Approximately 1 in 4000 are born partially or totally without a corpus callosum. These people, for the most part, do not have the severe changes as seen in surgical split brain individuals. This might be due to the brain compensating for the lack of a main connection between the hemispheres.

University of Rochester

Beginning in the 1960s, techniques used in studies with split-brain cats were used in order to study split-brain humans. This meant selectively presenting stimuli to one of the hemispheres to test their capabilities.

Studies of Split-Brain Humans

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

In some studies, words or pictures were projected to either the right side or left side of the visual field. As a result, split brain individuals could easily identify and communicate words and pictures projected to the left hemisphere (via right visual field), but couldn't do the same if it was projected to the right hemisphere. This shows how in most people, vocabulary and grammar are dominated in the left hemisphere.

Testing the Left Hemisphere in Split-Brain Humans

Testing the right hemisphere requires nonverbal methods. For example, a picture of a key is projected to the right hemisphere (via left visual field) and the participant is asked to touch different objects that they cannot see until they hold up the correct one. This task could be performed correctly by the left hand (controlled by hemisphere) but not by the right hand (controlled by left hemisphere). In this case, the left hemisphere doesn't know what the left hand is doing. This shows how in most people, the right hemisphere is responsible for processing spatial information, face preception, processing emotional aspects of language, and controlling attention.

Testing the Right Hemisphere in Split-Brain Humans

Callosal Agenesis

An example of split-brain behavior occurs when a picture is presented exclusively to a patient's left visual field, sending the information solely to the largely nonverbal right hemisphere. Consequently, the patient is unable to verbally name the object. However, they can recreate the picture using their left hand, which is also controlled by the right hemisphere. When the more verbal left hemisphere sees the picture that the hand drew, the patient is then able to name it, demonstrating how the hemispheres communicate via external actions.

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