Interference in dichotic listening: the effect of contralateral noise on oscillatory brain networks.

Coupling of thalamocortical networks through synchronous oscillations at gamma frequencies (30-80 Hz) has been suggested as a mechanism for binding of auditory sensory information into an object representation, which then becomes accessible for perception and cognition. This study investigated whether contralateral noise interferes with this step of central auditory processing. Neuromagnetic 40-Hz oscillations were examined in young healthy participants while they listened to amplitude-modulated sound in one ear and a multi-talker masking noise in the contralateral ear. Participants were engaged in a gap-detection task, for which their behavioural performance declined under masking. The amplitude modulation of the stimulus elicited steady 40-Hz oscillations with sources in bilateral auditory cortices. Analysis of the temporal dynamics of phase synchrony between source activity and the stimulus revealed two oscillatory components; the first was indicated by an instant onset in phase synchrony with the stimulus while the second showed a 200-ms time constant of gradual increase in phase synchrony after phase resetting by the gap. Masking abolished only the second component. This coincided with masking-related decrease of the P2 wave of the transient auditory-evoked responses whereas the N1 wave, reflecting early sensory processing, was unaffected. Given that the P2 response has been associated with object representation, we propose that the first 40-Hz component is related to representation of low-level sensory input whereas the second is related to internal auditory processing in thalamocortical networks. The observed modulation of oscillatory activity is discussed as reflecting a neural mechanism critical for speech understanding in noise.