The results were dramatic, says Kenet. While the brain region of the pups raised without exposure to the toxicant was developing typically, the brain region in the pups exposed to the toxicant in utero and while nursing was profoundly altered.

"The animals could hear, but their brains representation of what they heard was grossly disturbed," says Merzenich.

In one pronounced change, the balance of inhibitory and excitatory signaling between nerve cells, which contributes to the appropriately controlled responses of the brain to stimuli, was disrupted. Strong evidence indicates that there is imbalance in signaling throughout the brain of children with some developmental disorders, such as autism, says Merzenich.

In a secondary experiment, the toxicant-exposed pups were raised in a modified sound environment in which they were exposed to continuous tone or noise pulses. It was here that the auditory cortexs decreased capacity to change in response to sound was revealed. "This activity is crucial in the developing brain," says Merzenich. "Interruptions in these early-learning progressions contribute to learning-related challenges."

As the auditory cortex is the first sensory region to develop, its abnormal development in the rat pups could be just a hint of more pervasive effects of exposure, the scientists say.

PCB95 is closely related in its chemical structure to polybrominated diphenyl ether (PBDE), which is difficult to study and has only begun to receive attention for its environmental effects, says Kenet. It has been used in large quantities in the last 25 years, mostly in fire retardation in home and office furniture and electronics.

"Weve done as yet unpublished studies with PDBEs," says Pessah, director of the UC Davis Center for Children's Environmental Health. "The current finding could be just the tip of the iceberg."