Parkinson's disease, dementia with Lewy body, and other Lewy body disorders are characterized by the formation of Lewy bodies, which are α-synuclein (αSyn) aggregates, within nerve cells. The pathological process in Lewy body diseases begins in the olfactory bulb or the enteric nervous system and is thought to spread to the brain, leading to the onset of various neurological symptoms. We developed a marmoset model by inoculating synthetic αSyn fibers into the olfactory bulb and enteric nervous system, and demonstrated that this model faithfully recapitulates key features of Lewy body dementia, such as REM sleep behavior disorder and reduced brain activity in the occipital lobe. In this study, we aim to elucidate the neural circuits involved in the disease process and explore therapeutic strategies targeting the responsible brain regions.