This article is from the the official alt.spleen FAQ, by Andrew Stellman with numerous contributions by others.
Andrew says: NOTE: PLEASE DO NOT SEND ANY MAIL TO ME REQUESTING MEDICAL ADVICE! I AM NOT A DOCTOR, AND WILL NOT GIVE YOU ANY USEFUL INFORMATION.
Yes, it does. Generally, mental functions can be split into two groups: very important ones, and less important ones. The brain takes care of the less important mental functions, leaving the spleen free to carry out the much more important ones. Often, after trephination, less important mental functions that were carried out by the brain must now taken over by the spleen, interfering with its usual load of important functions, which can cause premature burnout and severe buildup (see above).