To investigate the involvement of the cortical cholinergic system in patients with Parkinson's disease (PD) and with progressive supranuclear palsy (PSP), we performed a comparative study of the short latency afferent inhibition (SAI) in ten patients with PD, in eight patients with PSP and in 15 healthy subjects. Six of the PD patients and four of the PSP patients had dementia. SAI was significantly increased in the PD patients, whereas it was not significantly different between PSP patients and the normal controls. Our findings demonstrate that the excitability of the motor cortex is differentially modulated by sensory afferents in PD and PSP and may indicate that the mechanisms of cholinergic dysfunction are different between the two diseases.
