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Katie Wagner, Karen Dobkins; Shape-color synesthesia in the first year of life: A normal stage of visual development?. Journal of Vision 2009;9(8):699. doi: 10.1167/9.8.699.
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Background: It has been proposed that synesthesia (a condition where a stimulus evokes an inappropriate, as well as appropriate, sensory response) may be a normal stage of infancy, resulting from more numerous and expansive neural projections in infant cortex. We investigated this hypothesis by asking whether infants experience grapheme-color synesthesia (graphemes evoke inappropriate sensations of color).
Methods: We used FPL to measure infants' looking preferences in response to combinations of colors and shapes (shapes being a precursor to graphemes). Stimuli consisted of a field of black shapes (triangles or circles) on a colored background, the left and right halves being isoluminant red and green, respectively (or vice versa). We predicted that if an infant perceives triangles as one color and circles as another, these “colored” shapes will interact with the background colors, such that preference for the red versus green background differs between the triangle and circle conditions. Blue/yellow backgrounds were also tested. Analysis: Associations made by adult synesthetes are highly individualized, making it essential that our analysis allow for individual differences (i.e. one infant may associate red with circles, another, green). To accomplish this, for each infant, the proportion of trials on which the red background was preferred to green was compared between the triangle and circle conditions. These individual comparisons were then compared as a group to the distribution expected from chance.
Results: We found a significant effect of shape on red/green preference at 2-months, but not 3- or 8-months. An effect of shape on blue/yellow preference was observed later at 3-months, but not 2- or 8-months.
Conclusions: Our data are consistent with infants experiencing synesthetic-like associations between colors and shapes for a brief developmental period. These associations occurred later for the blue/yellow backgrounds, possibly reflecting slower development of the blue/yellow system (koniocellular) than the red/green system (parvocellular).
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