Every experience is potentially a memory, but in most cases, what distinguishes experiences that endure in memory from those that pass without the note is that when they occurred, they either created or coincided with higher-than-normal levels of emotion.
Powerful emotion has an important role in the laying down of memories. Emotion is generated in a primitive part of the brain called the limbic system, but its effects radiate to the areas of the brain that process sensations and generate thoughts. One effect of emotional arousal is to direct attention to the events that provoke it, and attention, in turn, amplifies the brain activation associated with the event. Attention is effectively the first stage of laying down a memory.
For example, if you lazily surveyed a country view, your brain would be registering many different things – the sky, the clouds, the green of the fields, the twittering of the birds. None of these perceptions would be creating a particularly vivid experience – and the impressions of them in your brain would fade into oblivion almost as fast as they arose, leaving only a faint, general impression, like the backdrop to a painting. Later, you might be unable to bring any particular element to mind – the scene would have merged with your general memory of what it is like to be in the countryside. If, however, in the middle of this reverie, a fox ran across your path with a still-flapping bird in its mouth, the sight would provoke a sudden, strong emotion – horror, perhaps, or excitement, or even pleasure at witness signature in the raw. Later, the sight would stand out as a distinct memory. If it made a really strong emotional impression, it might come back from time to time, like a scene captured in a flash photograph.
Chemicals in the brain
We understand intuitively that exciting events are more clearly recalled than routine occurrences. One reason for this is that emotion changes the chemical balance of the brain. Excitement, for example, pushes up the levels of brain chemicals acetylcholine and noradrenalin in certain areas of the brain. At the same time, pleasure increases the production of dopamine, and disgust provokes the release of glutamate. These changes help to create links between whatever neurons happen to be firing together at the time. Noradrenalin, for example, encourages the growth of axons – the snake-like ten drills that reach out from one neuron to connect with another, and glutamate triggers the release of adhesion molecules that bind neurons together. Hence, emotion helps to preserve the pattern of neural activity associated with an event, making it more likely to be replayed in the future.
Emotions in recall
Emotional memories are most easily recalled when a person is again emotional in which the memory was laid down. That is why pleasurable events tend to pop back into memory during similarly enjoyable experiences and past losses are felt again when a person is grieving for some new loss. The emotional experience itself is laid down as a memory, and the more a particular experience is replayed, the more strongly it becomes engraved into the mind.
There are some clear advantages to a system that automatically preserves emotionally charged events; after all, these are likely to be the more significant events in our lives. From a practical viewpoint, retaining knowledge of such things can help to guide present and future actions. Frightening or dangerous events are likely to be ones that we will wish to avoid in the future and recognition of similar situations in the making acts as a warning. Conversely, memories of pleasurable incidents are likely to guide us to seek out more of them.
The Birth Of a Child
Jeremy vividly remembers the birth of his first child, William. ‘I was abroad when my wife went into labor – two weeks earlier than expected and I have a series of snapshot memories, almost like a cartoon strip, cf the frantic dash home. But the brightest and sharpest memory – so vivid that I can actually see it in space sometimes, as though it is displayed on an illuminated billboard – is of walking into the delivery room and seeing my wife with this little bundle of flesh on her breast. he looked up. He smiled, and all the stress of the last few hours disappeared. I can see that moment so clearly that I could tell you the position of every/ sweat-streaked hair on her head. It will never fade, and when I feel low, I sometimes call it up, like a mantra, to cheer me up.’
