I’ve just discovered a wonderfully simple finger touch procedure that can test the function of your corpus callosum, a key brain structure that connects the two cortical hemispheres.
It is called the ‘cross lateralization of fingertips test’ and was used in a 1991 study by Kazuo Satomi and colleagues.
It relies on the fact that different hemispheres are responsible for the movements and sensations from each hand.
In other words, each hand is connected to a different side of the brain, and, to allow you to co-ordinate both hands, the brain passes information between the two sides by using the corpus callosum.
The corpus callosum is the largest structure in the brain and works like a huge bundle of white matter ‘cables’, connecting different areas.
If this structure gets damaged, a patient might have trouble with coordinating their hands, preventing them from matching sensations on one hand with movement on the other, because the information doesn’t get to where it’s needed.
The test works like this: you need to ask someone to close their eyes and put their hands face up.
You then touch one of their fingertips with a pencil, and with the opposite hand the participant needs to touch the corresponding finger with thumb of the same hand.
For example, if you touched their right ring finger, they would need to touch their left ring finger with their left thumb, as shown in the diagram above.
You need to do this on both hands, with them always touching the corresponding finger on the opposite hand.
It’s important that the person keeps their eyes closed, because as soon as they look, they get information from the eyes, which goes to both hemispheres.
Patients who have damage to the corpus callosum (either because of acquired damage or because it just hasn’t developed) usually can’t do this test, because of the disruption in communication between the two hemispheres of the brain.
Of course, just to be sure its not a problem with movement or sensation in one hand only, the patient is also asked to do another quick test where they’re asked touch the exact finger you just touched.
For this part, the sensation and movement happen in the same hand, so information doesn’t need to cross the corpus callosum.
The test was shown to me by Dr Emma Barkus, who researches what neurological tests can tell us about psychosis and unusual experiences.