EFFECTS OF STIMULUS RATE AND PERIODICITY ON AUDITORY CORTICAL ENTRAINMENT AND THEIR RELATION TO SPEECH RHYTHMS

The neural mechanisms underlying rapid exogenous coding and the neural entrainment to auditory stimuli have been the subject of controversy. Using inter-trial phase-locking (ITPL) and phase-locking value (PLV) analyses applied to high-density human electroencephalogram (EEG) data, this dissertation investigated the degree to which the brain entrains to speech and non-speech (i.e., click) sounds and how changes in the rate and periodicity of the ongoing streams alter brain oscillation patterns. Temporal processing of cortico-acoustic tracking was investigated in N=24 normal young adults utilizing EEG time-frequency and source analyses that isolated neural activity stemming from both auditory temporal cortices. We manipulated the rate and periodicity of repetitive and continuous speech and click stimuli parametrically to investigate how the speed and jitter in ongoing auditory stimuli affect neural entrainment. Brain-behavior associations were investigated using analogous perceptual tests—temporal modulation transfer functions (TMTFs) and computerized adaptive beat alignment test (CA-BAT)—to tap listeners’ behavioral rate and periodicity sensitivity, respectively. Both stimulus domains (speech/non-speech) showed rightward hemisphere asymmetry in phase-locking strength; stronger response at the nominal speech syllable rate than at faster rates; and longer latency for speech vs. click. PLV in speech demonstrated a stark improvement in neuronal synchronization at 4.5 Hz that degraded at higher rates but with an opposite pattern for click. ITPL periodicity had stronger phase-locking in the right hemisphere for both stimulus types but longer latencies for speech. PLV, however, showed decreased phase-locking to speech with increasing jitter, yet entrainment to speech was still superior to that of clicks. However, click responses were not affected by periodicity. Even though the interpretation of ‘periodic speech preference’ can’t really be drawn from the presentation of only a syllable train with no comprehension task but overall, speech is more sensitive to changes in rhythm and periodicity in passive listening, maybe as a result of the greater neuronal entrainment it elicits in the brain. These findings explain fundamental response characteristics that will be critical for clinical and research applications of neural entrainment