Speech perception experiment using binaural integration of phonemic and prosodic information

1. Introduction Since Broadbent (1954) [1] first conducted a dichotic listening task by simultaneously presenting a pair of similar speech sounds at each ear, many aspects of dichotic listening have been studied, including Kimura's early interpretation from a neurological point of view [2]. In a dichotic listening task, a right-ear advantage is often observed, because language processing takes place in the left hemisphere of the brain, when the stimuli for the two ears are almost the same in terms of intensity, duration, and timing [3,4]. Zatorre also pointed out that the intonational information in a speech signal shows a right-hemispheric advantage [5,6]. In addition, Poeppel proposed the AST (asymmetric sampling in time) hypothesis of speech [7,8], where the time constant of temporal integration in the left-hemispheric auditory cortex is short (20–40 ms) and that of the right-hemispheric auditory cortex is long (150–250 ms). In near-infrared spectroscopy (NIRS) studies [9,10], hemispheric laterality using pairs of stimuli contrasting intonational and phonemic information was discussed. The right-handed participants of this experiment showed stronger performance in the left auditory cortex when presented with phoneme-contrasting stimuli and the right auditory cortex when presented with intonation-contrasting stimuli. On the basis of the studies of such laterality, the following evaluation method has been proposed to test central auditory processing disorders (CAPD) [11]:-dichotic listening tests,-monaural low redundancy speech tests,-temporal patterning tests, and-binaural interaction tests. In binaural interaction tests, it is necessary to integrate the information coming from both ears, because a speech signal is segmented into uniform-length frames (e.g., 200 ms) and alternately presented to each of the ears by means of RASP (rapidly alternating speech perception) [12]. In the present study, we likewise conduct a binaural integration test in which phonemic and prosodic information of a speech signal are