Synesthesia
From Psy3241
Synesthesia is a neurological syndromein which stimulation of one sensory modality automatically and uncontrollably triggers another sensory modality. The types of synesthesia discussed in class include:
- Color-graphemic synesthesia: written letters induce vivid color experiences
- Colored-hearing: color experiences induced by spoken words
- Numbers are projected into a spatial layout in front of the patient's chest, and are colored
- Periods of time are conceptualized in a colored spatial layout
Synesthesia often begins in early childhood, but it can be brought about as a result of brain injury or sensory dedeafferentation.
Most of the time, synesthesia is unidirectional (for example, most colored-hearing synesthetes do not hear sounds when they see colors). Even if patients have the same form of synesthesia, it is unlikely that their concurrents are the same: letters will not necessarily produce the same colors to different synesthetes.
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Color-graphemic
In Color-graphemic synesthesia, written words, letters, and numbers induce vivid color experiences. The actual experience of color tends to vary from patient to patient. Patients may report any of the following:
- As seen in the image, perceiving letters and numbers as being colored rather than monochrome (black).
- Seeing certain letters/words/numbers may cause the patient to see a screen of color "projected" onto their "mind's eye."
- As above, but rather than being fully colored, the screen is a blurry and colored version of the letter they are seeing.
- Patients may simply have an association between colors and letters/words/numbers without actually having a visual perception of color.
Sperling et. al (2006) found evidence that color-graphemic synesthesia is caused by an activation in the color areas (V4/V8) in the visual cortex as well as the inferior temporal lobe, another region known to be a cortical color processing system. A possible explanation for perceiving colors when seeing writing can be derived from activation of the superior temporal lobe and insula, which are hypothesized to mediate pathway convergence during the induction of a synesthetic-colour experience, leading to feedback-activation of visual areas responsible for color perception.
Colored-hearing
In colored-hearing synesthesia, sounds produce an extrasensory experience of color. This is usually in response to tones or other aspects of sound. The sounds that produce colors in colored-hearing synesthetes are musical sounds or environmental sounds (such as alarm clocks, or birds chirping). As with color-graphemic synesthesia, activation in V4/V8 has been associated with the experience of color in response to music or environmental sounds.
Number → Color & Shape
In (number)→(color/shape) synesthesia, the patient will project numbers in front of themselves in, for example, a colored arc.
Time → Location
In (time)→(location) synesthesia, the patient will arrange periods of time in a colored shape. For example, they may conceptualize the months of the year into a flat, horizontal loop surrounding them in which each month has its own color.
Causes
It is believed that synesthesia may have a genetic link, according to a twin study done by Hancock (2006). Hancock observed monozygotic twins, who both had color-number associations. The boys did not report seeing colors or even perceiving color experiences but, rather, had a simple association between numbers and colors, as if the numbers could be represented by colors. The genetic link was made possible by the discovery that the boys' mother also has a color-number association.
Synesthesia can also be a learned association. Witthoft and Winawer (2006) describe a case study in which their patient had a learned color-letter association as a byproduct of having colored refrigerator magnets as a child. Interestingly enough, this case also described the possibility that synesthetic color-letter associations can transfer between letters; their patient moved to Russia at the age of three, and her color-graphemic synesthesia transferred to Russian Cyrillik.
References
Grossenbacher, P.G., & Lovelace, C.T. (2001). Mechanisms of synesthesia: Cognitive and physiological constraints. TRENDS in Cognitive Sciences, 5(1), 36-41.
Hancock, P (2006). Monozygotic twins’ colour-number association: A case study. Cortex, 42, 147-150.
Sperling, J. M., Prvulovic, D., Linden, D.E.J., Singer, W., & Stirn, A. (2006). Neuronal correlates of a colour-graphemic synaesthesia: A fMRI study. Cortex, 42, 295-303.
Witthoft, N., & Winawer, J. (2006). Synesthetic colors determined by having colored refrigerator magnets in childhood. Cortex, 42, 175-183.
