| types of synesthesia
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It is important to note that synesthesia is not one singular condition. The word synesthesia is used to describe a number of different conditions (involving different or separate areas of the brain), ranging from smelling sounds, in which two basic senses come into play, to something as abstract as perceiving sexes with graphemes. Synesthesia of any kind involves a consciously perceived external stimulus which immediately and involuntarily causes a feedback loop in the brain, which in turn creates a secondary (or tertiary, etc.) perception of the stimulus, whether the secondary perception be formerly associated or whether it be arbitrary-but-situational.
As mentioned, synesthesia comes in a many varieties. When a person thinks of mixed senses, what generally comes to mind is the mixing of sight and sound, taste and touch, or maybe smell and sound. If only the five senses — sight, smell, sound, taste, and touch — were considered to be "mixable" synesthetically, there would theoretically be 20 different forms of synesthesia, as follows:
| n = 25 |
sight  sight | sight smell | sight sound | sight taste | sight touch | |
smell sight | smell smell | smell sound | smell taste | smell touch | |
sound sight | sound smell | sound sound | sound taste | sound touch | |
taste sight | taste smell | taste sound | taste taste | taste touch | |
touch sight | touch smell | touch sound | touch taste | touch touch
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Interestingly, while the forementioned forms of synesthesia do occur, there are other types that are possible. Firstly, there are subsets of the above senses:
 - subsets of sight: color, movement, shape (subsets of shape: graph*, grapheme*, written word*, person*)
- subset of sound: phoneme
- subsets of touch: pain, temperature, weight/pressure
Secondly, there are two other types of perceptions that I believe can trigger and be triggered by synesthesia: emotion and span/unit of time*. There are three possible subsets of emotion:
- subsets of emotion: age*, personality*, sex*
Emotion and span/unit of time bring the total of possible main types of synesthesiae up to 49. Finally, there may be one more type of synesthesia (more info to come later), but that type would only be a one-way street; it would bring the total up to 56.
* Synesthesia elicited from time, graph, grapheme, written word, personality, or thought/memory is known as conceptual synesthesia.
| n = 49 |
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sight sight | sight smell | sight sound | sight taste | sight touch | sight emotion | sight time | |
smell sight | smell smell | smell sound | smell taste | smell touch | smell emotion | smell time | |
sound sight | sound smell | sound sound | sound taste | sound touch | sound emotion | sound time | |
taste sight | taste smell | taste sound | taste taste | taste touch | taste emotion | taste time | |
touch sight | touch smell | touch sound | touch taste | touch touch | touch emotion | touch time | |
emotion sight | emotion smell | emotion sound | emotion taste | emotion touch | emotion emotion | emotion time | |
time sight | time smell | time sound | time taste | time touch | time emotion | time time
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