Grapheme-color synesthesia, seeing letters and numbers as certain colors, is one of the more common forms of synesthesia, with about 68% of synesthetes affected (Day, 2005). It is of intrigue because of its intramodal nature, meaning neuroimaging investigations into it may be a good place to begin in terms of synesthesia research.One of the emerging ideas regarding grapheme-color synesthesia suggests that there is physically increased connectivity in the brain of these synesthetes. For example, through diffusion tensor imaging, it has been shown that synesthete brains have more coherent white matter. Specifically, greater connectivity was seen in the inferior temporal cortex (Rouw & Scholte, 2007), an area associated with visual stimuli processing. Even more surprising has been the discovery of perhaps a globally altered structurally network topology in synesthete brains. Increased clustering, decreased betweenness centrality, and increased intermodular and intramodular connectivity, specifically between the fusiform gyrus and intraparietal sulcus (Hanggi et al., 2011). The FuG is an area likely responsible for processing color information as well as word recognition, which is consistent with grapheme-color synesthesia. Additionally, the posterior temporal grapheme areas (PTGA) are activated nearly simultaneously as color processing area V4 when synesthetes are presented with achromatic graphemes (Brang et al., 2010). Overall however, this begs the question of one: why is the architecture of synesthete brains so different, and two, perhaps more interestingly: why does a somewhat innocuous intramodality effect appear to be the only phenotypic expression of a seemingly extensive structural difference?
Another interesting area looking into grapheme-color synesthesia, is whether or not there is some malleability in the phenomenon. For instance, the frequency of digits has been shown to in part shape the color perception (Beeli et al., 2007). This suggests some aspect of learned synesthesia, at least as far as the actual associations or pairings are concerned. One study was able to trace color-grapheme pairings in 11 synesthetes to their childhood toys, indicating some role of learning and memory. Synesthesia is not simply, in this case, an overlap of two visual processing levels, but involves the “retrieval of highly specific mnemonic associations… akin to mental imagery” (Witthoft & Winawer, 2013). In contextual priming experiments, we see high contextual congruity of synesthetic colors with digits, meaning there is an automatic connection between the color and number. Parsing out how synesthesia seems to be both automatic as well as in part learned required further research. Perhaps a longitudinal study could be useful in this regard. Synesthesia is also interesting as we see increased color sensitivity in color-based synesthetes, which makes sense, but also decreased motion sensitivity (Banissy et al., 2013). This may indicate that increased connections in some areas of the brain result in decreases in connectivity in other parts. Therefore, is there some upper limit of connectivity supported within the brain?
Incongruency is another fascinating related subtopic around synesthesia. Synesthetes actual report feeling of unease when presented with a colored letter or number that does not match their internal pairing. In fact, it’s been shown that these incorrect pairings affect the emotional valence of the grapheme (Callejas et al., 2007). Thus there may be additional emotional intermodulation involved in synesthesia than previously thought. In general, it also may point to a learned affective reaction to stimuli. The underlying neural mechanism should be investigated.
The study of synesthesia is further complicated by individual differences in synesthetes. This supports the notion of learned inter or intramodal pairings. It is important to keep in mind that synesthetes are a heterogeneous group. Amongst synesthetes, those who perform the best on behavioral experiments such as texture segregation, show the greatest fMRI response in various V1 and higher visual areas (Hubbard et al., 2005).
The mechanism by which cross-activation occurs is also up for debate, either being bottom-up from the grapheme processing area, or through a top-down pathway in higher-order parietal areas. In fact there are actually two different ways that synesthetes perceive color, “projector” in which color is externally experienced versus “associators” whom report an internally evoked association. FMRI scanning has been used to show synesthesia induced via a bottom-up pathway for projector synesthetes, but via a top-down pathway in associators (van Leeuwen et al., 2011). This finding suggests that neural pathway directionality, even if it is of the same regions, impacts perceptual outcomes.
Overall, I find synesthesia to be a remarkable phenomenon, particularly as it seems unverifiable at first glance, at least without the advent of neuroimaging technology. As a research topic, it offers many insights into how both brain connectivity and pathways operate. There also seems to be some implications for the general fields of learning and memory, given the findings which have already been discovered. More comprehensive studies of synesthetic participants is necessary, perhaps even tracking grapheme-color pairings over time, seeing if there are any difference in perception of neural messaging speeds.