Split-Brain Functioning

While reports of physical differences between the two halves of the brain had been reported intermittently since the late 1800s, these differences were generally considered relatively minor and too small to explain observed differences in functioning of the left and right hemispheres. In 1968, however, research was reported that found strong and clear anatomical differences between the two hemispheres in areas believed to be of great importance for speech and language.

This research found a longer temporal plane in the left hemisphere than in the right in 65 of 100 brains examined at autopsy. Eleven brains had a longer temporal plane in the right hemisphere, and the remaining 24 showed no difference. On the average, the temporal plane was one-third longer in the left hemisphere than in the right. A number of studies have supported these findings, and on average, approximately 70% of the brains studied showed longer or larger temporal planes in the left hemisphere than in the right.

The temporal plane lies in a region of the brain called Wernicke's area. This area was named after Karl Wernicke because he is credited with being the first to observe that injuries in this region often leads to various symptoms of aphasia. Aphasia is a general term describing any partial or complete loss of linguistic abilities that is caused by a lesion in the brain. Wernicke's area seems to play a strong role in various language functions.

Another anatomical difference in the hemispheres involves the corpus callosum, which has been found to be larger in left-handers and those who are ambidextrous (showing no strong hand preference) than in those who consistently prefer their right hand. Some researchers believe it may be larger in these individuals because mental functions seem to be spread more equally across their hemispheres and this may necessitate a greater amount of interaction and thus connection between the hemispheres.

In addition to these larger physiological differences between the two brain hemispheres, there seem to be more microscopic differences, such as the dispersion of different types of brain cells. Examination of brains at autopsy has revealed some consistent differences, in the number and size of certain neurons between the two hemispheres. A region of the temporal lobe that is part of the auditory association cortex, which is involved in higher-level processing of auditory information and especially speech sounds, is larger on the left side of the brain. And an area lying mainly on the angular gyrus between the temporal and parietal lobes was also found to be larger on the left side. Lesions to this area have been associated with problems in naming objects and in word-finding tasks. Interestingly, enlargement of these areas in the left hemisphere is associated with having a larger left-temporal plane, so that larger anatomical asymmetries seem to be related to more microscopic asymmetries.

Thus it can be seen that there are some relatively consistent anatomical differences between the two hemispheres of the brain. Whether these physical differences are causally related to observed behavioral differences between the two hemispheres, however, is still unclear. This is partially due to the fact that much of this information has come from the study of brains post-mortem so that there is often little knowledge of the types of behavioral asymmetries these individuals may have exhibited before death.