Everything is blooming most recklessly; if it were voices instead of colors, there would be an unbelievable shrieking into the heart of the night. ~ Rainer Maria Rilke.
The beauty of spring inspired the poet to write this verse, but my neuroscientist mind reading it always thinks of Synesthesia. The word, from Greek, means “to perceive together” and refers to the experience of atypical sensory responses to stimuli. One of the most common responses is sound-color synesthesia – when you see sounds as colors. There are reports of the reverse - color evoking sound, or grapheme-color synesthesia where words or letters are associated with color, or even emotions like fear triggering visual imagery. People who experience things this way are called synesthetes. Famous synesthetes include Vincent van Gogh who associated musical notes with colors, and Vladimir Nabokov who saw letters in color.
Because of its subjective quality, the phenomenon of synesthesia has been viewed by many with skepticism. However, studies have shown that while there is a lot of variability between individuals in the type and strength of synesthetic association a stimulus like sound evokes, for a particular individual the experience is reproducible, idiosyncratic, involuntary, and usually cannot be suppressed. It is also generally present from a very young age and may be shared by first degree relatives, although a proven genetic basis for synesthesia remains elusive.
For years, the consensus has been that like other sensory experiences, synesthesia has a neural basis. However, the neural correlates of synesthesia are not known. Most neural models of synesthesia propose that it arises from atypical connectivity (structural or functional) between different sense processing regions of the brain. As one example, color centers in the visual cortex, which are typically activated by color, would be expected to be activated when sound-color synesthetes hear a sound or imagine it. Neuroscientists have used neurophysiological and imaging techniques like fMRI, PET scan, and EEG to study the brains of synesthetes. In cases of sound-color synesthesia, several studies did find increased activity in the color visual cortex in response to sound as expected. However, the studies were limited by small numbers of subjects, insignificant findings, variability among subjects and poor controls. An ambitious 2015 review and re-examination of all studies that had searched for neural correlates of synesthesia found no consistent structural or functional changes in the brains of synesthetes to explain synesthesia.
Alternative suggested hypotheses view synesthesia as simply a different subjective experience, a learned skill, or possibly a childhood memory of a special kind based on repeatedly experiencing senses together e.g., learning letters from colored alphabet books or blocks. A few small studies examining these ideas have not come to any reliable conclusions. Another possibility is raised by studies that reveal multisensory interactions in typical (non-synesthete) human brains at a subconscious level. For example, stimulating the visual cortex can improve the detection of smell, or sound can modulate activity in the visual cortex. So, neuroscientists are starting to look at the possibility that synesthetes may not have extraordinary sensory associations, but simply somehow become aware of them.
Whatever we eventually learn about the neural basis of synesthesia, the phenomenon is another reminder that our brain has the capacity to create our individual perceptual reality, and points to the likelihood of myriad unique and wonderful varieties of human sensory experience.
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Post by: Nadia Fike
Read more:
1.Ward, J. (2013). Synesthesia. Annu. Rev. Psychol. 64, 49–75. doi: 10.1146/annurevpsych-113011-143840
2.Hupe, J-M, and Dojat, M. (2015). A critical review of the neuroimaging literature on synesthesia. Front. Hum. Neurosci.| https://doi.org/10.3389/fnhum.2015.00103
3. Jadauji, J.B. et al. (2012). Modulation of Olfactory Perception by Visual Cortex Stimulation. J. Neurosci. 32 (9) 3095-3100. https://doi.org/10.1523/JNEUROSCI.6022-11.2012