This study investigates the neurophysiological basis of aperiodic EEG and the background spectral trend. Using biophysical modeling, the authors demonstrate that aperiodic neural activity can generate detectable scalp potentials and shape broadband EEG features, but these signals do not significantly perturb brain rhythm quantification. The model analysis further shows that rhythmic EEG signals are corrupted by shifts in synapse properties. To test these predictions, the authors recorded EEGs of human subjects under the influence of propofol, a general anesthetic and GABA receptor agonist. The results show that propofol administration caused broadband EEG changes that matched its known effects on GABA receptors. Correcting for these confounding broadband changes revealed that delta power increased within seconds of individuals losing consciousness. The study concludes that EEG signals are shaped by neurophysiological factors other than brain rhythms and highlights the importance of detrending EEG spectra to accurately quantify differences in brain rhythms.This study investigates the neurophysiological basis of aperiodic EEG and the background spectral trend. Using biophysical modeling, the authors demonstrate that aperiodic neural activity can generate detectable scalp potentials and shape broadband EEG features, but these signals do not significantly perturb brain rhythm quantification. The model analysis further shows that rhythmic EEG signals are corrupted by shifts in synapse properties. To test these predictions, the authors recorded EEGs of human subjects under the influence of propofol, a general anesthetic and GABA receptor agonist. The results show that propofol administration caused broadband EEG changes that matched its known effects on GABA receptors. Correcting for these confounding broadband changes revealed that delta power increased within seconds of individuals losing consciousness. The study concludes that EEG signals are shaped by neurophysiological factors other than brain rhythms and highlights the importance of detrending EEG spectra to accurately quantify differences in brain rhythms.