Refuting silent memory: the brain actively processes inattentive information

Understanding how the human brain stores information and then uses it to perform various tasks has long been a major goal of research in neuroscience and psychology. Scientists have previously identified different types of memory, each with its own characteristics and functions.

One such type is working memory, which involves the short-term storage and processing of important information, especially that needed to perform logical tasks or make decisions in the near future. Research shows that this temporary storage of information is associated with the constant and sustained activity of certain neurons in the brain.

Most past studies on working memory have used tasks in which participants were asked to remember all the information presented to them.

However, very few studies have attempted to understand how the brain stores “inattentional” information—that is, stimuli that are not currently the focus of attention and are not directly relevant to the task at hand.

Scientists from the Polish Academy of Sciences, SUNY Upstate, the Military Hospital in Elk and the Medical University of Wroclaw decided to test the validity of a theoretical model that suggests the existence of an “activity-silent mechanism” responsible for storing inattentive information.

Their results, published in the journal Nature Human Behaviour, challenge this theoretical assumption and instead show that storing inattentional information is also associated with neural activity.

“We know that elements of our working memory – our thoughts – are represented by the activity of specialized neurons,” senior author Jan Kaminski told Medical Xpress.
“When we need to hold something in our mind, certain neurons become more active. For example, when we memorize a phone number, certain neurons temporarily become more active, encoding that information.”
“However, recent research has suggested that if a memory element is temporarily out of focus, the activity of the neurons returns to background levels – for example, when we need to remember a phone number but temporarily switch to another task.”

These assumptions were based largely on data obtained using non-invasive methods such as EEG and fMRI. However, these methods measure the average activity of hundreds of thousands of neurons, which means that the activity of a small group of cells can be “swallowed up” by the inactivity of surrounding neurons.

“Our lab specializes in directly recording neural activity during invasive clinical procedures, such as implanting electrodes in patients’ brains to monitor epilepsy,” says Kaminski.
“This provides a unique opportunity to test the silent activity hypothesis directly. In this study, we recorded activity from neurons in the temporal lobe, known for their role in working memory.”

Participants were shown two pictures and asked to remember both, but to focus on only one in the first part of the experiment. Later, they either had to continue to focus on the same picture or switch to the previously "out of focus" one.

“This experimental design is called the double retro-cue paradigm and has been used in previous studies,” explains Katarzyna Paluch, the first author of the paper.
“To record activity, we used intracranial EEG, a clinical procedure in which electrodes are surgically placed directly into the patient’s brain, for example to diagnose epilepsy. This allowed us to record the activity of individual neurons.”

As participants switched their attention from one picture to another, the scientists recorded the activity of neurons in the temporal lobe to understand how the brain stores “non-focus” information.

"To our surprise, we found that even the image that the participant was not focusing on continued to be actively represented through neural firing," Kaminski said.
"This contradicts the silent storage hypothesis and shows that out-of-focus items in working memory are also represented by active neural activity. Our results suggest that a large part of our working memory — this mental 'sketch' — is maintained by active neural firing."

Thus, thoughts and other information that is outside the focus of attention are still supported by active neurons, and not by some “silent” mechanism, as previously assumed.

In addition to their fundamental importance for understanding how memory works, these findings may also influence the development of treatments for mental disorders that involve working memory impairment, such as ADHD, OCD, and schizophrenia.

“For example, our results allow us to think about creating neural implants or electrical stimulators that can help maintain specific information in working memory, expanding the possibilities of therapy,” added Kaminski.
“Our lab is currently continuing to study working memory and its neural mechanisms using direct recordings of brain activity.
In the future, we plan to study how the brain switches between maintaining current information and remembering new information, which is critical for flexible cognitive activity.”