Developmental Differences of the Brain’s Corpus Callosum:

Cross-Lateralization Fingertips Test


Brooks A. Kanski



            This study tested the difference of cross-lateralization reaction times between left-handed and right-handed individuals in an effort to clarify whether the corpus callosum is more developed in left-handed individuals. This study is a result of learning of a statistic which indicates that the corpus callosum of the brain is more developed in left-handed individuals. It is believed that because left-handed individuals use both hemispheres to a greater extent, the corpus callosum (deep white matter of brain connecting both cerebral hemispheres) is faster and stronger. This and further background research led to the hypothesis that the corpus callosum is more developed in left-hand dominant humans.

            To test this hypothesis, a cross-lateralization fingertips test (CLFT) was performed on six different subjects; three left-handed subjects (two male, and one female) and three right-handed subjects (two male, and one female). The CLFT was performed in a silent room. Subjects sat with their eyes closed and hands open, palm up, on the table. In random order, each digit of the subject’s recessive hand was stimulated by the touch of a pencil. Following each stimulation, the subject was asked to immediately respond by touching the corresponding digit of their dominant hand with their dominant thumb. Three random trials were performed for each subject, in which an average reaction time was taken and calculated into a general average for both the left-handed subjects and the right-handed subjects. It was predicted that because the corpus callosum of left-handed individuals is believed to be more developed, the average reaction time of the left-handed subjects would be faster.

            The study’s results indicated that the mean reaction time of the left-handed subjects (mean=.678889) was faster than that of the right-handed subjects (mean=1.016111, P = 0.07256).

            The study’s results insignificantly support (P=0.07256) the hypothesis that the corpus callosum of left-handed individuals is more developed than the corpus callosum of right-handed individuals. Potential errors that may have affected accuracy were the methods of how the reaction times were timed by bystanders. Future studies could be improved by utilizing more accurate, finger-touch timing devices. These timing devices would eliminate the inaccuracies of a bystander timing, which result from a bystander’s own reaction time and a bystander’s judgment of the start and stop point. A similar study measured transfer time between both sides of the brain via reaction time to the flashing of white lights to the left and the right of a cross screen (Cherbuin et al. 2006). The study found that left-handed subjects reacted faster, but not necessarily as accurately as right-handed subjects. Cherubin’s studies encourage the modification of the above hypothesis as follows: the corpus callosum is faster in left-hand dominant humans than right-hand dominant humans.



Cherbuin, N. 2006. “Hemispheric Interaction: when and why is yours better than mine?”. Australian

Digital Theses Program.