Fibers in the corpus callosum provide abundant bidirectional neural connections between most of the respective cortical areas of the two cerebral hemispheres except for the anterior portions of the temporal lobes; these temporal areas, including especially the amygdala, are interconnected by fibers that pass through the anterior commissure.
Because of the tremendous number of fibers in the corpus callosum, it was assumed from the beginning that this massive structure must have some important function to correlate activities of the two cerebral hemispheres. However, when the corpus callosum was destroyed in laboratory animals, it was at first difficult to discern deficits in brain function. Therefore, for a long time, the function of the corpus callosum was a mystery. However, properly designed experiments have now demonstrated extremely important functions of the corpus callosum and anterior commissure.
One of the functions of the corpus callosum and the anterior commissure is to make information stored in the cortex of one hemisphere available to corresponding cor tical areas of the opposite hemisphere. The following important examples illustrate such cooperation between the two hemispheres:
1. Cutting the corpus callosum blocks transfer of information from Wernicke’s area of the dominant hemisphere to the motor cortex on the opposite side of the brain. Therefore, the intellectual functions of Wernicke’s area, located in the left hemi sphere, lose control over the right motor cortex that initiates voluntary motor functions of the left hand and arm, even though the usual subconscious movements of the left hand and arm are normal.
2. Cutting the corpus callosum prevents transfer of somatic and visual information from the right hemisphere into Wernicke’s area in the left dominant hemisphere. Therefore, somatic and visual information from the left side of the body frequently fails to reach this general interpretative area of the brain and therefore cannot be used for decision making.
3. Finally, people whose corpus callosum is completely sectioned have two entirely separate conscious portions of the brain. For example, in a teenage boy with a sectioned corpus callosum, only the left half of his brain could understand both the written word and the spoken word because the left side was the dominant hemisphere. Conversely, the right side of the brain could understand the written word but not the spoken word. Furthermore, the right cortex could elicit a motor action response to the written word without the left cortex ever knowing why the response was performed. The effect was quite different when an emotional response was evoked in the right side of the brain: In this case, a subconscious emotional response occurred in the left side of the brain as well. This response undoubtedly occurred because the areas of the two sides of the brain for emotions, the anterior temporal cortices and adjacent areas, were still communicating with each other through the anterior commissure that was not sectioned. For instance, when the command “kiss” was written for the right half of his brain to see, the boy immediately and with full emotion said, “No way!” This response required function of Wernicke’s area and the motor areas for speech in the left hemisphere because these left-side areas were necessary to speak the words “No way!” When asked why he said this, however, the boy could not explain it.
Thus, the two halves of the brain have independent capabilities for consciousness, memory storage, communication, and control of motor activities. The corpus callosum is required for the two sides to operate cooperatively at the superficial subconscious level, and the anterior commissure plays an important additional role in unifying the emotional responses of the two sides of the brain.
