The study of consciousness is problematic and, in order to combat this, has become an interdisciplinary effort that touches upon a plethora of aspects of human cognition and brain function. Originally, a neurological approach looking at selective deficits in patients was adopted to learn about normal and abnormal brain function. Of late, there has been an increasing interest in the study of positive symptoms, like those related to synaesthesia. The phenomenon of synaesthesia can essentially be described as a ‘union of the senses’ (“UK Synaesthesia Association”, 2018), whereby an inducer (e.g. the letter A) elicits an unrelated synaesthetic association (e.g. the colour red). Explaining the reason for and the mechanisms behind this combination of two different cognitive streams (Sobczak-Edmans and Sagiv, 2013) is one of the ‘hard’ problems in consciousness research (Chalmers, 1995), as it is in essence a subjective experience of ‘what things feel like’. While early research into synaesthesia merely provided documentation, more recent work endeavours to explain the perceptual experience of synaesthetes and consider how synaesthesia can inform perception and consciousness as a whole (e.g. Cohen Kadosh, Gertner and Terhune, 2012). Several researchers have made cases for using synaesthesia as a model problem to better understand consciousness: for example Gray (2005), who suggested that instead of functionalism (where there was a difference in function, there should be a consequential difference in experience), in synaesthesia, two different functions can lead to the same subjective experience.
The advantages of using synaesthesia as a model problem were most effectively put forward by Sagiv and Frith (2013): they stated that synaesthesia is phenomenologically defined while its properties can be studied in detail. For example, it is characterised by atypical perceptual experiences, which can then be compared with cases where it is absent (while the subjective accounts of individuals’ conscious experience act as a dependent variable; Baars, 2003). The perceptual experience of synaesthetes is uncommon but its existence highlights the same general problem, including how these experiences arise and with what neural basis (Sagiv and Frith, 2013). The second feature of synaesthesia that makes it an appropriate model problem is the immense variety of sub-types, proposed to be more than 80 (“Synesthesia”, 2017). These involve diverse blends of sensory modalities and types of experiences, which provides researchers with a great number of opportunities to observe and test theories of brain function and associated mental states. Furthermore, it invites a debate on individual differences in subjective experience of the world around us. The third, most practical reason that favours using synaesthesia as a model problem is that synaesthetes are, generally, healthy and willing research participants in comparison to using neurological/neuropsychiatric patients who have perceptual abnormalities. Synaesthesia is more common than originally thought – estimated to occur in up to 5% of the population (Sagiv and Ward, 2006) – creating a large pool of potential participants. In addition, a review by Luke and Terhune (2013) outlines the current standing of research into chemically induced synaesthesia: their findings stated that serotonin agonists (Nichols, 2004) (e.g. LSD, mescaline) were able to induce synaesthesia in non-synaesthetes and enhance it in synaesthetes (Luke et al., 2012), with auditory-visual synaesthesia the most commonly chemically induced. This line of research is incomplete, but, if chemically induced synaesthesia is found to consistently mimic congenital synaesthesia, could further widen opportunity for research into the neural mechanisms underpinning synaesthesia and therefore consciousness as a whole.
One of the areas synaesthesia has been thought to be of most value as a model problem is in the study of the neural correlates of consciousness, which can be informed by the neural correlates of synaesthesia, and more specifically in identifying the minimal sets of neural mechanisms which, when activated, trigger a synaesthetic experience. It is necessary to differentiate between the neural correlates of synaesthetic experience and those of awareness of the inducing stimulus. Between-subjects designs provide relatively straightforward means for a constant stimulus while comparing synaesthetes and non-synaesthetes. However, synaesthetes cannot be treated as one, as even though the labels might be the same the manifestation often is not. Furthermore, the comorbidity of synaesthesia with other types may limit any investigation. It has long been suspected that V4 is essential for colour vision (e.g. Zeki, 1990) but in order to conclude that it is essential to colour consciousness, it would have to be shown there is no colour experience without V4. Studies of synaesthetic colour experience show patterns of V4 activation (e.g. Van Leeuwen et al., 2010), which would lend credibility to this theory. Despite improvement in the mapping of different cortical areas’ functions, the patterns of cortical connectivity on cognition, brain function and consciousness, and their effects, are not well understood. Synaesthesia offers opportunities to examine these patterns from another perspective, especially considering the increasing interest in connectivity in developmental conditions (e.g. Rippon et al., 2007), not to mention its unlimited capacity in informing the relationship between brain function, plasticity and conscious experience. Studies of synaesthetes under ‘rest’ circumstances (e.g. Tomson et al., 2013) support the theory that altered network function is directly related to altered conscious experience. The neural correlates of changes in these experiences cannot yet, however, explain how physiological mechanisms can create the experiences.
The individual differences in the way people perceive the world is another area which synaesthesia research may be able to inform. At its core, synaesthesia is about atypical experience; in Sagiv and Robinson’s (2005) investigation, for synaesthete AD the letter C is yellow, whereas for synaesthete CP it is blue. The individuality in the manifestation of synaesthesia can echo an old question in consciousness: do individuals agree on colour names but experience them differently? While behavioural measures have helped to exclude some potential transformations of individuals’ colour space (Palmer, 1999a), generally it is hard to empirically compare experiences. An exception is colour blindness (Palmer, 1999b), where looking at the neurophysiology of sensory systems has provided some explanation to the difference in experience: individuals who have one of three types of cone photoreceptors missing are unable to discriminate between colours that the typical population experience as distinctive.
The processing of information that comes from various sensory modalities is not only combined, but can also influence each other: an example would be how visual input (e.g. watching a scene on a screen) can influence sound localisation (e.g. the sound is coming from actors’ mouths, not speakers elsewhere in the room) (Macaluso and Driver, 2005). Sensory interactions are also possible when just one sensory modality is stimulated, like in synaesthesia. Synaesthesia exemplifies the conclusions that we construct our perceived world, which is not an exact copy of the external world. Frith (2007) stated that perception is a fantasy that happens to coincide with reality, meaning perception is an inferential process. Inferences about the world are made based on best available input and prior experience, but these can be wrong, resulting in perceptual illusions (Gregory, 1980). Like the rest of the world, synaesthetes can only explore the world within their senses, despite their perceptual experiences being seemingly richer.
Alongside these areas where synaesthesia and consciousness research may mutually inform the other, the construction of social reality, agency and thought processes coincide and may also benefit from using synaesthesia as a model problem.