Hearing Impaired Participants Improve More Under Envelope-Transcranial Alternating Current Stimulation When Signal to Noise Ratio Is High

Abstract

An issue commonly expressed by hearing aid users is a difficulty to understand speech in complex hearing scenarios, that is, when speech is presented together with background noise or in situations with multiple speakers. Conventional hearing aids are already designed with these issues in mind, using beamforming to only enhance sound from a specific direction, but these are limited in solving these issues as they can only modulate incoming sound at the cochlear level. However, evidence exists that age-related hearing loss might partially be caused later in the hearing processes due to brain processes slowing down and becoming less efficient. In this study, we tested whether it would be possible to improve the hearing process at the cortical level by improving neural tracking of speech. The speech envelopes of target sentences were transformed into an electrical signal and stimulated onto elderly participants' cortices using transcranial alternating current stimulation (tACS). We compared 2 different signal to noise ratios (SNRs) with 5 different delays between sound presentation and stimulation ranging from 50 ms to 150 ms, and the differences in effects between elderly normal hearing and elderly hearing impaired participants. When the task was performed at a high SNR, hearing impaired participants appeared to gain more from envelope-tACS compared to when the task was performed at a lower SNR. This was not the case for normal hearing participants. Furthermore, a post-hoc analysis of the different time-lags suggest that elderly were significantly better at a stimulation time-lag of 150 ms when the task was presented at a high SNR. In this paper, we outline why these effects are worth exploring further, and what they tell us about the optimal tACS time-lag.

Keywords: EEG; aging; entrainment; hearing aid; hearing impairment; speech envelope; speech processing; tACS.

Figure 1.

Hearing levels of participants at different frequencies. Prior to partaking in the experiment, participants had their hearing capabilities assessed using pure tone audiometry using air conduction. Hearing thresholds were obtained for different frequencies ranging from 125 to 8000 Hz for both ears separately. Participants were rated as normal hearing or hearing impaired as defined by the WHO. Out of the 20 hearing impaired participants, 18 had been using a hearing aid for at least 1 year.

Figure 2.

(a) Envelope-tACS setup. Stimulation shape and locations of the stimulation electrodes; positions are mirrored on the other side of the head. The shape of the tACS waveform is derived from the speech envelope of a presented sentence. (b) Different tACS time-lags. Whilst the electrical stimulation of the cortex is virtually instantaneous, there is a delay between the presentation of the speech stimulus and the cortical processing of said stimulus; the exact length of this delay varies per participant. To compensate for this, different delays between the presentation of the auditory stimulus and the onset of electrical stimulation are used. Although most participants cannot differentiate whether they received electrical stimulation or not, 2 sham conditions were used; 1 with short peaks of stimulation before and after presentation of the sentence to invoke the feeling of being stimulated (sham), and 1 devoid of any electrical stimulation (control).

Figure 3.

Differences in SNR between sham condition and no stimulation control condition. To control for any placebo effects caused by potentially perceived stimulation, we compared performance between the sham and control condition. There was no significant difference between performance in the sham stimulation conditions (light grey) and the no stimulation control conditions (dark grey). To avoid multiple comparisons, the no stimulation control condition was therefore removed from further analysis. When comparing the sham stimulation conditions, normal hearing (NH) participants were significantly better at the task than hearing impaired (HI) participants, and participants arrived at a significantly lower SNR in the SRT20 condition compared to the SRT80 condition.

Figure 4.

Distribution of baseline-corrected score (ΔSNR) for the 5 different time-lags, separate for the 2 different task difficulties (SRT20 and SRT80) and separate for normal hearing (NH) and hearing impaired (HI) participants. Black crosses represent the performances of the excluded participant. After resolving a 3-way interaction between task difficulty, hearing impairment and the 5 different time-lags, a significant difference in between the 100 and 150 ms time-lags was revealed for the hearing impaired participants in the SRT80 condition.

Figure 5.

Distribution of optimal time-lags, separate for hearing impairment (NH, normal hearing and HI, hearing impaired) and task difficulty (SRT20 and SRT80). Optimal time-lags were uniformly distributed except for SRT80_HI.

Figure 6.

(a) For each participant, their best performing time-lag out of the 5 time-lags was selected separately for the SRT20 and SRT80 conditions. Hearing impaired participants performed significantly better (ie, at a lower ΔSNR) in the SRT80 condition compared to their performance in the SRT20 condition. (b) As hearing impaired participants performed exceptionally well in the 150 ms time-lag of the SRT80 condition, several of them had their best time-lag at 150 ms in this condition. Therefore, it could be argued that the effect found of task difficulty was only caused by the effect in this 150 ms time-lag condition. However, after re-selecting participants’ best time-lags whilst excluding the 150 ms condition, the effect persisted.