Thursday, May 24, 2007

Synesthetes Show Greater Connectivity in the Inferior Temporal Cortex

Ah ah
Something ain't right
I'm gonna get myself
I'm gonna get myself
I'm gonna get myself connected

--Stereo MC's, Connected

SYNESTHESIA can be defined as unusual conscious experience, in which stimulation of one sensory modality leads to a sensory experience in a second, unstimulated sensory modality. For example, seeing letters might lead some people to see colors.
Previous neuroimaging studies have observed activations in V4 (a color processing region of visual cortex) when "colored hearing" synesthetes listened to spoken words (Nunn et al., 2002; Paulesu et al., 1995) and when grapheme-color synethetes viewed letters (Hubbard et al., 2005; Sperling et al., 2006). It has long been hypothesized (Ramachandran & Hubbard, 2001), but never proven, that people with synesthetic abilities have an unusual degree of connectivity between the brain regions involved in coding two (or more) sensory modalities. A brand new study by Rouw and Scholte (2007) used diffusion tensor imaging, a magnetic resonance imaging method that provides images of white matter tracts, to demonstrate just that. Although the authors found increased anisotropy in multiple areas [explained by hand-waving], indicative of greater connectivity, they emphasized the differences in inferior temporal cortex because of its role in processing color and letter/word stimuli.
Increased diffusion anisotropy reflects increased or more coherent connectivity due to microstructural aspects such as degree of myelination, axonal diameter, and density and coherence in fiber orientation as well as macrostructural features such as intravoxel fiber-tract coherence.

Figure 2. Increased brain activation and increased anisotropy in the inferior temporal cortex in grapheme-color synesthetes. The white matter skeleton (green) is projected on an MNI brain (grayscale). Greater BOLD signal (blue) as well as greater anisotropy (yellow) underlie synesthetic experiences in color-grapheme synesthesia. [from Rouw & Scholte, 2007]
Rouw R & Scholte HS (2007). Increased structural connectivity in grapheme-color synesthesia. Nature Neuroscience. Published online: 21 May 2007.

Diffusion tensor imaging allowed us to validate for the first time the hypothesis that hyperconnectivity causes the added sensations in synesthesia. Grapheme-color synesthetes (n = 18), who experience specific colors with particular letters or numbers (for example, 'R is sky blue'), showed greater anisotropic diffusion compared with matched controls. Greater anisotropic diffusion indicates more coherent white matter. Anisotropy furthermore differentiated subtypes of grapheme-color synesthesia. Greater connectivity in the inferior temporal cortex was particularly strong for synesthetes who see synesthetic color in the outside world ('projectors') as compared with synesthetes who see the color in their 'mind's eye' only ('associators'). In contrast, greater connectivity (as compared with non-synesthetes) in the superior parietal or frontal cortex did not differentiate between subtypes of synesthesia. In conclusion, we found evidence that increased structural connectivity is associated with the presence of grapheme-color synesthesia, and has a role in the subjective nature of synesthetic color experience.
Or, as this witty headline puts it, it's a...
Red-letter day for brain connectivity
More neural connections may explain synesthesia
Staff Writers 21/05/2007 11:24:24

People with grapheme-colour synesthesia - who see a cascade of colours associated with individual letters when looking at a page of text - appear to have more neural connections in areas of the brain involved in word processing and binding perceptions together, a new study shows.

Hubbard EM, Arman AC, Ramachandran VS, Boynton GM. (2005). Individual differences among grapheme-color synesthetes: brain-behavior correlations. Neuron 45:975-85.

Nunn JA, Gregory LJ, Brammer M, Williams SC, Parslow DM, Morgan MJ, Morris RG, Bullmore ET, Baron-Cohen S, Gray JA. (2002). Functional magnetic resonance imaging of synesthesia: activation of V4/V8 by spoken words. Nat Neurosci. 5:371-5.

Paulesu E, Harrison J, Baron-Cohen S, Watson JD, Goldstein L, Heather J, Frackowiak RS, Frith CD. (1995). The physiology of coloured hearing. A PET activation study of colour-word synaesthesia. Brain 118:661-76.

Ramachandran VS & Hubbard, EM. (2001). Psychophysical investigations into the neural basis of synaesthesia. Proc. R. Soc. Lond. B 268:979–983.

Rich AN, Williams MA, Puce A, Syngeniotis A, Howard MA, McGlone F, Mattingley JB. (2006). Neural correlates of imagined and synaesthetic colours. Neuropsychologia 44:2918-25.

If you make sure you're connected
The writing's on the wall
But if your mind's neglected
Stumble you might fall
Stumble you might fall
Stumble you might fall

--Stereo MC's, Connected

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At May 24, 2007 4:08 AM, Blogger Kristjan said...

I currently reading Nabokov's "Memory, Speak" which includes a fascinating chapter about synesthesia. He describes in great detail which colors, feelings etc are associated with specific letters and speech sounds. Like "The 'a' of the English alphabet has for me the stint of a weathered wood, but a French 'a' evokes polished ebony." As also with his case, the phoneme, a sound, not the grapheme, a written letter, gets a color association.

At May 24, 2007 12:58 PM, Blogger The Neurocritic said...

Nabokov was certainly more eloquent about his synesthesia than most. Sean Day and Wikipedia have extensive lists of famous [and not-so-famous] people with synesthia.


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