Lateralization of categorical perception of color changes with color term acquisition

Abstract

Categorical perception (CP) of color is the faster and more accurate discrimination of two colors from different categories than two colors from the same category, even when same- and different-category chromatic separations are equated. In adults, color CP is lateralized to the left hemisphere (LH), whereas in infants, it is lateralized to the right hemisphere (RH). There is evidence that the LH bias in color CP in adults is due to the influence of color terms in the LH. Here we show that the RH to LH switch in color CP occurs when the words that distinguish the relevant category boundary are learned. A colored target was shown in either the left- or right-visual field on either the same- or different-category background, with equal hue separation for both conditions. The time to initiate an eye movement toward the target from central fixation at target onset was recorded. Color naming and comprehension was assessed. Toddlers were faster at detecting targets on different- than same-category backgrounds and the extent of CP did not vary with level of color term knowledge. However, for toddlers who knew the relevant color terms, the category effect was found only for targets in the RVF (LH), whereas for toddlers learning the color terms, the category effect was found only for targets in the LVF (RH). The findings suggest that lateralization of color CP changes with color term acquisition, and provide evidence for the influence of language on the functional organization of the brain.

Fig. 1.

Characteristics of the stimulus display and stimuli. (A) Illustration of the display. Black circle shows target, 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 five Munsell hue units apart. The target was either in the same color category as the background (5BG and 10BG, both green) or in the adjacent category (5BG and 10G, green and blue). The dashed line indicates the category boundary.

Fig. 2.

Between-category targets are detected faster than within-category, yet only when targets appear in the LVF for those learning blue and green color terms (Learners), and only when targets appear in the RVF for those who have learned blue and green color terms (Namers). The dependent measure is the log transformation of initiation time (ms), and error bars are within-subjects 95% confidence intervals calculated by using the error term from the three-way interaction (22). ***, indicates significant difference at P < 0.005.

Fig. 3.

When age was added as a covariate to the analysis, the pattern of lateralization of CP for the Namers and Learners remained. As in Fig. 2, there is a category effect only for targets in the LVF for Learners and only for targets in the RVF for Namers. The dependent measure is the estimated means of the log transformation of initiation time (ms), estimated for when variance due to age is accounted for. Error bars are within-subjects 95% confidence intervals calculated by using the error term from the three-way interaction (22).