The aim of the PI-PEG study is to explore the taste functions of the following 3 groups of participants: * healthy volunteers * patients with early Parkinson's disease * patients with incipient Lewy body disease. To this end, the results obtained from taste evoked potentials in each of the 3 groups of participants will be compared with each other and with different nutritional, motor and cognitive data. This study could reveal a difference in cortical processing of gustatory sensory information between patients who have had idiopathic Parkinson's disease progressing for 3 years or less, and patients who have had Lewy body disease progressing for 3 years or less. Indeed, a modification of taste evoked potentials (in terms of latencies) proportional to the degree of cerebral degeneration could be observed.