Can you get amnesia from a blow to the head?
After years and years (and years) of watching TV, I've begun to wonder: Can people really get amnesia from a bonk on the head? Can they get their memories back from another bonk on the head? What really causes amnesia and what things do you forget? Is there any way I can induce amnesia without a cranial accident of some sort?
We need to clarify your question, Zeb. It's not whether you can get amnesia from a blow to the head (or, for that matter, from some purely psychological trauma). It's whether you can get Hollywood amnesia, defined for our purposes as forgetting all or part of your past with no impairment of your ability to deal with the here and now--from which condition, moreover, you can recover in time for the final reel. The brain being the mysterious organ that it is, I won't say that such things never happen. But they don't happen a whole hell of a lot.
Amnesia following violent but nonpenetrating head trauma, known in the business as "closed-head injury," is common and reasonably well understood. Typically the victim suffers two types of memory defect: retrograde amnesia, forgetting pretrauma events, and anterograde amnesia, not retaining what's happening now. Retrograde amnesia is thought to be a failure of the brain's playback mechanism--the memories are still in there, you just can't get at them. The length of the period rendered inaccessible can range from minutes to years. Some amnesiacs remember autobiographical information but not public events, though more often it's the other way around. Anterograde amnesia, by contrast, is a failure of the recording mechanism--new information never gets stored away. Memories that don't get recorded are gone for good, but it's possible to get lost pretrauma memories back.
Don't count on it, though. Memory loss due to a bonk on the head, stroke, alcoholism, etc, is considered "organic"--there's some definite physical cause. Usually that cause involves brain damage; usually the damage and the memory loss are both permanent. (In such a case another bonk will probably just make things worse.) Sometimes, admittedly, organic memory loss turns out to be temporary. I have here a 1995 paper charmingly entitled "The 'Petites Madeleines' Phenomenon in Two Amnesic Patients"--petites madeleines, as the Teeming Millions surely recall, are the little cakes that trigger a flood of memories in Proust's Remembrance of Things Past. In the first case, an artist had a stroke and suffered severe retrograde and anterograde amnesia, forgetting his family, his friends, his work--everything but (brain damage can be so cruel) the faces of famous politicians. A year later the patient was back in the hospital to have a pacemaker implanted. While lying on the operating table he suddenly recalled having been in a similar position 25 years earlier during surgery for a hernia. With that his memories poured back, starting the guy on a two-day talking jag while he got things sorted out. Upshot: total recovery of past memories, although the ability to form new ones was still impaired. In the second case, a young man wrecked his car at high speed. Though his injuries seemed minor at first and his short-term memory remained normal, after a few hours his pretrauma memory went completely blank--a rare condition known as pure retrograde amnesia. A month later, while playing tennis (amnesiacs commonly retain or easily relearn old skills), he realized he was making the same mistake he'd made in a match years before. Memories rushed back and he recovered completely.
As I say, though, this kind of thing doesn't happen often--certainly not as often as daytime television would have you believe. Granted, there's also psychogenic amnesia--or, as it was called in Freud's day, hysterical amnesia--which is purely psychological in origin. It only comes a little closer to approximating the Hollywood version, though. One form of psychogenic amnesia is called dissociative fugue--the patient forgets his current identity and wanders off, yet remains mentally together enough to start a new life, sometimes remembering his old self only years later. Often we're dealing here with fragile minds: one study of 25 fugue victims found that most had a history of depression, and many were compulsive liars or had attempted suicide.
Not all psychogenic amnesiacs are unstable, though--in one recent case, a man without apparent physical or psychological problems suffered a two-day bout of anterograde amnesia after a vivid dream that his son had joined the marines and died in Iraq. However, the common belief that traumatic experiences alone often lead to memory suppression is a questionable one--some studies of World War II concentration camp survivors, for example, have found little evidence of amnesia that exceeds normal forgetfulness. If you're a screenwriter, though, not to worry--so long as movie pistols never run out of bullets and movie heroes can survive leaps from tenth-story windows, few moviegoers are going to beef about the old "bonk on the head" trick.