Assessment of Audio-Tactile Sensory Substitution Training in Participants with Profound Deafness Using the Event-Related Potential Technique

The study was reviewed and approved by the Neuroscience Institute's Ethics Committee (ET062010-88, Universidad de Guadalajara), ensuring all procedures were conducted in accordance with the Declaration of Helsinki. All participants agreed to participate voluntarily and gave written informed consent (when underaged, parents signed consent forms).

1. Experimental design

1. Stimulus preparation
   1. Search in Creative Commons licensed sound databases to select a set of animal sounds in .wav format. The stimuli in this study consisted of five different animal sounds: dog barking, cow mooing, horse neighing, donkey braying, and elephant trumpeting.
      NOTE: The sound stimuli used here were previously selected as a collection of sounds for the vibrotactile discrimination training program in our earlier studies^9,10.
   2. Edit the sound files using a free, open-source audio editor to standardize the intensity and length of the stimuli to 1500 ms. For this protocol, standardize at a linear scale from 0 to 8000 Hz, at a gain of 20 dB, and at a range of 80 dB based on the parameters established in the previous studies^9,10 using the same vibrotactile stimulation system.
   3. Save the formatted audio files in a 32-bit float format with a 48,000 Hz project rate.
2. Paradigm setup in the electrophysiology presentation software
   1. Design a continuous performance task (CPT) using an experimental design and stimulus presentation software, assigning the stimuli to one of the two conditions: (a) target (T) stimulus (dog barking in 20% of trials) and (b) non-target (NT) stimuli (the remaining four animal sounds for the other 80%).
      NOTE: Each condition was labeled with the same code to synchronize stimulus presentation marks when programming the EEG protocol in the recording software.
   2. Build a pseudo-randomized stimulus-presentation using the software platform in which the five animal sounds (dog, cow, horse, donkey, and elephant) are each presented 20% of the time. Check that the target stimulus (dog barking) never occurs more than twice in succession.
   3. Specify the desired interstimulus interval (ISI) and the total response time, and select the response keys that will be used to automatically collect behavioral data for target (T) stimuli responses. Here, a fixed 2000 ms ISI list for 150 trials and the correct response for the T stimuli were programmed via the left control key on a standard computer keyboard. Participants were given a 3500 ms time window for a behavioral response (starting at stimulus presentation).

2. Participant selection

1. Recruit potential participants with profound bilateral sensorineural hearing loss diagnosis and collect demographic data, including age, sex, hand preference, and educational history.
2. Conduct semi-structured clinical interviews to screen participants for personal or family history of psychiatric, neurological, or neurodegenerative illness and to collect information pertaining to deafness clinical history: the age of onset, etiology, and hearing-aid use history, as well as their preferred communication mode (oral, manual, or bilingual).
3. Conduct audiological tests (pure-tone air hearing-thresholds) using an audiometer to confirm the severity of hearing loss.
   1. In a sound-attenuated room, sit directly in front of the participant and properly place headphones on them.
   2. Instruct the participants to raise their dominant hand to signal whenever they can hear the tone being presented through the headphones.
   3. Ranging from 20 dB to 110 dB intensity levels, present a pure-tone at six octaves in the following ascending order: 250, 500, 1000, 2000, 4000, and 8000 Hz, starting with the left ear and repeating the same steps for the right ear.
      1. Calculate the patient pure-tone average (PTA) by averaging the hearing thresholds at 500, 1000, 2000, and 4000 Hz for each ear. The hearing-loss severity inclusion criteria for the study is a bilateral pure tone average (PTA) greater than 90 dB.
      2. Select participants based on the eligibility criteria. Inclusion criteria additionally include no personal or family history of psychiatric, neurological, or neurodegenerative illness and non-syndromic, prelingual profound bilateral deafness. Obtain informed consent and explain the experimental procedures to the participants.
         ​NOTE: All the forms, questionnaires, and instructions used in the study were translated to Mexican Sign Language (MSL) by a professional MSL interpreter and were presented in video format using a tablet computer. In addition, an MSL interpreter was present during all study procedures.

3. Pre-training EEG recording session

1. Participant preparation
   1. Verify that the participants have come to the recording session with clean and dry hair, having not used any hair gel, conditioner, or other hair products that affect electrode impedance.
   2. Ask the participants to sit in a comfortable position, approximately 60 cm away from the stimulus screen, and use the tablet device to play the MSL videoclip with the preparation procedure description.
   3. Clean the areas where reference and electrooculogram (EOG) electrodes will be placed (earlobes, forehead, outer canthus, infraocular orbital ridges, etc.). First, wipe the skin with an alcohol swab, and then apply EEG abrasive prepping gel gently with a cotton swab to exfoliate dead skin cells on the surface.
   4. Fill the electrode gold cup with conductive electrode paste and place an electrode on each reference site, usually on the right and the left earlobes or mastoids. Repeat the steps to place at least one vertical EOG at the outer canthus and one horizontal EOG at the infraocular orbital ridge to monitor oculomotor activity (blinks and saccades). Hold the single electrodes in place with a piece of 1 in micropore tape.
   5. Ask the participants to hold their arms straight horizontally and then fit the body harness tightly but comfortably around the chest under the armpits with the snaps in the middle of the chest.
   6. Place the EEG commercial electro-cap with 19 Ag/AgCl electrodes (Fp1, Fp2, F3, F4, F7, F8, C3, C4, P3, P4, O1, O2, T3, T4, T5, T6, Fz, Cz, and Pz) topographically arranged according to the International 10-20 system. Use a measuring tape to check the participant's head circumference to ensure you use the proper cap size.
   7. Align the Cz electrode with the nose and then measure the distance from the nasion to the inion so that the Cz electrode falls precisely in the middle. Button the adjustable straps on the sides of the cap to the body harness so that the electro-cap is firmly tightened.
   8. Place the gel-filled blunt-needle syringe inside the electrode, circle the needle to remove hair, and then gently abrade the scalp region under the electrode before applying the conductive gel. Don't apply too much gel to avoid electrical bridging with neighboring electrode sites.
   9. Allow the EEG conductive gel to dry at cool room temperature.
2. Setting up the EEG recording equipment
   1. Calibrate the EEG system as per the instrument's instructions, then connect the electro-cap to the amplifier set at a bandpass of 0.05-30 Hz (3 dB cutoff points of 6 dB/octave roll-off curves), a 60 Hz notch-filter, and a 200 Hz sampling rate equal to a 5 ms sampling period.
   2. Check that the impedance is below 5 KΩ (for a low impedance system) in all electrode sites and check on the monitor that all channels are smoothly registering the electrical signals.
3. Running the experimental task
   1. Position the participant in front of the computer monitor and place the keyboard at a comfortable distance.
   2. Connect the cable of the portable stimulator device (see Figure 1) to the computer system speaker's outlet and set the speaker volume to the maximum intensity level.
   3. Adjust the portable stimulator system on the participant's right index fingertip and test.
   4. Using the tablet device, play the experiment instructions and execute a practice trial to familiarize the subject with the portable stimulator device, the audio-tactile stimuli, and the task. Repeat the MSL instructions and verify comprehension.
   5. Remind the participant to respond to the dog bark stimulus by pressing the left control key with their left index finger only upon target stimulus detection and to withhold their response when any of the other four animal sounds are perceived. The CPT experimental paradigm is represented in Figure 2.
   6. Provide clear instructions for how to minimize artifacts and demonstrate the effect of artifacts on the EEG in real-time before you begin recording (recommended as a standard recording procedure in research with clinical populations²⁰).
   7. Before starting the CPT task, check that the event-synchronization between the cognitive stimulation computer and the EEG recording computer is working properly. To do so, start recording the EEG signal and click on the communication icon in the stimulus presentation software interface. Upon clicking, the event-synchronized pulses appear at the bottom of the EEG recording screen.
   8. Run the experimental task. Carefully observe the participant and monitor alertness, response execution, and excessive movement or blinking.
   9. Pause and allow the participant a short break in the middle of the experiment (at 4 min in the experiment) to allow them to blink, relax, and move around if needed. Finish running the experiment.

4. Audio-tactile sensory substitution training program

1. Consult Supplementary File 1, which contains a detailed description of the five-session program, to perform the training. Automatize the activities described using a spreadsheet to make the training more systematic and engaging for the participants. Use original images and audio recordings from⁹ and ask the participants to respond by tapping on a laptop touch-screen monitor.
   ​NOTE: The content and tables in this file have been reprinted with permission from⁹.

5. Post-training EEG recording session

1. Repeat the exact same steps as specified in section 3.

6. EEG analysis

NOTE: The EEG acquisition steps were done using the EEG recording software, and the EEG processing steps were done using a separate EEG analysis software.

1. EEG raw signal pre-processing
   1. Define and select epochs of 1100 ms in the continuous EEG data, without the use of additional digital filters, using stimulus onset as the initial time instant (t0), and including a 100 ms pre-stimulus used for baseline correction. Supplementary Figure 1 illustrates how the 1100 ms epochs were selected according to the EEG analysis commercial software installed in the EEG recording equipment.
   2. During artifact rejection, exclude epochs of data on all channels when the voltage in a given recording epoch exceeds 100 µV on any EEG or EOG channel. Also, reject artifacts by visual inspection of the epochs. See Supplementary Figure 2, which provides an example of epochs that were manually rejected due to ocular artifacts.
2. Signal averaging
   1. Select an equal number of artifact-free epochs for each stimulus condition (target and non-target) in both the pre- and post-training conditions. Select the maximum epochs possible to improve the signal-to-noise ratio. Do this for each EEG record.
      NOTE: In this protocol, we selected an average of 25 correct response epochs per condition at each timepoint since we were interested in evaluating target discrimination. Keep in mind that some ERP components do not require overt behavioral responses to be observed. Participants with less than 15 artifact-free epochs in each condition were excluded from the study.
   2. Click on the Operations menu and select the EEG window averaging option to average individual ERPs.
   3. First, select the Independent Average option to average target trials only. Then, select the other four non-target stimuli and click on the Average Together option to average.
   4. Repeat steps 6.2.2 and 6.2.3 for each participant's EEG recording in the pre-training condition and then for the post-training condition.
   5. Once all the individual ERPs are calculated, average them together to obtain the grand-mean waveforms per stimulus condition for pre- and post-training. Open any individual EP average, then go to the Operations menu and select Grand-mean averaging option. Select the participant's individual averages to be included in the group average.
   6. Choose all pre-training target averages from the drop-down list, then click the Average button, type the desired file name, and press the Return key to save. Then select all pre-training non-target averages from the drop-down list, click the Average button, type the desired file name, and again press the Return key to save.
   7. Repeat the previous steps for the post-training condition.
3. ERP visualization and analyses
   1. Select the Operations menu to see the list of saved grand means. Then click on the group averages that you wish to plot. Next, click the Montage button to select the channels you want to plot.
   2. Go to the Tools menu, then click on Visualize Options to select each waveform's color and line width. Then click on the Signal menu, check the DC correction box, type in the desired baseline stimulus interval, then press the Return key.
   3. Carefully inspect the plotted grand-mean waveforms to identify the components of interest and their corresponding time windows.
      NOTE: For this experiment, we knew that the waveforms, because of the task design and sensory pathways understudy for P3, would very likely be a positive component appearing later than 300 ms in centroparietal electrodes and with greater voltage amplitudes in the target condition.
   4. Export individual peak amplitude latencies and voltages, and then import data on a spreadsheet to build the database. Conduct a repeated-measures Analysis of Variance (ANOVA) using a statistics software.