Cortical response to categorical color perception in infants investigated by near-infrared spectroscopy

Abstract

Perceptual color space is continuous; however, we tend to divide it into only a small number of categories. It is unclear whether categorical color perception is obtained solely through the development of the visual system or whether it is affected by language acquisition. To address this issue, we recruited prelinguistic infants (5- to 7-mo-olds) to measure changes in brain activity in relation to categorical color differences by using near-infrared spectroscopy (NIRS). We presented two sets of geometric figures to infants: One set altered in color between green and blue, and the other set altered between two different shades of green. We found a significant increase in hemodynamic responses during the between-category alternations, but not during the within-category alternations. These differences in hemodynamic response based on categorical relationship were observed only in the bilateral occipitotemporal regions, and not in the occipital region. We confirmed that categorical color differences yield behavioral differences in infants. We also observed comparable hemodynamic responses to categorical color differences in adults. The present study provided the first evidence, to our knowledge, that colors of different categories are represented differently in the visual cortex of prelinguistic infants, which implies that color categories may develop independently before language acquisition.

Keywords: categorical color perception; cortical response; infant; visual development.

Fig. 1.

Experimental procedure and location of the NIRS probes. (A) In each trial, the baseline phase consisted of figures changing in shape, which had a duration of at least 10 s. The test phase consisted of color changes between categories (G1/B1) or within a category (G1/G2), which had a duration of 10 s. The presentation order of the between- and within-category phases was counterbalanced across infants. (B) Location of the NIRS probes and the measurement channels in the probe system for infants. The probe holders were placed on the left and right OT regions, slightly below T5 and T6 of the international 10–20 system in the main measurement. The probe holder was placed slightly above Oz for the measurement of the occipital region responses. The distance between the emitter and detector probes was set at 2 cm.

Fig. 2.

Results of the NIRS measurements in infants. (A) Channels with significant changes in oxy-Hb in the OT regions. The size of each red circle represents the number of participants who showed significant changes (P < 0.01) in the corresponding channel. The spatial arrangement of the NIRS channels is illustrated in Fig. 1B. (B) Mean Z-scores of NIRS response in 5- to 7-mo-old infants. Mean Z-scores of data in infants for each of the left temporal (Left), right temporal (Middle), and occipital (Right) regions, respectively, are shown. Each bar represents the mean Z-score of oxy-Hb averaged across 2–6 s in the stimulus onset latency. Dark and light bars represent the results for the between- and within-category conditions, respectively. The error bars represent ±1 SEM. Asterisks indicate the significance level of statistical differences: *P < 0.05; **P < 0.01. (C) Time course of changes in oxy-Hb concentrations was averaged among 5- to 7-mo-old infants during the between- and within-category conditions measured in the left and right OT regions and the occipital region. The deoxy-Hb and total-Hb changes in infants’ bilateral OT and occipital regions are illustrated in Figs. S1 and S2. Thick red and blue lines in each panel represent the mean Z-score in the between- and within-category conditions, respectively. The broken lines represent the range of ±1 SEM. The horizontal axis represents the time from the onset of the test stimulus in seconds; the vertical dashed lines at 0 and 10 s denote the onset and offset of the test stimulus presentation, respectively.

Fig. S1.

Result of the deoxy-Hb and total-Hb changes in infants’ OT regions (related to Fig. 2). Time course of the changes in deoxy-Hb, and total-Hb concentrations averaged among 5- to 7-mo-old infants during the between- and within-category conditions. Thick red and blue lines in each panel represent the mean Z-score in the between- and within-category conditions, respectively. The broken lines represent ±1 SEM. The horizontal axis represents the time from the onset of the test stimulus in seconds; the vertical dashed lines at 0 and 10 s denote the onset and offset of the test stimulus presentation, respectively. (Left) Graphs show the hemodynamic changes in the left hemisphere (1–12 channels). (Right) Graphs show the hemodynamic changes in the right hemisphere (13–24 channels).

Fig. S2.

Results of the deoxy-Hb and total-Hb changes in infants’ occipital regions (related to Fig. 3). The time course of the average change in deoxy-Hb and total-Hb measured in the occipital region of infants is shown. Axes and line colors/styles are the same as in Fig. S1.

Fig. S3.

Results of adults’ OT regions. The time course of the average change in oxy-Hb, deoxy-Hb, and total-Hb measured in the OT region of adults. Axes and line colors/styles represent the same as in Fig. S1.

Fig. 3.

Mean preference scores for novel stimuli in the habituation experiment. Error bars indicate ±SEM. Results for the pretest phase are indicated by light gray bars, and results for the posttest phase are indicated by dark gray bars. A significant difference was found only in the between-category pair. Asterisks indicate a statistically significant difference: **P < 0.01.