Inside Couey's brain

Making a case for John Couey's "bad brain," Dr. Joseph Wu of the University of California at Irvine, showed scans of the defendant's brain function to jurors, contrasting it with images from a normal adult.

PET scans can be combined with other sources of information to make a clinical diagnosis.

Wu said he noted an asymmetry in the function of Couey's brain, which could be consistent with psychosis, mood disregulation and low IQ scores.

The temporal lobe, where Wu noted the significant asymmetry, can affect impulse control. Abnormalities of this kind are often associated with aggression and hypersexuality.

Inherent flaws in brain structure, combined with a bad rearing environment, can lead to a "catastrophic ability to regulate impulses," he said.

-- Elena Lesley