This study used electroencephalographic (EEG) measures to investigate language processing in 7-month-old infants. Infants receive different language input than adults. Individuals often modulate prosody of normal speech, or adult directed speech (ADS), in characteristic ways when addressing infants, producing infant directed speech (IDS). This study focused on how infants process (1) IDS versus ADS and (2) words versus sounds in the theta functional power band (3.6 – 5.6 Hz). Three experiments used time-frequency analysis to explore how vocalizations, differing along speech register and linguistic properties, affect how intensely and where in the brain infants experience theta oscillations. Experiments 1 (N = 20) and 2 (N = 23) focused on how infants process IDS and ADS. Auditory stimuli were presented alternatingly in ID and AD speech and power spectral density (μV2) in the theta band was recorded. Results showed no significant deviations in how infants processed IDS and ADS stimuli (p > .05). Experiment 3 (N = 17) focused on how infants process words (“Guck mal”) and sounds (“UA”). Stimuli were presented alternately, always spoken in IDS. Theta activation did not differ significantly between word and sound conditions (p>.05). Results predominantly suggest that, regarding theta activation, 7-month-old infants do not differentiate in their processing of IDS versus ADS vocalizations or in their processing of words versus sounds. Investigating the location of primary activation revealed that increased μV2 emerged in patterns. The frontal-midline theta activation pattern observed is congruous with attention modulation as well as language and familiar/unfamiliar auditory stimulus processing.
