² We can frame the problem in terms of Teller's ([1983]; Teller & Pugh Jr. [1983]; Teller [1984]) notion of linking propositions. Linking propositions specify the mappings between the forms of neural responses and the qualities of percepts.

³ See Gorea [1991] for a discussion of the relation of modern psychophysics to Müller's concept.

⁴ The analogy with richness breaks down somewhat because one could claim that richness does not in fact feel like anything at all. Another difference with richness is that the "knowing" we refer to in the case of visual sensation is much more implicit and non-propositional.

⁵ This classical reasoning is an instance of what Pessoa, Thompson, & Noë [1998] call "analytic isomorphism," that is, the view that at the neural substrate of an experience there must be an isomorophism between percept and substrate. Analytic isomorphism comes up again in our discussion of the neural basis of vision in Section 6.

⁶ Koenderink [1984] has a very perspicacious discussion of what it is to perceive, rather than simply to record information, where he makes this point.

⁷ Heil [1983], agreeing with Gibson and rejecting Müller's idea of physiological "channels" associated with different senses, also attempts a taxonomy of the different senses but does not suggest the idea that it could be the laws obeyed by the sensorimotor contingencies that are the essential fact that differentiates them.

⁸ Note that it could be claimed that Müller's idea of specific nerve quality could be salvaged by supposing that what differentiates the senses is different calculations that are done in the different pathways. This was suggested by Wittmann, Pöppel and Schill, reviewers of the original version of this manuscript. In a way this is what is being proposed by the present approach, although the present approach emphasizes that the calculation itself is not enough. What is needed is for the structure of the input/output relationships to obey different laws.

⁹ Note that we have been careful not to say that vision or horseriding provide different experiences: the experience is the fact of engaging in the activities. The activities, we claim, are not providing an experience -- though people often use the word provide in this way, we claim this is a figure of speech, and not indication of a true experience-generating mechanism. It is precisely this kind of misunderstanding which gives rise to the problem of the explanatory gap. Cf Section 3.7.

¹⁰ Merleau-Ponty [1968] has also compared vision to palpation.

¹¹ Minsky [1988] also suggested a similar idea.

¹² It is often assumed that transients must necessarily direct attention to a location. But presumably location is only one feature of visual stimuli, and in the brain, location may have a similar status to other features, like color, orientation, contrast, etc. Could it be that attention can be directed to aspects of a stimulus defined by such other features? For example is it possible to direct attention to all the red items in a scene, or to scene region constituted by a 3D surface? C.f Pylyshyn [1988] on this issue.

¹³ It is worth noting, in addition, that although people certainly say that their dream images are pictorial in character, there is reason to doubt that this is really so (or perhaps even that it is even intelligible). Is it supposed that dreaming is like looking at pictures? Clearly it is not. Is the idea that when we dream, or when we imagine, it is as if the dream represents the dream world with the same level of detail as one would find in a picture? If this is what the claim that "dreams are pictorial in character" comes to, then it is not at all clear that it is true. Does it really seem to you, when you dream, as if all the dreamt environmental detail is represented in sharp focus and uniform detail from the center out to the periphery? Note, it seems pretty certain that that is not what actually seeing is like. It might very well be the case that only those currently processed features of the dream-world are in fact really experienced. A somewhat paradoxical conclusion, but by no means obviously mistaken.

¹⁴ They observed no eye movement advantage but less than 50% correct performance in counting a grating pattern of identical vertical bars: it may be that the observers were using a strategy of estimating the number of bars by evaluating the number on the basis of the overall width of the pattern.

¹⁵ A portion of the existing data purportedly measuring the extraretinal signal under conditions of normal viewing can, to some degree, probably be explained by assuming that they are due to purely retinal effects (smear, retinal persistence, differences in spatio-temporal effects in central and peripheral vision; c.f. O'Regan [1984]).

¹⁶ It could be argued that people actually do have a detailed, picture-like internal representation of the outside world, but that it is destroyed at each saccade or on interruption by flicker and other transients. Alternately, as suggested by reviewers Wittmann, Schill and Pöppel of our manuscript, it may be that we deceive ourselves as to the amount of detail that we think we see in the representation. Such arguments are hard to square with data showing interaction of the change blindness effects with central/marginal interest manipulations, and with the data from the "mudsplash" experiments, among others. (cf. O'Regan, Rensink, & Clark [1996; Rensink, O'Regan, & Clark [1997; Rensink, O'Regan, & Clark [2000]). Similar alternatives have also been discussed by Simons [2000].

¹⁷ D'Zmura & Lennie [1986] have a theory of color constancy that depends on eye movements.

¹⁸ Broackes additionally notes: "And if it is puzzling how a dynamic property can make itself manifest in a static perception ('how can a disposition to present a variety of appearances be visible in a single appearance?'), then we already have, in familiar discussions of aspect-shift, the theoretical apparatus for a solution. It is because there is 'the echo of a thought in sight'". Broackes quotes Strawson [1974], p. 52-53 who says: "To see [a newly presented object] as a dog, silent and stationary, is to see it as a possible mover and barker, even though you give yourself no actual images of it as moving and barking."

¹⁹ Broackes says that contrary to what he said in Broackes [1992], he is either protanomolous or protanope. (Broackes, personal communication).

²⁰ Cole [1991] has also invoked these studies in a functionalist defense against the inverted spectrum problem.

²¹ Curiously, many people wearing normal glasses seem to voluntarily peer over the rims of their glasses when they look at you, as though this procured some kind of advantage in seeing.

²² Dolezal stresses that the use of the terms upside down and right side up is confusing, and guards against saying that the world comes again to appear right side up. He says that in his experiment the final state of adaptation could be distinguished from the state before the experiment. However presumably this could be because the duration of the adaptation was perforce limited, and because use of inverting goggles necessarily involves other constraints like the limited field of view and the weight of the apparatus. Howard & Templeton [1966] also stress the need to be wary of the terms upside down and right side up (see also Smith & Smith [1962]).

²³ Chapter 8 in Taylor's book contains a detailed, behaviorist theory of the effects of inversion of the visual world, referring to specific results of Stratton, Ewert and Kohler. The outcome appears to be that the observed adaptation effects are to be expected, and that the nativist theory is "shattered" (p. 168).The chapter includes a mathematical appendix by Seymour Papert, who was the subject in Taylor's left-right inversion experiment.

²⁴ The situation may be similar to what happens when you move to a new town, and attempt to orient yourself. It takes some time before local and global landmarks merge into a coherent representation of the town. Until that happens (and it may never do), you may make gross mistakes. For example you may be perfectly able to orient yourself locally, but be unable to correctly indicate the direction of a well-known global landmark.

²⁵ Bedford [1995] has a theory of perceptual learning which is related to the theory presented here.

²⁶ Movies of the demonstrations can be found on the first author's web page at http://nivea.psycho.univ-paris5.fr. See also http://www.cbr.com/~rensink/flicker/flickDescr.html.

²⁷ Movie demonstrations of some film-cut and other effects can be found on Simons' page on http://coglab.wjh.harvard.edu.

²⁸ The following quote from Haines [1991] is an example: 'Pilot F was a high-flight-time Captain who demonstrated exceptionally good performance both with and without HUD. The runway obstruction run was his seventh data run. He indicated his "Decision (140 ft) . . to land (110 ft)," and proceeded to do so. The experimenter terminated the run at an altitude of 50 ft. * The pilot was surprised. Captain: "Didn't get to flare on this one." First Officer "No you didn't . . . I was just looking up as it (the picture) disappeared, and I thought I saw something on the runway. Did you see anything?" Captain: "No, I did not." The experimenters suggested that an equipment failure was probably to blame. Both of these pilots saw the obstruction during the second exposure without HUD (13 runs and 21 runs later, respectively) and executed missed approaches. Later, when he was shown the videotape of this run, Pilot D said, "If I didn't see it (the tape), I wouldn't believe it. I honestly didn't see anything on that runway."'

²⁹ This demonstration may not work if the file is being viewed on the web or has been printed with the option of substitution of typography enabled. The point is that there are two ways of forming an "a"; one similar to the hand-written a (a circle with a line next to it), and one similar to a typewriter a. If hand-written-like "a"'s are mixed into a text, provided they have the same height and density as normal a's, this will generally not be noticed.

³⁰The word "of" is repeated. Repetitions of the word "the" can also be easily missed.

³¹ There are nine f's. Many people fail to count the f's in the three occurrences of the word "of".

³² In fact sensation itself is an abstraction, as already noted by James [1890/1950] (Vol 2, p. 3).

³³ Howells [1944] cited by Taylor [1962] p. 246 is an interesting example where association of a low and high pitched tone with red and green respectively, over 5000 trials, gave rise to a perception of white being tinged with red and green when white was associated with the tones.

³⁴ Bedford [1995] has a theory bringing together the McCollough effect and adaptation to prism displacement which is similar in concept to the present theory.

³⁵ In a criticism of Crick & Koch's arguments, Cogan [1995] also suggests that the notion of "perceptual moment" may not be useful. Dennett [1991] notes a similar point in his "multiple drafts" theory of consciousness.

³⁶ A similar argument was made in Section 5.5 with regard to the "filling in" of the blind spot: there may actually be what look like filling in processes in the brain, but these can not be what provide us with the impression of the blind spot being filled in.

³⁷ There is a large family of similar debates, e.g. the hypothesis of the inverted spectrum Block [1990]; Harman [1990]; Shoemaker [1970]; Palmer [1999]; the what-Mary-knows puzzle Nemirov [1984]; Lewis [1988]; Jackson [1986].

³⁸ This has been shown by Chun & Nakayama [2000] in the context of experiments on change blindness (cf. also Chun & Jiang [1998]).

³⁹ The phenomenological movement in philosophy is, to some extent, concerned precisely with the development of such first-personal methods.

⁴⁰ Varela et al. [1991] and Thompson, Palacios, & Varela [1992] also make this point. In their terminology, consciousness is something we enact.