Electrical stimulation of the median nerve is known to activate somatosensory pathways and elicit somatosensory evoked potentials (SEPs), measurable with the electroencephalogram (EEG). In contrast to the traditional stimulation with individual pulses, alternative approaches indicate that periodic pulses may elicit, at some frequencies of stimulation, a steady-state SEP in which the dominant frequency of the EEG corresponds with that of the stimulation. Even though this steady-state approach presents several practical benefits and it may enable new applications, its study in the somatosensory system has been limited so far, especially with electrical stimulation.
To bridge this gap, this study aimed to identify measurable changes in the brain response to periodic electrical stimulation when stimulation frequency is increased, as well as to identify a potential threshold frequency at which the steady state appears.
With this purpose, transcutaneous electrical stimulation was used to stimulate the median nerve of 9 healthy subjects using pulse trains with frequencies of 3, 7, 13, 19 and 36 Hz. Time and frequency domain analyses were used to compare responses across all stimulation frequencies.
Results of the analysis in the time domain showed a significant decrease in the amplitude of SEP component P40 for the highest frequencies (19 and 36 Hz) compared to the lowest frequency (3 Hz), as well as a maximal activation centered at the somatosensory cortex, contralateral to the stimulated side. Results of analysis in the frequency domain showed salient peaks in the frequency spectra at the frequencies of 19 and 36 Hz, which were not present for the lower frequencies. Additionally, the power distribution across the scalp at these frequencies showed higher values at the side contralateral to the stimulation.
According to previous definitions of a steady-state in evoked potentials, our results indicate that a steady-state SEP can be elicited by transcutaneous electrical stimulation at frequencies of 19 Hz and above. These observations could be interpreted as a transition in the nature of the response due to the activation of neural pathways different to those activated for lower frequencies. However, further research is needed to confirm this hypothesis.