What’s the biggest single difference between you and a dandelion?

Think about that for a moment. You both take in nutrients, air and water; you both grow, you both age. So what’s the difference?

Remember the last time you saw a dandelion? An argument could be made (and in fact I’m going to make it) that the biggest difference between the two of you is that you can remember that meeting and it can not.

Memory not only sets us apart from plant life, it sets us apart from each other. The kind of people we are, the way we do things, the way we talk, the things we talk about and our reactions to new events all depend on our memories. It’s the source of our sense of identity. Physically, you’re not the same person you were 10 years ago. Your only link with that past self is your memory. If you couldn’t remember from second to second any of your previous actions, you would have no more sense of who you are than — well, than a dandelion.

But what is memory? How is it formed and where is it stored?

Ah, now that’s the whatever-a-Nobel-Prize-is-worth-these-days question.

A few things seem pretty obvious. There are apparently three kinds of memory: sensory, short-term and long-term. Sensory memory is the information from our senses which is stored just long enough for our brains to make sense of it. The best-known example of this is the momentary retention of images that allows us to see a rapidly-changing series of still photographs as a motion picture.

Short-term memory (also called working memory) is the brief retention (for about 20 seconds) of information that’s not needed permanently. For example, you remember a phone number you look up in the book long enough to pick up the telephone and dial, but two days later you probably won’t remember the number, and maybe not even making the call.

However, you will if that information has been stored in long-term memory. If, when you made that call, the person on the other end informed you you had won $20 million, you’ll probably remember making that call for the rest of your life, just as you remember your first dog or your vacation to Fiji four years ago.

Memory can also be divided into declarative and procedural. Declarative memory involves facts such as names, dates, phone numbers and hockey scores. Procedural memory is acquired by practice or conditioning. Touch-typing and skateboarding skills would be examples of procedural memory.

Still, classifying types of memory isn’t the same thing as explaining it. It’s here that things get difficult, because nobody is really sure how memory works in the human brain. But we have some ideas . . .

Some basic research into the subject has been conducted by neurobiologist Eric Kandel of the Howard Hughes Medical Institute at Columbia University. Rather than tackle the daunting complexity of the human brain, he has done research with the sea slug Aplysia, which has only 20,000 neurons (nerve cells), which are both very large and naturally coloured yellow-orange, making them easy to identify.

Even with only 20,000 neurons, sea slugs are pretty good students. Squirt a slug’s tail with water, apply an electric shock, and repeat a few times, and the slug soon learns to jerk in its tail and gills at the first squirt, in expectation of the shock to follow.

To travel from one nerve cell to another, nerve impulses must jump across a gap called a synapse. This leap is made by a chemical called a neurotransmitter. What Kandel found in the sea slug is that repeated shocks sensitize neurons along the path followed by the nerve impulses, increasing their production of neurotransmitters and thereby enabling a quicker response to the stimulus.

That sensitivity vanishes quickly, however, unless the shocks recur. If stimulated often enough, the “memory” becomes permanent, moving from short-term storage to long-term. It appears that genes within the neurons are “switched on” and began physically restructuring the synapse so that no matter how infrequently the tail is squirted in the future, the slug is still prepared to batten down the hatches against the expected shock. This restructuring of neurons has been observed in higher animals such as rats, as well.

This new built-in sensitivity of nerve cells to specific stimuli is called long-term potentiation, and was the first demonstration of a possible mechanism for the formation of long-term memory formation.

Memory is a very hot topic of research right now — some interesting work is also underway at the University of Toronto using nematodes, worms no bigger than pinheads. Even these tiny creatures, consisting of a mere 1,000 cells (of which 302 are brain cells) can learn and remember, and using them researchers hope to gain new insights, particularly into whether memory is stored in individual brain cells.

Of course, it’s a long way from the tail-withdrawing reflex of a sea slug and the 302-cell brain of a nematode to the full-colour Dolby stereo memories you have of your third-grade teacher. But scientists are beginning to take the steps that may some day lead us to a full understanding of how and why we remember the dandelion.

See, thanks to memory, you knew what dandelion I was talking about. Let’s see a garden weed do that!

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