What do we think of when we think of sleep? Part of it is surely the absence of thinking itself, a shutting down of the hectic business of continually interfacing with the surrounding world—silencing all of the myriad inputs that we're otherwise forced to deal with for 16 or 18 hours a day. We may dream, we may even walk around or raid the fridge, but it is mostly a realm of darkened peace.
And yet maybe we're not quite so free in our states of sleep. New open-access research published in the journal Current Biology describes experiments in which unconscious subjects were tasked with word classification problems, with the goal of testing to see whether or not brains might remain responsive even when the subjects were unresponsive (asleep).
Indeed, the findings indicate that we're not quite so shut-down as was previously thought. According to EEG readings of neural activity, the brains processed the tasks it was asked to perform, and were even able to reach the correct conclusions.
The experiment began while the subjects were awake. They were presented with sequences of words, and were asked simply to press one button with one hand if the word was an animal, or a button in their other hand if the word was an object. During this process, the researchers were able to map out the brain's activity patterns for either response, such that, eventually, they were able to see a subject's answers without the need for an actual button: one part of the brain flashes for animal, and another part flashes for object.
What the research team found was that the brain continued to respond to the classification problems, albeit more slowly, when it was in a state of sleep. In neuro-speak, the team discovered a "covert activation of [the] motor cortex even when those stimuli are masked and presented below the threshold of consciousness." The brain anticipates the button.
"This explains some everyday life experiences such as our sensitivity to our name in our sleep, or to the specific sound of our alarm clock, compared to equally loud but less relevant sounds," Sid Kouider, a cognitive neuroscientist at the Ecole Normale Superieure, said in a statement. "Far from falling [into] a limbo when we fall asleep, parts of our brain can routinely process what is going on in our surroundings and apply a relevant scheme of response." So: perception without awareness.
This new research adds to a growing body of work that demonstrates the unconscious brain's capabilities. Notable examples: a 2012 study finding that sleeping brains were capable of learning new information (associating different smells with different sounds); a 1998 study showing how the sleeping brain responds differently to various patterns of related and unrelated word pairs; a 2007 meta-analysis concluding that the unconscious brain maintains a significant degree of conscious organization.
We're left to consider the possibilities of this activity and potential activity. If it's there, are we currently wasting that potential? While any interactivity with the sleeping brain must remain automated rather than executive (or else it wouldn't be a sleeping brain, of course), this might not exclude the possibility of sleep-learning. Kouider cautions, however, that "Research focusing on how to take advantage of our sleeping time must consider what is the associated cost, if any, and whether it is worth it."
We still need our sleep to be sleep, after all.