Synesthesia is a condition where individuals claim to experience the world in unusual ways. For example, in color-grapheme synesthesia, individuals may claim that letters or numbers have each their own color which they inherently seem to be. When shown a letter that is actually colored, they can say the correct color, but tend to be measurably slower if the actual color is different than the color they say they experience as associated with the letter.
Nunn et al.'s paper "Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words" studies a different type of synesthesia: color-hearing synesthesia, where individuals report color experiences when hearing spoken words. In a previous experiment, positron emission topography (PET) showed that synesthetes have some unusual brain activity in response to spoken words, but not in the exact regions that would be expected. Nunn et al. used more precise fMRI techniques to search for activity where it hadn't been found before.
They exposed color-hearing synesthetes to words while under fMRI, and found that synesthetes had activity in the V4/V8 region of the visual cortext, but not the V1 and V2 regions. The researchers did extra tests to verify that synesthetes in many ways normal sensory cortices, so the results cannot be accounted for in terms of general abnormalities.
The V1 and V2 regions are the primary visual cortex, the first stop for visual information after leaving the lateral geniculate nuclei in the thalamus. The V4/V8 region, in contrast, is an early part of the object-recognition pathway. This research not only tells us about synesthesia, but also gives us more information about these brain regions. It suggests that the primaray visual cortex is not essential for producing conscious experience, and the V4/V8 regions may be more important for this. More broadly, this research illustrates how the tools of hard neuroscience can be used to study consciousness, which is normally considered so elusive.
In addition to simply working out what is going on in synesthetes, researchers showed that normal people trained on word-color associations still do not have the kind of brain activity seen in synesthetes when shown words. This provides evidence that synesthesia is something much stronger and deeper than a mere trained association. This is important because it places synesthesia in the context of existing research on the differences between imagery and full-blown experience (whether from real objects or hallucinations). It also provides evidence against the theory of synesthesia which says synesthesia is a strong form of learned associations, suggesting synesthesia is hard-wired.
More V4
First, I just want to add that many researchers think that V4 is responsible for color detection. However, there is controversy on the matter, since some have shown that there are no more color receptors in V4 than in V1. The other interesting thing about V4 is that it is somewhat dependent on attention; so it would be interesting to see if a synesthete could be distracted from his or her color-word association. This review also reminded me of something I learned in psychology of perception: it is possible for color blind people to see an object when it is moving, even though they normally wouldn’t be able to see it. That idea has always intrigued me – the parallel processing of the brain and I think that it ties in with this article, which also shows that the connections we assume must exist, don’t always do exist.
fMRI and processing
I found your article very interesting, as one of my posts had to do with fMRI and blood oxygen level dependent responses in rat brains. I feel that by using fMRI technology, we as a society can learn a great deal about what may be going on in an individual’s brain, at least at the blood flow level; what regions activate, when they activate to what stimuli, and ultimately, someday we may discover the why. It was interesting to note that the authors suggest that the primary visual cortex may not be “essential” for the production of our conscious experiences. I say this because this is where information from the visual world first enters and then gets processed. I just found that to be an interesting suggestion from their study. The idea of synesthesia is also intriguing and I believe it will be interesting to find out whether or not the theory gains more evidence in support of it or refuting it.
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