Sample entropy can be used to assess BOLD signal complexity and predictability in resting-state and task fMRI. Task-relevant brain regions tend to display significantly lower entropy than less relevant regions. This trend appears most distinctly in tasks that involve cortical structures, such as a motor task involving the motor cortex, as opposed to tasks that involve subcortical nuclei, such as an emotion task involving the amygdala. Additionally, there is a strong negative correlation between BOLD signal amplitude and entropy that appears only in task-based fMRI, indicating that BOLD signal, as recruited by a task, becomes more predictable as amplitude increases.
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