Brain Dynamics When Alertness Fluctuates but Attentional Demands Persist
Alertness waxes and wanes throughout the day. Some tasks demand our unwavering attention, and when alertness dips, we still need to stay focused and engaged. This thesis examines how the human brain functionally reorganises when alertness levels decline but attentional demands persist. We focused on sustained auditory and visual attention using functional magnetic resonance imaging (fMRI). First, we examined how sustained auditory attention impacts brain functional reorganisation during sleep onset. We compared how brain dynamics change when alertness decreases during active and passive levels of auditory task engagement. We found that when alertness decreased, passively listening to auditory tones led to increased fMRI synchronisation in the parietal lobe, while actively performing an auditory task led to increased frontoparietal synchronisation. We also found that with declining alertness, passive listening, but not active task engagement, was associated with widespread increased thalamocortical synchronisation. In contrast, active task engagement, but not passive listening, led to increased synchronisation between the auditory cortex and the rest of the brain. These findings suggested that sustained auditory attention during sleep onset recruited auxiliary brain resources. Next, we sought to confirm that these alertness-related changes in brain dynamics directly contributed to cognition. We shifted our spotlight to sustained visual attention. To examine long windows of data, we created and validated an fMRI-based alertness classifier. Then, we examined how stimulus-driven brain dynamics reflect alertness level during passive movie-viewing in a large fMRI database. We found that with declining alertness arose a more functionally distributed visual attention brain network. We also found increased thalamo-prefrontal and frontoparietal stimulus-driven brain dynamics with declining alertness. Furthermore, we found functional relationships between frontoparietal and thalamo-prefrontal dynamics, as well as prefronto-insular and thalamo-insular dynamics. This work showed that different levels of alertness procured different stimulus-driven brain dynamics on minute-long timescales, and it is the first study to date to examine within-individual fluctuations during naturalistic neuroimaging. Altogether, this thesis reveals that sustained attention during low alertness is followed with additional frontoparietal, thalamocortical, and sensory cortex functional reorganisation. We show a strong justification for an examination of human cognition as it relates to alertness. Future directions and applications are discussed.
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