Originally published 11 January 1988
If I remember rightly, it was back about 1963 that I first got interested in the biochemistry of memory. My curiosity was sparked by some remarkable experiments with flatworms — tiny, extremely primitive animals with rudimentary brains and nervous systems.
The worms were subjected to a Pavlovian training routine, in which a pulse of strong light was followed by an electric shock. After a while, the worms cringed with the onset of the light, even before they received the shock.
The little fellows had learned to anticipate the shock, and had therefore “remembered” what was coming. But what was most remarkable was this: When trained worms were cut up and fed to untrained worms, the untrained cannibals also anticipated the shock. They had apparently ingested a learned behavior with dinner!
Well, maybe. It turned out that the flatworm experiments were not easy to replicate, and have since been called into question. But at the time, the implication of these and related experiments seemed clear: Memories were being stored in nerve cells in a molecular way, perhaps as a sequence of chemical units in RNA or protein molecules, in much the same way as molecular DNA stores genetic information. Only molecules, it was thought, and not neural networks, could have survived the transfer process from worm to worm.
There was something satisfying about the idea of memory as molecules. It seemed to endow memories with a kind of material permanence, something more akin to “hard copy” than to “floppy disks.”
Familiar faces without names
But, alas, the memory-as-molecules theory has faded from fashion, as indeed, in my own case, memory itself is fading away. I have reached an age (on the slippery side of 50) when I am increasingly afflicted by familiar faces that have no names, forgotten appointments, unpaid bills, things misplaced. I have this scary premonition that I will wake up one morning to find that all of my internal floppy disks have been inadvertently erased.
There is not much solace to be had from current theories of memory. Most contemporary research confirms that memories are stored in the brain as networks of interacting nerve cells, or neurons. The effect of experience is to somehow fine-tune the connections between cells, the synapses, creating a different “trace” of interconnected cells for each memory. Webs of fine-tuned synapses seem distressingly more erasable than molecules.
But if truth be told, we still don’t know much about how the human brain stores information, or how it is able to call up information at will. Many neurologists believe that we are on the verge of substantial breakthroughs in understanding the biochemistry of memory, but my guess is that progress will be painfully slow.
There are perhaps as many as a hundred billion nerve cells in the human brain. Each cell is in communication, through a treelike array of synapses, with thousands of other cells. The possibilities of interconnection are staggeringly intricate, and the problem of understanding them correspondingly difficult.
Faced with the intractable complexity of the human brain, many memory researchers choose to work with simpler organisms. The California sea snail Aplysia, a creature about the size of the human hand, has been a popular candidate for investigation. The sea snail’s nervous system contains only about 18,000 cells, and many of those are big enough to see with the naked eye. The snails can be trained to exhibit certain behaviors in response to stimuli, and changes in their nervous systems can be monitored with relative ease.
Doubtless, much can be learned by studying sea snails, but there are vast differences between Aplysia’s “brain” and that of humans. Do sea snails suffer embarrassment when they forget a name? Do sea snails put important papers in a safe place and then forget where they put them? Has a sea snail ever forgotten its mate’s birthday? Can a creature with only 18,000 nerve cells be said to properly remember at all?
A perplexing problem
No more challenging riddle remains to be solved by science than how it is that from a lifetime of experiences we can summon up remembrances of things past — sights, sounds, tastes, smells, ideas, skills, convictions. No one but the most obdurate mind-body dualists doubt that memories are somehow physically stored in that vast electro-chemical system called the brain, but exactly how and where remains uncertain.
Meanwhile, I start down the slippery slope of forgetfulness. Do nerve cells die with age, not to be replaced, taking memories irreversibly with them? Or is it only the ability to process memories — to call them up from the data banks — that undergoes alteration with age? A huge body of scientific literature has accumulated on the subject of memory and aging, but the answers remain frustratingly elusive.
There was one more thing I wanted to say about memory. Now, let’s see, what was it? It’s just on the tip of my tongue.
