Synesthetes are often sorted into two groups: Associators and Projectors.

Projectors have a more 'physical' union of the senses. For example, when they hear sound they may see color. The color is projected to them (Dixon, Smilek & Merikle, 2004; Dixon & Smilek, 2005). However, for an associator, it is more of a strong internal feeling. For example, they may associate a song with the color blue (Smilek, Dixon, Cudahy, & Merikle, 2001).

Now, what is happening in the brain to cause this?

Since synesthesia is a largely individual experience, activation regions vary across the community. However, some connections have been found.

Rouw and Scholte (2010) found that generally projectors have increased activity in the sensory areas, and associators typically have increased activity in the hippocampus and angular gyrus (an area involved in making associations across different information types).

Developed by Eagleman, Kagan, Nelson, Sagaram, and Sarma in 2007, the Synesthesia Battery works to provide quantitative data for synesthesia. The 'Battery' is not one test, but a collection of tests that can be used for testing different kinds of synesthesia.

The sound-color test involves the participant hearing a sound, and then selecting the connected color from a pallet of over 16 million colors. The inducer (the sound) will be played 3 times during the test, and the selected color is tested for consistency by measuring red, green, blue geometric color-space distance. The participant is then given a variation score which is used to determine if they have synesthesia.

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