Hemispheric decoupling of awareness-related activity in human auditory cortex under informational masking and divided attention

Abstract

Whether unattended sound streams reach perceptual awareness, and the extent to which they are represented in the central auditory system, are fundamental questions of modern hearing science. We examined these questions utilizing M/EEG, multi-tone masking, and a dual-task dichotic listening paradigm. Listeners performed a demanding primary task in one ear—detecting isochronous target-tone streams embedded in random multi-tone backgrounds and counting within-target-stream deviants—and retrospectively reported their awareness of similar masker-embedded targets in the other ear. Irrespective of attention or stimulation ear, left-AC activity strongly covaried with target-stream detection starting as early as 50 ms post-stimulus. In contrast, right-AC activity, while highly sensitive to stimulation ear, was unmodulated by detection until later, and then only weakly. Thus, under certain conditions, human ACs can functionally decouple, such that one—here, right—is automatic and stimulus-driven while the other—here, left—supports perceptual and/or task demands, including basic perceptual awareness of nonverbal sounds both within and outside the focus of top-down selective attention.