Neurotoxicity of methylmercury in squirrel monkeys. Cerebral cortical pathology, interference with scotopic vision, and changes in operant behavior.

Blood mercury was raised to levels exceeding 1,000 ng/gm by weekly oral doses of methylmercury hydroxide to squirrel monkeys. Operant behavior tests were employed and neurological signs were recorded on film. Sudden visual disturbance occured with subacute exposure. Prolonged exposure resulted in impaired coordination with impairment of scotopic vision and possibly also sensory disturbances. Scotopic vision was tested by determining the critical fusion intensity (cfi) at 10 cps. An increase in cfi was the earliest neurological sign appearing in some monkeys months before other signs could be detected. Poisoned monkeys exhibited typical cerebral cortical lesions. The visual cortex was invariably involved. Extension to adjacent cortical areas increased with increasing duration of exposure and increasing brain mercury. The microgram per gram brain tissue was the lowest concentration of methylmercury seen wiht morphological lesions.