Mediums have always answered my questions regarding this by stating that the mind is set free upon death and is allowed to reach its full potential without the physical limits of the brain.
However my argument has always been that without the physical limitations or disabilities of a brain the person ceases to be the same.
For example your grandmother who was forgetful, or even senile may have lived like that for 20 years, the way she is, is WHO she is, regardless of reasons.
If you remove the limitations or disabilities then if you spoke to your death grandmotehr she would be unrecognisable.
Mediums can never answer this.