Did you taste the purple coming from that song? Synesthesia is a mind boggler.

Seeing sounds and tasting colors

By: Michael Spelman

Alright, alright, take a deep breath and get ready for a trip through what could be described as the brainchild of Hunter S. Thompson, Dr. Suess and Salvador Dali after they all got together for a party that would have put to shame the entire decade of the 1960s. 

But first some background. The concept of perception, be it taste, smell, sound, touch or sight, has been a prominent focus of philosophical experimentation since Plato and Aristotle first asked "Why?". In Plato's Allegory of the Cave, it is posited that the ideas we form from sensations we experience, our perceptions, are more important in generating "reality" than the things eliciting the sensation in the first place. Essentially, reality is based more more in how we perceive things, than the things themselves. But what happens when the mechanisms of our perception get mixed up? How would one's concept of reality be affected? I will delve into that soon, but first let's overview some of the mechanisms behind sensation and perception.

Chemoreception, Mechanoreception, and Photoreception:

Our five senses (seeing, hearing, smelling, feeling, and tasting) result from the stimulation of certain types of sensory receptors in our bodies. Mechanoreceptors in our skin and our inner ears are activated by signals such as pressure that cause a physical or mechanical change in sensory neurons. This physical change is communicated to the brain by way of an electrical signal and is perceived as touch or sound, respectively. Chemoreceptors on our tongues and in our nose communicate with the brain in a similar manner, but in response to the presence of some combination of chemicals. These chemical combinations result in the perception of various tastes and smells. My personal favorite sense, sight, comes from photons of light entering our eyes and being transduced, or changed, into an electrical signal that is communicated to our brains. This is such a gross oversimplification it hurts my Biopsychology bachelor's degree, but it will suffice for now.

Alright, so the question that should be on your minds at this point is "if all of our senses get to the brain by way of these electrical signals, then how are they perceived as different from each other?" The answer lies in the complexity of our brain. The image here shows a schematic representation of our thalamus, the brain region responsible for organizing incoming sensations and sending the information to the appropriate cortical region for further processing (Krettek and Price, 1977). 

Wait, what? What is a cortical region? The cortex is the outermost portion of our brains, essentially the part that anyone would see and say "oh, thats a brain!". Each portion of the cortex is responsible for a specific set of functions, as shown in the image below. For instance, the rear-most portion of the cortex is known as the occipital lobe and is responsible for interpreting visual inputs and telling our brains what we are seeing. There is a dedicated brain area for interpreting each of the five senses after being relayed by the thalamus. 

As you might be thinking, the incoming signals to the thalamus, and subsequently to the cortex are awfully close to one another. And since they are all electrical inputs, couldn't the brain occasionally make a mistake or let one signal go to the wrong area of the brain? Surprisingly, our brain is remarkably efficient in getting messages where they need to go and rarely makes mistakes (unlike the U.S. postal service, am I right?). However when things go wrong things can get real weird, real quick.

Synesthesia, LSD, and Contemporary Art 

When the brain makes mistakes in interpreting incoming signals that are not due to normal aging processes, psychological conditions such as behavioral and mood disorders, and schizophrenia can occur. However, it doesn't always result in such devastating conditions. Synesthesia is one such condition that could, in theory, actually be really friggin cool. 

Synesthesia is defined as the involuntary physical experience of a cross-modal association (Cytowic, 1995). In layman's terms, it means that experiencing one sense, simultaneously activates another sense. For instance, seeing sounds, or hearing colors, or tasting geometric shapes. It may seem like something out of a science fiction book, but it is actually a diagnosable psychological condition that is estimated to occur in 1 in 2,000 people (Martino and Marks, 2001). Though, synesthesia is not found in the Diagnostic and Statistics Manual of Mental Disorders because it does not generally interfere with daily life, and synesthetes generally enjoy their added sensory perceptions (Jensen, 2007). Martino and Marks described that synesthesia can be either strong or weak; strong synesthesia refers to the full experience of a sensation not related to the modality being experienced, whereas weak synesthesia encompasses such experiences as seeing colors when reading words or numbers. 

I know, I know, you're thinking to yourself "how could this possibly happen, and how can I get in on this action?" Well, as with any other psychological disorder, it is largely unknown how synesthesia comes about. According to Grossenbacher and Lovelace (2001), synesthesia is thought to have a genetic basis and is thought to be inherited according to X-linked dominant patterns. There are a number of theories on the physical basis of synesthesia. The one that seemed most reasonable and worth mentioning was that of Local Crossactivation (Hubbard and Ramachandran, 2005). Essentially, neurons existing in brain areas connecting the sensory input from a certain modality to the brain region that interprets it actually send unusual connections to brain areas involved in perceiving other senses: basically the brain of synesthesetes (people who have synesthesia) have extra wiring that usually is not present in the brain. A large amount of research has supported this theory using neuroimaging techniques such as diffusion-tensor imaging (DTI) and functional magnetic resonance imaging (fMRI).

It has also been hypothesized that there are three classifications of synesthetes. The first type of synesthete experiences strong synesthesia from an early age and is classified as a developmental synesthete. The second is considered an acquired synesthete, and the condition can arise later in life as a result of injury. Now for the part Mr. Thompson was an expert on. The third type of synesthete is an induced synesthete, and their experience of synesthesia results from the ingestion of hallucinogenic substances such as LSD or mescaline (by no means am I promoting the recreational use of such compounds, but as a result of my undergraduate studies I find the subject totally fascinating). 

Although it is likely the quickest way to induce synesthesia, drugs are far from the only thing that can evoke the experience of multiple senses at the same time. In fact, we experience synesthesia on a regular basis (to a much milder extent). Reading a fiction novel for example can stimulate our imagination so much so that we create a visual representation of the most minor details from the story. Advertisers and cinematographers regularly generate imagery that is meant to evoke and stimulate emotions. Music videos such as this Blockhead video, directly pair sound with visual effects to create a really awesome video (and this song/artist is just the bees knees overall).

My favorite part

Now for the REAL trippy philosophical stuff. As a Neurologist, Dr. Richard Cytowic has extensively studied perception and synesthesia, interviewing patients and documenting various case studies. From his experiences he has expanded on the hypothesis of "form constants". Such form constants are what Cytowic and Dr. Heinrich Kluver describe as the four basic types of hallucinatory constants. These include gratings and honeycombs, cobwebs, tunnels and cones, and spirals. 

What do these form constants have to do with synesthesia? Kluver proposed that the experience of form constants is due to some fundamental aspect of visual perception. Amazingly, the visual cortex is mapped in such a way that the perception of these shapes correlates to a sweeping activation of neurons responding to individual line orientations, similar to when we perceive a moving object! (more on visuospatial mapping here) Now get this. The likely reason that these hallucinatory forms are "constant" is due to the presence of the Golden Ratio in nature, and our brains innate propensity to process it. While only a theory, the golden ratio has been posited as the mathematical explanation of perceived beauty, and patterns in nature. So much so that some little-known artists such as Leonardo DaVinci and Salvador Dali prominently planned paintings around this golden ratio. The band Tool has even made this awesome song lyrically centered on it (and paired some cool imagery with it too). 

Now, if you'd please humor me while I do some philosophizing of my own. What if, because the incoming information from the outside world (such as a vibrating photon of light entering our eyes, or vibrating molecules of air on our skin or eardrums, or vibrating chemoreceptor channel enzymes in response to chemical binding) is so similar across modalities, it could be considered abnormal NOT to experience synesthesia? What if we were meant to experience more than one sense at a time, and synesthesia as a trait was evolutionarily selected for? Think about THAT.

Bibliography:

Cytowic, R.E. 1995. Synesthesia: Phenomenology and neuropsychology. PSYCHE 2(10).

Grossenbacher, P.G., and C.T. Lovelace. 2001. Mechanisms of synesthesia: cognitive and physiological constraints. TRENDS in Cognitive Sciences 5:36-41.

Hubbard, E.M., and V.S. Ramachandran. 2005. Neurocognitive mechanisms of synesthesia. Neuron 408:509-520.

Jensen, A. 2007. Synesthesia. Lethbridge Undergraduate Research Journal 2(1).

Krettek, J. E. and Price, J. L. (1977), The cortical projections of the mediodorsal nucleus and adjacent thalamic nuclei in the rat. J. Comp. Neurol. 171:157–191.

Martino, G., and L.E. Marks. 2001. Synesthesia: Strong and weak. Current Directions in Psychological Science 10:61-65.

Parts of the brain thalamus of anatomy. (2012). Retrieved from http://www.rudyard.org/parts-of-the-brain-thalamus/

Sanderson, K. J. (1971), The projection of the visual field to the lateral geniculate and medial interlaminar nuclei in the cat. J. Comp. Neurol. 143:101–117. 

Ramachandran, V.S., and E.M. Hubbard. 2003. Hearing colors, tasting shapes. Scientific American 288:52-59.