The neuroanatomy of grapheme-color synesthesia (original) (raw)
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Increased structural connectivity in grapheme-color synesthesia
Nature Neuroscience, 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 graphemecolor synesthesia, and has a role in the subjective nature of synesthetic color experience.
The Neural Bases of Grapheme-Color Synesthesia Are Not Localized in Real Color- Sensitive Areas
The subjective experience of color by synesthetes when viewing achromatic letters and numbers supposedly relates to real color experience, as exemplified by the recruitment of the V4 color center observed in some brain imaging studies. Phenomenological reports and psychophysics tests indicate, however, that both experiences are different. Using functional magnetic resonance imaging, we tried to precise the degree of coactivation by real and synesthetic colors, by evaluating each color center individually, and applying adaptation protocols across real and synesthetic colors. We also looked for structural differences between synesthetes and nonsynesthetes. In 10 synesthetes, we found that color areas and retinotopic areas were not activated by synesthetic colors, whatever the strength of synesthetic associations measured objectively for each subject. Voxel-based morphometry revealed no white matter (WM) or gray matter difference in those regions when compared with 25 control subjects. But synesthetes had more WM in the retrosplenial cortex bilaterally. The joint coding of real and synesthetic colors, if it exists, must therefore be distributed rather than localized in the visual cortex. Alternatively, the key to synesthetic color experience might not lie in the color system.
The Importance of Individual Differences in Grapheme-Color Synesthesia
Neuron, 2005
The subjective reports of synesthetes whom we have interviewed, however, suggest that not all grapheme-Bremmer, F., Schlack, A., Shah, N.J., Zafiris, O., Kubischik, M., Hoff-Smilek, D., and Dixon, M.J. (2002). Psyche 8 (http://psyche.cs. monash.edu.au/v8/psyche-8-01-smilek.html). Smilek, D., Dixon, M.J., Cudahy, C., and Merikle, P.M. (2001). J. Cogn. Neurosci. 13, 930-936.
NeuroImage, 2014
In this study we show, for the first time, a correlation between the neuroanatomy of the synesthetic brain and a metric that measures behavior not exclusive to the synesthetic experience. Grapheme-color synesthetes (n = 20), who experience colors triggered by viewing or thinking of specific letters or numbers, showed altered white matter microstructure, as measured using diffusion tensor imaging, compared with carefully matched non-synesthetic controls. Synesthetes had lower fractional anisotropy and higher perpendicular diffusivity when compared to non-synesthetic controls. An analysis of the mode of anisotropy suggested that these differences were likely due to the presence of more crossing pathways in the brains of synesthetes. Additionally, these differences in white matter microstructure correlated negatively, and only for synesthetes, with a measure of the vividness of their visual imagery. Synesthetes who reported the most vivid visual imagery had the lowest fractional anisotropy and highest perpendicular diffusivity. We conclude that synesthetes as a population vary along a continuum while showing categorical differences in neuroanatomy and behavior compared to nonsynesthetes.
Frontiers in psychology, 2013
Synaesthesia is a heritable condition in which particular stimuli generate specific and consistent sensory percepts or associations in another modality or processing stream. Functional neuroimaging studies have identified potential correlates of these experiences, including, in some but not all cases, the hyperactivation of visuotemporal areas and of parietal areas thought to be involved in perceptual binding. Structural studies have identified a similarly variable spectrum of differences between synaesthetes and controls. However, it remains unclear the extent to which these neural correlates reflect the synaesthetic experience itself or additional phenotypes associated with the condition. Here, we acquired both structural and functional neuroimaging data comparing thirteen grapheme-color synaesthetes with eleven non-synaesthetes. Using voxel-based morphometry and diffusion tensor imaging, we identify a number of clusters of increased volume of gray matter, of white matter or of in...
Dynamic phenomenology of grapheme-color synesthesia
2010
Abstract. In grapheme-color synesthesia, observers perceive colors that are associated with letters and numbers. We tested the dynamic limits of this phenomenon by exposing two synesthetes to characters that rotate smoothly, that morph into other characters, that disappear abruptly, or that have colors either consistent or inconsistent with the corresponding synesthetic color. Rotating letters changed their synesthetic colors abruptly as letter identification changed or failed.
Effective Connectivity Determines the Nature of Subjective Experience in Grapheme-Color Synesthesia
The Journal of …, 2011
Synesthesia provides an elegant model to investigate neural mechanisms underlying individual differences in subjective experience in humans. In grapheme–color synesthesia, written letters induce color sensations, accompanied by activation of color area V4. Competing hypotheses suggest that enhanced V4 activity during synesthesia is either induced by direct bottom-up cross-activation from grapheme processing areas within the fusiform gyrus, or indirectly via higher-order parietal areas. Synesthetes differ in the way synesthetic color is perceived: “projector” synesthetes experience color externally colocalized with a presented grapheme, whereas “associators” report an internally evoked association. Using dynamic causal modeling for fMRI, we show that V4 cross-activation during synesthesia was induced via a bottom-up pathway (within fusiform gyrus) in projector synesthetes, but via a top-down pathway (via parietal lobe) in associators. These findings show how altered coupling within the same network of active regions leads to differences in subjective experience. Our findings reconcile the two most influential cross-activation accounts of synesthesia.
Grapheme-color synesthesia is a condition in which seeing letters and numbers produces sensations of colors (e.g., the letter R may elicit a sky-blue percept). Recent evidence implicates posterior parietal areas, in addition to lower- level sensory processing regions, in the neurobiological mechanisms involved in synesthesia. Furthermore, these mechanisms seem to differ for “projectors” (synesthetes who report seeing the color “out there in the real world”) versus “associators” (synesthetes who report the color to be only an internal experience). Relatively little is known about possible electrophysiological characteristics of grapheme-color synesthesia. Here we used EEG to investigate functional oscillatory differences among associators, projectors, and non-synesthetes. Projectors had stronger stimulus-related alpha-band (~10 Hz) power over posterior parietal electrodes, compared to both associators and non-synesthetes. Posterior alpha activity was not statistically signi␣cantly different between associators from non- synesthetes. We also performed a test-retest assessment of the projector-associator score and found strong retest reliability, as evidenced by a correlation coef␣cient of .85. These ␣ndings demonstrate that the projector-associator distinction is highly reliable over time and is related to neural oscillations in the alpha band.
The Long-Term Potentiation Model for Grapheme-Color Binding in Synesthesia
Sensory Integration and the Unity of Consciousness, D. Bennett and C. Hill (Eds.)
The phenomenon of synesthesia has undergone an invigoration of research interest and empirical progress over the past decade. Studies investigating the cognitive mechanisms underlying synesthesia have yielded insight into neural processes behind such cognitive operations as attention, memory, spatial phenomenology and inter-modal processes. However, the structural and functional mechanisms underlying synesthesia still remain contentious and hypothetical. The first section of the present paper reviews recent research on grapheme-color synesthesia, one of the most common forms of synesthesia, and addresses the ongoing debate concerning the role of selective attention in eliciting synesthetic experience. Drawing on conclusions of the first half, the paper’s second half examines the various models proposed to explain the cognitive mechanisms behind grapheme-color synesthesia, and discusses the explanatory virtues of a new model suggesting that grapheme-color synesthesia is grounded in memory. The last section offers an examination of some of the broader philosophical implications of synesthesia.