2.7.4. Discussion

Although clear asymmetry was found in the color detection (Experiment 1-2), no hemispheric difference was observed in the color discrimination reaction time without using language cues. Consistent with this, Danilova and Mollon (2009) recently reported that no asymmetry in color discrimination. However, before draw a conclusion we should consider a possibility that a longer reaction time has a larger variation. Both in Experiments 5 and 6, discrimination reaction time was longer than simple reaction time. This may easily obscure a subtle difference in reaction time and make it difficult to detect any difference in discrimination reaction time. To elucidate a small but definite difference in reaction time, it is important to adopt some explicit and refine methods to reduce individual variation (Sasaki et al. 2008). The net discrimination time, however, showed a slight tendency of shorter reaction time in left hemisphere (R-R) than right hemisphere (L-L) for chromatic stimuli, no significant difference was found. These findings suggest that color detection is specialized for the right hemisphere, while color discrimination is not specialized for any hemisphere.

It was shown that emotional stimuli are perceived more efficiently by the right hemisphere than by the left hemisphere (McKeever and Dixon, 1981; Smith et al., 2004; Sato and Aoki, 2006). And the right hemisphere plays an important role in producing emotions (Ladavas et al., 1984). Effect of right amygdala in discriminating emotional faces without primary visual cortices has been suggested (Pegna et al., 2005). A subcortical pathway to the right amygdala, via superior colliculus and pulvinar of the thalamic nucleus, provides a rout for processing unconscious identification of affective expressions in parallel to a cortical route necessary for conscious identification (Morris et al., 1999).

Verbal and non-verbal communication is important for social behaviors in humans. The non-verbal communication includes gestures, eye contact, and expression of emotion, such as disappointment, fear, pleasure, and surprise. Because the right hemisphere is closely related to emotion, as described above (Ladavas et al., 1984), it is suggested that the right hemisphere plays an important role in the non-verbal communication, by contrast the verbal communication in the left hemisphere.

3. EXPERIMENT 2: PREPULSE INHIBITION OF STARTLE BLINK

RESPONSE USING COLOR PREPULSE

3.1. Background

In the second experiment, contributions of color stimulus to modulation of the corneal blink reflex were examined in healthy human subjects.

3.1.1. Startle Response

A sudden intense stimulus produces a startle response in many species of animal (Landis and Hunt, 1936; see Davis, 1984). Startle response is one of defensive reflex to protect from harmful or noxious stimulus.

In human studies, eyeblink reflex was usually recorded as the startle response (Graham, 1975; Lipp and Siddle, 1998). The blink reflex elicited by a corneal stimulation is a component of the startle response, as well as the acoustic startle blink reflex (Krauter, 1987;