Therefore, the REA is indicated if the accuracy for the right-ear stimuli is higher than that for the left-ear stimuli. In the non-forced-attention condition, participants are required to allocate attention to both ears and need attention, which is considered endogenous (top-down). Alternatively, Hugdahl and Andersson (1986) detected the REA in a free-report (non-forced-attention) condition.
In previous studies on the REA, directed DL tasks were used in which participants were instructed to pay attention to the left or right ear. These studies used neurophysiological responses as an index of selective attention. (2001) used functional magnetic resonance imaging (fMRI) and demonstrated that activity in the left AC increased when selectively attending to right-ear sounds, and vice versa, right hemispheric activity increased when attending to the left-ear sounds. (2017) also revealed that significant increase of alpha power in the parietal and right frontal-parietal areas during right-ear attention conditions.
The sustained field of the event-related potential was stronger in the left auditory cortex (AC) than in the right in both non-instruction and right-ear attention conditions ( Alho et al., 2012). Additionally, neurophysiological results using directed DL in REA suggest the involvement of selective attention ( Jäncke et al., 2001 Alho et al., 2012 Payne et al., 2017). The structural model of the REA does not incorporate the fact that attention may play a role in right ear bias, as shown in a recent study ( Payne et al., 2017). Listeners use selective attention to focus on auditory information from the right and left ears without moving their heads. This model associates the REA with the combined effect of specialization of the left hemisphere for language processing and contralateral dominance of the auditory pathway ( Kimura, 1967). The structural model originally suggested by Kimura (1961a, b) is the most widely accepted explanation for REA. This preference for the right ear is termed right-ear advantage (REA) during DL. In particular, during dichotic listening (DL) to speech stimuli, listeners reported stimuli more correctly from the right than the left ear. The contralateral pathway is stronger than the ipsilateral pathway ( Rosenzweig, 1951 Hiscock and Kinsbourne, 2011). Additionally, projections from each ear to the bilateral auditory cortices are commonly asymmetric ( Hakvoort et al., 2016 Mei et al., 2020). It is known that the left hemisphere plays a role in language function. Hemispheric asymmetry in audition is even more complicated. The asymmetry of hemispheric organization of brain function is a major topic of research. The right-ear advantage in the free-response dichotic listening was also found in neural activities in the left auditory cortex, suggesting that it was related to the allocation of attention to both ears. We detected a correlation between the attention-related increase in the amplitude of the auditory steady-state response and the laterality index of behavioral accuracy. The amplitude of the auditory steady-state response was larger when attending to the stimuli compared to passive listening. Behavioral tests confirmed the right-ear advantage, with higher accuracy for stimuli presented to the right ear than to the left. Such frequency-tagging allowed the selective quantification of left and right auditory cortex responses to left and right ear stimuli. The amplitudes of the stimuli were modulated at 35 Hz in one ear and 45 Hz in the other. We recorded magnetoencephalography data while participants listened to pairs of Japanese two-syllable words (namely, “/ta/ /ko/” or “/i/ /ka/”). Our study aimed to investigate the role of attention in the right-ear advantage. It is assumed to result from prominent projection along the auditory pathways to the contralateral hemisphere and the dominance of the left auditory cortex for the perception of speech elements. Right-ear advantage refers to the observation that when two different speech stimuli are simultaneously presented to both ears, listeners report stimuli more correctly from the right ear than the left. 6Department of Physiology, School of Medicine, International University of Health and Welfare, Narita, Japan.5Department of Speech Language and Hearing Sciences, International University of Health and Welfare, Narita, Japan.4Faculty of Human Sciences, University of Tsukuba, Tsukuba, Japan.3Faculty of Health Sciences, Hokkaido University, Sapporo, Japan.2Baycrest Centre, Rotman Research Institute, Toronto, ON, Canada.1Department of Science and Engineering, Tokyo Denki University, Saitama, Japan.Keita Tanaka 1 * Bernhard Ross 2 Shinya Kuriki 1,3 Tsuneo Harashima 4 Chie Obuchi 5 Hidehiko Okamoto 6
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