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    • In order to successfully study early auditory processing in

    2018-11-15

    In order to successfully study early auditory processing in infants, age-appropriate techniques that do not require overt responses are needed, and examination of dense-array auditory evoked-response buy T-5224 potentials (dEEG/ERPs) provides non-invasive functional brain measurements of these skills in infancy. In the first year of life, auditory ERPs are characterized by a positive deflection (P1) at about 150ms from stimulus onset, followed by a negative peak (N2) at 200–250ms (e.g., Ceponiene et al., 2002). These two early peaks are referred to as obligatory responses, and are thought to be associated with auditory detection (P1), and feature processing (N2) (e.g., Ceponiene et al., 2008). However, in older children the N2 amplitude has been shown to increase with stimulus repetition (Karhu et al., 1997) and for this reason it was suggested that the N2 indexes the build-up of a neural representation or a sensory memory trace of the repeated stimulus. Recent literature suggests that in typical “oddball” paradigms (where deviants are presented within a series of standard or frequent stimuli), the N2 in response to deviant stimuli might represent the beginning point/onset of the discrimination response (Choudhury and Benasich, 2011). For this reason, in the present manuscript we have labeled this peak on the deviant waveform as “N2*” to emphasize that this peak could well be functionally different from the N2 on the standard wave. Finally, in oddball paradigms the electrophysiological pattern characterized by the P1/N2 (N2*) peaks is generally followed by a mismatch response (MMR), thought to reflect a neural change detection process. This component does not require conscious attention to the stimuli, and thus provides a measure of fine acoustic discrimination abilities even in preverbal infants. Within the infant literature, the MMR is usually characterized by a large positivity at about 300ms from deviant stimulus onset (e.g., Kushnerenko et al., 2002). The reasons for the positive polarity of the MMR in infancy (as compared to the typical mismatch negativity elicited within the same paradigms in older children and adults) are still not clear, and researchers hypothesize that this polarity shift could be related to the level of alertness or the sleep stage of sleeping infants (e.g., Friederici et al., 2002); to the maturational level of the infant (e.g., Leppänen et al., 2004); to the magnitude of the deviant stimulus change (e.g., Morr et al., 2002); or to particular filter settings (e.g., He et al., 2007). In the present paper the mismatch response will be referred to as the P3 component—labeled to reflect its polarity and average time of onset. ERPs have been extensively used in the first year of life in order to investigate infants’ ability to discriminate changes that occur in the processing of speech, including changes in a single phonetic feature in the consonant or vowel of a syllable, (e.g., Dehaene-Lambertz and Dehaene, 1994; Rivera-Gaxiola et al., 2005) and/or changes in the duration of the vowel (e.g., Friederici et al., 2002; Guttorm et al., 2005; Leppänen et al., 1999) or of the consonant (e.g., Kushnerenko et al., 2001a; Leppänen et al., 2002). Discrimination of changes in non-speech signals have also been investigated, including frequency changes (e.g., Fellman et al., 2004; Leppänen et al., 2010; Wunderlich et al., 2006) and changes in the temporal aspects of auditory stimulation, such as sound duration (Cheour et al., 2002; Kushnerenko et al., 2001b). Overall, these studies suggest that these acoustical features are encoded in auditory sensory memory, as reflected in the elicitation of the aforementioned ERP responses (especially N2/N2* and P3), although with differences in the morphology, latency and amplitude of these peaks/components. These cross-stimulus feature differences might imply that speech vs. non-speech stimuli, as well as sound frequency vs. sound duration are encoded differently in the auditory cortex and thus are reflected differently in the ERPs (Ceponiene et al., 2002).