Neural Entrainment to Speech Analyzed with EEG : A Review of Contemporary Theories about the Underlying Mechanisms of Speech Processing

Abstract: Neural entrainment quite recently became considered an important mechanism used by the brain to process stimuli with periodic qualities, such as the frequency and duration time of signals reaching sensory organs. An increasing amount of data strongly implies that the brain might be using neural entrainment as a mechanism to either directly process speech and/or to facilitate speech interpretation. Neural entrainment is therefore a promising marker to use for research of speech perception. This literature review aims to summarize the most recent findings within this area with the end-goal to be used as a basis for designing an EEG experiment intended to analyze speech perception as a means to distinguish human voices.    For this reason, data was collected from the scientific databases Europe PMC, Academic Search Premier, PsycINFO, PubMed, Scopus and Web of Science, where the keywords “EEG” + either the phrase “neural entrainment”, “neural oscillation”, or “cortical oscillation” were used to gather articles. Inclusion and exclusion criteria were then applied and the data was analyzed with the intention to answer the following research questions: “is it possible to observe neural entrainment to human voice/speech using EEG?”, “if so, what are the possibilities to use such neural entrainment as a marker for differentiating human voices from each other?” and “what is the nature of the mechanisms used by the brain to attain this entrainment?”. The resulting data from the articles indicated that, in order to yield reliable results when investigating neural entrainment to speech, the technique for analysis of brain activity could be done with EEG, a number of participants between 15-30 persons is enough, the spectral bands of interest are delta (<3 Hz), theta (4-8 Hz), beta (15-35 Hz) and gamma (>40 Hz), the method of analysis could be looking at both frequency and amplitude in the speech envelope, and finally the anatomical areas for investigating the brain’s ability to distinguish human voices using speech entrainment could be either areas within the auditory cortex or prefrontal areas involved in behavioral responses to speech processing.