Choline crystals. Choline is B fatty acid primarily involved in the production of neurotransmitters in the brain that regulate mood, appetite, behavior, and memory. It is most effective in the phosphatidyl choline form. Studies indicate that it improves cognitive performance. Blood levels of choline decrease during prolonged exercise. Choline is the basic constituent of lecithin that is found in many plant and animal organs and is readily made in the body. Polarized LM X50. Dennis Kunkel Microscopy, Inc.

The sweet, searing pain of your first tattoo, and the knowing ache of your second. Your dog limping into the corner of the room, collapsing after a lifetime of dutiful, loving service. The rush of an exuberant "I do!"... and the draining divorce that followed.

It's all there, somewhere - and seemingly forever. We can't control those memories; they control us. One of the great paradoxes of memory is that even the most painful recollections shape us, for better and worse. Which leads to the obvious and probably quixotic question: Why can't we just choose to forget the bad stuff? Flush it like we do a hard drive or a photo archive?

Research into memory has proliferated in the past decade, as scientists pursue new avenues, from the way memories are formed during sleep to the effects of fear-association and shock therapy, but the news is not encouraging for fans of Eternal Sunshine of the Spotless Mind. Selectively deleting memories may never be possible, and judging from a batch of new findings, may not even be all that desirable.

The main obstacle is how memories are stored and entwined. When we learn or experience something new, be it a blissful summer weekend on Cape Cod or a new mathematical equation, the data get stored in individual neurons in our brain. But, depending on the type of memory being formed, certain neurons will communicate with those in other parts of the brain. Your grandmother's sweet potato pie has neuronal connections to the olfactory components of your brain (smell) and its visual components.

A newly formed memory grows stable over time, until it gets planted as a long-term memory, more or less impermeable to change.

But that doesn't mean we can't change how we feel when we look back on past events. Rebecca Spencer, who researches the formation of memories via sleep at the University of Massachusetts, Amherst, says that in her lab, they've found that by asking a person to recall a specific memory, and subsequently introducing a new set of cues to associate with that memory, they can replace preexisting emotions with more positive ones. For example, a soldier may come back from a war terrified of any loud noise, having "learned" that loud noise usually means something terrible has just happened. This is a typical symptom of PTSD, and can be debilitating - after all, it's no good to have a major stress response to a dropped garbage can lid. There's no way to delete the soldier's memories of bullets fired and bombs exploded. But mental health professionals can remove the emotional component of those memories.

No, it's not as exciting as being able to delete memories from a pulldown menu, but this method of curing people of fear-based mental illness does less harm than complete deletion. Not that we haven't tried: After World War II, the Veterans Administration attempted to delete memories of about 2,000 shell-shocked soldiers by performing lobotomies, thinking that they could just excise the traumatized parts of their brains. It didn't work.

Consider how memories are structured. The web of neurons held together by synapses doesn't encode one isolated incident after another. Deleting the memory of an apparently isolated event actually cuts out huge chunks of time - months, even years - from a person's memories. This is problematic for obvious reasons, so it is much safer to keep the memories and instead attempt to erase the fear associated with the bad ones.

We could do that through extensive therapy, as Spencer has shown in her studies in Massachusetts. But we may be edging closer and closer to a chemical cure, at least according to a study published early last year. Dr. Kerry Ressler and his colleagues at Emory University, University of Miami, and Scripps Research Institute took some test mice and really stressed them out - to the point where they essentially had PTSD. These shell-shocked mice would freeze in place whenever they thought they were in danger (not an advantageous move, for obvious reasons).

But Ressler's team had a cure: a compound called SR-8993 that almost completely forestalled the fear responses in those mice. In other words, mice administered the compound began to act normally around danger again. The ultimate goal is for fearful memories to be seen as just bad memories, like that of stubbing a toe or missing the bus - memories we all have, but which don't take control of our lives. Ultimately, Ressler hopes compounds like SR-8993 will make their way into human trials and eventually become approved pharmacological approaches.

In the meantime, it's important to remember that deleting memories isn't picking the tomatoes out of your salad; it's removing the salt from your soup. If you want to remove one ingredient, you have no choice but to destroy the dish entirely.