The brain has two halves, or hemispheres, right and left. In many respects they are similar in appearance and function. In other respects they are dissimilar and somewhat specialized. This is referred to as hemispheric specialization or brain lateralization. In the popular press, hemispheric specialization has been greatly simplified. It is frequently presented in such a way that one could draw the conclusion that each half of the brain operates independently of the other. We read about how you can learn to draw with the right side of the brain and thus tap its (unused?) creative and intuitive potential. There is also an overabundance of popular articles describing how you can find out whether you are “left-brained” or “right-brained. This is not a realistic representation of scientific findings. The two hemispheres are connected by a large bundle of fibers, the corpus callosum, which carries information between the two hemispheres. Thus, in healthy individuals the two halves of the brain always function together in a coordinated fashion. We can not shut out the influence of one hemisphere. While our talents and abilities may certainly differ and evolve in various ways, most of us are both right- and left-brained.
Can we do without the corpus callosum?
Well, not quite. Yet, extensive damage of the corpus callosum can be surprisingly inconspicuous without the help of special testing procedures. Minor damage to the corpus callosum can still have dramatic consequences. In the case presented below we see how a damaged corpus callosum impedes the neural impulses crossing between the hemispheres, resulting in writing problems (agraphia) in one hand only.
Left unilateral agraphia
The patient described is left-handed but could not write (i.e., spell) with his left hand. He could still do so with his right hand. A problem on one side of the body is referred to as unilateral (i.e., one-sided). This patient, therefore, had left unilateral agraphia. A syndrome such as left unilateral agraphia is called a disconnection syndrome because the nerve fibers connecting the two hemispheres (corpus callosum) are partially disconnected. In order for us to write a word with the left hand the linguistic information must cross over to the right hemisphere from the left hemisphere as the right hemisphere controls the left side of the body. However, in cases such as the one described here the linguistic information could not pass into the right hemisphere in order to be written by the left hand due to a damaged corpus callosum.
A 62-year-old left-handed Icelandic man suffered a cerebrovascular accident when an aneurysm (http://www.brainaneurysm.com/) burst in his brain, and there was bleeding into the corpus callosum. The resulting brain damage led to personality changes and problems writing with his left hand. He could spell out aloud so he still knew how words should be spelled. He could write individual letters but chose the wrong ones so that the result was unrecognizable as a word. This frustrated him and he complained that he would not be able to write his Christmas cards himself (this was prior to Facebook and electronic greeting cards!). He tried to practice while in the rehabilitation center but to no avail. I suspected, however, that he might be able to write correctly with his right hand, which was not his dominant hand, and suggested this to him. He thought that this was an outrageous idea but was finally convinced to give it a go. And, lo and behold! When writing with his right hand he wrote understandable words and they were mostly correct.
In the accompanying pictures you see a few examples of the patient´s handwriting. The words are in Icelandic but they still demonstrate the difference between the patient´s spelling with his left (dominant) and right (non-dominant) hand.