Relationships between changes in frontal-occipital coherence and respiratory depression by morphine. There was a significant relationship between change in F4-M1 and O2-M1 coherence due to morphine and respiratory rate depression for frequency band β₂ (R = 0.745, P = 0.021, n = 9), but not for δ₂, δ₁, θ, α, and β₁ bands. These results suggest that a reduction in frontal-occipital coherence is associated with respiratory rate depression. Gray area indicates 95% CIs of the linear regressions. *Significant regression with P < 0.05.