The above discussion leads me to consider how the traditional distinction between primary and secondary qualities of objects can shed light on the nature of synesthetic experience.

Some background first: John Locke famously distinguished between primary and secondary qualities of objects (15). Primary qualities are those qualities that have to do with the object’s microstructure and, according to Locke, are inseparable from the external object itself, such as size, shape, mass, number, and motion. Secondary qualities, however, are those qualities which are “nothing in the objects in themselves but powers to produce various sensations or ideas in us,” such as colors, tastes, and sounds. It is only ideas of primary qualities of a perceptual object that really resemble what is in the object whereas secondary qualities are merely caused by the object’s microstructure. According to Locke’s representative realism, primary qualities are “really out there” mind-independently, especially as compared to secondary qualities which are mind-dependent to some extent. Primary qualities would still exist without minds to perceive them but there would not be any secondary qualities. Locke explains:

“…I think it easy to draw this observation, that the ideas of primary qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves, but the ideas produced in us by these secondary qualities have no resemblance to them at all. There is nothing like our ideas existing in the bodies themselves. They are, in the bodies we denominate from them, only a power to produce those sensations in us; and what is sweet, blue, or warm in idea is but the certain bulk, figure, and motion of the insensible parts, in the bodies themselves, which we call so” (15, Book II, Ch. 8, sec. 15).

There is of course still significant debate today as to how best to characterize the mind-dependence or mind-independence of secondary qualities (especially with respect to color). The question might be framed as “What kinds of properties are colors?” or “Are colors mind-independent in some sense?” For example, “primitivism” about colors holds that colors are primitive properties, that is, simple, sui generis, qualitative properties that physical bodies possess or appear to possess. A “reductive physicalist” holds that colors are “hidden” properties of bodies, that is, complex, physical properties that dispose bodies to look blue, pink, yellow, and so on. Another view is “dispositionalism” such that colors are perceiver-dependent, dispositional properties; that is, powers to look in distinctive ways to appropriate perceivers, in appropriate circumstances (16, p. 9). Note, however, that we still have the problematic and ambiguous expressions “appropriate perceivers” and “appropriate circumstances.”

Sollberger also recognizes the issue at hand. He explains that:

“Accepting that synesthetic experiences can be veridical will, of course, have important ramifications for what a metaphysical theory of color properties can look like. For instance, it seems to be immediately ruled out that colors could be construed as intrinsic, categorical properties of physical objects. Instead, it marries up more easily with a form of psychological-dispositionalism about color, according to which x’s property of having a certain color, such as red, is analyzed in terms of dispositions and powers…. In this way, grapheme-color synesthesia can be veridical because there is nothing incoherent in the idea that an object can have the disposition to appear black and the disposition to appear red to the synesthete simultaneously…Moreover, dispositionalists can insist that perceiving distal objects as colored does not involve a kind of massive error or systemic illusion, for the dispositional properties can be grounded in the categorical bases of the objects themselves” (4, p. 183).

Of course, an idealist, such as Bishop Berkeley, would say that even the so-called primary qualities are mind-dependent (at least dependent on God’s mind). I do not wish to try to settle these disputes here. For our purposes, let us simply assume that there are mind-independent objects and that we often do experience secondary qualities as properties of external objects, for example, that objects appear to us as colored. Still, some secondary qualities seem better described as caused by external objects, such as the sound of a guitar string vibration or the smell of a specific food.

So it makes sense that secondary qualities are almost always those experienced as concurrents. In this way, synesthetes can have an intrasubjective coherent stream of conscious perceptions. Since secondary qualities are at least not entirely mind-independent in some sense, they perhaps matter less to coherent conscious experience in the sense that one can have an individual and idiosyncratic way of experiencing the color, taste, and smell of objects. If we treat secondary qualities as themselves appearances of objects, then concurrents are more like unusual appearances of objects or, perhaps even better, “appearances of appearances” in at least cases where a secondary quality inducer triggers a concurrent experience of another secondary quality (such as in sound-color, smell-touch, or sound-taste synesthesia). That is, we can understand many instances of synesthesia to involve what we might call “second-order secondary properties,” namely, the experiences of (secondary) properties or qualities of objects induced by the secondary qualities of those objects (as opposed to primary qualities). So it is crucial to notice that the concurrent is virtually always a secondary quality of objects, such as a taste, smell, or color. Although the inducer is often a secondary quality, it can also be a primary quality such as size, shape, and motion.

Perhaps the fact that the concurrent is rarely a primary quality also accounts for why synesthesia is almost always one-directional, that is, synesthetes who experience an inducer-concurrent pair (I, C) will not experience that pair in reverse (C, I). One exceptional instance is Julie Roxburgh who sees color when she hears sounds and hears sounds when she sees colors (7, pp. 102-103). Each color produces a musical note. However, as we might expect, this leads her to have a kind of psychopathology where there is sensory overload and she has serious problems functioning in everyday life, including walking and navigating through traffic: “The onslaught of cacophony results in considerable perceptual interferences and causes her distress” (7, p. 102). She “feels frightened and exhausted… (it is) difficult to avoid traffic and people and to keep control…every one of her senses is ‘being battered’… (the) neon lights are shouting (and)…flashing lights give her a tactile sensation in her fingers” (7, p. 102). This description of her life, at least, certainly runs counter to Sollberger’s characterization of synesthesia as enhancing cognitive fitness and not as a disabling or dysfunctional biological trait. Nonetheless, Sollberger may unknowingly be pointing to the reason why bi-directionality is so rare.

Perhaps even more important for my immediate purposes is the fact that there are rarely, if ever, cases where there is a secondary quality inducer and a primary quality concurrent. There are some unusual forms of synesthesia which might appear to fit this description, such as audio-motor synesthesia (7, p. 40). However, in this case, we have a boy who felt compelled to move his body into various poses in response to the sounds of words. It was not as if he experienced the motion of outer objects when he heard these words. Otherwise, I would think that his daily life would be extremely difficult as a practical matter, analogous to Julie Roxburgh.

It seems to me that, as long as there is some internal individual experiential consistency among experienced concurrents, there is little worry about incoherent and very disruptive experiences. In contrast, the potential for disruption and difficulty successfully interacting with the world results (or would result) more often in cases where concurrents involve primary qualities such as size, shape, and motion. It is true that some synesthetes do talk about seeing a black letter as, say, both black and orange which would seem contradictory and potentially disruptive. However, these synesthetes are presumably not quite saying that they experience objects or letters as black and orange all over at the same time and nothing is changing in location or size. Some will describe the two colors as though a colored transparency (e.g. orange) is placed on top of the black grapheme. On the other hand, it would be much more difficult to understand what it would possibly be like, say, if a type of colored object appeared to be both large and small, or in motion and at rest, or both square and triangular. Similarly, it is difficult to see how one could coherently experience one type of food smell as inducing the experience of that food moving or being larger than the other food on a plate. The same can be said for systematically experiencing a type of colored object, say, moving in ways that other colored objects do not. For one thing, many objects have more than one color. And try to imagine, for example, judging the distance between objects. How could one drive or play a sport without becoming paralyzed into inaction? How could one engage in the most basic interactions with others? This would seem not only to threaten the coherence of such conscious experiences but, we might say, the very unity of consciousness. Further, these kinds of abnormal conscious experience would most certainly be more noticeable to others and highly debilitating to the people in question.

As a matter of fact, some of the possibilities might even resemble some rather bizarre known psychopathologies. For example, Alice in Wonderland Syndrome (AiWS) is a disorienting neuropsychological condition that affects object size perception (17). People experience distortions in visual perception such as objects appearing small (micropsia), objects appearing large (macropsia), (objects appearing to be closer than they are (pelopsia), or objects appearing to be further away than they are (teleopsia). Size distortion may occur with the other senses as well. AiWS is often associated with severe migraines, brain tumors, and psychoactive drug use. AiWS can be caused by abnormal amounts of electrical activity resulting in abnormal blood flow in the parts of the brain. Although this condition is more often found in young people and often clears up on its own, it is clear that AiWS has a negative impact one’s everyday life.

Let us explore another psychopathology sometimes discussed in connection with synesthesia.

Autism and synesthesia seem to be in opposition with regard to neural connectivity (19, 7, pp. 241-245). Recall that there is some evidence for neural “hyperconnectivity” in synesthetes, that is, a “cross-activation” or “cross-wiring” of adjacent brain regions (1). For example, the parietal cortex especially has been found to be hyperactivated in different types of synesthesia, which also suggests the idea of top-down modulation of sensory areas by this higher-order associative region (9).

It has been observed that those with autism seem to have the reverse condition, that is, decreased neural connectivity in certain brain areas. Cytowic and Eagleman explain that “it is enticing to consider (autism’s) opposite nature from synesthesia. Neural cross talk is reduced in autism but increased in synesthesia” (and) “autistics are less likely to be fooled by certain illusions…less susceptible to visually induced motion” (7, p. 242). Hirstein also argues independently that some recent evidence points to widespread underconnectivity in autistic brains (19).

The matter is not so simple, however. Hirstein also points out that this lack of frontal connectivity appears to be accompanied by increased local connectivity in the posterior cortex. He explains that “Monk et al. found increased connectivity between the posterior cingulate cortex and temporal regions in subjects with autism. In addition, they found that increased repetitive behaviors, a core diagnostic symptom of autism, were associated with increased connectivity between the posterior cingulate cortex and the parahippocampal gyrus” (19, p. 254; 20). It is also worth noting that Baron-Cohen and colleagues published a case study on a rather unusual man, Daniel Tammet, with synesthesia, autism, and savantism (21, 22). They suggested that co-occurrence of ASD and synesthesia might increase the likelihood of savantism but there is also evidence for a link between ASD and synesthesia, which is perhaps further support for the notion that synesthesia can benefit those who have synesthesia.

Adams and Shreve have argued that both synesthesia and autism are potential problems for the view that perceptual states have conceptual content as well as for the higher-order thought (HOT) theory of consciousness (23, 24, 25, 26). They focus on what we might call “the perceptual-conceptual divide.”

For those unfamiliar with the higher-order thought (HOT) theory, it says that what makes a mental state M a conscious mental state is that there is a HOT to the effect that “I am in mental state M.” One question that should be answered by any theory of consciousness is: What makes a mental state a conscious mental state? So, for example, my desire to drink some water becomes conscious when I am (non-inferentially) “aware” of the desire. Intuitively, it seems that conscious states, as opposed to unconscious ones, are mental states that I am “aware of” being in some sense. For various reasons, HOT theorists believe that it is best to construe such “meta-awareness” as thoughts constituted by concepts. Conversely, the idea that I could be having a conscious state while totally unaware of being in that state seems odd or perhaps even contradictory. A mental state of which the subject is completely unaware is clearly an unconscious state. For example, I would not be aware of having a subliminal perception and thus it is an unconscious perception. HOTs, since they are thoughts after all, are constituted by concepts. It is worth noting also that when a conscious mental state is a first-order world-directed state the higher-order thought (HOT) is not itself conscious. When the HOT is itself conscious, there is a yet higher-order (or third-order) thought directed at the second-order state. In this case, we have introspection which involves a conscious HOT directed at an inner mental state. When one introspects, one’s attention is directed back into one's mind.

So Adams and Shreve first explain that:

“Ramachandran was thoroughly investigating as many ways as he and his researchers could think of to test whether (grapheme-color synesthesia) was conceptual (or ‘top-down’) vs. perceptual (not driven by conceptual association or deployment) (23). In a ‘pop-out’ experiment, Ramachandran produced a grid of 5s and 2s that were mirror images of one another (27). The grid was presented for about one half second. To a non-synesthete, looking at the grid produced only the experience of random figures. The subjects had to press one of two buttons on a computer depending upon whether they saw a triangle or a circle….Twenty ‘normal’ subjects scored about 50% on whether the shapes were circles or triangles….However, when subjects with synesthesia looked at the grid, the colors that they saw on the numbers caused the shapes to pop out. That is, the 2s were arranged either in a circular pattern or a triangular pattern among the 5s (which were randomly placed). The colors seen when observing the 2s and their shapes in circular or triangular pattern were apparent to them at a hit rate of 80-90%. For example, a pattern of 2s that was triangular jumped out as a red triangle….The subjects with synesthesia experienced something the subjects without synesthesia did not. The reason this is interesting in regard to HOT theories, is that the ‘popout’ phenomena is a bottom-up visual experience. The subjects did not first see the shape (triangle or circle) and then have the higher-order thought (‘triangle’ or ‘circle’) causing the experience of the shape to become conscious. Rather, the perceptual pop-out produced the conscious visual experience of the shape prior to the having of the thought about the shape experienced” (23, pp. 253-254).

The experimental results themselves are uncontroversial since it seems rather well established that they show the popout experience is very real to synesthetes. Still, it can be argued that HOT theory does have the resources to account for synesthesia and the specific worries that they advance in their paper, such as the relationship between concepts and experience and the ability to explain instances of “pop-out” experiences (28, 29). Recall from section one that there are so-called “higher” and “lower” synesthetes in grapheme-color synesthesia (1, 7). Notably, lower synesthesia is rare. Adams and Shreve seem to have ignored the prevalence of higher synesthesia which explicitly involves the “meaning” or “concept” of the grapheme (23). It is the meaning of the grapheme that induces color, not the visual shape itself. Letter capitalization and font size generally do not change an induced color. For example, J, j, and J evoke the same color experience. Much the same goes for numbers and number concepts, that is, higher synesthetes experience the same color (e.g. blue) when seeing both the Arabic numeral ‘5’ and the Roman numeral ‘V.’ More generally, the claim might be framed as the view that “conscious perceptual experiences can represent objects as falling into fairly abstract conceptual categories” (22, p. 152). The matter is even more complicated, for example, some grapheme-color synesthetes may have different color experiences when seeing a “5” as opposed to seeing numerous “2’s” arranged in the shape of a “5.” Although HOTs, with their constituent concepts, are necessary for conscious states, the evidence here also seems to indicate that a conceptual component is intimately involved in these synesthetic experiences. This concept application itself can of course also occur unconsciously and almost instantaneously. The conscious experience of the colored number need not occur prior to the HOT and concept application.

The idea that concepts and cognitive states can affect one’s very perceptual experiences is more recently referred to as “cognitive penetration” (30, 31, 32) which also seems supported by the neuroscientific evidence on synesthesia. Recall again our discussion of “disinhibited cortical feedback” between brain areas such that information is processed in a bottom-up fashion but also that later stage brain activation feeds back to activate earlier stages. It is this abnormal feedback that causes these unusual synesthetic experiences (6). In addition, Cytowic and Eagleman explain that, for higher synesthetes, V4 cross-activiates with the anterior inferior temporal (AIT) cortex which processes conceptual representations of words, letters, and numbers (7). Further, it seems that semantic memory can affect sensory perceptions (33).

Thus, I disagree with Adams and Shreve when they suppose that thoughts, unlike experiences, involve concepts. Perceptual experiences are not concept-free. With regard to synesthesia, then, it seems to me that their either/or question presents a false dichotomy: is “synesthesia a conceptual or perceptual phenomenon?” (23, p. 253). My own view is that such experiences are both conceptual and perceptual as is the case with all conscious experience (26).

Regarding autism, Adams and Shreve say that:

“subjects with severe forms of autism are susceptible to pop-out synesthesia of the kind that we described in our initial paper (24, 23). Now a hallmark of severe autism is what Baron-Cohen called ‘mind-blindness (18).’ This is the inability to apply mental concepts to self or others. People with severe autism have no trouble understanding people as physical systems with physical properties…But when it comes to beliefs, desires, intentions, hopes, fears, wishes and other mental causes, severely autistic individuals simply do not understand behavior originating from these causes. Such purposive behavior is a complete mystery to them. Thus, they do not engage in applying mental concepts to themselves or others. Consequently, when a person with severe autism consciously experiences the pop-out of synesthesia, it cannot be the result of applying an HOT to their experience because they don’t employ HOTs about mental states (of self or others)” (24, p. 133).

But the ‘mind-blind’ characterization of autism, even in the more severe cases, is mistaken or at least greatly exaggerated. It is not at all clear that autistic people cannot have or apply any mental concepts to themselves or others (26, 28). One problem with the autism literature is that some authors who argue for a deficiency in ‘self-consciousness’ among autistic individuals leave the term undefined. This is important especially since it seems that self-consciousness, self-concepts, I-thoughts, concept possession, and so on can come in degrees. At the most sophisticated level, there is introspection or reflection. Even if there are deficiencies in introspection, it does not follow that there are no I-thoughts or metacognitive states at all. It is one thing to suppose that autistic people have abnormal or impaired self-consciousness, but quite another to claim that there is no self-consciousness at all. Indeed, despite their own skepticism regarding autistic self-consciousness, Frith and Happé themselves quote numerous cases of first-person reports from autistic people (34, pp. 11-14).