COMT and Parkinson disease: In terms of planning ability, previous studies [11, 12] hypothesized that based on the inverted-U rule, PD Met homozygotes, which code for COMT with poor dopamine metabolism, were postulated to have poorer planning performances compared to those of PD Val homozygotes, due to the hyperdopaminergic state in the prefrontal cortex (PFC) in early PD.