In a provocative review paper just published, French neuroscientists Jean-Michel Hupé and Michel Dojat question the assumption that synesthesia is a neurological disorder.

In synesthesia, certain sensory stimuli involuntarily trigger other sensations. For example, in one common form of synesthesia, known as ‘grapheme-color‘, certain letters are perceived as allied with, certain colors. In other cases, musical notes are associated with colors, or smells.

The cause of synesthesia is obscure. Many neuroscientists (including Hupé and Dojat) have searched for its brain basis. One theory is that it’s caused by ‘crossed wires’ – abnormal connections among the sensory processing areas of the brain.

But – according to Hupé and Dojat – the studies to date have failed to find anything, and the only conclusion we can draw from these studies is that “the brains of synesthetes are functionally and structurally similar to the brains of non-synesthetes.”

To reach this conclusion they reviewed 19 studies of brain grey and white matter structure (using MRI and DWI) in synesthetes, comparing them to people without the condition. They conclude

We did not find any clear evidence of structural brain alterations in synesthetes, either local differences or differences in connectivity, at least when considering the data with no a priori… More structural results exist in favor of the role of the parietal cortex in synesthesia. However… there was no consistency across studies about the precise anatomical location of which part of the parietal cortex was supposed to be involved.

Hupé and Dojat then considered 25 studies of brain activity in synesthetes, but, they say, these didn’t paint a consistent picture either. Considering the case of color synesthesia, for instance:

A few significant differences (in six studies) between synesthetes and controls were reported in the frontal and parietal cortex (whole brain analysis). When restricting the analysis to the visual cortex, only a few results (in five studies) were compatible with the involvement of color regions in synesthesia.

In other words, the majority of studies suggest that the experience of synesthetic colors is not caused by neural activity in the brain’s color-detecting cortex, which is inconsistent with the most straightforward version of the crossed wires idea. Hupé et al.’s own 2012 paper is one of the studies that didn’t find evidence of color cortex activity.

So what does this mean? The authors suggest that maybe synesthesia isn’t a brain condition at all:

If none of the proposed structural or functional differences [claimed to exist in synesthesia are] confirmed, this would speak against synesthesia being a neurological condition. But, then, what could be the nature of synesthesia?

This is where it gets a bit more speculative. Hupé and Dojat suggest that the source of synesthesia may lie in childhood memories. On this view, synesthesia would have a neural basis, but only in the trivial sense that all memories do. The authors suggest that grapheme-color synesthesia, for instance, might represent a kind of vivid memory of colored alphabet blocks or fridge magnets. They admit, however, that there’s not much direct evidence for thus yet.

Childhood toys also can’t easily explain other more abstract associations, e.g. between sounds and tastes. The authors suggest that the “creative mind of children” sometimes constructs these synesthetic patterns of associations. They cite some attempts to “trace back the origin” of these patterns in particular cases of synesthesia. This seems to me a bit close to Freudian dream interpretation, to be honest – with enough effort, you can trace anything back to anything.

Overall, Hupé and Dojat make a convincing case that we’ve yet to discover consistent neural correlates of synesthesia using neuroimaging. It’s worth remembering, though, that neuroimaging is a blunt instrument and can’t tell us anything about the fine scale of brain organization – i.e about individual neurons and circuits. Synesthesia could yet be a neurological disorder, just one we lack the technology to understand. Even some severe forms of epilepsy are not associated with any brain changes visible to MRI.

Incidentally, dedicated Neuroskeptic readers may remember Hupé and Dojat from their 2012 paper on the effects of eye blinks on fMRI signals, that I blogged about as The Blinking Brain.

Hupé JM, & Dojat M (2015). A critical review of the neuroimaging literature on synesthesia. Frontiers in Human Neuroscience, 9PMID: 25873873