Turtles All The Way Down

There is an old anecdote of a respected philosopher giving a public lecture on astronomy, in which he explains the data showing that the Earth orbits the Sun, and the Sun in turn, revolves around the center of the Milky Way galaxy. After the lecture, the lore goes, an old lady tells the philosopher that his ideas are nonsense. “The world stands on the back of a giant turtle,” she says. Amused, the philosopher — in some tellings it’s William James, in others Bertrand Russell — asks the lady what the turtle is standing on. Unfazed by the man’s smug question, the woman replies, “why, it’s turtles all the way down.” 

This infinite regress parallels the way that the mind has been depicted in popular culture - with a tendency to explain its workings, even when mechanistically, in terms of other minds. 

Despite evidence from neuroscience and complexity theory that the mind is a product of billions of mindless neurons, the nonsensical portrayal of the mind as a collection of other minds persists. 

In neuroscience, the problem comes in the form of the homunculus, a metaphorical person lodged deep in our brains. The homunculus observes incoming senses, thinks about what they mean, and decides what actions to make. As Fred Rieke, David Warland,  Rob de Ruyter van Steveninck, and William Bialek wrote in their book on neural data analysis, Spikes, this homunculus “observes the responses of his own sensory neurons and finally forms the percepts that the organism experiences.” 

The metaphor was made a bit more mechanical in Woody Allen’s 1972 film, Everything You Always Wanted to Know About Sex, where a man’s brain is depicted as a Mission Control Center, with specialized (human) workers for individual cognitive functions. While the protagonist is enjoying dinner with his date, the staffers in his brain work diligently to make sure the date goes well. As we get a glimpse of Mission Control, one worker takes a call from a department called Motor Function 12. He listens and relays the information to another worker (played by a laid-back Burt Reynolds):

“His leg itches.”  

Reynolds then calls another department:

“Scratch the, ugh, left leg”   

“The leg has been scratched,” someone else responds. 

So goes the important work of Mission Control, relaying information from one body part to another, asking the eyes for visual input, telling the stomach to get ready for fettuccini (“Jesus Christ!,” the stomach operator responds), and so on. To be sure, the brain does relay and process information in such mechanical ways; the chief difference between the movie portrayal and the real thing is that the latter has no human personalities inside. 


While the film’s depiction of the brain seems more technical than a homunculus, the explanations for how the brain actually does anything are still missing. “The problem with this picture is that it never gets to the essence of what it means to perceive and to experience the world,” Rieke and company wrote. ”On the other hand, as explorers of the nervous system we place ourselves, inevitably, in the position of the homunculus - we observe the responses of sensory neurons and try to decide what these responses could mean to the organism.” Whether in pop culture or in the lab, humans have a hard time thinking of the brain without anthropomorphizing its parts or functions. Perhaps the issue comes from the limits of our imaginations, or the incomprehensibility of nothingness. How could interactions of billions of mindless elements together produce something so seemingly irreducible as the mind? What would life be like without consciousness?

The reality of the mind is that its Mission Control is staffed by billions of mindless neurons which act as nodes of computation by taking in inputs through their dendrites, performing calculations on them, and passing on the results out through their axons. The currency of these inputs and outputs is millisecond-long fluctuations in the neurons’ voltage called action potentials. The coordination of the activity of these billions of neurons is what creates the mind.

The fact that neurons are mindless isn’t to say that they’re stupid; on the contrary, individual neurons can perform extraordinarily complex computations, and networks produce wonderfully complex patterns of activity. Their mindlessness just means that neurons don’t know or care about the outside world, unlike the people in Woody Allen’s Mission Control. As soon as the outside world begins to interact with our bodies, be it light hitting the retina, or chemicals binding to olfactory receptors, it is converted to electrochemical messages that neurons operate on. 

Understanding these messages, which are often referred to as the neural code, is one of the central aims of neuroscience. In my own work, I observe the spiking activity of neurons in visual cortex of rats as the animals look at various images on a monitor. One of the questions I seek to answer is how the visual scene is encoded in the patterns of spikes. In that capacity, I treat the population of neurons I observe as a homunculus that only speaks the language of spikes. 

My job is to translate from spikes to the visual language of images. When successful, this approach allows one to use the spiking patterns to predict what image the animal was looking at. This is achieved using linear algebra and statistics (“machine learning”), and is done not only for a cool mind-reading effect, but also to ask what kind of code a hypothetical downstream neuron might use to read out the information in these spike patterns. Asking what information is present in a given brain region, and how it gets transformed and passed on to other regions is central to the work of many neuroscientists today


More than forty years after Allen’s Mission Control depiction of the brain, the homunculus is alive and well, most recently in the Pixar film Inside Out, where a girl’s mind is charmingly portrayed as a group of five human-like emotions. 

The persistence of the homunculus, despite its absurdity, shows how strong our need to anthropomorphize is. There just isn’t anything it’s like to be a neuron or a brain region. And this is perhaps one of the most wondrous aspects of consciousness: it is a property that emerges seemingly out of nothing. 

When it comes to portraying the inner workings of a conscious being, just like the old lady debating the philosopher on the Solar System, we can’t imagine nothingness. When looking into the brain, we’ve been saying: it’s turtles all the way down.