A brain-imaging study shows adolescents who abuse alcohol and marijuana show poorer white-matter integrity, with alcohol associated with continuing damage to wiring in prefrontal regions.

A meta-analysis has concluded that low-level exposure to organophosphates has a small-to-moderate negative effect on cognitive function.

A mouse study demonstrates that prolonged social isolation can lead to a decrease in myelin, an effect implicated in a number of disorders, including age-related cognitive decline.

A large study reveals that a diet with high levels of carbohydrate and sugar greatly increases the chance of developing MCI or dementia, while high levels of fat and protein reduce the risk.

• A large study of older adults shows that physical exercise is associated with less brain atrophy and fewer white matter lesions. A small study shows that frail seniors benefit equally from exercise.

A mouse study adds to evidence that green tea may help protect against age-related cognitive impairment, by showing how one of its components improves neurogenesis.

Preliminary results for a small study indicate metabolic syndrome is linked to significantly reduced blood flow in the brain, perhaps explaining its link to cognitive impairment.

Persistent marijuana use beginning before age 18 (but not after) is associated with a significant drop in IQ in a large, long-running study.