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I recently stumbled accross the article "When the Brain's Mailbox Is Full", which can be found here: http://www.nytimes.com/1999/07/27/health/when-the-brain-s-mailbox-is-full.html.

The article claims that:

''As we get older, we run out of places to store new information,'' said Dr. H. Lee Swanson, a psychologist at the University of California at Riverside and the author of the study. ''We have a limited amount of space in our memory system.''

Do we really have a such a limited capacity for knowledge that we risk slowing down our brain functions if we study too much?

Swanson, H. L. (1999). What develops in working memory? A life span perspective. Developmental psychology, 35(4), 986. LINK

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  • $\begingroup$ Metaphors will only carry one so far in such instances. Memories are linked together by different modalities (as hearing a familiar tune may help evoke the memory of long-forgotten song lyrics), and represent much more than "mailboxes". $\endgroup$
    – Chuck Sherrington
    Commented Dec 21, 2012 at 19:50
  • $\begingroup$ Read some about Kim Peek (aka Real Rain Man), he got photographic memory and remembered more than any of us. $\endgroup$
    – LifeH2O
    Commented Dec 28, 2012 at 6:40

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As Chuck pointed out in the comments, it's important not to take a metaphor too literally. Comparing our memory to a mailbox may have some validity, it is not true that our memory can "fill up"-- i.e., that we have a limited capacity for knowledge in general. No matter how old or how many facts you have learned, you will always be capable of learning new ones.

In the NYT article, prominent aging research Timothy Salthouse points out this research is controversial:

"It seems unlikely that any single study, regardless of its quality, will be able to resolve the issue. We don't know how to measure storage capacity."

Additionally, your question asks whether we risk "slowing down our brain functions" if we study too much. This study is about age-related deficits, and there is no evidence that simply learning more information will reduce our memory capacity.

The study also is referring only to working memory, and not long-term memory. While some researchers believe working memory is simply a subset of attended items in long-term memory (e.g. Cowan, 1999; Anderson J.R. et al., 2004), others believe they are unique constructs (e.g. Baddeley, 2000). It is important not to conflate the two.

While learning new facts cannot reduce our overall memory capacity, it may affect our ability to retrieve specific items. This is known as memory inhibition. For instance, recalling semantically related words can inhibit retrieval of a target word (Anderson, M.C. et al., 1994).

In sum, learning new information may prevent us from retrieving older information through inhibition, but it is not related to overall memory capacity, or to the age-related deficits mentioned in the NYT article. We don't risk "slowing down our brain functions" by studying too much.

Anderson, M. C., Bjork, R. A., & Bjork, E. L. (1994). Remembering can cause forgetting: retrieval dynamics in long-term memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20(5), 1063. PDF

Anderson, J. R., Bothell, D., Byrne, M. D., Douglass, S., Lebiere, C., & Qin, Y. (2004). An integrated theory of the mind. Psychological review, 111(4), 1036. PDF

Cowan, N. (1999). An embedded-processes model of working memory.Models of working memory: Mechanisms of active maintenance and executive control, 62-101. PDF

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  • $\begingroup$ 'it is not true that our memory can "fill up"-- i.e., that we have a limited capacity for knowledge in general'. I agree with most of your answer and it probably never does fill up in a lifetime (and I don't know what to make of speculative estimates like these, but of course our brains do not have the capacity to store the position of every atom in our brains or something like that. $\endgroup$
    – Ruben
    Commented Dec 27, 2012 at 11:16
  • $\begingroup$ @Ruben i agree there is a fundamental limit, but in practice there is not. see my related answer on bio.SE $\endgroup$
    – Jeff
    Commented Dec 27, 2012 at 18:27
  • $\begingroup$ I find the question why we we filter what more interesting too. Still, back-of-the-envelope calculations (other estimates are way below your 2^86bn) like that don't tell us very much about the amount of meaningful information that can be stored and retrieved again. An important question from evolutionary psych would be: Why do we "afford" brains that can store more information than we could ever hope to learn in the information age, even less in the ancestral environment? How many people can go past fluency in four languages? How many TV series until you start confusing characters? $\endgroup$
    – Ruben
    Commented Dec 28, 2012 at 10:48
  • $\begingroup$ yes, sorry, i didn't mean to imply that figure had anything to do with functional capacity. as for why our functional capacity is seemingly far below that, i think it's a matter of efficiency. if old memories didn't decay, it would be computationally impossible to access recent memories quickly. See Anderson (1990) and Anderson & Schooler (1991). at least, that's the position i take--there are others. $\endgroup$
    – Jeff
    Commented Dec 29, 2012 at 17:35
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There is strong evidence that suggests there are changes in neural pathways, synapses, and even birth of new neurons due to changes in behavior, environment and neural processes. That is known as Neuroplasticity, which suggests that our brain's capacity can be "recycled" and even augmented in many cases.

If I had to choose a metaphor to make an analogy of our brain compared to something else, I would say it is more like a muscle or a bone. They have limitations regarding their capabilities, but they can change to fit demand.

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There is research showing that Black Cab taxi drivers while training with 'the knowledge' (2 months on a moped with an A-Z, learning every road/route in London) grew larger areas of brain grey matter concerned with memory. Therefore it cannot be fixed and fillable, neuroplasticity will grow more where required. Although this is not the reference I was referring to above, it is on the same lines. https://onlinelibrary.wiley.com/doi/abs/10.1002/hipo.20233

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