What is a zombie?

Perhaps a whole lot more than you thought...  thinking on Dennett and the zombie concept, mostly drawn from Intuition Pumps (2013).

The philosophical concept of the zombie seems to start from a fairly simple definition: "a hypothetical being that is indistinguishable from a normal human being except in that it lacks conscious experience, qualia, or sentience." (drawn from Wikipedia)

Dennett's important point is that given this definition, it must be true that this zombie has very complex functional abilities - it must have functionality to support all sorts of things that normal humans can do - such as visual-auditory-olfactory-touch-taste sensory input, memory (perhaps slightly faulty), color recognition, facial recognition & linking face to name, ability to know that recognition of friends & family should trigger different behavior than recognition, say, of a politician, and so much more.  While by definition it does not have conscious experience, it's hard to say how one could ever confirm that this was the case.

Dennett then goes on to examine what he calls a subset of zombies, those which have "equipment that permits it to monitor its own activities, both internal and external, so it has internal (nonconscious) higher-order informational states that are about its other internal states." (p. 290). He calls these 'zimboes' but it's unclear to me whether such equipment is actually necessary in all zombies in order to produce the definitional behavior of being indistinguishable.  Dennett claims that only a zimbo can "hold its own in everyday interactions" - and that's my sense as well - to be indistinguishable from a normal human.  So I guess I'm unsure of why Dennett creates this new category of zimbo, if the zimbo has equipment that all zombies must have.

At a later point, Dennett examines a couple cases of non-normal human pathologies around facial recognition. Prosopagnosics are people who do not recognize people's faces, and people with Capgras delusion who can recognize people but believe they are 'imposters' - not truly the person they resemble. Research on brain function seems to indicate that there are at least two mechanisms at work in normal facial recognition - there's unconscious visual processing going on that also ties into emotional recognition, and there's conscious recognition of 'knowing who it is'.  If you can show that the unconscious mechanisms are broken (as apparently in the case of Capgras), leading to an altered conscious experience (sense of an imposter), then it appears the qualia is quite tightly tied to the unconscious mechanisms (but by definition qualia is supposed to be the conscious bit).

In other words, it's hard to draw a neat line around qualia when you look closely.  Again pointing out the difficulty of truly imagining the zombie.

I would argue that regardless of the brain mechanisms in use (and I do agree that many modules or mechanisms are used), the subjective experience as a whole is the emergent phenomenon of interest, and the prosopagnosic indeed has a different subjective experience than normal people, as does the Capgras subject. It is a fact that the Capgras subject is deluded about reality, but that fact doesn't alter the subjective experience of seeing people as impostors.