Neural response phase tracks how listeners learn new acoustic representations.

Humans are remarkable at rapidly learning regularities through experience from a dynamic environment. For example, long-lasting memories are formed even for auditory noise patterns after short, repeated exposure in an unsupervised manner. Although animal neurophysiological and human studies demonstrate adaptive cortical plasticity after sensory learning and memory formation, the mechanisms by which the auditory system extracts and encodes holistic patterns from random noise, which contains neither semantic labels nor prominent acoustic features to facilitate encoding, remains unknown. Here we combined magnetoencephalography (MEG) with psychophysics to address the issue. We demonstrate that the establishment of a reliable neuronal phase pattern in low-frequency (3-8 Hz) auditory cortical responses mirrors the noise memory formation process. Specifically, with repeated exposure, originally novel noise patterns are memorized, as reflected in behavior, and gradually produce robust phase responses in auditory cortex. Moreover, different memorized noises elicit distinguishable phase responses, suggesting their specificity to noise structure. The results indicate that the gradual establishment of low-frequency oscillatory phase patterns in auditory neuronal responses mediates the implicit learning process by which originally undifferentiated noises become new auditory objects.