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

Abstract

Auditory externalization refers to the perception of a sound as originating from a source outside the head, as typically occurs in natural listening environments. Conversely, sounds may be perceived inside the head (internalization) when spatial cues are impoverished, as in some headphone-based listening conditions. However, the neural correlates of auditory externalization 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: two types of externalized sounds (reverberant and anechoic sounds) and one internalized sounds (diotic anechoic sounds). 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 supported 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.