Characterizing endogenous delta oscillations non-invasively in humans

Rhythmic activity in the delta frequency range (0.5 – 3 Hz) is a prominent feature of brain dynamics, mainly found in invasive recordings. In humans, stimulus-brain coherence in the delta band is commonly reported, and has been shown to influence behaviour. However, periodic brain dynamics observed during periodic sensory inputs cannot be interpreted as endogenous oscillations, and hence are not sufficient to confirm the influential theory of neural entrainment. To fill this gap, we examined whether spontaneous delta oscillations, as found in invasive recordings in awake animals, can be observed in non-invasive recordings performed in humans using M/EEG during resting. To increase our chances for observing oscillatory brain dynamics, we additionally analysed two conditions in which participants engaged in spontaneous finger tapping and silent counting, arguing that internally rhythmic behaviours could incite an otherwise silent neural oscillator. Using a novel set of analysis steps allowed us to show narrow spectral peaks in the delta frequency range in rest and during overt and covert rhythmic activity. Additional analyses in the time domain, however revealed that only the resting state condition warranted an interpretation of these peaks as endogenously periodic neural dynamics. In sum, this work shows that using advanced signal processing techniques, it is possible to observe endogenous delta oscillations in non-invasive recordings of human brain dynamics.