Amnesia is a popular plot device in Hollywood; it gives screenwriters a chance to create a mysterious backstory for their character. It can follow their path to discovering their own identity, or it can provide a quirky opportunity for romantic hijinks. But while on-film amnesia offers the chance for drama and laughs, how do its representations stack up to real life? From a spy out for revenge to everyone’s favorite talking blue fish, here’s how four films portray memory loss — and how much they got right.

The Bourne Trilogy

Watching Matt Damon use martial arts and participate in dangerous car chases to take down bad guys would probably be entertaining without any memory loss, but his character’s amnesia adds a layer of suspense to the film.

Jason Bourne is found floating in the Mediterranean with no memory of who he is, where he came from, or why he has two bullets in his back. His near drowning, which would have cut off oxygen to his brain, implies that he’s suffered a traumatic brain injury. And as a result, he’s come down with retrograde amnesia, which has caused him to lose memories of events that occurred before the injury was sustained. He subsequently spends the rest of the film trying to find his origin and how he ended up in the sea.

This “loss of identity and autobiographical knowledge” after a brain injury is unrealistic, according to clinical neuropsychologist Sallie Baxendale, of the Institute of Neurology in London, who wrote a review of amnesia’s representation in movies.

According to NeuroPsiFi, it is anterograde amnesia that’s more common after a traumatic brain injury. This kind of memory loss occurs when a person loses the ability to create new memories after the event that led to their amnesia. Sometimes, it is accompanied by mild retrograde amnesia, but the total loss of long-term memories, not so much. It isn’t totally impossible, however, and if a person were to experience retrograde amnesia alone, it would be because of damage to the frontal lobes, temporal lobes, or posterior regions.

The most realistic part of Bourne’s situation is that he retained his procedural memory, or how to perform tasks, and his general knowledge. For example, he knows how to tie his shoes, operate a car, and perform martial arts. This would be expected with most types of amnesia, according to research.

Finding Nemo

The loveable blue fish voiced by Ellen DeGeneres in Finding Nemo has become so popular that she’s getting her own sequel, and kids refer to the blue tang fish breed as a Dory fish. Part of Dory’s likeability comes from her lack of short-term memory, which plays into amusing situations such as introducing herself several times.

The animated fish, ironically, is one of very few accurate representations of amnesia. Her memory deficit reflects the issues real people face when they suffer from anterograde amnesia, according to Baxendale’s study. She faces profound difficulty in retaining new information, learning, and even knowing where she is going once she sets off.

Dory gets frustrated by her own limitations, as do those around her, but one of the most touching moments of the film represents an important neurological truth as well. Dory tells Marlin, her at-first-unwilling companion, that she remembers things better around him, highlighting the importance of social support and relationships in helping amnesiacs work through their situation.

50 First Dates

Though it makes for charming romantic situations, the clockwork-like amnesia afflicting Drew Barrymore’s character is not exactly possible. The movie describes her condition as only the “loss of her short-term memory,” and depicts the woman losing each day’s memories every time she goes to bed. Adam Sandler’s character takes a liking to the woman, but must introduce himself over and over — hence 50 first dates. Barrymore’s character’s family deals with her condition by devoting themselves to it, helping her live an illusion that time has stopped. This is not only unhelpful in terms of treating her condition, but also quite unrealistic, since most families do not have the time and resources to do this.

There are also no documented cases of amnesia where new memories are wiped clean only during sleep. Though the basis of anterograde amnesia is correct, “50 First Dates maintains a venerable movie tradition of portraying an amnesiac syndrome that bears no relation to any known neurological or psychiatric condition,” Baxendale wrote. Real anterograde memory loss would have Barrymore struggling to maintain information during the day as well as after a night’s sleep.

One part of the movie that has a bit of actual medical background is when Barrymore tells Sandler, after several dates with him, that she doesn’t think she knows him, but that she has been dreaming of him. In a famous case, a man named named H.M., who suffered from severe anterograde amnesia, seemed to build up a subconscious recognition of his doctor over time even though he was never able to explicitly recognize him.

Eternal Sunshine of the Spotless Mind

While many films focus on memory loss as a result of traumatic injury or an illness, Eternal Sunshine of the Spotless Mind is unique in that the characters’ amnesia was voluntary — and very specific. Jim Carrey and Kate Winslet’s characters had been in a relationship that went sour, and Winslet decided to undergo a special therapy to help her erase the entire relationship from her mind. Carrey then tries to follow suit, only to end up chasing himself around inside his own head, trying to safeguard his memories. The unconventional premise made the movie a cult classic, but how feasible would it be to actually erase a memory?

The first, and most important, difference between real memories and the movie’s representation is that the way we remember things is truly a brain-wide process, and memories “aren’t absolute records you can just look up,” according to LiveScience. Every time we recall a memory, we’re actually reconstructing it, physically changing the memory with every firing and rewiring of the neurons involved. Though the couple retains an emotional connection during the movie despite having their memories erased, it’s more likely that the opposite would occur in real life. A study published in Nature suggests it may be possible to “reverse the emotional associations of specific memories by using light to control the activity of neurons.” In other words, by targeting the emotional triggers associated with a memory, scientists may be able to reduce the impact of those memories.