Categorical perception of color is lateralized to the right hemisphere in infants, but to the left hemisphere in adults

Abstract

Both adults and infants are faster at discriminating between two colors from different categories than two colors from the same category, even when between- and within-category chromatic separation sizes are equated. For adults, this categorical perception (CP) is lateralized; the category effect is stronger for the right visual field (RVF)-left hemisphere (LH) than the left visual field (LVF)-right hemisphere (RH). Converging evidence suggests that the LH bias in color CP in adults is caused by the influence of lexical color codes in the LH. The current study investigates whether prelinguistic color CP is also lateralized to the LH by testing 4- to 6-month-old infants. A colored target was shown on a differently colored background, and time to initiate an eye movement to the target was measured. Target background pairs were either from the same or different categories, but with equal target-background chromatic separations. Infants were faster at initiating an eye movement to targets on different-category than same-category backgrounds, but only for targets in the LVF-RH. In contrast, adults showed a greater category effect when targets were presented to the RVF-LH. These results suggest that whereas color CP is stronger in the LH than RH in adults, prelinguistic CP in infants is lateralized to the RH. The findings suggest that language-driven CP in adults may not build on prelinguistic CP, but that language instead imposes its categories on a LH that is not categorically prepartitioned.

Fig. 1.

Presentation and Munsell codes for target detection task stimuli. (a) Illustration of the display. The black circle indicates the target, and white circles show other possible target locations. (b) Munsell codes of the stimuli; stimuli varied in hue at constant value and chroma. Hue separations were 2.5 steps apart (adults) and 10 steps apart (infants). The target was either in the same color category as the background (e.g., 6.25BG and 3.75BG, both green) or in the adjacent category (e.g., 6.25BG on 8.75BG, green and blue). The dashed line indicates the category boundary.

Fig. 2.

The category effect for adults is significant in both visual fields, but is larger in the RVF than in the LVF. The category effect for infants using the same task and measure is only significant in the LVF. (a) For adults, the difference in time to initiate an eye movement to the target for within- and between-category is larger in the RVF than LVF. (b) For infants, the difference in time to initiate an eye movement to the target for within- and between-category is larger in the LVF than RVF. Error bars are within-subjects 95% confidence intervals (30).