Isotopes have the same number of protons (atomic number) but differ in the number of neutrons (hence also in their mass numbers). Isotopes are named according to their mass numbers; example carbon-12, and carbon-14. Their atomic numbers remain as 6 but neutron numbers differ (6 and 8).
In a mammalian brain, there are four major lobes that are present and one of them is the frontal lobe.
The frontal lobe is mainly rich in dopaminergic neurons and thus, it is mainly concerned with functions such as memory, reward, motivation, planning, attention, etc.
The process of break down of tissues and cell death in the tissues is referred to as atrophy and since the frontal lobe is associated with the functions as mentioned above its atrophy would lead to a slow decline in these functions.
Since Mr. Klondlike behavior is changing his blunt comments are most likely a result of atrophy of the frontal lobe.
