More than one student has complained that the space in their head is limited, and new information is simply pushing the old information out. In the terms of memory research, this is retroactive interference: learning new information causes you to forget old information.
The way this is typically studied in the laboratory is to have the participant learn something -- often a paired associate (think "Concentration") -- then learn something else, and then finally be tested on the original memory item(s). This way, one can vary that middle task in order to study how different activities cause different amounts/types of retroactive interference.
The is another type of interference: proactive interference. This is the effect that learning one piece of information has on future learning. That is, the books a student has already read make it harder to learn new information.
Just like retroactive interference, proactive interference is seen in both short-term and long-term memory.
Memory Systems: How Does Memory Work?
The existence of interference tells us a lot about how memory works, because there is nothing necessary about it.
Consider a computer. We don't expect each new file we add to our computer to cause the computer to lose other files, short of copying over those original files. Similarly, the file I added today should not affect a file I add tomorrow, short of causing me to run out of disk space.
So why is human memory affected this way?
There are a couple reasons it could be. One is that memory is probably overlapping. A computer -- at least, in its basic forms -- saves each file in a unique place in memory. The human brain, however, probably reuses the same units for different memories. Memories are overlapping.
How exactly this works is still very much a matter of research and debate, but it makes a certain amount of sense. Suppose you have several different memories about your mother. It would make sense for your mental representation of your mother to show up in each of those memories. For one thing, that should make it easier to relate those memories to one another.
Searching for Memories
Another way interference might appear in memory is in how it effects memory retrieval. The more files you put on your computer, the harder it is to find the files you want. This is especially true if you keep them all in one directory and use keyword searches.
Human memory retrieval probably does not work like a keyword search, but nonetheless it is reasonable to assume that the more memories you have, the more similar memories you have. Thus, finding the right memory is harder, because you have to distinguish it from similar memories.
How exactly this plays out depends on your model of memory. I will talk about one I particularly like in a future post.
Although my main research is in semantics and pragmatics -- aspects of language -- I have also worked on working memory. I have a paper coming out shortly based partly on an experiment I ran at my Web-based lab. Over the next week or two, I plan to write about some of the fundamental questions about memory addressed in that paper, as well as write about the paper and lay out its results.