It has long been suspected that, in perceiving the same thing in the outside world, people may have variable qualia of that thing appearing in their minds, that is, qualia of the same thing may vary in their appearances and appear different in people’s minds. For example, when people look at the same color C, the color C qualia may appear as the red color quale in some people and as the blue, green, or another color quale in other people [Figure 9.1]. It is also possible that, in some people, the color C qualia may appear as some extraordinary color qualia that are not in the color spectrum that general people perceive, too. Yet, all people will call what they see in their minds the same, “color C”, even if the manifestations of the color qualia in their minds are different (red, blue, green, etc.). Even more bizarre possibilities are that the color C qualia may appear in some people as other kinds of qualia, such as auditory qualia, olfactory qualia, or qualia of unusual percept that do not occur in general people, instead of the color qualia! And all of them will still call what they see in their minds “color C”! It is remarkable that, at least at present, there is no way to prove definitely that qualia of the same thing appear identically or differently in people’s minds.
Figure 9.1 Variable color qualia among people
However, theoretically, are these possibilities real – can there be qualia that are variable and appear different in people’s minds? This chapter will examine this issue in details.
Hypothetically, based on what causes variable qualia and on how variable qualia occur among people, variable qualia can be classified into several types as listed below.
9.1. Physically-originated variable qualia
9.2. Non-physically-originated variable qualia
9.2.1. Randomly variable qualia
- 188.8.131.52. Completely randomly variable qualia
- 184.108.40.206. Partially randomly variable qualia
9.2.2. Restrictedly variable qualia
- 220.127.116.11. Inverted qualia
- 18.104.22.168. Shifted qualia
- 22.214.171.124. Identical-structure qualia
Now, let’s examine them one by one to see whether which one of them can really occur.
9.1. Physically-originated variable qualia
Physically-originated variable qualia are qualia that are variable and appear different in people because of physical causes. This kind of variability in qualia occurs because many physical factors such as environmental, anatomical, and physiological factors are involved in the process of perception and because these factors – which have effects on perception neural processes, their signaling patterns, and the associated perception mental processes and qualia – are different among people. For example, in the case of perceiving a color, the involved environmental factors in the outside world (such as the light that shines on the color, the light’s angle of incidence and of reflection, and the intervening medium, e.g., air, water, and glass, between the color and the observer’s eyes) and the anatomy and the physiology of the whole visual perception pathway (from the corneas of the eyes to the neural circuits for visual perception) are slightly different among people. So, the resulting perception neural processes and their signaling patterns, together with their mental processes and qualia, will be slightly different, and it is very likely that people will consciously experience (mentally see) the same color differently, but only subtly if their brains are normal and if other factors are very similar. Variable qualia that result from differences in physical factors are called physically-originated variable qualia in this theory. The differences of physically-originated variable qualia of the same thing among people are usually subtle because the differences of the resulting neural processes and signaling patterns are only slight, as noted. Drastic differences of physically-originated variable qualia of the same thing under similar perceiving conditions in normal people are not possible and do not occur.
Synesthesia is a condition in which a stimulus in one sensory modality automatically and consistently triggers a concurrent percept in another modality [1-18], or in another type of the same modality. For example, people with color-music synesthesia will have color perceptions occurring when they hear musical sounds, and people with grapheme-color synesthesia will have perceptions of colors occurring when viewing letters or numbers. Many scientists believe that synesthesia occurs because there are unusual neural connections between perception neural circuits of different modalities such that, when a perception neural circuit of one modality (such as auditory) is stimulated to function by some stimulus, another perception neural circuit of different modality (such as visual) will function simultaneously (such as in people with color-music synesthesia). In other types of synesthesia, the unusual neural mechanisms activate perception neural circuits of different types but of the same modality, such as the visual perception neural circuit of form and the visual perception neural circuit of color in people with grapheme-color synesthesia . Other possible underlying mechanisms for synesthesia generations are disinhibited feedback, hyper-connectivity/hyperbinding, or enhanced white matter connectivity . However, the evidence from some studies have shown that synesthetic experience (e.g., synthetic perception of colors) is not equivalent to true perception (e.g., perception that occurs in seeing colors)  and that the cause of synesthesia may be something else, such as a special kind of childhood memory that is recalled when there is some sensory perception  or distributed processing of synesthetic associations . However, despite the uncertainty of what the exact mechanisms that generate synesthesia are, it can be concluded, in general, that they are physical mechanisms involving neural connections and/or functions.
Qualia that occur in people with synesthesia thus differ from qualia that occur in general people who experience the same thing because there is more than one type of qualia occurring simultaneously. Because these extra qualia occur due to some physical mechanisms, qualia in these people vary from qualia in general people because of physical causes and thus are physically-originated variable qualia.
Physically-originated variable qualia are not the evidence that Theorem VIa (identical neural processes have identical qualia) is wrong because the neural processes that create physically-originated variable qualia are not identical. Theorem VIa is valid only for identical neural processes.
9.2. Non-physically variable qualia
Non-physically-originated variable qualia are qualia that are variable and appear different by themselves without physical causes. Theoretically, there can be several types of non-physically-originated variable qualia as listed at the beginning section of this chapter. They are as follows:
9.2.1. Randomly variable qualia (RVQ)
Randomly variable qualia (RVQ) are qualia that are variable and appear different randomly by themselves. RVQ can be sub-classified into 2 groups, depending on whether the randomness is complete or partial.
126.96.36.199. Completely randomly variable qualia (CRVQ) are qualia that are variable and appear different randomly in a complete manner. This means that CRVQ of something will appear different no matter whether they occur in the identical or non-identical neural processes and no matter whether those neural processes are in one individual or in separate individuals. For example, if the color C qualia are CRVQ, they will appear sometimes as the red color quale, sometimes as the blue color quale, and sometimes as another color quale no matter whether they occur among identical or non-identical neural processes, in one individual or in separate individuals, which means that the color C qualia will appear sometimes as the red color quale, sometimes as the blue color quale, and sometime as another color quale even among identical/similar neural processes in the occipital area of the same individual! So, CRVQ are variable and will appear different both among separate individuals and even in one individual. They are thus complete in their randomness.
188.8.131.52. Partially randomly variable qualia (PRVQ) are qualia that are variable and appear different randomly in a partial (incomplete) manner. PRVQ of something will appear different only when they occur in different individuals but will not appear different in the same individual. For example, if the color C qualia are PRVQ, they will always appear as the red color in some individuals, as the blue color qualia in some other individuals, as the green color qualia in yet some other individuals, and so on. So, PRVQ are not variable and will not appear different in one individual; only among separate individuals will they be variable and appear different. They are thus only partial in their randomness because they require separate individuals to manifest their variability.
Now, the question is whether RVQ do occur in the real world. To answer this question, we shall examine the two types of RVQ one by one as follows.
184.108.40.206. Completely randomly variable qualia (CRVQ)
For CRVQ – qualia that are variable and appear randomly different among neural processes in all situations, no matter whether the neural processes are identical or not and no matter whether the neural processes are in one individual or in separate individuals – there is evidence that they do not occur in the real world. Consider the following example:
A visual image that occurs in one’s mind is not an all-or-none phenomenon, i.e., it is not that the image must occur as the whole image or not occur at all. Instead, the whole visual image is composed of millions of independent tiny visual images, joined seamlessly together by the visual perception neural processes. Each tiny visual image is created by a tiny perception neural process of that point in the visual field. If there is damage to some of these tiny neural processes, the damage will affect only them and will be reflected in the corresponding points of the visual field, leaving the non-affected part intact. For example, a lesion of a portion of an occipital cortex can result in a scotoma (defect) in the corresponding portion of the visual field, leaving the rest of the visual field undisturbed. Also, as there are separate perception neural processes for each specific characteristic (color, shape, movement, etc.) [21,22], various types of defects can occur separately and independently of each other. For example, a unilateral occipito-temporal infarction can result in a contralateral hemi- or quadrantic achromatopsia (cerebral color blindness in a half or a quarter of the opposite visual field) [23-26] while other components (shape, movement, etc.) of the vision are still intact. Thus, the whole visual image in the mind is composed of millions of independent, tiny visual images, each composed of several independent types of visual characteristics.
Now, suppose one is looking at a screen of homogeneous color “C” (Figure 9.2A).
All the tiny neural processes for color perception must be creating the same signals – the color “C” signal – separately, and the quale for each tiny neural process must be occurring separately. If CRVQ are possible, each quale of the millions of tiny neural processes that generate the whole image should manifest itself differently as red, blue, green, or other colors randomly, and the whole visual image should not be a homogeneous color but a mixture of myriad different color bits (as in Figure 9.2B). But in reality, this does not happen. A screen of pure red color is always perceived as a pure red screen (as in Figure 9.3A), not a screen of numerous different colors (as in Figure 9.3B).
Similarly, a pair of same-color discs (such as red and red) is always perceived as a pair of same-color discs (red and red, as in Figure 9.4A), not a pair of different-color discs (such as red and blue, as in Figure 9.4B), which can occur if CRVQ are possible.
Even a pair of same-color discs (such as red and red) that are separated in different visual fields is always perceived as a pair of same-color discs (such as red and red, Figure 9.5A), not a pair of different-color discs (such as red and blue, Figure 9.5B), which can occur if CRVQ are possible.
So, it means that, for the same color in the outside world, the same color qualia occur in all those millions of tiny neural processes and that CRVQ do not occur. Also, it never happens that color qualia occur at some points in the visual field, but auditory, olfactory, or other kinds of qualia occur at other points in the visual field. This means that only visual qualia, not any other kind of qualia, occur for all signals from the tiny neural processes for color perception,
Similarly, with the same consideration, there is no evidence that CRVQ ever occur in other sensory perceptions either. For example, when someone hears two identical musical instruments playing the same musical note “M” (M stands for a certain musical note), the quale of each sound has the chance to manifest itself differently if CRVQ are possible: one may appear as the piano sound of note C while the other may appear as the violin sound of note G. This is, of course, absurd and never happens – he/she always hears the two sounds as the same musical note of the same musical instrument, such as the flute sound of note A if it is the flutes playing the note A. The same scenario is similar for other sensory perceptions, such as the feeling of the same touches on both arms, the pain from the needle stabbing with similar strength on both legs, and the taste of the same honey on both sides of the tongue – the qualia of the same things on both sides are always the same, not randomly different.
Thus, evidently, there is no randomness in the manifestation of qualia in any single individual. This “no randomness in manifestation in a single individual” of qualia has been consistently true for a long time in billions of humans nowadays and in the past – no normal person has ever reported having a mixture of variable qualia occurring when perceiving a homogenous stimulus or identical stimuli. Thus, even though CRVQ have had virtually infinite chances to manifest themselves, they have never ever done so. Therefore, in reality, it can be concluded that completely randomly variable qualia (CRVQ) do not occur. Also, it is noteworthy that the non-occurrence of CRVQ is evidence that qualia are neural-process dependent phenomena, because if qualia were neural-process independent phenomena, CRVQ would occur – they would appear randomly different independently of neural processes, and all the strange phenomena in the above examples would occur.
220.127.116.11. Partially randomly variable qualia (PRVQ)
However, is it possible that randomly variable qualia do not occur in a single individual because all the separate qualia occur in one single person, with one consciousness to experience the qualia, but that they will occur among different individuals, with different consciousness to experience the qualia? If so, it means that, even though all the neural processes in any single individual are not the same one and are at different anatomical positions and even though the neural processes on the right and on the left hemisphere are widely separated, these dissimilarities are not enough for qualia to manifest themselves differently and that it requires separate individuals for qualia to do so. If this is the case, then such qualia are not “completely randomly variable qualia” but are “partially randomly variable qualia” (PRVQ) because they do not appear completely randomly different; only in some situations – among different individuals – will they appear so.
Now, is there evidence that PRVQ do occur or do not occur? The answer is that there is evidence that PRVQ do not occur either. Let’s consider a case of visual qualia: if PRVQ is possible, then a certain color may manifest itself as a red color quale in some people but as a blue, green, yellow, or another color quale in other people (that is, my red is your blue as some people suspect). To see whether PRVQ really happen, consider a pair of red disc and blue disc in Figure 9.6A below. Everyone with normal color vision will agree that the two colors are markedly contrasting in their hue. However, if PRVQ are possible, the red disc can appear as a yellow disc and the blue disc as a green disc in some people by chance, and the pair will be of yellow and green color discs as in Figure 9.6B. In those people in whom the qualia appear as yellow and green will say that the two colors are not markedly contrasting in their hue, in disagreement with people who see them as red and blue. The disagreement will be even more obvious if the pair of red and blue discs is perceived as a pair of yellow and light-yellow discs as in Figure 9.6C, which can occur in some people if PRVQ are possible. But such disagreement never occurs in billions of people with normal color vision. This is evidence that even PRVQ do not occur.
Another visual evidence is a color wheel. Everyone with normal color vision will agree that the hue of a color in the color wheel like the one in Figure 9.7A gradually changes from one disc to another successive disc. However, if PRVQ occur, the color quale of each disc in the wheel will manifest itself haphazardly and result in a color wheel that the hues of successive discs do not gradually change, like the color wheel in Figure 9.7B, in some people. But this never happens in billions of people with normal color vision. They always agree that the hues of the colors in the color wheel like the one in Figure 9.7A gradually change from one successive disc to another. So, PRVQ do not occur in this case either.
The evidence in auditory perception is that, if PRVQ occur, the sound qualia of musical notes must be randomly different among people. For example, some may perceive the sound quale of a certain musical note M as the note C, some as the note F, and some as the note A#, and for the next sound that is one semitone higher, or note M#, some people must randomly perceive it as the note C#, some as the note Cb, and some as the note G, by chance. A musical scale must sound different among people if PRVQ occur. Also, a train of musical notes that have sound qualia as C-D-E-C-E-C-E in some people may have sound qualia as F-E-G-F-G-F-G in some people and as something else in others; a train of musical notes will appear as music for some people, but as an erratic train of sounds in others. But these strange phenomena never happen. People agree on the orderly sounds of a musical scale, the sound that is music, and the sound that is chaotic. This is similarly true for other sensory perceptions. For example, the sensory perception of a series of smoothly-increasing-vibration stimuli or a series of smoothly-increasing-sweetness stimuli is always that the stimuli are smoothly increasing in strength, in all individuals. It is not that, in some individuals, the perception is of haphazardly-changing-strength stimuli – which can occur in some individuals if PRVQ are possible.
Thus, evidently, there is no randomness in the manifestation of qualia among people. This “no randomness in manifestation” of qualia has been consistently true for a long time in billions of people nowadays and in the past. Thus, even though PRVQ have had virtually infinite chances to manifest themselves, they have never ever done so. It can, therefore, be concluded that PRVQ do not occur among people in the real world.
9.2.2. Restrictedly variable qualia
Still, is it possible that qualia can manifest themselves differently among people in some restricted forms that people cannot recognize as variable qualia? This kind of variable qualia may be called restrictedly variable qualia because they do not and cannot appear randomly different in a non-restricted way – there are some rules that restrict how they appear different among people so that people cannot tell that they are experiencing variable qualia. There are several possible kinds of restrictedly variable qualia. Let’s examine some forms of restrictedly variable qualia to see which kind can occur among people without people knowing that they are experiencing variable qualia.
18.104.22.168. Inverted qualia
Inverted qualia are qualia that have a certain characteristic inverted. For visual qualia, there are a few kinds of inverted qualia. One of them is the inverted light spectrum qualia in Figure 9.8.
Figure 9.8 Inverted light spectrum with respect to the wavelength
Generally, when people with normal color vision see a spectrum of light like the one in panel A, they will agree that there are definite changes in color hue around wavelengths of 410, 470, 490, 570, 590, and 610 nm. If the qualia of the light spectrum are inverted with respect to the wavelength in some people, the light spectrum will be like the one in panel B. Evidently, people with inverted qualia would not agree with the previous observation – for them, definite changes in color hue occur around wavelengths of 510, 530, 550, 630, 650, and 710 nm. But people with normal color vision never disagree around what wavelengths a light spectrum changes color hues. So, there is no evidence that inverted color qualia of this kind occur in people.
Another way to invert the color spectrum is to invert each color to its complementary color (color inversion). This will result in an inverted spectrum that looks like the one in panel B, Figure 9.9.
Figure 9.9 Inverted color spectrum by color inversion
The inverted spectra in this way still have problems of different changes of hue along the spectra. For example, from wavelengths 580 to 640 nm, the upper spectrum (A) changes from yellow to red, the definitely different hues, but the lower spectrum (B) changes from dark blue to light blue, the slightly different hues. Also, between wavelengths 510 and 590 nm, the upper spectrum (A) appears as green and yellow, the fairly close hues, but the lower spectrum appears as magenta and dark blue, the quite different hues. Therefore, if this kind of inverted spectrum really occurs among people, people will be able to tell that they are experiencing different color spectra. But this does not happen. So, there is no evidence that inverted color qualia of this kind really occur in people, either.
For auditory qualia, inverted chromatic scales in equal temperament, such as scale 1 and scale 2 below (the primed C, or C’, is one octave higher than the unprimed C), will have same chords, which are constructed from equivalent notes on each scale, sound different. For example, the chord constructed from the 1st, 5th, and 8th notes (C-E-G) of Scale 1 will sound like C Major, but the chord constructed from the 1st, 5th, and 8th notes (C’-G#-F) of Scale 2 will sound like F Minor.
Scale 1: C C# D D# E F F# G G# A A# B C’
Scale 2: C’ B A# A G# G F# F E D# D C# C
So, the same chords constructed from the inverted scales will sound different, and people, especially musicians, will not agree on what the mood of each chord is, which is not the case in the real world. Thus, it can be concluded that such inverted auditory qualia do not happen.
22.214.171.124. Shifted qualia
Figure 9.10 Shifted light spectrum along the wavelength
Shifted qualia are qualia that have their spectrum shifted. The situation in shifted qualia of a light spectrum is like that in inverted qualia. If the spectrum of light is shifted, such as shifted to the reader’s left side as in panel B (Figure 9.10), although we cannot say definitely about the new color qualia that occur in the vacated portion on the right side, it is evident that people with shifted qualia will not agree with people with non-shifted qualia about at what wavelengths the light definitely changes color hues in the remaining portion. But this kind of disagreement never happens. So, like inverted-spectrum qualia, there is no evidence that shifted light-spectrum qualia occur among people.
For shifted sound-pitch qualia, if the spectrum of sound-pitch qualia shifts up or down among people, it will be impossible to prove behaviorally whether shifted sound-pitch occur or not. This is because shifted sound-pitch qualia have the characteristics of the next type of restrictedly variable qualia – the identical-structure qualia – which enable them to occur among people without people knowing that they are experiencing variable qualia.
126.96.36.199. Identical-structure qualia
It is possible that there are other kinds of inverted or shifted qualia that may occur among people and that people may not be able to recognize as different qualia (please see more discussion in ref 27). However, one kind of different qualia that can theoretically occur in people without people being able to tell that they are experiencing different qualia is identical-structure qualia. Identical-structure qualia come in sets. Each set is composed of qualia that have different phenomenal characteristics from those of their counterparts in other sets. But all sets have identical qualia structures**, that is, they have the same number and types of their phenomenal characteristics and have the same spectral characteristics. One example of identical-structure qualia is the shifted sound qualia discussed above. Shifted sound qualia in each set have sound pitches that are shifted relative to their counterparts in other sets. But they have the same number and types of their phenomenal characteristics: pitch, loudness, timbre, envelope, and duration. And they have identical spectral characteristics, that is the internal relationships of sound qualia in each set are identical to those in other sets, such as the identical repeated sound of each note (in different pitches) every octave and the identical acoustic relations among the notes of the same chords (such as the identical consonance between the root, the major third, and the perfect fifth notes of the same major chord). So, all sets of shifted sound qualia behave identically, and people will not be able to tell the differences between them.
(** See more details about qualia structure in PQ2.6. Chapter 4.)
For another example, consider the color spectra of two sets of identical-structure qualia in Figure 9.11. The lower panel has qualia that have different phenomenal characteristics from their counterparts in the upper panel and that occur in only some people (so we cannot know what the lower panel’s qualia are like and cannot demonstrate them here in the picture). But the qualia in both panels have the same number and types of their phenomenal characteristics: color, brightness, shape, dimension, acuity, and velocity. And the spectral characteristics or all the internal relationships among qualia in each spectrum of both panels are the same. For example, the way the colors change their hues along the spectrum, the apparent brightness at all wavelengths along the spectrum, and the perceived hue difference between colors at different wavelengths (such as between the color at 440 nm vs. the one at 700 nm [which is overt in the upper panel] and between the color at 520 nm vs. the one at 560 nm [which is subtle in the upper panel]) are the same in both panels. Thus, if such identical-structure visual qualia occur among people, people will not be able to tell that they are experiencing different visual qualia.
Figure 9.11 Two color spectra of identical-structure qualia
Therefore, identical-structure qualia can occur among people, and, behaviorally, people will not be able to tell that they are experiencing different qualia. However, it is important to note that identical-structure qualia are restrictedly different qualia – they cannot occur haphazardly but must occur in some restricted ways, with some rules governing their manifestations. The governing rules for them (identical-structure qualia) must be that
- They must have different phenomenal characteristics among people.
- They must have the same number and types of their phenomenal characteristics.
- They must have the same spectral characteristics.
In humans, the spectral characteristics of each kind of qualia are different from those of other kinds and may be very complex and peculiar, such as the spectral characteristics of the light spectrum, which have the color hues and brightness change in an idiosyncratic way along the spectrum, and the spectral characteristics of the musical scale, which have similar sounds repeated in different pitches every octave. Therefore, if identical-structure qualia do occur, the physical problem about them is “what is the mechanism that keeps qualia of each kind always manifesting themselves restrictedly according to the complex rules of that kind correctly among different individuals at different places and different times?”. And the philosophical question about them is “Why should qualia manifest themselves differently in only restricted ways, why not in a totally random way, or why not not-differently at all?”.
From all the evidence at present, it can be concluded that, in reality,
– physically-originated variable qualia can occur,
– completely randomly variable qualia do not occur,
– partially randomly variable qualia do not occur, and
– restrictedly variable qualia in some forms, such as identical-structure qualia, may occur, and, behaviorally, people will not be able to tell that they are experiencing variable qualia.
However, theoretically, these issues can be investigated by examining the signaling patterns of those qualia, and according to Theorem VIa and Theorem VIb, it will be found that the signaling patterns of qualia of the same things under the same conditions are identical or similar and that, because those signaling patterns themselves are qualia, those qualia are identical or similar. Therefore, variable qualia that are not physically-originated variable qualia do not occur.
Predictions about different qualia are the same as predictions for Theorem VIa and Theorem VIb. Please see Predictions 1 and 2 in section 7.4.
- Banissy MJ, Jonas C, Kadosh RC. Synesthesia: An introduction. Front Psychol. 2014;5:1414. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4265978/
- Carmichael DA, Julia Simner J. The immune hypothesis of synesthesia. Front Hum Neurosci. 2013;7:563. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3769635/
- Hänggi J, Wotruba D, Jäncke L. Globally altered structural brain network topology in grapheme-color synesthesia. Journal of Neuroscience. 2011 Apr;31(15):5816-5828. http://www.jneurosci.org/content/31/15/5816.long
- Hubbard EM, Ramachandran VS. Neurocognitive mechanisms of synesthesia: A review. Neuron. 2005 Nov;48:509–520. http://cbc.ucsd.edu/pdf/neurocog_synesthesia.pdf
- Hubbard EM. Neurophysiology of synesthesia. Curr Psychiatry Rep. 2007 Jun;9(3):193-199. https://www.hal.inserm.fr/file/index/docid/150599/filename/Hubbard_CurrPsychReports.pdf
- Neckar M, Bob P. Neuroscience of synesthesia and cross-modal associations. Rev Neurosci. 2014;25(6):833-840.
- Neufeld J, Sinke C, Zedler M, Dillo W, Emrich HM, Bleich S, et al. Disinhibited feedback as a cause of synesthesia: Evidence from a functional connectivity study on auditory-visual synesthetes. Neuropsychologia. 2012 Jun;50(7):1471-1477.
- Ro T, Ellmore TM, Beauchamp MS. A neural link between feeling and hearing. Cereb Cortex. 2013 Jul;23(7):1724–1730. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673182/
- Rothen N, Terhune DB. Increased resting state network connectivity in synesthesia: Evidence for a neural basis of synesthetic consistency. Journal of Neuroscience. 2012 Oct;32(40):13641-13643. http://www.jneurosci.org/content/32/40/13641.long
- Rouw R, Scholte SH. Neural basis of individual differences in synesthetic experiences. Journal of Neuroscience. 2010 May;30(18):6205-6213. http://www.jneurosci.org/content/30/18/6205.long
- Rouw R, Scholte HS, Colizoli O. Brain areas involved in synaesthesia: A review. J Neuropsychol. 2011 Sep;5(2):214-242. http://mobile.www.daysyn.com/Rouwetal2011.pdf
- Neufeld J, Sinke C, Dillo W, Emrich HM, Szycik GR, Dima D, et al. The neural correlates of coloured music: A functional MRI investigation of auditory–visual synaesthesia. Neuropsychologia. 2012;50:85-89.
- Tomson SN, Narayan M, Allen GI, Eagleman DM. Neural networks of colored sequence synesthesia. J Neurosci. 2013 Aug;33(35):14098–14106. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4050198/
- Ward J.Annu Rev Psychol. 2013;64:49-75.
- Ward J. Cognitive neuroscience of synesthesia: Introduction to the special issue. Cogn Neurosci. 2015;6(2-3):45-47.
- Zamm A, Schlaug G, Eagleman DM, Loui P. Pathways to seeing music: Enhanced structural connectivity in colored-music synesthesia. Neuroimage. 2013 Jul;74:359–366. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3643691/
- van Leeuwen TM, Singer W, Nikolić D. The Merit of Synesthesia for Consciousness Research. Front Psychol. 2015;6:1850. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4667101/
- van Leeuwen TM, E. M. den Ouden H, Hagoort P. Effective connectivity determines the nature of subjective experience in grapheme-color synesthesia. J Neurosci. 2011 Jul;31(27):9879-9884. http://www.jneurosci.org/content/31/27/9879.long
- Hupé JM, Bordier C, Dojat M. The neural bases of grapheme–color synesthesia are not localized in real color-sensitive areas. Cereb Cortex. 2012;22(7):1622-1633. https://academic.oup.com/cercor/article/22/7/1622/293312/The-Neural-Bases-of-Grapheme-Color-Synesthesia-Are
- Hupé JM, Dojat M. A critical review of the neuroimaging literature on synesthesia. Front Hum Neurosci. 2015 Mar;9(103):1-45. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4379872/
- Dragoi V. Chapter 15: Visual processing: Cortical pathways. Neuroscience Online. The University of Texas Health Science Center at Houston (UTHealth). Retrieved 2017 Feb 13 from http://nba.uth.tmc.edu/neuroscience/s2/chapter15.html
- Fitzpatrick D. Chapter 11. Central Visual Pathway. In: Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, McNamara JO, Williams SM, editors. Neuroscience. 3rd ed. Sunderland, Massachusetts: Sinauer Associates Inc; 2004. ISBN-13: 9780878937257 ISBN-10: 0878937250. https://www.hse.ru/data/2011/06/22/1215686482/Neuroscience.pdf
- Bartolomeo P, Bachoud-Lévi AC, Thiebaut de Schotten M. The anatomy of cerebral achromatopsia: A reappraisal and comparison of two case reports. Cortex. 2014 Jul;56:138-144. https://www.researchgate.net/profile/James_Pickles/publication/273787629_Auditory_pathways_Anatomy_and_physiology/links/571781ab08ae09ceb264aa17.pdf
- Kölmel HW. Pure homonymous hemiachromatopsia. Findings with neuro-ophthalmologic examination and imaging procedures. Eur Arch Psychiatry Neurol Sci. 1988;237(4):237-243.
- Paulson HL, Galetta SL, Grossman M, Alavi A. Hemiachromatopsia of unilateral occipitotemporal infarcts. Am J Ophthalmol. 1994 Oct;118(4):518-523.
- Short RA, Graff-Radford NR. Localization of hemiachromatopsia. Neurocase. 2001;7(4):331-337.
- Byrne A. Inverted qualia. Zalta EN, editor. The Stanford Encyclopedia of Philosophy (Winter 2016 Edition). Retrieved 2017 Jun 01 from https://plato.stanford.edu/archives/win2016/entries/qualia-inverted/
Keywords: Different qualia, Inverted qualia, Shifted qualia, Identical-structure qualia