2.6.2. Methods

2.6.2.1. Participants

Ten right-handed undergraduate students (5 males and 5 females) with normal or corrected-to-normal vision (mean age 23.0 years, SD 1.0) participated in the present experiment. All of these participants were newly selected from ten groups of eight subjects in the preliminary experiment as in Experiment 1-1.

2.6.2.2. Apparatus

One of three chromatic solid circles with a diameter of 2 deg (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) were presented at 4 deg from the center of the CRT with a uniform gray background of 10 cd/m2 as in Experiment 1-2. The luminance of the chromatic stimuli was adjusted using the flicker photometry method as described in Experiment 1-2.

2.6.2.3. Procedures

The subjects were required to press a key quickly after they perceived the color that was not told before each session. For example, if the designated color was red, then target colors were green and blue. Each block consisted of 15 trials, 5 trials each for 3 conditions (target color was either non-red, non-green, or non-blue) with a randomized intertrial interval of 20 sec ranging from 15 sec to 30 sec. A total of 6 blocks, 3 blocks for L-L and 3 blocks for R-R were performed with a quasi-random order. Mean correct response rate was 94.8±1.03 % (± SE).

2.6.3. Results

Figure 14 shows that the mean discrimination reaction time was significantly shorter in R-R than L-L condition (367.2±20.98 ms vs. 385.6±21.15 ms, mean ± SE, paired-t(9)=2.718, p<0.05). The left hemispheric superiority in color discrimination based on the verbal cues was found in 8 of 10 participants (Fig. 15). These findings suggest that right-handed participants have left hemispheric predominance in color discrimination task with verbal cues.

2.6.4. Discussion

The discrimination reaction time was longer than the simple reaction time. This is consistent with a view that a serial processing of signal detection and discrimination and that the discrimination needs more process than the detection of color (Schmidt and Lee, 1998). The present finding that the discrimination reaction time was shorter in left hemisphere than in right hemisphere might suggest that the left hemisphere is dominant in color discrimination. However, before draw a conclusion, we must consider about effect of verbal processing in the present discrimination task. It is well known that the verbal processing is dominant in the left hemisphere (LeDoux et al., 1977; Gazzaniga et al., 1977), although it is also clear that there are complementary specializations of the right hemisphere (Sperry, 1982). Thus, the present finding that the discrimination reaction time was shorter in left hemisphere might be associated mainly with language processing in the left hemisphere.