Garbage Smells Green and Gunshots Are Rainbows

The minds of individuals are like parallel universes, forever inaccessible to one another.  Never do we truly see through the eyes of another person.  It is common for us to wonder if other people experience the world in the same way we do.  Is your green my red?  Is my yellow your blue?

While we can never directly answer these questions, we know that some people report a very different consciousness of the outside world than what is commonly experienced.  This different conscious experience is known as synesthesia, as it is a seamless blending of sound with color, tastes with words, color with letters, or even space with numbers.  While it is normal for individuals to integrate different senses through what is known as multimodal integration — an important process for perceptual tasks such as lip reading — synesthesia is a lower level blending of different sensory and cognitive pathways such that one automatically evokes the other.  Synesthesia is a lifelong ability and often considered a gift, allowing one to perceive the world through a truly unique lens.

Recently, Knowing Neurons interviewed three individuals with synesthesia.  Listen to their stories in the video below!


Although reports of synesthesia began at least as early as the 19th century with scientists such as Francis Galton, research on synesthesia did not begin in earnest until the very late 20th century.  Synesthesia research is important because it gives us deep insight into brain development and the nature of human perception.  Being one of the most common forms of synesthesia, color-grapheme synesthesia is perhaps the most studied form of this phenomenon.

How do we know that synesthetes are not “faking” this ability?  Could synesthesia be a huge lie invented for attention, amusement, or some other purpose?  Probably not. The abilities of synethetes can be evaluated through test-retest reliability.  When asked to select the color evoked by stimuli, synesthetes will reliably chose the exact same color about 90% of the time.  This performance can be obtained even when years pass between the first and second testing.  Non-synesthetes perform much worse on this task, choosing the same color about 30-40% of the time.

Neuroimaging experiments provide evidence for neural differences between synesthetes and non-synesthetes, and even predict the type of synesthesia experienced by synethetes.  Using a brain imaging technology known as diffusion tensor imaging, or DTI, neuroscientists have demonstrated increased wiring between parts of the brain which process colors and letters in color-grapheme synesthetes.  DTI uses diffusion of water molecules to trace the white matter of the brain formed by axon tracts.  Researchers found that the strength of cross-wiring in the inferior temporal cortex predicts whether synethetes merely perceive the colors in their mind’s eye — so called associators — or actually see the colors on the letter or number they are looking at — so called projectors — with projectors exhibiting stronger cross-wiring than associators.  Most importantly, this study provides evidence for the theory that cross-activation between anatomically adjacent brain regions is at least partially responsible for color-graphemes synethesia.  It remains possible that other forms of synesethesia are supported by other mechanisms.

While neuroscience is slowly unmasking the mysteries of synesthesia, there is still much for us to understand regarding this incredible phenomenon.  What developmental processes allow some individuals to have synethesia and not others?  Do all children begin life as synethetes, or is synethesia an ability that is gained?  How do rarer forms of synethesia, which cannot be explained by cross-activation between adjacent anatomical brain regions, arise?

And what if we are thinking about synesethesia in completely the wrong way?  What if synethesia has little to do with sensation, but rather semantics?  A recent school of thought which seeks to call synesthesia “ideasthesia” argues that this phenomenon has more to do with the meaning of stimuli and less to do with their sensory properties.  For instance, for graphemes that appear similar such as the number 5 and the letter S, the color evoked by the stimulus may be context dependent in color-graphemes synesthesia … or should I say, ideasthesia.  Another example of ideasthesia is individuals who involuntary experience numbers as appearing mapped onto geometrical space in what are known as “number forms.”  Similarly, individuals with time-space ideasethesia experience months or years as having geometric shapes.  Such abstract forms of synesethesia cannot easily be explained in terms of low level sensory properties of the triggering stimuli.

Whether we choose to call it synethesia or ideasthesia, neuroscience has revealed that the remarkable experiences described by synethetes have a real, neural basis.  It’s easy to imagine how claims of hearing colors and seeing sounds could have easily been dismissed in earlier times.  Perhaps synesethesia is a reminder that when we read or hear about someone’s incredible private experiences, we should be careful not to dismiss such claims until science has had the opportunity to investigate.


Special thanks to Michael, Sylvia, and Julia for being a part of our film!

Hosted and Edited by Joel Frohlich.

Camera work by Kate Fehlhaber.

Animations by David Jaimes.

Audio work by Anosh Kermani.

Image by Jooyeun Lee.

Don’t forget to subscribe to the Knowing Neurons YouTube channel!



Brang, David, et al. “Magnetoencephalography reveals early activation of V4 in grapheme-color synesthesia.” Neuroimage 53.1 (2010): 268-274.

Cytowic, Richard E. (2002). Synesthesia: A Union of the Senses (2nd edition). Cambridge, Massachusetts: MIT Press. ISBN 0-262-03296-1. OCLC 49395033.

Jürgens, Uta Maria, and D. Nikolic. “Synaesthesia as an Ideasthesia—cognitive implications.” Synaesthesia and Children—Learning and Creativity (2014).

Mroczko-Wasowicz, Aleksandra, and Danko Nikolic. “Semantic mechanisms may be responsible for developing synesthesia.” Developing Synaesthesia (2015).

Rouw, Romke, and H. Steven Scholte. “Increased structural connectivity in grapheme-color synesthesia.” Nature neuroscience 10.6 (2007): 792-797.

Joel Frohlich

Joel Frohlich is a postdoc studying consciousness in the lab of Martin Monti at UCLA. He is interested in using brain activity recorded with EEG to infer when a person is conscious. Joel earned his PhD from UCLA in 2018 studying EEG markers of neurodevelopmental disorders in the lab of Shafali Jeste. You can also check out Joel's blog Consciousness, Self-Organization, and Neuroscience on Psychology Today. For more about Joel's research and writing, please visit Joel's website at

One thought on “Garbage Smells Green and Gunshots Are Rainbows

  • September 16, 2016 at 12:15 am

    “The minds of individuals are like parallel universes, forever inaccessible to one another.”

    This is not entirely true. Else, why write the article? The article is a glimpse into your mind as you have glimpses into other minds. The fact is that we compare notes on experience all the time and thus have a pretty good idea what people’s minds are like. Of course we cannot *be* another person, other minds are certainly inaccessible in *that* sense; but we can certainly know other people intimately. We are all (potentially) accessible to each other in significant ways. If we were not, for example, we could not communicate or cooperate. And we seem to both tasks fairly well.

    Imagine what drama or literature would be like if the minds of others were “forever inaccessible to one another”. No drama would make sense, because we would have no conception of the motivations of the actors. No desires or aversions would be communicated to us. There could be no society, no morality, etc.

    I wish we could move on from this inaccurate unhelpful characterisation of the mind.

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