ABSTRACT
Participants and Methods: Fifteen healthy male volunteers (mean age 27 .4 years) meeting MR inclusion criteria took part in the study. Subjects were performing a sustained attention task in a MR scanner five times during the day: at 6 am, l O am, 2 pm, 6 pm, and 1 0 pm. Each session was combined with 9 blocks (lasted 30 sec each) : 5 blocks of fixation point presentations (control conditions) and 4 blocks of targets presentations, each containing 1 8 stimuli (task conditions) . Subjects were instmcted to react, by pressing a button, when two identical words appeared sequentially one after another. They were asked to identify the word's semantic meaning and ignore the color of ink in which the word was printed. MR imaging was performed on the 1 . 5 T General Electric Signa scanncr. Images were analyzed with AFNI software . Interaction effect between times of day was detected using ANOV A. Results : The analyses revealed significant diurnal differences in activations in brain regions linked with alerting (parietal lobe BA 40), orienting (frontal eye fields - FEF), and executive attention subsystem (fronto-insular cortex FIC, presupplementary motor area - pre SMA, basal ganglia, dorsolateral prefrontal cortex - DLPFC and ventrolateral prefrontal cortex - VLPFC) . Diurnal patterns of neuronal activity of three attention subsystems showed similar profiles with significantly higher activations observed at 6 am in comparison to the other times of day. In case of right BA 40, right and left FIC, activities at 6 am were significantly higher than activities measured at 10 am, 2 pm, and 1 0 pm. Conclusion : Observed diurnal profiles appear to present significantly higher levels at 6 am. There is also a visible, but not statistically significant increase of activation at 6 pm in almost all neuronal structures showing diurnal variability . Therefore, both 6 am and 6 pm seem to be related with higher effort invested in perfonning cognitively complex vigilance task.
