Genuine synesthesia is present at birth and persists throughout adulthood, staying relatively stable in terms of associations between inducer and concurrent. Synesthesia can be seperated into three general types, genuine (also referred to as developed and constitutional), acquired, and drug-induced. Quick Overview of the Types of Synesthesia (Genuine, Acquired, Drug-induced) However, there has been some debate over whether these experiences constitute as genuine synesthetic experiences. More recently, there has been research into whether synesthesia can be artificially developed in individuals either through reading in coloured letters or through sensory substitution. Specific research into the phenomenon of synesthesia for pain and phantom limbs may give insight into human perception of pain and the role of empathy in the processing of pain. A form of synesthesia which appears to be exclusive in acquired synesthetes is the experience of pain in response to seeing or thinking about pain. Other factors have been suggested as possible candidates for eliciting synesthetic experiences, such as the serotonin S2a receptor which appears to play a role in all three forms of synesthesia (developmental, acquired, and drug-induced). Due to the many forms of acquired synesthesia, there are some exceptions that do not appear to fit this model, thus the exact cause of synesthesia still remains unknown. The creation of these new associations are generally limited to regions of the brain close to the deafferentated sensory region. In most cases, it is believed this deafferentation leads to the rewiring of neural pathways such that stimulation of other sensory systems are able to elicit a response in the deafferentated sensory system. Acquired synesthesia is a phenomenon which is most commonly induced by physical trauma, neuropathology, or stroke leading to sensory deafferentation.

Auditory tactile synesthesia test