Scott LaFee, Contributor

"You never know when you are making a memory."

- Ricki Lee Jones, singer

In fact, there's almost never a time - or at least a waking moment - when you're not.

"Even if you try to clear your mind, say by listening to a song, you're making memories about that song," said John Wixted, chairman of the Psychology Department and a memory researcher at University of California, San Diego.

To be sure, the memory may be fleeting, the song soon forgotten. But that, too, is part of the marvel of memory. Why do we remember what we do? Why do some memories stick, but others do not? Why can't we recall the first few years of life, when our brains were like sponges, yet we retain other recollections for as long as we live? Such questions have long been the province of philosophy, but science is increasingly offering definitive, empirical answers. Equipped with new imaging technologies and an ever-fuller understanding of brain biology, researchers are peeling back and parsing memory's mysteries.

The long view

In his book, The Odd Brain, anthropologist Stephen Juan writes that the average brain on the average day generates about 70,000 thoughts. Many of these thoughts - including those that never rise to the level of consciousness - become memories.

Memories are categorised in many ways, most broadly as either short term or long term. Short-term memories last only as long as they are useful, from fractions of a second to mere minutes. Long-term memories can last a lifetime. They define us.

"We are not who we are simply because we think," write neuroscientists Larry Squire of UCSD and Eric Kandel of Columbia University in their 1999 book, Memory: From Mind to Molecules. "We are who we are because we can remember what we have thought about."

Long-term memories

Long-term memories, however, begin as short term. But how? Scientists think they know part of the explanation.

Imagine a countryside stroll on a sunny, windy day. You see a rabbit romping through a field of flowers. Your brain is bombarded with sensory input, which is sent to different parts of the brain keyed to different sensory experiences. These brain regions, from the occipital lobe that handles vision to Heschl's gyrus, part of the auditory system, process and encode the new data. As the information moves from neuron to neuron, it alters the structure of synapses it passes through.

"Every time information runs through it, the brain changes. It becomes a new machine," said Terry Sejnowski, a computational neurobiologist at the Salk Institute.

Neurons transmit information (in the form of electrical impulses) along an extension called an axon and receive information through dendrites. In the tiny gap where dendrite meets axon lies the synapse. To overcome the gap, neurons release chemicals called neurotransmitters that bind to specialised receptors on the dendrite, thus relaying the message.

Whenever this happens, the synapse is changed and certain receptors are strengthened. If the information is short term - let's say the sight of that running rabbit isn't particularly interesting - then the synaptical change is temporary. The pattern of neural firings created by the information fades, and the memory is quickly forgotten.

To become a long-term memory requires a permanent alteration in the structure and nature of the synapse. This can happen if a short-term memory is repeated often enough.

"For example, if your cat learns to connect the sound of a can opener with food," said Douglas R. Fields, chief of nervous system development and plasticity at the National Institutes of Health. "But to strengthen a connection between neurons permanently, some growth process must take place that expands the size of the synapse or sprouts new synapses. This requires that genes in the nucleus of the neuron become activated to make the proteins necessary for the synapse sprouting/growing process."

The act of forming long-term memories is really a cascade of biochemical processes. The brain needs time to make a long-term memory, a job involving lots of players: modulating neurotransmitters like dopamine and serotonin, synapse-building proteins and, of course, genes.

Genetics and memory

The precise role of genetics in memory is unclear, but obviously memory varies by individual genome. Some people simply have better memories. A recent study by Dietrich Stephan of the Translational Genomics Research Institute in Phoenix offers one potential clue: It found that participants in a memory skills test who placed in the top 25 per cent possessed a different variant of a gene called kibra than those who placed in the bottom 25 per cent.

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