Cortical and Subcortical Processing of Color |
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2.7. Experiment 1-6: Hemispheric Asymmetry of Color Discrimination without Verbal Cue in Right-Handed Individuals
2.7.1. Purpose
In this experiment, hemispheric lateralization in color discrimination was examined using a task which dose not requires verbal processing.
2.7.2. Methods
2.7.2.1. Participants
Eight right-handed undergraduate students (7 males and 1 female) were participated in the present experiment (mean age 22.0 years, SD 2.2). All of these participants were newly prepared. The participants were required to press a key when two stimuli were different in hue or luminance.
2.7.2.2. Apparatus
Two of three chromatic circles (red, x = 0.553, y = 0.313, CIE; green, x = 0.279, y = 0.577, CIE; and blue, x = 0.226, y = 0.151, CIE) or two of three achromatic ones (12, 14 and 18 cd/m2) were presented at 4 deg lateral to the center of the CRT either in the left or right visual field with a uniform gray background of 10 cd/m2. The diameters of circles were 2 deg. The chromatic stimuli had the same saturation of 60%. The luminance of the chromatic stimuli was adjusted to gray of 10 cd/m2 using the flicker photometry method.
2.7.2.3. Procedures
Participants were required to press a key as fast as possible when the two stimuli presented simultaneously were different either in hue (chromatic stimuli) or luminance (achromatic stimuli). Response was required to be done using the ipsilateral hand to the visual field. Which hand to use was instructed before starting each block.
A block consisted of 48 trials; 36 chromatic and 12 achromatic trials with a quasi-random order. Four trials were performed for each condition. For a chromatic trial, one of nine conditions (RR, RG, RB, GR, GG, GB, BR, BG and BB), and for an achromatic trial, one of three conditions (14-12 cd/m2, 14-14 cd/m2, and 14-18 cd/m2) was tested.
A total of four blocks (discrimination reaction time with R-R or L-L, and simple reaction time with R-R or L-L condition) were performed for each participant with a quasi-random order. Discrimination reaction times longer than 600 ms and simple reaction time longer than 400 ms were omitted from later analysis. Other procedures were the same as in Experiment 1- 2. Data from one subject was omitted from later analysis for achromatic stimuli, because he made no response to achromatic stimuli in L-L condition.
2.7.3. Results
Although the discrimination reaction time to chromatic stimuli was tend to be shorter in the left hemisphere (386.7±11.2 ms, mean ± SE) than in the right hemisphere (409.6±17.5 ms, mean ± SE), the difference was not statistically significant (paired-t(7)=1.722, p=0.1286) (Fig. 16). Similarly, although a net discrimination time which was obtained by subtracting simple reaction time from the discrimination reaction time was tend to be shorter in the left
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Hitoshi Sasaki |
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hemisphere (R-R: 123.3±10.8 ms, mean ± SE, L-L: 150.8±20.1 ms, mean ± SE), no statistically significant difference was found (paired-t(7)=1.867, p=0.1041) (Fig. 17). As for achromatic stimuli, there was no significant difference in achromatic discrimination reaction time (R-R: 514.7±10.1 ms, mean ± SE, L-L: 520.4±17.7 ms, mean ± SE, paired-t(6)=0.361, p=0.7303), or even after the net discrimination time was calculated (R-R: 258.3±12.4 ms, mean ± SE, L-L: 273.9±18.7 ms, mean ± SE, paired-t(6)=0.647, p=0.5414). From these findings we could not find any significant hemispheric asymmetry in color discrimination.
Figure 16. Discrimination reaction times without verbal cues to lateralized stimuli presented either in the right visual field responded by the right hand (R-R) or the left visual field responded by the left hand (L-L) in 8 right-handed participants. The discrimination task was not dependent on verbal cues. There was no significant difference between the reaction times both to chromatic and achromatic stimuli. Mean with SE.
Figure 17. Net discrimination times to stimuli presented either in the right visual field responded by the right hand (R-R) or the left visual field responded by the left hand (L-L) in 8 right-handed participants. The discrimination task was not dependent on verbal cues. The net discrimination time was calculated by subtracting simple reaction time from discrimination reaction time. There was no significant difference between the net discrimination times both to chromatic and achromatic stimuli. Mean with SE.