Just walking past the doorway where you stubbed your toe, or using the door you once shut your finger in, can make you wince at the memory. In 2006, researchers at the University of Arizona College of Medicine claimed this is because of a molecule called PKMzeta, which they argued strengthens the connections between neurons in the brain when we learn something new – and also creates physical sensitivity after a painful experience. Using mice, they demonstrated that, if they blocked this specific molecule in the spine, they could erase the extra sensitivity to pain in the mice. However, other researchers have since questioned this finding, showing that mice born without PKMzeta could still feel pain.
What about the uncomfortable or traumatic pain that can be associated with some medical procedures? When colonoscopies are done under conscious sedation, patients are sometimes given a drug called midazolam which makes them feel less anxious and also promotes anterograde amnesia – the inability to form new memories. Patients might wince or wriggle with discomfort during the procedure, but the hope is that they won’t remember any pain or feel disturbed by the procedure afterwards.
Some have questioned the ethics of this approach – and its implications. In a paper published last year Andrew Davidson, an anaesthetist at the Royal Children’s Hospital in Victoria, Australia, warned that although drugs such as midazolam can make it harder to form conscious memories, they can leave implicit memories intact. These are memories that affect us even though we can’t consciously recall them. So we can button a shirt or find our way along a familiar route to the station without thinking about when we actually made those memories. Troublingly, Davidson suggests that even if you don't remember a colonoscopy, you might feel “strangely uncomfortable when walking past a garden hose”. Would it be more ethical if patients were warned in advance that this might happen?
It’s even been suggested that to mitigate the psychological trauma of the very rare situations where patients wake up while under a general anaesthetic, they could be given midazolam as soon as the anaesthetist realises, to disrupt the formation of memories of the event. But this would only erase memories from the point when the drug was administered, not from the point of waking.
The ethics of the use of drugs like this will continue to be discussed. The very fact that they are being considered underlines the fact that – unfortunately – for people who have given birth or experienced other kinds of agony, it can be a myth that pain has no memory.
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Update: This article has been amended to reflect the evidence questioning the importance of the molecule PKMzeta.