A growing number of people who have recovered from COVID-19 say their minds still don’t feel the same.
They struggle to remember things, lose focus, and tire quickly are symptoms often described as “brain fog.”
Scientists have long documented these lingering effects but have not fully understood why they occur. Now, new research from Japan may offer a biological explanation.
Japanese study finds changes in brain activity
Researchers at Yokohama City University used advanced PET imaging to study the brains of 30 patients living with long COVID and compared them with 80 healthy volunteers.
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According to Science Daily, the team observed abnormal activity in AMPA receptors which is the brain’s most common type of receptor for transmitting signals between nerve cells.
These receptors play a crucial role in learning and memory.
The study, published in Brain Communication in October 2025, found that patients with brain fog had significantly higher receptor density.
The degree of change closely matched how severely they reported memory and concentration problems.
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Lead researcher Takuya Takahashi said the findings show that brain fog should be recognized as a genuine medical condition, rather than dismissed as a vague or psychological complaint.
Norwegian data support the findings
In Norway, scientists have tracked similar symptoms since the early days of the pandemic.
Dr. Arne Søraas, who leads the Norwegian Corona Study at Oslo University Hospital, said his team began noticing cognitive problems among patients as early as the summer of 2020.
"It is exciting that other researchers can now actually “see” what is happening in the patients’ brains", Arne Søraas told the Norwegian Forskning.no.
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His ongoing study, which follows thousands of participants, shows that nearly 12 percent now report memory problems, up from just 3 percent in 2020.
The symptoms are often persistent and affect both vaccinated and unvaccinated individuals, even after mild illness.
A step toward treatment
Currently, no specific medication exists to treat post-COVID brain fog.
However, Takuya Takahashi hopes the new findings will guide the development of diagnostic tools and therapies that target the underlying neurological changes.
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Arne Søraas agrees that understanding what happens inside the brain is key to helping long COVID patients recover.
Sources: Forskning.no, Yokohama City University, and Science Daily.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.