GPI and Parkinson disease: However, recent electrophysiological studies using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD monkeys have failed to detect an expected increase in GPi activity (Wichmann et al., 1999; Raz et al., 2000; Rivlin-Etzion et al., 2008).