What herb was worn as a wreath around the forehead for memory and brainpower?
This would probably be rosemary.
In Medieval times rosemary was considered to be a sign of remembrance and fidelity. A wreath of rosemary was placed on the bride's head to help the couple remember their wedding vows.
The Romans believed that rosemary improved memory. Students massaged rosemary oil into their forehead and temples before taking examinations.