Dogmas of neurobiology + Neuroscientists are baffled by representational drift

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We could be wrong about how the brain works

EXCERPTS: . . . The scientist looked around before answering, as if to make sure no one else could hear. “Well,” he said, almost apologetically, “I have sometimes seen waves of excitation slowly propagate across the brain in patterns that do not correspond to any known axonal pathways.”

I paused to consider what I’d just heard. The researcher had described something that wasn’t supposed to happen, long-distance transmission of neural excitation, not through axons (the equivalent of wires in electronic circuits), but by some other process. I asked, “You mean, spread by volume conduction?”

All I got for an answer was an amused “Who knows?” shrug.

I was about to ask why he hadn’t published his strange discovery or followed up with further research, but stopped myself when I realized the answer: He didn’t want to jeopardize his career by being labeled a crank for contradicting well established neuroscience “truths,” and, in any case, would never have gotten funding to pursue the outlandish idea that long-distance communication in the brain could occur outside of axons.

The sad truth is that [...] Although many neurobiologists (myself included) have stumbled across unexplainable phenomena at some point in their careers, few pursue their strange discoveries out of well-placed concern that such pursuits would end their careers.

Nevertheless, hints that the brain might work in ways not contemplated by “normal” neuroscience have emerged over the years. Here are a few examples.

Conventional wisdom has it that neurons transmit information to one another in one of two ways [...] Further, spread of this electrical excitation is believed to occur within individual neurons over short distances...

However, the findings of my colleague in Aspen suggest that the brain may have other ways of communicating within itself that arise from viewing the brain not as a collection of individual nerve cells and non-neuronal glia (e.g. nerve insulators), but, to put it crudely, as a bucket of gelatinous saltwater. In a bucket of salty Jell-o, an electrical current source in one region of the bucket will spread to other regions of the bucket not through dendrites or axons, but through three-dimensional ionic (dissolved salt) conduction channels...

[...] there is mounting evidence that magnetic fields, all by themselves, do play a role in neural information processing. A wide variety of animals, such as birds, are known to use the Earth’s magnetic field for long-distance navigation, possibly through nano-crystals of iron compounds that form in some neurons. Shinsuke Shimojo and colleagues at Cal Tech have recently shown that human brains do directly respond to weak magnetic fields, even though humans are not consciously aware of these responses.

Thus, as with volume-conducted EEG signals, changes in magnetic fields, generated both inside and outside the brain, conceivably could be used by the brain to create, process, or retrieve information.

Biologists have long dismissed the possibility that weird quantum mechanical effects [...] occur in the brain, or any other biological tissue. The reason is that living tissues are far too “hot and wet” to allow ultra-fragile quantum effects to occur in anything close to a stable, useful fashion. And yet, functionally significant quantum mechanical effects have been observed in some kinds of biological processes...

Peter Jedlicka of Goethe University in Germany points out that there is now good evidence for “non-trivial” [...] quantum mechanical effects in generation of neural signals in the retina through photopigment response to quantal photons of light, and in the olfactory system, where quantum mechanical effects might play a role in odor discriminations.

[...] I’m confident that, although we don't really know all of the physics the brain itself uses to do its work, most neuroscientists would dismiss volume conduction and magnetic and quantum phenomena as trivial and unimportant side effects of brain function. And, of course, such skepticism might be well placed.

But it might also be the case that those scientists, like all of us, form such opinions from cognitive biases that lead us—unconsciously—to believe only what we expect to be true and want to be true. [...] The irony of all ironies here would be that the biggest obstacle standing in the way of our ultimate understanding of the brain could be the biases of the brains seeking to understand themselves... (MORE - missing details)

Neuroscientists Have Discovered a Phenomenon That They Can’t Explain

EXCERPTS: Carl Schoonover and Andrew Fink are confused. As neuroscientists, they know that the brain must be flexible but not too flexible. It must rewire itself in the face of new experiences, but must also consistently represent the features of the external world. How? The relatively simple explanation found in neuroscience textbooks is that specific groups of neurons reliably fire when their owner smells a rose, sees a sunset, or hears a bell. These representations—these patterns of neural firing—presumably stay the same from one moment to the next. But as Schoonover, Fink, and others have found, they sometimes don’t. They change—and to a confusing and unexpected extent.

[...] This is, of course, just one study, of one brain region, in mice. But other scientists have shown that the same phenomenon, called representational drift, occurs in a variety of brain regions besides the piriform cortex. Its existence is clear; everything else is a mystery. Schoonover and Fink told me that they don’t know why it happens, what it means, how the brain copes, or how much of the brain behaves in this way. How can animals possibly make any lasting sense of the world if their neural responses to that world are constantly in flux? If such flux is common, “there must be mechanisms in the brain that are undiscovered and even unimagined that allow it to keep up,” Schoonover said. “Scientists are meant to know what’s going on, but in this particular case, we are deeply confused. We expect it to take many years to iron out.”

[...] It might be less common in other sensory hubs, such as the visual cortex, which processes information from the eyes. The neurons that respond to the smell of grass might change from month to month, but the ones that respond to the sight of grass seem to mostly stay the same. That might be because the visual cortex is highly organized. Adjacent groups of neurons tend to represent adjacent parts of the visual space in front of us, and this orderly mapping could constrain neural responses from drifting too far. But that might be true only for simple visual stimuli, such as lines or bars. Even in the visual cortex, Ziv found evidence of representational drift when mice watched the same movies over many days.

[...] How does the brain know what the nose is smelling or what the eyes are seeing, if the neural responses to smells and sights are continuously changing? One possibility is that it somehow corrects for drift. For example, parts of the brain that are connected to the piriform cortex might be able to gradually update their understanding of what the piriform’s neural activity means. The whole system changes, but it does so together.

Another possibility is that some high-level feature of the firing neurons stays the same, even as the specific active neurons change. As a simple analogy, “individuals in a population can change their mind while maintaining an overall consensus,” Timothy O’Leary, a neuroscientist at the University of Cambridge, told me. “The number of ways of representing the same signal in a large population is also large, so there’s room for the neural code to move.” Although some researchers have found signs of these stable, high-level patterns in other drifty parts of the brain, when Schoonover and Fink tried to do so in the piriform cortex, they couldn’t. Neither they nor their colleagues can conclusively say how the brain copes with representational drift. They’re also unsure why it happens at all.

Drift might simply be a nervous-system bug—a problem to be addressed. “The connections in many parts of the brain are being formed and broken down continually, and each neuron is itself continually recycling cellular material,” O’Leary said. Perhaps a system like this—a gray, goopy version of the ship of Theseus—is destined to drift over time. But that idea “is a little weak,” O’Leary told me. The nervous system can maintain precise and targeted connections, such as those between muscles and the nerves that control them. Drift doesn’t seem inevitable... (MORE - missing details)

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