Slide Slam F16
Enhancement of speech-in-noise comprehension through transcranial alternating current stimulation
Tobias Reichenbach1; 1Friedrich-Alexander-University Erlangen-Nuremberg
Auditory cortical activity tracks speech rhythms, in particular at the rate of words (1 - 4 Hz, delta band) and at the rate of syllables (4 - 8 Hz, theta band). The modulation of this cortical tracking through alternating current stimulation with the speech envelope has been found to influence the comprehension of speech in noise. However, the stimulation can be performed with different parameters, such as temporal delay, particular frequency band and a potential phase delay. The influence of these stimulation parameters on speech comprehension remains insufficiently understood. We presented human volunteers with single sentences that were embedded in noise to assess their speech comprehension. Simultaneously we stimulated the subject's auditory cortices through transcranial alternating current. The current waveforms were obtained from the speech envelope and were shifted by different temporal delays as well as phases. We also explored currents that were obtained from the speech envelope filtered in the delta and in the theta frequency bands. We first investigated two characteristic delays that emerge in the cortical tracking of speech rhythms, a short delay of 100 ms and a longer delay of 250 ms. We found that current stimulation at both delays influenced speech comprehension. Next, we applied current waveforms that followed either the delta- or the theta-band portion of the speech envelope. The theta-band current stimulation had a significant effect on speech comprehension, while the delta-band stimulation did not. We further investigated the influence of different latencies of the theta-band stimulation on the comprehension of speech in noise. We found that a latency of 0 ms yielded the highest speech comprehension, which was higher than under a sham stimulus. Last but not least, we showed that stimulation with the envelope of a distractor speaker could modulate spech comprehension as well. Taken together, our results demonstrate that the modulation of speech comprehension through transcranial alternating current stimulation is driven by the theta- but not by the delta band. Speech-in-noise comprehension can be enhanced when the current waveform is temporally aligned to the speech signal. Moreover, the delta rhythms of a distractor speaker appear to be processed in the brain as well, presumably competing with the processing of the target speaker.