differences in Working Memory Journal of Verbal Learning and Verbal Behavior
Hitch, G., (2006), Working Memory in Children, A Cognitive Approach in Bialystock, E., and Craik FIM., (Eds.) Lifespan Cognition. Oxford University Press, New York.