Synaesthesia refers to the phenomenon where certain perceptual stimuli induce an unrelated and illusory perception - for example, a digit-color synaesthete may experience a sensation of the color green whenever exposed to the number 3. The relationships between the inducers and the induced synaesthetic experience are widely considered random; one anecodotal explanation is that letter-color synaesthesia could reflect a childhood memory of the particular colors used inrefrigerator magnet letters.
A recent Current Biology article from Kadosh, Henik and Walsh turns this common wisdom on its head. By breaking colors down into their underlying psychometric dimensions (luminance, saturation, and hue), Kadosh et al demonstrate a linear relationship between the value of a digit and the luminance of the synaesthetically-induced color, as illustrated in the graph below the fold:
By presenting colors and asking synaesthetes to select the digit most associated with those colors, Kadosh et al were able to plot synaesthetes' digit-color associations in terms of hue, saturation and luminance. Only luminance explained variance in the colors associated with particular digits.
Luminance accounts for 68% of the variance in which digits are associated with which colors in synaesthesia. Importantly, it is the cardinal value of digits which is associated with luminance; for 8 of the 19 subjects included above who also experienced day-color synaesthesia, neither luminance, brightness or hue explained variance in which day they associated with each color. This indicates that it cannot be the ordinal or sequential value of digits which generate a relationship with color, since the same would then be expected of days (especially since the Hebrew days of the week - the language in which these studies were run - can be literally translated as "first day," "second day," etc).
Some reports suggest that synaesthesia is more common in children, but disappears by adulthood (presumably due to neural pruning). The authors point to previous work showing that children associate small objects with brightness and large objects with darkness, similar to the relationship observed here among synaesthetic adults.
It seems unlikely to me that different forms of synaesthesia result from the same underlying cause. For example, color-grapheme synaesthesia may relate to abnormal connectivity in the temporal lobe (but see this), whereas the form described above is more likely related to parietal function, considering the magnitude-luminance relationship observed here.
R COHENKADOSH, A HENIK, V WALSH (2007). Small is bright and big is dark in synaesthesia Current Biology, 17 (19) DOI: 10.1016/j.cub.2007.07.048
As an adult with synesthesia, I am always interested in learning more about it. Thank you for the update.