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R. Blake, T. Palmeri, R. Marois, W. Whetsell; Visual binding of synesthetic colors to achromatic forms. Journal of Vision 2001;1(3):66. doi: 10.1167/1.3.66.
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© ARVO (1962-2015); The Authors (2016-present)
A popular but unresolved issue in visual science is the binding problem: how does the brain unify distributed neural representations of diverse aspects of vision, such as form and color? We have studied an individual in whom visual “binding” is novel. Since early childhood, this 60 year-old male (WO) has experienced synesthesia in which achromatic words and numerals are seen in vivid, reliable colors. We administered a battery of visual and cognitive tests to WO, to validate the reality of his color experiences and to test hypotheses about their efficacy. WO's color vision, as assessed by the Munsell Hue Test and the Ishihara plates, is perfectly normal, as are his acuity and stereopsis. Repeated matches to his synesthetic colors are entirely reliable, and individual words and characters always elicit the same color terms. WO readily experiences colors when viewing numerals depicted in random-dot stereograms and numerals synthesized dichoptically, implicating central visual mechanisms. Surprisingly, WO does not experience colors when viewing achromatic afterimages, although he is perfectly capable of experiencing ordinary color afterimages. Unlike normal observers, WO's ability to name non-color words is impaired when those words are printed in ink colored differently from his synesthetic experience of those words (synesthetic Stroop effect). Moreover, unlike normals, WO experiences “pop-out” when viewing an achromatic “target” letter whose associated color differs from that of an array of achromatic “background” letters; the conspicuity of his synesthetic colors is also revealed in WO's faster reaction times in a standard “present/absent” visual search task. Evidently WO's synesthetic colors can be neurally “bound” to contour features at a relatively early stage in visual processing.
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