Synesthesia (or synaesthesia) is a psychological phenomenon where a person experiences one stimulation or use one sensory pathway and another sensory pathway activates simultaneously. This phenomenon effects around 4% of the population. For instance, as someone is listening to an orchestra performance, they may see a variety of colors dependent on the pitches of the piece or the piece as whole that is being performed. There are many types of synesthesia including grapheme-color synesthesia (associating a color to written numbers or letters) and chromesthesia (also known as "colored-hearing" where one sees colors when hearing certain sounds).