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James L. Dannemiller; A contrast polarity heterogeneity effect in infant visual orienting. Journal of Vision 2005;5(8):691. doi: 10.1167/5.8.691.
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Visual orienting in human infants has been modeled assuming dimensional switching across trials (Dannemiller, VSS, 2003). In displays with a single moving bar and 27 static bars, directional orienting is determined on some proportion of trials exclusively by the movement, but on the complementary proportion of trials, the stimulus dimensions that comprise the static elements (e.g., contrast polarity, color, size) control orienting. The observed proportion of trials with orienting toward the moving bar then conforms to a mixture model of these two proportions. A further test of this model was conducted, and the results required modification of the model in a theoretically interesting way. Orienting was measured at two ages: 7–11 weeks and 17–21 weeks in two experiments with 129 infants. Bars of positive and negative contrast polarity relative to the background luminance were used. The dimensional switching model accounted well at both ages for the observed proportion of trials with orienting toward the moving bar but only if it was assumed in the model that the strength of the internal motion signal depended on the heterogeneity of the contrast polarities of the bars in the display. In homogenous displays with all of the bars having the same contrast polarity, orienting did not depend on the absolute polarity of the moving bar. However, in heterogeneous displays in which half of the bars had positive polarities and half of the bars had negative polarities, orienting toward the moving bar occurred more frequently when it had negative polarity than when it had positive polarity. This same asymmetry held regardless of how bars of the two contrast polarities were spatially distributed in the display. These results could be explained by assuming that a type of contrast gain mechanism enhances the perceived contrasts of the negative polarity bars only when bars of both polarities are present in the display simultaneously. This polarity heterogeneity effect was equally strong at both ages.
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