Long-term learning deficits of mentally retarded monkeys.

During the first year of life, 12 rhesus monkeys were either subjected to diets high in phenylalanine or para-chlorophenylalanine either pre- or postpartum, leading to a condition similar to phenylketonuria (PKU); 12 other rhesus monkeys were controls. After removal from the PKU diets and return to normal diets for a period of 2 years, subjects were tested for learning ability on an extensive series of tasks. No support was found for suggestions that PKU monkeys (a) do worse if task difficulty is increased, (b) are less affected by negative reinforcement, (c) have an attentional or short-term memory storage deficit, and (d) do not attend to the relevant dimension. Support was found for the hypothesis that PKU subjects are more emotional, accounting for (a) disruption in performance following negative reinforcement and (b) difficulty in changing an initial or a learned response pattern.