Synesthesia is an unusual condition in which stimulation of one modality evokes 
sensation or experience in another modality. Although discussed in the 
literature well over a century ago, synesthesia slipped out of the scientific 
spotlight for decades because of the difficulty in verifying and quantifying 
private perceptual experiences. In recent years, the study of synesthesia has 
enjoyed a renaissance due to the introduction of tests that demonstrate the 
reality of the condition, its automatic and involuntary nature, and its 
measurable perceptual consequences. However, while several research groups now 
study synesthesia, there is no single protocol for comparing, contrasting and 
pooling synesthetic subjects across these groups. There is no standard battery 
of tests, no quantifiable scoring system, and no standard phrasing of questions. 
Additionally, the tests that exist offer no means for data comparison. To remedy 
this deficit we have devised the Synesthesia Battery. This unified collection of 
tests is freely accessible online (http://www.synesthete.org). It consists of a 
questionnaire and several online software programs, and test results are 
immediately available for use by synesthetes and invited researchers. 
Performance on the tests is quantified with a standard scoring system. We 
introduce several novel tests here, and offer the software for running the 
tests. By presenting standardized procedures for testing and comparing subjects, 
this endeavor hopes to speed scientific progress in synesthesia research.