Electrophysiological and behavioral measures of phonological processing of auditory nonsense V-CV-VCV stimuli.

An event-related potential, the Phonological Mapping Negativity (PMN), has been reported to reflect recognition of phonological mismatches in speech stimuli. The purpose of the present study was to explore how the PMN response to the auditory nonsense syllable reflects phonological processing in isolation without the letter prime or lexical/semantic context. Sixty-four nonsense syllable stimuli were composed for each of three stimulus conditions: phonological match (PM), phonological mismatch with similar sound (PMMS), and phonological mismatch with different sound (PMMD), making a total of 192 stimuli. The PMN was measured from fourteen normal-hearing listeners. Electroencephalogram (EEG) activity was recorded while subjects were listening to the stimuli and responding behaviorally. Subjects were asked to determine what vowel-consonant-vowel (VCV) (e.g. /apa/) would be formed from the combination of the preceding vowel (V) (e.g. /a/) and consonant-vowel (CV) (e.g. /pa/), and press a 'correct' or 'incorrect' response button as soon as they decided whether the target VCV stimulus matched their expectation. In this way, along with the PMN, behavioral response accuracy and reaction times were obtained. The results were as follows: (1) PMN amplitude was not different by stimulus condition, (2) PMN amplitude was larger over frontal and central than posterior regions, but not different between the left versus right hemisphere, (3) PMN was detected in the absence of N400, and (4) behavioral responses were more accurate and faster in PMMD than PM and PMMS. Results indicate that the PMN can reflect phonological processing of auditory nonsense syllables in isolation. The scalp distribution of PMN is most dominant in the fronto-central regions without lateralization. Lastly, behavioral response accuracy and reaction times appear to be influenced by the extent of the task difficulty or processing demand rather than by the extent of phonological violation.