A man doesn’t recognize his wife or children due to dementia, Alzheimer’s, etc., but if you ask him to fix a car engine (if he was a mechanic) or discuss a physics theory (if he was a physicist), he’s capable of doing it and can explain it. Is there medical term for this type of mental loss or degradation?
Selective amnesia, maybe.
It is certainly possible to lose specific types of ability, including the ability to recognize people’s faces, through brain damage in specific brain areas (as opposed to the widely diffused damage caused by things like Alzheimers), but although there may be specific names for the loss of specific abilities, I do not know that there is a general name for all of them. (Well, they are agnosias, but that is probably a wider term than what you are looking for.) An inability to recognize faces is called prosopagnosia. There is also a neurological syndrome where familiar people, such as family members, seem to be strangers (it is not so much that they are not recognized, but they seem to be imposters), but I can’t recall the name of it at the moment. Anyway, people suffering from syndromes like these could probably still fix a car engine or discuss physics (if they knew how to do so before.)
An interesting phenomenon.
I can’t answer your specific question but will point out that the cerebellum (at the back of the brain and traditionally believed to be involved only in things like coordination, balance, and fine motor control) also plays a role in memory. This role, not too surprisingly, involves so-called “motor memory”.
Alzheimer’s, which usually has catastrophic effects on a person’s memory, tends not to involve the cerebellum (or at least not nearly to the degree elsewhere in the brain). Perhaps that is why, then, a person with Alzheimer’s and the associated memory loss can sometimes still perform tasks or remember things, i.e. tasks and things arising from cerebellar memory.
(Please note that I really don’t know much about, if anything, in the area of cerebellar memory. So, take my comments with a huge grain of salt - they are nothing more than the speculations of a dilettante!)
ETA: I would further speculate that in your example above, fixing a car engine, but not discussing plasma physics, might involve cerebellar memory.
You might be interested in this wikipedia article. There are several terms used that might be what you are looking for:
You might be talking about prosopagnosia.
Oliver Sacks (who has prosopagnosia, as it happens) talks a lot about memory, of course. On the subject of “amnesia islands” (not an actual term), there are some interesting videos at http://musicophilia.com/index.htm
Also read The Abyss | The New Yorker
Sacks did a great radio interview about the guy in the New Yorker article: Podcast | Radiolab | WNYC Studios
Ah, I have remembered the name of the disorder (mentioned in my earlier post) where someone thinks that a person or people whom they know very well (such as close family members), have been replaced by an impostor. It is Capgras syndrome. In this disorder, basic recognition seems to be intact (the sufferer knows the person looks like who they are supposed to be), but, as I understand it, somehow the normal emotional response the person one is close to is no longer evoked. The person looks like one’s wife (or whatever) but does not arouse the emotions she previously aroused, so she does not feel like one’s wife any more.