EFFECT OF DOPAMINE AND ACETYLCHOLINE ON THE VISUAL-EVOKED POTENTIAL

Visual evoked potentials were measured on patients with Parkinson's di sease and Alzheimer's disease and normal controls to assess the functi on of dopamine and acetylcholine in the visual system. Dopamine is a n eurotransmitter known to be present in the retina of primates and is f ound 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 gros sly reduced in patients with Alzheimer's disease. Stimuli were designe d to preferentially activate functionally separate pathways in the vis ual system described as magnocellular and parvocellular. The four stim uli 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 indi cate that an acetylcholine deficit produces a delay to the flash P2 co mponent of the visual evoked potential. No change was detected when ot her stimuli were used.