A related anecdote: A few years ago, I asked my friend, a neuroscientist specializing in vision, why it's sometimes painful to look at the sky. After confirming that it's unrelated to cloud cover but vaguely correlated with the seasons, he suggested that it might be that I'm sensitive to the polarization of light, but said that this was unlikely since mammals in general are supposedly not capable of detecting it. Polarized glasses are cheap, though, so I tried them, and they entirely fixed the problem - notably, regular sunglasses do not - indicating that that is probably the case.
Even more interestingly, I discovered that some significant parts of my visual experience were related to polarization: Detecting the rotation of distant reflective objects (panes of glass, leaves) is helped by it, for one. More significantly, perceiving how far away objects are is affected by it: It's easier for me to tell how far away a small, relatively still object is without the glasses, but also much easier for me to tell how fast a large object is approaching with them, and I find crossing busy roads to be much less stressful that way. I also find certain kinds of reflective surfaces confusing with the glasses, but entirely sensible without them.
The human eye is slightly sensitive to the polarization of light. See Haidinger's brush, which is a yellowish bowtie shaped pattern some people can see in the center of their visual field when looking at the sky while facing away from the sun.
I may simply be unclear on what it means to be "wrong about the subjective quality of your own conscious experience," but it seems to me that this post is completely irrelevant to that question. All of the evidence shows flaws in our predictive ability, our memory, and our language. I don't see any contradictions or wrongness; indeed, I'm still unsure what such would look like. I'll go through it step by step.
Someone predicted that people couldn't experience echolocation. He was wrong. No evidence was offered that he could experience echolocation. Moreover, comparing the ability of the untrained to notice the difference between a T-shirt and a mixing bowl, and the ability of a bat or dolphin to render rich detail is disingenuous. But disputing the detail is besides the point: his mistake was about his abilities, not his actual experience. It's not like he experienced echolocation and didn't know it, or failed to and thought he did.
Does a coin look circular? This seems to be purely semantic and, if anything, a product of language. No one is disputing my ability to see a coin or predict or understand its properties. The problem is mostly whether we're describing the image o...
"We can be wrong about our subjective experiences" is a very broad and maybe misleading way of summarizing this information.
You've shown that we can sometimes fail to consciously pick up on certain channels of sensory information (like echolocation) or fail to consciously appreciate certain qualities of our sensory experience (like the ellipticalness of coins or the vagueness of imagery).
But all your examples have the quality that if we consciously think about them, we realize our mistake (I know this is kind of unfair to you as you couldn't have used an example that didn't have that quality because people wouldn't admit they were wrong about it). So this post only re-affirms that there is a conscious-unconscious distinction, and that lots of the things we think are fully conscious processes are the conscious shadows of larger unconscious processes.
When I think of being wrong about subjective experience, I think of being wrong about the "contents" of "my" "own" "consciousness". The problem here isn't that I'm wrong about my consciousness, it's that certain things never entered my consciousness to begin with and were handled by unconscious routines without me noticing. As soon as I think about them consciously, I'm no longer wrong.
This is also how people like Descartes used the idea of subjective experience, and I don't think knowing about these examples would make Descartes hesitate to say with certainty that he can be sure he's really thinking if he thinks he's thinking.
Here's a more straightforward case of being mistaken about one's experience that happened to me:
Schwitzgebel describes somewhere an experiment you can do with a random playing card. Draw it, and hold it facing you at arm's length directly to your left or right while focusing your eyes straight ahead. Slowly move it around in an arc at arm's length, so it goes through your peripheral vision bit by bit, and try to guess if it's red or black.
I tried this, and I had the weirdest experience. I thought I saw it as black(1), and then I realized it was red, and as the card moved, I did not experience a change in its apparent color; it just became plain that that color, the same one, was red.
So either the card looked black and then red without changing its apparent color (implausible), or I was mistaken about my subjective experience somewhere (the initial color perception, or the absence of a change perception).
(1) I don't remember which it was actually, but it was the wrong one.
Some neat tidbits about our ability to recall our conscious experience and about how difficult it is to hold a passing thought in memory long enough to analyze it, but it is a total strawman of Hume's (and many others') position that "I cannot be wrong about my subjective experience."
What you've done here is equivocate on the term "subjective experience," using it in the introduction as if it were going mean your current, right-this-moment experience (e.g., "I am in pain now," which imparts a huge wow factor for readers, so I can see why), then proceeded to give a bunch of examples where "subjective experience" means something you subjectively experienced in the past, or had trouble bringing fully into conscious awareness in the first place.
Then, at the very end, you equivocate back to the Humean sense of this-moment conscious experience and flounder into this whopper:
We can be wrong about our own subjective conscious experiences. Thus, they cannot serve as a bedrock for certainty and a priori truth.
Sure, past subjective conscious experience is something we can be wrong about. We can misremember things. Hume's (and others') point is that...
Since nobody's linked it directly here, I should probably point out that an old draft of Perplexities of Consciousness is available online.
Also it might be worth pointing in the post that blind echolocators typically do not realize they are echolocating unless it is pointed out to them.
In fact, let me go ahead and just paste in here another quote from that book (not as a suggestion to add to the post, I don't mean, but a neat thing to point out):
...You might think that the blind, whose abilities at echolocation are generally thought to be superior to those of normally sighted people, and who often actively use echolocation to dodge objects in novel environments, would be immune to such ignorance. Not so. For example, one of the two blind participants in Supa and colleagues’ 1944 study believed that his ability to avoid collisions was supported by cutaneous sensations in his forehead and that sound was irrelevant and distracted him (p. 144 and 146). Although asked to attend carefully to what allowed him to avoid colliding with silent obstacles, it was only after a long series of experiments, with and without auditory information, and several resultant collisions, that he was final
I think the temptation is very strong to notice the distinction between the elemental nature of raw sensory inputs and the cognitive significance they are the bearers of. And this is so, and is useful to do, precisely to the extent that the cognitive significance will vary depending on context and background knowledge, such as light levels, perspective, etc. because those serve as dynamically updated calibrations of cognitive significance. But these calibrations become transparent with use, so that we see, hear and feel vividly and directly in three dime...
You present some interesting material but I don't think it supports your conclusion. For example, that I am unaware of some of my perceptual abilities does not entail that I'm wrong about my own subjective experience. Subjective experience is generally taken to mean consciousness and I cannot be conscious of something of which I am unaware. Perhaps my hidden talent for echolocation has covertly aided me in the past but, by definition, it can't be part of what I was conscious of and therefore wasn't part of my subjective experience. This also applies to you...
Everybody's always citing Hume, but nobody ever seems to do him any justice. The OP is simply yet another example in this trend. I have no idea whether after reading the first paragraph of your post, Hume would agree that he couldn't "bring himself to seriously doubt the content of his own subjective experience", but I'm pretty sure that by the end of it, he would summarily reject your interpretation of his epistemology.
First of all, to make what I'm saying at least sound plausible, I need only give you one counter-example:
I think it's important to distinguish between subjective experience and verbal reports of subjective experience.
You've convinced me that my verbal reports of subjective experience are untrustworthy. The next time I make (or am tempted to make) an assertion like "I see a right angle," I will ask myself, "Do I really see a right angle? What is actually going on here?" whereas before I would not have bothered to do so. So, thank you for the help understanding myself. That's a big step.
On the other hand, you haven't quite made out the case ...
Daniel Dennett's Consciousness Explained is a very relevant piece of work here. Early in his book, he seeks to establish that our intuitions about our own perceptions are faulty, and provides several scientific examples to build his case. The Wikipedia entry on his multiple drafts theory gives a reasonable summary.
Neat. Your conclusions don't surprise me much, but I hadn't heard of the evaluating-your-mental-imagery exercise before. It's obvious to me now that my mental images are very foggy; they're mostly concept with a small amount of visual detail attached. However, I'm sure this a faculty that can be improved. Before I started my music training, songs stuck in my head were tuneless, basically the lyrics in rhythm but in a kind of monotone. After 7 years of being in bands and choirs, I can now hear tunes in key in my head (imagining melodies involves feeling the...
If they are genuinely learning the structure of reality in their studies (rather than, e.g., how to best pull off an "inside joke" among their fellow acoyltes), they ought to be capable of a lot more than just impressing classmates.
I agree with Paul Graham on this point, actually, which is that the first mover advantage is simply too strong to be overcome on any significant scale. Shakespeare is the playwright because he was massively prolific when plays were the hot new thing, and then inertia plus quality will propel him forward as the playwright until the end of humanity. There is some upper bound to the quality of plays, and Shakespeare is close enough that even though people who like plays will have contemporary favorites and those plays will be as good as or better than Shakespeare they will never significantly reduce his market share.
There's a similar experiment you can consider: start off with a bag containing one of each color of M&M. Remove one of the M&Ms chosen uniformly at random, and replace it and another M&M of the same color. What happens is that after 100 iterations, the bag will be almost all one color of M&M (this simulation is pretty easy to code; try it!). Even if you put in a new color of M&M that gets 2 new M&Ms of its color whenever it's picked, it won't dominate the old color since the old edge is too strong.
The conclusion may be a bit of overstretch (not that it isn't right in some sense, but relative to the argument in this post): it seems that the modern defenders of introspective knowledge of conscious experience (Mary seeing red, etc) have only a very limited sort of access in mind.
Today I added the 'Sound and vision biases' section to the original post, having just discovered the cited studies on auditory bias and the descent illusion.
I think part of the disagreement over this is that the usual, simple model of qualia is too limited. That's an easy mistake to make, given that the normal examples of qualia are things that are directly reported by sensory organs, but in practice qualia have much more to do with our brains than our eyes and ears and so on, and don't have a 1:1 correspondence with sensory information in most cases.
Take lukeprog's coin ellipse/circle example. It's pretty trivial to show that the outline of a coin from most perspectives is an ellipse, but that doesn't mean th...
Having read your post, and asking myself "Can I be wrong about my own subjective experience?", I find myself wondering why I'm even asking such a question. The answer seems to depend on what I want to achieve with it. Perhaps I should wait for your later posts in this series before I begin really answering. With an endgame in mind, the OP's question would be more productively and fruitfully answered.
A circular coin held at a slant to you doesn't look like a true ellipse. The half that's closer to you will be fuller and the half that's away from you will be flatter because of perspective.
ETA: Pfft points out below that circles actually map to ellipses under perspective projection. Before I posted this, I took a look at an angled circle, and it looked as I described it-- at the moment, they're looking like true ellipses.
Speaking of the yellow banana, people do a lot of filling in with color. Take a careful look at someone who's wearing a monochrome garm...
Speaking of the yellow banana, people do a lot of filling in with color.
One of Dennett's points is the misleading notion that our mind "fills in". In the case of vision, our brain doesn't "paint in" missing visual data, such as the area in our field of vision not captured by our fovea. Our brains simply lack epistemic hunger for such information in order to perform the tasks that they need to.
I've noticed that this account potentially explains how color works in my dreams. My dreams aren't even black and white - the visual aspects are mostly just forms. However, if the color has meaning or emotion, it's there. I recently had a dream where I looked up at the sky, and the moon was huge and black, moving in a rapid arc across the sky then suddenly diving into the Earth causing an apocalyptic wave of dirt to head towards me. The vivid blackness was present, because it meant something to me emotionally. The houses, in comparison, merely had form, but no color. In any case, it seems that the question "Do we dream in color?" can't be answered adequately if using a "filling in" model of the mind.
I've heard that lots of people used to end up dreaming in black and white as a result of watching lots of black-and-white TV.
What is the character of an imagined scene?
I believe that the best we can say about people's visual imagination is that there's a continuum. Some people have no visual imagination at all, and then there are people with eidetic memory so advanced I hazard to say they need to "imagine" anything at all, pun intended.
I've been thinking about this recently because my grandmother is dying and it bothers me that I can't imagine what she looks like. Or anyone, for that matter. All I end up imagining is a fragment of her hair, an eye, or some other piece of her face. It's disconcerting.
The elliptical coin case is a good one for understanding the concept of qualia. As you move your head, your cognitions about the coin don't change: you still affirm its circularity. Your visual perception remains consistent with that affirmation - but still, something about that perception is changing. The shape and extent of the copper-colored qualia are changing (the coin is a penny). If we want to, we can change our focus from the usual objective facts and describe these aspects of subjective experience. That's the point at which the word "el...
The section about "imagined scenes" reminds me very much of the post Generalizing from One Example. I think that post and this one can be taken together to imply that, since we can be wrong about our own subjective experiences, we can be even more wrong when we try to model others' subjective experiences naively (based on our own unexamined misinterpretations). That makes the reflective process, as you describe in the conclusion, even more important.
Also, I do dream in color (as far as I know).
Very interesting article, Luke, I think I might try some of those echolating exercises.
A somewhat random observation: I dream in color but I hallucinate in black and white. I've only ever hallucinated in dark rooms when I've just woken up so I suppose this is not terribly surprising.
Although I very rarely have hallucinations it happens that I did have one the night before last. I woke up in the middle of the night and saw a toddler climbing the door of the room I was in with unnatural jerky motions. The toddler was about the same age as my daughter but was...
Funnily enough, I have never once noticed the elliptical shape of a coin when viewed at an angle. It just looked like a perfectly round circle, only tilted slightly.
If I had to guess, I'd say my brain doesn't process spatial information all that well, so my looking at a tilted coin was very similar to a middle school student reading about trigonometry- no glint of recognition whatsoever. To them it just looks like funny words and numbers.
Today I learned of another example. I was unaware that humans perceive changes in 'approaching' sounds as greater than equivalent changes in 'receding' sounds. Also, 'approaching' sounds are perceived as occurring closer to us than equivalent receding sounds. The evolutionary story being told about the origin of this bias should be obvious.
Neuhoff (2001). An adaptive bias in the perception of looming auditory motion. Ecological Psychology, 13: 87-110.
It's not clear that there IS a fact of the matter on many of these questions. Not in the sense of being subjective, but simply the question not making sense to ask. In fact, this is how I experience it to be, but as we just established that could be wrong, and even if it's right I have reasons to believe my subjective experience might be different from that of most people.
Pre-scientific authors tended to assume they dreamed in color, while studies in the first half of the 20th century found very few people who reported dreaming in color.
This is extremely unsurprising, given that the world was in black and white until sometime in the 1930's.
ETA: I'd hope such a patently absurd statement would get people to follow the link. I was evidently wrong. If you follow the link (which is a classic!), you'll see the point was intended to be humorous. My bad.
As Psychohistorian already wrote my reaction to this post for me, I'll just leave this somewhat relevant story by Smullyan here.
Pre-scientific authors tended to assume they dreamed in color, while studies in the first half of the 20th century found very few people who reported dreaming in color. In the 1960s, this consensus was overturned, and recent studies show that today, more than 80% of people report that they dream in color.
I think I once heard someone hypothesize that people in the early/mid-20th century tended to dream in black and white as a result of watching lots of TV/films in black and white.
Hume was skeptical of induction and causality. Descartes began his philosophy by doubting everything. Both thought we may be in great error about the external world. But neither could bring themselves to seriously doubt the contents of their own subjective conscious experience.
Philosophers and non-philosophers alike often say: "I may not know whether that is really a yellow banana before me, but surely I know the character of my visual experience of a yellow banana! I may not know whether I really just dropped a barbell on my toe, but surely I know the subjective character of my pain experience, right?"
In this article I hope to persuade you that yes, you can be wrong about the subjective quality of your own conscious experience. In fact, such errors are common.
Thomas Nagel famously said that we cannot imagine the subjective experience of bat sonar:
Bat sonar, though clearly a form of perception, is not similar in its operation to any sense that we possess, and there is no reason to suppose that it is subjectively like anything we can experience or imagine.1
Hold up a book in front of your face at arm's length, close your eyes, and say something loudly. Can you hear the emptiness of the space in front of you? Close your eyes again, hold the book directly in front of your face, and say the book's name again. Can you now hear that the book is closer?
I'll bet you can, and thus you may be more bat-like than Nagel seems to think is possible, and more bat-like than you have previously thought. When I discovered this, I realized that not only had I been wrong about my perceptual capabilities, I had also been ignorant of the daily content of my subjective auditory experience.
Blind people can be especially good at using echolocation to navigate the world. Just like bats and dolphins and whales (but less accurately), humans can make sounds and then hear how nearby objects reflect and modify those sounds. People with normal vision can also be trained to echolocate to some degree with training, for example detecting the location of walls while blindfolded.2 After some practice, blindfolded people can use sound to distinguish objects of different shapes and textures (at a rate significantly better than chance).3
You can try this yourself. Get a friend to blindfold you and then move their hand to one of four quadrants of space in front of your face. Try hissing or talking loudly and see if you can tell something about where your friend's hand is. Have your friend move their hand to another quadrant, and try again. Do this a few dozen times. I suspect you will find that after a while you'll do better than chance at locating the quadrant your friend's hand is in, and you may be able to tell something about its distance as well. If so, you are echolocating. You are having an auditory experience of the physical location of an object - something you may not have realized that you can do, something you probably have been doing your whole life without much realizing it.
Alternatively, have a friend blindfold you and place you some unspecified distance from a wall. Step toward the wall a few inches at a time, speaking loudly, and stop when the wall is directly in front of you. Most people find they can do this quite reliably. But of course you can't see or touch the wall, and the wall is making no sound of its own. You are echolocating.
One final test to prove it to yourself, this one relevant to shape and texture. Close your eyes, repeat some syllable, and have a friend hold one of three objects in front of your face: a book, a wadded-up T-shirt, and a mixing bowl. I think you'll find that you can distinguish between these three silent objects better than chance, and that the book will sound solid, the T-shirt will sound soft, and the mixing bowl will sound hollow. You are echolocating shape and texture.
Set a coin on a desk or table three feet in front of you. Is its shape circular or elliptical?
One popular view is to say that we perceive the coin in two aspects. In one aspect, we perceive the raw sense data from our visual plane, which shows the coin as being elliptical (because one end of it is stretching away from us). In another aspect, we perceive it as circular because our minds have an intuitive physics about the shape permanence of solid objects. Perhaps our minds 'flip' between seeing the coin from the two aspects - a kind of 'Gestalt shift' - just as it flips between seeing a rabbit and a duck in Wittgenstein's famous duck-rabbit drawing.
Schwitzgebel (2011) reports his own confusion:
What exactly is my sensory experience as I stare at a penny? My first and recurring inclination is to say that the penny looks just plain circular, in a three-dimensional space - not elliptical at all... However, I also find that if I dip my head lower to view the penny from a flatter angle, I begin to see how one might think it looks elliptical. Closing one eye helps too... Am I experiencing the ellipse too? Maybe not. But neither can I say that I noticed any Gestalt shift... Could it be, simply, that my visual experience is disorganized, so that there is no simple relationship between viewing angle and apparent shape...?
Maybe my terms and concepts are muddled. What is it for something to 'look elliptical'? ...
Or am I simply a poor introspector? Maybe the fact that my own phenomenology in this case doesn't seem obvious to me reveals my introspective ineptitude... And yet I am not sure I should trust other [people's] introspections either.4
The character of our subjective experience of shape at varying angles and distances is widely debated by philosophers and psychologists,4 lending some support to the claim that we are unsure of it.
Close your eyes and picture the front of your house or apartment building from the street.
Presumably, you know that you experience an image, and you know some aspects of its content (that it is a house, from a certain angle). But what else do you know? Schwitzgebel questions:
How much of the scene can you vividly visualize at once? Can you keep the image of the chimney vividly in mind at the same time that you vividly imagine your front door, or how does the image of the chimney fade as you begin to think about the door? How much detail does your image have? How stable is it? If you can't visually imagine the entire front of your house in rich detail all at once, what happens to the aspects of the image that are relatively less detailed? If the chimney is still experienced as part of the imagery when your imagemaking energies are focused on the front door, how exactly is it experienced? Does it have determinate shape, determinate color? In general, do the objects in your image have color before you think to assign color to them, or do some of the colors remain indeterminate, at least for a while...? If there is indeterminacy of color, how is that indeterminacy experienced? As gray? Does your visual image have depth in the same way that your sensory experienced does... or is your imagery somehow flatter...? ...Do you experience the image as located somewhere in egocentric space - inside your head, or before your eyes, or in front of your forehead - or does it make no sense to attempt to assign it a position in this way?5
When questioned in this way, most people quickly become uncertain about the character of their own subjective conscious experience.
Most people, when asked, are fairly confident of their answer. But the answer given (in questionnaires or after awakened during REM sleep) has varied widely throughout history and between persons.6 Pre-scientific authors tended to assume they dreamed in color, while studies in the first half of the 20th century found very few people who reported dreaming in color. In the 1960s, this consensus was overturned, and recent studies show that today, more than 80% of people report that they dream in color. But there are also certain populations that overwhelming report dreaming in black and white.
Is there something in our genes or in the air that decides whether or not we dream in color, or are we confused about our own subjective experience?
Schwitzgebel reviews the arguments back and forth, but none give a clear answer. And unfortunately, there remains much disagreement about the neurology of color experience.7
Schwitzgebel asks:
Do you have constant tactile experience of your feet in your shoes?... Constant visual experience of the frames of your eyeglasses? ... Is consciousness abundant, the stream of experience bristling with phenomenology in a wide variety of modalities simultaneously (visual, auditory, tactile, olfactory, imagistic, proprioceptive, emotional), or is it sparse, limited to one or a few things at a time?
Suppose you have driven to work by the same route a thousand times. Today, you are absorbed in remembering an unpleasant interaction with your department head. Traffic is light, no dangerous situation occurs, and you drive habitually. You arrive at your usual parking area and seem to "wake up" - "Ah, I'm at work already!" - with virtually no memory of having driven there. Did you have visual experience while you were driving? You responded to events on the road, stopped at red lights, and stayed in your lane... But perhaps visual input can influence behavior without the involvement of consciousness.
These are difficult questions, and both experts and laymen disagree as to their answers.8
Have you noticed that you perceive vertical distance as greater when you are looking down than when you are looking up? Well, you do.9
Have you noticed that you perceive changes in 'approaching' sounds as being greater than equivalent changes in 'receding' sounds? Have you noticed that you perceived 'approaching' sounds as occurring more near to you than 'receding' sounds? You do.10
As many people who practices meditation seriously can tell you, the subjective contents of conscious experience are often surprising and uncertain. We can be wrong about our own subjective conscious experiences. Thus, they cannot serve as a bedrock for certainty and a priori truth. (At least, minimally complex subjective conscious experiences cannot.)
It seems that all we can do is exercise some naturalized epistemology: "reflecting on your mind's degree of trustworthiness, using your current mind as opposed to something else." Luckily, the brain is the lens that sees its flaws, as demonstrated by surprising studies in human echolocation, the color of dreams, and more.
1 Nagel (1974).
2 Supa et al. (1944); Ammons et al. (1953); Roseblum et al. (2000).
3 Hausfeld et al. (1982); Rosenblum & Robert (2007); Gordon & Rosenblum (2004).
4 Schwitzgebel (2011), chapter 2. Note that I also interviewed the author here. This post is basically a summary of a few sections of Schwitzgebel's book.
5 Schwitzgebel (2011), chapter 3.
6 Schwitzgebel (2011) reports that most people he ask about dreaming in color are fairly confident of their answer. Also see his Table 1.1, which summarizes the results of 21 studies on the reported color of dreams, some of which include confidence measures. Chapter 1 contains a summary of historical assumptions about dreaming in color.
7 Gegenfurtner & Kiper (2003); Solomon & Lennie (2007); Wade et al. (2008); Conway (2009).
8 Schwitzgebel (2011), chapter 6.
9 Jackson & Cormack (2008).
10 Neuhoff (2001).
Ammons, Worchel, & Dallenbach (1953). Facial vision: The perception of obstacles out of doors by blindfolded and blindfolded-deafened subjects. American Journal of Psychology, 66: 519-554.
Conway (2009). Color vision, cones, and color-coding in the cortex. Neuroscientist, 15: 274-290.
Gegenfurtner & Kiper (2003). Color vision. Annual Review of Neuroscience, 26: 181-206.
Gordon & Rosenblum (2004). Perception of sound-obstructing surfaces using body-scaled judgments. Ecological Psychology, 16: 87-113.
Hausfeld, Power, Gorta, & Harris (1982). Echo perception of shape and texture by sighted subjects. Perceptual and Motor Skills, 55: 623-632.
Jackson & Cormack (2008). Evolved navigation theory and the descent illusion. Evolution and Human Behavior, 29: 299-304.
Nagel (1974). What is it like to be a bat? Philosophical Review, 83: 435-450.
Neuhoff (2001). An adaptive bias in the perception of looming auditory motion. Ecological Psychology, 13: 87-113.
Roseblum, Gordon, & Jarquin (2000). Echolocating distance by moving and stationary listeners. Ecological Psychology, 12: 181-206.
Rosenblum & Robert (2007). Hearing silent shapes: Identifying the shape of a sound-obstructing surface. Ecological Psychology, 19: 351-366.
Schwitzgebel (2011). Perplexities of Consciousness. MIT Press.
Solomon & Lennie (2007). The machinery of colour vision. Nature Reviews Neuroscience, 8: 276-286.
Supa, Cotzin, & Dallenbach (1944). Facial vision: the perception of obstacles by the blind. ASmerican Journal of Psychology, 62: 133-183.
Wade, Augath, Logothetis, & Wandell (2008). fMRI measurement of color in macaque and human. Journal of Vision, 8(10): 1-19.
