Discharge responses of the optic tract to flash stimuli in Parkinson's disease

Dopamine has a significant role in retinal processing, and it has been demonstrated that retinal dopamine content is decreased in parkinsonian patients. We measured the latency of the evoked discharges in the optic tract (OT) to flash stimuli during stereotactic pallidal neurosurgery in 25 patients with Parkinson's disease (PD) (13 women and 12 men, age 38–78 years, unified Parkinson's disease rating scale (UPDRS) Motor Score in the Off state 11–54, Hoehn and Yahr stage in the Off state 1.5–5) and investigated the effects of age at surgery, disease duration, levodopa dose, and severity of parkinsonian symptoms on the latency. OT discharges were evoked by monocular flash stimuli delivered from a flashlight with a krypton bulb with a tungsten filament. The luminance at the eye measured ∼4 × 10(4) cd/m(2). The light wavelength of the stimulus was composed of a wide spectrum with its peak at around 800 nm or longer. The latency of OT discharges ranged 49–79 msec, and there was a significant positive correlation between the latencies of evoked activities in the OT to a flashlight and age (r = 0.59, P < 0.001, by Pearson correlation), but no correlation between the latency and the severity of parkinsonian symptoms and between the latency and duration of illness. These results indicate that the delay in visual processing and conduction at the level of the retina and the OT are substantially derived from age-related degenerative changes in the retina and visual pathway which are apparently unrelated to the striatal dopamine deficiency in PD.