Selective impairment of optic flow perception in amnestic mild cognitive impairment: evidence from event-related potentials.

Alzheimer's disease (AD) patients have visuospatial deficits due to parietal dorsal stream dysfunction. Two distinct dorsal flows have been proposed: the inferior parietal (ventro-dorsal (v-d)) and superior parietal (dorso-dorsal (d-d)) streams. We aimed to elucidate how the two dorsal streams are altered in patients with amnestic mild cognitive impairment (aMCI) and AD. Thus, the psychophysical threshold measurements and visual event-related potentials (ERPs) were recorded in patients with aMCI and AD, and in healthy old and young adults. The visual stimuli included radial optic flow (OF) derived from the v-d stream and horizontal (HO) motion conveyed from the d-d stream. The motion thresholds between aMCI patients and old adults were comparable. However, AD patients showed significantly higher motion thresholds for both stimuli compared with other groups. In lower-level ERPs, there were no significant differences in P1 (100 ms) and N1 (130 ms) for both stimuli among the groups. For higher-level ERPs, aMCI patients showed the prolonged latency of OF-specific P200 (v-d origin) and comparable latency of motion-related N170 (V5/MT origin) for both stimuli compared with old adults. In AD patients, both N170 and P200 latencies were significantly prolonged compared with other groups. P200 latency was closely correlated with the Mini-Mental State Examination score. These findings indicate that the v-d function related to OF perception is selectively impaired in aMCI, whereas AD has impairment of the distributed higher-level dorsal stream. Therefore, OF-specific P200 can be useful for detecting early functional changes of the brain in aMCI.