Charles 'Chuck' Stevens (photo: InSight Foto Inc., 2011)

Charles Stevens, a preeminent neurobiologist who revealed fundamental architectures in the brain and whose experimental techniques paved the way for decades of molecular neuroscience, passed away on October 21, 2022, in San Diego, CA. 

At the time of his passing Stevens, 88, was a distinguished professor emeritus at the Salk Institute for Biological Studies and a fellow of the Santa Fe Institute’s Science Board and External Faculty. He was also a long-time member of the National Academy of Sciences and the American Academy of Arts and Sciences (AAAS).

"Chuck had a procedural curiosity like almost no one I have ever met before," says SFI President David Krakauer. "Rather than read about a topic from outside of his orbit he would dedicate a year to applying distant ideas and methods to a data set from his own work. This is not finding and mastering a method that seems like a good fit to a problem but finding a fit because an idea feels so deep that it needs to be learned. And what better way to do this than by leveraging the experimental system over which he had a demonstrated mastery — the brain. And there was almost no area of inquiry that might not illuminate his subject, from feed-back control to information theory, compressed sensing to statistical mechanics."

When he first joined SFI in 1996 as a member of the Science Steering Committee, Stevens was revered in neuroscience circles for his foundational work in synaptic transmission. He had developed experimental techniques to analyze how neurons conduct electric currents across single acetylcholine ion channels. 

A consummate man of science, and a mentor throughout his career, Stevens shared his methods with research collaborators and mentees. His postdoctoral fellow at Yale University, Erwin Neher, further developed Stevens’ techniques and went on to co-create the patch clamp method for electrophysiology, for which Neher and Bert Sakmann shared the 1991 Nobel Prize in Physiology or Medicine. The ion patch clamp has since been widely used in electrophysiology for recording electrical currents at the cellular level.

Among Stevens' many distinguished mentees is SFI External Professor Vijay Balasubramanian (University of Pennsylvania), who credits Stevens with helping him move from physics into computational neuroscience. “He mentored so many people,” says Balasubramanian. "He was just a generous soul. And he never tried to take credit. Chuck always had a focus on the intellectual depth and content of the things he thought about. That’s what he really cared about."

In addition to training many neuroscientists who went on to become leaders in the field, Stevens was also highly respected by his contemporaries and his seniors. Balasubramanian gives the example of David Hubel, who shared a Nobel prize with Torsten Wiesel in 1981 for their work on the brain’s visual processing system. “David Hubel had enormous respect for Chuck,” Balasubramanian recalls. "I think Chuck was recognized as being one of the smartest people around. He really had a reputation for being completely brilliant."

Like many SFI scientists, Stevens’ work bridged physics and biology. He spent time at the Aspen Center for Physics, often planning his visits to coincide with Balasubramanian’s, and wrote a book called “The Six Core Theories of Modern Physics” (MIT Press, 1995), which Krakauer describes as an "extreme example" of his desire to integrate techniques from physics into his neuroscience research. "I am convinced Chuck would have loved to apply special relativity and quantum field theory to the brain," Krakauer says. "In lieu of finding compelling case studies — Chuck was far too much of an empiricist to write theory papers without experimental support — he wrapped up his yearly reading and research projects with a delightful theoretical physics monograph."

Moreover, Stevens excelled as both an empiricist and a theorist, and made great strides toward understanding the fundamental architectures of the brain across a variety of animal species. During his summers at SFI, he conversed with Geoffrey West, SFI Distinguished Shannan Professor, on the subject of scaling laws — the physical, quantifiable constraints that govern biological traits across species, that often give rise to predictable ratios between, e.g., body mass and metabolic rate. 

"He became intrigued by our work on scaling laws and we spent quite a bit of time struggling with what we might learn about the brain, and even cognition, by extending the ideas to neurological systems," says West. "He was one of my favorite people for talking science and for getting feedback on almost any issue."

Stevens and his colleagues at the Salk Institute went on to elucidate several fundamental scaling laws for the brain that revealed how brain structures can grow, adding more neurons, without having to re-organize. They found that a 2/3 ratio, for example, predicts, in goldfish, the area over which each retinal cell collects information from a visual scene, and the size of the retina itself. The ratio holds steady as the fish grow and generate more retinal cells that transmit visual information to their brains. Beyond goldfish, the same 2/3 ratio governs how the number of neurons in an animal’s brain increases with the size of its body. 

"In order for evolution to work, neural circuits have to have what the computer scientists call a scalable architecture," Stevens told PNAS in a 2012 interview. "That means that you have to be able to make the computer more powerful just by making it bigger — you don’t have the luxury of redesigning it; and so the question that I’m asking is: What are the design principles that brains use to give their circuits a scalable architecture?"

Stevens shared this work with colleagues and curious laypeople alike. A recording of his 2012 SFI Community Lecture, for the Santa Fe public, reveals his "old-school" presentation style, for which he was known and loved.

West writes: "In addition to his science, [Chuck] will be remembered as the last (and maybe the only) person who gave seminars without fancy slides, but drawing figures and writing equations in real time in his spidery handwriting on a computer which was projected on to a screen. And many of those figures were of the organization and structure of the frontal cortex, the cerebellum, or some sophisticated neural network associated with vision. And not just in seminars, but also colloquia and even in public lectures! Eccentrically charming, but it meant the speed of his talk was perfect for comprehension; alas no more."

"Chuck had a rather unique approach to giving talks and bridging the digital–analog divide," recalls Krakauer." Chuck loved to spend at least the first half of his talks drawing nerve cells. Over the years in seminars at the Santa Fe Institute, we would find Chuck's recapitulations of the anatomy of soma, axon, and dendrite rather amusing since these elementary facts invariably ate into the limited time required for the technical exposition that followed. I am convinced that Chuck did this to showcase his latest gadget which was always some kind of tablet that substituted for a blackboard. Every year the tablet improved and he showcased more of its advanced features. But always in the service of the same drawings. Given the depth of Chuck's ideas I think back to these meandering sketches in the foothills of thought as a welcome warm-up."

Many SFI colleagues recall Stevens’ remarkable work ethic. He was known for his 10–12-hour workdays during his summer visits and, simultaneously, his willingness to engage with everyone he met around the institute. He often enjoyed a glass of wine with his lunch, and Balasubramanian recounts that Stevens spent many lunch breaks with his wife, Jane Stevens (née Jane Robinson), with whom he had three beloved daughters. 

"Chuck and I were really in different fields, but his ability to translate difficult concepts into simple terms almost made me think we were in the same field,” says SFI Science Board Member Simon Levin (Princeton University). "His writings were elegant, deep, and pedagogical, and I learned much from them. He was a giant in his field, and I always enjoyed our interactions at Santa Fe. He will be missed."

Read the Salk Institute obituary 

Read the Retrospective in PNAS (February 21, 2023)