The leaf of this tree, native to China and Japan (and used in China for 5000 years), improves memory and brain function by helping blood flow to the brain. The active chemicals, known as gingkosides, help focus one's attention and is useful in counteracting the side effects of caffeine. It should be noted, though, that more than 3 grams per day may result in headaches and irritability, much like caffeine. Some sources indicate that 120-160 mg. is quite enough to produce the desired results.
In fact, gingko is a vasodilator, meaning that it dilates blood vessels throughout the body, improving cirdulation in general. Gingkosides help protect the body against free radicals, and the herb as a whole strengthens arterial tone and prevents blood platelets from clumping.
Gingko is helpful for men who have erectile problems due to poor circulation, making it useful to the study of aphrodisiacs even if it doesn't qualify specifically for the designation. Its stimulative effects, both general and brain-specific, make it worth noting in any case.