All methods described here have been approved by the Ethics Committee of Tokyo Denki University. For every participant, informed consent was obtained after the participant received a detailed explanation of the protocol.
1. Setup of the Left-Right Reversed Audition System
2. Validation of the Left-Right Reversed Audition System
NOTE: Perform the following steps to validate the left-right reversed audition system, irrespective of experiments studying adaptation to left-right reversal.
3. Studying the Adaptation to Left-Right Reversed Audition
The representative results shown here are based on Aoyama and Kuriki15. The present protocol achieved left-right reversed audition with high spatiotemporal accuracy. Figure 1 shows the sound source localization in directions over 360° before and immediately after putting on the left-right reversed audition system (Figure 1A), in six participants, as indicated by the cosine similarity. As shown in Figure 1B, the perceptual angles in the normal condition were quite well correlated with the physical angles (positive correlation, adjusted R2 = 0.99). The perceptual angles in the reversed condition were also well correlated with the physical angles (negative correlation, adjusted R2 = 0.96; see also Figure 4 in Aoyama and Kuriki15), although there existed a slight perceptual bias toward the counterclockwise rotation, especially for sounds coming from the right-front and the left-back directions. Notably, the perceptual angles in the reversed condition were more correlated with the oppositely arranged perceptual angles in the normal condition (adjusted R2 = 0.98) than the physical angles, as shown in Figure 1C. Furthermore, a potential delay of the system was estimated to be a constant 2 ms. The present protocol also achieved a natural wearing appearance, like listening to music with a mobile music player, thereby avoiding any stress of being noticed by other individuals.
Figure 1: Sound source localization in 360° directions, before and immediately after putting on the left-right reversed audition system, in six participants. (A) The constructed left-right reversed audition system. (B) Cosine similarity between perceptual angles and sign-regulated physical angles in the normal (blue) and reversed (red) conditions plotted against (unregulated) physical angles, respectively. While the physical angles are directly used for the cosine similarity in the normal condition, the signs of physical angles are inverted in the reversed condition. (C) Cosine similarity between perceptual angles in the reversed condition and oppositely arranged perceptual angles in the normal condition plotted against physical angles (purple). This figure has been modified from Aoyama and Kuriki15. Please click here to view a larger version of this figure.
The present protocol revealed perceptual changes to the reversed audition from a relatively early stage during the approximately 1-month exposure. Although a feeling of strangeness was reported just after the exposure, it began to decrease within a week of the exposure and continued to drop further over time. Mirror-image sounds were gradually perceived as normal, which also occurred with visual information and movements. One week after the end of the exposure period, all changes returned to the pre-exposure level. The present protocol detected not only perceptual but also behavioral and neural changes underlying the adaptation. Figure 2 shows changes in behavioral and neural responses during the selective reaction time task over the exposure time in a representative participant. As shown in Figure 2A, the mean reaction times for response-incompatible sounds were overall longer than those for response-compatible sounds from the pre-exposure period to the third week, but became slightly shorter in the fourth week. This relative inversion followed the transient elongation of the mean reaction times irrespective of compatibility in the second week. After the exposure, all mean reaction times returned to the initial level. The MNE intensities of the left and right N1m components exhibited similar trends to the mean reaction times, as shown in Figure 2B, although the compatible-incompatible relationship was inversed. The N1m components are distinct auditory evoked fields observed at about 90 ms after sound onset, and their source was confirmed to be located in the bilateral superior temporal planes using dSPMs. Overall, the intensities in the stimulus-response compatible conditions were higher than those in the incompatible conditions from the pre-exposure period to the third week, but were slightly lower in the fourth week. This relative inversion followed the transient enhancement of the intensities irrespective of compatibility and laterality in the second week. After the exposure, they returned to the initial levels.
Figure 2: Behavioral and neural responses during the selective reaction time task in a representative participant. (A) Mean reaction times for stimulus-response compatible and incompatible conditions. (B) Left and right auditory N1m intensities for stimulus-response compatible and incompatible conditions, as evaluated by minimum-norm estimates. Yellow zones indicate a period exposed to left-right reversed audition. This figure has been modified from Aoyama and Kuriki15. Please click here to view a larger version of this figure.
Furthermore, the present protocol revealed changes in the functional connectivity across the left and right auditory and motor areas during the selective reaction time task in two participants, as shown in Figure 3. The functional connectivity was tested with the Granger causality test at a threshold of p < 0.05. Initially, these auditory-motor areas communicated with each other irrespective of stimulus and response. However, after exposure to the reversed audition, the auditory-motor connectivity became unstable. Notably, in the second week, the auditory-motor connectivity was disrupted drastically, especially in the right motor-to-auditory feedback and left-to-right motor communication. Immediately after that, the connectivity recovered at the level of the first week, and returned to the initial level after the exposure.
Figure 3: Auditory-motor functional connectivity as tested by Granger causality tests during the selective reaction time task in two participants. Red, yellow, and no arrow(s) indicate the number of participants who showed significance at a threshold of p < 0.05 (N = 2, 1, and 0, respectively). LM and RM denote left and right motor areas, respectively, and LA and RA denote left and right auditory areas, respectively. This figure has been modified from Aoyama and Kuriki15. Please click here to view a larger version of this figure.
Linear pulse-code-modulation recorder | Sony | PCM-M10 | |
Binaural microphones | Roland | CS-10EM | |
Binaural in-ear earphones | Etymotic Research | ER-4B | |
Digital angle protractor | Wenzhou Sanhe Measuring Instrument | 5422-200 | |
Plane-wave speaker | Alphagreen | SS-2101 | |
Video camera | Sony | HDR-CX560 | |
MATLAB | Mathworks | R2012a, R2015a | R2012a for stimulation and R2015a for analysis |
Psychophysics Toolbox | Free | Version 3 | http://psychtoolbox.org |
Insert earphones | Etymotic Research | ER-2 | |
Magnetoencephalography system | Neuromag | Neuromag-122 TM | |
Electroencephalography system | Brain Products | acti64CHamp | |
MNE | Free | MNE Software Version 2.7, MNE 0.13 |
https://martinos.org/mne/stable/index.html |
The Multivariate Granger Causality Toolbox | Free | mvgc_v1.0 | http://www.sussex.ac.uk/sackler/mvgc/ |
An unusual sensory space is one of the effective tools to uncover the mechanism of adaptability of humans to a novel environment. Although most of the previous studies have used special spectacles with prisms to achieve unusual spaces in the visual domain, a methodology for studying the adaptation to unusual auditory spaces has yet to be fully established. This study proposes a new protocol to set-up, validate, and use a left-right reversed stereophonic system using only wearable devices, and to study the adaptation to left-right reversed audition with the help of neuroimaging. Although individual acoustic characteristics are not yet implemented, and slight spillover of unreversed sounds is relatively uncontrollable, the constructed apparatus shows high performance in a 360° sound source localization coupled with hearing characteristics with little delay. Moreover, it looks like a mobile music player and enables a participant to focus on daily life without arousing curiosity or drawing attention of other individuals. Since the effects of adaptation were successfully detected at the perceptual, behavioral, and neural levels, it is concluded that this protocol provides a promising methodology for studying adaptation to left-right reversed audition, and is an effective tool for uncovering the adaptability of humans to a novel environments in the auditory domain.
An unusual sensory space is one of the effective tools to uncover the mechanism of adaptability of humans to a novel environment. Although most of the previous studies have used special spectacles with prisms to achieve unusual spaces in the visual domain, a methodology for studying the adaptation to unusual auditory spaces has yet to be fully established. This study proposes a new protocol to set-up, validate, and use a left-right reversed stereophonic system using only wearable devices, and to study the adaptation to left-right reversed audition with the help of neuroimaging. Although individual acoustic characteristics are not yet implemented, and slight spillover of unreversed sounds is relatively uncontrollable, the constructed apparatus shows high performance in a 360° sound source localization coupled with hearing characteristics with little delay. Moreover, it looks like a mobile music player and enables a participant to focus on daily life without arousing curiosity or drawing attention of other individuals. Since the effects of adaptation were successfully detected at the perceptual, behavioral, and neural levels, it is concluded that this protocol provides a promising methodology for studying adaptation to left-right reversed audition, and is an effective tool for uncovering the adaptability of humans to a novel environments in the auditory domain.
An unusual sensory space is one of the effective tools to uncover the mechanism of adaptability of humans to a novel environment. Although most of the previous studies have used special spectacles with prisms to achieve unusual spaces in the visual domain, a methodology for studying the adaptation to unusual auditory spaces has yet to be fully established. This study proposes a new protocol to set-up, validate, and use a left-right reversed stereophonic system using only wearable devices, and to study the adaptation to left-right reversed audition with the help of neuroimaging. Although individual acoustic characteristics are not yet implemented, and slight spillover of unreversed sounds is relatively uncontrollable, the constructed apparatus shows high performance in a 360° sound source localization coupled with hearing characteristics with little delay. Moreover, it looks like a mobile music player and enables a participant to focus on daily life without arousing curiosity or drawing attention of other individuals. Since the effects of adaptation were successfully detected at the perceptual, behavioral, and neural levels, it is concluded that this protocol provides a promising methodology for studying adaptation to left-right reversed audition, and is an effective tool for uncovering the adaptability of humans to a novel environments in the auditory domain.