Entrainment of human EEG: Sensory versus electrical entrainment and simulations
Neural activity can be entrained by sensory and electromagnetic stimuli if stimulation is repetitive. We have performed two entrainment experiments to demonstarate this effect and have carried out simulations to better understand the phenomena. In the first experiment, 21 subjects were entrained with auditory stimuli that elicited steady-state auditory evoked potentials (SSAEPs) in the subjects’ electroencephalogram (EEG). The stimuli were amplitude-modulated sine tones whose modulation frequency ranged from 1 to 250 Hz. SSAEPs revealed amplitude maxima around 45 Hz stimulation frequency, probably reflecting a resonance frequency of auditory cortex. In addition, auditory evoked potentials (AEPs) in response to 1000 Hz tones were recorded from the same 21 subjects and AEPs were analyzed in the frequency domain. It turned out that the frequency of the evoked gamma-band response in AEPs correlated with the frequency of the maximum SSAEP. Thus, we speculate that the resonance frequencies determine the frequency spectrum of the AEP. In the second experiment, we applied transcranial alternating current stimulation (tACS) that has been introduced to directly modulate rhythmic brain activity by the application of oscillatory currents. Until now, the efficiency of tACS in modulating rhythmic brain activity has been mainly indicated by inference from the perceptual and behavioural consequences. We delivered tACS over the occipital cortex of 10 healthy participants to entrain the neuronal oscillations in their individual alpha frequency range and compared results with those from a separate group of participants receiving sham stimulation. The tACS but not the sham stimulation elevated EEG alpha power. The results demonstrate the ability of tACS to modulate oscillatory brain activity at a specific frequency. In a simulation study, we have analysed the responses of spiking neurons to repetitive stimulation of different frequencies and intensities. The resulting neuron activity showed so-called Arnold tongues indicating that the neurons respond more strongly to certain frequencies than others, i.e. the neurons have resonance frequencies.