Our protocol demonstrates how ERPs can be applied to explore learning-related neural changes in subjects with profound deafness after a short training period in vibrotactile discrimination of complex natural sounds. The main advantage of the ERP technique is that it allows the study of the precise temporal dynamics of electrical brain activity that underlies cognitive processing during audio-tactile sensory substitution. The implications of this technique could extend towards speech production therapy because audio-tactile sensory feedback could definitely facilitate early oral language development in patients with severe auditory deficits.
The current method contributes to the global search for alternatives to treat the specific sensory deficits and provides insights into a more in-depth understanding of the neural basis of sensory transduction. Visual demonstration of this protocol is critical to guarantee replication because so far, the experimentally employed vibrotactile simulation all have distinct approaches in methodology and instrumentation. Demonstrating the procedure will be Geisa, Assistant Professor, Ricardo, master student, Eduardo, student research assistant, and Deborah, MSL interpreter and student research assistant from our lab.
Recruit potential participants with profound bilateral sensorineural hearing loss diagnosis and collect demographic data including age, sex, hand preference, and educational history. Conduct semi-structured clinical interviews to screen participants for personal or family history to deafness clinical history. Conduct audiological tests using an audiometer to confirm the severity of hearing loss.
In a sound attenuated room, sit directly in front of the participant and properly place headphones on them. Instruct the participants to raise their dominant hand to signal whenever they can hear the tone being presented through the headphones. Ranging from 20 decibels to 110 decibels intensity levels, present a pure tone at six octaves in the ascending order, 250, 500, 1, 000, 2, 000, 4, 000 and 8, 000 hertz, starting with the left ear and repeating for the right ear.
Calculate the patient's pure tone average by averaging the hearing thresholds at 500, 1, 000, 2, 000, and 4, 000 hertz for each ear. To prepare the participant, verify that the participant have come to the recording session with clean and dry hair, having not used any hair products that affect electrode impedance. Ask the participant to sit in a comfortable position approximately 60 centimeters away from the stimulus screen and use the tablet device to play the MSL video clip with the preparation steps.
Clean the areas where reference and EOG electrodes will be placed. 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. Fill the electrode gold cup with conductive electrode paste and place an electrode on each reference site.
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. Hold the single electrodes in place with a piece of one micropore tape. 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.
Use a measuring tape to check the participant's head circumference to ensure you use the proper cap size. Place the EEG commercial electro cap with 19 silver chloride electrodes topographically arranged according to the international 10-20 system. Align the front midline electrode with the nose and then measure the distance from the nasion to the inion so that the central midline 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. 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.
Then allow the EEG conductive gel to dry at room temperature. Then calibrate the EEG system. Then connect the electro cap to the amplifier set at a bandpass of 0.05 to 30 hertz, a 60 hertz notch filter, and a 200 hertz sampling rate equal to a five millisecond sampling period.
Check that the impedance is below five kiloohm in all electrode sites and check on the monitor that all channels are smoothly registering the electrical signals. Then to run the experimental task, position the participant in front of the computer monitor and place the keyboard at a comfortable distance. Connect the cable of the portable stimulator device to the computer system speakers outlet and set the speaker volume to the maximum intensity level.
Adjust the portable stimulator system on the participant's right index fingertip and test. Using the tablet device, play the experiment instructions and execute a practice trial to familiarize the subject. Repeat the MSL instructions and verify comprehension.
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 the response when any of the other four animal sounds are perceived. Start recording the EEG signal and click on the communication icon in the interface before starting the CPT task to check that the event synchronization between the cognitive stimulation computer and the EEG recording computer is working properly. Upon clicking, the event synchronized pulses appear at the bottom of the EEG recording screen.
Run the experimental task and carefully observe the participant and monitor alertness, response execution, and excessive movement or blinking. Pause and allow the participant a short break in the middle of the experiment to allow them to blink, relax, and move around if needed. Then finish running the experiment.
For pre-processing EEG raw signal, define and select epochs of 1, 100 milliseconds in the continuous EEG data without additional digital filters using stimulus onset as the initial time instant and including 100 millisecond pre-stimulus used for baseline correction. During artifact rejection, exclude epochs of data on all channels when the voltage in a given recording epoch exceeds 100 microvolts on any channel, also reject artifacts by visual inspection. Then select an equal number of artifact-free epochs for each stimulus in both the pre and post-training conditions.
Do this for each EEG record. Click on the Operations menu and select the EEG window averaging option to average individual ERPs. 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. Repeat these steps for pre-training and post-training conditions. Open any individual EP average, then go to the Operations menu and select Grand Mean Averaging option.
Select the participants'individual averages to be included in the group average. Choose all pre-training target averages from the dropdown list, then click the Average button, type the desired filename and press the Return key to save. For ERP visualization and analyses, 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. 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. Carefully inspect the plotted grand mean waveforms to identify the components of interest and their corresponding time windows. The pre-training grand averages and post-training grand averages portrayed the main results of this investigation evaluating changes in P3 in a group of 17 profoundly deaf individuals.
The ERP waveforms were modified when plotted using a lowpass digital filter at five hertz. The ERP plots revealed changes in a P3-like central parietal positive waveform which was more robust for the target stimuli after training. In the pre-training condition, ERPs suggested that the target and non-target conditions were not as clearly distinguishable as in the post-training condition.
The most crucial step is to properly instruct the profoundly deaf participants and to adequately select the artifact-free EEG epochs for each condition before window averaging. Quantitative EEG and functional connectivity could be used to further analyze and interpret the data to describe the neurofunctional changes related to an alternative sensory stimulation training program.
