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So, how appropriate is it that your humble blogger forgot to post the blog on memory?
Much of this material has been covered in the earlier sections on hippocampus (right), so we are going to discuss what constitutes memory in today's blog, and discuss failures of memory tomorrow.
So, what is memory (in the brain)? In simplest terms, it is a pattern of activity in neurons that (A) represent an event in the "external" world (i.e. outside those neurons) and (B) can be either held in an active state or recalled once the external event is gone.
The simplest example is how most people remember a phone number - first they read or hear it, then they repeat it - often continuously until they can write it down or use it. Repeating the whole process makes it easier - because each time strengthens the connections between the neurons that are active with the pattern. Learning any skill is a prime example - repetition leads to alteration in the neural circuits that hold the code.
What makes memory *special* is that the information you want to learn is not the only thing that gets stored. To explain, let's take a small detour...
The detour is called "nonlinear systems analysis." You may have heard it under another name - "chaos theory" - but in this case that's largely a misnomer. Nonlinear theory states that the output of certain systems is not a linear function of the input. Consider a water hose - the input is a stream of water with defined volume, pressure, temperature, salinity, etc. The output, assuming no changes, is exactly the same. That's a linear function of *unity* (one) since the input and output are the same. But suppose you put a nozzle on the output end - now you transform the pressure - higher pressure in smaller diameter if the volume stays the same. That's a linear function, too.
Now consider a dripping faucet - time the space between drips - they are never the same (even if they repeat in *almost* the same sequence). You can't write an equation to *exactly* describe the inter-drip-interval - thus it is a *nonlinear* system. The reason is that there are *so* many factors that affect the water droplets - temperature, airflow, the spread of a film of water over the faucet head, the tiny irregularities in the worn valve and washer - that every new drop is a function of so many variables that *change* with each preceding drop. If you make a graph of *two* successive inter-drop-intervals, though, it makes an interesting pattern - seeming to repeat, but not exactly. It's close though. The equation is rightly termed "chaos", and the theory that has people excited is that there is an order underlying even the most chaotic system - we can't know the *exact* starting conditions for each drop - but we can estimate them, and if we measure enough, we can get pretty darned close to prediction.
So what does this have to do with memory? It's the nonlinear aspect - the concept that each pattern measured is the overlying information, plus the sum of all the little "initiation" variables. This is important to memory, because not only the main pattern gets stored. The little details - if they repeat - also get stored. Even it they don't repeat *exactly*, an approximation gets stored.
So memory is more appropriately an association. The key piece of information is stored *associated* with context. The words of a song with the melody. Grandma's face with the smells of her kitchen. The name of that memory person you dated with the smell of their perfume/cologne. (By the way, odor is a *powerful* associative cue for memory). When you practice a skill long enough, the knowledge gets associated with the motor movements required to perform it (so-called "muscle memory").
This also means that memory is *recalled* associatively. If you study with loud music, then try to take the test in the quiet, it will be more difficult to remember. Because emmory is also stored as *seqeunces* of associations, you may have to sing that verse all the way to the end and start over to remember the name of a song. You retrace your steps to remember something you may have forgotten. "Tip of the tongue" phenomenon occurs when something interferes with the associational tag that you were using to recall a fact.
The hippocampus is a key player in making associations, and in short term storage of memory, but not in long term storage. The hippocampus has capacity for about 7-11 (short) items such as a phone number of address, but only for about 5 to 10 minutes. After that, you have to put it in longer term memory. That happens in the Temporal and Parietal Lobes, and the associational functions for retrieval are shared between Frontal Lobe, Temporal Lobe and diencephalon.
So that's how we make and recall memory. Tomorrow's blog will look at how memory fails - by age, disease or abnormal conditions...
... and I promise not to forget!