Re: How does memory work? See specific questions below.

Dear Hugo,
You have quite a few questions! I'll answer as best I can...

1. I'm not exactly sure, but the heart pumps 4300 gallons of blood a day. That's about 550,000 ounces. That works out to be about 23,000 ounces an hour, or around 380 per minute. Since the heart beats around 70 times per minute, that's about 5.5 ounces per beat. So that would be about how much new blood enters the brain with each heartbeat, and how much leaves it.

2. This is a difficult question to answer, because human memory (unlike a computer's) cannot easily be measured in units like kilobytes. One reason is that there are different types of memory. There is what some call declarative memory: the type of facts such as what your phone number is, what the capitol of Tennessee is, etc. There is procedural memory: the memory of how to do something. These are skills like riding a bike or playing a piece of music from memory... you may not be able to say exactly how you do each one, but you can do them if asked. Finally there are episodic memories, such as memories of a trip or other event from childhood. They may not be remembered in specific details, but in general terms. There is also a difference between short-term memories (which last for a few minutes to hours) and long-term memories, which can last for decades.

A Shakespearean actor may have memorized dozens of his plays. A high school band teacher may know how to play a dozen different instruments. An elderly woman may have hundreds of memories of her childhood and adult life. It's not really possible to "measure" any of these memories or compare them to each other. Therefore it's very hard to say how much memory the brain will hold.

3. There isn't a specific age, and the good news is, most very old people do not lose all their memories or their ability to recognize people. Almost everyone experiences some loss of memory as they get older, but severe memory loss is seen only in people with disease conditions like Alzheimer's. Even then, the inability to recognize close friends and family is one of the last skills to be lost. My grandmother had Alzheimer's, and though she could not remember what had happened to her the day, or in some cases, even an hour before, she could still recognize most of her family and friends. And she could remember events from her childhood almost 90 years previously. It was only at the very last stages of the disease that she lost the ability to recognize her children. A question similiar to this was answered recently on another science site for kids: I'll direct you there for more information and links.
http://faculty.washington.edu/chudler/q1999.html#q21

4. Again, there isn't one specific site. Different parts of the brain work together to aquire, maintain and retrieve memories. Go here for a picture of a scan of a human brain showing different sites being activated during a memory task.

One very interesting part of the brain involved in memory is the hippocampus, which is in the temporal (or side) lobe of the brain. Hippocampus means "seahorse"; early anatomists thought it looked like one. In 1953 a man (who is called H.M. to protect his privacy) had both his hippocampi removed to treat his severe epilepsy. His intelligence and language skills were not affected much by the loss of this much of his brain, but he lost the ability to form new, long term memories. He now lives in a hospital, and has to be re-introduced to his doctors every day, even though many of them have treated him for years. He does not recognize terms like "Jacuzzi" or "VCR" or other things that have been invented since 1953. He knows his birthdate, but if you ask him his age, he'll underestimate, since he does not realize how much time has passed since 1953. He probably thinks of Ronald Reagan as an actor, not a US President.

The thing is, he does remember events from before his surgery, so it's not accurate to say memories are "stored" in the hippocampus. H.M.'s childhood memories, and his memory of how to dress himself, speak English, etc. must be "stored" someplace else. Rather, the hippocampus seems to be involved in forming new long-term memories.

H.M. also can learn some new things, particularly procedural memory tasks. For instance, he was once given a mirror drawing task: asked to trace a picture while looking at his hand in a mirror, rather than directly. (Try it, it's hard to do, but gets easier with practice.) When he was asked to practice this skill for a while each day, he gradually got better, just as a person with intact memory skills would. The thing is, at the start of each new session, he didn't remember ever doing this before. When asked how he could mirror-draw so well, he said he "was just good at this," whereas a person with normal memory would know it was because they had practiced. Again, this is an example of how different types of memories are processed differently by the brain. Certain types require the hippocampus, others do not.

I hope this gives you some idea about how complicated the study of human memories is.

I suggest Neuroscience for Kids as a good site for more information on the brain and memory.

Thank you for your question.

Louise Freeman