You might wonder, what do we use short-term memory for? Even though short-term is very short, you are, in fact, constantly using it. You use it to remember the beginning of this sentence as you get to the end. You use it to sustain a conversation, which involves listening, formulating what you are going to say, and then saying it. You use short-term memory when you are baking, to remember the quantity of flour you need to weigh out. Your waitress will use her working memory to write down your order as you’re speaking it – but note that if she takes the whole table’s order and then goes to the machine to punch it in, she’s probably transferring your order to long-term memory!

The reason why cognitive psychologists believe that there is something truly special about the 15-30 second range that can be separated from all other memory beyond that timeframe is that patients who present with apparently total memory loss are still able to keep things in memory for 15-30 seconds.

The first patient to demonstrate a profound loss of long-term memory along with perfectly intact short-term memory was called H.M. He was treated for epilepsy when he was in his 20s; since this was the 1950s and they didn’t know any better, the doctors removed part of his brain as an attempt to cure him of his fits. This did result in improvement in terms of epilepsy, but with huge consequences: H.M. also lost the ability to form new long-term memories. For the 40 years that he lived after his surgery, he didn’t form any meaningful new memories about his life.

If asked what he did yesterday, H.M. didn’t know, and if asked when he started suffering from memory loss (yes, he knew that something was wrong), he would say maybe a year, regardless of how many decades had passed. When he saw the researcher who tested him probably at least once a week for those 40 years, he would introduce himself anew every time. This is all to say that despite how severely his long-term memory was affected, his short-term memory remained just as good as mine, or yours. That is, if you read out a phone-number to H.M., he could repeat it back to you just as well as the next person. This also explains why H.M. was actually able to hold relatively normal-seeming conversations, as long as the topic did not extend beyond the present situation.

H.M. died in 2008, and the researcher who had studied him for his entire post-operative life recently came out with a book (1) about him (and then, sadly, also died). In this book – which I highly recommend – she uses a metaphor to distinguish between short and long-term memory, which I think is brilliant and worth repeating. Suzanne Corkin describes memory as if it were a hotel, with short-term memory represented by the lobby, and long-term memory represented by the guest rooms. Here is what she says about H.M.:

“The information could be collected in the hotel lobby of Henry’s brain, but it could not check into the rooms” (p. 53).

To illustrate how sensitive H.M.’s memory is to this distinction between short- (15-30 seconds) and long-term memory, consider the following experiment (2). H.M. was shown two shapes, one after the other, and his job was to indicate whether the shapes were the same, or different. The length of time between presentation of the two shapes varied between 15 and 60 seconds. Here are examples of same and different shape pairs: