<p>We present a case of developmental auditory–visual synesthesia and its neurophysiological investigation in a female patient in her late teens. She began experiencing persistent positive visual phenomena at age 14, followed by the development of auditory–visual synesthesia. Neurophysiological assessment was conducted using EEG testing. In this study, we focus on the resting EEG and its relationship to colored hearing synesthesia. During music performance, visual EEG inspection revealed a sequential frequency acceleration shifting from the left temporal regions (auditory cortex) to the left centroparietal regions at the onset of color perception, supporting the two-stage hyperbinding model of synesthesia.</p>