Neuronal firing disproves activity-silent hypothesis

Understanding how the human brain stores information and later uses it to complete various tasks has been a long-standing goal of neuroscience and psychology research. Past studies have identified different types of memory processes that have distinct roles and characteristics.

One of these, known as working memory, entails the storage and manipulation of important information for short periods of time, particularly information that is helpful for completing reasoning tasks or to make decisions in the short-term. Findings suggest that this temporary storage of information is associated with the continued and persistent firing of specific neurons in the brain.

Most past studies focusing on working memory processes relied on experimental tasks that require participants to prioritize and memorize all items they are presented with.

In contrast, very few have tried to understand how the human brain stores “unattended” items, or, in other words, stimuli that are momentarily not in focus and not immediately relevant for completing a task at hand.

Researchers at the Polish Academy of Sciences, SUNY Upstate Medical University, Military Hospital in Ełk and Wroclaw Medical University set out to test the validity of a theoretical model suggesting the existence of an “activity-silent” mechanism that is responsible for storing unattended memories.

Their findingspublished in Nature Human Behaviourdisprove this theoretical prediction, instead showing that the brain’s storage of unattended memories is also associated with the firing of neurons.

“We know that items in our working memory—our thoughts—are represented by the activity of specialized neurons,” Jan Kaminski, senior author of the paper, told Medical Xpress.

“Whenever we need to maintain something in mind, specific neurons in our brain increase their firing rates. For instance, when memorizing a phone number, certain neurons temporarily become more active as they encode this information.

“Recent studies, however, have suggested that if a memory item is not actively attended to, the activity of these neurons drops back to baseline. For example, when we must remember a phone number but temporarily perform a different task.”

Recent experimental evidence hinted at the possibility that unattended items are maintained in the brain via an activity-silent mechanism that is markedly different from the one supporting the storage of important information during working memory tasks. Nonetheless, most of this evidence was collected using non-invasive imaging techniques, such as electroencephalography (EEG) and functional magnetic resonance imaging (fMRI).

Although EEG and fMRI are widely used to conduct neuroscience research, they both record the “average” activity of hundreds of thousands of neurons in the human brain. As a result, studies relying on these imaging techniques might have failed to pick up the activity of a few neurons, as it was averaged with the inactivity of several other neurons.

“Our lab specializes in directly recording neural activity during invasive clinical procedures, such as electrode implantation in the human brain for epilepsy monitoring,” said Kaminski.

“This provides a unique opportunity to test the activity-silent hypothesis directly. As part of this study, we recorded from neurons in the temporal lobe known to be involved in working memory.”

The participants who took part in the team’s study were shown two images and asked to remember both images but focus on only one of them for the first part of the trial. Subsequently, they were either instructed to keep their attention on the same image or to shift their focus to the image that was previously unattended.

“This experimental designknown as the double retro-cue paradigm, has been employed in prior research,” explained Katarzyna Paluch, first author of the paper.

“To record brain activity, we used an invasive clinical procedure called intracranial EEG, where electrodes are surgically placed directly into patients’ brains for medical purposes, like epilepsy monitoring. This allowed us to record the activity of individual neurons.”

Kaminski and his colleagues later examined the activity of neurons in the temporal lobe when participants shifted their attention to one or the other image. This allowed them to better understand how unattended images were stored in the brain.

“To our surprise, we found that even the image outside the focus of attention was still actively represented by neuronal firing,” said Kaminski.

“This contradicts the activity-silent hypothesis and indicates that unattended items in working memory remain actively represented. Our findings suggest that a larger portion of our working memory—the mental workspace or ‘mind’s sketchpad’—is actively represented by neuronal activity.”

The findings of this study suggest that thoughts and other information that is outside a person’s immediate focus of attention is still represented by some neurons that remain active. In other words, the storage of this unattended information does not rely on activity-silent mechanisms, as some earlier works had suggested.

In addition to improving the present understanding of human memory processes, the results gathered by Kaminski and his colleagues could inform the future treatment of some psychiatric disorders that are sometimes accompanied by working memory deficits. These include attention deficit hyperactivity disorder (ADHD), obsessive compulsive disorder (OCD) and schizophrenia.

“For example, our findings raise the possibility of developing neural implants or electrical stimulators to help maintain specific information in working memory, expanding the scope of therapeutic interventions,” added Kaminski.

“Our lab is now continuing to focus on understanding working memory and its underlying neural mechanisms through direct recordings of neuronal activity in humans.

“In our future studies, we plan to explore how the brain switches between protecting current contents of working memory and encoding new information, which is critical for flexible cognitive functioning.”

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