By Scott Jon Shagin
I consume neuroscience, I don’t produce it. So when asked by a neuroscientist to consider which of the five major senses is most interesting, I was intrigued. Sight interests me the most. Watching each of my five major senses fail, each in its own way—to borrow from Tolstoy—gives me a ringside seat at this event. Faulty wiring of touch, taste, smell, hearing and visual systems is my lot. Progressive multiple sclerosis affords me an opportunity to experience the damage, impairment, plasticity, rehabilitation and absence of parts of my central nervous system. This requires me to place a value on my major senses and to prioritize attempts to keep them afloat.
“Interesting” is a loaded word in science, especially so in my case. Interesting because we know so much and can build on this edifice? Interesting because we know so little and the harvest appears more plentiful? Interesting because the ignorance that spawns the scientific investigation will result in more questions, more ignorance, and potentially interesting results? Of course, I mean ignorance in a fertile way; a wellspring of questions to seed creative scientific minds and fuel high value hypotheses. Or interesting because breakthroughs in the offing will soon lead to improvements in the lives of interested parties? Or interesting in that well conceived research projects in a given area are more likely to be funded, justifying an allocation of more resources and human endeavor? Has the creative quest to connect myriad and pressing causal neuroscientific dots been replaced by the reallocation of resources to connectomics, the mapping of the brain?
Consider a thought experiment. Which sense, from a neuroscience perspective, is most likely to help us survive, adapt and best equip us to live; the one it would be most difficult for the species to live autonomously without?
Touch is important, and if it includes thermoreceptors, nociception and proprioception, its absence would make survival for the species difficult. Touch allows me to put wine glass to lips, pen to paper and razor to skin without harm. Touch allows me to feel heat, pain and experience early warnings of tissue damage. My somatosensory system lets me walk without veering off course like a ship about to run aground. I have broken my share of wine glasses while unable to feel the stem or rim, lost and regained my ability to write, lost and regained my ability to walk without falling over, and cut my face while shaving with numb fingers. My thermoreceptor pathways sometimes fail me, at low cost, and there are times when no nociception would be a benefit, as anyone who has ever experienced trigeminal neuralgia can attest.
But vision helps me to compensate, helps me to remain independent, and helps me to continue to feel a member in good standing of the species. I adapt. Watching the glass more carefully while it is picked up, handled and used is one of many ways in which more conscious effort and the use of vision substitutes for loss of touch. Computer keyboard and screen provide a visual feedback loop that substitutes for numbness; something I appreciate as I type this. Razors and other sharp objects can be used more safely with increased visual attention substituted for touch. Heat, pain and pressure offer added risks, but visual attention frequently comes to the rescue. Visual cues can substitute for somatosensory pathway deficits; just watch the eyes of ballerinas or figure skaters focus on a fixed point as they spin.
Consider hearing. The world would be a lesser place without music and the sounds of love, joy and warnings of all sorts. But a silent world would not be uninhabitable and deaf humans would probably still be at the top of the primate hierarchy. Visual alerts such as flashing lights now regularly supplement auditory ones. I suffer from diminished hearing, tinnitus and something which—for lack of a better term—I’ll call rolling auditory brownouts; the loss of hearing briefly and sequentially first in my left ear, then in my right ear. All of these things pose problems, even interesting questions for the neuroscientist. But solving them is not as important as keeping my vision intact.
What about taste? For those with gustatory inclinations, loss of taste would be a hardship. Our species now exists largely beyond the need to taste food in order to determine if it is noxious. My diet would appall many. But it serves a trophic purpose and I have learned to eat to live, not live to eat. Our ancestors did this with good results. I can tell if something is on my diet just by looking at it or by reading the food labeling. Our species can survive without taste.
Taste offers some complications for those in search of “interesting.” The relationship between taste and smell is almost indistinguishable at times. Retronasal and orthonasal smell help define flavor. The floral, wine and perfume industries are built at the intersection of these senses. Does that make them neuroscientifically interesting?
One taste related nuance presents interesting questions and useful work to be done. Traditional thinking about the tongue, its function and taste was simplistic. The tongue contains taste buds that allowed us to experience taste as the food moved through our mouths and into the alimentary canal. The tongue was also critical to the development of speech. Simple enough, but it gets more interesting.
In some riveting work done in the late Paul Bach Y Rita’s lab in Madison, Wisconsin, neuroscientists have noted that the tongue is a portal to the brain. Different areas of the tongue can be mildly stimulated with electrical current to activate different neural circuits in the brain. The tongue’s connections to the brain can be mapped. This knowledge has been harnessed to therapeutic effect for people with central nervous system damage. The potential impact of this type of neuromodulation therapy is great. The questions it raises are also quite interesting. Why is the tongue so sensitive and so wired to the rest of the brain? Does the neuromodulation trigger synaptic plasticity? If so, how discretely can it be differentiated, targeted and harnessed? Can this technique be refined to substitute for invasive brain surgery? Can such stimulation extend and enhance the productive life of brains? What are the risks?
And smell? Some neuroscientists argue convincingly that there are very interesting reasons to pursue the study of olfaction. Maybe it will open a door to neurogenesis, something I—with a neurodegenerative disease--have a vested interest in seeing hastened. But the species would survive without olfaction, though the world would be a subjectively different place. In fact, my mind has taken olfaction to an extreme. I have cacosmia; olfactory hallucination. Always cigarette smoke, frequently but not always when I am tired and never when I first wake up. Since I don’t smoke and despise the smell, and since smoking is contraindicated not just for all humanity, but especially for those with MS, I stay as far away from cigarette smoke as possible. Yet this olfactory hallucination plagues me. Like my tinnitus, no external stimulus causes the sensation. I can look around and see that no one is smoking so I know it is a hallucination, but since the sensation is noxious I would be happy to have this distorted olfactory percept turn itself off.
Finally, return to sight. People survive and learn how to live without sight. But most who do so lose their independence. Neuroscientists could probably run a lab with one of the other senses impaired, but loss of sight would likely put them out of business. The loss of sight is one of my biggest MS concerns, and surviving it productively is something that I have thought about. There is a reason why over 12,000 neuroscientists have chosen to work on vision. Unlike the yarn about the scientist found crawling around in the dark under the street light looking for the car keys he lost elsewhere, it is not just because the light is brighter in vision research. Humans, with few exceptions, need sight to be productive. Perhaps that is the reason why the FDA first approved the use of the technology developed in the Bach Y Rita lab for a device that projects visual images through the tongue to the brain.
My neuro-ophthalmologist says my vision is “undergoing change.” I live in interesting times.
Sources
“Anna Karenina,” Leo Tolstoy, Translated by Marian Schwartz, Yale University Press, Margellos, 2014.
“McAlpine’s Multiple Sclerosis,” 4th Ed., Churchill Livingstone Elsevier, 2006.
“Ignorance: How It Drives Science,” Stuart Firestein, Oxford University Press, 2012.
“Failure: Why Science Is So Successful,” Stuart Firestein, Oxford University Press, 2016.
“Connectome: How The Brain’s Wiring Makes Us Who We Are,” Sebastian Seung, Houghton Mifflin Harcourt, 2012.
“Neurogastronomy: How The Brain Creates Flavor and Why It Matters,” Gordon M. Shepherd, Columbia University Press, 2012.
"The Secret of Scent: Adventures in Perfume and the Science of Smell," Luca Turin, HarperCollins, 2006.
"The Brain That Changes Itself," Norman Doidge, M.D., Viking, 2007.
"The Brain's Way of Healing," Norman Doidge, M.D., Viking, 2015.
