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Nathaniel S Witthoft, Jonathan A Winawer; Casting shadows on synesthesia. Journal of Vision 2003;3(9):619. doi: 10.1167/3.9.619.
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© ARVO (1962-2015); The Authors (2016-present)
Experiments with a color-grapheme synesthete AD have turned up several novel findings regarding the origin of the synesthetic colors and how they interact with normal visual processes.
No systematic organization to the color-grapheme associations has been previously reported either for a particular synesthete or between synesthetes. A number of theories have been put forward to explain the origin of synesthesia but do not address the source of the colors produced. However, in this case, the colors can be traced to a childhood letter set still in AD's possession. Furthermore, AD moved to Russia while a child, and experiences synesthetic colors in response to Cyrillic letters as well as the English alphabet. Where the Cyrillic letters are visually similar to English letters, the color is the same, otherwise, the color of the Cyrillic letter is determined by phonetic similarity to English.
The interaction of AD's photisms with normal visual processing was explored using the Adelson checkershadow illusion in which squares of identical luminance produce different subjective experiences of lightness depending on whether they appear to be in shadow or direct light. Normal controls attempting to match colored letters embedded in the illusion showed effects of context on their matches. AD's photisms were also affected by the illusion. This result suggests that the processes that influence lightness perception in normal vision also operate on synesthetic photisms.
There are also certain conditions under which AD's photisms can be abolished. Of particular interest is the finding that presenting letters in saturated colors in the vicinity of the hue opponent to her synesthetic color prevents AD from seeing her photisms. This novel result points to the involvement of low level mechanisms in AD's synesthesia.
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