Kermit the frog is hit by a taxi. He no longer recognises friends or even remembers his own name. When he later mocks the idea of a pig and a frog falling in love, Miss Piggy punches him. After recovering from being knocked out a second time, his memory returns.
The plot device of a second bang to the head relieving the amnesia caused by a first is far from unique to the film The Muppets Take Manhattan. It has also been used in Laurel & Hardy and Tarzan the Tiger, among others. I’m not suggesting we should look to the Muppets for accurate portrayals of the impacts of neurological conditions, but it’s an idea that many people believe to be realistic. In a study conducted in the US in 2004, almost 42% of respondents agreed with the statement: “Sometimes a second blow to the head can help a person remember things that were forgotten”. On the other side of the Atlantic in Britain, 26% of those surveyed believed it is true.
But useful as it is to scriptwriters, it is in fact a myth. A third of those who suffer brain injuries do experience so-called “islands of memory”, meaning they can only remember some things. The symptoms can last anything from minutes to months. They might get confused, falsely believing they are at work, late for an appointment or even that they’re being held prisoner.
Amnesia in real life is not as it is portrayed in films, where we see people waking from comas having forgotten their entire past. This can happen, but it's rare. Cases like these, and more common ones where individuals suffer partial-memory loss, are called retrograde amnesia. More common still is anterograde amnesia, which means people have difficulty forming new memories of the things that happen after their accident. This is what happened to HM, the most famous case study in the history of memory research, but he still knew who he was and where he grew up.
Whichever type of amnesia a person experiences following a brain injury, a second blow to head is not going to repair the initial damage. Unfortunately it’s not like banging a television to get it working again. However, it is true that people who suffer one traumatic brain injury are statistically more likely to have a second. A person’s attention or balance might have been affected in the initial accident, making a second more likely. For others of course, it is simply the fact that they continue to take part in high risk sports or have dangerous jobs, which makes them more likely than the average member of the public to sustain another injury.
A second blow to the head not only fails to cure the symptoms of the first, but could leave the brain more vulnerable and increase the likelihood of a second injury being fatal. This is known as second impact syndrome, but it is based on such a small number of cases that the diagnosis is controversial. There have been a few isolated cases of young people returning to sport after recovering from a concussion, getting hit on the head for a second time, and suddenly dying. These incidents have driven the guidelines in the National Football League in the US for the length of break a player must take before participating again after they’ve been knocked unconscious. Others, such as the neurologist Paul McCrory, of the University of Melbourne, believe second impact syndrome to be a myth. He doesn’t doubt the seriousness of the situation, but questions the evidence connecting these deaths to the initial injuries. Instead he thinks these sudden deaths are caused by a swelling across the brain due to the second injury alone.
Whoever is proved right in the long-run, it is clear that second injuries don’t help with amnesia. But there is one other condition which might explain where we get the popular idea that a second accident can bring memories flooding back. Patients with a rare and poorly understood psychiatric condition known as a dissociative fugue state, or simply fugue, can forget everything about their lives, including their own name, following an extremely traumatic event.
Occasionally individuals with the condition turn up in a new town, with no idea who they are. Eventually their memory returns, and it’s not understood exactly why. Afterwards their recollections of the time when their memory was absent might remain a blur. This fits in better with the pattern you tend to see in films, and may explain the origin of the myth. But neither the initial loss of memory, nor its recovery are caused by a physical blow to the head.
So while the dramatic potential of complete memory loss sandwiched between two accidents is unquestionable, its basis in reality certainly is.
You can hear more Medical Myths on Health Check on the BBC World Service.
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