In contrast to general anesthesia, in which the goals are well-defined — no pain, no awareness, no memory — conscious sedation is something of a gray zone, where the boundaries between consciousness and unconsciousness are blurred, where patients are sometimes expected to be semi-awake and responsive, and where their distress could potentially be ignored, because amnesia usually sets in.

“With conscious sedation, I think physicians recognize that quite a lot of the time their patients will actually be distressed, but they’re relying on the fact that most are not going to remember it,” Dr. Davidson said. It is obviously not the goal, he added, “but it’s hard to get the dose right.” As he wrote in the Journal of Medical Ethics, “In many cases, the patient will wince or wriggle during the procedure — some may even tell you it hurts.” But “the vast majority won’t remember a thing,”

In my case, plagued by my distressing memories, I was diagnosed with acute stress disorder, a transient form of PTSD. I spoke to a therapist, sought clarification from my physician, and am looking for ways to avoid a negative experience the next time.

But what if, like most people, I had actually forgotten that panic and struggle on the table? Might I still have experienced the nightmares and anxiety, with no explanation for them? And might someone with a tendency toward depression or anxiety trace their problems back to forgotten experiences under sedation?

“There’s plenty of evidence” that even without an explicit memory of surgery, humans can form implicit or subconscious memories under anesthesia, said Dr. Aeyal Raz, an anesthesiologist at the University of Wisconsin. “There is a trace of the event, a memory, left in the brain. However, it cannot be accessed with conscious thoughts and the person does not recall the learning event. Nonetheless, it may affect future feelings and behavior.”

“Research in psychology suggests that even this very rudimentary memory activity may have profound effects on behavior or emotion,” wrote Jackie Andrade, a psychologist at the University of Plymouth, in a review of the topic. “It seems plausible that negative experiences during surgery could reduce patients’ well-being on recovery, but it is hard to prove,” she told me.

But Dr. Davidson feels the evidence for implicit memory after anesthesia is “very mixed, and it is a very long bow to pull to say it has long lasting effects.”