Recent studies have reported that early right anterior negativity (ERAN) is elicited by harmonic irregularity in Western tonal music. We assumed that tonal hierarchy-as the basic structure for harmony processing-is expressed as a mental representation with spatial specificity. In this study, we examined whether this assumption was appropriate by investigating the effect of a mental rotation task on ERAN. If the representation of tonal hierarchy has a spatial structure, mental rotation of the structure will decrease the amplitude of ERAN. Twelve adult participants took part in the experiment, which comprised three task conditions (mental rotation, reading, and control). Event-related potentials to cadenzas including the Neapolitan sixth chord as the deviated tone were recorded under each condition. Results showed a decrease of ERAN amplitude in the mental rotation task, suggesting a relationship between spatial processing and the establishment of tonal hierarchy. Further research is necessary to confirm distinguish whether the effect is attentional or task specific.