When you're getting the kind of surgery that requires being put under, you definitely don't want to be awake for any of the slicing. But a new study shows that when it comes to some general anaesthetics, our brains might be in a more sleep-like state than previously realised.

The discovery suggests that far from being switched off, some parts of the brain are still capable of processing sensations from their environment, even if the patient can't recall any of it on waking.

Researchers from the University of Turku in Finland compared the neurological effects of a pair of drugs commonly used to render patients unresponsive before medical procedures.

Their goal was to determine whether the lack of responsiveness was largely due to the direct influence of the drugs themselves, or whether there was a knock-on effect that sends us into la-la land.

The difference isn't just academic. If it's a knock-on effect, anaesthesia simply triggers an altered state of consciousness rather than flicking our off switch, meaning some conscious processes are still ticking while we're knocked out.

As the researchers say in their report, "unresponsiveness does not equal unconsciousness, as one may have conscious experiences without behavioural responsiveness."

As far as modern day medical science goes, you'd think we'd have it all figured by now. Anaesthesia has been methodically used to knock patients out for about 170 years.

But for the wealth of knowledge we've collected on the topic, details on how these drugs actually change the brain's functions are still pretty vague.

This is in part because we also don't know a lot about how consciousness works. We can map functioning regions of the brain or record the wobble of brain wave activity, but separating cause and effect can be tricky.

Recordings of alpha, beta, and slow or 'delta' waves are routinely used to identify when the brain is awake and asleep.

Rapid beta waves are the 'awake' waves that fire when we're stimulated. Mid-ranged alpha waves hint at the kind of conscious activity we exhibit when we're chilled and contemplative. Delta is your slow, deep sleep wave.

In this study, 47 healthy volunteers were dosed with either the sedative dexmedetomidine or the general anaesthetic propofol before having their brain waves recorded.

Both drugs result in unresponsiveness at a certain dosages, though propofol is a far more powerful anaesthetic.

When barely out cold, half of the subjects in the dexmedetomidine group could be aroused with a brief shake and a loud shout. Surprisingly, 42 percent of those given propofol could also be woken into a groggy state.

Their electroencephalogram recordings showed an increase in alpha and slow waves, suggesting a reversal of deep sleep and a heightening of reflective consciousness.

Later, the volunteers in both the sedative and the anaesthetic groups could recall the event, if a little hazily.

"Nearly all participants reported dream-like experiences that sometimes mixed with the reality," says pyschologist Antti Revonsuo.

The two groups were also played recordings of sentences which ended unexpectedly, such as "The night sky was filled with shimmering tomatoes". Other recordings were completely normal.

Electroencephalogram recordings indicated volunteers who were deeply sedated could still hear the confusing phrases and were trying to make sense of it, even if later they couldn't remember hearing the sentences.

Those on propofol didn't even try to interpret the weird words, with their brain waves continuously snoozing.

That's all great, but when the patients in both groups were played unpleasant sounds they all seemed to be paying attention. On waking, the brains of both groups reacted significantly faster to the noise, as if they'd learned to recognise it.

"In other words, the brain can process sounds and words even though the subject did not recall it afterwards," says anaesthesiologist Harry Scheinin.

"Against common belief, anaesthesia does not require full loss of consciousness, as it is sufficient to just disconnect the patient from the environment."

While previous research has matched brain waves with states of awareness under anaesthesia, the fine control over the dosing in this study allowed researchers to identify changes due to the drugs and those due to changing consciousness.

The work falls in line with other studies that also suggest anaesthesia doesn't switch off parts of the brain, as much as prevents them from communicating freely.

If this sounds like nightmare fuel, take a deep breath – this isn't implying we can feel the cut of the surgeon's scalpel while lying trapped on a table.

Rather, much like natural sleep, our brain is still half-paying attention, even if our awareness is switched off.

This research was published in Anesthesiology.