When:

• Black printed letters or numbers have colour, or a personality, or
• music is seen in color, or
• days, weeks or months of the year appear in shapes like squares, circles and ovals, or
• sounds or words have a taste, or
• tastes have shapes, or
• faces have an aura, or
• sounds have smells or temperature

then synesthesia appears to be involved. (in fact about half the people experiencing synesthesia have more than one form of it)

Types of Synesthesia

Most definitions of synesthesia refer to cross-modal sensory experiences. That is: experiences where a percept in one sense initiates or co-occurs with a percept in another sense. This in itself is not uncommon. Scientists know that the brain integrates sensory information all the time. However, these sensations are idiosyncratic. Perhaps the way we see synesthesia will change over time as we understand it more and people discuss it more readily.

Some artists and musicians would argue that we all have some synesthesia (ability to join the senses) but that some people do it more readily and more permanently than others. In this section of the website we delve a little more deeply into the different types of synesthesia. 

There are various forms of synesthesia. Infact Sean Days website lists the frequency of them here.

The most common form of synesthesia (at least the one most widely studied) is grapheme / number to color. Graphemes (letters of the alphabet) or numbers visually presented in black typeface can initiate the experience of color. The synesthetes who experience this often report that the color is superimposed on the letter in the external environment but many also report seeing the color in the minds eye.  Exactly how these reports are different is difficult to determine from a scientific and research point of view, but one thing seems likely - these experiences are perceptually real and not imagined. That is, the synesthetes color experience may utilise similar pathways in the brain as normal color vision. Here are some types of synesthesia that have been reported.

Incidence of Idiopathic Synesthesia
Early estimates of synesthesia ranged from 1 in 4 (Calkins, 1895; Domino, 1989); 1 in 200 (Ramachandran and Hubbard 2001a), and 1 in 25,000 to 100,000 (Cytowic, 1993, 1997). The most commonly cited study shows a prevalence of “at least 1 in 2000'' with a female : male ratio of 5.5 : 1 (Baron-Cohen, Burt, Smith-Laittan, Harrison, & Bolton, 1996). An Australian investigation reports a prevalence of 2.4% and a female bias of 6.2 : 1 (Rich, Bradshaw, & Mattingley, 2005). Both these studies based their estimates on the number of respondents to newspaper advertisements, together with those newspapers' circulation figures. More recently however the apparent female bias has lowered to 2 : 1 (Simner et al., 2005). This was derived from a university student population.

The most comprehensive study to date assessed a large number of people (N = 1690) and verified the reports of synesthesia with objective tests for consistency over time (Simner et al., 2006). The prevalence for synesthesia reported in the study was 4.4%, with a female : male ratio of 1.1 : 1, which showed no significant sex bias.

Synesthesia tends to run in families.

Parents and children often have different forms of synesthesia and even when similar forms of synesthesia are expressed from parent to child these are still influenced by other factors (Barnett, Finucane et al., 2008). The prevalence of synesthesia among biological relatives of synesthetes surveyed by (Rich et al., 2005) was found to be much higher than that estimated for the general population with 36% reporting at least one relative with synesthesia. One of the difficulties in estimating the incidence of synesthesia accurately is the fact that idiopathic synesthesia is not a condition which creates difficulties for the person who experiences it. This may lead to under reporting.

Other types of Synesthesia 

One example of this is when you listen to music and you see visual images like colours or shapes. This form of coloured hearing is called Chromesthesia or music-color synesthesia.

There are however a number of synesthesia types where the percept initiates another class of percept in the same sense. One example of this is when you see black printed letters or numbers as having a color associated with them. This is referred to as grapheme / number to color  synesthesia. We are going to look at a few of the different types below.

There are a number of other types of synesthesia as well. Ordinal Personification for example. This is where a number elicits a personality. For example 3 is a young boy eating ice cream and 8 is his sister.

'Time' to 'Space' correlations are also reported. Here time units - like days or weeks  - are spatially linked to form shapes in external space. Each person experiences this differently so the shapes can vary from person to person. One research report (Smilek, et al, 2006) reports a subject who sees May as a blue area located on the right area of her body about an arms length away.