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Peter A. Butcher, Edward M. Hubbard, Vilayanur S. Ramachandran; Top-down influences affect the experience of synesthetically induced colors. Journal of Vision 2002;2(7):266. doi: 10.1167/2.7.266.
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Some otherwise normal individuals experience sensations in one sensory modality when a second modality is stimulated. For example, a synesthete may see any given number as always tinged a certain color (e.g. 5, may be green and 6, may be red). We have previously shown that synesthetically induced colors can influence perceptual grouping, can lead to pop-out and are not affected by crowding (a form of blindsight). This suggests that synesthesia is a sensory phenomenon caused by cross activation of brain maps corresponding to color (V4 or V8) and graphemes, which lie adjacent to each other in the fusiform gyrus (Ramachandran and Hubbard, SFN 2000, 2001 VSS 2001). We now present evidence that top-down influences can also affect synesthetic colors. We find: 1) When presented with hierarchical figures (e.g., a large 5 composed of small 2's) the colors depend on where a synesthete focuses his attention (the exact experience differs depending on whether letters or numbers are used) 2) For synesthetes who experience letters in color, unitization (for example, missing the “f’s in “of’) can affect whether or not a synesthete experiences colors for letters and 3) Most synesthetes report that imagining a grapheme evokes the corresponding color more strongly than actually seeing it. We tested two synesthetic subjects (JC and ER) on a variant of the Perky experiment, in which mental imagery interferes with visual perception. Subjects made a 2AFC identification of colored letters or numbers while imagining either consistent or inconsistent graphemes. When their performance with and without imagery were compared, synesthetes showed a much greater increase in threshold for detecting the presence of a grapheme when they were imagining another grapheme then did normals (a larger Perky effect). We therefore conclude that although synesthetic colors arise early in perception, they can also be influenced by top-down influences.
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