BAKERSFIELD, Calif. -- Chemotherapy patients sometimes experience what they call a "brain fog" -- increased forgetfulness and sluggish thinking, as if the brain is wrapped in cotton.

Now a new study has revealed the root of the problem.

Researchers at Stanford's Cancer Center performed MRI brain scans on 28 women, half of whom were undergoing chemotherapy for breast cancer.

The women had to remember common words such as tree or house. Results showed that both groups performed equally well on the memory test, but the chemo patients had to exert more brain power.

Areas that should have been resting were instead active during the test. And an area at the front part of their brains -- called the prefrontal cortex -- showed reduced activity as the patients were trying to memorize the words.

Doctors discovered that women taking a particular combination of drugs known as CMF had the most abnormal patterns of brain functioning, suggesting this regimen might have the worst effect.