Mode of stimulus presentation is one of the principal elements in recording auditory-evoked potentials. When a sound is simultane- ously presented to the two ears, a summation of acoustic energy reaches both ears. Binaural summation of loudness refers to the phe- nomenon that a sound captured by the two ears is usually judged to be louder than the same sound heard with one ear (Porsolt & Irwin, 1967). Reflections of binaural phenomena in brainstem processing of auditory stimuli have been widely sketched in the tone and click-ABR (Dobie & Berlin, 1979; Dobie & Norton, 1980; Ito et al, 1988), frequency following response (FFR) (Parthasarathy & Moushegian, 1993; Krishnan & McDaniel, 1998; Ballachanda & Moushegian, 2000), middle latency (McPherson et al, 1989; Suzuki et al, 1992), and late cortical auditory-evoked potentials (Debruyne, 1984; McPherson & Starr, 1993). However, similar parametric stud- ies have not been conducted for speech stimuli. Recently, speech-ABR was introduced as a pragmatic means to investigate sound structure encoding of speech syllables in the sub- cortical nuclei of the central auditory nervous system (King et al, 2002; Russo et al, 2004; Chandrasekaran & Kraus, 2009). Ample evidence is available to confirm the application of speech-ABR for the objective assessment of biological processes underlying auditory function and auditory processing deficits in normal subjects or clinical populations (Skoe & Kraus, 2010). The typical response to a stop consonant-vowel /da/ is composed of an onset response corresponding to the burst release of the stop consonant (includ- ing peaks V and A), transition from consonant to vowel (peak C), FFR (including peaks D, E, and F), and response to stimulus off- set (peak O). In addition, the interpeak differences between the FFR peaks significantly correspond to the wavelength of funda- mental frequency (F0) (Johnson et al, 2005; Banai et al, 2007). Speech-evoked auditory brainstem responses are sensitive to the manipulation of stimulus parameters, including how the stimulus is presented. Therefore, this study was designed to determine the optimal presentation mode for speech-evoked ABR features. On the other hand, another factor that may influence the speech- ABR is the right versus left ear asymmetry at the brainstem level. Clear evidence indexing asymmetric mechanisms within the lower brainstem and differences in ABR waveforms for stimuli presented to the right versus the left ear are reported in literature (Levine & McGaffigan, 1983; Levine et al, 1988; Sininger et al, 1998; Sininger Original Article Effects of stimulus presentation mode and subcortical laterality in speech-evoked auditory brainstem responses Mohsen Ahadi * , Akram Pourbakht * ,† , Amir Homayoun Jafari ‡,# & Shohre Jalaie $ * Department of Audiology, School of Rehabilitation, Iran University of Medical Sciences, Tehran, Iran, † Rehabilitation Research Center, Iran University of Medical Sciences, Tehran, Iran, ‡ Medical Physics and Biomedical Engineering Department, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran, # Research Center of Biomedical Technology and Robotics, Tehran University of Medical Sciences, Tehran, Iran, and $ Biostatistics Department, School of Rehabilitation, Tehran University of Medical Sciences, Tehran, Iran Abstract Objective: Speech-evoked auditory brainstem responses (ABR) are sensitive to the manipulation of stimulus parameters, including how the stimulus is presented, i.e. monaurally or binaurally. Therefore, this parametric study was designed to compare the effect of binaural and monaural stimuli presentation on speech-evoked ABR features in the parallel assessment of subcortical asymmetry of speech stimuli acoustic elements. Design: Electrophysiological responses to the speech syllable /da/ were recorded within three stimulus presentation modes. Study sample: Forty-eight normal hearing monolingual Persian speakers were included in the current study as volunteer cases. Results: Shorter right ear latency was observed for peaks A and E, but the overall response timing was comparable within different stimulus presentation modes. Binaural stimulation generally led to larger response than monaural stimulation and affected the encoding of speech spectral elements. Moreover, no significant interpeak interval difference was observed in the sustained portion of responses. Response to the right and left ear stimulation was highly correlated, and a symmetrical pattern was observed between the two ears. Conclusions: The timing of speech- evoked responses is not related to the stimulus presentation mode; however, binaural stimulation produces more robust responses. Lateral asymmetry in the representation of speech elements was not considerable at the brainstem level. Key Words: Auditory brainstem responses; speech encoding; auditory processing; diotic advantage Correspondent: Akram Pourbakht, Department of Audiology, School of Rehabilitation, Iran University of Medical Sciences, Nezam St., Shah-Nazari St., Madar Sq., Mirdamad Blvd., Tehran 1545913187, Iran. E-mail: a-pourbakht@tums.ac.ir (Received 13 May 2013; accepted 12 November 2013) ISSN 1499-2027 print/ISSN 1708-8186 online © 2014 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2013.866281 International Journal of Audiology 2014; Early Online: 1–7 Int J Audiol Downloaded from informahealthcare.com by 84.241.34.146 on 02/07/14 For personal use only.