Effect of dopamine and acetylcholine on the visual evoked potential

Visual evoked potentials were measured on patients with Parkinson's disease and Alzheimer's disease and normal controls to assess the function of dopamine and acetylcholine in the visual system. Dopamine is a neurotransmitter known to be present in the retina of primates and is found to be severely depleted in the substantia nigra of patients with Parkinson's disease. Acetylcholine is also known to be present in the retina, visual cortex, and superior colliculus and is found to be grossly reduced in patients with Alzheimer's disease. Stimuli were designed to preferentially activate functionally separate pathways in the visual system described as magnocellular and parvocellular. The four stimuli were a diffuse flash; an achromatic, 73' check counterphasing at 6 Hz at a contrast of 30%; an achromatic 10' check counterphasing at 2 Hz at a contrast of 85%; and an isoluminant red/green grating of 4 cpd presented using an on and off cosine ramp of 200 ms. The results indicate that an acetylcholine deficit produces a delay to the flash P2 component of the visual evoked potential. No change was detected when other stimuli were used.