Is self-report brain fog after mild COVID-19 truly induced by COVID-19 infection? A longitudinal fNIRS study

Introduction Brain fog (i.e., subjective cognitive deficits) is one of the most common long-lasting symptoms post-COVID-19. Cognitive deficits and psychological factors (such as depression) are possible causes of brain fog. This study is to determine whether brain fog post-COVID-19 results from cognitive deficits or depression induced by COVID-19 infection.Methods We implemented psychological (depression scale) and behavioural measures (dual task: subtraction while walking) with simultaneous functional near-infrared spectroscopy (fNIRS) recording in 25 healthy young adults pre-, 3, 7 and 14 weeks post-COVID-19. Depression level, dual task performance and fNIRS results were compared among sessions. The relationships between brain fog severity, depression and dual task performance were assessed.Results Depression level and dual task performance remained unchanged post-COVID-19. Brain fog severity was positively correlated depression at all sessions (p’s < 0.05). Brain activation was significantly reduced at all sessions post-COVID-19 (p’s < 0.001), while global efficiency was significantly reduced at 7 weeks post-COVID-19 (p = 0.003). Changes in left somatosensory and motor cortices activation were inversely correlated with changes in walking cost (p’s = 0.005 and 0.007, respectively), while changes in global efficiency were associated with changes in walking cost (p = 0.006) from pre- to 7 weeks post-COVID-19.Conclusions Mild COVID-19 infection does not lead to apparent cognitive deficits or depression, although brain activation and global efficiency are reduced post-COVID-19. Brain fog is likely to result from depression rather than cognitive deficits induced by COVID-19. People should pay attention to their psychological states during COVID-19 pandemic.