This method can be an effective tool for uncovering the adaptability of humans to a novel environment in the auditory domain. The main advantage with this technique is that long-term adaptation to precise left-right reversed audition can be tested in a variable manner in combination with neural imaging. Demonstrating the procedure will be Takayuki Hoshino, a grad student from my laboratory.
To begin, prepare the linear pulse code modulation recorder, binaural microphones and binaural in-ear earphones. First, cross-connect the left and right lines of the microphones to the recorder so that left-right reversed analog sound signals are digitalized. Then, connect the left and right lines of the earphones straight through to the recorder so that the reversed digitalized signals are played immediately.
Finally, put the bodies of the microphones and the earphones together for each ear with slight isolation by soundproofing materials. Then cover the microphones with dedicated windscreens for suppressing any wind noise. Next, insert rechargeable batteries and a large-capacity, high-speed memory card into the recorder and turn it on.
Then place the body of the system into a pocket-sized bag. Instruct the participant to insert the earphones tightly into the ear canals. Then disconnect the lines for the left and right microphones and connect the dominant ear side of the microphone straight through to the recorder.
Now, have them repetitively take off and put on the dominant ear side of the system while adjusting the sound volume of the recorder. This makes the subjective loudness of the reversed sounds equal. Ah, ah.
Check the loudness for the non-dominant ear, as well. Then reconnect all the system lines. Tell the participant that, during exposure, they should follow the same calibration procedure whenever they set up the system again.
Prior to reversed audition exposure, have the participant thoroughly practice the task that will be used during the neuroimaging experiments. A selective reaction time task can be used. Use a Psychophysics software Toolbox to present 1000 Hertz sounds at 65 decibels sound pressure level for 0.1 seconds, played 80 times per block.
Also use an interstimulus interval of 2.5 to 3.5 seconds, played pseudorandomly in either ear. The task should consist of two compatible and two incompatible blocks. In the compatible condition, the participant should respond immediately to the sound with the index finger on the same side of their body as the sound.
This should be alternated with the incompatible condition in which the participant responds immediately to the sound with the index finger on the opposite side of their body. Before the exposure to reversed audition, conduct a neuroimaging experiment using the trained task. Record either MEG or EEG responses, as well as the left and right finger responses.
To begin exposure, provide the participant with a sufficient number of spare rechargeable batteries and large-capacity, high-speed memory cards so they can replace them as needed. Then, remind them repeatedly of their right to quit the exposure at any time and instruct them to wear, calibrate and check the reversed audition system by themselves during the exposure period. The participant should perform daily life activities while wearing the system continuously for approximately a month, except while sleeping, bathing, neuroimaging or during an emergency.
When removed, the system should be immediately replaced with earplugs without producing any sound while in a silent area. The batteries and memory cards should also be replaced routinely, before battery exhaustion and memory overcapacity. To facilitate adaptation, the participant should experience situations involving high auditory input, such as playing video games, walking in a shopping mall or a campus and having a conversation with more than two persons for as long as possible.
They should also keep a diary or provide a subjective report to an observer as often as possible, detailing perceptual and behavioral changes, experienced events or any other details. During approximately a one month exposure to reversed audition, conduct neuroimaging experiments under the trained task every week without the reversed audition system in exactly the same way as in the preexposure experiment. Finally, after the target exposure period, the participant should take off the reversed audition system.
One week after exposure had ended, conduct a final neuroimaging experiment under the trained task. This figure shows the sound source localization in directions over 360 degrees before and immediately after putting on the left-right reversed audition system in six participants. Here, cosine similarity between perceptual angles and sine-regulated physical angles in the normal and reversed conditions are plotted against unregulated physical angles.
And here, we see cosine similarity between perceptual angles in the reversed condition and oppositely-arranged perceptual angles in the normal condition plotted against physical angles. This figure shows changes in behavioral and neural responses during the selective reaction time task before, during and after exposure in one participant. Yellow zones indicate a period exposed to left-right reversed audition.
Mean reaction times for the stimulus response compatible and incompatible conditions are seen here, while here we see left and right auditory N1m intensities for stimulus response compatible and incompatible conditions as evaluated by minimum norm estimates of MEG data. This figure shows auditory motor functional connectivity as tested by Granger causality tests during the selective reaction time task in two participants. The left and right motor and auditory areas are shown.
Red, yellow or lack of arrows indicate the number of participants who showed significance at a threshold of P less than 0.05. After watching this video, you should have a good understanding of how to study left-right reversed audition as a tool to uncover the adaptability of humans to a novel auditory environment.
