Thanks to memory, the brain is constantly travelling through time, pulling fragments of the past into the present. This ability is key to our entire existence. Without long-term memory you’d have no idea who you are, where you’re from, who you know, or the difference between a safe and a dangerous situation. Short-term memory is just as important. Without it, there would be no point reading this paragraph – by the time you’d reached the end, you’d have forgotten what it said at the beginning.

The following excerpts from The Rough Guide to The Brain, explain some of the common phenomena associated with memory.

Hangover amnesia
Aside from the severe, long-term damage that years of heavy drinking can do to the memory system, alcohol can also affect our ability to lay down memories on a much shorter time-scale. Indeed, many people have woken up after a heavy night to find themselves suffering not only a hangover but also a complete inability to recall the latter stages of the previous evening. In fact, it doesn’t take much alcohol to start to interfere with memory creation – even a couple of drinks will have some effect. Push the levels of alcohol in the blood above a critical threshold, however, and a kind of artificial anterograde amnesia kicks in. Sensory and short-term memories continue to function relatively well, but information ceases to be passed to the long-term store. The reason this happens is that high levels of alcohol cause the hippocampus – one of the most important relay stations in the formation of long-term memories – to malfunction.

Déjà vu
One occasional quirk of the human brain’s perception of time is déjà vu, or paramnesia: the strong and often disquieting sensation that you’ve experienced the current situation before. Déjà vu – which means “already seen” in French – is difficult to study, since it happens unpredictably, but it seems to occur when we briefly misinterpret the present as a recollection. That explains why we usually struggle to pinpoint the conditions under which we experienced the situation before – because it never happened.

Déjà vu highlights the fact that normal perception of time requires a healthy, functional brain. And, though most people experience the phenomenon from time to time, frequent déjà vu is associated with certain neurological conditions – including temporal lobe epilepsy and schizophrenia. This could be due to such conditions causing abnormal activity within an area deep within the temporal lobes – the entorhinal cortex. Essential for memory processing in its own right, this part of the brain is also located close to the hippocampus, another key component in memory formation.

Post-traumatic stress disorder
In general, our ability to recall events is a benefit, but in some extreme cases it can be a burden. Post-traumatic stress disorder (PTSD) is a debilitating psychological condition that can be triggered by an experience that leaves a person feeling helpless and deeply threatened. Soldiers are particularly at risk. Besides insomnia, nausea, irrational anger and feelings of alienation, PTSD can cause extremely vivid flashbacks, or intrusions. Entire scenes can be re-lived, accompanied by perceptions of sound and smell – far more realistic than a typical drug-induced hallucination. Moreover, memories involving emotions – including fear – are formed within the amygdala and, as such, can be associated with hormone release. This means that when a traumatic memory intrudes, it can be accompanied by increased heart rate and respiration, making the flashback all the more real.

Watch the video webcast on The Rough Guide to the Brain.