<P> The simplest approach is test - retest reliability over long periods of time, using stimuli of color names, color chips, or a computer - screen color picker providing 16.7 million choices . Synesthetes consistently score around 90% on reliability of associations, even with years between tests . In contrast, non-synesthetes score just 30--40%, even with only a few weeks between tests and a warning that they would be retested . </P> <P> Grapheme - color synesthetes, as a group, share significant preferences for the color of each letter (e.g. A tends to be red; O tends to be white or black; S tends to be yellow etc .) Nonetheless, there is a great variety in types of synesthesia, and within each type, individuals report differing triggers for their sensations and differing intensities of experiences . This variety means that defining synesthesia in an individual is difficult, and the majority of synesthetes are completely unaware that their experiences have a name . </P> <P> Neurologist Richard Cytowic identifies the following diagnostic criteria for synesthesia in his first edition book . However, the criteria are different in the second book: </P> <Ol> <Li> Synesthesia is involuntary and automatic . </Li> <Li> Synesthetic perceptions are spatially extended, meaning they often have a sense of "location ." For example, synesthetes speak of "looking at" or "going to" a particular place to attend to the experience . </Li> <Li> Synesthetic percepts are consistent and generic (i.e. simple rather than pictorial). </Li> <Li> Synesthesia is highly memorable . </Li> <Li> Synesthesia is laden with affect . </Li> </Ol>

What can cause a person to see colors and hear sounds