Much testing on the safety of chemical substances is done on rodents, specifically rats.
Rodent physiology is quite different than humans, and rodents metabolize chemicals differently.
Rats in the aspartame tests had to be fed 60 times more aspartame to approximate the equivalent effect on humans. Tests using rodent species relative to aspartame have also been conducted with guinea pigs, because much of the brain development occurs in utero, much like humans, and the guinea pig metabolizes phenylalanine in a manner more similar to humans than rats. In other words, the fetal brain was affected negatively by aspartame in areas responsible for learning tasks, e.g, the amygdala. This particular scientific experiment was conducted at the Laboratory of Cerebral Metabolism, Department of Neurosurgery, State University of New York, Health Science Center, Brooklyn, New York, and the Department of Psychology, San Diego State University, San Diego, California. Even though the experiments were conducted on guinea pigs, the report mentioned that aspartame, particularly phenylalinine, has been shown to be developmentally neurotoxic even in rats.