Into the Mind of the Synesthete

Synesthesia is a condition in which a stimulus from one sensory modality involuntarily triggers a separate experience in one or more different sensory modalities. Though originally believed to be brought about by vivid cross-modal linguistic associations (Marks, 1975) or even unconsciously-constructed metaphor used by those with strong artistic inclinations (Osgood, 1981), current research has shed light on both the genuine sensory phenomena experienced by synesthetes (Cytowic, 1989) and, with the advent of modern neuroscience, potential neural substrates of this phenomenon. Most of these physiologically-based theories describe a hyperconnectivity or "cross-wiring" of neural fibers in the relevant sensory loci of the brain (Blake, Palmeri, Marois & Kim, 2005). This review will discuss such mechanisms that have been proposed for grapheme-color synesthesia, the most common and well-documented form of synesthesia, as well as the impact of said research on this author as well as the scientific community at large.

Individuals with grapheme-color synesthesia experience color when presented with numbers and/or letters. For instance, a subject with grapheme-color synesthesia might perceive every number nine as having a vivid green hue and every number five being a de-saturated blue, even while being consciously aware of the blackness of the ink with which the numbers were printed (Cytowic, 1989). A huge leap towards uncovering the secrets behind this bewildering phenomenon was made when researchers identified a "number-grapheme area" of the fusiform gyrus, which happened to lie directly adjacent to hV4, an area of the extrastriate cortex responsible for color processing. Subsequent fMRI studies showed cross-activation of hV4 when subjects were presented with inducing grapheme stimuli (Hubbard, Ramachandran & Boynton, 2004). Ramachandran and colleagues proposed that this cross-activation occurs in a manner analogous to the cortical reorganization that occurs in patients with phantom limb sensations. Just as input from the facial areas of the somatosensory cortex begins to remap onto areas formerly devoted to the missing limb of amputees, so might connections in the number-grapheme area be anomalously mapped to hV4 at some stage in development (Ramchandran & Hubbard, 2003).

In recent years, many researchers have built on Ramachandran's pioneering discoveries. Scientists in Germany found higher-level activation in the left intraparietal cortex during the experience of grapheme-color synesthesia. Noting that this cortical area is implicated in sensory and premotor integration, they hypothesized that the left intraparietal cortex may "bind" input from the number-grapheme node and hV4 so that the synesthete experiences a single, coherent perception (Weiss, Zilles & Fink, 2005). Using voxel-based morphometry, Weiss and Fink further demonstrated that grapheme-color synesthetes had increased gray matter volume in both the fusiform gyrus and the intraparietal cortices, coupling structural evidence of his theory to previous activation evidence (Weiss & Fink, 2009). The debate over the exact mechanism of action continues, however. While cross-activation is evident in fMRI studies, it is possible that significantly variant neural pathways are functionally indistinguishable using this technique. Another study that employed diffusion tensor imaging was not able to detect abnormal connectivity in the fusiform gyrus among grapheme-color synesthetes (Jcke, Beeli, Eulig & Hggi, 2009). This does not disprove the previous implications of the fusiform gyrus in synesthesia, but rather shows that cross-activation may not be due to hyperconnectivity or cortical remapping. Tomson and colleagues propose that reduced functioning of inhibitory neural networks may instead be the root cause, and they are currently performing genetic studies to pin down the molecular basis of grapheme-color synesthesia (Tomson et al., 2011).

Thus, even though we understand a great deal about grapheme-color synesthesia, there is still much research that remains to be done to flesh out the neural basis of this fascinating condition. Furthermore, even more work remains to be done in other forms of synesthesia. While phenomenologically analogous to grapheme-color synesthesia, they may employ drastically different neural mechanisms about which we currently know next to nothing. Indeed, Ramachandran posits that synesthesia is a spectral condition that may affect a large portion of the population to a greater or lesser extent. He goes as far as speculating that human preponderance for metaphor may be a "higher order" level of synesthesia active in the angular gyrus rather than the "lower order" sensory areas in the fusiform gyrus (Ramachandran & Hubbard, 2003).  For this reason, I believe that the continued study of synesthesia is of utmost importance. I hope to become involved in this process soon, too. I recently applied for an undergraduate research assistanceship for David Eagleman's Laboratory for Perception and Action at Baylor College of Medicine. Dr. Eagleman does awesome research on synesthesia, among other things (and he recently published an excellent book on the subject called Wednesday is Indigo Blue: Discovering the Brain of Synesthesia.) The position is very competitive, though, so I'm crossing my fingers that I'll get lucky!





REFERENCES

Blake, R., Palmeri, T., Marois, R., & Kim, C. (2005).  On the perceptual reality of synesthetic color.

In L. Robertson & N. Sagiv (Eds.), Synesthesia: Perspectives from cognitive neuroscience (pp. 47-73). New York: Oxford University Press.

Cytowic, R. (1989).  Synesthesia: A union of the senses.

New York: Springer-Verlag New York, Inc.

Hubbard, E., Ramchandran, V., & Boynton, G. (2003). Cortical cross-activation as the locus of grapheme-          color synesthesia. Journal of Vision, 3(9), 621. doi: 10.1167/3.9.621

Jcke, L., Beeli, G., Eulig , C., & Hggi, J. (2009). The neuroanatomy of grapheme-color          synesthesia.European Journal of Neuroscience, 29, 1287-93. doi:10.1111/j.1460-9568.2009.06673.x

Marks, L. (1975). On colored-hearing synesthesia: Cross-modal translations of sensory          dimensions.Psychological Bulletin, 82(3), 303-331.

Osgood, C. (1981). The cognitive dynamics of synesthesia and metaphor. 

Review of research in visual arts education, 7(2), 56-80.

Ramachandran, V., & Hubbard, E. (2003). What neuroscience can teach us about human nature and the          potential for change. In M. Devlin (Ed.), The internet and the university (pp. 15-33).

Tomson, S. et al. (2011). The genetics of colored sequence synesthesia: suggestive evidence of linkage to 16q          and genetic heterogeneity for the condition.Behavioral Brain Research, 223, 48-52.          doi:10.1016/j.bbr.2011.03.071

Weiss, P., Zilles, K., & Fink, G. (2005). When visual perception causes feeling: Enhanced cross-modal          processing in grapheme-color synesthesia. NeuroImage, 28, 859-868.          doi:10.1016/j.neuroimage.2005.06.052

Weiss, P., & Fink, G. (2009). Grapheme-colour synaesthetes show increased grey matter volumes of parietal          and fusiform cortex. Brain, 132, 65-70.  doi:10.1093/brain/awn304