Our Quest to Understand the Human Brain Is Limited by Ethics, Not Science

Recording spikes from neurons deep in the brain is, by its very nature, invasive.

Adapted from The Spike: An Epic Journey Through the Brain in 2.1 Seconds ©2021 Mark Humphries, reprinted with permission from Princeton University Press.

We are facing a hard limit to how well we can understand the human brain. Not a limit created by the impassable boundaries of the physical world, like how the size of the observable universe is defined by the distance that light could have travelled since the Big Bang. Nor is it a limit of our technological prowess that we can aspire to overcome, like the challenge of developing near-weightless spacecraft that can travel at one-fifth of the speed of light to fulfill the dream of sending probes to Alpha Centauri within our lifetime. Rather, it is a limit we give ourselves, set by the ethical boundaries of what we will permit to be done to another human.

To understand brains, we record tiny pulses of electricity, the spikes sent from one neuron to another. Those spikes are you moving your eyes to read this text, cocking an eyebrow at the content therein; you sensing the gentle breeze and the waning sun on the skin; your vivid memory of strolling the streets of a beloved city. Spikes are the brain’s own language, so capturing them promises a profound understanding of how brains work.

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Vast public research programs, including the U.S.’s BRAIN Initiative and the EU’s Human Brain Project, are throwing billions of dollars at recording and understanding the signals passed between neurons. But these programs are fundamentally limited. They cannot ever arrive at that understanding of us because they can never do what matters most: record from the individual neurons of a healthy human brain.

Recording spikes from neurons deep in the brain must be, by its very nature, invasive. Electrodes are inserted deep into tissue; fiber optics or miniature microscopes are put into the brain to capture the light coming from neurons that are genetically engineered to fluoresce when they send spikes. We cannot invasively record from nor genetically engineer healthy humans. So the capturing of spikes is focused on the animal models—mice, flies, zebrafish—in which that capture is easiest, in part because those animals are also the workhorses of genetics. And so we know the spikes in these animals that mean place and time, “I am here” and “how long has it been?”; the spikes for holding things in mind, the short-term memory of something that just happened to them; the spikes that add up evidence towards a decision they are about to make; the spikes that encode the simplest elements of their perception—the edges they see, the pitches they hear, the touches they feel, the smells they inhale. But we have none of these spikes from humans.

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You might ask why we can’t just go ahead and invasively record from our own brains—why can’t we just give informed consent? Even for the promise of knowledge, it is difficult to contemplate recording neurons from within a healthy human brain. The risks of brain surgery are significant: Hemorrhage and infection are possible in any surgery; operating on the brain adds the risks of permanent brain damage or stroke. There is our hard ethical limit on understanding the human brain: that the risks of invasive recordings outweigh the unknown gains. This limit has been in place for decades, defining how we do science on the brain. But, unexpectedly, we will soon be asked if we want to break that limit.

Companies like Neuralink are ambitiously intent on this work. Their immediate goal is to make it possible to better record spikes—more, cleaner, and for longer—in cases where invasive intervention is already taking place, like the interfaces for restoring arm movement or walking in paralyzed patients. Their ultimate goal, though, is invading the healthy human brain; in Neuralink’s case, with a two-way interface between your brain and your computing devices, your phones, tablets, and laptops. As its website proclaims, “Neuralink’s long-term vision is to create BMIs [brain-machine interfaces] that are sufficiently safe and powerful that healthy individuals would want to have them.”

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We don’t know how companies building invasive devices to record the spikes from many single neurons are planning to deal with the ethical barrier of implanting them in healthy human brains; perhaps they don’t know, either. But it is a possibility we have to weigh seriously, for we know the currently unthinkable can become reality. After all, plastic surgery on perfectly healthy people, which carries some of these same risks, is now routine.

We already have tantalizing glimpses of what recording spikes in the human brain would tell us. The risks are far more bearable in surgeries to implant electrodes for treatment, not research—to allow a paralyzed patient to move their arm, or to help alleviate seizures, or to control the tremors of a Parkinson’s patient. Moving an arm is driven by spikes from the motor cortex translated, by machine learning, into patterns of stimulation of the muscles. Severe seizures are treated by surgical removal of their source, a group of neurons whose out-of-control spikes are detected by temporarily implanted electrodes. Tremors are treated by continually passing pulses of current at a high rate deep into the brain through the implanted electrode, an electrode from which spikes can only be recorded before the stimulating current is switched on. Research can piggy-back on the treatments, but it is highly limited, to just a handful or tens of neurons, in regions of the brain prescribed by the treatment, for short periods of time.

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With those glimpses to tempt us, crossing that final ethical line of invasively recording from the healthy human brain could be borne simply out of frustration, frustration with how we currently deal with the hard limit of understanding the spikes of the human brain.

Imaging the human brain is not the answer. fMRI is a wonderful technology, the best way we have of peering into the living human brain. But fMRI shows us bloodflow, not the brain’s electrical activity itself. And its scale is far less precise. Each tiny dot in its images is a cube containing at least 100,000 neurons. The spikes themselves, millions of them per second in each cube, are invisible.

We could tell ourselves that we can learn everything we want to know from non-human brains. Yet there are so many uniquely human things our minds can do—speech and its comprehension, reading and writing, the creativity of science and math, of painting and sculpture, our complex social webs, and our subjective experience—that this seems vanishingly unlikely.

Instead we could be reconciled to the hard limit, accepting that there are things we can never understand about our brains, and that’s OK. This is the pragmatist’s response, for so long as we can continue to improve how we treat the brain, aiding those with paralysis or those with mental disorders, and pursue cures where possible, then perhaps we can accept a limit on our understanding.

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Reconciliation is the realist’s response too: There’s a fair chance we’ll never truly understand even the simplest of brains, so why should we be heartbroken over ours? The knowledge we already have is overwhelming, with such a glut of data on the brains of worms, flies, fish, and mice that we are struggling to make sense of it. While this means the vast public funding of neuroscience is recouping its investment, that glut will only get worse. The rate at which we’re able to record single neurons is accelerating: A team from the Rockefeller University in New York demonstrated a recording of 1 million neurons at the same time in February 2021. Realists would argue that, paradoxically, as these data reveal ever more nuances of how brains work, the prospect of understanding even the simplest of brains is receding. To them there is little point, either pratically or scientifically, in recording spikes from the healthy human brain in the near future, because we are too far from being able to understand them, and we have so much to learn before then.

But these stances will not satisfy everyone—some will be uncomfortable separating treatment from understanding, others will refuse to accept there are things we can never understand about our own brains. And say new implantable devices from private companies prove their worth as prosthetics or treatments in the next decade, able to record far more neurons, for months or years at a time, without damaging the tissue or risking infection, from any region of the brain we desire. As the risks reduce and the research possibilities open up, then it is easy to imagine how we could slip, unnoticing, from thinking the recording of neurons in a healthy human brain is unimaginable to thinking it is something that needs doing to further our understanding of ourselves.

Sooner than we thought, we will face a deep ethical challenge, one that will force us to decide: How badly do we want to understand the human brain?


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