Synesthesia

Synesthesia is a fascinating neurological condition that causes an individual (proudly called a synesthete) to experience perceptual information through a sense modality that is unlinked to its source. This is a fancy way of saying that synesthetes may hear colors, smell noises, taste shapes, and even feel flavors. This experience is both involuntary and stable over time. Around 100 different types of synesthesia have been documented, and the condition affects nearly four percent of the general population. Synesthesia is thought to be an inherited trait affecting areas of the brain that communicate sensory information to one another.

Scientific milestones

In the late 19th and early 20th centuries, synesthesia enjoyed a flurry of scientific study, mostly descriptive. By the mid-20th century, however, synesthesia had fallen off scientists’ radar, a casualty of the behaviorism movement. The phenomenon began to resurface as a subject of psychological inquiry beginning in the 1970s, stimulated largely by the work of two scientists.

In 1975, Yale University psychologist Larry Marks, PhD, authored a review of the early history of synesthesia research in the journal Psychological Bulletin (Vol. 82, No. 3), the first major psychological treatment of the subject after a 30-year drought. Then, in the early 1980s, neurologist Richard E. Cytowic, MD, published several case reports of synesthesia. He proposed, provocatively, that the condition’s cause rests in the limbic system, a more emotional and “primitive” part of the brain than the neocortex, where higher order thinking occurs. Although that theory has not received widespread support, Cytowic’s case studies and his popular 1993 book, “The Man Who Tasted Shapes,” heightened synesthesia’s prominence and prompted psychologists and neuroscientists to examine the condition experimentally.

In 1987, a team led by Baron-Cohen found the first hard evidence that synesthetes’ experiences are consistent across time. The researchers asked a synesthete to describe the color that each of 100 words triggered. A year later, they repeated the test without warning and found that the associations between words and colors that their subject described were consistent with her initial responses more than 90 percent of the time. In contrast, people without synesthesia, asked to perform the same task but with only a two-week interval between the two tests, were consistent only 20 percent of the time.

In later research, Baron-Cohen’s group established that synesthesia is not only consistent across time but also concretely measurable in the brain. Using positron-emission tomography and functional magnetic resonance imaging, the researchers have found that for synesthetes who report colored hearing, visual areas of the brain show increased activation in response to sound. That isn’t the case for nonsynesthetes.

Other studies have demonstrated that synesthetic perception occurs involuntarily and interferes with ordinary perception. And last summer, University of Waterloo researchers Mike Dixon, PhD, Daniel Smilek, Cera Cudahy and Philip Merikle, PhD, showed that, for one synesthete, the color experiences associated with digits could be induced even if the digits themselves were never presented. These researchers presented a synesthete with simple arithmetic problems such as “5 + 2.” Their experiment showed that solving this arithmetic problem activated the concept of 7, leading their synesthete to perceive the color associated with 7.

This finding, published last July in the journal Nature (Vol. 406), was, according to Dixon, the first objective evidence that synesthetic experiences could be elicited by activating only the concepts of digits. As such, these results suggest that, at least for this synesthete, the color experiences were associated with the digit’s meaning, not just its form.

Together, the evidence shows that “something is going on in the sensory areas of the brain,” concludes Christopher Lovelace, PhD, a research fellow at Wake Forest University School of Medicine. “What we have to do now is try to figure out how the brain does it.”

Neural basis of synesthesia:

Cross-activation

Since regions involved in the identification of letters and numbers lie adjacent to a region involved in color-processing (V4), the additional experience of seeing colors when looking at graphemes might be due to “cross-activation” of V4 (Ramachandran & Hubbard 2001). This cross-activation may arise due to a failure of the normal developmental process of pruning, which is one of the key mechanisms of synaptic plasticity, in which connections between brain regions are partially eliminated with development. Similarly, lexical → gustatory synesthesia may be due to increased connectivity between adject regions of the insula in the depths of the lateral sulcus involved in taste processing that lie adjacent to temporal lobe regions involved in auditory processing. Similarly, taste → touch synesthesia may arise from connections between gustatory regions and regions of the somatosensory system involved in processing touch. However, not all forms of synesthesia are easily explained by adjacency.

Disinhibited feedback

Alternatively, synesthesia may arise through “disinhibited feedback” or a reduction in the amount of inhibition along feedback pathways (Grossenbacher & Lovelace 2001). It is well established that information not only travels from the primary sensory areas to association areas such as the parietal lobe or the limbic system, but also travels back in the opposite direction, from “higher order” cortical regions to early sensory areas. Normally, the balance of excitation and inhibition are maintained. However, if this feedback were not adequately inhibited, then signals coming from later stages of processing might influence earlier stages of processing, such that tones would activate visual cortical areas in synesthetes more than in non-synesthetes. In this case, it might be possible to temporarily have synesthetic experiences after taking drugs like LSD or mescaline. Indeed, some psychedelic drug users report synesthesia-like experiences, although the exact degree of similarity between these drug induced experiences and congenital synesthesia is still unclear (Luke & Terhune 2013).

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