Transcranial alternating current stimulation with the theta- but not delta band modulates speech-in-noise comprehension
Mahmoud Keshavarzi1, Mikolaj Kegler1, Shabnam Kadir2, Tobias Reichenbach1
1 Department of Bioengineering and Centre for Neurotechnology, Imperial College London, South Kensington Campus, SW7 2AZ, London, UK
2 School of Engineering and Computer Science, University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK
Background. When listening to speech, oscillatory activity in the auditory cortex entrains to the amplitude fluctuations. The entrainment can be influenced by non-invasive neurostimulation, which can thereby modulate the comprehension of a speech signal in background noise. However, cortical entrainment occurs predominantly in two frequency bands, the delta band (1 – 4 Hz) and the theta frequency band (4 – 8 Hz), that have been suggested to play different roles regarding speech processing. Here we therefore sought to tear apart the contribution of neurostimulation with the theta-band portion and with the delta-band portion of the speech envelope to the modulation of speech comprehension.
Method Seventeen native English speaking, right-handed participants with normal hearing took part in the experiment. For each participant we determined the signal-to-noise ratio (SNR) at which the participant understood 50% of a sentence embedded in speech-shaped noise. We then employed transcranial alternating current stimulation (tACS) with the speech envelope filtered into either the delta or the theta band, while assessing the speech comprehension at the SNR determined before. For both the delta-band and the theta-band current waveform, we further employed six different phase shifts. In addition, we assessed the speech-in-noise comprehension during a sham stimulation, as well as when the current waveform was based on the speech envelope of an unrelated sentence.
Results and Conclusion Our results show that the phase shifts of the theta-band current, but not of the delta-band current, influence speech comprehension. Moreover, the theta-band stimulation without phase shift leads to better speech comprehension than the sham stimulation. The modulation of speech comprehension through neurostimulation with the speech envelope accordingly results from the theta- but not from the delta-band portion of the envelope. Moreover, the transcranial current stimulation can not only modulate but in fact improve the comprehension of speech in background noise.