Synesthesia research, which just a decade ago was considered a decidedly esoteric, and perhaps even a rather dubious area of study for a ‘serious’ neuroscientist, has since emerged as an area of vigorous interest. On the one hand, there is simply no denying the fascination that this unusual condition engenders. More important though, are the potential implications of synesthesia for theories of functional brain connectivity, multisensory integration and perceptual binding. One of the most common and therefore widely studied forms of synesthesia is the grapheme-color (GC) variety, where standard monochromatic letters and digits elicit idiosyncratic color percepts. That is, what typical observers see as black letters on a white page appear as a multicolored array to these synesthetes. Recently, it has been shown that GC synesthesia can be bidirectional (Knoch et al., 2005; Cohen Kadosh & Henik, 2006; Cohen Kadosh et al., 2007), with visualized colors eliciting number or letter percepts. This finding adds a new dimension to synesthesia, previously thought to be a strictly one-way phenomenon. However, before this finding can be incorporated into models of synesthesia, it must first be established whether similar neuronal mechanisms underlie both directions of synesthesia.

In this issue of EJN, Gebuis et al. (2009) investigated this question by inducing bi-directional synesthesia whilst recording event related potentials (ERPs), and comparing the responses to both directions. GC synesthetes had longer reaction times for incongruent compared to congruent trials in both number[RIGHTWARDS ARROW]color and color[RIGHTWARDS ARROW]number priming tasks compared to controls. That is, synesthetes are negatively impacted (have longer reaction times) when the number is primed by a color, or color is primed by a number, other than the one that they have a specific color-number mapping for and vice-versa, whereas control subjects are oblivious to this distinction. Turning to the electrophysiological responses, incongruent trials in both tasks elicited larger frontal P3a amplitudes and longer parietal P3b latencies for the synesthetes. These effects did not differ between tasks, implying that the neurophysiological correlates underlying synesthesia are similar in both directions. However, the effects did differ between two types of synesthetes: those who had a small behavioral effect showed only the frontal modulation, while those who had a large behavioral effect showed both the frontal and the parietal modulations. Gebuis et al. (2009) interpret these findings as support for a distinction between ‘higher’ and ‘lower’ synesthetes (Hubbard et al., 2005). ‘Higher’ synesthetes’ experiences are elicited by the concept of the trigger (top-down attentional/inhibitory processes) whereas the experience of ‘lower’ synesthetes is elicited by the percept of the trigger (bottom-up perceptual processes). Based on their ERP results, Gebuis et al. (2009) hypothesize that synesthetic experiences require top-down processes, but for some synesthetes (the ‘lower’ group) bottom-up processes are also required. Presumably, these synesthetes also have a stronger behavioral effect.

Some of our own recent work using ERPs also concerns this issue of feedforward vs. feedback mechanisms (Barnett et al., 2008). We took a rather different approach in that we decided not to look at orthographic stimuli at all in our GC synesthetes. Instead, using ERP recordings, we simply asked if there were basic sensory processing differences early in the visual system in a large cohort of synesthetes in response to simple visual stimuli that did not induce synesthetic experiences. We found clear evidence of very early differences in visual sensory processing that began just 70–100 ms after stimulus presentation, which is only 25–30 ms after initial afference in the primary visual cortex. These data suggest a basic difference in the wiring of the synesthetic brain, a difference that impacts very early sensory processing and implicates bottom-up processes in GC synesthesia. While these results and those of Gebuis et al. (2009) add to the ongoing debate regarding whether synesthetic experience arises from early or later processing stages, they do not resolve this issue, but they stress how synesthesia is far from a unitary phenomenon, and the importance of taking into account individual differences among synesthetes in future research.

Interestingly, Gebuis et al. (2009) did not find any behavioral or electrophysiological differences between ‘projector’ and ‘associator’ synesthetes. The former are those who experience the grapheme ‘physically’ colored, the latter are those who experience the color in the ‘mind’s eye’. When Gebuis et al. (2009) divided their synesthetes into two groups based on their behavioral results, both groups had the same number of associators and projectors. Thus, contrary to the suggestion made by Dixon & Smilek (2005), these results imply that that these two dimensions of individual differences (high vs. low, projector vs. associator) are dissociable rather than correlated. While many theories have been posited to explain the synesthetic experience, the Gebuis et al. (2009) study leaves open the interpretation of the electrophysiological effects and does not distinguish between (for example) the disinhibited feedback model (Grossenbacher & Lovelace, 2001) or the cross-activation model (Ramachandran & Hubbard, 2001). However, these models will have to incorporate the bi-directional feature of synesthesia and the results of this provocative study by Gebuis and colleagues, which suggests that similar neural mechanisms subserve both directions.


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