On the relationship between cortical oscillatory activity and inner ear amplification gain

The role of the efferent system (descending projections from the auditory cortex to the lower auditory centres) in aiding speech processing and comprehension remains controversial in the auditory literature. In a previous approach (Hernandez-Perez et al., 2018), we addressed how the cochlear gain is differentially modulated by the efferent system depending on the auditory scene encountered (single vs multiple speech streams perception). To determine if this efferent control of the cochlear gain is related to word processing, we have explored in this study the relationship between oscillatory activity and amplification of the cochlear gain (measured using otoacoustic emissions: OAEs). In particular, we have focused our interest on the relationship between the cortical oscillatory activity in the theta band (4–8 Hz), that has been extensively linked to word processing (Ghitza, 2012; Ghitza, 2013), and amplification of the cochlear gain. Here concurrent EEG and OAE recording systems allowed us to monitor, at different stages of the auditory pathway, the effects of lexical decision-making as well as passive listening of words and non-words. In a series of three experiments, speech intelligibility was either manipulated by noise vocoding the speech signal or adding babble noise/speech shaped noise to the clean speech. Our results show that theta band energy is stronger in the power spectrum when participants are performing the lexical decision compared to passive listening of speech. Changes in oscillatory activity could also be accompanied by suppression of Click-Evoked OAE magnitude depending on the type of stimuli being processed. This study contributes to our better understanding of the likely role that the auditory efferent system plays in speech processing and comprehension.