Theta Oscillations as a Bridge between Perception and Memory

In natural speech comprehension, listeners rely on more than just the sound of a speaker's voice. Visual cues, such as facial expressions and lip movements, significantly enhance understanding, especially in noisy environments or when speech is fast and complex (Peelle and Sommers, 2015). As many academics may notice, watching the speaker during a lecture, instead of focusing solely on the slides, can improve both attention and comprehension. These observations highlight the importance of visual information in supporting auditory processing and the brain's ability to integrate multisensory inputs during language comprehension. But how does the brain optimally bind visual and auditory information into a coherent speech perception? One proposed mechanism is that aligned sensory inputs, reflected in synchronous cortical oscillations, are more effectively associated in the hippocampus through theta-phase-modulated plasticity. Speech has a natural temporal structure in which syllables typically occur at a rate of ∼4–8 Hz. Because lip movements align closely with syllable onsets, both visual and auditory speech signals are naturally organized at this timescale. As a result, neural activity in auditory and visual cortices tends to follow the stimulus rhythm, reflecting entrainment to the temporal structure of speech (Luo et al., 2010; Park et al., 2016). Notably, this frequency range overlaps with the theta band, which has been widely implicated in mnemonic processes in the hippocampus. Across species, hippocampal theta oscillations are associated with episodic memory encoding, attention, and navigation, and their phase modulates synaptic plasticity mechanisms that support memory formation (Jacobs, 2014). Synchronization between cortical inputs and hippocampal theta is therefore proposed to offer a temporal framework that favors both … Correspondence should be addressed to Beatriz Moura at beatriz.moura.076{at}ufrn.edu.br.