A wearable system for adaptation to left-right reversed audition tested in combination with magnetoencephalography

Abstract

Exposure of humans to unusual spaces is effective to observe the adaptive strategy for an environment. Though adaptation to such spaces has been typically tested with vision, little has been examined about adaptation to left-right reversed audition, partially due to the apparatus for adaptation. Thus, it is unclear if the adaptive effects reach early auditory processing. Here, we constructed a left-right reversed stereophonic system using only wearable devices and asked two participants to wear it for 4 weeks. Every week, the magnetoencephalographic responses were measured under the selective reaction time task, where they immediately distinguished between sounds delivered to either the left or the right ear with the index finger on the compatible or incompatible side. The constructed system showed high performance in sound localization and achieved gradual reduction of a feeling of strangeness. The N1m intensities for the response-compatible sounds tended to be larger than those for the response-incompatible sounds until the third week but decreased on the fourth week, which correlated with the initially shorter and longer reaction times for the compatible and incompatible conditions, respectively. In the second week, disruption of the auditory-motor connectivity was observed with the largest N1m intensities and the longest reaction times, irrespective of compatibility. In conclusion, we successfully produced a high-quality space of left-right reversed audition using our system. The results suggest that a 4-week exposure to the reversed audition causes optimization of the auditory-motor coordination according to the new rule, which eventually results in the modulation of early auditory processing.

Keywords: Auditory adaptation; Early auditory processing; Magnetoencephalography (MEG); Neural plasticity; Stimulus-response compatibility; Unusual environment.

Fig. 1

Left–right reversed audition system. Binaural microphones are cross-connected to a linear PCM recorder, in which the analog sound signals are digitalized. Digital signals are immediately played by in-ear earphones that are connected directly to the recorder. The microphones and the earphones are slightly isolated by soundproofing materials

Fig. 2

Experimental setup of sound source localization. In an anechoic room, 1000-Hz tones are presented from either of 72 sound sources ranging from −180 to 175 degrees through plane-wave speakers that are directed toward the center of the circle, where a digital angle ruler protractor is set. The sound at one location lasts for 10 s and is immediately switched to the next sound at another location. Each of sound sources is used once in a random order

Fig. 3

Selective reaction time task. Tone bursts of 1000 Hz are delivered to either the left or the right ear pseudorandomly. In compatible blocks, participants are instructed to respond immediately to the tone bursts with the index finger on the compatible side. In incompatible blocks, they are instructed to respond immediately to them with the index finger on the incompatible side. Each block has 80 presentations, and these blocks are alternatively arranged

Fig. 4

Results of localization of 72 sound sources ranging from −180 to 175 degrees based on the front direction in a clockwise manner averaged across six participants in the normal and reversed conditions. a Correlation between physical and perceptual angles in the normal (white dots; adjusted R ² = 0.99) and reversed (dark gray dots; adjusted R ² = 0.96) conditions. b Correlation between normal and reversed perceptual angles (light gray dots; adjusted R ² = 0.98). The reversed perceptual angles are more correlated (negatively) with normal perceptual angles than physical angles

Fig. 5

Mean reaction times during the selective reaction time task in a participant A and b participant B. The reaction times for response-incompatible sounds are longer than those for response-compatible sounds until the third week (W3), but becomes slightly shorter in the fourth week (W4). Moreover, the mean reaction times are the longest in the second week (W2) irrespective of compatibility and returns to the initial level after the exposure period (shaded in gray)

Fig. 6

Characteristics of N1m components under the selective reaction time task. a Dynamic statistical parametric mapping waveforms averaged across all conditions for the left and right cortices and source localization for N1m components. The N1m components are detected at about 90 ms after sound onset in the bilateral superior temporal planes (STPs). b, c Changes in the relationships between N1m intensities within the ROIs of STPs for the response-compatible sounds and response-incompatible sounds over exposure time in participants A and B, respectively. The N1m intensities in the compatible conditions tend to be larger than those in the incompatible conditions from Pre to the third week (W3) but reduce further in the fourth week (W4) in both hemispheres. The intensities are the largest in the second week (W2) irrespective of compatibility and laterality

Fig. 7

Temporal changes in functional connectivity across the left and right auditory (LA and RA, respectively) and motor (LM and RM, respectively) areas at a threshold of p < 0.05 during the selective reaction time task. White, gray, and black squares indicate the number of participants who showed significance for the Granger causality test (N = 0, 1, and 2, respectively). Initially-interplayed auditory-motor connectivity is unstable from the first week and transiently shows overall disruption in the second week. The connectivity disruption recovers thereafter, though unstable situation continues