Monday, May 06, 2013

Two Types of Hallucination

Oliver Sacks is one of the great essayists of our time. I have just finished his book Hallucinations.

Sacks does not, I think, adequately distinguish two types of hallucination. I will call them doxastic and phenomenal. In a phenomenal hallucination of A, one has sensory experience as of A. In a doxastic hallucination, one thinks one has sensory experience as of A. The two can come apart.

Consider this description, from page 99 of Hallucinations (and attributed to Daniel Breslaw via David Ebin's book The Drug Experience).

The heavens above me, a night sky spangled with eyes of flame, dissolve into the most overpowering array of colors I have ever seen or imagined; many of the colors are entirely new -- areas of the spectrum which I seem to have hitherto overlooked. The colors do not stand still, but move and flow in every direction; my field of vision is a mosaic of unbelievable complexity. To reproduce an instant of it would involve years of labor, that is, if one were able to reproduce colors of equivalent brilliance and intensity.
Here are two ways in which you might come to believe the above about your experience: (1.) You might actually have visual experiences of the sort described, including of colors entirely new and previously unimagined and of a complexity that would require years of labor to describe. Or (2.) you might shortcut all that and simply arrive straightaway at the belief that you are undergoing or have undergone such an experience -- perhaps with the aid of some unusual visual experiences, but not really of the novelty and complexity described. If the former, you have phenomenally hallucinated wholly novel colors. If the latter, you have merely doxastically hallucinated them.

The difference seems important -- crucial even, if we are going to understand the boundaries of experience as revealed by hallucination. And yet phenomenal vs. merely doxastic hallucinations might be hard to distinguish on the basis of verbal report alone, and almost by definition subjects will be apt to confuse the two. I can recall no point in the book where Sacks displays sensitivity to this issue.

Once I was attuned to it, the issue nagged at me again and again in reading:

Time was immensely distended. The elevator descended, "passing a floor every hundred years" (p. 100).

Then my whole life flashed in my mind from birth to the present, with every detail that ever happened, every feeling and thought, visual and emotional was there in an instant (p. 102).

I have had musical hallucinations (when taking chloral hydrate as a sleeping aid) which were continuations of dream music into the waking state -- once with a Mozart quintet. My normal musical memory and imagery is not that strong -- I am quite incapable of hearing every instrument in a quintet, let alone an orchestra -- so the experience of hearing the Mozart, hearing every instrument, was a startling (and beautiful) one (p. 213).

The possibility of merely doxastic hallucination might arise especially acutely when subjects report highly unusual, almost inconceivable, experiences or incredible detail beyond normal perception and imagery; but of course the possibility is present in more mundane hallucination reports too.

(A fan of Dennett might suggest that there's no difference between the phenomenal and doxastic hallucinations; but I don't know what Dennett himself would say -- probably something more complex than that.)


benj said...

If you are having a doxastic hallucination, do you think you are having a phenomenal hallucination? If so, when you think you are having a phenomenal hallucination, how do you know when you are? If you can't, why suppose there are any phenomenal hallucinations? If that supposition can't be justified, why suppose there is any such thing as a 'perceptual experience as of an F' -- as opposed to perceptual experiences of something or other thought to be perceptual experiences of an F? If that supposition can't be justified, how are we to understand the distinction Sacks has supposedly slurred over?

Eric Schwitzgebel said...

Hi Benj! I deleted your first comment, which I assume was meant to be replaced by your second. (I hope that's okay. I should have saved a copy, but I habitually hit the "remove forever" key which I'm used to using for spam.)

You write: "If you are having a doxastic hallucination, do you think you are having a phenomenal hallucination? If so, when you think you are having a phenomenal hallucination, how do you know when you are?"

I would assume that the default supposition would be that you know what your experiences are -- at least in crude outline. But, given my own views on the inaccuracy of our judgments about experience, I'm inclined to think that that default supposition can be defeated fairly easily if there are reasons to be suspicious about the accuracy of the judgment. For example, if your judgment is that you just experienced a wholly novel color, that might be suspicious on general theoretical grounds about the nature of color experience. That doesn't mean you're necessarily wrong that you have had such an experience, but it would probably be good to find some corroborating evidence of some sort.

That's where I'm going to resist the path of your argument. But maybe an absolute flat skeptic about phenomenal report would have to ride your argument to the end.

Howie Berman said...

How would you separate hallucinations from delusions for this reason: that delusions interpret experience and thus are experienced as perceptions?
I think this may effect your argument as delusions partake of the doxic

Michel Clasquin-Johnson said...

OK, let me be the curmudgeon today and see where it takes us.

The human brain and its associated visual apparatus is capable of detecting and interpreting a tiny part of the available photons around us. We somewhat chauvinistically call this "the visible spectrum".

We do know, from instruments we have built, that there are other colours that we have not evolved to see, off to the infrared on one side and the ultraviolet on the other. Other animals are able to see in either or both of those ranges (we can detect infrared in a crude way through the skin as heat, but that does not really affect the argument here since there is no distinction of colours in it, just more heat or less heat).

I have no problem with doxastic hallucinations. I can imagine new, impossible colours just as I can dream of physically impossible sexual positions. But for a phenomenal hallucination to occur, I would need a different kind of brain, one that had evolved to detect and interpret IR and UV colours. I therefore put it forward that the statement "I, a human being, have actually observed a previously unknown colour" contains a contradiction. There are no phenomenal hallucinations. Hallucinations convey no actual information. And "doxastic hallucination" is a tautology, in the rhetorical sense of the word.

One possible get-out-of-jail card: a distant (nocturnal?) ancestor of mine might have possessed the ability to see in IR, and the ability is still there in a latent state. It can be reactivated under the right condition, such as drugs. Possible, but that claim is in principle empirically testable, and becomes a scientific rather than a philosophical claim.

OK, start your flamethrowers.

Eric Schwitzgebel said...

@ Howie: One possibility: Merely doxastic hallucinations might be a *type* of delusion -- delusions concerning one's current or past sensory experience.

Eric Schwitzgebel said...

@ Michel: Color experience appears to be confined to the space of the color solid with hues appearing along the red-green and blue-yellow axes, so yes if a radical new color were to be something unrepresentable by that solid that would be, well, pretty radical, and not in accord with standard theories of color perception.

Of course what theorists think of as impossibilities (pain that is not aversive? tactile experience represented as outside one's body map?) sometimes later comes to be accepted as rare but possible.

Howard Berman said...

Yes, so if I have a delusion that I am Napoleon, what might be the link to my past or current experience?

Michel Clasquin-Johnson said...

Do I have to do ALL the work around here? Let me see if I can demolish my own argument.

While it is true that perception of colour is determined by our sensory apparatus, the interpretation of colour is socially determined and influenced by personal quirks. Why do we claim to see seven colours in a rainbow? Because Isaac Newton said that's how many there were (actually, he started out with five). Most ordinary people can't tell you where violet ends and indigo begins.

Dig around in classical literature and you find plenty of references to the THREE colours of the rainbow. Or think of Homer's "wine-dark sea"? Was the sea around Achaea permanently infested with red tide? Or does this tell us that the way we classify colour is not a divinely determined scheme, but one that is contingent on social consensus?

If we can agree on the latter (the Sapir-Whorf theory of colour perception, one might say), then seeing a new colour is easy. All you have to do is reject the consensus and decide to break up the spectrum in a different way. Drugs can do that. Or so I'm told, not that I would have any personal knowledge of that, of course.

Eric Schwitzgebel said...

Michel: Now we need Michel double-prime to respond to that argument! Or try this: If I pick out one weird-shaped patch from the color solid and call it "grek", I haven't thereby seen a new color in the sense that I *think* the quoted person was trying to convey.

Eric Schwitzgebel said...

Howard: There need be none, as far as I can see. That was my thought in saying that merely doxastic illusions might be a *type* of delusion -- a type in which the delusive content is about sensory experience (rather than, say, about one's identity).

Charles T. Wolverton said...

Another speculation on how a "new" color might be hallucinated.

Presumably, somewhere in the visual processing chain the fundamental entity that is the basis of color perception is a pattern of neural activity, and recognizing a "color" is really recognizing some manifestation of such a pattern (eg, visual imagery). Because of overlapping S-M-L response curves and opponent processing, it seems possible (even likely) that there are patterns that can't occur consequent to visual sensory stimulation but could occur in principle. In which case it's conceivable that a hallucinogen could cause their occurrence. Such patterns wouldn't be recognized (except perhaps from previous hallucinogenic episodes) and in that sense would constitute "new" color experiences.

Other novel features of the experience described in the quote might be explained similarly.

Eric Schwitzgebel said...

@ Charles: Yes, I could imagine things spinning out that way. It might help to know more about the physiology of the color-associated areas of the brain, but last I checked that was still very much a work in progress, scientifically.

Arnold Trehub said...

The passage you quote ("... my whole life ... every feeling and thought in an instant ...") seems more like an example of literary license than an account of a hallucinatory experience. If one were to truly believe that this full content is what one consciously experienced, then I would call it a delusion. If a belief were to cause a true sensory hallucination then I think it would properly be described as a phenomenal hallucination caused by that particular psychological state. Hallucinations in psychoses without evident brain toxicity or brain lesions might be considered to be of this kind.

Dregs said...


Perhaps I'm dense, but in your reply to benj I was unable to sort out what your reply actually was.

For example, you begin by saying that the default supposition would be that one knows what one's experiences are.

Whatever are you talking about?

By knowing what one's experiences are, do you mean that one knows whether they are phenomenal hallucinations as opposed to doxastic hallucinations and vv?

Eric Schwitzgebel said...

@ Dregs: My thought is that our default assumption usually is (and probably should be, despite some of my own skeptical work) that if someone thinks she is having a visual experience of a certain sort (e.g., that she is visually experiencing redness 5 degrees below the center of her visual field) that she probably is having that experience -- and thus either a veridical experience or a phenomenal hallucination (or, somewhere between, a phenomenal illusion) rather than a merely doxastic hallucination. But I think that default supposition can be overcome if the judgment is strange enough or there are other reasons to doubt, in which case it would be an open question whether the experience might be a merely doxastic hallucination.

Does that help?

Dregs said...

Thank you for replying, Eric.

My understanding of what you are saying is that if someone has a sensory experience that seems real to them, then it probably is a real experience--whether a true experience of the world or a phenomenal hallucination. However, based on considerations apart from the fact of the experience itself (such as its strangeness) it can be doubted as possibly a doxastic hallucination instead.

This is where I don't understand.

I can see how considerations such as the strangeness of an experience can support doubt that it is not veridical, but this seems to support only that it might be a hallucination in the general sense instead.

What I don't understand, then, is how any such consideration could indicate that one either is or might be having a doxastic hallucination as opposed to a phenomenal one.

My worry--I take this to be benj's worry as well--is that phenomenal hallucinations are identical either with sensory illusions (such as the bent stick in the water) or interpretive errors (such as mistaking a stranger for your neighbor). In other words, that they don't actually pick out hallucinations.

If this worry is misplaced, then there should be some way to distinguish a phenomenal hallucination from something like an optical illusion.

That way is what I'm missing.

Eric Schwitzgebel said...

Dregs, thanks for pushing on this. It's helpful.

Merely doxastic hallucinations might be reasonably conceivable as "interpretative errors". Whether to call them "hallucinations" would be a question of vocabulary. The person still sincerely says, "I saw a wholly novel color" (or whatever). You might then want to say, that's not really a hallucination just an interpretative error. Fine! But is there a reason to prefer that way of talking to the way I have proposed?

Guy Inchbald said...

Hi, to indroduce myself I am a nobody who once studied a little philosophy. I am sure that Charles Wolverton is onto something sound. We can construct an intermediate or "opponent" colour-cube with axes of Red-Green, Blue-Yellow and Black-White. This cube represents the "space" of possible signals generated by the associated outputs from that area of the brain. But normally this area is stimulated only by the perceived colours in the familiar RGB colour-cube or space. A simple mapping of the one space into the other, e.g. via a geometrical sketch, shows that all of the RGB cube colours fit inside the opponent cube space, and with room to spare. The gaps around the RGB cube represent a space of "impossible" colour signals which are never normally stimulated. But some neural misfunction, whether from drugs, synaesthesia or trauma, could stimulate such an "impossible" signal output to our conscious perceptions. Such colours would clearly be phenomenal, with doxastic colours appearing later in the chain of experiential processing. For example if someone reported seeing a million unknown colours, one might hypothesise that they had hallucinated the impression or belief rather than the actual colours and then, delusionally, come to accept it as a phenomenal hallucination.
How to distinguish between the various types? I would suggest that until the identification and analysis of brain activity becomes sufficiently mature, we can never do more than hazard a guess. Only hard correlates will lead us to the truth of the matter.
(This is a slightly expanded repeat of a previous, anonymous attempt at commenting)

Eric Schwitzgebel said...

Thanks for the thoughtful comment, Guy! That all seems very sensible to me.