Abstract
Working memory and conscious perception are thought to share similar brain mechanisms, yet recent reports of non-conscious working memory challenge this view. Combining visual masking with magnetoencephalography, we investigate the reality of non-conscious working memory and dissect its neural mechanisms. In a spatial delayed-response task, participants reported the location of a subjectively unseen target above chance-level after several seconds. Conscious perception and conscious working memory were characterized by similar signatures: a sustained desynchronization in the alpha/beta band over frontal cortex, and a decodable representation of target location in posterior sensors. During non-conscious working memory, such activity vanished. Our findings contradict models that identify working memory with sustained neural firing, but are compatible with recent proposals of ‘activity-silent’ working memory. We present a theoretical framework and simulations showing how slowly decaying synaptic changes allow cell assemblies to go dormant during the delay, yet be retrieved above chance-level after several seconds. , Many everyday activities require you to store information in your brain for immediate use. For example, imagine that you are cooking a meal: You have to remember the ingredients, add them in the correct order, and operate the stove. This ability is called working memory. Researchers have long believed that, whenever we store information in our working memory, we are conscious of that information. That is, if someone asks you, you can report the information. Scientists usually also think that working memory comes with constant brain activity. This means that for as long as you have to remember something, the cells in your brain that code for that information will be active. Trübutschek et al. now show that we can sometimes store information in working memory without being conscious of it and without the need for constant brain activity. As part of the experiment, a barely visible square-shaped target was briefly flashed in 1 of 20 different locations on a computer screen. Human volunteers had to locate the square and indicate whether they had seen it or not. Importantly, they had to guess the location of the target whenever they had not seen it. While the volunteers performed this task, their brain activity was monitored using magnetoencephalography, a noninvasive technique that captures the magnetic fields created by electrical signals in the brain. Even when the volunteers had not seen the target, they could often correctly guess where it had been up to four seconds later, more often than would be predicted by chance alone. The experiment ruled out the possibility that this so-called “blindsight” was simply due to the volunteers accidentally reporting not having seen a target, when they had actually seen it. It also excluded the possibility that the volunteers guessed the location long before they had to report it and simply consciously stored that guess. Instead, without the participant knowing, the brain appears to have stored the target location in working memory using parts of the brain near the back of the head that process visual information. Importantly, this non-conscious storage did not come with constant brain activity, but seemed to rely on other, “activity-silent” mechanisms that are hidden to standard recording techniques. Although Trübutschek et al. show that the brain can unknowingly store information, they did not test other aspects of working memory. Future studies are needed to examine whether the brain can also non-consciously manipulate or use information in its working memory. In addition, future research also needs to investigate the exact mechanism that stores information without constant brain activity.
Darinka Trübutschek
MSCA Research Fellow
I am a cognitive neuroscientist, trying to understand how our brain generates and stores subjective experience. Beyond that, I am also a newly minted mother *2.