In humans, motion information is mainly processed by the dorsal visual stream. This stream consists of two functional streams: the ventro-dorsal (v-d) and dorso-dorsal (d-d) streams. Patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI) exhibit an impairment in motion perception. By using visual event-related potentials (ERPs), we have previously demonstrated that v-d function related to optic flow (OF) perception is selectively impaired in the dorsal stream in MCI patients. The present study is aimed at examining the difference in the changes in two functional dorsal streams among MCI and AD patients and healthy controls. Therefore, we recorded ERPs in response to OF and horizontal (HO) motion stimuli in patients with AD and MCI, and healthy controls. In all groups, motion-related N170 (V5/middle temporal (MT) origin) and OF-specific P200 (v-d origin) were elicited as major components. MCI patients showed a prolonged latency of P200 for OF but not of N170 for both stimuli compared with healthy controls. In contrast, the latencies of N170 for both stimuli and P200 for OF were significantly prolonged in AD patients compared with other groups. These findings indicate that MCI patients exhibit a selective impairment in the v-d stream, whereas AD patients have impairments in distributed dorsal stream function. These ERP results may reflect the progression of pathological changes in the course of the disease. Therefore, motion-related ERPs are useful to detect and track changes in the brain function of patients with MCI and AD.