LACK OF EFFECT OF METHYLMERCURY EXPOSURE FROM BIRTH TO ADULTHOOD ON INFORMATION-PROCESSING SPEED IN THE MONKEY

Although it is established that developmental methylmercury exposure p roduces severe motor and sensory impairment, the effect on cognitive f unction is less clear. To explore this issue, monkeys with robust meth ylmercury-induced deficits in visual, auditory, and somatosensory func tion were tested on a series of tasks assessing central processing spe ed, which is highly correlated with intelligence in humans. Five monke ys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 mu g/kg/day of mercury as methylmercuric chloride. Blood mercury le vels were stable at 0.8-1.1 mu g/g until cessation of dosing. When the y were 20 years old, these monkeys and four age-and rearing-matched co ntrols were tested on a series of simple and complex reaction lime tas ks. The monkey sat in a primate chair with a stainless steel bar cente red at waist height. Four push buttons equidistant from the steel bar were mounted on a vertical Plexiglas panel in front of the monkey. The monkey was required to make contact with the bar, then release the ba r and push the appropriate button in response to a change in stimulus conditions. For the first task (simple reaction time), the monkey was required to respond on a button when it changed from unlit to red. The monkey then performed a sequence of complex reaction time tasks: two- button, four-button, and several tasks of increasing complexity using four buttons and multiple colors. For each task, the latency to releas e the bar after the stimulus change (central processing speed) and to move the hand from the bar to the button (motor speed) were determined . Lastly, the monkey was required to make the quickest possible motor response on the simple reaction time task. There were no differences b etween groups on any aspect of the experiment. These data provide furt her evidence for absence of cognitive impairment in monkeys exposed de velopmentally to methylmercury. Published by Elsevier Science Inc.