Electrophysiological Manifestations of Speech Processing and their Promise as Objective Measures

Background: Hearing is the primary sensory modality of human communication: speech. Almost without exception, clinical assessment of auditory function addresses hearing sensitivity (e.g., threshold, discrimination) and not auditory processing. Unlike hearing sensitivity, that has objective electrophysiological tests, speech comprehension is only assessed behaviorally, with no direct measure of the brain processing involved. Electrophysiological functional imaging of speech processing in the brain is now available and may be used in speech perception assessment.

Objective: Study brain activity specific to speech sounds and to language as well as audio-visual integration in speech perception. Thus, we recorded electrophysiological manifestations of processing the same sounds when perceived as speech or as non-speech, as words in first or second language and while lip-reading that is congruent or incongruent with the speech sound presented concurrently.

Methods: Subjects listened to pairs of utterances and indicated whether pair members were the same or not. Source current densities of event‐related potentials to each utterance in the pair were estimated.

Results: The distribution and magnitude of brain activity associated with speech processing varied across multiple time scales and settings with early processing (<300 ms), which mostly related to the acoustics of the sound, and later processing (>300 ms), which was more cognitive-, context- and task-related. Brain processing of speech changed according to contexts and settings, be they linguistic, type of acoustic cues discriminating the speech or even treating the very same stimulus as speech or non-speech.

Conclusion: Electrophysiological functional imaging of speech processing differs when the same sounds are perceived as speech or non-speech, as congruent or incongruent with lip reading that accompanies them and between the same words in first and second language. This sensitivity can be used, with some further development, for objective assessment of speech perception and discrimination in monitoring the progress of patients undergoing training following cochlear or brainstem implants and the efficacy of using lip-reading to aid their hearing.