Neural mechanisms of auditory awareness underlying verbal transformations.

Prolonged listening to a repeated word without a pause produces a series of illusory transitions of the physically unchanging word, which is called verbal transformation. Verbal transformations provide a rare opportunity to examine how auditory percepts are formed in the brain. We found that verbal forms are affected by phonetic reorganization of a word, rather than by auditory adaptation and lexical distortion of it. We identified brain activity leading to individual differences between perceptual transitions and tone detection. An event-related fMRI analysis revealed that the left inferior frontal cortex (IFC), anterior cingulate cortex (ACC), and the left prefrontal cortex were activated when perceptual transitions from one verbal form to another occurred, but not when tone pips were detected. The number of perceptual transitions showed positive and negative correlations with signal intensity in the left IFC and the left ACC, respectively. The results suggest that active generation of verbal forms is linked with articulatory gestures for speech production and that the frequency of perceptual transitions is determined by a balance of the activations between the two brain regions. Structural equation modeling demonstrated that individual differences in the number of perceptual transitions rely on negative feedback from the ACC to the IFC via the posterior insula. These findings suggest that distributed frontal areas are involved in auditory awareness underlying verbal transformations.