The similarity of reports from different cultures and different times across the century. The familial pattern to the condition. Clinical Diagnosis of Synesthesia. Psychologists and more recently Neuropsychologists have documented the nature of synesthetic experience in a useful manner for over a hundred years. Synesthesia is involuntary but elicited.
Synesthesia is projected.
If visual, a photism will appear outside the body in the region close to the face. Synesthetic percepts are durable and discrete. The associations for an individual Synesthete are stable over their lifetime. If a sound is blue, it will always be blue. Synesthetic experience is memorable.