Mahmoud Keshavarzi1,2,3, Enrico Varano1, Tobias Reichenbach1,4
1 Department of Bioengineering and Centre for Neurotechnology, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK; 2 Centre for Neuroscience in Education, Department of Psychology, University of Cambridge, Cambridge, CB2 3EB, UK; 3 Cambridge Hearing Group, Department of Psychology, University of Cambridge, Cambridge, CB2 3EB, UK; 4 Department Artificial Intelligence in Biomedical Engineering, Friedrich-Alexander-University Erlangen-Nuremberg, Henkestrasse 91, 91056 Erlangen, Germany
Background: To comprehend speech in busy environments such as pubs and restaurants, human listeners need to selectively attend a target voice while ignoring interfering voices. Such speech-in-noise processing has recently been found to involve vortical tracking of the target speaker. In particular, altering this cortical entrainment through transcranial alternating current stimulation (tACS) in the theta band has been found to modulate the comprehension of speech in noise. However, the functional role the cortical tracking of an ignored speech signal and its impacts on the behaviour remain unclear. Here we therefore sought to investigate the impact of tACS with the envelope of a distractor voice on the comprehension of a target speech signal.
Methods: Eighteen right-handed native English speakers with self-reported normal hearing and no history of mental health problems or neurological disorders took part in the experiment. The individual SNR was determined such that the participant understood 50% of a target sentence in the presence of a distractor voice. We then employed tACS with either the target or the distractor envelope filtered in theta band, while evaluating the speech comprehension at the individual SNR. As a control condition, we further evaluated the speech comprehension when a sham stimulus was employed as a current waveform.
Results & Conclusion: We found that tACS with the distractor envelope influenced comprehension of the target speaker, suggesting that the cortical tracking of the ignored speaker plays a functional role in speech processing. tACS with the target envelope also influenced speech comprehension in a very similar manner. Particularly, both types of neurostimulation caused a significant modulation of speech comprehension that varied sinusoidally, at the longest possible period, with the applied phase shifts. Moreover, both types led to the modulation of speech comprehension that had a significantly consistent phase dependency across different participants.