How the Brain Distinguishes Internal and External Sounds: An fMRI Investigation of Auditory Sound Externalization

Auditory externalization, the perception of a sound source as located outside the head, is essential for spatial hearing and auditory scene analysis. However, its neural correlates remain poorly understood. This study investigated differences in brain activation elicited by externalized versus internalized sound sources. Twenty-nine healthy participants underwent a 3T functional magnetic resonance imaging (fMRI) scan while listening to auditory stimuli presented in three spatialization conditions: reverberant externalized sounds (highest externalization), anechoic externalized sounds (intermediate externalization) and diotic anechoic sounds (internalized). Whole-brain analyses revealed greater activation for externalized compared to internalized sound sources in the left superior temporal gyrus, including the planum temporale, the cerebellum and the left posterior cingulate gyrus. Internalized sounds elicited greater relative activity in the left inferior temporal gyrus. Direct comparison between the two externalized conditions revealed stronger left superior temporal gyrus activation for reverberant sounds, while anechoic sounds preferentially activated the right middle temporal gyrus. These findings confirmed the key role of the planum temporale in auditory externalization and the involvement of higher-order brain regions, suggesting broader networks underpinning the perception of sound location.