Right hemispheric dominance in color detection.

HITOSHI SASAKI AND YUTAKA FUKUDA
Department of Physiology and Biosignaling, Osaka University Graduate School of Medicine

  It is well known that language is processed mainly in the left brain, while non-verbal information such as visual image and emotional feeling is mainly processed in the right brain. This also leads to the notion that left brain is dominant in the processing of logical thinking@of complex matter and problem solving@using language, while the right brain is dominant in processing of  intuitive comprehension of total feature such as recognition of well-known face and appreciation of arts.  On the other hand, simple sensory features such as lightness and color have been thought to be processed equally in the right and left brains without any asymmetry. Here, we present the first experimental evidence in humans  that the color information is processed asymmetrically with the right brain dominance. This finding has been revealed by using a simple reaction time task under adequately  controlled visual stimulations in human subjects.

  Reaction time is a minimal time from sensory inputs to motor outputs. If we look at a point in the visual field, visual signals from objects on the left side of the point are sent  to the right brain, while signals from the right side  to the left brain. Similarly, movements of the  right hand are controlled by the left brain and vise visa. Using this double crossed projections in sensory and motor projections, we could study the functions of right and left brains separately by presenting visual stimuli and asking the subjects to respond with the hand of the same side to the visual stimulation.  If we could obtain any difference in  reaction time between the activations of right and left brains, we could estimate the time difference spent in the two brains because the time spent in sensory afferents  and motor outputs are the same.
We measured these two reaction times to small color (Red, Green, or Blue) and white light spots presented on a computer display set in from of the subjects. Eye movements were recorded to monitor their fixation. Sixteen volunteer right-handed undergraduate students, with normal vision of both gender, attended in two series of experiments. In the first experiment, the white circle appeared at the center of visual field and they were asked to press a button as fast as possible either by right or left hand. The experimenter told which hand should be used beforehand, and the order of hand used was random for each subject. There was no difference between reaction times by right and left hand, regardless of their preference in right hand. In the second experiment, the colored and white targets appeared either on the right or left side from the fixation point. Reaction times by the left hand to colored targets presented in the left side were significantly shorter than those by the right hand to the targets in the right side. On the other hand, there was no difference between reaction times to white visual targets. 

  These data show that the right brain is dominant in color information processing, while there is no asymmetry of brain in the processing of lightness. Our findings show that even the simple sensory processing  such as color is processed in the brain with a clear asymmetry with the dominance of right brain. However, functional dominance was only observed for color detection task and was not for detection of luminance change task, suggesting that not all of the sensory information but color information is processed with asymmetry. This finding casts a new insight to the understanding of the function of right brain. In addition, our findings suggest that color stimulation on the left visual field  can effectively activate color centers in the right brain. This might be used for a method of effective stimulation of right brain of children to promote development of the right brain. @

  In the next step, we should clarify whether or not the other features of color information processing, such as color discrimination and color memory, are also processed dominantly in the right brain. In addition, similar study with left-handed subjects will provide information about the role of speech center in the asymmetry of color processing, because the speech center is almost localized in the left brain in right-handed people while it is not so in the left-handed.